Combustion Diagnostics by Multiangular Absorption

Absorption techniques are being applied to three-dimensional combustion diagnostics. By using multiangular scanning, the traditional “onion peeling” method can be extended from axisymmetrical flows to arbitrary distributions of radicals and pollutants in the flow. Since scattering “point” techniques are limited by their very small cross section, such an extension would be a step improvement in sensitivity, down to low radical concentrations. Convolution Fourier transforms and iterative algorithms have already been proven in X-ray absorption tomography and interferometric applications. They are currently tested and compared on typical pollutant and radical concentration as they appear in flames or exhausts. The effect of the number of scans is analyzed for parallel beams. A trade-off exists between accuracy and the number of viewing angles. A five-angle procedure gives 10% accuracy with a moderately filtered convolution algorithm. An experiment feasibility study shows that near time-continuous three-dimensional maps of low concentrations (1 ppm) can be obtained at repetition rates up to 20 kHz. Specific applications to radical and pollutant mapping are discussed, as well as multiangular scanning strategies.

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