| The focus of this study is on the transient two-phase flow resulting from the fast depressurization of vaporizing liquids where a cover gas has been employed. An experimental test facility was designed and fabricated to study two-species, two-phase flows for assessment to two predictive models, a state-of-the-art computer code and a simplified homogeneous flow model. In our experiments, a volatile fluid is held in a tank at high pressure, in equilibrium with a blanket of nitrogen gas, which dissolves into the fluid. At the start, the tank is rapidly depressurized, the liquid vaporizes and the nitrogen in the liquid degasses, leading to the formation of multiphase (i.e., gas and liquid) flow in the exit lines. Two-phase phenomena was observed in all of the 21 experiments conducted, where pressure decay in the system showed distinct regions where two-phase phenomena occurred with an initial region of the system filling with fluid, to a two-phase discharge region, to a final venting of the remaining gas and vapor. Measurements of void fraction and mass flow were also obtained and reiterated these observations by showing the regions with high void and when the fluid had finished its discharge. A one-dimensional, transient, two-phase model was developed to explain the observed results. The dynamic equations are modeled using a single-phase incompressible fluid model as a base, with the addition of a nitrogen release model to give a simple, two-phase predictive model, which was then compared to experimental data to judge its effectiveness. The model did exceptionally well when modeling the discharge rate of two different test fluids containing dissolved nitrogen gas, refrigerants HFC-227ea and HFC-125, over a range of initial operating pressures. The predictive ability of the output from the current state-of-the-art computer code FSP was also assessed by the experimental data and compared to the simplified model. This code uses a two-fluid model of two-phase flow with constitutive equations to specify the interfacial transport terms. It was found that the data was a thorough benchmark for testing this code, which in turn proved to do a reasonable job in predicting the transient, two-phase data. |
