Boiling on enhanced surfaces

Abstract

A review of recent literature on pool and flow boiling with a focus on improving the boiling process using enhanced surfaces is presented here. A discussion on the mechanisms for the reported improvements in boiling performance is also provided to help guide the design of enhanced surfaces. The key surface enhancements reported in the field of pool boiling have been aimed at (a) tuning surface wettability to promote enhanced nucleation of bubbles, (b) modifying surface morphology by structuring at the micro/nanoscale to enhance surface wettability and its wicking ability, and (c) varying surface thermal conductivity to delineate zones of liquid supply to the heated surface and nucleation of bubbles from the surface. The modification of surface wettability has been achieved via two methods: surfactants added to the operating fluid and hydrophobic coatings deposited on the heated surface. Surface micro/nanoscale morphology can be tailored via the use of surface structures including micropillar arrays, nanowire clusters, sintered particle wicks, microchannels, and hierarchical structures (i.e., a combination of various length scale structures to create a tier of structure size scales). In the area of flow or convective boiling, surface enhancements have focused on mitigating flow instabilities that plague microchannel two-phase flow. Specifically, inlet restrictors and artificial nucleation sites have been proposed (and demonstrated) to prevent upstream compressibility and explosive bubble growth instabilities. Microchannel flow boiling performance has also been improved by the incorporation of surface structures on the microscale and nanoscale, the former to promote capillary wicking and delayed dryout and the latter to increase the number nucleation sites in addition to promoting capillary wicking. In the chapter summary, a brief discussion is provided on the areas within these fields where opportunities exist to better understand the behavior in these systems and to create reliable solutions for industrial applications.