Mitigating diffusion limited heat transfer effects through controlled bubble departure during buoyancy-opposed nucleate pool boiling: Whole field measurements

Downward-facing pool boiling with buoyancy-opposed conditions holds importance in nuclear safety applications such as external reactor vessel cooling (ERVC). Despite extensive research, existing studies have primarily focused on film boiling, elongated vapor slug dynamics, and nucleate boiling involving multiple interacting bubbles, largely relying on high-speed visualization and providing limited insight into the associated thermal field. In the present work, a comprehensive experimental investigation of single bubble dynamics and coupled thermal transport on a downward facing heated surface is carried out using Rainbow Schlieren Deflectometry (RSD). This optical technique enables simultaneous, non-intrusive and time-resolved measurements of vapor bubble dynamics and two-dimensional temperature gradients near the heater surface and around the vapor bubble. Experiments were performed under subcooled pool conditions (ΔTsub = 3 K) at heat fluxes of 10 and 20 kW/m². Time-resolved measurements of bubble equivalent diameter, base diameter and aspect ratio are obtained to characterize bubble growth and deformation under buoyancy-opposed conditions. The reconstructed temperature and its gradient fields reveal pronounced thermal boundary layer development near the heater surface, with maximum near wall temperature gradients increasing from 15,000 °C/m at 10 kW/m² to 20,000 °C/m at 20 kW/m². Heat partitioning analysis indicates that both evaporative and condensation heat transfer components increase with heat flux, with the evaporative contribution showing an enhancement of nearly 150% at higher heat flux. Furthermore, a noticeable enhancement in the local heat transfer coefficient is observed after bubble departure compared to before bubble nucleation. The heat transfer coefficient attains higher values after the bubble leaves the heater surface due to turbulence, with average increases of approximately 9% and 13% at heat fluxes of 10 and 20 kW/m², respectively.