Mechanical Engineering / Makina Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4129
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Article Citation - WoS: 17Citation - Scopus: 18Electric Field Controlled Heat Transfer Through Silicon and Nano-Confined Water(Taylor & Francis, 2019) Yenigün, Onur; Barışık, MuratNanoscale heat transfer between two parallel silicon slabs filled with deionized water was studied under varying electric field in heat transfer direction. Two oppositely charged electrodes were embedded into the silicon walls to create a uniform electric field perpendicular to the surface, similar to electrowetting-on-dielectric technologies. Through the electrostatic interactions, (i) surface charge altered the silicon/water interface energy and (ii) electric field created orientation polarization of water by aligning dipoles to the direction of the electric field. We found that the first mechanism can manipulate the interface thermal resistance and the later can change the thermal conductivity of water. By increasing electric field, Kapitza length substantially decreased to 1/5 of its original value due to enhanced water layering, but also the water thermal conductivity lessened slightly since water dynamics were restricted; in this range of electric field, heat transfer was doubled. With a further increase of the electric field, electro-freezing (EF) developed as the aligned water dipoles formed a crystalline structure. During EF (0.53 V/nm), water thermal conductivity increased to 1.5 times of its thermodynamic value while Kapitza did not change; but once the EF is formed, both Kapitza and conductivity remained constant with increasing electric field. Overall, the heat transfer rate increased 2.25 times at 0.53 V/nm after which it remains constant with further increase of the electric field.Article Citation - WoS: 8Citation - Scopus: 8Pore Connectivity Effects on the Internal Surface Electric Charge of Mesoporous Silica(Springer, 2019) Şen, Tümcan; Barışık, MuratNano-scale confinements within mesoporous systems develop overlapping electric double layers (EDL) such that the existing theoretical models cannot predict the electric potential distributions and resulting surface charges. In addition, ionic conditions undergo local variation through connections between the pore voids and pore throats. For the first time in literature, we studied the charging behavior of mesoporous silica in terms of the pore to throat size ratio (R-pt) to characterize the pore connectivity effects, in addition to porosity (epsilon) and pore size (H). Both local and average surface charge densities inside mesoporous silica were examined by varying these parameters systematically. Results showed that the magnitude of surface charge density decreased with increasing EDL overlap and decreasing connectivity effects. We formulized this behavior and developed an extended model to predict mesoporous silica's internal charge as a function of porosity, pore size, and pore to throat size ratio.Article Citation - WoS: 12Citation - Scopus: 12Numerical Determination of Interfacial Heat Transfer Coefficient for an Aligned Dual Scale Porous Medium(Emerald Group Publishing, 2018) Sabet, Safa; Mobedi, Moghtada; Barışık, Murat; Nakayama, AkiraPurpose Fluid flow and heat transfer in a dual scale porous media is investigated to determine the interfacial convective heat transfer coefficient, numerically. The studied porous media is a periodic dual scale porous media. It consists of the square rods which are permeable in an aligned arrangement. It is aimed to observe the enhancement of heat transfer through the porous media, which is important for thermal designers, by inserting intra-pores into the square rods. A special attention is given to the roles of size and number of intra-pores on the heat transfer enhancement through the dual scale porous media. The role of intra-pores on the pressure drop of air flow through porous media is also investigated by calculation and comparison of the friction coefficient. Design/methodology/approach To calculate the interfacial convective heat transfer coefficient, the governing equations which are continuity, momentum and energy equations are solved to determine velocity, pressure and temperature fields. As the dual scale porous structure is periodic, a representative elementary volume is generated, and the governing equations are numerically solved for the selected representative volume. By using the obtained velocity, pressure and temperature fields and using volume average definition, the volume average of aforementioned parameters is calculated and upscaled. Then, the interfacial convective heat transfer coefficient and the friction coefficient is numerically determined. The interparticle porosity is changed between 0.4 and 0.75, while the intraparticle varies between 0.2 and 0.75 to explore the effect of