Reaction zone structure and scalar dissipation rates in turbulent diffusion flames

Images of mixture fraction, temperature, scalar dissipation rates and OH concentrations in turbulent diffusion flames of methane-air and hydrogen-carbon dioxide are presented. The images are derived from Rayleigh scattering, fuel Raman scattering and OH-LIF. The images reveal that the reaction zones in these flames are strongly affected by the turbulence within them and that they become broadly distributed as the velocity is increased. The reaction zone width, as estimated from the measured OH profiles, is found to increase with the increase in jet Reynolds number of the flames. Local turbulence affects the OH profiles and causes a variation in the OH concentrations with little apparent variation in the corresponding mixture fraction and temperature images. This is seen in flames which are far from blow off and is not thought to be a local extinction effect but the direct influence of turbulence in the reaction zone. High scalar dissipation rates are not measured in the reaction zone where unburnt samples are encountered. Local nonburning may be due to a lack of ignition of premixed fluid or to local quenching by large eddy entrainment of cold fluid. The measured scalar dissipation rates do not increase significantly with an increase in the jet velocity. This may be due to the decrease in the gas diffusivity with decreasing temperature which offsets the increase in the spatial mixture fraction gradient. The scalar dissipation rates are believed to reduce in regions of local nonburning to the associated reduction in the temperature.