Elucidating dominant flow channel size for capillary performance of open-cell porous wicks

The capillary pressure (Delta Pcap) and permeability (K) of porous wicks, both of which depend on the size of the flow channel, are crucial for the performance of heat transfer devices including loop heat pipes. However, the flow channel in actual porous wicks shrinks at the connective part between adjacent pores called the throat and does not have a constant size. The dominant flow channel size (average size or throat size) for Delta Pcap and K remains unclear. To elucidate the dominant flow channel size, porous aluminum with constant pore size and various throat sizes was controllably fabricated by the space holder method using sodium chloride (NaCl) particles. Two types of pore size (simple pore size reflecting the space holder size and connectivity-reflected pore size) were characterized by X-ray computed tomography. The Delta Pcap and K were evaluated by capillary rise tests using water. The average simple pore size was almost constant, but the average connectivity-reflected pore size changed depending on the porosity controlled by the amount of the NaCl space holder because the throat was enlarged, and the connectivity was improved by increasing porosity. It was clarified that Delta Pcap changed depending on the average connectivity-reflected pore size regardless of the constant simple pore size. In contrast, K was determined by neither the simple nor connectivity-reflected pore sizes and could be dominated by local size around the throat. These results provide an insight into the design of optimal porous structure appropriate for porous wicks applied to heat transfer applications.