High Temperature Microporous Coatings: The Effects Of Wetting And Wicking On Nucleate Boiling And Chf

Abstract

Porous coatings have long been a method to enhance boiling heat transfer. Previous researchers have thoroughly explained most of the physics behind the enhancement, but some holes still exist. Correlations, setup to predict nucleate boiling, are often only accurate in specific scenarios. CHF correlations that take into account wettability do not consider microstructure changes that decrease contact angle, but do not enhance CHF. A solution to these problems is presented via the wicking and wetting of liquid on a porous surface. CHF is found to be enhanced with increases in wicking speeds and flow rate. Wetting, measured by apparent contact angle, is found to coincide with changes in nucleate boiling. Contact angle measurements may be able to indicate the level of hydraulic resistance where decreases in apparent contact angle led to decreases in nucleate boiling performance. Nucleate boiling predictions made by wetting measurements were not found to always correctly predict enhancement or degradation. The degree of wetting or wicking is manipulated via corrosion, oxidation, material change, and particle size. It is believed that the CHF of porous materials is determined by either the wicking limit (capillary limit) or the macrolayer dryout limit. The exact mechanism was found to depend on the coating thickness and wicking flow rate.