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Title: Fluid flow and heat transfer in porous media manufactured by a space holder method
Author: Lu, Xianke
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2020
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Porous metals are attracting growing attentions due to their unique properties such as light weight, high surface area, fluid permeability and good thermal conductivity, and a wide range of applications in many fields such as thermal management, chemical catalysis, energy absorption and filtration. In particular, as technology advances, the conventional cooling methods and components have become difficult to meet the rapidly growing cooling demand in modern electrical and electronic devices. Opencell porous metals have been proven to be an ideal and effective cooling solution because of their excellent thermal and permeability properties. The Lost Carbonate Sintering (LCS) process, invented in the University of Liverpool, is an efficient and low cost manufacturing process to produce open-cell porous copper with controllable porosity, pore size and pore shape. The heat transfer performance of porous metal is determined by three factors, identified as the thermal properties of the solid skeleton, the thermal properties of the fluid within the skeleton and the interactions between the solid and the fluid flow. Fluid flow behaviour in porous metal is also crucial when considering the heat transfer performance of porous metal. However, the flow behaviour in porous media and how it affects the heat transfer performance of the solid-fluid system have not been fully studied in the past. The aim of this study is to understand the interactions between fluid flow and heat transfer in porous media. Four kinds of porous media, porous copper and porous glass with LCS structure, sintered copper and sintered glass, were manufactured. The fluid flow behaviour was visualized and the pressure drop was measured in the porous glass. Flow behaviour in sintered glass was also studied to explore the commonality of the experimental results. The heat transfer performance such as heat transfer coefficient and thermal conductivity of porous copper with wide ranges of porosity and pore size was studied. Effect of flow regime on heat transfer performance of porous copper was evaluated. Pressure drop in porous media was measured at a wide range of flow rate using a purpose-built apparatus. The relationship between pressure drop and flow velocity fitted well with the Forchheimer extended Darcy equation, from which four flow regimes, pre-Darcy, Darcy, Forchheimer and turbulent, were identified. The flow regime boundaries were different for each sample and also different from the porous media reported in previous studies. In sintered glass samples, the permeability increased and form drag coefficient decreased dramatically with increasing particle size. In porous samples, the permeability increased and form drag coefficient decreased with increasing porosity. The values of permeability and form drag coefficient depended on the flow regime in which they were calculated. Flow visualization showed that the velocity magnitude distribution at the pore scale was similar to that in an open pipe flow, with the highest velocity located at the centre and gradually decreasing to zero towards the wall. Velocity profiles across a specific channel/pore were parabolic in the Darcy and Forchheimer regimes, and became more uniform in the turbulent regime. Strong velocity fluctuations were located at the channel junctions in sintered glass samples and at the near wall region in porous glass samples with LCS structure. The critical Reynolds number identifying the transition from laminar to turbulent flow obtained by quantifying velocity fluctuations agrees well with the critical Reynolds number from Forchheimer to turbulent flow identified from the pressure drop measurements. The thermal conductivity of porous copper was greatly affected by porosity and copper particle size, but was less affected by pore size. It decreases along a power law with porosity, with exponent around 2.3. For a given porosity, the porous copper with the copper particle size range of 45-70 μm had the highest thermal conductivity. Fluid in the porous copper also contributed to thermal conductivity through natural convection. For high porosity samples, the contribution could account for up to 50% of the total thermal conductivity. The heat transfer performance of porous copper through natural convection and forced convection was characterised by measuring the heat transfer coefficient. Under natural convection, the heat transfer coefficient increased nearly linearly with temperature difference. For a given porosity, the porous copper with a pore size range of 700-1000 μm performed best. The effect of porosity on natural convective heat transfer performance was insignificant. Sample orientation also affected the heat transfer performance; orientating the larger surface on the horizontal plane increased the heat transfer coefficient by more than 10%. Under forced convection, there was an optimal porosity (around 0.65) for heat transfer performance at a given pore size and flow rate. The relationship between Nusselt number and Reynolds number can be divided into three sections, each of which followed a power function, Nu = CtRen , where Ct and n are parameters constant for each section. The value of n was largely independent of porosity, except at the lowest porosity. The three sections broadly corresponded to the pre-Darcy, Darcy and non-Darcy regimes.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral