Effects of subcooling on downward facing boiling heat transfer with micro-porous coating formed by Cold Spray technique

•Formation of micro-porous coatings by an innovative Cold Spray technique.•Performed transient quenching experiments under simulated IVR-ERVC conditions.•Studied boiling phenomena by visual observation and quantitative measurements.•Enhanced local CHF limits significantly by coatings formed by Cold Spray.•Proposed CHF correlations for both bare and micro-porous coated vessels. External reactor vessel cooling (ERVC) is an effective strategy to achieve in-vessel retention (IVR) of core melt in the reactor pressure vessel (RPV) under severe accident conditions. Among the available strategies, micro-porous coating technique has been known to enhance the thermal margin of the RPV. In this study, a new and versatile micro-porous coating technique applicable to commercial size reactors known as “Cold Spray” has been developed to coat a hemispherical test vessel. Quenching boiling experiments at different degrees of subcooling (10°C, 5°C, 3°C, 1°C, and 0°C) were performed using bare and micro-porous coated vessels. Visual observations of the quenching process along with quantitative analyses of the boiling data were performed. It was found that the critical heat flux (CHF) limit varies significantly with the angular location at all subcooled conditions. Higher cooling rates and CHF limits were obtained with higher degrees of subcooling. A micro-porous coating formed by Cold Spray significantly improved the CHF limit compared to the bare vessel. In fact, nearly 90% enhancement was achieved using the Cold Spray coated vessel. CHF correlations for both bare and micro-porous coated vessel have been proposed capturing the effects of subcooling and angular variation along the outer surface of the hemispherical test vessels.