A Study on Nucleate Boiling Heat Transfer Enhancement by Thermal Spray Coating

Abstract

In order to enhance nucleate boiling heat transfer it is efficient to increase nucleation site density. This study deals with heat transfer enhancement surface manufactured by thermal spraying. Two thermal spraying methods using copper as a coating material, wire flame spraying (WFS) and vacuum plasma spraying (VPS), were applied to the outside of copper cylinder with 20 mm OD. The surface structure by WFS was denser than that by VPS. The heat transfer performance around the horizontal tubes under microgravity was evaluated in pool boiling experiments using HCFC123 for heat fluxes between 1.0 and 160 kW/m2 and saturation temperature of 30 °C. The microgravity experiments were carried out during a parabolic flight of an airplane. As a result, the surface by VPS produced higher heat transfer coefficient and lower superheat at boiling incipiency under microgravity. For the smooth surface, the effect of gravity on boiling heat transfer coefficient was small. For the coated surface, a large difference in heat transfer characteristics by gravity was observed in the moderate heat flux range. The heat transfer was deteriorated by the change from the normal gravity to hyper-gravity, while it was improved by the transition from the hyper-gravity to microgravity. The difference in heat transfer coefficient was a little between the normal gravity and microgravity. The effect of the coatings on the CHF was not observed.