Experimental assessment of liquid-wall contacts in post-CHF convective boiling

Abstract

Convective boiling in the post critical-heat-flux regime is of interest for analysis of LOCA scenarios in nuclear reactor safety. Most models for post-CHF heat transfer concentrate on cooling of the superheated channel walls by vapor convection. The possibility of additional cooling by direct liquid droplet impingement has been a matter of debate. The present work is an attempt to experimentally assess whether contacts between entrained liquid droplets and the superheated walls occur in convective-post-CHF flows. A micro-thermocouple probe, capable of detecting liquid-induced cooling by tracking transient temperatures approximately 0.016 mm beneath the surface, was used in reflood experiments with vertical upflow of water in a superheated tube. Liquid contacts were detected at wall superheat temperatures up to 380°C.