Investigation of combustion enhancement by ozone additive in CH4/air flames using direct laminar burning velocity measurements and kinetic simulations

The effect of ozone additive on the enhancement of the burning velocity for premixed methane–air flames is investigated by both experimental measurements and kinetic simulations. Laminar burning velocities with and without O3 were directly measured using the Heat Flux method. The O3 molecules were introduced into the system by a dielectric-barrier-discharge ozone generator installed in the O2 gas line, which provided prompt control of on/off of the O3 feed into the system, enabling a precise comparison of the measured burning velocity with and without ozone additives. Noticeable burning velocity enhancement was observed at off-stoichiometric conditions rather than stoichiometric conditions. With 3730 ppm O3 additive in the oxidizer, experimental data shows ∼8% burning velocity increase in fuel-rich mixtures and ∼3.5% burning velocity increase for the stoichiometric mixture. With 7000 ppm ozone additive in the oxidizer, maximum ∼16% burning velocity increase was observed at fuel-lean conditions while ∼9.0% was found at fuel-rich conditions. An O3 kinetic mechanism involving 16 elementary reactions together with the GRI-Mech 3.0 was composed and validated through CHEMKIN calculations, which gives good predictions of the burning velocities with and without O3 additives. Extra O radicals contributed by O3 molecules in the pre-heat zone initiate and accelerate the chain-branching reactions and consequently increase the burning velocity.