Large-eddy simulation of tri-fuel ignition: diesel spray-assisted ignition of lean hydrogen–methane–air mixtures

We present 3D numerical results on tri-fuel (TF) combustion using large-eddy simulation and finite rate chemistry. The TF concept was recently introduced by Karimkashi et al. (Int. J. Hydrogen Energy, 2020) in 0D. Here, the focus is on spray-assisted ignition of methane–hydrogen blends. The spray acts as a high-reactivity fuel (HRF) while the ambient premixed methane-hydrogen blend acts as a low-reactivity fuel (LRF) mixture. Better understanding on such a TF process could enable and motivate more extensive hydrogen usage in e.g. compression ignition marine engines where spray-assisted dual-fuel (DF) combustion is already utilised. The studied spray set-up is based on the modified ECN Spray A case, see Kahila et al. (Combustion and Flame, 2019) for DF combustion. The ambient pressure and temperature are (Formula presented.) 900 K and (Formula presented.) 60 bar. The hydrogen content of the LRF blend is varied systematically by changing the molar fraction (Formula presented.), (Formula presented.). The main added value of the study is that we extend the TF concept to 3D. The particular findings of the study are as follows: 1) Consistent with Karimkashi et al. 2020, hydrogen delays ignition also in 3D and the effect becomes significant for (Formula presented.). 2) The ratio between the first- and second-stage ignition delay times (Formula presented.) and (Formula presented.). Furthermore, the ratio between 3D and 0D ignition delay times is given as (Formula presented.) for all TF cases. 3) Finally, consistent with Karimkashi et al. 2020, also in 3D the high-temperature combustion heat release mode is shown to appear stronger in TF than the low-temperature combustion mode compared to DF methane–diesel combustion.