Theoretical Consideration of a Boiling Transition Model for the Low Flow Region.

Abstract

To introduce a theoretical model of boiling transition under low flow conditions, which is related closely to start-up condition in natural circulation BWR's, consideration on the nature of boiling transition models are carried out on the basis of previous experiments and theoretical studies. Two local models, the “Stability of dry patch” and the “Liquid Sublayer Dryout” model are investigated in detail according to their applicability and capability. The boiling transition model “Stability of dry patch” is based on the force balance. To enclose this model, parameters are assumed, which are discussed according to the system conditions of this study. The boiling transition model “Liquid Sublayer Dryout” is based on the energy balance and was extended to the annular flow region. The flow parameters of these models were determined by a drift flux model. The numerical prediction of boiling transition heat flux confirms with the experimental results under low flow condition using the liquid sublayer model and the dry patch model. However both models axe showing discrepancies in the prediction for larger mass fluxes, related to the inaccuracy in the prediction of the complex flow condition.