Effect of gap size and inclination angle on Critical Heat Flux (CHF) of pool boiling based on bubble behavior observations

Abstract

In this study, the effect of gap size and inclination angles on the Critical Heat Flux (CHF) on bare copper surfaces is investigated in the context of vertical-facing saturated pool boiling. The research endeavor aims to unravel the intricate relationships between these variables and their collective impact on CHF. Through hypothesis formulation and analysis of the collected experimental data, it has been determined that the effects of the two variables, inclination angle and gap size, on CHF are approximately relatively independent. Based on previous research, experimental data has validated the conclusions that CHF increases with both increasing inclination angle and gap size. Furthermore, an empirical formula has been developed that closely aligns with experimental data, incorporating both contemporary experimental findings and established theoretical models. Comparison with previous data and models has shown that it fits well with the derived formula. The author argues that both variables—inclination angle and gap size—have a significant impact on CHF by modulating the Bubble Film Departure Frequency (BFDF) and the average width of bubbles formed during the boiling process. However, these two variables do not directly affect CHF, instead, they influence CHF by affecting bubble behavior. Bubble behavior is the direct factor in the occurrence of CHF, while gap size and inclination angle are indirect factors. Through analyzing bubble behavior, we can gain a better understanding of how gap size and inclination angle influence the average bubble width and BDFM, as well as how the average bubble width and BDFM affect CHF. This research offers reference for IVR strategy implementation: the In-vessel Corium Retention (IVR) strategy, crucial for mitigating core meltdown accidents, ensures the reactor pressure vessel (RPV) lower head remains intact during severe incidents. The Critical Heat Flux (CHF) on the RPV lower head surface is the main barrier to IVR. Consequently, CHF research is essential for IVR application. While there's extensive research on heating surface inclination and gap size effects on CHF, less focuses on bubble behavior. Instead, they influence CHF by affecting bubble behavior. Bubble behavior is the direct factor in the occurrence of CHF, while gap size and inclination angle are indirect factors. Through analyzing bubble behavior, we can gain a better understanding of how gap size and inclination angle influence the average bubble width and BDFM, as well as how the average bubble width and BDFM affect CHF.