Experimental and Numerical Investigation of Single Bubble Condensation: Influence of the Shrinkage-Induced Volume Effect

In heat exchange equipment such as Pressurized Water Reactor (PWR) cores and steam generators, the wall heat flux and vapor phase distribution are critical parameters that directly govern the heat transfer, turbulence intensity, and flow instability within the flow channels. Bubble condensation represents pivotal processes that significantly influence these parameters. While the wake structures of single bubbles under adiabatic conditions have been extensively investigated, research regarding the wake characteristics of a single condensing bubble remains scarce. In this study, experiments on single bubble condensation in subcooled pool boiling were conducted. An experimental database concerning the variation of single bubble diameter was established within a subcooling range of 4.5 K to 29.3 K, providing detailed benchmark data for numerical simulations involving vapor-liquid condensation. Numerical simulations of single bubble condensation were performed by coupling the Volume of Fluid-Continuous Surface Force (VOF-CSF) and Lee models. Furthermore, a semi-empirical prediction model for the phase change coefficient (L) was developed based on the thermal diffusivity (α), initial bubble diameter (D0), and Jakob number (Ja), extending the applicability of the Lee model. Moreover, numerical simulations were conducted to investigate single bubble condensation under zero-gravity conditions using the improved Lee model, providing a detailed analysis of the impact of condensation as the primary driving force on flow field characteristics. The ”volume effect” induced by the volumetric shrinkage of the condensing bubble was identified. Furthermore, the concept of a ”Condensation-affected Zone” was proposed, and its length (Lc) was quantitatively defined. Additionally, numerical simulations of single bubble condensation under gravity conditions were performed. Comparisons with the flow field of an adiabatic rising bubble revealed that the volume effect caused by condensation shrinkage expands the range of the bubble wake region, intensifies the vortices, velocity, and velocity gradients within the wake, and attenuates potential flow disturbances. Results show that the condensation volume effect on wake velocity characteristics becomes more pronounced with larger bubble diameters and higher subcooling degrees.