Use of the “Lagrangian and Eulerian Points of View” in the Transient Critical Heat Flux Calculations for BWR Rod Bundles and Experimental Verifications

The (X, L_B) correlation, which is currently used to predict the critical heat flux (CHF) onset in simple and complex geometries under a steady-state regime, is capable of a dynamic interpretation. The two-phase flow mean particles, climbing the channel at linear velocities corresponding to their mass velocity and local densities, reach the CHF conditions—i.e., zero thickness of the liquid film on the rod—when they have traveled a certain distance and have achieved a certain quality. According to this model, the CHF would be predicted in transient condition, when the boiling length and the mass velocities change with space and time, by applying the steady-state CHF (X, L_B) correlation to the actual paths of the mean fluid particles. The calculations performed in comparison of the “Lagrangian point of view,” outlined above by means of the DOLCE computer code, with the local space-time approach of the “Eulerian point of view” indicate that the two methods give substantially equivalent results and predict satisfactorily the onset of the transient CHF for the Centro Informazioni Studi Esperienze annuli experimental data and General Electric Company 16-rod bundles data under typical boiling water reactor transients, including loss-of-coolant accident simulations.