Critical flow in convergent-divergent nozzles with cavity nucleation model

The transient critical flow experiment with convergent-divergent nozzles as the break geometry was conducted in high pressure steam-water test loop of Xi'an Jiaotong University. The test parameters are as follows: pressure 3.0∼16.0 MPa, inlet liquid subcooling 0∼60 degree, corresponding critical mass flow rate 40∼120 x 103 kg/m2s. The flashing incipient and the choking plane were assumed to occur at the throat (minimum) section. The total pressure difference between inlet pressure and pressure at throat location was divided into two parts, one part only determined by the inlet fluid properties, the other being the pressure undershoot. The problem of computing critical mass flow rate is to find the pressure undershoot at choking plane. Our experimental results show that the pressure undershoot and the liquid superheat at the choking plane attain maximum values with saturated inlet condition. With increasing the inlet liquid subcooling, the thermal nonequilibrium is decreased. A new wall surface cavity nucleation model was developed for prediction of the pressure undershoot. the model includes a coefficient, and it is verified by the experiment that such coefficient is only the function of inlet liquid subcooling. Based on the present theory, the predicted critical mass flow rates agree well with the experimental data