Effects of thermal coupling and diffusion on the mechanism of H₂ oxidation in steady premixed laminar flames

The article considers the question why steady premixed laminar flames can be successfully described by highly reduced models, whereas the underlying mechanism is inherently complex. The calculations are performed on H₂air systems. Sensitivity functions are evaluated and studied for diffusion-free situations, both isothermal and adiabatic, as well as for steady premixed flames. In the diffusion-free cases most reactions of a 38-step mechanism are shown to be influential in a distinct fashion. The form of sensitivity functions is, however, radically changed and rendered self-similar by simultaneous thermal coupling and diffusion that introduce strong nonlinear coupling among the variables. Due to self-similarity, the mechanism can be reduced to 15 reactions, while keeping the temperature profile and the mass fraction profiles of molecular species almost unchanged in flame calculations. Furthermore, there exists an invariant subspace in the space of kinetic parameters such that large parameter perturbations along any vector in this subspace result in relatively small changes of the computed flame properties. By giving mechanistic interpretation to such parameter perturbations, the model can be simplified in many ways. In particular, a sequence of models is constructed in the stoichiometric H₂air flame problem that converge to a nine-step reduced mechanism with quasi-steady-state assumptions in radicals except H, thereby resulting in a two-step quasi-global model. All these approximations are unfeasible without the presence of molecular and thermal diffusion.