Laser Induced Breakdown (LIB) has been used for flame ignition and simultaneous gas concentration/density measurements in a supersonic combustor devised for hypersonic air-breathing propulsion. A Nd:YAG laser emitting 532 nm radiation provides photons that are focused at ignition/measurement locations to generate plasma. The plasma emission spectrum, which contains gas concentration/density information, is collected during a minimal time period (10 nanoseconds) approximately 75 ns after the arrival of the photons at the plasma location to minimize the measurement uncertainty in the turbulent, compressible, high-speed, and chemically reacting environment. The plasma emission is collected in the backward direction along the laser beam path, which is useful in practical measurement environments with limited optical accesses. The laser pulse energy absorbed in the gaseous medium is sufficient to initiate combustion reactions while the measurements are completed prior to the occurrence of reactions. The plasma is relocated at various locations in the supersonic combustor to investigate the ignition processes in the complex compressible flows as well as quantitative gas property measurements over the entire cavity flameholder area. The 2D gas density/concentration fields in planes parallel to the freestreams on the center- and off-center planes are measured using the technique developed in this study.

© 2015 Optical Society of America

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