Laminar flame speeds and kinetic modeling of H2/O2/diluent mixtures at sub atmospheric and elevated pressures

Recognizing the need to improve the predictability of the foundational H2/O2 chemical kinetic mechanism over extensive parametric ranges of pressure, adiabatic flame temperature and composition, laminar flame speeds extracted from constant-pressure expanding flames were experimentally determined for H2/O2/diluent flames spanning the parameter range of: equivalence ratio from ultra-lean to ultra-rich (0.3 and 2.5), flame temperature from 1300 to 2200K, pressure from sub-atmospheric to elevated values of 0.25–20atm, and dilution of He up to 65% and CO2 up to 34%. A new kinetic model, HP-Mech, based on evaluation of the rate coefficients from recent high-level quantum chemistry calculations and shock tube measurements, was developed and subsequently used to investigate the controlling reaction steps of these flames. In particular, the overall reaction order was extracted, allowing for the pressure dependence of the adiabatic flame temperature. The results show that it can assume values not only less than zero at high pressures, as was reported before, but also values larger than 2 at lower pressures. Sensitivity and reaction path analyses show that these responses are controlled by the competition between two opposing pathways involving reaction R9: H+O2(+M)=HO2(+M).