intra-pore on heat transfer enhancement. Findings The obtained Nusselt number values are compared with corresponding mono-scale porous media, and it is found that heat transfer through a porous medium can be enhanced threefold (without the increase of pressure drop) by inserting intraparticle pores in flow direction. For the porous media with low values of interparticle porosity (i.e. = 0.4), an optimum intraparticle porosity exists for which the highest heat transfer enhancement can be achieved. This value was found around 0.3 when the interparticle porosity was 0.4. Research limitations/implications The results of the study are interesting, especially from heat transfer enhancement point of view. However, further studies are required. For instance, studies should be performed to analyze the rate of the heat transfer enhancement for different shapes and arrangements of particles and a wider range of porosity. The other important parameter influencing heat transfer enhancement is the direction of pores. In the present study, the intraparticle pores are in flow direction; hence, the enhancement rate of heat transfer for different directions of pores must also be investigated. Practical implications The application of dual scale porous media is widely faced in daily life, nature and industry. The flowing of a fluid through a fiber mat, woven fiber bundles, multifilament textile fibers, oil filters and fractured porous media are some examples for the application of the heat and fluid flow through a dual scale porous media. Heat transfer enhancement. Social implications The enhancement of heat transfer is a significant topic that gained the attention of researchers in recent years. The importance of topic increases day-by-day because of further demands for downsizing of thermal equipment and heat recovery devices. The aim of thermal designers is to enhance heat transfer rate in thermal devices and to reduce their volume (and/or weight in some applications) by using lower mechanical power for cooling. Originality/value The present study might be the first study on determination of thermal transport properties of dual scale porous media yielded interesting results such as considerable enhancement of heat transfer by using proper intraparticle channels in a porous medium.Article Citation - WoS: 26Citation - Scopus: 27Internal Surface Electric Charge Characterization of Mesoporous Silica(Nature Publishing Group, 2019) Şen, Tümcan; Barışık, MuratMesoporous silica is an emerging technology to solve problems of existing and to support projected revolutionary applications ranging from targeted drug delivery to artificial kidney. However, one of the major driving mechanisms, electric charging of internal mesoporous surfaces, has not been characterized yet. In the nanoscale confinements of mesoporous structures made of pore throats and pore voids, surface charges diverge from existing theoretical calculations and show local variation due to two occurrences. First, when the size of pore throat becomes comparable with the thickness of ionic layering forming on throats' surfaces, ionic layers from opposite surfaces overlap so that ionic concentration on the surface becomes different than Boltzmann distribution predicts, and there will no longer be an equilibrium of zero electric potential at pore throat centers. Second, when this non zero potential inside throats becomes different than the potential of pore voids, ionic diffusion from void to throat creates axial ionic variation on surfaces. For such a case, we performed a pore level analysis on mesoporous internal surface charge at various porosities and ionic conditions. Pore parameters strongly affected the average internal charge which we characterized as a function of overlap ratio and porosity, first time in literature. Using this, a phenomenological model was developed as an extension of the existing theory to include nano-effects, to predict the average mesoporous internal surface charge as a function of EDL thickness, pore size and porosity.Article Citation - WoS: 26Citation - Scopus: 27Electric Charge of Nanopatterned Silica Surfaces(Royal Society of Chemistry, 2019) Özçelik, H. Gökberk; Barışık, MuratThe most recent technologies employ nanoscale surface patterning or roughening in order to engineer desired properties on a surface. Electrokinetic properties at the interface of such surfaces and ionic liquids show different behavior to the well-known theoretical descriptions. Basically, the ionic distribution on the surface differs due to electrical double layer overlap effects in the pits and curvature effects at the tips of surface structures. Generally, the charge density of a surface is assumed to be a material property and surface roughness effects are overlooked in most of the literature. In contrast, we properly calculated the local surface charges based on surface chemistry