A Study of Flame Observables in Premixed Methane - Air Flames

Abstract

The use of particular experimental flame observables as flame markers, and as measures of flame burning and heat release rates requires the establishment of robust correlations between the particular observable and the rate in question. In this work, we use a compilation of results from numerical computations of the interaction of a premixed methane flame with a two-dimensional counter-rotating vortex pair using detailed kinetics. The data set involves the use of two different chemical mechanisms, a two-fold variation in flow time scales, and the examination of both stoichiometric and rich methane flames. Correlations between a number of flame observables and heat release and burning rates are examined. We study HCO, ∇ · v, OH, CH, CO, CH3, CH2O, CH2*, and C2H2, as well as various concentration products (surrogates for production rates) including [OH][CH2O], [OH][CH4], and [OH][CO]. Other concentration products expected to relate to chemiluminescent observables such as CH*, OH* and CO2* are also studied. HCO mole fraction is found to have the best correlation with flame burning and heat release rates for all cases studied. Results suggest that significant scatter due to flow unsteadiness is expected from correlations of peak ∇ · v, CO mole fraction gradient, C2H2 mole fraction, and CH* with heat release. Changes in stoichiometry are found to adversely affect the correlation expected from peak CO, OH, OH gradient, CH, CH3, and [OH][CH2O]. Little scatter is observed in the [OH][CH2O] data, highlighting its utility in the absence of significant variation of reactants composition. We observe evidence of useful correlations of peak [OH][CH4] and [OH][CO]. Concentration products of the precursors of OH* and CO2* are also found to correlate well with peak heat release rate. Peak CH2O data is found to have good correlation with peak burning and heat release rates, with small scatter, and little correlation shift due to changes in reactants composition.