3d cfd simulation of flashing flows in a converging-diverging nozzle

Flashing of initially sub-cooled water in a converging-diverging nozzle is simulated with two-fluid model incorporating drag and non-drag forces. Phase change is assumed to be induced by interphase heat transfer. Comparison with experimental data is performed for 14 test runs under different temperature and pressure conditions. Good agreement is achieved for mass flow rate and cross-section averaged parameters. The reliability of the CFD predictions is obviously better than the ones obtained in 1D code. The transversal (radial) distribution of void fraction is however not satisfyingly reproduced. It is mainly caused by the neglect of nucleation as well as other uncertainties related to the prediction of mean bubble diameter. A poly-disperse approach with consideration of all relevant bubble dynamics is recommended for further work. For this purpose, reliable closure models and experimental data of bubble size information is required. In addition, cases characterised with large pressure-undershoot exhibit significant mechanical non-equilibrium.