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

TL;DR

This study numerically investigates combustion modes in stratified dual-fuel mixtures, developing a theoretical model and validating it with simulations to predict ignition behavior under various conditions.

Contribution

It introduces a theoretical framework and the $eta$-curve for predicting combustion modes in stratified dual-fuel mixtures, validated by detailed numerical simulations.

Findings

Two combustion modes identified: spontaneous ignition and deflagrative propagation.

The $eta$-curve accurately predicts phase borders between modes.

Convection influences combustion mode switching, confirmed by turbulent simulations.

Abstract

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<$\lambda$<15 mm). In contrast, methanol is assumed to be fully premixed with air at equivalence ratio 0.8. First, one-dimensional setup is investigated by hundreds of chemical kinetics simulations in (Y,$\lambda$) parameter space. Further, the concepts by Sankaran et al. 2005 and Zeldovich 1980 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 ...