Evaluation of thermal and fluid dynamic performance parameters in aluminum foam compact heat exchangers

This paper analyses the behavior of heat exchangers with metal foam. A numerical study in a laminar steady state two-dimensional model is provided to evaluate the thermal - fluid dynamic performance of the system. An aluminum foam block embedded with an array of five circular tubes and assigned uniform temperature, constitute the physical domain. The open-celled metal foam is considered as a porous medium in local thermal non equilibrium (LTNE) and the Darcy - Forchheimer - Brinkman model is assumed. The metal foam has a porosity equal to 0.9353 and a pore density equal to 20 pores per inch (PPI). The numerical simulations are carried out considering air as the cooling fluid with several mass flow rate for assigned temperature of the tube surface. The Reynolds number, Re, based on tube diameter, ranges from 56 to 1120, the ratios between the thickness of metal foam and tube diameter, L-mf/d, and between the tube pitch and tube diameter, 2H/d, are in the ranges from 1 to 20 and 1.5-25, respectively. The goal of this investigation is to find the compact heat exchanger size for an assigned metal foam that allows trade-off between the improvement of the heat transfer and the increment of pressure drop, and to study the fluid dynamics and thermal behavior of these configurations. The study focuses on thermal performances of a metal foam heat exchanger using several conventional parameters employed in the analysis of porous medium heat exchangers to find optimal geometrical configurations. Results are given in terms of Colburn factor, friction factor and area goodness factor as a function of Re, L-mf/d and 2H/d. It is shown that the compact heat exchanger, examined in the investigation, presents optimum Reynolds number values; where the ratio between the Colburn and the friction factor, AG, attains a maximum value for metal foam, with different thickness and distance between the tubes of the heat exchanger. Two correlations for AG(max) are proposed for 56 <= Re-d <= 1120; one in terms of dimensionless thickness and another for the dimensionless pitch.