Subcooled forced convection boiling on a vertical surface and gas-liquid structure near the heat transfer surface

This study examines the mechanism of Departure from Nucleate Boiling (DNB)-type Critical Heat Flux (CHF) during subcooled forced convection boiling on a vertical heated surface. For high-velocity experiments (1.5 m/s), the distance between the 90-degree elbow and the rectangular nozzle outlet was set to 115 mm to ensure fully developed flow; for low-velocity experiments (0.5 m/s), the distance was 60 mm. A custom apparatus enabled visualization of near-wall two-phase behavior using a high-resolution micro-scale conductance probe positioned perpendicular to the surface. Vapor plug transit time and liquid film thickness were measured across various subcooling and flow velocity conditions. Results indicate that CHF increases with both subcooling and flow velocity. Void fraction profiles revealed a liquid-rich region between the vapor plug and the heating surface, supporting the liquid-film evaporation model. Vapor plug transit time decreased with higher subcooling and velocity, while liquid film thickness increased. Empirical correlations between these parameters and heat flux were established using least-squares fitting and incorporated into a mechanistic CHF prediction model. Predicted CHF values matched experimental data within a 10% margin. These findings show that DNB-type CHF can be accurately predicted using a liquid film evaporation model that incorporates near-wall vapor-liquid dynamics. Future research should validate this approach under high-pressure conditions relevant to pressurized water reactors (PWRs). Unlike macroscopic CHF models such as that of Lee and Mudawar, this method emphasizes near-wall measurements and vapor-liquid interactions.