Heat and Fluid Sciences
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2021 
Yenigun, Onur; Barisik, Murat Local Heat Transfer Control using Liquid Dielectrophoresis at Graphene/Water Interfaces Journal Article INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 166 , 2021, ISSN: 00179310. @article{ISI:000609976900040, title = {Local Heat Transfer Control using Liquid Dielectrophoresis at Graphene/Water Interfaces}, author = {Onur Yenigun and Murat Barisik}, doi = {10.1016/j.ijheatmasstransfer.2020.120801}, issn = {00179310}, year = {2021}, date = {20210201}, journal = {INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, volume = {166}, abstract = {Graphenebased materials are considered for the solution of the thermal management problem of current and next generation micro/nanoelectronics with high heat generation densities. However, the hydrophobic nature of fewlayer graphene makes passing heat to a fluid very challenging. We introduced an active and local manipulation of heat transfer between graphene and water using an applied, nonuniform electric field. When water undergoes electric field induced orientation polarization and liquid dielectrophoresis, a substantial increase in heat transfer develops due to a decrease in interfacial thermal resistance and increase in thermal conductivity. By using two locally embedded pin and plate electrodes of different sizes, we demonstrated a twodimensional heat transfer control between two parallel fewlayer graphene slabs. We obtained local heat transfer increase up to nine times at pin electrode region with an ultralow Kapitza resistance through the studied nonuniform electric field strength range creating highlyordered compressed water in the experimentally measured density limits. With this technique, heat can be (i) distributed from a smaller location to a larger section and/or (ii) collected to a smaller section from a larger region. Current results are important for hot spot cooling and/or heat focusing applications. (C) 2020 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphenebased materials are considered for the solution of the thermal management problem of current and next generation micro/nanoelectronics with high heat generation densities. However, the hydrophobic nature of fewlayer graphene makes passing heat to a fluid very challenging. We introduced an active and local manipulation of heat transfer between graphene and water using an applied, nonuniform electric field. When water undergoes electric field induced orientation polarization and liquid dielectrophoresis, a substantial increase in heat transfer develops due to a decrease in interfacial thermal resistance and increase in thermal conductivity. By using two locally embedded pin and plate electrodes of different sizes, we demonstrated a twodimensional heat transfer control between two parallel fewlayer graphene slabs. We obtained local heat transfer increase up to nine times at pin electrode region with an ultralow Kapitza resistance through the studied nonuniform electric field strength range creating highlyordered compressed water in the experimentally measured density limits. With this technique, heat can be (i) distributed from a smaller location to a larger section and/or (ii) collected to a smaller section from a larger region. Current results are important for hot spot cooling and/or heat focusing applications. (C) 2020 Elsevier Ltd. All rights reserved. 
2020 
Coskun, Turgay; Cetkin, Erdal A review of heat and fluid flow characteristics in microchannel heat sinks Journal Article HEAT TRANSFER, 49 (8), pp. 41094133, 2020, ISSN: 26884534. @article{ISI:000595699600002, title = {A review of heat and fluid flow characteristics in microchannel heat sinks}, author = {Turgay Coskun and Erdal Cetkin}, doi = {10.1002/htj.21819}, issn = {26884534}, year = {2020}, date = {20201201}, journal = {HEAT TRANSFER}, volume = {49}, number = {8}, pages = {41094133}, abstract = {Heat transfer and flow characteristic in microchannel heat sinks (MCHS) are extensively studied in the literature due to high heat transfer rate capability by increased heat transfer surface area relative to the macroscale heat sinks. However, heat transfer and fluid flow characteristics in MCHS differ from conventional ones because of the scaling effects. This review summarizes the studies that are mainly based on heat transfer and fluid flow characteristic in MCHS. There is no consistency among the published results; however, everyone agrees on that there is no new physical phenomenon in microscale that does not exist at macroscale. Only difference between them is that the effect of some physical phenomena such as viscous dissipation, axial heat conduction, entrance effect, rarefaction, and so forth, is negligibly small at macroscale, whereas it is not at microscale. The effect of these physical phenomena on the heat transfer and flow characteristics becomes significant with respect to specified conditions such as Reynolds number, Peclet number, hydraulic diameter, and heat transfer boundary conditions. Here, the literature was reviewed to document when these physical phenomena become significant and insignificant.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Heat transfer and flow characteristic in microchannel heat sinks (MCHS) are extensively studied in the literature due to high heat transfer rate capability by increased heat transfer surface area relative to the macroscale heat sinks. However, heat transfer and fluid flow characteristics in MCHS differ from conventional ones because of the scaling effects. This review summarizes the studies that are mainly based on heat transfer and fluid flow characteristic in MCHS. There is no consistency among the published results; however, everyone agrees on that there is no new physical phenomenon in microscale that does not exist at macroscale. Only difference between them is that the effect of some physical phenomena such as viscous dissipation, axial heat conduction, entrance effect, rarefaction, and so forth, is negligibly small at macroscale, whereas it is not at microscale. The effect of these physical phenomena on the heat transfer and flow characteristics becomes significant with respect to specified conditions such as Reynolds number, Peclet number, hydraulic diameter, and heat transfer boundary conditions. Here, the literature was reviewed to document when these physical phenomena become significant and insignificant. 
Toprak, Kasim; Ouedraogo, Kiswendsida Elias Performance Analysis of Thermal Storage Assisted Cooling Tower with Night Cooling Journal Article JOURNAL OF POLYTECHNICPOLITEKNIK DERGISI, 23 (4), pp. 10271035, 2020, ISSN: 13020900. @article{ISI:000581901200009, title = {Performance Analysis of Thermal Storage Assisted Cooling Tower with Night Cooling}, author = {Kasim Toprak and Kiswendsida Elias Ouedraogo}, issn = {13020900}, year = {2020}, date = {20201201}, journal = {JOURNAL OF POLYTECHNICPOLITEKNIK DERGISI}, volume = {23}, number = {4}, pages = {10271035}, abstract = {As global warming and water scarcity issues continue to grow, it is essential to increase resources efficiency for air conditioners and power plants. In order to increase the efficiency, the systems need to be modified to take the advantages of the low night temperature and thermal storage tanks. In this study, the low night temperature and thermal storage tanks effects on the cooling tower is studied using TRNSYS. Using a chiller operating from 8:00 to 16:00 as a case study, hot water from the condenser is partially stored on daytime and cooled slowly during the night. The storage tank volume is optimized by considering two big tanks and five small tanks. The results show that night cooling reduces cooling water temperature by 5.8 degrees C or 21.8% while the cooling efficiency is increased by 36%. The thermal storage tanks enable to have the low continuous flow rate and help to reduce the fan power by 67.1%. On the storage side, compared to two tanks system, the tanks volume is reduced by 16.5% when 5 tanks are used. In theory this reduction can go up to 50% by increasing the number of tanks and reducing their individual size.}, keywords = {}, pubstate = {published}, tppubtype = {article} } As global warming and water scarcity issues continue to grow, it is essential to increase resources efficiency for air conditioners and power plants. In order to increase the efficiency, the systems need to be modified to take the advantages of the low night temperature and thermal storage tanks. In this study, the low night temperature and thermal storage tanks effects on the cooling tower is studied using TRNSYS. Using a chiller operating from 8:00 to 16:00 as a case study, hot water from the condenser is partially stored on daytime and cooled slowly during the night. The storage tank volume is optimized by considering two big tanks and five small tanks. The results show that night cooling reduces cooling water temperature by 5.8 degrees C or 21.8% while the cooling efficiency is increased by 36%. The thermal storage tanks enable to have the low continuous flow rate and help to reduce the fan power by 67.1%. On the storage side, compared to two tanks system, the tanks volume is reduced by 16.5% when 5 tanks are used. In theory this reduction can go up to 50% by increasing the number of tanks and reducing their individual size. 
Ozcelik, Gokberk H; Satiroglu, Ezgi; Barisik, Murat Size dependent influence of contact line pinning on wetting of nanotextured/patterned silica surfaces Journal Article NANOSCALE, 12 (41), pp. 2137621391, 2020, ISSN: 20403364. @article{ISI:000584907600029, title = {Size dependent influence of contact line pinning on wetting of nanotextured/patterned silica surfaces}, author = {Gokberk H Ozcelik and Ezgi Satiroglu and Murat Barisik}, doi = {10.1039/d0nr05392a}, issn = {20403364}, year = {2020}, date = {20201101}, journal = {NANOSCALE}, volume = {12}, number = {41}, pages = {2137621391}, abstract = {Wetting behavior on a heterogeneous surface undergoes contact angle hysteresis as the droplet stabilized at a metastable state with a contact angle significantly different from its equilibrium value due to contact line pinning. However, there is a lack of consensus on how to calculate the influence of pinning forces. In general, the pinning effect can be characterized as (i) microscopic behavior when a droplet is pinned and the contact angle increases/decreases as the droplet volume increases/decreases and (ii) macroscopic behavior as the pinning effects decrease and ultimately, disappear with the increase of the droplet size. The current work studied both behaviors using molecular dynamics (MD) simulation with more than 300 different size water droplets on silica surfaces with three different patterns across two different wetting conditions. Results showed that the contact angle increases linearly with increasing droplet volume through the microscopic behavior, while the droplet is pinned on top of a certain number of patterns. When we normalized the droplet size with the corresponding pattern size, we observed a ``wetting similarity'' that linear microscopic contact angle variations over different size heterogeneities continuously line up. This shows that the pinning force remains constant and the resulting pinning effects are scalable by the size ratio between the droplet and pattern, independent of the sizescale. The slope of these microscopic linear variations decreases with an increase in the droplet size as observed through the macroscopic behavior. We further found a universal behavior in the variation of the corresponding pinning forces, independent of the wetting condition. In macroscopic behavior, pinning effects become negligible and the contact angle reaches the equilibrium value of the corresponding surface when the diameter of the freestanding droplet is approximately equal to 24 times the size of the surface structure. We found that the pinning effect is scalable with the droplet volume, not the size of the droplet base.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Wetting behavior on a heterogeneous surface undergoes contact angle hysteresis as the droplet stabilized at a metastable state with a contact angle significantly different from its equilibrium value due to contact line pinning. However, there is a lack of consensus on how to calculate the influence of pinning forces. In general, the pinning effect can be characterized as (i) microscopic behavior when a droplet is pinned and the contact angle increases/decreases as the droplet volume increases/decreases and (ii) macroscopic behavior as the pinning effects decrease and ultimately, disappear with the increase of the droplet size. The current work studied both behaviors using molecular dynamics (MD) simulation with more than 300 different size water droplets on silica surfaces with three different patterns across two different wetting conditions. Results showed that the contact angle increases linearly with increasing droplet volume through the microscopic behavior, while the droplet is pinned on top of a certain number of patterns. When we normalized the droplet size with the corresponding pattern size, we observed a ``wetting similarity'' that linear microscopic contact angle variations over different size heterogeneities continuously line up. This shows that the pinning force remains constant and the resulting pinning effects are scalable by the size ratio between the droplet and pattern, independent of the sizescale. The slope of these microscopic linear variations decreases with an increase in the droplet size as observed through the macroscopic behavior. We further found a universal behavior in the variation of the corresponding pinning forces, independent of the wetting condition. In macroscopic behavior, pinning effects become negligible and the contact angle reaches the equilibrium value of the corresponding surface when the diameter of the freestanding droplet is approximately equal to 24 times the size of the surface structure. We found that the pinning effect is scalable with the droplet volume, not the size of the droplet base. 
Yakin, Fetiye Esin; Barisik, Murat; Sen, Tumcan Pore Size and Porosity Dependent Zeta Potentials of Mesoporous Silica Nanoparticles Journal Article JOURNAL OF PHYSICAL CHEMISTRY C, 124 (36), pp. 1957919587, 2020, ISSN: 19327447. @article{ISI:000571497000020, title = {Pore Size and Porosity Dependent Zeta Potentials of Mesoporous Silica Nanoparticles}, author = {Fetiye Esin Yakin and Murat Barisik and Tumcan Sen}, doi = {10.1021/acs.jpcc.0c04602}, issn = {19327447}, year = {2020}, date = {20200901}, journal = {JOURNAL OF PHYSICAL CHEMISTRY C}, volume = {124}, number = {36}, pages = {1957919587}, abstract = {Mesoporous silica nanoparticles (MSNPs) are synthesized in the various forms of porous structures according to an application's needs, while their zeta potentials play a major role in their function. We show that variation in pore size and/or porosity yields a substantial decrease in MSNP zeta potential up to 25% lower than the theoretical zeta potential predictions for a flat surface at the corresponding ionic conditions in moderate pH range. By considering surface chemistry as a function of local ionic conditions (charge regulation), we calculated local zeta potentials around the MSNP which showed variation between pore openings and solid surfaces. Through a systematic study, we evaluated an average threedimensional zeta potential for MSNPs with various conditions, based on the ratio of the area covered by pore openings to the rest of the MSNP surface area as a function of threedimensional porosity and pore size. Results show that the high overlap of ionic layers inside the pores creates electric potentials close to zeta potential of the remaining surface, but large pore size and/or high ionic salt concentration yields divergence. We characterized the variation of MSNP zeta potential in terms of porosity (epsilon(3D)), pore size (Dpore), and ionic condition quantified by Debye length (lambda) and obtained unified behavior as a function of the nondimensional group of epsilon(3D)(Dpore/lambda). For epsilon(3D)(Dpore/lambda) < 0.01, MSNP zeta potential remains similar to flat plate predictions, but it decreases by increasing epsilon(3D)(Dpore/lambda) value. The influence of pore entrances on surface zeta potential increases nonlinearly by the increase of porosity and/or decrease of EDL overlap, similar to a change of area to volume ratio. The current findings are important for the understanding and further control of mesoporous particle transport in various promising and groundbreaking applications such as targeted drug delivery.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Mesoporous silica nanoparticles (MSNPs) are synthesized in the various forms of porous structures according to an application's needs, while their zeta potentials play a major role in their function. We show that variation in pore size and/or porosity yields a substantial decrease in MSNP zeta potential up to 25% lower than the theoretical zeta potential predictions for a flat surface at the corresponding ionic conditions in moderate pH range. By considering surface chemistry as a function of local ionic conditions (charge regulation), we calculated local zeta potentials around the MSNP which showed variation between pore openings and solid surfaces. Through a systematic study, we evaluated an average threedimensional zeta potential for MSNPs with various conditions, based on the ratio of the area covered by pore openings to the rest of the MSNP surface area as a function of threedimensional porosity and pore size. Results show that the high overlap of ionic layers inside the pores creates electric potentials close to zeta potential of the remaining surface, but large pore size and/or high ionic salt concentration yields divergence. We characterized the variation of MSNP zeta potential in terms of porosity (epsilon(3D)), pore size (Dpore), and ionic condition quantified by Debye length (lambda) and obtained unified behavior as a function of the nondimensional group of epsilon(3D)(Dpore/lambda). For epsilon(3D)(Dpore/lambda) < 0.01, MSNP zeta potential remains similar to flat plate predictions, but it decreases by increasing epsilon(3D)(Dpore/lambda) value. The influence of pore entrances on surface zeta potential increases nonlinearly by the increase of porosity and/or decrease of EDL overlap, similar to a change of area to volume ratio. The current findings are important for the understanding and further control of mesoporous particle transport in various promising and groundbreaking applications such as targeted drug delivery. 
