Application of a fully implicit scheme for the prediction of pressure drop in a downcomer pipe during a depressurization process

The paper predicts the pressure and voidage transients occurring in a downcomer pipe of an experimental boiler when the boiler is subjected to a sudden depressurization. The theoretical model used in the study is that of the combined flow, three-equation model and it also incorporates the effects of relative velocity between the liquid and vapour phases, the thermal non-equilibrium and surface heat loss. Comparison of the predictions with the Authors' experimental data shows reasonably good agreement in terms of the pressure difference across the downcomer pipe as well as the rate of propagation of the voidage front. The study shows that the fully implicit numerical solution procedure can be used with the computational time-step being much greater than that specified by the Courant-Friedrich-Levy requirement.