Shock Event Analysis, Characteristic Considerations and Conservativeness Influence in High Pressure Injection System Cavitating Flow Simulations

A comparison between conservative and nonconservative models has been carried out for evaluating the influence of conservativeness on predicting transient flows in presence of cavitation induced discontinuities inside high-pressure injection systems. Even if nonconservative models can assure satisfactory accuracy in the evaluation of the wave propagation phenomena, they introduce fictitious source terms in the discretized equations. Such terms are usually negligible, but can play a significant role when discontinuities in the flow properties occur, producing appreciable errors on the pressure wave speed estimation. An analysis based on fluid characteristics around both the rarefaction and compression wave fronts has been carried out, showing that cavitation desinence is a shock occurrence, leading to a transition from a supersonic to a subsonic flow. For a significant evaluation of conservative and nonconservative model performances a conventional pump-line-nozzle injection system was considered because the pipe flow presented interesting cases of cavitation-induced shocks. The validity of the conservative model is substantiated by the comparison between computed pressure time-histories and experimental results at two pipe locations. The Rankine-Hugoniot jump conditions have been usefully applied to the numerical results obtained by the conservative model in order to calculate the sound speed of the traveling compression waves in the presence of cavitation. A novel algorithm of general application to calculate the shock speed predicted by nonconservative models, which points out the contribution of the internal fictitious fluxes in the wrong estimation of the shock velocity, has been introduced and validated through its application to Burgers' equation.