Degenerate Four-Wave Mixing Measurements of Methyl Radical Distributions in Hydrocarbon Flames: Comparison with Model Predictions

Abstract

We report an investigation of the use of degenerate four-wave mixing (DFWM) to measure CH3 radical distributions in premixed hydrocarbon flames. Experimental results are compared with flame model calculations. By exciting the Herzberg _1 band near 217 nm, we monitored DFWM signals from CH3 as a function of height above a laboratory flat-flame burner at atmospheric pressure. Coherent anti-Stokes-Raman spectroscopy (CARS) of N2 was performed to obtain temperature profiles. The dependence of the DFWM signal on temperature was shown theoretically to be reduced by the short predissociative lifetimes of the CH3 upper states. A two-color FWM mixing experiment indicated that DFWM from CH3 in CH4 flames was predominately the result of population gratings. Using population-grating theory, the DFWM signal profile was corrected for varying temperature, beam absorption, and lower-state lifetimes. The results were in reasonable agreement with flame calculations and demoustrate that DFWM can provide useful relative concentration profiles of a polyatomic radical in rich hydrocarbon flames. However, we found that a weak DFWM background signal limited detection sensitivity ≈70 ppm of CH3 in CH4 flames. The background was comparable to the CH3 signal in C3H8 flames, and it obscured the CH3 signal in rich C2H2 and C2H4 flames. Strategies to alleviate the background are discussed.