Diode Laser-Based Detection of Combustor Instabilities with Application to a Scramjet Engine

Fluctuations in temperature non-uniformity along the line-of-sight of a diode laser absorption sensor in a model scramjet are found to precede backpressure-induced unstart (expulsion of the isolator shock train). A novel detection strategy combining Fourier analysis of temperature time series to determine low-frequency heat release fluctuations with simultaneous measurements of multiple absorption features of H2O to identify temperature non-uniformities was applied to the scramjet combustor. Time-resolved absorption is measured using wavelength modulation spectroscopy for three transitions chosen with different temperature-dependent absorption characteristics. The line-of-sight (LOS)-averaged temperature inferred from the ratio of absorption from one pair of transitions is highly sensitive to low-temperature nonuniformities along the absorption path while the other ratio is less sensitive. The fraction of fluctuations in the range 1 < f < 50 Hz is determined from short-time Fourier transforms (STFTs) of the measured temperatures from both transition pairs.
Published in the Proceedings of the Combustion Institute, v32 p831-838, 2009. Prepared in collaboration with Stanford University, Stanford, CA, and with Innovative Scientific Solutions, Inc., Dayton, OH.