Effects of ambient pressure and nozzle diameter on ignition characteristics in diesel spray combustion

Numerical simulations are performed to investigate the effects of ambient density (ρ_am) and nozzle diameter (D_noz) on the ignition characteristic of diesel spray combustion under engine-like conditions. A total of nine cases which consist of different ρ_am of 14.8, 30.0, and 58.5 kg/m³ and different D_noz of 100, 180, and 363 μm are considered. The results show that the predicted ignition delay times are in good agreement with measurements. The current results show that the mixture at the spray central region becomes more fuel-rich as D_noz increases. This leads to a shift in the high-temperature ignition location from the spray tip towards the spray periphery as D_noz increases at ρ_am of 14.8 kg/m³. At higher ρ_am of 30.0 and 58.5 kg/m³, the ignition locations for all D_noz cases occur at the spray periphery due to shorter ignition timing and the overly fuel-rich spray central region. The numerical results show that the first ignition location during the high-temperature ignition occurs at the fuel-rich region at ρ_am⩽30.0 kg/m³ across different D_noz. At ρ_am=58.5 kg/m³, the ignition occurs at the fuel-lean region for the 100 and 180 μm cases, but at the fuel-rich region for the 363 μm nozzle case. This distinctive difference in the result at 58.5 kg/m³ is likely due to the relatively longer ignition delay time in the 363 μm nozzle case. Furthermore, the longer ignition delay time as D_noz increases can be related to the higher local scalar dissipation rate in the large nozzle case.