Sen, Tumcan; Barisik, Murat Slip Effects on Ionic Current of Viscoelectric Electroviscous Flows through Different Length Nanofluidic Channels Journal Article LANGMUIR, 36 (31), pp. 91919203, 2020, ISSN: 07437463. @article{ISI:000562137700018, title = {Slip Effects on Ionic Current of Viscoelectric Electroviscous Flows through Different Length Nanofluidic Channels}, author = {Tumcan Sen and Murat Barisik}, doi = {10.1021/acs.langmuir.0c01457}, issn = {07437463}, year = {2020}, date = {20200801}, journal = {LANGMUIR}, volume = {36}, number = {31}, pages = {91919203}, abstract = {The pressure driven slip flow of an electrolyte solution is studied through different nanofluidic channel lengths at varying salt concentrations. The viscousthickening due to the electrostatic interactions within the electric double layer and the reverse ionic transport due to the streaming potential are developed. The influence of the Navier slip boundary condition is described under both electroviscous and viscoelectric effects with a surface charge regulation (CR) model while the observed behavior is compared and validated with molecular dynamic (MD) calculations from multiple studies. Results show that electroviscous and viscoelectric effects decrease transport. Earlier studies at the no slip boundary presented an increase of ionic current by increasing salt concentration and decreasing channel length. In contrast, our study found that the ionic current occurred almost independent of both salt concentration and channel length, except for very short channels and very low salt concentrations, when electroviscous and viscoelectric effects were considered. In the case of the constant slip length condition, ionic conduction was enhanced, but velocity slip developing on surfaces showed significant variation based on the salt concentration and channel length. This is due to the natural CR behavior enhancing the surface charge and consequential near surface electrohydrodynamics as a result of increase in salt concentration and/or decrease of channel length. Considering that the electroviscous effect alone creates up to 70% lower velocity slips than Poiseuille flow predictions, while further including the viscoelectric effect, results in an almost noslip condition at high salt concentrations and/or short channels. As a result, the ionic current of a viscoelectric electroviscous slip flow is found to be equal to 1/3 of an electroviscous slip flow and to decrease with a decrease in the channel length.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The pressure driven slip flow of an electrolyte solution is studied through different nanofluidic channel lengths at varying salt concentrations. The viscousthickening due to the electrostatic interactions within the electric double layer and the reverse ionic transport due to the streaming potential are developed. The influence of the Navier slip boundary condition is described under both electroviscous and viscoelectric effects with a surface charge regulation (CR) model while the observed behavior is compared and validated with molecular dynamic (MD) calculations from multiple studies. Results show that electroviscous and viscoelectric effects decrease transport. Earlier studies at the no slip boundary presented an increase of ionic current by increasing salt concentration and decreasing channel length. In contrast, our study found that the ionic current occurred almost independent of both salt concentration and channel length, except for very short channels and very low salt concentrations, when electroviscous and viscoelectric effects were considered. In the case of the constant slip length condition, ionic conduction was enhanced, but velocity slip developing on surfaces showed significant variation based on the salt concentration and channel length. This is due to the natural CR behavior enhancing the surface charge and consequential near surface electrohydrodynamics as a result of increase in salt concentration and/or decrease of channel length. Considering that the electroviscous effect alone creates up to 70% lower velocity slips than Poiseuille flow predictions, while further including the viscoelectric effect, results in an almost noslip condition at high salt concentrations and/or short channels. As a result, the ionic current of a viscoelectric electroviscous slip flow is found to be equal to 1/3 of an electroviscous slip flow and to decrease with a decrease in the channel length. 
Gocmen, Sinan; Gungor, Sahin; Cetkin, Erdal Thermal management of electric vehicle battery cells with homogeneous coolant and temperature distribution Journal Article JOURNAL OF APPLIED PHYSICS, 127 (23), 2020, ISSN: 00218979. @article{ISI:000542961200001, title = {Thermal management of electric vehicle battery cells with homogeneous coolant and temperature distribution}, author = {Sinan Gocmen and Sahin Gungor and Erdal Cetkin}, doi = {10.1063/5.0004453}, issn = {00218979}, year = {2020}, date = {20200601}, journal = {JOURNAL OF APPLIED PHYSICS}, volume = {127}, number = {23}, abstract = {Electric vehicles play an integral role in eliminating pollution related to transportation, especially if the electricity is generated via renewable sources. However, storing electricity onboard requires many battery cells. If the temperature of the cells is not strictly regulated, their capacity decreases in time, and they may burn or explode due to thermal runaway. Battery thermal management systems emerged for safe operations by keeping the battery cell temperatures under limit values. However, the current solutions do not yield uniform temperature distribution for all the cells in a pack. Here, we document that constant temperature distribution can be achieved with uniform coolant distribution to the channels located between batteries. The design process of the developed battery pack begins with a design used in current packs. Later, how the shape of the distributor channel affects flow uniformity is documented. Then, the design complexity was increased to satisfy the flow uniformity condition, which is essential for temperature uniformity. The design was altered based on a constructal design methodology with an iterative exhaustive search approach. The uncovered constructal design yields a uniform coolant distribution with a maximum of 0.81% flow rate deviation along channels. The developed design is palpable and easy to manufacture relative to the tapered manifold designs. The results also document that the peak temperature difference between the cells decreases from a maximum of 12K to 0.4K. Furthermore, homogenous distribution of air is one of the limiting factors of the development of metalair batteries. This paper also documents how air can be distributed uniformly to metalair battery cells in a battery pack.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Electric vehicles play an integral role in eliminating pollution related to transportation, especially if the electricity is generated via renewable sources. However, storing electricity onboard requires many battery cells. If the temperature of the cells is not strictly regulated, their capacity decreases in time, and they may burn or explode due to thermal runaway. Battery thermal management systems emerged for safe operations by keeping the battery cell temperatures under limit values. However, the current solutions do not yield uniform temperature distribution for all the cells in a pack. Here, we document that constant temperature distribution can be achieved with uniform coolant distribution to the channels located between batteries. The design process of the developed battery pack begins with a design used in current packs. Later, how the shape of the distributor channel affects flow uniformity is documented. Then, the design complexity was increased to satisfy the flow uniformity condition, which is essential for temperature uniformity. The design was altered based on a constructal design methodology with an iterative exhaustive search approach. The uncovered constructal design yields a uniform coolant distribution with a maximum of 0.81% flow rate deviation along channels. The developed design is palpable and easy to manufacture relative to the tapered manifold designs. The results also document that the peak temperature difference between the cells decreases from a maximum of 12K to 0.4K. Furthermore, homogenous distribution of air is one of the limiting factors of the development of metalair batteries. This paper also documents how air can be distributed uniformly to metalair battery cells in a battery pack. 
Alan, Oyku B; Barisik, Murat; Ozcelik, Gokberk H Roughness Effects on the Surface Charge Properties of Silica Nanoparticles Journal Article JOURNAL OF PHYSICAL CHEMISTRY C, 124 (13), pp. 72747286, 2020, ISSN: 19327447. @article{ISI:000526328500032, title = {Roughness Effects on the Surface Charge Properties of Silica Nanoparticles}, author = {Oyku B Alan and Murat Barisik and Gokberk H Ozcelik}, doi = {10.1021/acs.jpcc.0c00120}, issn = {19327447}, year = {2020}, date = {20200401}, journal = {JOURNAL OF PHYSICAL CHEMISTRY C}, volume = {124}, number = {13}, pages = {72747286}, abstract = {The surface charge property of silica nanoparticles plays a key role in their function. Previous studies assumed surface charge as a homogeneously distributed constant material property, independent of the nanoparticle size and surface condition. Instead, this study considered surface chemistry as a function of local ionic conditions (Charge Regulation) to calculate the local surface charges around a rough nanoparticle, as an extension to our earlier study (J. Phys. Chem. C 2014, 118 (4), 18361842). For the current surface heterogeneity in the form of concave and convex circles, the surface charge showed a distinct local variation: decrease due to the electrical double layer (EDL) overlap in the valleys and increase due to curvature effects on the hills of the surface structure. The average of local surface charges decreased with the decrease of the roughness size (DR), depending on the particle size (DP) and pH. We characterized the variation of the average surface charge by a nondimensional group we formed as a measure for the EDL overlap and curvature effects [(DR/lambda) x (DR/DP)]. Based on this, we devised a phenomenological model as an extension to the existing flat surface theory, which can successfully predict the average surface charge around a rough/patterned nanoparticle as a function of the particle size, roughness size, and pH.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The surface charge property of silica nanoparticles plays a key role in their function. Previous studies assumed surface charge as a homogeneously distributed constant material property, independent of the nanoparticle size and surface condition. Instead, this study considered surface chemistry as a function of local ionic conditions (Charge Regulation) to calculate the local surface charges around a rough nanoparticle, as an extension to our earlier study (J. Phys. Chem. C 2014, 118 (4), 18361842). For the current surface heterogeneity in the form of concave and convex circles, the surface charge showed a distinct local variation: decrease due to the electrical double layer (EDL) overlap in the valleys and increase due to curvature effects on the hills of the surface structure. The average of local surface charges decreased with the decrease of the roughness size (DR), depending on the particle size (DP) and pH. We characterized the variation of the average surface charge by a nondimensional group we formed as a measure for the EDL overlap and curvature effects [(DR/lambda) x (DR/DP)]. Based on this, we devised a phenomenological model as an extension to the existing flat surface theory, which can successfully predict the average surface charge around a rough/patterned nanoparticle as a function of the particle size, roughness size, and pH. 
Toprak, K; Ouedraogo, K E Effect of storage tanks on solarpowered absorption chiller cooling system performance Journal Article International Journal of Energy Research, 44 (6), pp. 43664375, 2020. @article{Toprak20204366, title = {Effect of storage tanks on solarpowered absorption chiller cooling system performance}, author = {K Toprak and K E Ouedraogo}, url = {https://www.scopus.com/inward/record.uri?eid=2s2.085079718680&doi=10.1002%2fer.5210&partnerID=40&md5=aa6c4b8eb29c64ec8849ffdd43934cc6}, doi = {10.1002/er.5210}, year = {2020}, date = {20200101}, journal = {International Journal of Energy Research}, volume = {44}, number = {6}, pages = {43664375}, abstract = {Thermal storage, low power tariff at night, and low nocturnal temperature can be used in synergy to reduce the cooling costs of the solarpowered absorption chiller cooling systems. This study aims to reduce the required cooling capacity of an absorption chiller (ACH) powered by a solar parabolic trough collector (PTC) and a backup fuel boiler by integrating thermal storage tanks. The thermal performance of the system is simulated for a building that is cooled for 14 h/day. The system uses 1000 m2 PTC with 1020 kW ACH. Chilled water storage (CHWS) and cooling water storage (CWS) effects on the system performance for different operation hours per day of the ACH under Izmir (Turkey) and Phoenix (USA) weather conditions are analyzed. When the ACH operates 14 h/day as the load for both systems and both locations, the variations of the solar collector efficiency and the cooling load to heat input ratio remain less than 4% after the modifications. From the addition of CHWS to the reference system, a parametric study consisting of changing the ACH operation hours per day shows that the required cooling capacity of the ACH can be reduced to 34%. The capacity factor of the ACH is improved from its reference value of 41% up to 96%. © 2020 John Wiley & Sons Ltd}, keywords = {}, pubstate = {published}, tppubtype = {article} } Thermal storage, low power tariff at night, and low nocturnal temperature can be used in synergy to reduce the cooling costs of the solarpowered absorption chiller cooling systems. This study aims to reduce the required cooling capacity of an absorption chiller (ACH) powered by a solar parabolic trough collector (PTC) and a backup fuel boiler by integrating thermal storage tanks. The thermal performance of the system is simulated for a building that is cooled for 14 h/day. The system uses 1000 m2 PTC with 1020 kW ACH. Chilled water storage (CHWS) and cooling water storage (CWS) effects on the system performance for different operation hours per day of the ACH under Izmir (Turkey) and Phoenix (USA) weather conditions are analyzed. When the ACH operates 14 h/day as the load for both systems and both locations, the variations of the solar collector efficiency and the cooling load to heat input ratio remain less than 4% after the modifications. From the addition of CHWS to the reference system, a parametric study consisting of changing the ACH operation hours per day shows that the required cooling capacity of the ACH can be reduced to 34%. The capacity factor of the ACH is improved from its reference value of 41% up to 96%. © 2020 John Wiley & Sons Ltd 
2018 
Kor, Orcun; Acarer, Sercan; Ozkol, Unver Aerodynamic optimization of throughflow design model of a high bypass transonic aeroengine fan using genetic algorithm Journal Article PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART AJOURNAL OF POWER AND ENERGY, 232 (3), pp. 211224, 2018, ISSN: 09576509. @article{ISI:000435491500001, title = {Aerodynamic optimization of throughflow design model of a high bypass transonic aeroengine fan using genetic algorithm}, author = {Orcun Kor and Sercan Acarer and Unver Ozkol}, doi = {10.1177/0957650917730466}, issn = {09576509}, year = {2018}, date = {20180501}, journal = {PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART AJOURNAL OF POWER AND ENERGY}, volume = {232}, number = {3}, pages = {211224}, abstract = {This study deals with aerodynamic optimization of a high bypass transonic aeroengine fan module in a throughflow inverse design model at cruise condition. To the authors' best knowledge, although the literature contains throughflow optimization of the simplified cases of compressors and turbines, an optimization study targeting the more elaborate case of combined transonic fan and splitter throughflow model is not considered in the literature. Such a throughflow optimization of a transonic fan, combined with bypass and core streams separated by an aerodynamically shaped flow splitter, possesses significant challenges to any optimizer, due to highly nonlinear nature of the problem and the high number of constraints, including the fulfillment of the targeted bypass ratio. It is the aim of this study to consider this previously untouched area in detail and therefore present a more sophisticated and accurate optimization environment for actual bypass fan systems. An inhouse optimization code using genetic algorithm is coupled with a previously developed inhouse throughflow solver which is using a streamline curvature technique and a set of inhouse calibrated empirical models for incidence, deviation, loss and blockage. As the throughflow models are the backbone of turbomachinery design, and great majority of design decisions are taken in this phase, such a study is assessed to result in significant guidelines to the gas turbine community.}, keywords = {}, pubstate = {published}, tppubtype = {article} } This study deals with aerodynamic optimization of a high bypass transonic aeroengine fan module in a throughflow inverse design model at cruise condition. To the authors' best knowledge, although the literature contains throughflow optimization of the simplified cases of compressors and turbines, an optimization study targeting the more elaborate case of combined transonic fan and splitter throughflow model is not considered in the literature. Such a throughflow optimization of a transonic fan, combined with bypass and core streams separated by an aerodynamically shaped flow splitter, possesses significant challenges to any optimizer, due to highly nonlinear nature of the problem and the high number of constraints, including the fulfillment of the targeted bypass ratio. It is the aim of this study to consider this previously untouched area in detail and therefore present a more sophisticated and accurate optimization environment for actual bypass fan systems. An inhouse optimization code using genetic algorithm is coupled with a previously developed inhouse throughflow solver which is using a streamline curvature technique and a set of inhouse calibrated empirical models for incidence, deviation, loss and blockage. As the throughflow models are the backbone of turbomachinery design, and great majority of design decisions are taken in this phase, such a study is assessed to result in significant guidelines to the gas turbine community. 
2017 
Cetin, Eylem; Cetkin, Erdal The effect of cavities and Tshaped assembly of fins on overall thermal resistances Journal Article INTERNATIONAL JOURNAL OF HEAT AND TECHNOLOGY, 35 (4), pp. 944952, 2017, ISSN: 03928764. @article{ISI:000429132900030, title = {The effect of cavities and Tshaped assembly of fins on overall thermal resistances}, author = {Eylem Cetin and Erdal Cetkin}, doi = {10.18280/ijht.350430}, issn = {03928764}, year = {2017}, date = {20171201}, journal = {INTERNATIONAL JOURNAL OF HEAT AND TECHNOLOGY}, volume = {35}, number = {4}, pages = {944952}, abstract = {In this study, authors show that maximum excess temperature on a heat generating cylindrical solid domain can be minimized with numerically optimized rectangular cavities and Tshaped fins. The effect of the cavities and the fins on overall thermal resistances were compared while their volume fraction in a unit volume element is fixed. Furthermore, the designs correspond to the minimum thermal resistance were uncovered for two types of flows; parallel and crossflow. The governing equations of the heat transfer and the fluid flow were solved simultaneously in order to show the effects of design on the flow characteristics and the thermal performance. Twodimensional solution domain was used to uncover the thermal performance in crossflow case because the flow direction is perpendicular to the heat transfer surface area of the heat generating domain. However, threedimensional domain was used in parallel flow case because the fluid flows along the outer surface of the heat generating domain. For the crossflow case, the results show that Tshaped assembly of fins with longer stem and shorter tributaries correspond to the lower peak temperature. In addition, the results also show that there is an optimal cavity shape that minimizes the peak temperature. This optimal shape becomes thinner when the number of the cavities increase. In parallel flow case, fins with thicker and shorter stem and longer tributaries correspond to the minimum excess temperature. In addition, the longer and thinner cavities increase the thermal performance in parallel flow case.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this study, authors show that maximum excess temperature on a heat generating cylindrical solid domain can be minimized with numerically optimized rectangular cavities and Tshaped fins. The effect of the cavities and the fins on overall thermal resistances were compared while their volume fraction in a unit volume element is fixed. Furthermore, the designs correspond to the minimum thermal resistance were uncovered for two types of flows; parallel and crossflow. The governing equations of the heat transfer and the fluid flow were solved simultaneously in order to show the effects of design on the flow characteristics and the thermal performance. Twodimensional solution domain was used to uncover the thermal performance in crossflow case because the flow direction is perpendicular to the heat transfer surface area of the heat generating domain. However, threedimensional domain was used in parallel flow case because the fluid flows along the outer surface of the heat generating domain. For the crossflow case, the results show that Tshaped assembly of fins with longer stem and shorter tributaries correspond to the lower peak temperature. In addition, the results also show that there is an optimal cavity shape that minimizes the peak temperature. This optimal shape becomes thinner when the number of the cavities increase. In parallel flow case, fins with thicker and shorter stem and longer tributaries correspond to the minimum excess temperature. In addition, the longer and thinner cavities increase the thermal performance in parallel flow case. 
