Development of a two-phase flow mass quality correlation in the post-dryout DFFB regime during reflood transients

Abstract

A new semi-empirical two-phase flow mass quality correlation was developed for the post-dryout dispersed flow film boiling (DFFB) regime in a 7 × 7 rod bundle geometry. Starting from the fundamental physics describing the two-phase flow in the dispersed droplet flow regime, one-dimensional conservation governing equations were formulated for the liquid and vapor phases by considering a detailed characterization of the liquid droplet phase, based on which the liquid phase velocity was related to the key flow parameters. A scaling analysis was then performed within a sub-channel of the rod bundle to obtain an expression for the vapor drift velocity, from which a suitable relationship for the two-phase flow quality was derived using the drift flux model. It was found that the vapor drift velocity in the DFFB regime is affected by many variables including: the droplet size, velocities of the liquid and vapor phases, void fraction, vapor superheat and fluid properties, all of which are important in characterizing the dispersed droplet flow. Based on the results of the RBHT reflood tests, which simulate the reflood transients of a LWR during an accident scenario, a new mass quality correlation was developed that is applicable to the conditions with system pressure ranging from 138 to 414 kPa, rod bundle peak power between 0.98 and 1.97 kW/m, inlet liquid subcooling temperature from 11 to 83 K, and inlet flooding rate ranging from 0.0191 to 0.0508 m/s. Comparisons with experimental data indicated that the new correlation is able to predict the actual mass quality in the post-dryout two-phase flow regime with significantly improved accuracy not only for the rod bundle geometry but also for the tube geometry.