Semi-quantitative measurements of CH concentration in atmospheric-pressure counterflow diffusion flames using picosecond laser-induced fluorescence

Measurements of relative CH concentrations and absolute fluorescence lifetimes are reported in atmospheric pressure, laminar, counterflow diffusion flames using picosecond laser-induced fluorescence combined with a novel photon-counting technique. Three fuels consisting of nitrogen-diluted methane, ethane, or acetylene were used with global strain rates varying from 20 s^-1 to 60 s^-1. The concentrations were normalized by a measurement in one of the methane flames. The resulting (CH) ratios are corrected for quenching effects and are quantitative with uncertainties of typically ±7% (95% confidence interval). The results were compared to predictions from OPPDIF using two chemical mechanisms, GRI-2.11 and GRI-3.0. Both mechanisms correctly predict the relative CH concentrations between the ethane and methane flames but overpredict those between the acetylene and methane flame. Predictions of fluorescence lifetimes were computed from the OPPDIF species and temperature profiles using species-dependent quenching cross sections from the literature. The predicted fluorescence lifetimes are larger than the measurements by 10-15% in the methane and acetylene flames and by 50% in the ethane flames.