Celebi, Alper Tunga; Barisik, Murat; Beskok, Ali Electric field controlled transport of water in graphene nanochannels Journal Article JOURNAL OF CHEMICAL PHYSICS, 147 (16), 2017, ISSN: 00219606. @article{ISI:000414177600067, title = {Electric field controlled transport of water in graphene nanochannels}, author = {Alper Tunga Celebi and Murat Barisik and Ali Beskok}, doi = {10.1063/1.4996210}, issn = {00219606}, year = {2017}, date = {20171001}, journal = {JOURNAL OF CHEMICAL PHYSICS}, volume = {147}, number = {16}, abstract = {Motivated by electrowettingbased flow control in nanosystems, water transport in graphene nanochannels is investigated as a function of the applied electric field. Molecular dynamics simulations are performed for deionized water confined in graphene nanochannels subjected to opposing surface charges, creating an electric field across the channel. Water molecules respond to the electric field by reorientation of their dipoles. Oxygen and hydrogen atoms in water face the anode and cathode, respectively, and hydrogen atoms get closer to the cathode compared to the oxygen atoms near the anode. These effects create asymmetric density distributions that increase with the applied electric field. Forcedriven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths. Apparent viscosity of water increases and the slip length decreases with increased electric field, reducing the flow rate. Increasing the electric field above a threshold value freezes water at room temperature. Published by AIP Publishing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Motivated by electrowettingbased flow control in nanosystems, water transport in graphene nanochannels is investigated as a function of the applied electric field. Molecular dynamics simulations are performed for deionized water confined in graphene nanochannels subjected to opposing surface charges, creating an electric field across the channel. Water molecules respond to the electric field by reorientation of their dipoles. Oxygen and hydrogen atoms in water face the anode and cathode, respectively, and hydrogen atoms get closer to the cathode compared to the oxygen atoms near the anode. These effects create asymmetric density distributions that increase with the applied electric field. Forcedriven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths. Apparent viscosity of water increases and the slip length decreases with increased electric field, reducing the flow rate. Increasing the electric field above a threshold value freezes water at room temperature. Published by AIP Publishing. 
Kalyoncu, Gulce; Barisik, Murat Analytical solution of micro/nanoscale convective liquid flows in tubes and slits Journal Article MICROFLUIDICS AND NANOFLUIDICS, 21 (9), 2017, ISSN: 16134982. @article{ISI:000410286400004, title = {Analytical solution of micro/nanoscale convective liquid flows in tubes and slits}, author = {Gulce Kalyoncu and Murat Barisik}, doi = {10.1007/s1040401719855}, issn = {16134982}, year = {2017}, date = {20170901}, journal = {MICROFLUIDICS AND NANOFLUIDICS}, volume = {21}, number = {9}, abstract = {Analytical solutions examining heat transport in micro/nanoscale liquid flows were developed. Using the energy equation coupled with fully developed velocity, we solved developing temperature profiles with axial conduction and viscous dissipation terms. A comprehensive literature review provided the published range of velocity slip and temperature jump conditions. While molecular simulations and experiments present constant slip and jump values for a specific liquid/surface couple independent of confinement size, nondimensional forms of these boundary conditions were found appropriate to calculate nonequilibrium as a function of flow height. Although slip and jump conditions are specific for each liquid/surface couple and hard to obtain, we proposed modeling of the slip and jump as a function of the surface wetting, in order to create a general, easy to measure methodology. We further developed possible correlations to calculate jump using the slip value of the corresponding surface and tested in the results. Fully developed Nu showed strong dependence on slip and jump. Heat transfer stopped when slip and jump coefficients became higher than a certain value. Strong variation of Nu in the thermal development length was observed for low slip and jump cases, while an almost constant Nu in the flow direction was found for high slip and jump coefficients. Variation of temperature profiles was found to dominate the heat transfer through the constant temperature surface while surface and liquid temperatures became equal at heat transfer lengths comparable with confinement sizes for nodissipation cases. In case of nonnegligible heat dissipation, viscous heating dominated the Nu value by enhancing the heating while decreasing the heat removal in cooling cases. Implementation of proposed procedure on a microchannel convection problem from a microfluidics application showed the dominant effect of the model defining the slip and jump relationship. Direct use of kinetic gas theory resulted in an increase of Nu by an increase in nonequilibrium, while models developed from published liquid slip and jump values produced an opposite behavior.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Analytical solutions examining heat transport in micro/nanoscale liquid flows were developed. Using the energy equation coupled with fully developed velocity, we solved developing temperature profiles with axial conduction and viscous dissipation terms. A comprehensive literature review provided the published range of velocity slip and temperature jump conditions. While molecular simulations and experiments present constant slip and jump values for a specific liquid/surface couple independent of confinement size, nondimensional forms of these boundary conditions were found appropriate to calculate nonequilibrium as a function of flow height. Although slip and jump conditions are specific for each liquid/surface couple and hard to obtain, we proposed modeling of the slip and jump as a function of the surface wetting, in order to create a general, easy to measure methodology. We further developed possible correlations to calculate jump using the slip value of the corresponding surface and tested in the results. Fully developed Nu showed strong dependence on slip and jump. Heat transfer stopped when slip and jump coefficients became higher than a certain value. Strong variation of Nu in the thermal development length was observed for low slip and jump cases, while an almost constant Nu in the flow direction was found for high slip and jump coefficients. Variation of temperature profiles was found to dominate the heat transfer through the constant temperature surface while surface and liquid temperatures became equal at heat transfer lengths comparable with confinement sizes for nodissipation cases. In case of nonnegligible heat dissipation, viscous heating dominated the Nu value by enhancing the heating while decreasing the heat removal in cooling cases. Implementation of proposed procedure on a microchannel convection problem from a microfluidics application showed the dominant effect of the model defining the slip and jump relationship. Direct use of kinetic gas theory resulted in an increase of Nu by an increase in nonequilibrium, while models developed from published liquid slip and jump values produced an opposite behavior. 
Cetkin, Erdal JOURNAL OF HEAT TRANSFERTRANSACTIONS OF THE ASME, 139 (8), 2017, ISSN: 00221481. @article{ISI:000426124400018, title = {Constructal Microdevice Manifold Design With Uniform Flow Rate Distribution by Consideration of the TreeBranching Rule of Leonardo da Vinci and HessMurray Rule}, author = {Erdal Cetkin}, doi = {10.1115/1.4036089}, issn = {00221481}, year = {2017}, date = {20170801}, journal = {JOURNAL OF HEAT TRANSFERTRANSACTIONS OF THE ASME}, volume = {139}, number = {8}, abstract = {In this paper, we show how the design of a microdevice manifold should be tapered for uniform flow rate distribution. The designs based on the treebranching rule of Leonardo da Vinci and the HessMurray rule were considered in addition to the constructal design. Both da Vinci and HessMurray designs are insensitive to the inlet velocity, and they provide better flow uniformity than the base (not tapered) design. However, the results of this paper uncover that not only pressure drop but also velocity distribution in the microdevice play an integral role in the flow uniformity. Therefore, an iterative approach was adopted with five degreesoffreedom (inclined wall positions) and one constraint (constant distribution channel thickness) in order to uncover the constructal design which conforms the uniform flow rate distribution. In addition, the effect of slenderness of the microchannels (Svelteness) and inlet velocity on the flow rate distribution to the microchannels has been documented. This paper also uncovers that the design of a manifold should be designed with not only the consideration of pressure distribution but also dynamic pressure distribution especially for nonSvelte microdevices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this paper, we show how the design of a microdevice manifold should be tapered for uniform flow rate distribution. The designs based on the treebranching rule of Leonardo da Vinci and the HessMurray rule were considered in addition to the constructal design. Both da Vinci and HessMurray designs are insensitive to the inlet velocity, and they provide better flow uniformity than the base (not tapered) design. However, the results of this paper uncover that not only pressure drop but also velocity distribution in the microdevice play an integral role in the flow uniformity. Therefore, an iterative approach was adopted with five degreesoffreedom (inclined wall positions) and one constraint (constant distribution channel thickness) in order to uncover the constructal design which conforms the uniform flow rate distribution. In addition, the effect of slenderness of the microchannels (Svelteness) and inlet velocity on the flow rate distribution to the microchannels has been documented. This paper also uncovers that the design of a manifold should be designed with not only the consideration of pressure distribution but also dynamic pressure distribution especially for nonSvelte microdevices. 
Cetkin, E VASCULAR STRUCTURES FOR SMART FEATURES: SELFCOOLING AND SELFHEALING Journal Article JOURNAL OF THERMAL ENGINEERING, 3 (4, 5), pp. 13381345, 2017, ISSN: 21487847. @article{ISI:000407804200004, title = {VASCULAR STRUCTURES FOR SMART FEATURES: SELFCOOLING AND SELFHEALING}, author = {E Cetkin}, doi = {10.18186/journalofthermalengineering.330185}, issn = {21487847}, year = {2017}, date = {20170801}, journal = {JOURNAL OF THERMAL ENGINEERING}, volume = {3}, number = {4, 5}, pages = {13381345}, abstract = {Here we show how smart features of selfcooling and selfhealing can be gained to mechanical systems with embedded vascular structures. Vascular structures mimic the circulatory system of animals. Similar to blood distribution from heart to the animal body, vascular channels provide the distribution of coolant and/or healing agent from a point to the entire body of a mechanic system. Thus the mechanic system becomes capable of cooling itself under unpredictable heat attacks and capable of healing itself as cracks occur due to applied mechanical loads. These smart features are necessary for advanced devices, equipment and vehicles. The essential design parameter is vascularization in order to provide smart features. There are distinct configurations for vascularization such as radial, treeshaped, grid and hybrids of these designs. In addition, several theories are available for the shape optimization of vascular structures such as fractal theory and constructal theory. Unlike fractal theory, constructal theory does not include constraints based on generic algorithms and dictated assumptions. Therefore, constructal theory approach is discussed in this paper. This paper shows how smart features can be gained to a mechanical system while its weight decreases and its mechanical strength increases simultaneously.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Here we show how smart features of selfcooling and selfhealing can be gained to mechanical systems with embedded vascular structures. Vascular structures mimic the circulatory system of animals. Similar to blood distribution from heart to the animal body, vascular channels provide the distribution of coolant and/or healing agent from a point to the entire body of a mechanic system. Thus the mechanic system becomes capable of cooling itself under unpredictable heat attacks and capable of healing itself as cracks occur due to applied mechanical loads. These smart features are necessary for advanced devices, equipment and vehicles. The essential design parameter is vascularization in order to provide smart features. There are distinct configurations for vascularization such as radial, treeshaped, grid and hybrids of these designs. In addition, several theories are available for the shape optimization of vascular structures such as fractal theory and constructal theory. Unlike fractal theory, constructal theory does not include constraints based on generic algorithms and dictated assumptions. Therefore, constructal theory approach is discussed in this paper. This paper shows how smart features can be gained to a mechanical system while its weight decreases and its mechanical strength increases simultaneously. 
Celik, Hasan; Mobedi, Moghtada; Manca, Oronzio; Ozkol, Unver A pore scale analysis for determination of interfacial convective heat transfer coefficient for thin periodic porousmedia undermixed convection Journal Article INTERNATIONAL JOURNAL OF NUMERICAL METHODS FOR HEAT & FLUID FLOW, 27 (12), pp. 27752798, 2017, ISSN: 09615539. @article{ISI:000416431600006b, title = {A pore scale analysis for determination of interfacial convective heat transfer coefficient for thin periodic porousmedia undermixed convection}, author = {Hasan Celik and Moghtada Mobedi and Oronzio Manca and Unver Ozkol}, doi = {10.1108/HFF0120170036}, issn = {09615539}, year = {2017}, date = {20170101}, journal = {INTERNATIONAL JOURNAL OF NUMERICAL METHODS FOR HEAT & FLUID FLOW}, volume = {27}, number = {12}, pages = {27752798}, abstract = {Purpose  The purpose of this study is to determine interfacial convective heat transfer coefficient numerically, for a porous media consisting of square blocks in inline arrangement under mixed convection heat transfer. Design/methodology/approach  The continuity, momentum and energy equations are solved in dimensionless form for a representative elementary volume of porous media, numerically. The velocity and temperature fields for different values of porosity, Ri and Re numbers are obtained. The study is performed for the range of Ri number from 0.01 to 10, Re number from 100 to 500 and porosity value from 0.51 to 0.96. Based on the obtained results, the value of the interfacial convective heat transfer coefficient is calculated by using volume average method. Findings  It was found that at low porosities (such as 0.51), the interfacial Nusselt number does not considerably change with Ri and Re numbers. However, for porous media with high Ri number and porosity (such as 10 and 0.51, respectively), secondary flows occur in the middle of the channel between rods improving heat transfer between solid and fluid, considerably. It is shown that the available correlations of interfacial heat transfer coefficient suggested for forced convection can be used for mixed convection for the porous media with low porosity (such as 0.51) or for the flow with low Ri number (such as 0.01). Originality/value  To the best of the authors' knowledge, there is no study on determination of interfacial convective heat transfer coefficient for mixed convection in porous media in literature. The present study might be the first study providing an accurate idea on the range of this important parameter, which will be useful particularly for researchers who study on mixed convection heat transfer in porous media, macroscopically.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Purpose  The purpose of this study is to determine interfacial convective heat transfer coefficient numerically, for a porous media consisting of square blocks in inline arrangement under mixed convection heat transfer. Design/methodology/approach  The continuity, momentum and energy equations are solved in dimensionless form for a representative elementary volume of porous media, numerically. The velocity and temperature fields for different values of porosity, Ri and Re numbers are obtained. The study is performed for the range of Ri number from 0.01 to 10, Re number from 100 to 500 and porosity value from 0.51 to 0.96. Based on the obtained results, the value of the interfacial convective heat transfer coefficient is calculated by using volume average method. Findings  It was found that at low porosities (such as 0.51), the interfacial Nusselt number does not considerably change with Ri and Re numbers. However, for porous media with high Ri number and porosity (such as 10 and 0.51, respectively), secondary flows occur in the middle of the channel between rods improving heat transfer between solid and fluid, considerably. It is shown that the available correlations of interfacial heat transfer coefficient suggested for forced convection can be used for mixed convection for the porous media with low porosity (such as 0.51) or for the flow with low Ri number (such as 0.01). Originality/value  To the best of the authors' knowledge, there is no study on determination of interfacial convective heat transfer coefficient for mixed convection in porous media in literature. The present study might be the first study providing an accurate idea on the range of this important parameter, which will be useful particularly for researchers who study on mixed convection heat transfer in porous media, macroscopically. 
