Numerical Investigation of the Effect of Dissolved Non-Condensable Gases on Hydraulic Flip in Cavitating Nozzles
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This work focuses on hydraulic flip in cavitating nozzles and systematically investigates the effect of dissolved non-condensable gases on the concurrent phase change processes. First, different sets of equations known as 6-, 5-, and 4-equation models are evaluated. All models use a bisection-based Gibbs free energy relaxation algorithm for phase change after consecutively imposing velocity equilibrium, pressure equilibrium, and temperature equilibrium either in a hyperbolic solver or in subsequent relaxation steps. The well-established stiffened-gas equation-of-state is employed for thermodynamic closure. The impact of the different equilibrium assumptions on the resulting mixture speed of sound is discussed and the necessity of high-order numerics is investigated. Second, simulations of injector nozzles at gasoline direct injection (GDI) conditions were performed, resulting in flow regimes dominated by either hydraulic flip or cavitation. The underlying mechanism of hydraulic flip is analyzed and the competitive relationship between mechanical gas expansion and vapor formation in the cavitating flow regime is emphasized. It is shown that the amount of dissolved non-condensable gases has a strong, non-linear impact on the effective fuel cross section in the nozzle. Finally, the effect of viscosity on this non-linear behavior is studied.