Capillary-driven evaporation of superhydrophilic copper foams with ultra-high porosity

Capillary-driven evaporation in porous wicks widely exists in vapor chambers, heat pipes, etc. Porous wicks with high porosity are expected to increase critical heat flux (CHF). Here, the superhydrophilic copper foam with 92.5% porosity is adopted, with nanostructures coated on its skeleton. The effect of wicking length (L) on CHF and thermal resistance (R_th) is experimentally studied. The capillary rise includes saturated and partially saturated section. A high CHF (about 400 W/cm²) is obtained in saturated section. In the partially saturated section (L > 5.8 cm,) there are wall-attached and nano structure-dominated capillary flow. The former (liquid film on the interface between copper substrate and copper foam) exists when 5.8 cm < L < 9.25 cm, but disappears when L > 9.25 cm. As L increases, therefore, the R_th at CHF (R_th,CHF) decreases in different manners and even increases when L > 9.75 cm. As L increases, the decreasing liquid replenishment reduces the impedance to vapor discharge but easily generates local dried-out. Therefore, R_th versus heat flux varies in different L. The largest CHF/R_th,CHF, meaning high CHF with low R_th,CHF, is achieved at L = 7 cm. These insights help to enhance the capillary-driven evaporation for high porosity porous wicks.