Exploring wall heat transfer characteristics in the inverted annular/slug film boiling regimes by direct hot-patch technique+

• Quasi steady-state subcooled film boiling experiments were conducted with water. • Wall heat transfer characteristics of the IAFB and ISFB regimes were investigated. • Void fraction data were acquired to understand heat transfer and flow characteristics. • A parametric study revealed the effects of key parameters on wall heat transfer. • Local subcooling in subcooled regime, and the mass flux and pressure in saturated regime significantly affect wall heat transfer. Inverted annular film boiling (IAFB) and inverted slug film boiling (ISFB) are highly complicated two-phase phenomena exhibiting considerable uncertainties in predicting the wall heat transfer characteristics due to lack of comprehensive experimental data and understanding on the underlying mechanisms resulting from challenging experimental conditions. Therefore, in this study, a series of quasi steady-state IAFB/ISFB experiments has been conducted using a direct hot-patch technique to improve our understanding of the wall heat transfer characteristics in these post-critical heat flux (post-CHF) regimes. Experimental conditions include the inlet subcooling up to 30 °C, mass flux between 150 and 450 kg/m 2 -s, and system pressure varying from 25 to 60 psia (0.172 to 0.412 MPa) to explore quantitative relationships between the wall convective heat transfer coefficient in the IAFB/ISFB regimes and the variables of the liquid subcooling, mass flux, and system pressure. Detailed test section wall temperature measurements are acquired using thermocouples while the void fraction and flow topology are measured via an X-ray radiography system and a gamma densitometry. In addition, analyses are carried out using a computer code for film boiling based on a one-dimensional two-fluid model. The predictive capability of the code is validated using the void fraction data obtained by radiation-attenuation techniques. The physical mechanisms are identified to explain how the key variables affect the wall heat transfer variations in the IAFB/ISFB regimes. In subcooled conditions, the wall convective heat transfer coefficient is solely determined by the local subcooling, as it is governed the interfacial heat transfer and heat conduction through the vapor film. In saturated conditions, the wall heat transfer in both the IAFB and ISFB regimes increases with increasing mass flux and system pressure because convection in the vapor phase and interfacial area concentration, respectively, dominate the heat transfer process. The acquired IAFB/ISFB data including the void fraction and flow topology, and identified physical mechanisms based on the comprehensive data are expected to help improve the predictive capability of post-CHF heat transfer in light water reactor system-level thermal–hydraulic analysis codes.