at the corresponding local ionic concentration (charge regulation) for various surface roughness and solution conditions. The results showed that the surface charge density of silica decreased at the pits but increased at the tips of surface patterns. Even for the simplest case of self-repeating surface structures, the average of local surface charges becomes lower than the theoretical predictions. Based on numerical calculations, a phenomenological model was developed as an extension to the existing flat surface theory, which can successfully predict the average surface charge on a nano patterned surface as a function of the surface pattern size, ionic concentration and pH.Article Citation - WoS: 21Citation - Scopus: 24Effect of Nano-Film Thickness on Thermal Resistance at Water/Silicon Interface(Elsevier Ltd., 2019) Yenigün, Onur; Barışık, MuratParallel to the developments in micro/nano manufacturing techniques, component sizes in micro/nano electro mechanical systems have been decreasing to nanometer scales. Decrease in lengths in heat transfer direction below the heat carrier phonon length scales reduces thermal conduction in semiconductors. This study shows that such altered phonon spectrums with the decrease of size also reduce the heat transfer at the solid/liquid interfaces and can be correlated with the thermal conductivity of the slab. Using Molecular Dynamics (MD), we measured heat transfer between water and silicon of different thickness between 5 nm and 60 nm. Silicon slabs exhibit a linear temperature profile through the bulk where thermal conductivities measured based on Fourier law decreased by the decreasing slab thickness. We applied a semi-theoretical formulism on variation of conductivity by slab thickness. At the interface of these slabs and water, heat passage is disturbed due to the phonon mismatch of dissimilar materials, which is mostly considered as solid/liquid couple interface properties by the earlier literature. Resistance for phonon passage characterized as Kapitza length (L-K) is measured for different slab thicknesses at different surface wetting conditions varying between hydrophilic to hydrophobic. Increasing surface wetting decreases the L-K while at a certain wetting, decreasing the slab thickness increases the L-K. Once the L-K of different size slabs normalized by its bulk value (assumed to be the L-K of the thickest slab at the corresponding wetting), L-K variation by silicon thickness shows a universal behavior independent of surface wetting. A mathematical model describing the exponential increase of L-K by decreasing thickness was developed and validated by an earlier model. We further developed a correlation between the corresponding changes of L-K and conductivity with respective to their bulk values by analytically combining two models as (L-K/L-K-(Bulk)) = exp (3.94(k(Bulk) - k)/(k x k(Bulk))), using which L-K can be predicted from available thermal conductivities of a certain material. Results are crucial for thermal management of current and future electronics. (C) 2019 Elsevier Ltd. All rights reserved.Article Citation - WoS: 23Citation - Scopus: 24An Extended Kozeny-Carman Model for Gas Permeability in Micro/Nano-porous Media(American Institute of Physics, 2019) Sabet, Safe; Barışık, Murat; Mobedi, Moghtada; Beşkök, AliGas transport in micropores/nanopores deviates from classical continuum calculations due to nonequilibrium in gas dynamics. In such a case, transport can be classified by the Knudsen number (Kn) as the ratio of gas mean free path and characteristic flow diameter. The well-known Klinkenberg correction and its successors estimate deviation from existing permeability values as a function of Kn through a vast number of modeling attempts. However, the nonequilibrium in a porous system cannot be simply modeled using the classical definition of the Kn number calculated from Darcy's definition of the pore size or hydraulic diameter. Instead, a proper flow dimension should consider pore connectivity in order to characterize the rarefaction level. This study performs a wide range of pore-level analysis of gas dynamics with different porosities, pore sizes, and pore throat sizes at different Kn values in the slip flow regime. First, intrinsic permeability values were calculated without any rarefaction effect and an extended Kozeny-Carman model was developed by formulating the Kozeny-Carman constant by porosity and pore to throat size ratio. Permeability increased by increasing the porosity and decreasing the pore to throat size ratio. Next, velocity slip was applied on pore surfaces to calculate apparent permeability values. Permeability increased by increasing Kn at different rates depending on the pore parameters. While the characterization by the Kn value calculated with pore height or hydraulic diameter did not display unified behavior, relating permeability values