EXPERIMENTAL INVESTIGATION OF MINIMUM FILM BOILING TEMPERATURE DURING REFLOOD TRANSIENTS

Knowledge of the post-critical heat flux (post-CHF) heat transfer and accurate prediction of the boiling characteristics are important during nuclear safety assessments. The minimum film boiling temperature (Tmin) separates the film boiling regime and the transition boiling regime and its value is used to determine the boiling regime in thermal-hydraulics codes. In this study, the Tmin was investigated during constant reflood experiments under different flow conditions. Reflood experiments were performed at the NRC-PSU Rod Bundle Heat Transfer (RBHT) facility. The 7×7 bundle is electrically heated, and Inconel was used as the cladding material. Inverse heat conduction was used to determine the minimum film boiling temperature (Tmin) at different elevations in the bundle for a broad range of liquid subcooling and reflood rates. The liquid subcooling varied from 10 to 79 K (18 to 144 °F), whereas the flow rate ranged between 0.0254 m/s to 0.05 m/s (1 to 2 in/s). All experiments were conducted at 275 kPa (40 psi) pressure. The results show that under the same inlet conditions, Tmin varied with elevation. The flooding rate had a significant effect on Tmin, whereas liquid subcooling had a minor effect. At lower elevations of the rod bundle, Tmin tends to increase with increasing flooding rate and liquid subcooling. However, the effect of subcooling on Tmin at a higher elevation was found to be insignificant since the liquid would reach saturation temperature regardless of the inlet subcooling.