Axially Varying Vapor Superheats in Convective Film Boiling

Abstract

Axially varying vapor superheats in convective film boiling have been measured for water flowing in a vertical tube at low to moderate pressures and mass flow rates. Using a slow “reflood” process, measurements of wall temperature and nonequilibrium vapor temperature were obtained as functions of distance from the quench front. With the low quench front velocity, the time required to progapate the front a few millimeters corresponds to many fluid residence times, and the thermal hydraulic data thus obtained are quasi-steady state. These experimental results indicate a zone near the quench front where the vapor generation rate is relatively high, followed by a far zone where the generation rate drops off to a relatively low magnitude. The data obtained agree with the very limited previously reported steady-state data. Comparison with existing heat transfer models shows the models give poor predictions of vapor superheats but reasonable predictions of wall heat fluxes.