A comparative study on methanol and n-dodecane spray flames using Large-Eddy Simulation

Methanol (CH₃OH) is an attractive alternative fuel that can reduce net carbon release and decrease pollutant emissions. In this study, methanol and n-dodecane spray flames were investigated using Large-Eddy Simulation (LES) and direct coupling with finite-rate chemistry. The selected ambient conditions are relevant to engines and were previously unreported for numerical methanol spray studies, i.e. high pressure (60 bar) and temperature (900 – 1200 K) with high injection pressure (1500 bar). The Engine Combustion Network (ECN) Spray A case was used to validate the n-dodecane spray flame. For methanol, a modified ECN Spray A condition was used with a high initial ambient temperature (1100 K-1200 K) to ensure fast enough ignition relevant to engine time scales. The performed homogeneous reactor (0D) simulations revealed a new phenomenon of a two-stage ignition process for methanol, confirmed by the 3D LES at high pressure, high temperature, and lean conditions. The present numerical results also show that: 1) there is a strong ambient temperature sensitivity for methanol ignition delay time (IDT) with a five-fold decrease in IDT (IDT_1100K/IDT_1200K=5) and a factor of 2.6 decrease in the flame lift-off length (FLOL_1100K/FLOL_1200K=2.6) as the ambient temperature is increased from 1100 K to 1200 K, 2) methanol spray ignition takes place at a very lean mixture (ϕ_MR≈0.2) consistent with the 0D predicted most reactive mixture fraction (Z_MR), 3) on average, methanol sprays are significantly leaner than n-dodecane sprays at quasi-steady-state (ϕ_meoh,ave≈0.2 vs ϕ_ndod,ave≈0.7), implying very low soot emissions, and 4) the methanol spray flames could have similar temperatures as the n-dodecane sprays depending on the initial conditions, thus a similar level of NO_x emissions.