A numerical investigation on the laminar flame speed of methane/air and iso-octane/air mixtures with ozone addition

A numerical study has been carried out to evaluate the influence of ozone addition on the laminar flame speed for both methane/air and isooctane/air mixtures under thermodynamic conditions typical of internal combustion engines. Ozone is a highly oxidising chemical species and modifies the fuel oxidation reaction pathways, mainly in the initial stages of combustion. In the case of alkanes, such as methane and isooctane, ozone enhances the laminar flame speed. This study aims to analyse similarities and differences of the flame structure of methane and isooctane for different values of ozone concentration and equivalence ratio and for a wide range of pressure and temperature conditions. A 1-D numerical model, validated against several experimental data taken from the scientific literature, has been employed and simulations have been carried out by using a chemical kinetic reaction mechanism for methane and three different mechanisms for isooctane. The results show that the laminar flame speed increases with ozone concentration in the range 0–7000 ppm. At 1 bar, the percentage enhancement in terms of LFS with the addition of 7000 ppm of ozone, compared to the case without ozone, is about 4% for both fuels, under stoichiometric conditions, as long as temperature is lower than about 540 K. On the other hand, at higher temperatures ozone addition enables the formation of a cool flame for isooctane/air mixtures, and the reactions follow a different path compared to the case with methane. This leads to a greater influence of ozone on the isooctane laminar flame speed. Besides, for isooctane/air stoichiometric mixtures, at 560 K with 7000 ppm ozone, LFS shows a non-monotonic behaviour with pressure, being higher at 20 bar than at 15 bar, which is somehow unexpected. If the reactants temperature increases, the non-monotonic trend appears at lower pressures and lower ozone concentrations and is due to the fast ozone decomposition, promoted by the pressure increase. Such a decomposition enables fuel oxidation reactions by OH radicals in the cool flame region. This phenomenon does not occur for the methane case.