Development of the film-splitting look-up table applicable to mechanistic annular film dryout model in annulus geometry

Abstract

Annular flow in annulus geometry is characterized as two liquid films flowing along the inner heated rod and outer unheated wall. Critical heat flux (CHF) occurs when the liquid film on the inner heated wall dries out, while there still exists the liquid film on the outer cold wall. In the safety analysis code, film dryout is calculated by a mechanistic model or CHF table look-up method. The mechanistic film dryout is a complex function of film flow rate, applied heat flux and entrainment/deposition rate, etc. and is determined by the hydrodynamic solution. However, both models were not able to distinguish the liquid films on the cold surface from that on the hot surfaces in a calculation cell, that is, the cold wall effect. This resulted in over-estimation of the calculated CHF in the single-channel modeling of annulus geometry, so it needs a new model that could consider the cold wall effect mechanistically in the single-channel modeling. In order to consider the cold wall effect, a mechanistic film-splitting model look-up table was developed, in which the inner and outer liquid film fractions are solved analytically. The new look-up table was assessed using Wurtz experimental data and was assessed indirectly using several annulus geometry CHF test data.