Numerical study on tri-fuel combustion: Ignition properties of hydrogen-enriched methane-diesel and methanol-diesel mixtures

Simultaneous and interactive combustion of three fuels with differing reactivities is investigated by numerical simulations. In the present study, conventional dual-fuel (DF) ignition phenomena, relevant to DF compression ignition (CI) engines, are extended and explored in tri-fuel (TF) context. In the present TF setup, a low reactivity fuel (LRF), methane or methanol, is perfectly mixed with hydrogen and air to form the primary fuel blend at the lean equivalence ratio of 0.5. Further, such primary fuel blends are ignited by a high-reactivity fuel (HRF), here n-dodecane under conditions similar to HRF spray assisted ignition. Here, ignition is relevant to the HRF containing parts of the tri-fuel mixtures, while flame propagation is assumed to occur in the premixed LRF/H-2 containing end gas regions. The role of hydrogen as TF mixture reactivity modulator is explored. Mixing is characterized by n-dodecane mixture fraction xi, and molar ratio x = x(H2)/x(H2) + x(LRF). When x < 0.6, minor changes are observed for the first- and second-stage ignition delay time (IDT) of tri-fuel compared to dual-fuel blends (x = 0). For methane, when x > 0.6, first- and second-stage IDT increase by factor 1.4-2. For methanol, a respective decrease by factor 1.2-2 is reported. Such contrasting trends for the two LRFs are explained by reaction sensitivity analysis, indicating the importance of OH radical production/consumption in the ignition process. Observations on LRF/H-2 end gas laminar flame speed (S-l) indicate that Si increases with x due to the highly diffusive features of H-2. For methane, S-l increase with x is more significant than for methanol. (C) 2019 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.