Optimization of Metal Foam Heat Sinks for Electronic Air Cooling: Part I: Porosity

Open-Cell metal foams (MFs), e.g., aluminum, copper, nickel, have a web-like structure with open connected pores. The foams can have porosities exceeding 90%; and they are isotropic and exhibit uniform average geometrical properties. Thermal advantages of MFs stem from their high solid thermal conductivities, huge surface area density (up to 10,000 m
²
/m
³
), and high permeability of about 10–
⁸
m
²
, yielding a relatively small impact on pressure drop. Additionally, the nature of foams' internal structure causes vigorous mixing which augments convection. MFs have been sought for compact heat exchangers, and for thermal management of fuel cells, high-power batteries, and portable and other electronic systems. It is well established that the effective thermal conductivity of MFs is largely governed by the porosity of a given foam- the lower the porosity the higher the effective conductivity. A metal-foam heat sink with higher effective thermal conductivity can remove more heat from electronic systems. However, as the porosity decreases, the pressure drop increases rapidly. Therefore, there has to be a tradeoff between the enhanced heat transfer and the increased pressure drop (or pumping power). In this paper, an optimization study was conducted on an air heat sink made from metal foam having 10 pores per inch. Using simulations, the porosity of the foam was varied from 90% to 60% in steps of 5%. The total size of the heat sink was 109x110x25 mm
³
, and 55.3 W was applied uniformly at the base. The heat sink was cooled by a fan providing 0.33 m
³
/min, with air inlet temperature of 85°C. Results show that the optimal porosity is 75%. Details of the optimization work are presented and some recommendations are made.