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 T-amb = 900K and p(amb) = 60 bar. The hydrogen content of the LRF blend is varied systematically by changing the molar fraction 0 <= x <= 1, x = [H-2]/[H-2]+[CH4]. 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 x >= 0.5. 2) The ratio between the first- and second-stage ignition delay times (tau(2)/tau(1))(0D) approximate to 2 +/- 0.1 and (tau(2)/tau(1))(3D) approximate to 3 +/- 0.3. Furthermore, the ratio between 3D and 0D ignition delay times is given as tau(3D)(2)/tau(0D)(2) approximate to 2 +/- 0.2 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.