Boiling performance enhancement and self-recovery of nucleate boiling regime on micro- and nanostructured porous surfaces

Abstract

In this paper, we report the pool boiling experimental results for significant enhancements in the boiling heat transfer coefficient (BHTC) and CHF on various microporous surfaces. The microporous surfaces were fabricated by metal (Copper) powder sintering process, and nanostructured porous surfaces were additionally fabricated through the thermal oxidation of the metal porous surfaces. Among the examined substrates, a boiling surface with a thin metal porous layer and millimeter-sized porous pillar structures exhibited optimal boiling performance; the BHTC and CHF of the microporous surface were measured to be approximately 500 % and 270 % higher than those of a smooth reference surface, respectively. We conjecture that the results come from the combined effect of active nucleation site density enhancement, capillary wicking promotion, and separation of liquid-vapor flow paths on the microporous surface with porous pillars. On the other hands, the porous pillar surface with needle-like nanostructures showed the interesting phenomena that can be regarded to the recovery of the boiling regime from transition boiling to nucleate boiling. At the CHF, the temperature increase on the surface was slower than that on the other surfaces. Additionally, at high heat fluxes below the CHF, the overheated surface self-recovered to the nucleate boiling state, indicating the rewetting of local dry spots. We consider that the capillary wicking capability strongly improved by the nanostructures on the surface was presumably responsible for inhibiting the irreversible expansion of local dry spots. From the experimental observations throughout this study, we propose the following key requirements to design an ideal boiling surface: increasing the nucleation site density, ensuring a liquid supply path by capillary wicking, separating the liquid and vapor flow paths, and improving the liquid wettability of the solid surface by forming nanostructures.