Joint planar CH and OH lif imaging in piloted turbulent jet diffusion flames near extinction

Simultaneous planar LIF images of the OH and CH radicals have been obtained in piloted turbulent diffusion flames, extending previous single-point Raman/Rayleight/OH LIF measurements in the same flames. The results give visual confirmation of inferences from the point measurements that at sufficiently high mixing rates, the flame is locally interrupted and extinguished some 10-30 diameters from the nozzle, and is re-lit further downstream. The images reveal thin, tendril-like structures of CH closely wrapping around the rich side of the OH profiles. Air dilution of the flame substantially thickens the OH reaction zone to the lean side of stoichiometric, and comparisons with laminar flame calculations indicate that whilst flamelet concepts may be applicable to the inner reaction zone where CH is produced and consumed, they are not valid for the thickened recombination zone.