Mathematical Model Predicting the Critical Heat Flux of Nuclear Reactors

Boiling heat transfer system keeps a nuclear power plant safe without getting over-heated. Crisis will occur if the dissipated heat flux exceeds the critical heat flux value. This study assumes the flow boiling system at high heat flux is characterized by the existence of a very thin liquid layer, known as the "sublayer", which is trapped between the heated surface and the vapor blankets. In the present study, it is hypothesized that the heat transfer through the liquid sublayer is dominated by the heat conduction and the sublayer is dried out due to occurrence of Helmholtz instability as the relative velocity of the vapor blanket to the local liquid in the sublayer reaches a critical value. By recognizing this hypothesis, a theoretical model for low-quality flow is developed to predict boiling heat transfer and Critical Heat Flux (CHF). To verify the validity of the present model, the predictions are compared with the experimental data of flow boiling heat transfer in the simulation of Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) conditions. For the PWR low-quality flow, the comparison demonstrates that the Helmholtz instability is the trigger condition for the onset of CHF.