CHF mechanism in flow boiling from a short heated wall—II. Theoretical CHF model

Abstract

A theoretical CHF model is presented which is based upon the flow visualization of the wavy liquid-vapor interface near the heated surface as reported in Part I of this study. At approximately 90% of CHF, bubbles coalesced into a continuous wavy vapor layer and vigorous boiling was observed in a liquid sub-film beneath the vapor layer. This efficient boiling mechanism was transformed to boiling at isolated regions, wetting fronts, at approximately 95% of CHF. Wetting fronts were established when minimum points in the wavy vapor interface made contact with the heater surface. Regions surrounding the wetting fronts remained dry as the supply of liquid was consumed. CHF was triggered by intense vapor production which lifted the upstream wetting front away from the heater surface, cutting the supply of liquid locally and causing the heat flux to become more concentrated at the remaining wetting fronts. Soon after, remaining wetting fronts were also lifted from the heater surface and the surface temperature increased more rapidly. The new mechanistic CHF model incorporates classical interfacial instability theory for a confined two-dimensional wave, a separated two-phase flow model and a criterion for separation of the liquid-vapor interface from the heater surface. The model predictions show good agreement with the experimental data.