Modelling thermal effects in cavitating high-pressure diesel sprays using an improved compressible multiphase approach

In this study, the influence of in-nozzle phenomena including flow separation, cavitation, turbulence and hydraulic flip on the morphology of the spray emerging from a convergent-divergent-convergent diesel injector is investigated numerically. Non-linear equations of state for the liquid diesel, diesel vapour and chamber gas are employed for the simulation of high pressure diesel injection and atomisation processes. A modified multiphase mixture energy equation which takes into account enthalpy of phase change due to cavitation is integrated into a previously developed compressible, multiphase Volume of Fluid Large Eddy Simulation. The mass transfer source terms are modelled using a modified Schnerr and Sauer cavitation model. The numerical method is validated by comparing simulated mass flow rates, momentum fluxes, effective injection velocities and discharge coefficients at different injection conditions against published experimental data obtained using the same injector. Favourable comparison between simulations and experimental measurements is achieved with minor discrepancies attributable to unknown experimental uncertainties and assumptions made in numerical modelling. Calculation of in-nozzle flow and primary spray breakup reveals that interfacial instabilities generated due to in-nozzle flow separation, cavitation and liquid-wall shear contribute greatly to the jet fragmentation. The increase in sensible enthalpy due to wall shear induced viscous heating together with enthalpy of condensation increase the surface temperature of the exiting jet. Comparison of the flow physics before and after the onset of hydraulic flip indicates that wall shear is one of the main mechanisms inducing most of the energy for jet breakup. This modelling shows that vapour production at nozzle entrance remains after the onset of hydraulic flip, limiting the extent of ambient air influx. In addition, the onset of hydraulic flip causes production of near nozzle shockwaves as a result of significantly increased injection velocity attributable to minimised wall shear. This aspect needs more experimental evidence and simulations to confirm and validate.