The Effect of Ozone Addition on Flame Propagation

Abstract

The effect of ozone (O3) addition on flame propagation speeds (SL) at pressures ranging from sub-atmospheric to 2.5 atm were investigated experimentally and numerically with Bunsen flames using different fuels including CH4, C2H4 and C3H8. Significant increase of SL enhancement at elevated pressure was observed by comparing SL measured with and without O3 addition. Experiments showed that the enhancement in the stoichiometric CH4/air SL for 6334 parts per million (ppm) O3 addition was increased from 7.7% at atmospheric pressure to 11% at 2.5 atm. This enhancement growth occurs because elevated pressure promotes O3 decomposition, which provides O atoms, and suppresses diffusion of H, which weakens the O3+H = OH+O2 reaction. In contrast, increasing the initial reactant temperature, T0, accelerates both O3 decomposition and the O3+H = OH+O2 reaction and results in less enhancement of flame propagation. Modeled CH4 SL enhancement with O3 addition of 6334 ppm decreased from 8.7% at T0 =300K to 7.8% at 400K. C3H8 SL with O3 addition at atmospheric pressure and room temperature were measured at different equivalence ratios, and the enhancement was well predicted by the simulations. In contrast to these saturated hydrocarbon fuels, both of detrimental and enhancing effects of O3 addition on unsaturated hydrocarbon SL were observed, depending on operating conditions. With O3 addition, C2H4/air SL decreased at room temperature and pressure, owing to the ozonolysis reaction between O3 and C2H4, but increased if reactants were cooled to 200 K or pressures was decreased below 0.2 atm. Experimental results were successfully explained by a numerical model that includes a new ozonolysis sub-mechanism and considers energy loss during the mixing process.