Polarized/depolarized Rayleigh Scattering for Determining Fuel Concentrations in Flames

Rayleigh scattering has been shown to be a useful diagnostic technique for two-dimensional imaging studies of reacting and non-reacting flows. For example, by combining Rayleigh scattering with a simultaneous measurement of the fuel concentration (e.g., using Raman scattering), mixture fraction and temperature can be determined in flames. In this work, it is demonstrated that the fuel concentration can be obtained by measuring the polarized and depolarized components of the Rayleigh signal and taking their difference or a suitable linear combination. While the depolarized Rayleigh signal is smaller than the polarized signal by a factor of 100, this is still a factor of 10 larger than the Raman scattering. Application of the technique requires that one of the primary constituents of the fuel stream possess a depolarization ratio sufficiently different from that of the oxidizer. Methane is a convenient candidate as it has no measurable depolarization. Results are shown for methane flames diluted by argon as well as air.