Enhanced pool boiling heat transfer during quenching of water on superhydrophilic porous surfaces: Effects of the surface wickability

Porous surfaces were prepared by chemical etching with hydrofluoric acid on stainless steel spheres. The etched surfaces were shown to be superhydrophilic with near zero static contact angles as well as wickable by taking advantage of the capillary microstructures. Using the capillary tube method, the surface wickability of the etched samples was characterized by the transient absorbed liquid volume and the wicked volume flux. The wickability was able to be varied by controlling the etching time. Quenching experiments were then performed on the unmodified and etched samples in saturated water at atmospheric pressure. It was found that the quenching process is significantly accelerated in the presence of the etched wickable surfaces, because of the enhancement of boiling heat transfer, especially within the transition boiling regime, and improvement in both the critical heat flux (CHF) and Leidenfrost point as well. Even at a relatively high wall superheat, the vapor film is highly destabilized due to the localized evaporation of the wicked liquid in the capillary structures on the etched surfaces, in addition to the effects of increased surface roughness and wettability. It was also shown that the extent of quenching acceleration is closely related to the surface wickability. In the present work, the most wickable surface leads to the fastest quenching with the mostly improved boiling heat transfer. Based on the classical Kandlikar's model, a linear correlation was proposed between the increase ratio of the CHF and the square of the wicking number that quantifies the surface wickability.