Initiation of laminar flames in near-limit H2/O2/H2O mixtures

A one-dimensional spherically symmetrical flame model including a detailed chemical mechanism for hydrogen oxidation, multispecies transport, and thermal radiation is used to examine the ignition and flame propagation properties for homogeneous hydrogen/oxygen/steam mixtures at elevated temperatures up to about 500 ii and pressures ranging from 0.15 to 3 MPa. The model takes into account 26 elementary: reactions and absorption of thermal radiation by water molecules. Time-dependent conservation equations of mass, species, and energy are integrated numerically to investigate the details of flame structure and calculate the flame speed. Kinetic sensitivity analysis of the burning velocity is used to elucidate the importance of individual elementary reactions at various pressures. Laminar flame propagation in stoichiometric mixtures heavily diluted with steam is investigated near the flammability limit through numerical simulation of the evolution of the spherical flame kernel initiated by a spatially: localized ignition source. Critical conditions for flame propagation are analyzed in terms of ignition energy and flame kernel radius. The effect of pressure on these critical conditions is revealed and investigated. The predicted ignition energy and limiting hydrogen concentration nonmonotonically depend on pressure. The computed results are in reasonable agreement with available experimental data on flammability limits.