A dynamic characteristics on the boiling crisis in downward facing flow boiling, part I: Motion of the CHF front and factors investigation

This study investigated boiling dynamics near the Critical Heat Flux (CHF) threshold, utilizing a downward-facing flow setup on a polished copper surface to examine distinct thermal behaviors in both upstream and downstream regions concurrently.It discovered that in flow boiling scenarios, the downstream region often reaches CHF levels earlier than the upstream region, prompting the introduction of the “CHF front” concept. This concept delineates the transition from nucleate to film boiling, marking an imminent boiling crisis. Measurements revealed CHF front velocities of several millimeters per second, physically representing the expansion rate of irreversible dry spots that lead to film boiling. The research also uncovered an inverse relationship between CHF front velocity and variables such as flow rate and surface roughness. Additionally, the study observed a linear but inverse relationship between CHF and CHF front velocity, leading to the development of a balance model within the microlayer region. These findings indicate that CHF enhancement factors, particularly increased microlayer thickness, help to delay the formation and expansion of irreversible dry spots, thus reducing CHF front velocity. This study provides essential insights for future CHF front velocity modeling and underscores the importance of understanding CHF front motion in boiling heat transfer analysis. [Display omitted] •Introduction to CHF front motion at the boiling crisis signifying the transition from nucleate to film boiling.•Definition and quantification of CHF front velocity as a metric to elucidate the boiling dynamics observed on heated surfaces.•Examination of the CHF front velocity across various surface roughness levels and flow rates to assess how these factors influence the magnitude of this velocity term.•The motion of the CHF front is physically linked to the expansion behavior of irreversible dry spots, for which a balance model in the microlayer region has been proposed.