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Large Eddy Simulation (LES) is used to compute the spark ignition in a turbulent methane jet flowing into air. Full ignition sequences are calculated for a series of ignition locations using a one-step chemical scheme for methane combustion coupled with the thickened flame model. The spark ignition is modeled in the LES as an energy deposition term added to the energy equation. Flame kernel formation, the progress and topology of the flame propagating upstream, and stabilization as a tubular edge flame are analyzed in detail and compared to experimental data for a range of ignition parameters. In addition to ignition simulations, statistical analysis of non-reacting LES solutions are carried out to discuss the ignition probability map established experimentally.

The effects of equivalence ratio variations on flame structure and propagation have been studied computationally. Equivalence ratio stratification is a key technology for advanced low emission combustors. Laminar counterflow simulations of lean methaneair combustion have been presented which show the effect of strain variations on flames stabilized in an equivalence ratio gradient, and the response of flames propagating into a mixture with a time-varying equivalence ratio. 'Back supported' lean flames, whose products are closer to stoichiometry than their reactants, display increased propagation velocities and reduced thickness compared with flames where the reactants are richer than the products. The radical concentrations in the vicinity of the flame are modified by the effect of an equivalence ratio gradient on the temperature profile and thermal dissociation. Analysis of steady flames stabilized in an equivalence ratio gradient demonstrates that the radical flux through the flame, and the modified radical concentrations in the reaction zone, contribute to the modified propagation speed and thickness of stratified flames. The modified concentrations of radical species in stratified flames mean that, in general, the reaction rate is not accurately parametrized by progress variable and equivalence ratio alone. A definition of stratified flame propagation based upon the displacement speed of a mixture fraction dependent progress variable was seen to be suitable for stratified combustion. The response times of the reaction, diffusion, and cross-dissipation components which contribute to this displacement speed have been used to explain flame response to stratification and unsteady fluid dynamic strain. Nomenclature Englishproperties at location of peak heat release rate m value midway between boundary values Subscripts fu fuel st stoichiometric max maximum R component due to chemical reaction D component due to molecular diffusion CR component due to cross-dissipation IntroductionProgress in combustion technology depends heavily on the ability of engineers to combine demands for higher efficiency with those for lower emissions from stable combustion devices. Stratified combustion is becoming an increasingly widespread strategy for internal combustion engines and gas turbine engines to meet these combined requirements. Stratified combustion occurs when an inhomogeneous fuel-air mixture is either entirely lean or entirely rich, precluding the occurrence of edge flames. Nevertheless this mode of combustion has in the past received relatively little research attention, in comparison with the dominant premixed and diffusion flame accounts of combustion. Current capabilities for the measurement [1-5] and simulation [6-8] of turbulent stratified flames have begun to allow some interrogation of turbulent stratified flame structure and propagation. In