Experimental analyses of flow boiling heat transfer on downward-facing surface at low inclination: Critical heat flux and vapor bubble dynamics

Numerous studies on critical heat flux (CHF) on a downward-facing heating surface in a low inclination channel have been reported, yet a profound understanding of CHF is still lacking under these conditions, which reflect the optimized design of thermal systems with a large surface that manage a large amount of heat. Flow boiling experiments were conducted in this work to evaluate boiling heat transfer and CHF with a downward-facing heating surface and very low inclination (10 degrees) flow channel. The experimental conditions in this study were determined from the configuration of the core catcher as a representative system utilizing the flow boiling phenomenon on a 10 degrees inclined large heating surface. Mass flux and subcooling effects on boiling heat transfer and CHF were evaluated, and analyses were performed on the heater and fluid temperature measurement results. Furthermore, an optical probe with two fiber sensor tips and a high-speed camera were used to estimate the vapor bubble characteristics. Results indicate that CHF increases with increasing mass flux and subcooling, while other distinct phenomena were observed including an overheated fluid region around the heater surface, condensation-induced water hammer events under 15 K subcooling, and converged local void fraction near the CHF.