Transient condensation dynamics and interfacial heat transfer of free-rising steam bubbles in subcooled water under rapid pressurization

Existing interfacial heat transfer correlations for steam bubble condensation are predominantly derived from subcooled boiling or steam injection configurations, where wall effects and jet-induced momentum disturbances hinder accurate representation of freely rising bubble behavior. This study employs a rapid pressurization method to investigate free-rising steam bubble condensation in subcooled water while substantially reducing the influences of wall heating and jet-induced disturbances on the observed condensation process. High-speed visualization experiments were conducted at system pressures of 0.10–1.14 MPa and liquid subcooling of 3–73 K. Bubble morphology evolution under varying initial size, subcooling, and system pressure is systematically characterized; elevated system pressure is found to intensify early-stage interfacial deformation and induce complex non-spherical shapes, while increased subcooling significantly shortens both condensation lifetime and rise distance. The effects of initial bubble size, subcooling, and system pressure on condensation lifetime and rate are further analyzed. Unsteady bubble rising velocity during condensation is observed, and a transient correction factor ( f tr = C D / C D,SS ) and a dimensionless transient number are introduced to quantify transient inertial effects arising from bubble acceleration and condensation-induced volume change. Based on energy conservation incorporating the temporal variation of vapor density under rapid pressurization, a new interfacial heat transfer correlation in terms of Nusselt, Reynolds, and Jakob numbers is proposed, with prediction errors within ±20%. Comparisons with existing correlations further demonstrate the strong dependence of interfacial condensation heat transfer on bubble-generation mode and hydrodynamic boundary conditions. The present results provide fundamental experimental data and modeling information for the development and validation of steam-bubble condensation models.