2016 
Kalyoncu, Gulce; Barisik, Murat The extended Graetz problem for microslit geometries; analytical coupling of rarefaction, axial conduction and viscous dissipation Journal Article INTERNATIONAL JOURNAL OF THERMAL SCIENCES, 110 , pp. 261269, 2016, ISSN: 12900729. @article{ISI:000382793600021, title = {The extended Graetz problem for microslit geometries; analytical coupling of rarefaction, axial conduction and viscous dissipation}, author = {Gulce Kalyoncu and Murat Barisik}, doi = {10.1016/j.ijthermalsci.2016.07.009}, issn = {12900729}, year = {2016}, date = {20161201}, journal = {INTERNATIONAL JOURNAL OF THERMAL SCIENCES}, volume = {110}, pages = {261269}, abstract = {In order to support the recent MEMS and Labonachip technologies, we studied heat transport in microscale slit channel gas flows. Since the micro convection transport phenomena diverges from conventional macroscale transport due to rarefaction, axial conduction and viscous heating, an accurate understanding requires a complete coupling of these effects. For such cases, we studied heat transfer in hydrodynamically developed, thermally developing gas flows in microslits at various flow conditions. The analytical solution of the energy equation considered both the heat conduction in the axial direction and heat dissipation of viscous forces. Furthermore, updated boundary conditions of velocity slip and temperature jump were applied based on Knudsen number of flow in order to account for the non equilibrium gas dynamics. Local Nusselt number (Nu) values were calculated as a function of Peclet (Pe), Knudsen (Kn) and Brinkman (Br) numbers which were selected carefully according to possible microflow cases. Strong variation of Nu in thermal development length was found to dominate heat transfer behavior of microslits with short heating lengths for early slip flow regime. For this instance, influence of axial conduction and viscous dissipation was equally important. On the other hand, high Kn slip flow suppressed the axial conduction while viscous heating in a small surfacegas temperature difference case mostly determined the fully developed Nu and average heat transfer behavior as a function of Kn value. (C) 2016 Elsevier Masson SAS. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In order to support the recent MEMS and Labonachip technologies, we studied heat transport in microscale slit channel gas flows. Since the micro convection transport phenomena diverges from conventional macroscale transport due to rarefaction, axial conduction and viscous heating, an accurate understanding requires a complete coupling of these effects. For such cases, we studied heat transfer in hydrodynamically developed, thermally developing gas flows in microslits at various flow conditions. The analytical solution of the energy equation considered both the heat conduction in the axial direction and heat dissipation of viscous forces. Furthermore, updated boundary conditions of velocity slip and temperature jump were applied based on Knudsen number of flow in order to account for the non equilibrium gas dynamics. Local Nusselt number (Nu) values were calculated as a function of Peclet (Pe), Knudsen (Kn) and Brinkman (Br) numbers which were selected carefully according to possible microflow cases. Strong variation of Nu in thermal development length was found to dominate heat transfer behavior of microslits with short heating lengths for early slip flow regime. For this instance, influence of axial conduction and viscous dissipation was equally important. On the other hand, high Kn slip flow suppressed the axial conduction while viscous heating in a small surfacegas temperature difference case mostly determined the fully developed Nu and average heat transfer behavior as a function of Kn value. (C) 2016 Elsevier Masson SAS. All rights reserved. 
Yenigun, O; Cetkin, E Experimental and numerical investigation of constructal vascular channels for selfcooling: Parallel channels, treeshaped and hybrid designs Journal Article INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 103 , pp. 11551165, 2016, ISSN: 00179310. @article{ISI:000384777800107b, title = {Experimental and numerical investigation of constructal vascular channels for selfcooling: Parallel channels, treeshaped and hybrid designs}, author = {O Yenigun and E Cetkin}, doi = {10.1016/j.ijheatmasstransfer.2016.08.074}, issn = {00179310}, year = {2016}, date = {20161201}, journal = {INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, volume = {103}, pages = {11551165}, abstract = {In this paper, we show experimentally and numerically how a plate which is subjected to a constant heat load can be kept under an allowable temperature limit. Vascular channels in which coolant fluid flows have been embedded in the plate. Three types of vascular channel designs were compared: parallel channels, treeshaped and their hybrid. The effects of channel design on the thermal performance for different volume fractions (the fluid volume over the solid volume) are documented. In addition, the effects of the number of channels on cooling performance have been documented. Changing the design from parallel channels to treeshaped designs decreases the order of pressure drop. Hence increase in the order of the convective heat transfer coefficient is achieved. However, treeshaped designs do not bathe the entire domain, which increases the conductive resistances. Therefore, additional channels were inserted at the uncooled regions in the treeshaped design (hybrid design). The best features of both parallel channels and treeshaped designs are combined in the hybrid of them: the flow resistances to the fluid and heat flow become almost as low as the treeshaped and parallel channels designs, respectively. The effect of design on the maximum temperature shows that there should be an optimum design for a distinct set of boundary conditions, and this design should be varied as the boundary conditions change. This result is in accord with the constructal law, i.e. the shape should be varied in order to minimize resistances to the flows. (C) 2016 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this paper, we show experimentally and numerically how a plate which is subjected to a constant heat load can be kept under an allowable temperature limit. Vascular channels in which coolant fluid flows have been embedded in the plate. Three types of vascular channel designs were compared: parallel channels, treeshaped and their hybrid. The effects of channel design on the thermal performance for different volume fractions (the fluid volume over the solid volume) are documented. In addition, the effects of the number of channels on cooling performance have been documented. Changing the design from parallel channels to treeshaped designs decreases the order of pressure drop. Hence increase in the order of the convective heat transfer coefficient is achieved. However, treeshaped designs do not bathe the entire domain, which increases the conductive resistances. Therefore, additional channels were inserted at the uncooled regions in the treeshaped design (hybrid design). The best features of both parallel channels and treeshaped designs are combined in the hybrid of them: the flow resistances to the fluid and heat flow become almost as low as the treeshaped and parallel channels designs, respectively. The effect of design on the maximum temperature shows that there should be an optimum design for a distinct set of boundary conditions, and this design should be varied as the boundary conditions change. This result is in accord with the constructal law, i.e. the shape should be varied in order to minimize resistances to the flows. (C) 2016 Elsevier Ltd. All rights reserved. 
Pham, An Truong; Barisik, Murat; Kim, BoHung Interfacial thermal resistance between the graphenecoated copper and liquid water Journal Article INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 97 , pp. 422431, 2016, ISSN: 00179310. @article{ISI:000374616900038, title = {Interfacial thermal resistance between the graphenecoated copper and liquid water}, author = {An Truong Pham and Murat Barisik and BoHung Kim}, doi = {10.1016/j.ijheatmasstransfer.2016.02.040}, issn = {00179310}, year = {2016}, date = {20160601}, journal = {INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, volume = {97}, pages = {422431}, abstract = {The thermal coupling at watersolid interfaces is a key factor in controlling thermal resistance and the performance of nanoscale devices. This is especially important across the recently engineered nanocomposite structures composed of a graphenecoatedmetal surface. In this paper, a series of molecular dynamics simulations were conducted to investigate Kapitza length at the interface of liquid water and nanocomposite surfaces of graphenecoatedCu(111). We found that Kapitza length gradually increased and converged to the value measured on pure graphite surface with the increase of the number of graphene layers inserted on the Cu surface. Different than the earlier hypothesis on the ``transparency of graphene,'' the Kapitza length at the interface of monolayer graphene coated Cu and water was found to be 2.5 times larger than the value of bare Cu surface. This drastic change of thermal resistance with the additional of a single graphene is validated by the surface energy calculations indicating that the monolayer graphene allows only similar to 18% van der Waals energy of underneath Cu to transmit. We introduced an ``overall interaction strength'' value for the nanocomposites based the quantitative contribution of pair interaction potentials of each material with water into the total surface energy in each case. Similar to earlier studies, results revealed that Kapitza length shows exponentially variation as a function of the estimated interaction strength of the nanocomposite surfaces. The effect of Cu/graphene coupling on thermal behavior between the nanocomposite with water was characterized. The Kapitza length was found to decrease significantly with increased Cu/graphene strength in the case of weak coupling, while this behavior becomes negligible with strong coupling of Cu and graphene. (C) 2016 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The thermal coupling at watersolid interfaces is a key factor in controlling thermal resistance and the performance of nanoscale devices. This is especially important across the recently engineered nanocomposite structures composed of a graphenecoatedmetal surface. In this paper, a series of molecular dynamics simulations were conducted to investigate Kapitza length at the interface of liquid water and nanocomposite surfaces of graphenecoatedCu(111). We found that Kapitza length gradually increased and converged to the value measured on pure graphite surface with the increase of the number of graphene layers inserted on the Cu surface. Different than the earlier hypothesis on the ``transparency of graphene,'' the Kapitza length at the interface of monolayer graphene coated Cu and water was found to be 2.5 times larger than the value of bare Cu surface. This drastic change of thermal resistance with the additional of a single graphene is validated by the surface energy calculations indicating that the monolayer graphene allows only similar to 18% van der Waals energy of underneath Cu to transmit. We introduced an ``overall interaction strength'' value for the nanocomposites based the quantitative contribution of pair interaction potentials of each material with water into the total surface energy in each case. Similar to earlier studies, results revealed that Kapitza length shows exponentially variation as a function of the estimated interaction strength of the nanocomposite surfaces. The effect of Cu/graphene coupling on thermal behavior between the nanocomposite with water was characterized. The Kapitza length was found to decrease significantly with increased Cu/graphene strength in the case of weak coupling, while this behavior becomes negligible with strong coupling of Cu and graphene. (C) 2016 Elsevier Ltd. All rights reserved. 
Vo, Truong Quoc; Barisik, Murat; Kim, BoHung Atomic density effects on temperature characteristics and thermal transport at grain boundaries through a proper bin size selection Journal Article JOURNAL OF CHEMICAL PHYSICS, 144 (19), 2016, ISSN: 00219606. @article{ISI:000377712600039, title = {Atomic density effects on temperature characteristics and thermal transport at grain boundaries through a proper bin size selection}, author = {Truong Quoc Vo and Murat Barisik and BoHung Kim}, doi = {10.1063/1.4949763}, issn = {00219606}, year = {2016}, date = {20160501}, journal = {JOURNAL OF CHEMICAL PHYSICS}, volume = {144}, number = {19}, abstract = {This study focuses on the proper characterization of temperature profiles across grain boundaries (GBs) in order to calculate the correct interfacial thermal resistance (ITR) and reveal the influence of GB geometries onto thermal transport. The solidsolid interfaces resulting from the orientation difference between the (001), (011), and (111) copper surfaces were investigated. Temperature discontinuities were observed at the boundary of grains due to the phonon mismatch, phonon backscattering, and atomic forces between dissimilar structures at the GBs. We observed that the temperature decreases gradually in the GB area rather than a sharp drop at the interface. As a result, three distinct temperature gradients developed at the GB which were different than the one observed in the bulk solid. This behavior extends a couple molecular diameters into both sides of the interface where we defined a thickness at GB based on the measured temperature profiles for characterization. Results showed dependence on the selection of the bin size used to average the temperature data from the molecular dynamics system. The bin size on the order of the crystal layer spacing was found to present an accurate temperature profile through the GB. We further calculated the GB thickness of various cases by using potential energy (PE) distributions which showed agreement with direct measurements from the temperature profile and validated the proper binning. The variation of grain crystal orientation developed different molecular densities which were characterized by the average atomic surface density (ASD) definition. Our results revealed that the ASD is the primary factor affecting the structural disorders and heat transfer at the solidsolid interfaces. Using a system in which the planes are highly closepacked can enhance the probability of interactions and the degree of overlap between vibrational density of states (VDOS) of atoms forming at interfaces, leading to a reduced ITR. Thus, an accurate understanding of thermal characteristics at the GB can be formulated by selecting a proper bin size. Published by AIP Publishing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } This study focuses on the proper characterization of temperature profiles across grain boundaries (GBs) in order to calculate the correct interfacial thermal resistance (ITR) and reveal the influence of GB geometries onto thermal transport. The solidsolid interfaces resulting from the orientation difference between the (001), (011), and (111) copper surfaces were investigated. Temperature discontinuities were observed at the boundary of grains due to the phonon mismatch, phonon backscattering, and atomic forces between dissimilar structures at the GBs. We observed that the temperature decreases gradually in the GB area rather than a sharp drop at the interface. As a result, three distinct temperature gradients developed at the GB which were different than the one observed in the bulk solid. This behavior extends a couple molecular diameters into both sides of the interface where we defined a thickness at GB based on the measured temperature profiles for characterization. Results showed dependence on the selection of the bin size used to average the temperature data from the molecular dynamics system. The bin size on the order of the crystal layer spacing was found to present an accurate temperature profile through the GB. We further calculated the GB thickness of various cases by using potential energy (PE) distributions which showed agreement with direct measurements from the temperature profile and validated the proper binning. The variation of grain crystal orientation developed different molecular densities which were characterized by the average atomic surface density (ASD) definition. Our results revealed that the ASD is the primary factor affecting the structural disorders and heat transfer at the solidsolid interfaces. Using a system in which the planes are highly closepacked can enhance the probability of interactions and the degree of overlap between vibrational density of states (VDOS) of atoms forming at interfaces, leading to a reduced ITR. Thus, an accurate understanding of thermal characteristics at the GB can be formulated by selecting a proper bin size. Published by AIP Publishing. 
Barisik, Murat; Beskok, Ali ``Law of the nanowall'' in nanochannel gas flows Journal Article MICROFLUIDICS AND NANOFLUIDICS, 20 (3), 2016, ISSN: 16134982. @article{ISI:000372866300003, title = {``Law of the nanowall'' in nanochannel gas flows}, author = {Murat Barisik and Ali Beskok}, doi = {10.1007/s1040401617136}, issn = {16134982}, year = {2016}, date = {20160301}, journal = {MICROFLUIDICS AND NANOFLUIDICS}, volume = {20}, number = {3}, abstract = {Molecular dynamics simulations of forcedriven nanochannel gas flows show two distinct flow regions. While the bulk flow region can be determined using kinetic theory, transport in the nearwall region is dominated by gaswall interactions. This duality enables definition of an innerlayer scaling, y*, based on the molecular dimensions. For gaswall interactions determined by LennardJones potential, the velocity distribution for y* <= 3 exhibits a universal behavior as a function of the local Knudsen number and gaswall interaction parameters, which can be interpreted as the ``law of the nanowall.'' Knowing the velocity and density distributions within this region and using the bulk flow velocity profiles from BeskokKarniadakis model (Beskok and Karniadakis in Microscale Thermophys Eng 3(1): 4377, 1999), we outline a procedure that can correct kinetictheorybased mass flow rate predictions in the literature for various nanochannel gas flows.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Molecular dynamics simulations of forcedriven nanochannel gas flows show two distinct flow regions. While the bulk flow region can be determined using kinetic theory, transport in the nearwall region is dominated by gaswall interactions. This duality enables definition of an innerlayer scaling, y*, based on the molecular dimensions. For gaswall interactions determined by LennardJones potential, the velocity distribution for y* <= 3 exhibits a universal behavior as a function of the local Knudsen number and gaswall interaction parameters, which can be interpreted as the ``law of the nanowall.'' Knowing the velocity and density distributions within this region and using the bulk flow velocity profiles from BeskokKarniadakis model (Beskok and Karniadakis in Microscale Thermophys Eng 3(1): 4377, 1999), we outline a procedure that can correct kinetictheorybased mass flow rate predictions in the literature for various nanochannel gas flows. 
