Injected Droplet Size Effects on Diesel Spray Results with RANS and LES Turbulence Models

Injection spray dynamics is known to be of great importance when modeling turbulent multi-phase flows in diesel engines. Two key aspects of spray dynamics are liquid breakup and penetration, both of which are affected by the initial sizes of the injected droplets. In the current study, injection of liquid n-heptane is characterized with initial droplet sizes with diameters on the order of 0.10 - 0.25 nozzle diameters. This is done for a Reynolds Averaged Navier-Stokes (RANS) RNG k-ε turbulence model with a minimum grid size of 125 μm and for a Large Eddy Simulations (LES) viscosity turbulence model with a minimum grid size of 62.5 μm. The results of both turbulence models are validated against non-reacting experimental data from the Engine Combustion Network (ECN).

The results show that the injected droplet sizes have a significant impact on both liquid and vapor penetration lengths. In addition, the primary breakup parameter B₁ and the sizes of the injected droplets are shown to be equally important for predicting the liquid penetration length. Both turbulence models match experimental data of liquid penetration for injected droplets with diameters of 17.5 μm. However, the vapor penetration is under-predicted at this diameter which can be addressed by decreasing the droplet-per-parcel ratio. Fuel mass fractions and temperature gradients are in good agreement with experiment.