AN EXPERIMENTAL STUDY OF THERMAL NON EQUILIBRIUM CONVECTIVE BOILING IN POST-CRITICAL-HEAT-FLUX REGION IN ROD BUNDLES.

摘要

Prediction of heat transfer beyond critical-heat-flux (CHF) is an important aspect for boiling systems, particularly for nuclear reactors. Significant differences occur when these correlations are used in advanced computer programs to estimate the thermal-hydraulic conditions in a rod bundle assembly. In order to develop more accurate correlations, there is a need for a full size test set up and measurements of nonequilibrium vapor temperatures, wall temperatures and wall heat fluxes.; In this study, an experimental investigation of convective heat transfer beyond CHF has been performed with a full size test set up. Post-CHF experiments with simultaneous measurements of nonequilibrium vapor temperature at two axial locations has been successfully performed for a 3 x 3 rod bundle array, under stabilized post-CHF conditions. In addition, some transient (slowly moving CHF) experiments have also been conducted. Total of 374 data points, 98 steady-state with fixed CHF point and 276 transient with moving CHF point, have been obtained. The range of the data included conditions pertinent to reflood and quench phase of light water reactors.; Very high vapor and wall superheats were measured. Typical vapor superheat encountered in this research was 85 percent of corresponding wall superheat, and was in the range of 400 to 600(DEGREES)C. The experimental results indicated the existence of two different regions downstream from the CHF point, namely near region and far region. Modeling of heat transfer process in convective film boiling must account for these two regions of behavior. Significant variations in radial vapor temperature, up to 150(DEGREES)C, was found. It showed that use of one dimensional models for prediction of thermal-hydraulic conditions in a rod bundle is not proper. Available models, developed from single tube data base, showed no reasonable prediction of heat transfer for the new rod bundle data.