Abstract
The relative importance of molecular transport and chemical kinetics on flame ignition was investigated through detailed numerical simulations. The study was conducted in stagnation-type flows for atmospheric, laminar premixed and non-premixed iso-C 8H 18, n-C 7H 16, and H 2 flames. Ignition of premixed flames was studied by: (1) increasing the temperature of a N 2 jet counterflowing against a fuel/air jet, (2) increasing the temperature of a solid wall against which a fuel/air jet was injected. Ignition of non-premixed flames was studied by increasing the temperature of an air jet counterflowing against a fuel-containing jet. The simulations were performed along the stagnation streamline, and included detailed descriptions of chemical kinetics, molecular transport, and radiative heat transfer. Sensitivity analyses of the ignition temperatures to the diffusion coefficients of the reactants as well as to the kinetics were performed. Results revealed that premixed flame ignition is rather sensitive to the fuel diffusivity in the opposed-jet configuration, and notably less in the jet-wall. This is due to the diffusive transport that is required to convey the reactants towards the ignition kernel in the opposed-jet. It was found that the two approaches result in similar ignition temperatures only for fuel-rich cases and that the ignition temperatures tend to be lower as the equivalence ratio increases in the opposed-jet configuration. However, the ignition temperatures were found to depend mildly on the equivalence ratio in the jet-wall configuration. The sensitivity of ignition to diffusion in non-premixed systems was found to also be notable, especially for cases in which the fuel is highly diluted by an inert. For both premixed and non-premixed flames, the sensitivity of ignition to diffusion coefficients was found to be of the same order or larger than that to kinetics. This is important when flame ignition data are used to validate kinetics, as rate constants could be potentially falsified.
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