CHF and near-wall boiling behaviors in pool boiling of water on a heating surface coated with nanoparticles

Abstract

This study investigated the saturated pool boiling of water on a 12mm diameter horizontal heating surface coated with titanium oxide (TiO2) nanoparticles under atmospheric pressure. The TiO2 coated surface was formed during nucleate boiling of TiO2-water-surfactant nanofluid on the plain heating surface at high heat flux conditions. It was found that the CHF of water boiling on the TiO2 coated surface increased up to about 1.8 times the CHF for the uncoated (plain) surface. To examine the mechanism of the CHF enhancement by TiO2 nanoparticle coating, liquid–vapor behaviors close to the heating surface were measured using a conductance probe with a tip diameter smaller than 5μm. The probe signals and the void fraction distributions showed that there is little difference in the liquid–vapor structure in the boiling on the uncoated surface and on the TiO2 coated surface, that a liquid rich layer (a so called macrolayer) remains on the heating surface, and that in boiling on the TiO2 coated surface it does not dry out even at heat fluxes far higher than the CHF of the uncoated surface. The thickness of the macrolayer formed beneath large vapor masses was determined from the location where the probe signals corresponding to the large vapor masses disappear. It was found that the macrolayers formed on the TiO2 coated surface are thicker than those on the uncoated surface, and it is considered that this is the most likely to be a cause of the CHF enhancement with the TiO2 coated heating surface.