Investigation of capillary properties of copper metal foams by the rate of rise method in the presence of evaporation

An experimental study has been done to define the capillary characteristics of a novel a type of copper metal foam which is to be used as a wick in flat heat pipes for electronic cooling. Capillary properties such as permeability (K) and effective pore radius (r_eff) are critical in defining the capillary limit of heat pipes, because their ratio (K/r_eff) i s a measure of pumping capacity of the wicking material. Foam strips with porosities of 68%, 75% and 82% are tested by the rate of rise method to measure K and r_eff. Impact of evaporation on the capillary rise has been studied by using acetone as test liquid in an open and a partially saturated ambient to reduce the evaporation rate. It was found out that the rate of evaporation while the liquid is rising is less than the evaporation rate of a saturated sample with stationary liquid. This will allow ignoring the evaporation effect in the mathematical model used to extract K and r_eff. By this simplification, rate of increasing of acetone and water weight with time is measured with a high precision balance and then, by using an appropriate mathematical model, K and r_eff are extracted. Results show that 75% porosity foam has the highest K/r_eff ratio which makes it a suitable candidate for a heat pipe wicking material.