Thermal conductance of biporous evaporator wicks in thin film evaporation and boiling regimes

We present detailed data on the performance of microstructured geometries for use in the evaporator section of a vapor chamber heat pipe. The central innovation of the geometries is their hierarchical structure, involving the use of large microchannels in order to reduce liquid flow drag while fabricating microscale pin fin arrays whose small pores increase capillary suction. The overall conductance in such a geometry is dependent on the extent of thin liquid film (thickness ∼few microns), which is manipulated by increasing the surface area-to-volume ratio through the use of microstructuring. Experiments were conducted for a heater area of 1cm², with the wick in a vertical orientation. Results are presented for fixed microchannel widths of 30-60 microns, with pin fin diameters ranging from 4 to 32 microns, and pin fin array widths of 150 to 300 microns. The competing effects of increase in surface area due to microstructuring, and the suppression of evaporation due to reduction in pore scale are explored. In the evaporative regime, conductances of the order of 6 W/cm²-K are attained at heat fluxes of up to 140 W/cm², until the capillary limit is reached and the wick dries out.