Investigation of combustion enhancement by ozone additive in CH(4)/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 O(3) were directly measured using the Heat Flux method. The O(3) molecules were introduced into the system by a dielectric-barrier-discharge ozone generator installed in the O(2) gas line, which provided prompt control of on/off of the O(3) 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 O(3) additive in the oxidizer, experimental data shows similar to 8% burning velocity increase in fuel-rich mixtures and similar to 3.5% burning velocity increase for the stoichiometric mixture. With 7000 ppm ozone additive in the oxidizer, maximum similar to 16% burning velocity increase was observed at fuel-lean conditions while similar to 9.0% was found at fuel-rich conditions. An O(3) 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 O(3) additives. Extra O radicals contributed by O(3) molecules in the pre-heat zone initiate and accelerate the chain-branching reactions and consequently increase the burning velocity. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved. (Less)