Abstract
Concentration fluctuations of specific chemical species in unsteady mixed gas flow were studied by fast Fourier transform (FFT) analysis of Raman intensity. Average concentration and the amplitude and frequency distribution of fluctuations about that average (at a specific Raman shift) can be determined by the FFT/Raman technique. The fluctuation spectra can be modeled to yield information on the lifetimes, periodicity, amplitudes, and the statistical distribution of these in unsteady or turbulent motion. A simple burner with a concentric outer air sheath and inner CH4 jet was used in the preliminary experiments for both cold gas and flame studies. Interference from flame emission (uncorrelated to Raman scattering) was minimized by chopping the Raman exciting beam and observing the FFT signal near the chopper frequency. Peak height at the chopper frequency indicates average concentration, while the sidebands around this frequency contain information on the fluctuations in concentration of the species under observation. With the present system, fluctuations in the frequency range of 0–1 kHz having an amplitude of 5% of the average concentration of certain species whose average partial pressure is as little as 1% of atmospheric pressure could be measured. The fluctuations are observed from sampling volumes as small as ∼10−4 mm3 at any position in the flow. Improvements in the technique and extensions to other systems are discussed.
Published Version
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