Analyzing Flow Rate Impact on CHF Front Behavior During Boiling Crisis in Downward Flow Boiling

Abstract Our research thoroughly examines the Critical Heat Flux (CHF) in downward-facing flow boiling, essential for comprehending the intricacy of the CHF phenomenon. Conducted on a downward-facing copper surface within a flow loop, our research uncovered significant differences in CHF between upstream and downstream regions, with CHF predominantly occurring downstream first. High-speed camera analysis during boiling crisis led to the identification of the ‘CHF front,’ marking the transition from nucleate to film boiling phases. This CHF front typically originates downstream and progresses upstream, significantly influencing final CHF thresholds. This movement, linked to dry spot formation on the heated surface as per dry-out theories, is a key focus of our analysis. Furthermore, our experiments on smooth copper surfaces under varied flow rates revealed that increased flow rates decrease the CHF front velocity, delaying CHF onset and resulting in higher final CHF values. This observed effect is attributed to the enhanced supply of liquid to the micro-layer beneath the bubble, which increases its thickness and thereby prevents the formation of dry spots. While our study offers deep insights, we recognize uncertainties in our data. Future research will focus on various factors for a more comprehensive understanding of CHF front velocity. These findings are crucial for predicting CHF levels in various systems, marking a significant contribution to the field.