Numerical study of choked cavitation in high temperature hydrocarbon liquid jets

A numerical study was conducted on a practical plain orifice injector issuing pressurized high-temperature aviation fuel, in order to simulate injection of fuel after use as a coolant in the active cooling system of a hypersonic vehicle. A three-dimensional unstructured mesh inside the orifice was created using ICEMCFD^TM S/W, and the CFD analysis was performed using FLUENT^TM S/W. A multiphase mixture model was used to simulate cavitating two-phase flow, and the full cavitation model was activated to predict the mechanism and effects of cavitation induced by the high fuel vapor pressures at elevated temperature conditions. The simulation was performed for fuel heated up to 553 K (280 °C) at an upstream pressure (P_inj) of up to 1.0 MPa, and various ambient pressures (P_∞). The results were compared with experimental data, and the simulation was found to predict the discharge coefficient (C_d) with respect to the fuel injection temperature (T_inj) quite well at the given conditions. The CFD analyses for high fuel temperature conditions revealed that the mainstream flow inside the injector separates from the orifice wall at the vena contracta due to the generated fuel vapor cavity, and the attached flow at the end of the cavity separates again to produce a very small recirculation zone. In addition, for a given pressure drop, the sharply decreasing trend of the mass flow rate (or C_d) with increasing T_inj varies depending on P_∞, because the mass flow choking is determined by the relationship between P_∞ and the vapor pressure (P_sat) at T_inj. Finally, C_d with respect to cavitation number was found to follow an almost identical line, even at different P_∞. This confirms that choked cavitation at high fuel temperature conditions depends on the downstream pressure of the orifice, and the effect of cavitation on C_d at high T_inj is well represented by the cavitation numbers, regardless of P_inj, P_∞, and T_inj.