Boiling Heat Transfer and Countercurrent Gas-Liquid Flow in Narrow Gap with Closed Bottom.

Abstract

For severe accident assessment in a light water reactor (LWR), heat transfer models in a narrow annular gap between the overheated core debris and the reactor pressure vessel (RPV) are important to evaluate the integrity of the RPV and emergency procedures. This paper discusses the boiling heat transfer and the average critical heat flux (CHF) restricted by countercurrent flow limiting (CCFL) based on existing data. At the cooling-down process, the heat flux in a narrow gap is similar to the pool boiling curve but the superheat at corresponding heat fluxes is higher than in pool boiling except in the case of film boiling, which can be expressed by the Bromley's pool film boiling correlation. However, experiments should be required to derive a heat flux correlation in the case of nucleate boiling. We derived an average CHF correlation using the Kutateladze-type CCFL correlation, with its empirical constant Cκ as a function of the dimensionless superheat. Average CHF at the cooling-down process was 3-4 times larger than CHF at the heat-up process and the CHF difference was well predicted by different flow patterns in the gap and the momentum balance between gas and liquid phases.