Numerical Analysis of Flashing Jet Structure and Its Generated Droplet Size Characteristics

摘要

In this paper, the flashing jets are numerically simulated using the MPS method. The boiling mode for flashing is identified as surface boiling mode, based on our postulation of jets from a short nozzle with high depressurization. The Homogeneous Non-equilibrium Relaxation Model (HRM) is used for calculating the evaporation rate of flashing. The numerical simulation results show that these flashing jets comprise an inner intact core which is surrounded by two-phase droplet flow. The effect of degree of superheat on the jet topological geometry is investigated. With increasing degree of superheat, the topological shape of jet evolves from cylindrical core for low degree of superheat to cone-shaped core for high degree of superheat, and meanwhile the extinction length of jet comes to decreases and tends asymptotically to a constant as the initial injection temperature approaches the saturation temperature corresponding to the injection pressure. Two modes of droplet generation are revealed in the course of jet atomization: entrainment into vapor from the jet surface and pitching-off from the jet tip; the latter generation mode is present exclusively for low degree of superheat. The mean droplet size decreases with increasing degree of superheat.