Analytical, Experimental and Computational Investigation of the Influence of Stoichiometric Mixture Fraction on Structure and Extinction of Laminar, Nonpremixed Methane Flames and Ethane Flames

摘要

Fundamental studies on combustion in laminar, nonpremixed flames are often carried out using conserved scalar quantities. These conserved scalar quantities, represented here by mixture fraction, ξ, are used as independent variables in activation-energy asymptotic analysis and in rate-ratio asymptotic analysis. These analyses are carried out in the asymptotic limit of large Damkohler number, with chemical reactions presumed to take place in a thin reaction zone, that is located at ξ = ξ_(st). The quantity ξ_(st) is the stoichiometric mixture fraction. A characteristic diffusion time is given by the reciprocal of the scalar dissipation rate, χ. Previous computational studies have shown that the scalar dissipation rate at extinction depends on ξ_(st) and the maximum flame temperature, T_(st). Here a rate-ratio asymptotic analysis is carried out using reduced chemistry to elucidate the influence of ξ_(st) on critical conditions of extinction. The scalar dissipation rate at extinction was predicted as a function of ξ_(st) with the mass fractions of reactants so chosen that the adiabatic flame temperature, T_(st), is fixed. The predictions of the analysis show that with increasing values of ξ_(st), the scalar dissipation rate at extinction first increases and then decreases. To test the predictions of the asymptotic analysis critical conditions of extinction are measured on nonpremixed methane flames stabilized in the counterflow configuration. With increasing values of stoichiometric mixture fraction, the strain rate at extinction was found to increase and the scalar dissipation rate at extinction was found to first increase and then decrease. The predictions of the asymptotic analysis agreed with experiments. A key outcome of the analysis is that with increasing ξ_(st) the thickness of the regions where oxygen and fuel are consumed first increase and the decrease. This is responsible for the observed non-monotonic changes in the values of the scalar dissipation rate at extinction with changes in ξ_(st).