Combustion and Flame, Vol.211, 147-157, 2020
Investigation of methanol ignition phenomena using a rapid compression machine
The ignition phenomena of stoichiometric methanol/oxygen/argon mixture are comprehensively investigated at p= 12-24 bar, T= 840-1000 K, using a rapid compression machine (RCM). A strong tendency of stochastic ignition followed by non-forcible flame propagation at a speed of similar to 11 m/s is demonstrated. Such an event may be responsible for the pre-ignition or super-knock issue in methanol engines. Under engine relevant spark ignition conditions, different ignition regimes are observed in the end-gas, including thermal explosion, supersonic deflagration, and detonation, characterized by the Chapman-Jouguet velocity criterion. All modes originate from a similar, early auto-ignition ahead of the spark-triggered flame. The auto-ignition process is proved to be dominated by chemical kinetics, where the Livengood-Wu correlation is applicable. In addition, the transition mechanism of different ignition regimes is thoroughly validated against previous ignition theories. Basically, the detonation onset is closely related to the initial thermodynamic condition of the mixture. At a given temperature, the decrease of pressure induces the gradual substitution of detonation by supersonic deflagration and thermal explosion due to a smaller reactivity gradient in the end-gas. The epsilon -xi diagram proposed by Bradley is adopted to interpret the transition mechanism of methanol, which turns out to be different from previous results of isooctane. A moderate burned mass fraction range of 0.35-0.45 is found when detonation is initiated. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.