Numerical analysis of high-pressure ammonia/n-dodecane dual-fuel combustion

Ammonia is a promising zero-carbon fuel and hydrogen carrier for marine engines; however, its low reactivity presents significant challenges for compression-ignition combustion. Dual-fuel strategies employing a diesel-like pilot fuel offer a practical solution to enhance ignition and combustion stability. This study presents a three-dimensional computational fluid dynamics, and Reynolds-averaged Navier-Stokes investigation of ammonia combustion assisted by an n-dodecane pilot spray, using orthogonally arranged engine combustion network spray D for ammonia and spray C for n-dodecane injectors. After validating the numerical framework against available non-reactive and reactive experimental data, the model is applied to examine the influence of injection dwell time, chamber thermodynamic conditions, and ammonia injection pressure on ignition and combustion behaviour. The results show that delaying the pilot injection relative to ammonia substantially enhances ammonia ignition and flame development. Increasing ammonia injection pressure and ambient temperature promotes spray evaporation, mixture preparation, and combustion intensity, while lower ambient pressure extends spray penetration and improves fuel-air mixing. Among the investigated conditions, the optimal case increases the peak OH* signal by up to 65% and reduces unburned ammonia by 66% compared to the baseline condition. The parametric analysis further reveals that ammonia oxidation under compression-ignition engine-relevant conditions is primarily controlled by spray evaporation and mixture formation rather than chemical kinetics. This work provides new insights into the interaction between ammonia and pilot-fuel sprays and identifies operating strategies for improving ammonia combustion in future marine dual-fuel engines.