Experimental validation of positive relationship between the heat fluxes at departure from nucleate boiling and large bubble formation

Avoiding the occurrence of departure from nucleate boiling (DNB) in subcooled flow boiling is of considerable practical importance to ensure the safety of various high power density industrial plants such as pressurized water reactors, since it leads to an abrupt rise in the temperature of the heat transfer surface. However, due to complexity of the phenomenon, accurate prediction of the heat flux causing the DNB is still a challenging issue. Recently, a semi-mechanistic method was developed to estimate the DNB heat flux, and its high predictive performance in wide range of thermal-hydraulic conditions was demonstrated. In this method, the heat flux at the large bubble formation (LBF) is first calculated mechanistically, and then the DNB heat flux is calculated using an empirically derived dimensionless correlation for the ratio of the heat fluxes at the DNB and LBF. In the present work, experiments were carried out to determine the conditions under which the LBF and DNB are initiated in subcooled flow boiling. For this purpose, the vapor bubbles and the wall temperature distribution were observed simultaneously using a high-speed camera and an infrared camera, respectively. It was confirmed that the positive relationship is present between the heat fluxes at the LBF and DNB, as hypothesized in the semimechanistic model for the DNB heat flux. In particular, the DNB was initiated as soon as the LBF condition was satisfied, when the large bubble formation following the global bubble coalescence occurred at high wall heat flux.