Hydrodynamic saturation of vapor transport as a precursor to critical heat flux in pool boiling

This study investigates the precursor behavior of Critical Heat Flux (CHF) through quantitative analysis of mushroom bubble dynamics in pool boiling using high-speed visualization and image-based measurements. A characteristic interfacial transport velocity, U M = f M × D 32 , is introduced by combining a representative temporal scale (mushroom bubble departure frequency, f M ) and a spatial scale (Sauter mean diameter, D 32 ). The results show that, while D 32 increases monotonically with heat flux due to enhanced coalescence, f M exhibits saturation at elevated heat flux conditions. Consequently, U M transitions from a growth-dominated regime to a transport-limited regime associated with increasing liquid replenishment constraints. The transition is identified using a piecewise regression framework and is shown to be statistically robust, occurring at approximately 72 % of the measured CHF. Furthermore, the precursor behavior remains clearly identifiable under dimensionless scaling, providing a normalized representation of the observed transport saturation. These findings suggest that the approach to the boiling crisis is preceded by the emergence of a transport-limited interfacial regime. The saturation of the characteristic interfacial transport velocity therefore provides a physically interpretable indicator of CHF precursor behavior, connecting experimentally measurable mushroom bubble dynamics with the hydrodynamic transport processes underlying CHF.