Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7148
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Article Citation - WoS: 5Citation - Scopus: 6Active Heat Transfer Enhancement by Interface-Localized Liquid Dielectrophoresis Using Interdigitated Electrodes(Elsevier, 2022) Yenigün, Onur; Barışık, MuratWe introduced an active heat transfer control between graphene and water using interdigitated electrodes (IDEs). Oppositely charged co-planer electrodes embedded on a graphene surface created a non-uniform electric field. Resulted interface localized liquid dielectrophoresis (LDEP) perpendicular to surface enhanced the water/graphene coupling and decreased interfacial thermal resistance (ITR) substantially. We correlated the theoretical calculations of average electric field strength near surface with Kapitza values measured at corresponding electrode configurations. We obtained a unified linear variation of Kapitza as a function of average electric strength independent of electrode size and charge. By increasing the electric field strength, we measured up to 96% decrease of Kapitza near electrodes. Since the IDEs generated electric field was only interface localized, it required lower electrode charges than any parallel-plate capacitor systems. We showed that ITR remains effective in heat transfer behavior for systems as big as 100nm such that interface localized electric field can at least increase the heat removal 50% by eliminating the ITR from both graphene/water interfaces of a channel system. By converting hydrophobic few-layer graphene to super-hydrophilic condition with ultra-low Kapitza, current results are important for graphene-based materials considered for the solution of the thermal management problem of current and next generation micro/nano-electronics.Article Citation - WoS: 9Citation - Scopus: 13Local Heat Transfer Control Using Liquid Dielectrophoresis at Graphene/Water Interfaces(Elsevier Ltd., 2021) Yenigün, Onur; Barışık, MuratGraphene-based materials are considered for the solution of the thermal management problem of current and next generation micro/nano-electronics with high heat generation densities. However, the hydrophobic nature of few-layer 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, non-uniform 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 two-dimensional heat transfer control between two parallel few-layer graphene slabs. We obtained local heat transfer increase up to nine times at pin electrode region with an ultra-low Kapitza resistance through the studied non-uniform electric field strength range creating highly-ordered 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. © 2020Article 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: 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: 9Citation - Scopus: 12Void Fraction and Speed of Sound Measurement in Cavitating Flows by the Three Pressure Transducers (3pt) Technique(Elsevier, 2020) Esposito, Claudia; Yenigün, Onur; Gouriet, Jean-Baptiste; Steelant, Johan; Vetrano, Maria RosariaSpeed of sound and void fraction are two key parameters in the characterisation of two-phase flows. However, accurate measurements require either intrusive or complex techniques. This paper reports on the Three Pressure Transducers (3PT) technique, which derives the speed of sound by measuring pressure fluctuations and which, thanks to its robustness and simplicity, could be applicable in harsh conditions. Therefore, the aim of this paper is to study in detail the feasibility of this technique against its limits and constraints in a cavitating flow. First, a numerical assessment of the technique is proposed to determine both the optimal transducers configuration and the sampling frequency. Then, the implemented algorithm was applied to a two-phase air-water mixture with well-known properties. Finally, the 3PT algorithm was used to study the behaviour of a cavitating flow induced by an orifice. This last application highlighted the possibility to use this technique to characterize the bubble flow generated by an orifice without the use of any optical access and by using a very compact experimental arrangement. The results obtained are also qualitatively compared to the images of the flow simultaneously acquired by a high-speed camera.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: 3Citation - Scopus: 5Constructal Tree-Shaped Designs for Self-Cooling(Edizioni ETS, 2016) Yenigün, Onur; Çetkin, ErdalIn this paper, we show how a plate which is subjected to a heating load can be kept under an allowable temperature. Vascular channels in which coolant fluid flows have been embedded in the plate. Two types of vascular channel designs were compared: radial and tree-shaped. The effects of channel design on the thermal performance for different volume fractions (the fluid volume over the solid volume) are documented. Changing the design from radial to tree-shaped designs decreases the order of pressure drop. Hence increase in the order of the convection coefficient is achieved. However, treeshaped designs do not bath the entire domain. Therefore, we have inserted additional branches at the uncooled regions. Then, we have compared the peak temperatures of radial, traditional tree-shaped and improved tree-shaped designs. 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.Article Citation - WoS: 33Citation - Scopus: 35Experimental and Numerical Investigation of Constructal Vascular Channels for Self-Cooling: Parallel Channels, Tree-Shaped and Hybrid Designs(Elsevier Ltd., 2016) Yenigün, Onur; Çetkin, ErdalIn 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, tree-shaped 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 tree-shaped designs decreases the order of pressure drop. Hence increase in the order of the convective heat transfer coefficient is achieved. However, tree-shaped designs do not bathe the entire domain, which increases the conductive resistances. Therefore, additional channels were inserted at the uncooled regions in the tree-shaped design (hybrid design). The best features of both parallel channels and tree-shaped designs are combined in the hybrid of them: the flow resistances to the fluid and heat flow become almost as low as the tree-shaped 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.