2015 
Cetkin, Erdal Constructal Vascular Structures With HighConductivity Inserts for SelfCooling Journal Article JOURNAL OF HEAT TRANSFERTRANSACTIONS OF THE ASME, 137 (11), 2015, ISSN: 00221481. @article{ISI:000362512900008, title = {Constructal Vascular Structures With HighConductivity Inserts for SelfCooling}, author = {Erdal Cetkin}, doi = {10.1115/1.4030906}, issn = {00221481}, year = {2015}, date = {20151101}, journal = {JOURNAL OF HEAT TRANSFERTRANSACTIONS OF THE ASME}, volume = {137}, number = {11}, abstract = {In this paper, we show how a heatgenerating domain can be cooled with embedded cooling channels and highconductivity inserts. The volume of cooling channels and highconductivity inserts is fixed, so is the volume of the heatgenerating domain. The maximum temperature in the domain decreases with highconductivity inserts even though the coolant volume decreases. The locations and the shapes of highconductivity inserts corresponding to the smallest peak temperatures for different number of inserts are documented}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this paper, we show how a heatgenerating domain can be cooled with embedded cooling channels and highconductivity inserts. The volume of cooling channels and highconductivity inserts is fixed, so is the volume of the heatgenerating domain. The maximum temperature in the domain decreases with highconductivity inserts even though the coolant volume decreases. The locations and the shapes of highconductivity inserts corresponding to the smallest peak temperatures for different number of inserts are documented 
Vo, Truong Quoc; Barisik, Murat; Kim, BoHung Nearsurface viscosity effects on capillary rise of water in nanotubes Journal Article PHYSICAL REVIEW E, 92 (5), 2015, ISSN: 15393755. @article{ISI:000364413300003, title = {Nearsurface viscosity effects on capillary rise of water in nanotubes}, author = {Truong Quoc Vo and Murat Barisik and BoHung Kim}, doi = {10.1103/PhysRevE.92.053009}, issn = {15393755}, year = {2015}, date = {20151101}, journal = {PHYSICAL REVIEW E}, volume = {92}, number = {5}, abstract = {In this paper, we present an approach for predicting nanoscale capillary imbibitions using the LucasWashburn (LW) theory. Molecular dynamics (MD) simulations were employed to investigate the effects of surface forces on the viscosity of liquid water. This provides an update to the modified LW equation that considered only a nanoscale slip length. An initial water nanodroplet study was performed to properly elucidate the wetting behavior of copper and gold surfaces. Intermolecular interaction strengths between water and corresponding solid surfaces were determined by matching the contact angle values obtained by experimental measurements. The migration of liquid water into copper and gold capillaries was measured by MD simulations and was found to differ from the modified LW equation. We found that the liquid layering in the vicinity of the solid surface induces a higher density and viscosity, leading to a slower MD uptake of fluid into the capillaries than was theoretically predicted. The nearsurface viscosity for the nanoscaleconfined water was defined and calculated for the thin film of water that was sheared between the two solid surfaces, as the ratio of water shear stress to the applied shear rate. Considering the effects of both the interface viscosity and slip length of the fluid, we successfully predicted the MDmeasured fluid rise in the nanotubes.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this paper, we present an approach for predicting nanoscale capillary imbibitions using the LucasWashburn (LW) theory. Molecular dynamics (MD) simulations were employed to investigate the effects of surface forces on the viscosity of liquid water. This provides an update to the modified LW equation that considered only a nanoscale slip length. An initial water nanodroplet study was performed to properly elucidate the wetting behavior of copper and gold surfaces. Intermolecular interaction strengths between water and corresponding solid surfaces were determined by matching the contact angle values obtained by experimental measurements. The migration of liquid water into copper and gold capillaries was measured by MD simulations and was found to differ from the modified LW equation. We found that the liquid layering in the vicinity of the solid surface induces a higher density and viscosity, leading to a slower MD uptake of fluid into the capillaries than was theoretically predicted. The nearsurface viscosity for the nanoscaleconfined water was defined and calculated for the thin film of water that was sheared between the two solid surfaces, as the ratio of water shear stress to the applied shear rate. Considering the effects of both the interface viscosity and slip length of the fluid, we successfully predicted the MDmeasured fluid rise in the nanotubes. 
Ma, Zhiyuan; Korucu, Ayse; Miller, Richard Steven A priori analysis of subgrid scale pressure and heat flux in high pressure mixing and reacting shear layers Journal Article COMBUSTION THEORY AND MODELLING, 19 (6), pp. 807832, 2015, ISSN: 13647830. @article{ISI:000366248600006, title = {A priori analysis of subgrid scale pressure and heat flux in high pressure mixing and reacting shear layers}, author = {Zhiyuan Ma and Ayse Korucu and Richard Steven Miller}, doi = {10.1080/13647830.2015.1100753}, issn = {13647830}, year = {2015}, date = {20151101}, journal = {COMBUSTION THEORY AND MODELLING}, volume = {19}, number = {6}, pages = {807832}, abstract = {Direct Numerical Simulation (DNS) data on high pressure H2/O2 and H2/air flames using the compressible flow formulation, detailed kinetics, a real fluid equation of state, and generalised diffusion are analysed. The DNS is filtered over a range of filter widths to provide exact terms in the Large Eddy Simulation (LES) governing equations, including unclosed terms. The filtered pressure and the filtered heat flux vector are extensively compared with the pressure and the heat flux vector calculated as a function of the filtered primitive variables (i.e. the exact LES term is compared with its form available within an actual LES). The difference between these forms defines the subgrid pressure and the subgrid heat flux vector. The analyses are done both globally across the entire flame, as well as by conditionally averaging over specific regions of the flame; including regions of large subgrid kinetic energy, subgrid scalar dissipation, subgrid temperature variance, flame temperature, etc. In this work, although negligible for purely mixing cases, the gradient of the subgrid pressure is shown to be of the same order as, and larger than, the corresponding divergence of the turbulent subgrid stresses for reacting cases. This is despite the fact that all species behave essentially as ideal gases for this flame and holds true even when the ideal gas law is used to calculate the pressure. The ratio of the subgrid pressure gradient to the subgrid stress tensor divergence is shown to increase with increasing Reynolds number. Both the subgrid heat flux vector and its divergence are found to be substantially larger in reacting flows in comparison with mixing due to the associated larger temperature gradients. However, the divergence of the subgrid heat flux vector tends to be significantly smaller than other unclosed terms in the energy equation with decreasing significance with increasing Reynolds number.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Direct Numerical Simulation (DNS) data on high pressure H2/O2 and H2/air flames using the compressible flow formulation, detailed kinetics, a real fluid equation of state, and generalised diffusion are analysed. The DNS is filtered over a range of filter widths to provide exact terms in the Large Eddy Simulation (LES) governing equations, including unclosed terms. The filtered pressure and the filtered heat flux vector are extensively compared with the pressure and the heat flux vector calculated as a function of the filtered primitive variables (i.e. the exact LES term is compared with its form available within an actual LES). The difference between these forms defines the subgrid pressure and the subgrid heat flux vector. The analyses are done both globally across the entire flame, as well as by conditionally averaging over specific regions of the flame; including regions of large subgrid kinetic energy, subgrid scalar dissipation, subgrid temperature variance, flame temperature, etc. In this work, although negligible for purely mixing cases, the gradient of the subgrid pressure is shown to be of the same order as, and larger than, the corresponding divergence of the turbulent subgrid stresses for reacting cases. This is despite the fact that all species behave essentially as ideal gases for this flame and holds true even when the ideal gas law is used to calculate the pressure. The ratio of the subgrid pressure gradient to the subgrid stress tensor divergence is shown to increase with increasing Reynolds number. Both the subgrid heat flux vector and its divergence are found to be substantially larger in reacting flows in comparison with mixing due to the associated larger temperature gradients. However, the divergence of the subgrid heat flux vector tends to be significantly smaller than other unclosed terms in the energy equation with decreasing significance with increasing Reynolds number. 
Barisik, Murat; Yazicioglu, Almila Guevenc; Cetin, Barbaros; Kakac, Sadik Analytical solution of thermally developing microtube heat transfer including axial conduction, viscous dissipation, and rarefaction effects Journal Article INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER, 67 , pp. 8188, 2015, ISSN: 07351933. @article{ISI:000362143700012, title = {Analytical solution of thermally developing microtube heat transfer including axial conduction, viscous dissipation, and rarefaction effects}, author = {Murat Barisik and Almila Guevenc Yazicioglu and Barbaros Cetin and Sadik Kakac}, doi = {10.1016/j.icheatmasstransfer.2015.05.004}, issn = {07351933}, year = {2015}, date = {20151001}, journal = {INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER}, volume = {67}, pages = {8188}, abstract = {The solution of extended Graetz problem for microscale gas flows is performed by coupling of rarefaction, axial conduction and viscous dissipation at slip flow regime. The analytical coupling achieved by using GramSchmidt orthogonalization technique provides interrelated appearance of corresponding effects through the variation of nondimensional numbers. The developing temperature field is determined by solving the energy equation locally together with the fully developed flow profile. Analytical solutions of local temperature distribution, and local and fully developed Nusselt number are obtained in terms of dimensionless parameters: Peclet number, Knudsen number, Brinkman number, and the parameter Kappa accounting temperaturejump. The results indicate that the Nusselt number decreases with increasing Knudsen number as a result of the increase of temperature jump at the wall. For low Peclet number values, temperature gradients and the resulting temperature jump at the pipe wall cause Knudsen number to develop higher effect on flow. Axial conduction should not be neglected for Peclet number values less than 100 for all cases without viscous dissipation, and for short pipes with viscous dissipation. The effect of viscous heating should be considered even for small Brinkman number values with large length over diameter ratios. For a fixed Kappa value, the deviation from continuum increases with increasing rarefaction, and Nusselt number values decrease with an increase in Knudsen number. (C)2015 Published by Elsevier Ltd.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The solution of extended Graetz problem for microscale gas flows is performed by coupling of rarefaction, axial conduction and viscous dissipation at slip flow regime. The analytical coupling achieved by using GramSchmidt orthogonalization technique provides interrelated appearance of corresponding effects through the variation of nondimensional numbers. The developing temperature field is determined by solving the energy equation locally together with the fully developed flow profile. Analytical solutions of local temperature distribution, and local and fully developed Nusselt number are obtained in terms of dimensionless parameters: Peclet number, Knudsen number, Brinkman number, and the parameter Kappa accounting temperaturejump. The results indicate that the Nusselt number decreases with increasing Knudsen number as a result of the increase of temperature jump at the wall. For low Peclet number values, temperature gradients and the resulting temperature jump at the pipe wall cause Knudsen number to develop higher effect on flow. Axial conduction should not be neglected for Peclet number values less than 100 for all cases without viscous dissipation, and for short pipes with viscous dissipation. The effect of viscous heating should be considered even for small Brinkman number values with large length over diameter ratios. For a fixed Kappa value, the deviation from continuum increases with increasing rarefaction, and Nusselt number values decrease with an increase in Knudsen number. (C)2015 Published by Elsevier Ltd. 
Cetkin, Erdal; Oliani, Alessandro The natural emergence of asymmetric treeshaped pathways for cooling of a nonuniformly heated domain Journal Article JOURNAL OF APPLIED PHYSICS, 118 (2), 2015, ISSN: 00218979. @article{ISI:000357961000033, title = {The natural emergence of asymmetric treeshaped pathways for cooling of a nonuniformly heated domain}, author = {Erdal Cetkin and Alessandro Oliani}, doi = {10.1063/1.4926620}, issn = {00218979}, year = {2015}, date = {20150701}, journal = {JOURNAL OF APPLIED PHYSICS}, volume = {118}, number = {2}, abstract = {Here, we show that the peak temperature on a nonuniformly heated domain can be decreased by embedding a highconductivity insert in it. The trunk of the highconductivity insert is in contact with a heat sink. The heat is generated nonuniformly throughout the domain or concentrated in a square spot of length scale 0.1 L0, where L0 is the length scale of the nonuniformly heated domain. Peak and average temperatures are affected by the volume fraction of the highconductivity material and by the shape of the highconductivity pathways. This paper uncovers how varying the shape of the symmetric and asymmetric highconductivity trees affects the overall thermal conductance of the heat generating domain. The treeshaped highconductivity inserts tend to grow toward where the heat generation is concentrated in order to minimize the peak temperature, i.e., in order to minimize the resistances to the heat flow. This behaviour of highconductivity trees is alike with the root growth of the plants and trees. They also tend to grow towards sunlight, and their roots tend to grow towards water and nutrients. This paper uncovers the similarity between biological trees and highconductivity trees, which is that trees should grow asymmetrically when the boundary conditions are nonuniform. We show here even though all the trees have the same objectives (minimum flow resistance), their shape should not be the same because of the variation in boundary conditions. To sum up, this paper shows that there is a highconductivity tree design corresponding to minimum peak temperature with fixed constraints and conditions. This result is in accord with the constructal law which states that there should be an optimal design for a given set of conditions and constraints, and this design should be morphed in order to ensure minimum flow resistances as conditions and constraints change. (c) 2015 AIP Publishing LLC.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Here, we show that the peak temperature on a nonuniformly heated domain can be decreased by embedding a highconductivity insert in it. The trunk of the highconductivity insert is in contact with a heat sink. The heat is generated nonuniformly throughout the domain or concentrated in a square spot of length scale 0.1 L0, where L0 is the length scale of the nonuniformly heated domain. Peak and average temperatures are affected by the volume fraction of the highconductivity material and by the shape of the highconductivity pathways. This paper uncovers how varying the shape of the symmetric and asymmetric highconductivity trees affects the overall thermal conductance of the heat generating domain. The treeshaped highconductivity inserts tend to grow toward where the heat generation is concentrated in order to minimize the peak temperature, i.e., in order to minimize the resistances to the heat flow. This behaviour of highconductivity trees is alike with the root growth of the plants and trees. They also tend to grow towards sunlight, and their roots tend to grow towards water and nutrients. This paper uncovers the similarity between biological trees and highconductivity trees, which is that trees should grow asymmetrically when the boundary conditions are nonuniform. We show here even though all the trees have the same objectives (minimum flow resistance), their shape should not be the same because of the variation in boundary conditions. To sum up, this paper shows that there is a highconductivity tree design corresponding to minimum peak temperature with fixed constraints and conditions. This result is in accord with the constructal law which states that there should be an optimal design for a given set of conditions and constraints, and this design should be morphed in order to ensure minimum flow resistances as conditions and constraints change. (c) 2015 AIP Publishing LLC. 
Barisik, Murat; Beskok, Ali Molecular free paths in nanoscale gas flows Journal Article MICROFLUIDICS AND NANOFLUIDICS, 18 (56), pp. 13651371, 2015, ISSN: 16134982. @article{ISI:000353819900057, title = {Molecular free paths in nanoscale gas flows}, author = {Murat Barisik and Ali Beskok}, doi = {10.1007/s1040401415353}, issn = {16134982}, year = {2015}, date = {20150501}, journal = {MICROFLUIDICS AND NANOFLUIDICS}, volume = {18}, number = {56}, pages = {13651371}, abstract = {Average distance traveled by gas molecules between intermolecular collisions, known as the mean free path (MFP), is a key parameter for characterizing gas flows in the entire Knudsen regime. Recent literature presents variations in MFP as a function of the surface confinement, which is in disagreement with the kinetic theory and leads to wrong physical interpretations of nanoscale gas flows. This controversy occurs due to erroneous definition and calculation practices, such as consideration of gas wall collisions, using local bins smaller than a MFP, and utilizing time frames shorter than a mean collision time in the MFP calculations. This study reports proper molecular MFP calculations in nanoscale confinements by using realistic molecular surfaces. We utilize molecular dynamics (MD) simulations to calculate gas MFP in threedimensional periodic systems of various sizes and for forcedriven gas flows confined in nanochannels. Studies performed in the transition flow regime in various size nanochannels and under a range of gassurface interaction strengths have shown isotropic mean travelled distance and MFP values in agreement with the kinetic theory regardless of the surface forces and surface adsorption effects. Comparison of the velocity profiles obtained in MD simulations with the linearized Boltzmann solutions at predicted Knudsen values shows good agreement in the bulk of the channels, while deviations in the near wall region due to the influence of surface forces are reported.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Average distance traveled by gas molecules between intermolecular collisions, known as the mean free path (MFP), is a key parameter for characterizing gas flows in the entire Knudsen regime. Recent literature presents variations in MFP as a function of the surface confinement, which is in disagreement with the kinetic theory and leads to wrong physical interpretations of nanoscale gas flows. This controversy occurs due to erroneous definition and calculation practices, such as consideration of gas wall collisions, using local bins smaller than a MFP, and utilizing time frames shorter than a mean collision time in the MFP calculations. This study reports proper molecular MFP calculations in nanoscale confinements by using realistic molecular surfaces. We utilize molecular dynamics (MD) simulations to calculate gas MFP in threedimensional periodic systems of various sizes and for forcedriven gas flows confined in nanochannels. Studies performed in the transition flow regime in various size nanochannels and under a range of gassurface interaction strengths have shown isotropic mean travelled distance and MFP values in agreement with the kinetic theory regardless of the surface forces and surface adsorption effects. Comparison of the velocity profiles obtained in MD simulations with the linearized Boltzmann solutions at predicted Knudsen values shows good agreement in the bulk of the channels, while deviations in the near wall region due to the influence of surface forces are reported. 
