A short mechanism consisting of seven elementary reactions, of which only three are reversible, is shown\ud to provide good predictions of hydrogen–air lean-flame burning velocities. This mechanism is further\ud simplified by noting that over a range of conditions of practical interest, near the lean flammability\ud limit all reaction intermediaries have small concentrations in the important thin reaction zone that\ud controls the hydrogen–air laminar burning velocity and therefore follow a steady state approximation,\ud while the main species react according to the global irreversible reaction 2H2 + O2 → 2H2O. An\ud explicit expression for the non-Arrhenius rate of this one-step overall reaction for hydrogen oxidation\ud is derived from the seven-step detailed mechanism, for application near the flammability limit. The\ud one-step results are used to calculate flammability limits and burning velocities of planar deflagrations.\ud Furthermore, implications concerning radical profiles in the deflagration and reasons for the success\ud of the approximations are clarified. It is also demonstrated that adding only two irreversible direct\ud recombination steps to the seven-step mechanism accurately reproduces burning velocities of the full\ud detailed mechanism for all equivalence ratios at normal atmospheric conditions and that an eight-step\ud detailed mechanism, constructed from the seven-step mechanism by adding to it the fourth reversible\ud shuffle reaction, improves predictions of O and OH profiles. The new reduced-chemistry descriptions can\ud be useful for both analytical and computational studies of lean hydrogen–air flames, decreasing required\ud computation times

This paper investigates the inner structure of the thin reactive layer of hydrogen–air fuel-lean deflagrations close to the flammability limit. The analysis, which employs seven elementary reactions for the chemistry description, uses the ratio of the characteristic radical and fuel concentrations as a small asymptotic parameter, enabling an accurate analytic expression for the resulting burning rate to be derived. The analysis reveals that the steady-state assumption for chemical intermediaries, applicable on the hot side of the reactive layer, fails, however, as the crossover temperature is approached, providing a nonnegligible higher-order correction to the burning rate. The results can be useful, for instance, in future investigations of hydrogen deflagration instabilities near the lean flammability limit.