Numerical Analysis of Conjugate Heat Transfer In the Presence of Porous Medium

Abstract

The intent of the current research is to emphasize the computational modelling of forced convection heat dissipation in the presence of high porosity and high thermal conductivity metallic foam in a circular pipe for different regimes of fluid flow for a range of Reynolds number. For a constant heat flux condition, the goal is to find out the efficient metal foam and configurations when air is considered as a working fluid. Flow dynamics and heat transport phenomenon are captured using Darcy Extended Forchheimer (DEF) flow and local thermal non-equilibrium (LTNE) models within the porous filled region of the pipe. The numerical results are initially matched with experimental and analytical results for the purpose of validation. Initially, the effect of fully filled foam (i.e., L􀭤 = 0.6L (i.e., 60% L), 0.8L (i.e., 80% L) and L (i.e., 100% L), i.e., L􀭤 = length of the foam, L = length of test section), and discrete filled foam (i.e., L􀭤 = 0.6L (i.e., 60% L) and 0.8L (i.e., 80% L)), in a pipe is accomplished to decrease and increase the pressure drop and heat transfer rate, respectively. The average Nusselt number for fully filled foam (L􀭤 = L) is found to be higher compared to other filling rate of metallic foams and the clear pipe at the cost of pressure drop. Further, in the presence of discrete filled foam (L􀭤 = 0.8L), the heat transfer rate deteriorates significantly while increases considerably for fully filled foam (L􀭤 = 0.8L) accompanied with the same pressure drop. As an important finding, it has been observed that the laminar and transition flow gives higher heat transfer enhancement ratio and thermal performance factor compared to turbulent flow. This work resembles numerous industrial applications such as solar collectors, heat exchangers, electronic cooling, and microporous heat exchangers. The novelty of the work is the selection of suitable flow and thermal models in order to clearly assimilate the flow and heat transfer in metallic foam. The parametric study proposed in this work surrogates the complexity and cost involved in developing an expensive experimental setup. Further in this contemporary research, a parametric analysis of partially filled high porosity metallic foams in a horizontal pipe is performed to augment heat transfer with reasonable pressure drop. The investigation includes six different models filled partially with aluminium foam by varying internal diameter of foams from the wall side and external diameter of foam from the core of the pipe. The pore density of the foam ranges from 10 to 45 PPI (pores per inch) and their porosity varies from 0.90 to 0.95. The results showed that the thermal performance factor of 10 PPI aluminium foam performs close to the 10 PPI expensive copper foam. The performance factor is found to be higher for 30 PPI aluminium foam amongst the PPI’s of the foam considered. However, the performance factor is found to be 2.93, 2.22 and 1.73 for 30PPI, 45 PPI and 20PPI with their porosities of 0.92, 0.90 and 0.90, respectively for the model 1, model 2 and model 3 at lower Reynolds number of 4500 and then it decreases progressively with increasing flow rates of the fluid. Further, optimization study is proposed to optimize for various (six) filling rate of the metallic foam in a horizontal circular pipe. Optimization study in flow through metal foams for heat exchanging applications is very much essential as it involves variety of fluid flow and structural properties. Moreover, the identification of best combinations of structural parameters of metal foams for simultaneous improvement of heat transfer and pressure drop is a pressing situation. In this work, six different metal foam configurations are considered for the enhancement of heat transfer in a circular pipe. The foam is aluminium with PPI varying from 10 to 45 and almost the same porosity (0.90-0.95). The aluminium foams are chosen from the available literature and they are partially filled in the pipe to reduce the pressure drop. A special attention is paid to the preference between pressure drop and heat transfer enhancements. Hence, Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) optimization techniques with five different criteria (contains the combination of the weightage/priority of heat transfer and pressure drop) is used. Based on the numerical results of heat and fluid flow in pipe, it is found that when an equal importance is given to both heat transfer and friction effect, 30 PPI aluminium foam with 80% filling on the inner lateral of the pipe provides the best score as 0.8197.