Nuclear Engineed.ng aml Nuclear Engineering and Design 146 (1994) 15-24 ELSEVIER Flow visualization study of post,critical heat flux in inverted flow I. Babelli, S.T. Revankar, M. Ishii Thermal-hydraulics and Reactor Safety Laboratory, School of Nuclear Engineering, Purdue University, West Lafayette, IN 47907, USA Abstract A visual study of film boiling was carded out to determine the flow regime transition in the post-CHF region for a transient bottom reflooding of a hot transparent test section. The effect of test liquid subcooling and inlet velocity on flow transition as well as on the quench front propagation was investigated. The respective ranges for liquid velocity and subcooling were 1.8-26.8 era/s, and 20-45°C, respectively. The test liquid was Freon 113 which was introduced into the bottom of the quartz test section whose walls were maintained well above the film boiling temperature of the test liquid, via a transparent heat transfer fluid. The flow regimes observed down stream of the upward moving quench front were the rough wavy, the agitated, and the dispersed droplet/ligaments in agreement with a steady state, two-phase core injection study carried on recently by one of the authors. A correlation for the flow regime transition between the inverted annular and the dispersed droplet/ligament flow patterns was developed. The correlation showed a marked dependence on the void fraction at the CHF location and hence on the flow regime encountered in the pre-CHF region. 1. Introduction Inverted annular flow, which consists of a liq- uid core surrounded by a vapor annulus, as shown in Fig. 1, is of considerable importance in the areas of confined, low quality film boiling appli- cations such as LWR accident analysis. While many analytical and experimental heat transfer studies for this flow situation have been carried out, the relevant hydrodynamics of the post-CHF flow field received less attention. A more thor- ough understanding of the hydrodynamics of the post-CHF flow is necessary in order to ade- quately assess post-CHF heat transfer due to the coupled thermo-hydraulic nature of confined flow film boiling phenomena. Consequently, the deter- mination and characterization of the various two-phase flow regimes both before and after CHF become important in determining heat 0029-5493/94/$07.00 © 1994 Elsevier Science B.V. All rights SSDI 0029-5493(93)E0216-7 transfer (as well and mass and momentum trans- fer). 2. Review of experimental studies of post-CHF Most of the research on the post-CHF region dealt mainly with heat transfer. Excellent reviews of the widespread literature were provided by Jordan [1] and Clemments and Clover [2]. A subsequent review by Kalinin et al. [3] considered both free and forced convective film boiling in great detail. Complete tabulations as well as ex- tensive graphical comparisons of the various pre- dictive equations for heat transfer were given. However, because heat transfer is significantly influenced by the regime of film boiling, some of the disagreement among the various correlations are not so readily explained due to insufficient reserved