Single and two phase heat transfer in novel porous foams

• Main Author: Li, Hongying
• Other Authors: Leong Kai Choong
• Format: Theses and Dissertations
• Language: English
• Published: 2011
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/46544
• Institution: Nanyang Technological University

The rapid development of technology in the last decades brings increased challenges in the cooling of electronic devices. Densely packed electronic systems require more effective methods to dissipate the heat generated by the chips and circuits. Porous media with high effective thermal conductivity and large contact surface area between the solid and fluid phases are well recognised as good heat sinks. However, given the complex micro-scale structures involved in fluid flow and heat transfer in porous media, numerical modelling plays an important role in understanding the physics in both single and two-phase heat transfer processes. The present study develops in-house codes to investigate fluid flow and heat transfer in porous media. Two-dimensional (2D) and three-dimensional (3D) codes based on the finite volume method (FVM), for both single and two phases are developed. These codes are validated against the existing published works. With the validations made, single-phase flow and heat transfer in porous media of different configurations are investigated. The predicted pressure drops and heat transfer performance are compared favourably with the experimental data. The codes for two-phase heat and fluid flow are used to investigate flow boiling characteristics in both horizontal and vertical channels fully-filled with porous media. The experiments are performed with water as the working fluid. The numerical predictions are in reasonable agreement with those of the experiments. In single-phase flow, a simplified numerical procedure is proposed to deal with the interface between the porous and open regions. This procedure is applied successfully to a variety of well-known problems in a 2D setting for validation purpose. With this achieved, the procedure is extended to a 3D setting in a dimension-by-dimension manner. Such a 3D code is employed to study fluid flow and heat transfer in the configurations of zigzag and baffle graphite foams which is a novel open-cell porous medium. This 3D code is aimed at the simulation of the transport phenomena in porous media with complex structures which serve as heat sinks in thermal management systems. For further validation of the developed codes for simulation of single-phase flow and heat transfer in porous media, the experimental studies are performed to compare with the simulation results. The configurations of zigzag and baffle are manufactured based on graphite foam. The physical properties of this graphite foam are obtained from the experiments. The temperature distributions at the substrate of different configurations of graphite foam are measured to compare with the simulation results. Good agreements are achieved between the simulation and experimental results. In two-phase flow, a numerical procedure is presented to study flow boiling process in porous media. The “modified” Kirchhoff method is introduced to treat the discontinuity of the diffusion coefficient. The numerical procedure proposed is validated against the existing experimental data and then applied successfully to horizontal and vertical porous channels to reveal the evolutions of the boiling process. Upon achieving that, a 3D code is developed for a more realistic simulation of two-phase flow and heat transfer in porous channels. This 3D code is aimed at the simulation of a large class of phase change flow problems to compare with the actual situations. In addition to the numerical simulation of phase change in porous media, the experimental study for flow boiling is also carried out. The heat flux for the onset of nucleate boiling and the local temperature distribution measured from the experiments are compared against the simulation results. The reasonable agreements demonstrate the capability of the developed code in the prediction of two-phase heat transfer in porous media. It is hoped that the developed codes for both single and two-phase flow and heat transfer in porous media can be used as guidelines for the preliminary design of heat sinks in thermal systems.