Subcooling effect on boiling heat transfer of inclined downward-facing surface under low flow and pressure

The present study investigated the physical processes responsible for the variation in the boiling curve and critical heat flux (CHF) caused by liquid subcooling under atmospheric pressure in a rectangular flow channel; the flow channel was oriented 10° upward from the horizon. Bubble dynamics were examined using a high-speed camera and optical fiber microprobes. A solid copper block was utilized as a test heater and mounted above the flow channel to simulate the passive cooling system of an ex-vessel core catcher designed for nuclear power plants. Low mass flux and subcooling conditions ranging from 40-300 kg/m2 s and 5-25 K, respectively, were applied at the inlet of the test section. At the mass flux value of 40 kg/m2 s, large sliding bubbles were attributed to a key criterion for enhanced boiling heat transfer when the liquid subcooling was varied up to 15 K. The results showed that the CHF was weakly dependent on the degree of liquid subcooling, which deviates from the general trend reported by many researchers. A repetitive flow reversal along with a pressure shock appeared, owing to the rapid condensation at the exit, which added complexity to the analysis of the CHF. This study provides physical insights for understanding the subcooled flow boiling heat transfer mechanism (including the CHF) based on sophisticated experimental measurements, such as the visual capture of boiling dynamics using high-speed video and local void fraction.