A SHUTTER-BASED LINE-IMAGING SYSTEM FOR SINGLE-SHOT RAMAN SCATTERING MEASUREMENTS OF GRADIENTS IN MIXTURE FRACTION

摘要

A line-imaging system is described that uses a high-speed, rotating shutter to reject background luminosity and allow one-dimensional Raman scattering measurements to be obtained without the limitations in dynamic range and signal-to-noise ratio that are associated with image intensifiers. The rotating shutter replaces the entrance slit of an imaging spectrograph, which is combined with a cryogenically cooled, back-illuminated CCD array. This system was developed for the purpose of obtaining simultaneous single-shot measurements of the major species concentrations, the mixture fraction, and the gradients of these scalars along a line in turbulent non-prernixed flames, including those with simple hydrocarbon fuels. The performance of the shutter-based system was demonstrated through experiments in a laminar jet flame having the same fuel composition (25% CH_4 and 75% air) used in previous experiments on piloted jet flames. Time-averaged, high-resolution spectra are presented to document the nature of hydrocarbon fluorescence interference in such flames. Single-shot measurements are obtained using on-chip binning to form superpixels, a process that minimizes instrument noise. The demonstration experiment made use of f/3.45 collection optics and 450 mJ/pulse of laser energy at 532 nm. The precision of the measurement of species concentrations, the mixture fraction, and the radial gradient of mixture fraction in the laminar flame is analyzed in terms of signal-to-noise ratios and standard deviations. This analysis indicates that planned experiments using f/2 optics and approximately 2.6 J/pulse should yield relative standard deviations of the conditional scalar dissipation of roughly 13% at χ_(st) = 10 s~(-1) and 6% at χ_(st) = 160 s~(-1). This level of precision will be useful in validating models for conditional scalar dissipation in turbulent non-premixed flames.