HEAT-TRANSFER IN OPEN-CELL METAL FOAMS

The paper explores the use of open-celled metal foams as compact hear exchangers, exploiting convective cooling. An analytical model is deve loped for model foams with simple cubic unit cells consisting of heate d slender cylinders, based on existing heat transfer data on convectiv e crossflow through cylinder banks. A foam-filled channel having const ant wall temperatures is analyzed to obtain the temperature distributi on inside the channel as a function of foam density, cell size and oth er pertinent heat transfer parameters. Two characteristic length scale s of importance to the problem are discussed: the minimum channel leng th required for heating the fluid to its goal temperature and the ther mal entry length beyond which the transfer of heat between fluid and c hannel wall assumes a constant coefficient, The overall heat transfer coefficient of the heat exchanging system is calculated, and the press ure drop experienced by the fluid flow obtained. These results are use d to analyze and guide the design of optimum foam structures that woul d maximize heat transfer per unit pumping power. Two examples are give n to demonstrate the applicability of the analytical model: heat sinks for high power electronic devices and multi-layered heat exchangers f or aeronautical applications. The present model perhaps oversimplifies the calculation of transport in a metal foam consisting of non-circul ar, possibly sharp-edged ligaments, and so likely leads to overestimat es. Nevertheless the trends of heat transfer predicted by the model (f or dependence on foam relative density, duct geometries, fluid velocit y, etc.) are expected to be valid for a wide range of open-cell foams and are in reasonable agreement with available experimental data on al uminum foams (Bastawros and Evans, Proc. Symp. Application of Heat Tra nsfer in Microelectronics Packaging. [MECE, Dallas, TX, 1997). (C) 199 8 Acta Metallurgica Inc.