On the Structure of Hydrogen Diffusion Flames with Reduced Kinetic Mechanisms

Structure of hydrogen/air diffusion flames has been analyzed by adopting a stagnant diffusion layer as a model problem. Starting from a twelve-step hydrogen oxidation mechanism, a two-step reduced mechanism has been deduced by applying appropriate steady-state and partial equilibrium assumptions. Various flame structures have been identified depending on the strain rate which affects the relative rate of the recombination and branching reactions. In the small strain rates, all the radicals can be assumed in steady-state that a one-step reduced mechanism can describe the flame structure in which the branching and recombination reactions occur simultaneously in a thin reaction zone. Radical profiles and their relative ratios have been derived and the results agree well with the numerical calculations. In the moderate strain rates, the steady-state assumption for the hydrogen radical breaks down that the two-step mechanism should be used. The flame has a structure of a thin branching zone imbedded in a relatively broad radical recombination zone. The analysis shows that the hydrogen radical has profile of the Airy function in the oxidizer side and is inversely proportional to the square of the mixture fraction in the fuel side. The maximum hydrogen radical concentration depends on the one-third power to the scalar dissipation rate. In the high stretch regime, distributed constant reaction rate model has been proposed and subsequently analyzed accounting the two-step reduced mechanism through which extinction conditions have been identified.