Critical flow in convergent-divergent nozzles with cavity nucleation model

A transient critical flow experiment with convergent-divergent nozzle as the break geometry was conducted in the high-pressure steam-water test loop of Xian Jiaotong University. The test parameters were pressure 3.0-16.0 MPa, inlet liquid stagnation subcooling 0-60 degrees C, and corresponding critical mass flow rate (40-120) x 10(3) kg/(m(2) s). The concept of the incipient flashing, with the choking plane occurring at the throat location, was applied in a wall surface cavity nucleation model. The total pressure difference between the inlet pressure P-0 and the pressure at the throat location P-1 was divided into two parts, one determined by the inlet fluid properties and the other being the pressure undershoot. Our experimental results show that the pressure undershoot and the liquid superheat at the incipient flashing location reach their maximum values with saturated inlet conditions; with increased inlet subcooling, thermal nonequilibrium decreased. A cavity nucleation model was developed for prediction of the pressure undershoot. The model includes a discharge coefficient, and it has been verified experimentally that this coefficient is a function of only the inlet liquid subcooling. Based on the present theory, the predicted critical mass flow rates are compared with not only our own experimental data but also other experimental data, and good agreement is achieved. (C) Elsevier Science Inc., 1997.