Post-dryout heat transfer in circular tubes using R-134a: experiment and correlation assessment

Post-dryout heat transfer plays an important role in the safety analysis of nuclear reactors or in the design of once through steam generators, and thus attracts great interest in the research community and technical applications. In the present study, experiments on post dryout heat transfer were conducted in a uniformly heated tube test section using Freon R-134a with the following range of parameters; pressure from 1.1 to 2.8 MPa, mass flux from 300 to 2000 kg/m² s, heat flux from 20 to 140 kW/m², and local thermal equilibrium vapor quality of more than 100%. Both procedures with increasing and decreasing heat flux were applied, to examine the hysteresis phenomenon. In general, excellent reproducibility of experiments is proven. The behavior of wall temperature and the effect of various parameters on post-dryout heat transfer can be well explained with mechanistic processes. In total, about 9000 data points were obtained and provide a valuable data base for future development of prediction models. Based on the test data gathered, five widely applied correlations of post-dryout heat transfer were selected and assessed. It was found that both correlations using thermal equilibrium approach have a much better prediction capability than the other three correlations based on thermal non-equilibrium conditions. Further analysis reveals that the main deficiency in the thermal non-equilibrium correlations is in the prediction of actual superheating of vapor, which requires obviously further improvement.