GCRIS

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Numerical Investigation of Gas Transport Through Micro/Nano-scale Porous Media at Slip Flow Regime
(Izmir Institute of Technology, 2021) Sabet, Safa; Barışık, Murat
Gas flow in micro/nano-scale porous systems is observed in many applications and technologies. Gas dynamics at such small scales differ from conventional fluid dynamics estimations due to rarefaction effects. In the literature, the Knudsen number (Kn) for the characterization of rarefaction effects on permeability is calculated based on a characteristic flow height estimated from the pore size, while the geometric parameters such as pore shape and pore-throat ratios are mostly ignored. Therefore, an accurate characterization of rarefaction effects could not be ascertained. For the first time in literature, a general characterization of gas transport through systems at different porosity and pore throat size values and at different rarefaction levels was obtained using a modified Kn definition. The characteristic height required for an accurate Kn of a porous system is defined using the "equivalent diameter" calculated from the corresponding permeabilities. Pore-level calculations were performed in a wide range of systems while the observed permeability variation by porous parameters was successfully described by an extended volume-averaged model developed as a combination of the Darcy, Kozeny-Carman, and Klinkenberg models. The characterization systematic and volume-averaged model was applied for various cases of (i) two-dimensional porous, (ii) two-dimensional multi- porous, and (iii) three-dimensional complex porous system. For all these systems, the permeability values could be estimated in terms of the geometric parameters of the porous structures and rarefaction levels. In addition, the rarefaction effects on heat convection in metal foams were studied through Darcy to Forchheimer flow regimes using the Kelvin Cell structure. A 60% increase in permeability and a substantial decrease in inertial effects developed due to rarefaction, while Nusselt numbers were found mostly related to Reynolds number. Further, the influence of variation in gas thermophysical properties coupled with rarefaction as a function of increasing gas temperature for high heat flux applications was described. A 40% decrease in hydraulic conductivity for a temperature increase from 300K to 400K is observed, independent from the Kn number.
Molecular Dynamics Studies on Interface Heat Transfer Control Using Electric Field
(Izmir Institute of Technology, 2021) Yenigün, Onur; Barışık, Murat
Thermal management is considered as a bottleneck for the development of next generation micro/nano-electronics with high heat dissipation rates. When component sizes decrease to nanoscales, increase in surface to volume ratio leads the interfacial thermal resistance (ITR) to dominate the heat transfer behavior. The current study focuses on characterizing ITR at molecular level and exploring smart thermal management concepts for nano-scale systems. In sequence, the effect of solid thickness on ITR was investigated such that the altered phonon spectrum inside the solid domain creating the size dependency on thermal conductivity was also found to create a size dependency in ITR. Next, an active and local manipulation of heat transfer between water and various solids by an applied uniform and/or non-uniform electric field was examined. When the water molecules underwent electric field induced orientation polarization and liquid dielectrophoresis (LDEP), a substantial increase in heat transfer was developed due to the decrease in ITR and increment in thermal conductivity. Finally, an interface-localized heat transfer control technique was proposed, where interdigitated electrodes (IDEs) were embedded into the heat dissipating surface. IDEs created an electric field gradient exclusively near the electrode surface which resulted in LDEP forces on the water dipoles at near surface region enhancing solid/liquid interface energy and almost eliminating the ITR. We developed semi-empirical and theoretical relations to describe ITR variation by the electric field, which will be important for thermal management of current and future technologies.
Multiphysics Modeling of Surface Charge and Pressure-Driven Electrokinetic Flow in Micro/Nano Scale Porous Media
(Izmir Institute of Technology, 2021) Şen, Tümcan; Barışık, Murat
Accurate characterization of fluid transport in micro/nano confinements is essential for numerous applications from industrial, agricultural, and medical sciences. In these applications, electrokinetic interactions dominate the fluid behavior, which causes conventional fluid dynamics to become incomplete. Specifically, near-wall hydrodynamics and liquid/solid coupling at the interface varies by electrokinetic effects. Therefore, the current study focuses on characterization of the fluid transport at various porous systems and ionic conditions. The Poisson-Nernst-Planck (PNP) equations were numerically solved coupled with the Navier-Stokes (NS) equations. Charge regulation (CR) boundary condition is employed to calculate the charging behavior of the surfaces. First, the surface charging of nano-scale systems was analyzed by considering the electric double layer (EDL) overlap and inlet/outlet effects. While EDL overlap decreased the surface charge, inlet/outlet effects presented an opposite behavior. Then, transport is characterized by calculating the hydraulic conductivity from Darcy's law under electrokinetic and boundary slip effects. The results showed that electrokinetic effects decrease the hydraulic conductivity with increasing concentrations and decreasing confinement sizes. At slipping condition with a constant slip length applied, velocity slip developing on surface showed strong dependence on porosity and ionic conditions. For low porosities and high concentrations almost no-slip conditions were observed even at high slip lengths. Results showed that the transport in micro/nano-scale porous systems is dominated by electrokinetic interactions depending on porous system parameters and ionic conditions.

Phd Degree / Doktora

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