High resolution imaging of flameless and distributed turbulent combustion

Planar laser-induced fluorescence (PLIF) and Rayleigh scattering measurements were used for the study of turbulence/combustion interactions in distributed reaction regimes including flameless or MILD combustion. A novel laboratory scale burner (Distributed and Flameless Combustion Burner – DFCB) was used to reach uniquely high Karlovitz numbers, presently reported up to 14,400. It consists of a highly turbulent piloted high speed jet burner with a vitiated coflow. Six cases are reported whereas two of them (leaner cases) led to an invisible reacting zone, though still emitting light in the UV and near infrared range. Simultaneous OH/CH2O PLIF image with 50 μm spatial resolution were achieved to capture the variation of intermediate species in the reaction layer. When complemented with temperature images obtained by Rayleigh scattering measurement, it provided insights of the reaction front structures as well as measures of the flame brush thicknesses. In particular, variations in the jet velocity highlighted the influence of turbulent mixing (hence turbulence/chemistry interaction) on the flame structures as depicted by the formation of relatively large pools of CH2O. Further, variations in the jet stoichiometry impacted on the reaction zone visibility but only marginally on the intensity and moderately on the overall shape of the OH and CH2O signals.