Viskoelastik akış soğutmalı mikro kıvrımlı kanallarda akış ve ısı transferi

In recent years, the microelectronic industry has shown significant improvement in terms of manufacturability, integration, functionality and enhanced performance. It has been known that microelectronic devices are strongly affected by temperature as well as thermal environment. This leads to an increasing demand for highly efficient cooling technologies. Thermal management often imposes the main obstacle for scaling down the size of modern electronic devices. Microchannel heat exchanger is an ideal candidate for small electronic devices due to their high surface-to-volume ratio as well as their small volumes. Microchannel heat sinks constitute an innovative cooling technology for the removal of a large amount of heat from a small area. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 100 micrometers. Following the Tuckerman and Pease (1981) at Stanford University work on single layer parallel flow micro-channel heat sink, many other studies on microchannel heat sink have been carried out. Since then, this technology has received considerable attention in microelectronics and other major application areas. Microchannel heat sinks can be used in a wide variety of applications, including electronics cooling, air-conditioning, space application, refrigeration, heat recovery ventilation (HRV), and automotive applications, to name a few. Micro serpentine channels have been extensively used in liquid cooling due to their high heat transfer coefficients. Various enhancement methods have been proposed in order to improve the heat transfer characteristic of a microchannel. The majority of these methods share a common objective, i.e., to interrupt the boundary layer on the solid surface, and replace it with fluid from the core, thus creating a new boundary layer with an increased temperature gradient. Also, it is desirable to employ the method that gives the minimum pressure drop, and the highest heat transfer rate. In the current study, an incompressible viscoelastic fluid has been introduced as a coolant in a micro serpentine channel for heat transfer enhancement due to presence of three-dimensional purely-elastic instabilities at vanishingly small Reynolds numbers. The aim of the current study is to show that a micro serpentine channel with a viscoelastic coolant provide the higher heat transfer efficiency at vanishingly small Reynolds numbers compared to that of Newtonian flow. In order to assess the heat and mass transfer characteristics of such heat sinks, an unstructured finite volume method based on three dimensional solver has been developed. The solver utilizes a method that is based on the side-centered arrangement of flow variables with an exact mass conservation. The numerical algorithm is based on side-centered finite volume method where the velocity vector components and temperature are defined at the midpoint of each cell face while the pressure term and extra stress tensor are defined at the element centroid. The present arrangement of the primitive variables leads to a stable numerical scheme. The resulting algebraic linear systems are solved using the FGMRES(m) Krylov iterative method with the restricted additive Schwarz preconditioner with a block-incomplete factorization within each portioned sub-domain. To speed up the iterative solver per time-step, the former solutions can be used as the initial solution. The computer program used in this study was written in Fortran. The implementation of the preconditioned Krylov subspace algorithm, matrix-matrix multiplication and the multilevel preconditioner were carried out using the PETSc (Portable, Extensible Toolkit for Scientific Computation) software package developed at the Argonne National Laboratories. The coolant, viscoelastic fluid is modeled as an incompressible Oldroyd-B fluid. The heat and mass transfer characteristic of the flow is studied numerically by varying a series of parameters such as Reynolds, Prandtl and Weissenberg numbers systematically. As the Weissenberg number is increased, the viscoelastic instability becomes significant. As a result of this, flow complexity and the strength of streamwise vortices increase. This shows not only hot fluid being transported away from the walls into the center but also the fluid near the wall continuously being refreshed. Based on initial evaluation of the present results, it is concluded that purely-elastic instabilities are responsible for heat transfer enhancement.