Measurement of the Superheated Steam Temperature in a Post-CHF Dispersed Droplet Flow Regime

Heat transfer in the post-dryout dispersed flow regime has often been modeled with the assumption of thermodynamic equilibrium between liquid and vapor phases. In such equilibrium models, the heat energy transferred from the wall is assumed to be totally absorbed by the evaporation of the liquid phase. The vapor temperature would thus be equal to the local saturation temperature, so long as the equilibrium quality of the two-phase mixture does not exceed unity. Other more phenomenological models have attempted to consider the possible existence of thermodynamic non-equilibrium, where superheated vapor would coexist with entrained liquid droplets. Some of the thermodynamic non-equilibrium models and correlations are history-dependent in the sense that knowledge of the location and conditions of the fluid at the dryout point is required. Other groups of nonequilibrium correlations for dispersed flow boiling attempts to use only local parameters to predict wall heat transfer. All the analytical models and correlations, whether of the history-dependent or local parameter class, have emphasized the need for determining the nonequilibrium vapor temperature as a first step in calculating post-dryout heat transfer. Challenges to verify or improve the correlations and models, however, have been greatly hindered by a lack of experimental data on the existence and magnitude of thermodynamic non-equilibrium. The main topic of this study is how to measure the superheated steam temperature in a post- CHF dispersed droplet flow. As a first step of this attempt, two different kind of superheated steam probes were devised. One is lake-type and the other is enveloped-type, described in the following section