Cetkin, Erdal CONSTRUCTAL STRUCTURES FOR SELFCOOLING: MICROVASCULAR WAVY AND STRAIGHT CHANNELS Journal Article JOURNAL OF THERMAL ENGINEERING, 1 (5, 1), pp. 166174, 2015, ISSN: 21487847. @article{ISI:000434616100004, title = {CONSTRUCTAL STRUCTURES FOR SELFCOOLING: MICROVASCULAR WAVY AND STRAIGHT CHANNELS}, author = {Erdal Cetkin}, doi = {10.18186/jte.10873}, issn = {21487847}, year = {2015}, date = {20150201}, journal = {JOURNAL OF THERMAL ENGINEERING}, volume = {1}, number = {5, 1}, pages = {166174}, abstract = {This paper shows that a conductive domain which is subjected to heating from its bottom can be cooled with embedded microvascular cooling channels in it. The volume of the domain and the coolant are fixed. The actively cooled domain is mimicked from the human skin (which regulates temperature with microvascular blood vessels). The effect of the shape of cooling channels (sinusoidal or straight) and their locations in the direction perpendicular to the bottom surface on the peak and average temperatures are studied. In addition, the effect of pressure difference in between the inlet and outlet is varied. The pressure drop in the sinusoidal channel configurations is greater than the straight channel configurations for a fixed cooling channel volume. The peak and average temperatures are the smallest with straight cooling channels located at y = 0.7 mm. Furthermore, how the cooling channel configuration should change when the heat is generated throughout the volume is studied. The peak and average temperatures are smaller with straight channels than the sinusoidal ones when the pressure drop is less than 420 Pa, and they become smaller with sinusoidal channel configurations when the pressure drop is greater than 420 Pa. In addition, the peak and average temperatures are the smallest with sinusoidal channels for a fixed flow rate. Furthermore, the peak temperatures for multiple cooling channels is documented, and the multiple channel configurations promise to the smallest peak temperature for a fixed pressure drop value. This paper uncovers that there is no optimal cooling channel design for any condition, but there is one for specific objectives and conditions.}, keywords = {}, pubstate = {published}, tppubtype = {article} } This paper shows that a conductive domain which is subjected to heating from its bottom can be cooled with embedded microvascular cooling channels in it. The volume of the domain and the coolant are fixed. The actively cooled domain is mimicked from the human skin (which regulates temperature with microvascular blood vessels). The effect of the shape of cooling channels (sinusoidal or straight) and their locations in the direction perpendicular to the bottom surface on the peak and average temperatures are studied. In addition, the effect of pressure difference in between the inlet and outlet is varied. The pressure drop in the sinusoidal channel configurations is greater than the straight channel configurations for a fixed cooling channel volume. The peak and average temperatures are the smallest with straight cooling channels located at y = 0.7 mm. Furthermore, how the cooling channel configuration should change when the heat is generated throughout the volume is studied. The peak and average temperatures are smaller with straight channels than the sinusoidal ones when the pressure drop is less than 420 Pa, and they become smaller with sinusoidal channel configurations when the pressure drop is greater than 420 Pa. In addition, the peak and average temperatures are the smallest with sinusoidal channels for a fixed flow rate. Furthermore, the peak temperatures for multiple cooling channels is documented, and the multiple channel configurations promise to the smallest peak temperature for a fixed pressure drop value. This paper uncovers that there is no optimal cooling channel design for any condition, but there is one for specific objectives and conditions. 
Cetkin, E; Lorente, S; Bejan, A Vascularization for cooling and reduced thermal stresses Journal Article INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 80 , pp. 858864, 2015, ISSN: 00179310. @article{ISI:000345202100080, title = {Vascularization for cooling and reduced thermal stresses}, author = {E Cetkin and S Lorente and A Bejan}, doi = {10.1016/j.ijheatmasstransfer.2014.09.027}, issn = {00179310}, year = {2015}, date = {20150101}, journal = {INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, volume = {80}, pages = {858864}, abstract = {This paper documents the effect of thermal expansion on a vascularized plate that is heated and loaded mechanically. Vascular cooling channels embedded in a circular plate provide cooling and mechanical strength. The coolant enters the plate from the center and leaves after it cools the plate to an allowable temperature limit. The mechanical strength of the plate decreases because of the embedded cooling channels. However, cooling the plate under an allowable temperature level decreases the thermal stresses. The mechanical strength of the plate which is heated and loaded mechanically at the same time can be increased by inserting cooling channels in it. The mechanical and thermofluid behavior of a vascularized plate was simulated numerically. The cooling channel configurations that provide the smallest peak temperature and von Mises stress are documented. There is one cooling channel configuration that is the best for the given set of boundary conditions and constraints; however, there is no single configuration that is best for all conditions. (C) 2014 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } This paper documents the effect of thermal expansion on a vascularized plate that is heated and loaded mechanically. Vascular cooling channels embedded in a circular plate provide cooling and mechanical strength. The coolant enters the plate from the center and leaves after it cools the plate to an allowable temperature limit. The mechanical strength of the plate decreases because of the embedded cooling channels. However, cooling the plate under an allowable temperature level decreases the thermal stresses. The mechanical strength of the plate which is heated and loaded mechanically at the same time can be increased by inserting cooling channels in it. The mechanical and thermofluid behavior of a vascularized plate was simulated numerically. The cooling channel configurations that provide the smallest peak temperature and von Mises stress are documented. There is one cooling channel configuration that is the best for the given set of boundary conditions and constraints; however, there is no single configuration that is best for all conditions. (C) 2014 Elsevier Ltd. All rights reserved. 
Cetkin, Erdal Constructal vascularized structures Journal Article OPEN ENGINEERING, 5 (1), pp. 220228, 2015, ISSN: 23915439. @article{ISI:000218406100024, title = {Constructal vascularized structures}, author = {Erdal Cetkin}, doi = {10.1515/eng20150017}, issn = {23915439}, year = {2015}, date = {20150101}, journal = {OPEN ENGINEERING}, volume = {5}, number = {1}, pages = {220228}, abstract = {Smart features such as selfhealing and selfcooling require bathing the entire volume with a coolant or/and healing agent. Bathing the entire volume is an example of point to area (or volume) flows. Point to area flows cover all the distributing and collecting kinds of flows, i.e. inhaling and exhaling, mining, river deltas, energy distribution, distribution of products on the landscape and so on. The flow resistances of a point to area flow can be decreased by changing the design with the guidance of the constructal law, which is the law of the design evolution in time. In this paper, how the flow resistances (heat, fluid and stress) can be decreased by using the constructal law is shown with examples. First, the validity of two assumptions is surveyed: using temperature independent HessMurray rule and using constant diameter ducts where the duct discharges fluid along its edge. Then, point to area types of flows are explained by illustrating the results of two examples: fluid networks and heating an area. Last, how the structures should be vascularized for cooling and mechanical strength is documented. This paper shows that flow resistances can be decreased by morphing the shape freely without any restrictions or generic algorithms.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Smart features such as selfhealing and selfcooling require bathing the entire volume with a coolant or/and healing agent. Bathing the entire volume is an example of point to area (or volume) flows. Point to area flows cover all the distributing and collecting kinds of flows, i.e. inhaling and exhaling, mining, river deltas, energy distribution, distribution of products on the landscape and so on. The flow resistances of a point to area flow can be decreased by changing the design with the guidance of the constructal law, which is the law of the design evolution in time. In this paper, how the flow resistances (heat, fluid and stress) can be decreased by using the constructal law is shown with examples. First, the validity of two assumptions is surveyed: using temperature independent HessMurray rule and using constant diameter ducts where the duct discharges fluid along its edge. Then, point to area types of flows are explained by illustrating the results of two examples: fluid networks and heating an area. Last, how the structures should be vascularized for cooling and mechanical strength is documented. This paper shows that flow resistances can be decreased by morphing the shape freely without any restrictions or generic algorithms. 
Cetkin, Erdal INVERTED FINS FOR COOLING OF A NONUNIFORMLY HEATED DOMAIN Journal Article JOURNAL OF THERMAL ENGINEERING, 1 (1), pp. 19, 2015, ISSN: 21487847. @article{ISI:000434614600001, title = {INVERTED FINS FOR COOLING OF A NONUNIFORMLY HEATED DOMAIN}, author = {Erdal Cetkin}, doi = {10.18186/jte.12488}, issn = {21487847}, year = {2015}, date = {20150101}, journal = {JOURNAL OF THERMAL ENGINEERING}, volume = {1}, number = {1}, pages = {19}, abstract = {This paper shows that the peak temperature of a nonuniformly heated region can be decreased by embedding highconductivity treeshaped inserts which is in contact with a heat sink from its stem. The volume fraction of the highconductivity material is fixed, and so is the volume of the solid region. The length scale of the solid domain is L. Inside there is a cubeshaped region with length scale of 0.1L and heat production 100 times greater than the rest of the domain. The location of this hot spot was varied to uncover how its location affects the peak temperature and the design of inverted fins, i.e. highconductivity treeshaped inserts. The volume fraction of the highconductivity tree was varied for number of bifurcation levels of 0, 1 and 2. This showed that increasing the number of the bifurcation levels decreases the peak temperature when the volume fraction decreases. The optimal diameter ratios and optimal bifurcation angles at the each junction level are also documented. Yshaped trees promise smaller peak temperatures than Tshaped trees. The location of the vascular tree in the z direction also affects the peak temperature when the heat generation is nonuniform. In addition, the peak temperature is minimum when z = 0.65L even though the hot spot is located on z = 0.75L.}, keywords = {}, pubstate = {published}, tppubtype = {article} } This paper shows that the peak temperature of a nonuniformly heated region can be decreased by embedding highconductivity treeshaped inserts which is in contact with a heat sink from its stem. The volume fraction of the highconductivity material is fixed, and so is the volume of the solid region. The length scale of the solid domain is L. Inside there is a cubeshaped region with length scale of 0.1L and heat production 100 times greater than the rest of the domain. The location of this hot spot was varied to uncover how its location affects the peak temperature and the design of inverted fins, i.e. highconductivity treeshaped inserts. The volume fraction of the highconductivity tree was varied for number of bifurcation levels of 0, 1 and 2. This showed that increasing the number of the bifurcation levels decreases the peak temperature when the volume fraction decreases. The optimal diameter ratios and optimal bifurcation angles at the each junction level are also documented. Yshaped trees promise smaller peak temperatures than Tshaped trees. The location of the vascular tree in the z direction also affects the peak temperature when the heat generation is nonuniform. In addition, the peak temperature is minimum when z = 0.65L even though the hot spot is located on z = 0.75L. 
2014 
Toprak, Kasim; Bayazitoglu, Yildiz Interfacial thermal resistance of CuSWCNT nanowire in water Journal Article INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 79 , pp. 584588, 2014, ISSN: 00179310. @article{ISI:000343781900056, title = {Interfacial thermal resistance of CuSWCNT nanowire in water}, author = {Kasim Toprak and Yildiz Bayazitoglu}, doi = {10.1016/j.ijheatmasstransfer.2014.08.024}, issn = {00179310}, year = {2014}, date = {20141201}, journal = {INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, volume = {79}, pages = {584588}, abstract = {The conduction along the radial direction of CuNWSWCNT nanocomposite surrounded with water is examined. Due to its simplicity and adaptability, a simple pointcharge water model is implemented. Using the NoseHoover thermostat, the copper core is kept at a uniform temperature as a heat source, and a circular edge layer of water is kept at a lower temperature as a heat sink in order to impose a radial temperature distribution. The thermal boundary resistance was predicted as 0.1732 x 10(8) m(2) K/W at the interface of CuNWwater, 3.16 x 10(8) m(2) K/W at the interface of SWCNTwater, and 0.743 x 10(8) m(2) K/W at the interface of (CuNWSWCNT)water. (C) 2014 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The conduction along the radial direction of CuNWSWCNT nanocomposite surrounded with water is examined. Due to its simplicity and adaptability, a simple pointcharge water model is implemented. Using the NoseHoover thermostat, the copper core is kept at a uniform temperature as a heat source, and a circular edge layer of water is kept at a lower temperature as a heat sink in order to impose a radial temperature distribution. The thermal boundary resistance was predicted as 0.1732 x 10(8) m(2) K/W at the interface of CuNWwater, 3.16 x 10(8) m(2) K/W at the interface of SWCNTwater, and 0.743 x 10(8) m(2) K/W at the interface of (CuNWSWCNT)water. (C) 2014 Elsevier Ltd. All rights reserved. 
Cetkin, Erdal Threedimensional highconductivity trees for volumetric cooling Journal Article INTERNATIONAL JOURNAL OF ENERGY RESEARCH, 38 (12), pp. 15711577, 2014, ISSN: 0363907X. @article{ISI:000342133500008, title = {Threedimensional highconductivity trees for volumetric cooling}, author = {Erdal Cetkin}, doi = {10.1002/er.3176}, issn = {0363907X}, year = {2014}, date = {20141001}, journal = {INTERNATIONAL JOURNAL OF ENERGY RESEARCH}, volume = {38}, number = {12}, pages = {15711577}, abstract = {Here, we show how the cooling performance of a volumetrically heated solid can be increased by embedding highconductivity treeshaped designs in it. The volume fraction occupied by the highconductivity material is fixed. Embedding the highconductivity material as trees in the solid decreases the maximum temperature more than three times compared with distributing the highconductivity material uniformly throughout the solid. The maximum temperature decreases as the number of the bifurcation levels and the volume fraction of the highly conductive material increase. The thermal resistance of the cube is the lowest when the diameter ratio of the mother and daughter branches at each pairing junction is 2. Changing from Tshaped to Yshaped designs and from twodimensional to threedimensional designs decrease the maximum and the volume averaged temperatures. The peak temperature is the lowest in threedimensional and Yshaped designs. This paper shows that the peak temperature of the heated solid can be decreased by only varying the shape of the highconductivity tree embedded in it. Copyright (c) 2014 John Wiley & Sons, Ltd.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Here, we show how the cooling performance of a volumetrically heated solid can be increased by embedding highconductivity treeshaped designs in it. The volume fraction occupied by the highconductivity material is fixed. Embedding the highconductivity material as trees in the solid decreases the maximum temperature more than three times compared with distributing the highconductivity material uniformly throughout the solid. The maximum temperature decreases as the number of the bifurcation levels and the volume fraction of the highly conductive material increase. The thermal resistance of the cube is the lowest when the diameter ratio of the mother and daughter branches at each pairing junction is 2. Changing from Tshaped to Yshaped designs and from twodimensional to threedimensional designs decrease the maximum and the volume averaged temperatures. The peak temperature is the lowest in threedimensional and Yshaped designs. This paper shows that the peak temperature of the heated solid can be decreased by only varying the shape of the highconductivity tree embedded in it. Copyright (c) 2014 John Wiley & Sons, Ltd. 
Ozgumus, Turkuler; Mobedi, Moghtada; Ozkol, Unver DETERMINATION OF KOZENY CONSTANT BASED ON POROSITY AND PORE TO THROAT SIZE RATIO IN POROUS MEDIUM WITH RECTANGULAR RODS Journal Article ENGINEERING APPLICATIONS OF COMPUTATIONAL FLUID MECHANICS, 8 (2), pp. 308318, 2014, ISSN: 19942060. @article{ISI:000337224100010, title = {DETERMINATION OF KOZENY CONSTANT BASED ON POROSITY AND PORE TO THROAT SIZE RATIO IN POROUS MEDIUM WITH RECTANGULAR RODS}, author = {Turkuler Ozgumus and Moghtada Mobedi and Unver Ozkol}, doi = {10.1080/19942060.2014.11015516}, issn = {19942060}, year = {2014}, date = {20140601}, journal = {ENGINEERING APPLICATIONS OF COMPUTATIONAL FLUID MECHANICS}, volume = {8}, number = {2}, pages = {308318}, abstract = {KozenyCarman permeability equation is an important relation for the determination of permeability in porous media. In this study, the permeabilities of porous media that contains rectangular rods are determined, numerically. The applicability of KozenyCarman equation for the periodic porous media is investigated and the effects of porosity and pore to throat size ratio on Kozeny constant are studied. The continuity and NavierStokes equations are solved to determine the velocity and pressure fields in the voids between the rods. Based on the obtained flow field, the permeability values for different porosities from 0.2 to 0.9 and pore to throat size ratio values from 1.63 to 7.46 are computed. Then Kozeny constants for different porous media with various porosity and pore to throat size ratios are obtained and a relationship between Kozeny constant, porosity and pore to throat size ratio is constructed. The study reveals that the pore to throat size ratio is an important geometrical parameter that should be taken into account for deriving a correlation for permeability. The suggestion of a fixed value for Kozeny constant makes the application of KozenyCarman permeability equation too narrow for a very specific porous medium. However, it is possible to apply the KozenyCarman permeability equation for wide ranges of porous media with different geometrical parameters (various porosity, hydraulic diameter, particle size and aspect ratio) if Kozeny constant is a function of two parameters as porosity and pore to throat size ratios.}, keywords = {}, pubstate = {published}, tppubtype = {article} } KozenyCarman permeability equation is an important relation for the determination of permeability in porous media. In this study, the permeabilities of porous media that contains rectangular rods are determined, numerically. The applicability of KozenyCarman equation for the periodic porous media is investigated and the effects of porosity and pore to throat size ratio on Kozeny constant are studied. The continuity and NavierStokes equations are solved to determine the velocity and pressure fields in the voids between the rods. Based on the obtained flow field, the permeability values for different porosities from 0.2 to 0.9 and pore to throat size ratio values from 1.63 to 7.46 are computed. Then Kozeny constants for different porous media with various porosity and pore to throat size ratios are obtained and a relationship between Kozeny constant, porosity and pore to throat size ratio is constructed. The study reveals that the pore to throat size ratio is an important geometrical parameter that should be taken into account for deriving a correlation for permeability. The suggestion of a fixed value for Kozeny constant makes the application of KozenyCarman permeability equation too narrow for a very specific porous medium. However, it is possible to apply the KozenyCarman permeability equation for wide ranges of porous media with different geometrical parameters (various porosity, hydraulic diameter, particle size and aspect ratio) if Kozeny constant is a function of two parameters as porosity and pore to throat size ratios. 
