Thermodynamic nonequilibrium in dispersed two-phase boiling

Convective boiling beyond critical-heat-flux (CHF) (post-dryout) is encountered in a number of applications, including steam generators, nuclear reactors, cryogenic systems, and metallurgical processing. Post-CHF boiling is traditionally divided into the regimes of flow film boiling and transition boiling. Both regimes are further categorized according to the distribution of the vapor and liquid phases. In this paper, a framework is presented to examine phenomenological questions pertinent to our understanding of the transport mechanisms in convective post-dryout boiling heat transfer. In applications, the need is to predict the thermal-hydraulic conditions downstream from the dryout (CHF) point as a function of position and time. In the most general sense, this may be regarded as a transient, entrance-region problem with axial coordinate (Z) measured from the CHF location. Historically, various approaches have been taken to address this problem, from the very simple approximations to the highly complex numerical models. Various experimental investigations have also been pursued to delineate the physical process and to obtain quantitative data to guide the development of predictive models. As a result of extensive experimental and theoretical studies, thermodynamic nonequilibrium, with potential for significant vapor superheating, must be taken into account. Simple equilibrium models are insufficient except for very special situations. On the other hand, the experimental data base for nonequilibrium needs to be significantly extended in order to test the various proposed models and to aid in development of improved models