Vapor mass formation and characteristic liquid-film thickness in pool boiling on a vertical surface relevant to macrolayer-based CHF mechanisms

Accurate prediction of critical heat flux (CHF) is essential for the design of high-heat-flux thermal systems utilizing boiling heat transfer. Macrolayer-based CHF models propose that CHF occurs when the liquid macrolayer trapped beneath a vapor mass completely evaporates. However, the conditions for vapor mass formation and quantitative information on macrolayer thickness remain insufficiently understood, mainly due to the lack of direct measurements. In this study, saturated pool boiling experiments were conducted on a vertical heating surface at atmospheric pressure using water. High-speed visualization was employed to clarify the conditions leading to vapor mass formation, and a characteristic liquid-film thickness scale detectable under conditions where vapor mass formation is established was nonintrusively measured using a spectral-interference laser displacement meter. The results show that vapor mass formation is promoted by bubble coalescence under conditions where the apparent bubble-covered area ratio approaches or exceeds unity, indicating unavoidable bubble overlap. The most frequent detected liquid-film thickness was approximately 45–70 μm and remained nearly independent of heat flux after vapor-mass formation was established. This characteristic thickness scale is consistent with macrolayer thicknesses estimated at the critical heat flux using existing correlations. These findings provide experimental insight into representative liquid-layer thickness scales relevant to macrolayer-based CHF mechanisms.