Simulation of R718 flash boiling flow inside motive nozzle of ejector.

Flash boiling steam/water flow phenomenon is encountered when a saturation mixture flow of R718 refrigerant experiences a depressurization rapidly inside motive nozzle of ejector. In this work, the phenomenon is modelled via CFD approach through a mixture model with slip velocity between two phases. To account for thermal non-equilibrium, the authors use evaporation/condensation model with evaporation/condensation frequency (time delay of boiling/condensing) is extracted from experimental data. This study tested on a 2D convergent-divergent nozzle with a range of total pressure-inlet (134 – 189 kPa)
and range of inlet liquid mass fraction (0 - 0.36) for case of saturation vapor/liquid R718 flow. Results of simulation in this case are in agreement with experimental data when maximum relative error is 8.2 % . The same geometry and numerical model is also applied for superheated vapor R718 flow in range of total pressue-inlet (91 – 141 kPa) and total inlet temperature (378 – 399 K), maximum relative error between simulation and experimental data in this case is 9.2%.