Development of electric probes to detect phase change at a heated surface.

Abstract

Two types of electric resistance probe have been developed to detect the rewetting of a heated surface, during quenching experiments. Measurements of electric resistance were made using two different methods: (i) Two-Electrode measurement, through a thermocouple probe; (ii) Probe-to Wall measurement, through a zirconium-platinum probe. Forced vertical flow experiments were conducted for water at atmospheric pressure, mass fluxes at 25,000 and 50,000 lbm/hr-ft2 and inlet subcooling ranging from 0 to 70°F. These experiments were preceded by a pool boiling experiment to relate visually the probe output signals to different boiling regimes. The zirconium-platinum probe showed advantages over the thermocouple probe and could detect the onset of rewetting at 440--480°F. the maximum heat flux point was also detected by both types of probe and was found to be in good agreement with those available from boiling curves. Several boiling phenomena recorded by the probes, are discussed. Finally, recommendations are made for alternative methods of detecting the transition from film boiling to nucleate boiling during a quenching process.