An improved correlation of wall convective heat transfer coefficient for inverted annular film boiling regime

Post-Critical Heat Flux (Post-CHF) is a highly complex two-phase phenomenon that greatly influences the cooling of nuclear fuel during loss of coolant accidents (LOCAs) in light water reactors. Extensive studies examined wall heat transfer characteristics in inverted annular film boiling (IAFB) and dispersed flow film boiling (DFFB) regimes to develop heat transfer correlations and models aimed at predicting the peak cladding temperature (PCT) of the fuel. However, the lack of comprehensive data to support the physical assumptions underlying the modeling of heat transfer in the IAFB regime has led to a limited predictive accuracy of the existing correlations and models, which are fraught with considerable uncertainties. This study assesses the predictive performance of several existing correlations and models for the IAFB regime using quasi-steady-state film boiling experimental data obtained from the Post-CHF Heat Transfer (PCHT) test facility at the University of Michigan to evaluate their applicability. By analyzing the wall convective heat transfer data, validating vapor film thickness results through void fraction measurements taken with radiation attenuation-based instruments, and considering the principles of laminar film theory, a new correlation is introduced and benchmarked in this research. In the benchmark analysis utilizing existing IAFB test data from the literature, the newly proposed correlation demonstrates enhanced predictive capability for IAFB wall heat transfer compared to existing correlations and models across a broad spectrum of flow conditions, including system pressure up to 8.0 MPa, inlet subcooling up to 50 °C, and coolant mass flux up to 929 kg/m 2 -s. • Limitations of existing IAFB wall heat transfer models are analyzed using PCHT IAFB data. • A new IAFB wall heat transfer correlation is developed based on the laminar film theory. • The constants in the correlation are determined by the acquired HTC and void fraction data. • The newly proposed correlation agrees with experimental data within a ±20 % band. • The proposed correlation can be applied to a wide range of IAFB flow conditions.