Quantitative Analysis of Mixing in Unlike Impinging Jets with Flashing

This study quantitatively investigates the effects of flashing on the mixing characteristics of unlike impinging jets, simulating the startup phase of a bipropellant thruster. Water and hydrofluoroether are used as propellant simulants, and optical patternator measurements are conducted over chamber pressures ranging from 0.005 to 0.10 MPa. The results show that oxidizer flashing below its vapor pressure degrades mixing by forming vapor barriers that prevent direct liquid-liquid contact during impingement. To evaluate this effect, we develop a method for estimating the oxidizer vapor fraction using the deflection of film cooling fuel jets, and the vapor fraction reaches 0.73 at the lowest chamber pressure. Furthermore, we extend the nondimensional injection parameter [Formula: see text] to include gas–liquid multiphase effects by introducing average density and velocity at the impingement point. The extended framework provides excellent agreement between theoretical predictions and experimental overlap ratios, demonstrating its validity across all pressure conditions. This unified approach enables accurate prediction of mixing degradation during startup, offering practical design tools for bipropellant injectors.