Experimental and numerical study on combustion characteristics of diesel spray in premixed ammonia/air mixtures using a constant-volume combustion vessel

The ammonia/diesel dual-fuel combustion strategy is gaining increasing attention for its potential to reduce engine CO2 emissions. This study investigates the combustion characteristics of diesel spray in premixed ammonia/air mixtures using constant-volume combustion vessel experiments and numerical simulations. The effects of premixed ammonia/air equivalence ratio (φa = 0 – 0.3), ambient temperature (Ta = 800 – 900 K) and diesel injection pressure (100 – 160 MPa) were examined. Experimental results demonstrate that the premixed ammonia/air mixtures delay auto-ignition of diesel spray, shift the initial auto-ignition locations toward the downstream region of the diesel spray, and increase flame lift-off length. Meanwhile, the recorded flame images indicate that the flame temperature decreases with increasing premixed ammonia/air equivalence ratio. Elevated ambient temperatures reduce ignition delays and increase flame temperatures at different premixed ammonia/air equivalence ratio conditions. Diesel injection pressure shows little effect on ignition delays and flame lift-off lengths at the studied conditions. Numerical simulations employed a reduced chemical kinetic model for ammonia/diesel blends, with diesel represented by a surrogate blend of n-cetane, iso-cetane, and α-methylnaphthalene. Simulated ignition delays and flame lift-off lengths show reasonable agreement with experimental data. The simulations indicate that premixed ammonia/air significantly increases NOx (NO, NO2 and N2O) emissions. NO forms primarily in post-flame, high-temperature regions (>2000K) for all studied cases. Substantial N2O and NO2 are produced in relatively lower-temperature regions (<1500K) at the flame base in the presence of premixed ammonia/air. Furthermore, under the studied conditions, increasing the premixed ammonia/air equivalence ratio promotes NOx formation, whereas a higher ambient temperature suppresses it.