A numerical study on combustion mode characterization for locally stratified dual-fuel mixtures

Combustion modes in locally stratified dual-fuel (DF) mixtures are numerically investigated for methanol/n-dodecane blends under engine-relevant pressures. In the studied constant-volume numerical setup, methanol acts as a background low-reactivity fuel (LRF) while n-dodecane serves as high-reactivity fuel (HRF), controlling local ignition delay time. The spatial distribution of n-dodecane is modeled as a sinusoidal function parametrized by stratification amplitude (Y') and wavelength (0.01 mm<15). In contrast, methanol is assumed to be fully premixed with air at equivalence ratio 0.8. First, onedimensional setup is investigated by hundreds of chemical kinetics simulations in (Y', lambda) parameter space. Further, the concepts by Sankaran et al. (2005, Proceedings of the Combustion Institute) and Zeldovich (1980, Combustion and Flame) on ignition front propagation speed are applied to develop a theoretical analysis of the time-dependent diffusion-reaction problem. The theoretical analysis predicts two combustion modes, (1) spontaneous ignition and (2) deflagrative propagation, and leads to an analytical expression for the border curve called beta-curve herein. One-dimensional chemical kinetics simulations confirm the presence of two combustion modes in (Y', lambda) parameter space while the beta-curve explains consistently the position of phase border observed in the simulations. Finally, the role of convective mixing is incorporated to the theoretical expression for the beta-curve. The effect of convection on combustion mode is assessed by carrying out two-dimensional fully-resolved simulations with different turbulence levels. Two-dimensional numerical simulation results give evidence on combustion mode switching, which is consistent with predictions of the modified beta-curve for turbulent cases. The practical output of the paper is the beta-curve which is proposed as a predictive tool to estimate combustion modes for various fuels or fuel combinations. (C) 2019 The Author(s). Published by Elsevier Inc. on behalf of The Combustion Institute.