One-step reduced kinetics for lean hydrogen–air deflagration

Abstract

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