Atalay, Selcuk; Barisik, Murat; Beskok, Ali; Qian, Shizhi Surface Charge of a Nanoparticle Interacting with a Flat Substrate Journal Article JOURNAL OF PHYSICAL CHEMISTRY C, 118 (20), pp. 1092710935, 2014, ISSN: 19327447. @article{ISI:000336509400046, title = {Surface Charge of a Nanoparticle Interacting with a Flat Substrate}, author = {Selcuk Atalay and Murat Barisik and Ali Beskok and Shizhi Qian}, doi = {10.1021/jp5023554}, issn = {19327447}, year = {2014}, date = {20140501}, journal = {JOURNAL OF PHYSICAL CHEMISTRY C}, volume = {118}, number = {20}, pages = {1092710935}, abstract = {Electrostatic interactions between two charged dielectric objects highly depend on their surface charge. Most existing studies assume constant surface charge densities between the two interacting objects regardless of the separation distance between them. The surface charge of a spherical silica nanoparticle interacting with a flat silica plate is investigated numerically as a function of the separation distance normalized with the electrical double layer thickness (kappa h), pH, and background salt concentration. The numerical model employs PoissonNernstPlanck equations for ionic mass transport and considers surface charge regulation in the presence of multiple ionic species. Relatively weak interactions between the nanoparticle and the plate are observed for kappa h >> 1, resulting in uniform surface charge densities. Because of curvature, the surface charge density of the nanoparticle is higher than that of the plate. Strong interactions are observed for kappa h <= 1, leading to spatially nonuniform surface charge densities on the nanoparticle and the plate. This effect increases with decreased separation distance (kappa h). Enhanced proton concentration in the gap between the particle and the plate leads to reduced surface charge densities on the two objects.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Electrostatic interactions between two charged dielectric objects highly depend on their surface charge. Most existing studies assume constant surface charge densities between the two interacting objects regardless of the separation distance between them. The surface charge of a spherical silica nanoparticle interacting with a flat silica plate is investigated numerically as a function of the separation distance normalized with the electrical double layer thickness (kappa h), pH, and background salt concentration. The numerical model employs PoissonNernstPlanck equations for ionic mass transport and considers surface charge regulation in the presence of multiple ionic species. Relatively weak interactions between the nanoparticle and the plate are observed for kappa h >> 1, resulting in uniform surface charge densities. Because of curvature, the surface charge density of the nanoparticle is higher than that of the plate. Strong interactions are observed for kappa h <= 1, leading to spatially nonuniform surface charge densities on the nanoparticle and the plate. This effect increases with decreased separation distance (kappa h). Enhanced proton concentration in the gap between the particle and the plate leads to reduced surface charge densities on the two objects. 
Pham, An Truong; Barisik, Murat; Kim, Bohung Molecular Dynamics Simulations of Kapitza Length for ArgonSilicon and WaterSilicon Interfaces Journal Article INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING, 15 (2), pp. 323329, 2014, ISSN: 22347593. @article{ISI:000331786500017, title = {Molecular Dynamics Simulations of Kapitza Length for ArgonSilicon and WaterSilicon Interfaces}, author = {An Truong Pham and Murat Barisik and Bohung Kim}, doi = {10.1007/s125410140341x}, issn = {22347593}, year = {2014}, date = {20140201}, journal = {INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING}, volume = {15}, number = {2}, pages = {323329}, abstract = {A comprehensive understanding of heat conduction between two parallel solid walls separated by liquid remains incomplete in nanometer scale. In addition, the solid/liquid interfacial thermal resistance has been an important technical issue in thermal/fluid engineering such as micro electromechanical systems and nano electromechanical systems with liquid inside. Therefore, further advancements in nanoscale physics require an advanced understanding of momentum and energy transport at solid/liquid interfaces. This study employs threedimensional molecular dynamics (MD) simulations to investigate the thermal resistance at solid/liquid interfaces. Heat conduction between two parallel silicon walls separated by a thin film of liquid water is considered. The density distribution of liquid water is discussed with the simulation results to further understanding of the dynamic properties of water near solid surfaces. Meanwhile, temperature profiles appear discontinuous between liquid and solid temperatures due to the dissimilarity of thermal transport properties of the two materials, which validates thermal resistance (or Kapitza length) at solid/liquid interfaces. MD results also investigate the temperature dependence of the Kapitza length, demonstrating that the Kaptiza lengths fluctuate around an average value and are independent of the wall temperature at solid/liquid interfaces. Our study provides useful information for the design of thermal management or heat dissipation devices across silicon/water and silicon/argon interfaces in nanoscale.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A comprehensive understanding of heat conduction between two parallel solid walls separated by liquid remains incomplete in nanometer scale. In addition, the solid/liquid interfacial thermal resistance has been an important technical issue in thermal/fluid engineering such as micro electromechanical systems and nano electromechanical systems with liquid inside. Therefore, further advancements in nanoscale physics require an advanced understanding of momentum and energy transport at solid/liquid interfaces. This study employs threedimensional molecular dynamics (MD) simulations to investigate the thermal resistance at solid/liquid interfaces. Heat conduction between two parallel silicon walls separated by a thin film of liquid water is considered. The density distribution of liquid water is discussed with the simulation results to further understanding of the dynamic properties of water near solid surfaces. Meanwhile, temperature profiles appear discontinuous between liquid and solid temperatures due to the dissimilarity of thermal transport properties of the two materials, which validates thermal resistance (or Kapitza length) at solid/liquid interfaces. MD results also investigate the temperature dependence of the Kapitza length, demonstrating that the Kaptiza lengths fluctuate around an average value and are independent of the wall temperature at solid/liquid interfaces. Our study provides useful information for the design of thermal management or heat dissipation devices across silicon/water and silicon/argon interfaces in nanoscale. 
Cetkin, Erdal EMERGENCE OF TAPERED DUCTS IN VASCULAR DESIGNS WITH LAMINAR AND TURBULENT FLOWS Journal Article JOURNAL OF POROUS MEDIA, 17 (8), pp. 715722, 2014, ISSN: 1091028X. @article{ISI:000342967400005, title = {EMERGENCE OF TAPERED DUCTS IN VASCULAR DESIGNS WITH LAMINAR AND TURBULENT FLOWS}, author = {Erdal Cetkin}, doi = {10.1615/JPorMedia.v17.i8.50}, issn = {1091028X}, year = {2014}, date = {20140101}, journal = {JOURNAL OF POROUS MEDIA}, volume = {17}, number = {8}, pages = {715722}, abstract = {Here we show that tapered ducts emerge in volumetrically bathed porous materials to decrease the resistance to the flow in laminar and turbulent flow regimes. The fluid enters the volume from one point and it is distributed to the entire volume. After bathing the volume, it is collected and leaves the volume from another point, i.e., two trees matched canopy to canopy. This paper shows that the flow architecture (i.e., design of the void spaces in a porous material) should be changed to obtain the minimum resistance to the flow as its size increases. Tapering the ducts decreases the order of the transition size, i.e., the size for changing from one construct to another to obtain the minimum pressure drop. The decrease in the pressure drop is 16% and 38% with the tapered ducts when the flow is laminar and turbulent, respectively. In addition, the volume ratios and the shape of the tapered ducts are documented. There is no design existing in nature with diameters of constant size in order to distribute and/or collect heat, fluid, and/or stress such as bones, rivers, veins, and tree branches. The emergence of the tapered ducts in designed porous materials is natural.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Here we show that tapered ducts emerge in volumetrically bathed porous materials to decrease the resistance to the flow in laminar and turbulent flow regimes. The fluid enters the volume from one point and it is distributed to the entire volume. After bathing the volume, it is collected and leaves the volume from another point, i.e., two trees matched canopy to canopy. This paper shows that the flow architecture (i.e., design of the void spaces in a porous material) should be changed to obtain the minimum resistance to the flow as its size increases. Tapering the ducts decreases the order of the transition size, i.e., the size for changing from one construct to another to obtain the minimum pressure drop. The decrease in the pressure drop is 16% and 38% with the tapered ducts when the flow is laminar and turbulent, respectively. In addition, the volume ratios and the shape of the tapered ducts are documented. There is no design existing in nature with diameters of constant size in order to distribute and/or collect heat, fluid, and/or stress such as bones, rivers, veins, and tree branches. The emergence of the tapered ducts in designed porous materials is natural. 
2013 
Toprak, K; Bayazitoglu, Y Numerical modeling of a CNTCu coaxial nanowire in a vacuum to determine the thermal conductivity Journal Article INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 61 , pp. 172175, 2013, ISSN: 00179310. @article{ISI:000318260200018, title = {Numerical modeling of a CNTCu coaxial nanowire in a vacuum to determine the thermal conductivity}, author = {K Toprak and Y Bayazitoglu}, doi = {10.1016/j.ijheatmasstransfer.2013.01.082}, issn = {00179310}, year = {2013}, date = {20130601}, journal = {INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, volume = {61}, pages = {172175}, abstract = {A molecular dynamics simulation is created to predict the thermal conductivity of a (7,7) single wall carbon nanotube filled with a copper nanowire for lengths ranging from 9.957 nm to 63.091 nm. In the simulations, a temperature difference is created using NoseHoover thermostats at the two ends with 320 K and 280 K. The thermal conductivity of the carbon nanotubecopper nanowire nanostructure is calculated to be 24% higher than that of a corresponding pure single wall carbon nanotube and estimated to be 40% lower than that of a pure copper nanowire. (C) 2013 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A molecular dynamics simulation is created to predict the thermal conductivity of a (7,7) single wall carbon nanotube filled with a copper nanowire for lengths ranging from 9.957 nm to 63.091 nm. In the simulations, a temperature difference is created using NoseHoover thermostats at the two ends with 320 K and 280 K. The thermal conductivity of the carbon nanotubecopper nanowire nanostructure is calculated to be 24% higher than that of a corresponding pure single wall carbon nanotube and estimated to be 40% lower than that of a pure copper nanowire. (C) 2013 Elsevier Ltd. All rights reserved. 
Ozgumus, Turkuler; Mobedi, Moghtada; Ozkol, Unver; Nakayama, Akira Thermal Dispersion in Porous MediaA Review on the Experimental Studies for Packed Beds Journal Article APPLIED MECHANICS REVIEWS, 65 (3), 2013, ISSN: 00036900. @article{ISI:000329612200001, title = {Thermal Dispersion in Porous MediaA Review on the Experimental Studies for Packed Beds}, author = {Turkuler Ozgumus and Moghtada Mobedi and Unver Ozkol and Akira Nakayama}, doi = {10.1115/1.4024351}, issn = {00036900}, year = {2013}, date = {20130501}, journal = {APPLIED MECHANICS REVIEWS}, volume = {65}, number = {3}, abstract = {Thermal dispersion is an important topic in the convective heat transfer in porous media. In order to determine the heat transfer in a packed bed, the effective thermal conductivity including both stagnant and dispersion thermal conductivities should be known. Several theoretical and experimental studies have been performed on the determination of the effective thermal conductivity. The aim of this study is to review the experimental studies done on the determination of the effective thermal conductivity of the packed beds. In this study, firstly brief information on the definition of the thermal dispersion is presented and then the reported experimental studies on the determination of the effective thermal conductivity are summarized and compared. The reported experimental methods are classified into three groups: (1) heat addition/removal at the lateral boundaries, (2) heat addition at the inlet/outlet boundary, (3) heat addition inside the bed. For each performed study, the experimental details, methods, obtained results, and suggested correlations for the determination of the effective thermal conductivity are presented. The similarities and differences between experimental methods and reported studies are shown by tables. Comparison of the correlations for the effective thermal conductivity is made by using figures and the results of the studies are discussed.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Thermal dispersion is an important topic in the convective heat transfer in porous media. In order to determine the heat transfer in a packed bed, the effective thermal conductivity including both stagnant and dispersion thermal conductivities should be known. Several theoretical and experimental studies have been performed on the determination of the effective thermal conductivity. The aim of this study is to review the experimental studies done on the determination of the effective thermal conductivity of the packed beds. In this study, firstly brief information on the definition of the thermal dispersion is presented and then the reported experimental studies on the determination of the effective thermal conductivity are summarized and compared. The reported experimental methods are classified into three groups: (1) heat addition/removal at the lateral boundaries, (2) heat addition at the inlet/outlet boundary, (3) heat addition inside the bed. For each performed study, the experimental details, methods, obtained results, and suggested correlations for the determination of the effective thermal conductivity are presented. The similarities and differences between experimental methods and reported studies are shown by tables. Comparison of the correlations for the effective thermal conductivity is made by using figures and the results of the studies are discussed. 
Cetkin, E; Lorente, S; Bejan, A Constructal paddle design with ``fingers'' Journal Article JOURNAL OF APPLIED PHYSICS, 113 (19), 2013, ISSN: 00218979. @article{ISI:000319295200065, title = {Constructal paddle design with ``fingers''}, author = {E Cetkin and S Lorente and A Bejan}, doi = {10.1063/1.4804961}, issn = {00218979}, year = {2013}, date = {20130501}, journal = {JOURNAL OF APPLIED PHYSICS}, volume = {113}, number = {19}, abstract = {Here, we show how the performance of a paddle that pushes a fluid can be increased by making parallel slits through it. The slit spacing is varied to see its effect on the drag force and the maximum stress in the paddle. The effect of water speed and paddle dimensions is documented. Designs with one or more slits are investigated. The drag force is maximum when the slit spacing matches the boundary layer thickness of the flow through the slit. Furthermore, the drag force is greater when the slit spacing is nonuniform: larger in the central slits than in the peripheral slits. The paddle with slits of one size performs almost as well as the best design with nonuniform spacings. The paddle design with slits achieves the same drag force and maximum stress with less material compared with a paddle without slits. (C) 2013 AIP Publishing LLC.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Here, we show how the performance of a paddle that pushes a fluid can be increased by making parallel slits through it. The slit spacing is varied to see its effect on the drag force and the maximum stress in the paddle. The effect of water speed and paddle dimensions is documented. Designs with one or more slits are investigated. The drag force is maximum when the slit spacing matches the boundary layer thickness of the flow through the slit. Furthermore, the drag force is greater when the slit spacing is nonuniform: larger in the central slits than in the peripheral slits. The paddle with slits of one size performs almost as well as the best design with nonuniform spacings. The paddle design with slits achieves the same drag force and maximum stress with less material compared with a paddle without slits. (C) 2013 AIP Publishing LLC. 
