A short review of dispersed flow heat transfer in post-dryout boiling

Abstract

Convective boiling beyond critical-heat-flux (CHF) is encountered in a number of applications including steam generators, nuclear reactors, cryogenic systems, and metallurgical processing. At high void fractions (moderate to high vapor qualities) post-CHF heat transfer occurs with dispersed flow where the liquid phase is distributed as droplets entrained in the continuous vapor phase. It appears today that there is strong theoretical and experimental cause to believe that thermodynamic nonequilibrium is highly likely in dispersed flow, convective post-CHF boiling heat transfer. The experimental data base on the degree of nonequilibrium is very sparse and needs to be greatly expanded as the technical community seek to quantify this important phenomenon. Recently, experiments at Lehigh University and Idaho National Engineering Laboratory obtained measurements of vapor nonequilibrium superheat at various distances downstream from the dryout CHF point. This group of three experimental studies provide the most detailed measurements of nonequilibrium, dispersed flow, post-dryout boiling heat transfer available to date. In addition to the normal measurements of wall heat flux, wall temperature, system pressure, flow rate, and equilibrium quality, obtained in earlier experiments, these new experiments added the critical additional measurement of nonequilibrium bulk vapor temperature. With the availability of these new experimental results, it is now possible and in fact necessary to assess all nonequilibrium models not just for heat flux and wall temperature, but also for the vapor temperature. In fact, all proposed models and correlations should be required to satisfactorily predicts both the vapor temperature and the wall heat flux (or wall temperature) simultaneously and correctly.