Multiplex CARS temperature measurements in a coal-fired MHD environment

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Abstract
Multiplex coherent anti-Stokes Raman spectra (CARS) of nitrogen were recorded in an environment that simulates the postmagnet gas stream of a coal-fired MHD generator. The presence of coal fly ash and potassium seed created a weakly ionized, highly luminous medium with a high number density of relatively large (1–50-μm) diam particles. Maximum temperatures of 2500 K were measured with a spatial resolution of 5 mm. The precision optical alignment necessary for folded BOXCARS phase-matching was maintained for the long distances (>10 m) necessary to route the laser beams from the CARS instrument to the combustion facility. The increased luminosity caused by injection of potassium seed did not impede the recovery of good quality spectra. The coal fly ash particles precipitated laser-induced breakdown, which, in turn, led to the generation of a coherent interference with N2 spectra. Techniques to overcome this problem are discussed. A nonlinear least-squares fitting computer program was used to examine systematically the error in inferred temperatures caused by uncertainties in the parameters used in the Kataoka-Teets algorithm employed to reduce the data. The accuracy of the temperature measurements is estimated to be ±3%.