The Discussion of Ignition Criterion About the Mechanical Spark Ignition of Near-Limit Methane-Air Mixtures.

An optimal ignition criterion must be selected for the numerical prediction of the ignition parameters for mechanical spark ignition of near-limit methane-air fuel mixtures based on fuel mixture equivalency ratio; wherein, the vast majority of ignition criteria can be used for stoichiometric fuel mixtures. However, the viability of each criterion must be individually determined for near-limit fuel mixtures. This study developed a high-resolution algorithm that can be used to model the mechanical spark ignition and combustion of methane-air fuel mixtures of varying equivalence ratios. Three ignition criteria, Van't Hoff, reaction heat release rate, and -OH concentration, were evaluated at varying equivalence ratios based on their respective distributions at varying times. The widely used Van't Hoff criterion cannot be used to predict near-limit fuel mixtures due to a high required ignition temperature and low rates of combustion. -OH concentration criterion is significantly affected by the rate of combustion and reaction temperature; therefore, it tends to erroneously overestimate ignition parameters for near-limit fuel mixtures. This study has found that reaction heat release rate criterion can be used at all equivalence ratios and faithfully produces ignition parameter trends for fuel mixture reaction schemes of varying simplifications. Reaction heat release rate is a valid ignition criterion because the location of peaks in reaction released heat at varying times corresponds to flame front location, which is its physical counterpart.