Injection of Fuel at High Pressure Conditions: LES Study 2011-24-0041

This paper presents a large eddy simulation study of the liquid spray mixing with hot ambient gas in a constant volume vessel under engine-like conditions with the injection pressure of 1500 bar, ambient density 22.8 kg/m₃, ambient temperature of 900 K and an injector nozzle of 0.09 mm. The simulation results are compared with the experiments carried out by Pickett et al., under similar conditions. Under modern direct injection diesel engine conditions, it has been argued that the liquid core region is small and the droplets after atomization are fine so that the process of spray evaporation and mixing with the air is controlled by the heat and mass transfer between the ambient hot gas and central fuel flow. To examine this hypothesis a simple spray breakup model is tested in the present LES simulation. The simulations are performed using an open source compressible flow solver, in OpenFOAM. It is found that with the simple spray breakup model and almost unadjusted model parameters, the global quantities simulated from LES agree fairly well the experiments in terms of the vapor fuel penetration length and the vapor fuel mass fraction profiles along the axis of the injector and along radial direction at different axial positions downstream the injector nozzle. Sensitivity study is performed to examine the effect of the model parameters such as the number of injected parcels and the size of the injected parcels, as well as LES grid resolution on the numerical results. It is seen that the liquid penetration length, the flow velocity and vapor distribution in the near-nozzle region can be moderately sensitive to the model parameters. However, the vapor fuel penetration and distribution in the downstream region is rather insensitive to the model parameters. The good agreement between the numerical results and experiments is largely owing to the capability of the LES approach for capturing the shear layer instability and transition to turbulence. It implies the dominant effect of the large scale mixing of ambient air with the fuel on the spray injection and mixing process.