Two-dimensional scalar measurements for turbulent flame characterization

This thesis describes an investigation of planar laser diagnostics developed to characterize parameters of interest in turbulent nonpremixed flames. Nitrogen Raman scattering was imaged with Rayleigh scattering to compare a passive conserved scalar to a conserved scalar based on fuel concentration and temperature. The results indicate good agreement between the two approaches in a turbulent flame when the mixture fraction dependent terms are properly defined. An alternative approach for describing the local fuel concentration is demonstrated based on the Rayleigh depolarization ratio for different gases. The technique shows promise when the fuel and oxidizer streams have sufficiently different depolarization ratios. Isotropic molecules such as methane have zero depolarization, and the difference between the normalized vertically and horizontally polarized Rayleigh scattering can be an order of magnitude larger than the corresponding Raman signal. This approach is extended to include simultaneous laser induced fluorescence of the CH radical which occurs in a narrow spatial region at the flame front. The fluorescence signal is linear, and quantitative CH concentrations are obtained based on simplifying assumptions regarding the local quenching environment taken from strained laminar flame calculations. Reynolds numbers of 15,100, 18,600, and 21,500 are examined for a piloted 25% methane - 75% air turbulent flame showing the transition from stability to local extinction 12 jet diameters downstream.; Finally, optical flow is examined as a methodology for extracting velocity information from a pair of temporally separated dense scalar images. The advantage of such a technique is compatibility with the mixture fraction imaging by eliminating the need for seed particles as in particle image velocimetry. Optical flow uses the structure inherent in images of turbulent flows to determine velocities. However, systematic errors are identified for two exemplar optical flow routines applied to a non-reacting and a reacting turbulent flow. Angular and absolute magnitude errors increase as a function of distance from the jet centerline, which is likely due to the decreased scalar intensity and increased homogeneity.