A dynamic characteristic on the boiling crisis in downward facing flow boiling, Part II: Model development and validation

This paper advances our understanding of boiling dynamics at the Critical Heat Flux (CHF) level within a downward flow boiling scenario. Building on initial visual observations, this study clarifies the mechanisms driving CHF front motion and introduces a model to track its progression. It reveals that the CHF front advances when the evaporation rate in the microlayer surpasses the liquid inflow. This insight has facilitated further modeling of the relationship between the CHF threshold and CHF front velocity using a balance model in the microlayer. We observe an inverse yet linear relationship where the ratios $R_m/{\delta }_0$ $-$ the microlayer radius to its thickness $-$ and ${\bar{U}}_X$ $-$ the mean liquid inflow rate $-$ show negligible variation despite changes in surface roughness and flow rates. Additionally, we have refined our CHF prediction model to incorporate effects of surface roughness and flow rate, linking it with the CHF front velocity model. This model, which balances three velocity terms at the CHF front, demonstrates substantial predictive accuracy and suggests that enhancements to the CHF threshold improve rewetting flow, thereby delaying surface dry-out, reducing CHF front velocity and enhancing CHF performance. These findings provide crucial insights for advancing thermal management technologies.