2012 
Barisik, Murat; Beskok, Ali Surfacegas interaction effects on nanoscale gas flows Journal Article MICROFLUIDICS AND NANOFLUIDICS, 13 (5), pp. 789798, 2012, ISSN: 16134982. @article{ISI:000310640900009, title = {Surfacegas interaction effects on nanoscale gas flows}, author = {Murat Barisik and Ali Beskok}, doi = {10.1007/s1040401210000}, issn = {16134982}, year = {2012}, date = {20121101}, journal = {MICROFLUIDICS AND NANOFLUIDICS}, volume = {13}, number = {5}, pages = {789798}, abstract = {Molecular dynamics (MD) method is used to simulate shear driven argon gas flows in the early transition and free molecular flow regimes to investigate surface effects as a function of the surfacegas potential strength ratio (epsilon(wf)/epsilon(ff)). Results show a bulk flow region and a near wall region that extends three molecular diameters away from the surfaces. Within the near wall region the velocity, density, and shear stress distributions exhibit deviations from the kinetic theory predictions. Increased epsilon(wf)/epsilon(ff) results in increased gas density, leading toward monolayer adsorption on surfaces. The near wall velocity profile shows reduced gas slip, and eventually velocity stick with increased epsilon(wf)/epsilon(ff). Using MD predicted shear stress values and kinetic theory, tangential momentum accommodation coefficients (TMAC) are calculated as a function of epsilon(wf)/epsilon(ff), and TMAC values are shown to be independent of the Knudsen number. Presence of this near wall region breaks down the dynamic similarity between rarefied and nanoscale gas flows.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Molecular dynamics (MD) method is used to simulate shear driven argon gas flows in the early transition and free molecular flow regimes to investigate surface effects as a function of the surfacegas potential strength ratio (epsilon(wf)/epsilon(ff)). Results show a bulk flow region and a near wall region that extends three molecular diameters away from the surfaces. Within the near wall region the velocity, density, and shear stress distributions exhibit deviations from the kinetic theory predictions. Increased epsilon(wf)/epsilon(ff) results in increased gas density, leading toward monolayer adsorption on surfaces. The near wall velocity profile shows reduced gas slip, and eventually velocity stick with increased epsilon(wf)/epsilon(ff). Using MD predicted shear stress values and kinetic theory, tangential momentum accommodation coefficients (TMAC) are calculated as a function of epsilon(wf)/epsilon(ff), and TMAC values are shown to be independent of the Knudsen number. Presence of this near wall region breaks down the dynamic similarity between rarefied and nanoscale gas flows. 
Cetkin, E; Lorente, S; Bejan, A Vascularization for cooling a plate heated by a randomly moving source Journal Article JOURNAL OF APPLIED PHYSICS, 112 (8), 2012, ISSN: 00218979. @article{ISI:000310597500155, title = {Vascularization for cooling a plate heated by a randomly moving source}, author = {E Cetkin and S Lorente and A Bejan}, doi = {10.1063/1.4759290}, issn = {00218979}, year = {2012}, date = {20121001}, journal = {JOURNAL OF APPLIED PHYSICS}, volume = {112}, number = {8}, abstract = {Here, we show that a plate heated by a moving beam can be cooled effectively by fluid that flows through a vasculature of channels embedded in the plate. The vascular designs studied are radial, grid and hybrid (radial + grid). The peak temperature of the plate changes with the path and direction of the moving beam. The strength, size and speed of the beam vary. The peak temperature increases as the beam strength and size increase and as the speed of the beam decreases. The grid and hybrid designs are robust because of loops present in the flow structure. The pressure difference that drives the fluid flow varied. The channel diameter ratios that provide greatest flow access are reported. The cooling performance of the multiscale grid structures is less sensitive to the changes in beam path than the cooling performance of the other structures studied. The effect of adding a vascular structure to the design is dramatic. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759290]}, keywords = {}, pubstate = {published}, tppubtype = {article} } Here, we show that a plate heated by a moving beam can be cooled effectively by fluid that flows through a vasculature of channels embedded in the plate. The vascular designs studied are radial, grid and hybrid (radial + grid). The peak temperature of the plate changes with the path and direction of the moving beam. The strength, size and speed of the beam vary. The peak temperature increases as the beam strength and size increase and as the speed of the beam decreases. The grid and hybrid designs are robust because of loops present in the flow structure. The pressure difference that drives the fluid flow varied. The channel diameter ratios that provide greatest flow access are reported. The cooling performance of the multiscale grid structures is less sensitive to the changes in beam path than the cooling performance of the other structures studied. The effect of adding a vascular structure to the design is dramatic. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759290] 
Lorente, S; Cetkin, E; BelloOchende, T; Meyer, J P; Bejan, A The constructallaw physics of why swimmers must spread their fingers and toes Journal Article JOURNAL OF THEORETICAL BIOLOGY, 308 , pp. 141146, 2012, ISSN: 00225193. @article{ISI:000307030100015, title = {The constructallaw physics of why swimmers must spread their fingers and toes}, author = {S Lorente and E Cetkin and T BelloOchende and J P Meyer and A Bejan}, doi = {10.1016/j.jtbi.2012.05.033}, issn = {00225193}, year = {2012}, date = {20120901}, journal = {JOURNAL OF THEORETICAL BIOLOGY}, volume = {308}, pages = {141146}, abstract = {Here we show theoretically that swimming animals and athletes gain an advantage in force and speed by spreading their fingers and toes optimally. The larger force means larger body mass lifted and greater speed, in accord with the constructal theory of all animal locomotion. The spacing between fingers must be twice the thickness of the boundary layer around one finger. This theoretical prediction is confirmed by computational fluid dynamics simulations of flow across two and four cylinders of diameter D. The optimal spacing is in the range 0.2D0.4D, and decreases slightly as the Reynolds number (Re) increases from 20 to 100. For example, the total force exerted by two optimally spaced cylinders exceeds by 53% the total force of two cylinders with no spacing when Re=20. These design features hold for both timedependent and steadystate flows. (C) 2012 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Here we show theoretically that swimming animals and athletes gain an advantage in force and speed by spreading their fingers and toes optimally. The larger force means larger body mass lifted and greater speed, in accord with the constructal theory of all animal locomotion. The spacing between fingers must be twice the thickness of the boundary layer around one finger. This theoretical prediction is confirmed by computational fluid dynamics simulations of flow across two and four cylinders of diameter D. The optimal spacing is in the range 0.2D0.4D, and decreases slightly as the Reynolds number (Re) increases from 20 to 100. For example, the total force exerted by two optimally spaced cylinders exceeds by 53% the total force of two cylinders with no spacing when Re=20. These design features hold for both timedependent and steadystate flows. (C) 2012 Elsevier Ltd. All rights reserved. 
Shi, Ziyuan; Barisik, Murat; Beskok, Ali Molecular dynamics modeling of thermal resistance at argongraphite and argonsilver interfaces Journal Article INTERNATIONAL JOURNAL OF THERMAL SCIENCES, 59 , pp. 2937, 2012, ISSN: 12900729. @article{ISI:000306765700005, title = {Molecular dynamics modeling of thermal resistance at argongraphite and argonsilver interfaces}, author = {Ziyuan Shi and Murat Barisik and Ali Beskok}, doi = {10.1016/j.ijthermalsci.2012.04.009}, issn = {12900729}, year = {2012}, date = {20120901}, journal = {INTERNATIONAL JOURNAL OF THERMAL SCIENCES}, volume = {59}, pages = {2937}, abstract = {Heat conduction between two parallel solid walls separated by liquid argon is investigated using threedimensional molecular dynamics (MD) simulations. Liquid argon molecules confined in silver and graphite nanochannels are examined separately. Heat flux and temperature distribution within the nanochannels are calculated by maintaining a fixed temperature difference between the two solid surfaces. Temperature profiles are linear sufficiently away from the walls, and heat transfer in liquid argon obeys the Fourier law. Temperature jump due to the interface thermal resistance (i.e., Kapitza length) is characterized as a function of the wall temperature. MD results enabled development of a phenomenological model for the Kapitza length, which is utilized as the coefficient of a Naviertype temperature jump boundary condition using continuum heat conduction equation. Analytical solution of this model results in successful predictions of temperature distribution in liquid argon confined in silver and graphite nanochannels as thin as 7 nm and 3.57 nm, respectively. (C) 2012 Elsevier Masson SAS. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Heat conduction between two parallel solid walls separated by liquid argon is investigated using threedimensional molecular dynamics (MD) simulations. Liquid argon molecules confined in silver and graphite nanochannels are examined separately. Heat flux and temperature distribution within the nanochannels are calculated by maintaining a fixed temperature difference between the two solid surfaces. Temperature profiles are linear sufficiently away from the walls, and heat transfer in liquid argon obeys the Fourier law. Temperature jump due to the interface thermal resistance (i.e., Kapitza length) is characterized as a function of the wall temperature. MD results enabled development of a phenomenological model for the Kapitza length, which is utilized as the coefficient of a Naviertype temperature jump boundary condition using continuum heat conduction equation. Analytical solution of this model results in successful predictions of temperature distribution in liquid argon confined in silver and graphite nanochannels as thin as 7 nm and 3.57 nm, respectively. (C) 2012 Elsevier Masson SAS. All rights reserved. 
2010 
Mobedi, Moghtada; Ozkol, Uenver; Sunden, Bengt Visualization of diffusion and convection heat transport in a square cavity with natural convection Journal Article INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 53 (13), pp. 99109, 2010, ISSN: 00179310. @article{ISI:000272877900012, title = {Visualization of diffusion and convection heat transport in a square cavity with natural convection}, author = {Moghtada Mobedi and Uenver Ozkol and Bengt Sunden}, doi = {10.1016/j.ijheatmasstransfer.2009.09.048}, issn = {00179310}, year = {2010}, date = {20100101}, journal = {INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, volume = {53}, number = {13}, pages = {99109}, abstract = {In this study, the total heatfunction equation which includes diffusion and convection transport is divided into the corresponding heatfunction equations. The superposition rule is used to obtain the mathematical definitions of diffusion and convection heatfunctions and corresponding boundary conditions. It is observed that the separate visualization of diffusion and convection heatlines provides significant information on understanding of the mechanism of heat transfer in a convective flow. The direction of the diffusion and convection heat transport as well as the strength of convection compared to the conduction in entire or in a portion of a domain can be visualized. The diffusion heatlines demonstrate a potential flow like behavior while convective heat flow rotates due to the source term of the convection heatfunction equation, similar to the rotation of fluid flow generated by fluid flow vorticity. The similarity between the streamfunction and the total heatfunction yields a concept of heat flow vorticity, Omega(t). The obtained results show that the maximum absolute value of the convection heatfunction may be an appropriate parameter for determination of the convection strength. The diffusion and convection heatfunction equations for natural convection in a differentially heated square cavity for four different length of the heated surface on the right vertical wall as s(p) = L/4, L/2, 3L/4 and L and a fixed length of the cooled surface on the right vertical wall as L/4 are obtained and corresponding heatlines are drawn. The values of the conduction heatfunction are positive while the sign of convection heatfunction values is negative for the studied cases. Based on the distribution of total heatlines, two regions are detected in the cavity, an active region with the positive values of heatlines signifying dominant conduction heat transfer and a passive region with the negative heatfunction values in where convection heat flow is dominant and heat only rotates in a closed contour pattern. The variations of average Nusselt number, average of heat flow vorticity. maximum absolute values of convection heatfunction and streamfunction at different Rayleigh numbers and lengths of the heated surface are presented. (C) 2009 Elsevier Ltd. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this study, the total heatfunction equation which includes diffusion and convection transport is divided into the corresponding heatfunction equations. The superposition rule is used to obtain the mathematical definitions of diffusion and convection heatfunctions and corresponding boundary conditions. It is observed that the separate visualization of diffusion and convection heatlines provides significant information on understanding of the mechanism of heat transfer in a convective flow. The direction of the diffusion and convection heat transport as well as the strength of convection compared to the conduction in entire or in a portion of a domain can be visualized. The diffusion heatlines demonstrate a potential flow like behavior while convective heat flow rotates due to the source term of the convection heatfunction equation, similar to the rotation of fluid flow generated by fluid flow vorticity. The similarity between the streamfunction and the total heatfunction yields a concept of heat flow vorticity, Omega(t). The obtained results show that the maximum absolute value of the convection heatfunction may be an appropriate parameter for determination of the convection strength. The diffusion and convection heatfunction equations for natural convection in a differentially heated square cavity for four different length of the heated surface on the right vertical wall as s(p) = L/4, L/2, 3L/4 and L and a fixed length of the cooled surface on the right vertical wall as L/4 are obtained and corresponding heatlines are drawn. The values of the conduction heatfunction are positive while the sign of convection heatfunction values is negative for the studied cases. Based on the distribution of total heatlines, two regions are detected in the cavity, an active region with the positive values of heatlines signifying dominant conduction heat transfer and a passive region with the negative heatfunction values in where convection heat flow is dominant and heat only rotates in a closed contour pattern. The variations of average Nusselt number, average of heat flow vorticity. maximum absolute values of convection heatfunction and streamfunction at different Rayleigh numbers and lengths of the heated surface are presented. (C) 2009 Elsevier Ltd. All rights reserved. 
Project Title  Director of the Project  Start Date  Funds 
Jeotermal enerji araştırmageliştirme, test ve eğitim Merkezi  Prof. Dr. Zafer İLKEN  2002  DPT 
Biyoteknoloji ve Biyomühendislik Araştırma Merkezi  Prof. Dr. Barış ÖZERDEM (Proje Eş Yürütücü)  2007  DPT 
Adsorpsiyonlu Isı Pompası  Doç. Dr. M. Mobedi  2006  DPT 
Evaluation Of Geothermal Developments In Turkey From The Clean Energy Point Of View. Case Studies: Balcova District Heating SystemIzmir And Kizildere Geothermal Power PlantDenizli  Prof. Dr. Gülden Gökçen  2005  TÜBİTAK 
Jet Akışlarındaki Kararsızlıkların Kontrolü İle Isı Transferinin Arttırılması  Yrd.Doç.Dr. Ünver Özkol  2008  TÜBİTAK 
İYTE Kampüs Alanındaki Rüzgar Enerjisi Potansiyelinin Ölçülmesi  Prof. Dr. Barış ÖZERDEM  2000  İYTEBAP 
Alüminyum Köpük Malzeme ile İmal Edilmiş Hava Tipi Güneş Kollektörlerinin Analizi  Prof.Dr. Zafer İLKEN  2001  İYTEBAP 
Jeotermal Enerjide Kullanılan Kuyu İçi Eşanjör Performanslarının Artırılması  Prof.Dr. Zafer İLKEN  2001  İYTEBAP 
Endüstriyel Ürün Tasarımının , Üretiminin ve Mühendisliğinin ( CAD/CAM/CAE) Profesyonel Amaçlı Bir Hizmet Sektörü Olarak İnternet Ortamında Gerçekleştirilmesine Yönelik İnteraktif Bir Stüdyo Modeli : E Stüdyo  Prof. Dr. Barış ÖZERDEM (Ortak Yürütücü)  2000  İYTEBAP 
Rüzgar Türbinlerinde Verim Arttırmaya ve Veri Toplamaya Yönelik Bir Bilgisayar Kontrol Sistemi Geliştirilmesi  Prof. Dr. Barış ÖZERDEM  2005  İYTEBAP 
Sinüsoidal ve “Offset Strip” tipindeki ısı değiştirici kanallarda akışın görüntülenmesi ve CDF ile karşılaştırılması  Yrd.Doç.Dr. Ünver Özkol  2008  İYTEBAP 
Kamu Binalarında Enerji Verimliliği: İyte İdari Bina’nın Enerji Performansının Belirlenmesi  Prof. Dr. Gülden Gökçen  2005  İYTEBAP 
Tarımsal Ürünlerin Kurutulmasında Kurutma Parametrelerinin Belirlenmesi  Prof. Dr. Gülden Gökçen  2006  İYTEBAP 
Moleküler Seviyede NanoÖlçek Gaz Akışlarının İncelenmesi  Yrd. Doç. Dr. Murat Barışık  2015  Marie SkłodowskaCurie COFUND 
Adsorpsiyon Yatak Isı Pompası Kullanılarak Buzdolabı Verimliliğinin Artırılması  Yrd. Doç. Dr. Murat Barışık  2015  SANTEZ Projesi 
Momentum Flux, Phase Doppler Anemonetry and High Speed Imaging Analysis of GDI injector under flash boiling conditions  Yrd. Doç. Dr. Alvero Diez Rodriguez  2014  TÜBİTAK 
Biodiesel Spray Investigation in a Constant Volume Combustion Chamber  Yrd. Doç. Dr. Alvero Diez Rodriguez  2013  TÜBİTAK 
Türkiye’nin Farklı İklim Koşullarında Isıl Konfor Sıcaklıklarına Bağlı Olarak Konutların Enerji Performanslarının Değerlendirilmesi  Prof. Dr. Gülden Gökçen  2012  TTMD 
Konutlarda Enerji Performansı Standard Değerlendirme Metodu (KEPSDM)  Prof. Dr. Gülden Gökçen  2010  TMMOB 