The Influence of Ozone on the Flame Propagation Behavior of n-Heptane/air and i-Octane/air Premixed Flame in an Engine-Relevant Environment

The promotional effect of ozone on lean premixed spherical flame propagation in an engine-relevant, high-temperature, high-pressure environment was investigated using an optically accessible rapid compression and expansion machine (RCEM). The laminar burning velocity, burned gas Markstein length, and initial flame development time based on mass fraction burned (MFB), were examined for n-heptane/air and i-octane/air premixture (phi = 0.55) with 15 [ppm] ozone, through self-emission observation of a propagating flame with a high-speed camera and combustion pressure analysis. The results showed that ozone reduced the initial flame development time and increased the laminar burning velocity for n-heptane/air premixture under temperature conditions in which the low-temperature oxidation (LTO) reaction was more clearly pronounced; but ozone did not enhance either the initial flame development or the laminar burning velocity in the case of i-octane/air premixture, where the LTO reaction was weak owing to the fuel molecular structure, even under the same temperature conditions. These results suggest that the enhancement of the LTO reaction by ozone, and the subsequent generation of thermal energy and CO radicals through an H2O2 loop reaction in the pre-flame region, were responsible for the increase in the laminar burning velocity. Furthermore, ozone increased the burned gas Markstein length, flame thickness, and Zeldovich number of the n-heptane/air mixture under the temperature conditions in which the laminar burning velocity was enhanced by ozone. This suggests that the chemical reaction in the reaction zone was more active in response to the increase in flame temperature.