with the Kn number calculated from the equivalent height definition created a general characterization based on the porosity independent from the pore to throat size ratio. Next, we extended the Klinkenberg equation by calculating unknown Klinkenberg coefficients which were found as a simple first order function of porosity regardless of the corresponding pore connectivity. The extended model as a combination of Kozeny-Carman for intrinsic permeability and Klinkenberg for apparent permeability correction yielded successful results. Published under license by AIP Publishing.Conference Object Molecular Modeling of Force Driven Gas Flows in Nano-Channels(International Conference on Computational Fluid Dynamics (ICCFD), 2016) Barışık, MuratNano-scale gas transport plays an important role in many micro/nanotechnology applications where the rarefied gas dynamics based solutions are frequently used by maintaining a “dynamic similarity” between low pressure (rarefied) and nano gas flows. However, such a consideration is incomplete since the surface force field effects dominant in nano-levels induce significant variations. In order to specify the surface force effects on gas transport, we define a new parameter (B) as the ratio of force penetration length to the channel height, and studied the length scales and conditions for applicability of current rarefied gas dynamics on nano-scale gas flows. Using Molecular Dynamics, force driven gas flows were characterized as a function of B parameter and Knudsen number. Results showed that for a negligible value of B parameter (B?0) transport can be described by rarefied gas dynamics in the whole Knudsen range while the velocity profiles are parabolic and the variation of mass flow rate shows the well-known Knudsen minimum around Kn=1. As the flow dimension decreases, B becomes a finite value indicating the dominancy of surface force effects over rarefaction, and gas velocities and mass transport significantly deviates from the kinetic theory predictions. © 2016 9th International Conference on Computational Fluid Dynamics, ICCFD 2016 - Proceedings. All rights reserved.Conference Object Effect of Electric Field on Interfacial Thermal Resistance Between Silicon and Water at Nanoscales(Avestia Publishing, 2019) Yenigün, Onur; Barışık, MuratIn this study, heat transfer rate of a nano-confined liquid is controlled by applying an electric field parallel to the heat transfer direction. Molecular Dynamics simulations are performed for deionized water confined between silicon slabs, where their surfaces oppositely charged to create an electric field perpendicular to the silicon wall to promote the electrowetting. Electric field strengths used in this study are 0, 0.18 and 0.35 V/nm. To investigate the effect of electric field on heat transfer, first water density profiles near the silicon walls are examined. Results shows that by applying electric field, water molecules near the silicon walls develop layering, which indicates the increased solid/liquid coupling. With the increasing electric field strength, an increase in the peak of the density layering is observed. Furthermore, heat transfer at the solid/liquid interface is characterized with the Kapitza length values. The results show that applying electric field reduces the interfacial thermal resistance between water and silicon due to the increased solid/liquid coupling and doubles the total heat flux. © 2019, Avestia Publishing.Article Citation - WoS: 47Citation - Scopus: 46Surface Charge-Dependent Transport of Water in Graphene Nano-Channels(Springer Verlag, 2018) Çelebi, Alper Tunga; Barışık, Murat; Beşkök, AliDeionized water flow through positively charged graphene nano-channels is investigated using molecular dynamics simulations as a function of the surface charge density. Due to the net electric charge, Ewald summation algorithm cannot be used for modeling long-range Coulomb interactions. Instead, the cutoff distance used for Coulomb forces is systematically increased until the density distribution and orientation of water atoms converged to a unified profile. Liquid density near the walls increases with increased surface charge density, and the water molecules reorient their dipoles with oxygen atoms facing the positively charged surfaces. This effect weakens away from the charged surfaces. Force-driven water flows in graphene nano-channels exhibit slip lengths over 60 nm, which result in plug-like velocity profiles in sufficiently small nano-channels. With increased surface charge density, the slip length decreases and the apparent viscosity of water increases, leading to parabolic velocity profiles and decreased flow rates. Results of this study are relevant for water desalination applications, where optimization of the surface charge for ion removal with maximum flow rate is desired.
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