Convective heat transfer in open-cell foams: The effects of porosity and velocity on representative volume element size

Because of features like high heat transfer area per unit volume and tortuosity, open-cell foams are used in applications which benefit from heat transfer enhancement, such as solar receivers, heat exchangers, and thermal energy storage systems. Numerical simulations are carried out to predict the heat transfer performance of foams, but their complex geometry makes the required computational power challenging. Therefore, the behaviour of a foam is predicted by simulations carried out on a computational domain far smaller than the volume of the foam. Reference is, then, made to a cubic sub-volume, named the Representative Volume Element (RVE), which has the same characteristics as those of the whole foam. The effects of porosity on RVEs for convective heat transfer in open-cell foams are analysed numerically in this study. Four 40 Pores Per Inch (PPI) open-cell foam samples with various nominal porosities are reconstructed by X-ray computed tomography. After building up a numerical grid with MATLAB, mass, momentum and energy equations are solved numerically with the COMSOL Multiphysics commercial finite-element code, under the assumption of uniform heat flux at the solid/fluid boundary. The interfacial convective heat transfer coefficient and volumetric heat transfer coefficient, for different nominal porosities of the foam and inlet velocities of the air, are predicted. The RVE length that minimizes the computational cost of simulations, under different nominal porosities and fluid velocities, is then determined.