Porous Counterflow Heat Exchanger Model: Experimental and Numerical Investigation

Abstract

An experimental and numerical investigation was performed in order to evaluate the performance of several heat transfer sub-models for conduction, convection and radiation in the prediction of flow and heat transfer through 10 ppi Al2O3 foams with Peclet number based on the pore size o(102). These sub-models comprise either effective conductivity models or two phase (gas and solid) models, corresponding to thermodynamic equilibrium or non-equilibrium assumptions, respectively. Experiments were conducted in a counterflow coaxial heat exchanger, where the hot outer air, flowing at a maximum temperature of 800°C, heats the counter cold inner pipe water flow. Temperature measurements were obtained at several locations inside the porous media, pipe walls and inlet/outlet ports. Two-dimensional finite volume calculations of the coupled phenomena in the full geometrical configuration of the heat exchanger were performed. This study shows that the effective conductivity sub-models derived for packed beds of spheres and arrays of cylinders do not provide satisfactory solutions when applied to ceramic foams. An inverse method was used to estimate the effective conductivity and contact resistance between the porous media and the inner pipe as a function of the reference temperature. Two phase flow models were scrutinised in order to discuss the influence of the relevant heat transfer parameters.