OME[formula omitted]/air mixtures: The structure and propagation mechanism of their laminar premixed flames

Abstract

Oxymethylene dimethyl ethers (CH3O(CH2O)xCH3 or OMEx) are synthetic substitutes of diesel and jet fuel. They can achieve soot-less combustion and have been shown to exhibit favourable ignition characteristics. With the aim to explore their combustion physics, the structure of their flames is analysed using the algorithmic tools of the Computational Singular Perturbation (CSP) method. It is demonstrated that the flame structure and the mechanism for its propagation is encapsulated in the fastest explosive mode, introduced by chemical dynamics in a narrow region of the flame front. This narrow region extends around the point where the maximum heat release rate (MHRR) is recorded. CSP diagnostics identify (i) reactions H + O2→ O + OH and CO + OH → CO2 + H beyond MHRR as initiating an upstream diffusion of heat and H radicals, both of which are generated by the exothermic reaction H2 + OH → H2O + H, (ii) a downstream convective motion that ensues and (iii) chemical activity that is initiated ahead of MHRR and sustained by the convective transport of fuel and oxygen. CSP also identifies differences in the action of the upstream diffusion of heat and H radicals and of the downstream transport of fuel and oxidiser. It is also shown that the fastest explosive mode far from the neighbourhood of MHRR has no influence on the structure and propagation of the flame front. Although OME2−4/air mixtures exhibit drastically different dynamics in the context of homogeneous autoignition, very small differences are recorded in terms of flame structure and propagation. The findings suggests that, since practically all major processes driving the flame front propagation are similar to those of hydrocarbons, OMEx fuels can be used as “drop-in” fuels in currently employed burners.