Convective film boiling in a rod bundle: axial variation of nonequilibrium evaporation rates

Abstract

An experimental study of convective film boiling (post-CHF) is carried out for two-phase steamwater flow in a nine-rod bundle test section. Measurements of wall heat flux, wall superheat, and vapor superheat are obtained as a function of axial distance from the critical heat flux (quench front) for a range of flow rates and flow qualities. These data permit the evaluation of the effective evaporation ratio (fraction of total heat input causing net vapor generation) as a function of test conditions and axial distance from the quench front. The results indicate the existence of a ‘near region’ with significant evaporation, followed by a far region where the effective evaporation ratio decreases to less than 10% of the total heat input. While the wall and vapor superheats measured in these rod bundle experiments differ in magnitude from those obtained in earlier single tube experiments, qualitative agreement in the axial behavior of the post-CHF process is observed. These data are presented in the hope of aiding attempts to develop improved mechanistic models for convective film boiling heat transfer.