Numerical simulation of in-nozzle flow characteristics under flash boiling conditions

•A modified EOS was used to calculated the properties of both liquid and vapor phases simultaneously.•The distributions of in-nozzle pressure, velocity and vapor volume fraction were discussed in detail to show their relationship with the in-nozzle flow.•The sound speed of the two-phase mixture would decrease as the liquid starts to evaporate and could be correlated to the nozzle choking phenomenon.•The ambient pressure, liquid temperature and injection pressure can affect the in-nozzle flow by different mechanisms. Flash boiling spray has been extensively investigated because it has the potential in improving spray atomization. However, some phenomena, such as flow rate reduction, exit velocity choke and drastic bubble generation near the exit, etc., cannot be explained by existing theory of subcooled liquid jet and spray atomization. The phase change process occurring inside the nozzle still needs further exploration and investigation. However, due to the small dimensions of the nozzle, it is challenging to measure the flow characteristics experimentally. In this research, a one-dimensional two-phase flow model incorporating modified Stiffened Gas Equation of State is used to simulate the in-nozzle flow characteristics under flash boiling conditions. The model was validated against previous experimental data. The simulation results of the phase change process inside the nozzle was compared qualitatively with experimental data taken from a 2D optical nozzle. Different from subcooled conditions, superheated working liquid inside the nozzle evaporates more drastically as the result of local pressure distribution change. The flow velocity and mass flow rate are affected accordingly as well. In addition, the effects of the injection pressure, ambient pressure, and liquid temperature on these phenomena are also discussed. It was found that the ambient pressure and liquid temperature can affect the bubble generation by altering the evaporation rate of the fuel, while the injection pressure only influence the flow velocity and change the evaporation time of the liquid.