Thermal hydraulic performance analysis of a post-CHF heat transfer test facility

Abstract

Post critical heat flux (post-CHF) heat transfer may occur in loss of coolant accidents (LOCAs) in water-cooled nuclear reactors when makeup water quenches the uncovered fuel rods in the reactor core. The post-CHF regimes are characterized by low heat transfer coefficient and high wall temperature due to the vapor-wall contact. A good understanding of the hydrodynamics and heat transfer process in the post-CHF regimes, especially low-quality film boiling, such as inverted annular film boiling (IAFB) and inverted slug film boiling (ISFB), is imperative for accurately modeling the post-CHF phenomena and predicting the fuel rod cladding temperature and void fraction during accidents. This paper first discusses limitations of existing post-CHF hydrodynamics and heat transfer models/correlations and then identifies needs of post-CHF experimental data under high-flow and high-pressure conditions for model improvement. To narrow down the gap in the available experimental data, a Post-CHF Heat Transfer (PCHT) test facility is designed to perform quasi-steady-state low-quality film boiling experiments under high-pressure (ranging from 0.14 to 3.45 MPa), high-flow (ranging from 150 to 2000 kg/m2-s), and high inlet subcooling (up to 50 °C) conditions. In addition to the experimental study, a numerical model using COMSOL Multiphysics tool is developed to inform the test section design and to investigate wall heat flux and wall temperature distributions in the post-CHF regimes. The COMSOL Multiphysics simulation results show that quasi-steady-state post-CHF phenomena can be achieved in the PCHT test section. Considering thermal non-equilibrium conditions in the post-CHF regimes, a one-dimensional two-fluid model is implemented into a computer code to predict the void fraction using the wall temperature information obtained from the COMSOL simulation results. A correlation for the liquid-side Nusselt number is obtained for the IAFB regime based on previous experimental data and is implemented into the code, which results in an improvement in the agreement between the calculated void fraction and corresponding experimental data. With the new Nusselt number correlation, a parametric study of the effects of the heat flux, pressure, mass flux, and inlet subcooling on the liquid-side Nusselt number is studied. In addition, flow regime transition in the post-CHF regimes is studied by comparing the existing correlation and experimental data. Furthermore, the Weber number and liquid-side Nusselt number are calculated to examine whether there exists any correlation between these two non-dimensional numbers and the flow regime transition. The simulation results provide insights for design improvement of the PCHT test section and assist the development of a test plan for future IAFB and ISFB tests under high-pressure, high-flow, and high-subcooling conditions in the PCHT test facility.