The effect of ozone addition on laminar flame speed

The effect of ozone (O3) addition on laminar flame speeds (SL) across a wide pressure range was investigated experimentally and numerically using three fuels, CH4, C2H4 and C3H8. Enhancement of SL due to O3 addition was consistently observed for CH4 and C3H8 mixtures over a range of lean to rich equivalence ratios, based on comparisons of SL measured with and without O3 addition. For both fuels, simulation results agree with experimental results, with the best predictions at near stoichiometric conditions and the largest discrepancies for fuel-rich cases. A significant increase in the SL enhancement was observed at elevated pressures: the enhancement in the measured SL for a stoichiometric CH4/air mixture with 6334 parts per million (ppm) O3 addition increased from 7.7% at atmospheric pressure to 11% at 2.5 atm. Elevated pressure both promotes O3 decomposition, which provides O atoms, and suppresses diffusion of H, which reduces the influence of the O3+H=OH+O2 reaction. Together, these lead to the increased SL enhancement with pressure. In contrast to the results for the two saturated hydrocarbons, both detrimental and beneficial effects due to O3 addition were observed for the unsaturated hydrocarbon fuel, C2H4 in this study. With O3 addition, C2H4/air SL decreased at room temperature and pressure, owing to the heat loss induced by the exothermic ozonolysis reaction between O3 and C2H4 in the mixing process, but increased as the ozonolysis reactions were minimized when reactants were cooled to 200 K or pressure was decreased below 0.66 atm. These experimental results were successfully explained by a numerical model that includes a new ozonolysis sub-mechanism.