Abstract
Time-series measurements of CH concentrations [CH] are reported in a series of methane/hydrogen/nitrogen diffusion flames. Power spectral densities (PSDs) and autocorrelation functions are computed from the time series, permitting a detailed investigation of [CH] fluctuation time scales. The effects of fluorescence lifetime fluctuations are found to be negligible for quantitative determination of the PSDs. A similar study of hydroxyl concentrations in nonpremixed hydrogen/argon flames recently demonstrated that OH PSDs collapse to a single curve when normalized by the integral time scale, in agreement with mixture fraction statistics. However, for the present measurements, [CH] data on the fuel-side of the peak [CH] location exhibit low-frequency differences in their PSDs with respect to both the [OH] and air-side [CH] PSDs. These differences could be associated with the closer proximity of the CH radical to the shear layer and thus to enhanced fluctuations in the local mixture fraction. This possibility can be examined via measurements of other scalars.
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