Effect of gravity on pool boiling heat transfer on thermal spray coating

Abstract

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 effect of gravity on boiling heat transfer coeffcient and wall superheat at the onset of boiling were experimentally evaluated under micro- and hyper-gravity condition during a parabolic trajectory flight of an airplane. Pool boiling experiments in saturated liquid of HCFC123 were carried out for heat fluxes between 1.0 and 160 kW/m2 and saturated temperature of 30 °C. As a result, the surface by VPS produced higher heat transfer coefficient and lower superheat at the onset of boiling under microgravity. For the smooth surface, the effect of gravity on boiling heat transfer coefficient was a little. For the coating, a large difference in heat transfer coefficient to gravity was observed in the moderate heat flux range. The heat transfer coefficinet decreased as gravity changed from the normal to hypergravity, and was improved as gravity changed from the hyperto microgravity. The difference in heat transfer coefficient between the normal and microgravity was a little. Heat transfer enhancement factor was kept over the experimental range of heat flux. It can be said that boiling behavior on thermal spray coating might be influenced by flow convection velocity.