Separated two-phase flow model: application to critical two-phase flow

A separated flow model has been developed to allow the calculation of critical flow rates for steam-water mixtures. This model considers hydrodynamic as well as thermal non-equilibrium effects which are present due to rapid depressurization. Thus, this model incorporates interphase interaction terms for momentum, energy and mass. The mass transfer, evaporation or condensation rate, is coupled with the heat transfer between the two phases. Certain empiricisms are necessary to be included into this model, e.g. the size and number of nucleation sites at the onset of flashing. Transitions from one flow regime to the other are assumed to occur at certain void fractions. Justification of these assumptions is only possible by comparison with experimental results of different authors which in general shows good agreement.