A critical heat flux model for saturated flow boiling on the downward curved heated surface

The critical heat flux (CHF) distribution on the outer surface of the lower head is a crucial parameter to assess the coolability limits of the in-vessel retention strategy through external reactor vessel cooling. Several CHF correlations concerning the orientation angle of heated wall have been proposed while the theoretical analysis is relatively insufficient. Based on the liquid microlayer dryout mechanism, a theoretical CHF model for the saturated flow boiling on the downward curved heated surface has been proposed in this work. With a thorough analysis of the vapor blanket behavior in the near-wall region, the effects of the mass velocity and orientation angle of the heated wall on the CHF are considered in present model. The well agreement between the predicted CHF and the experimental data suggests the validity and applicability of present CHF model to assess the CHF limits on the outer surface of the lower head under in-vessel retention.