Rotational and Vibrational Raman Spectroscopy for Thermochemistry Measurements in Supersonic Flames
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High speed chemically reacting flows are important in a variety of aerospace applications, namely ramjets, scramjets, afterburners, and rocket exhausts. To study flame extinction under similar high Mach number conditions, we need access to thermochemistry measurements in supersonic environments. In the current work a two-stage miniaturized combustor has been designed that can produce open supersonic methane-air flames amenable to laser diagnostics. The first stage is a vitiation burner, and was inspired by well-known principles of jet combustors. We explored the salient parameters of operation experimentally, and verified flame holding computationally using a well-stirred reactor model. The second stage of the burner generates an external supersonic flame, operating in premixed and partially premixed modes. The very high Mach numbers present in the supersonic flames should provide a useful test bed for the examination of flame suppression and extinction using laser diagnostics. We also present the development of new line imaging diagnostics for thermochemistry measurements in high speed flows. A novel combination of vibrational and rotational Raman scattering is used to measure major species densities (O_2, N_2, CH_4, H_2O,CO_2, CO, & H_2) and temperature. Temperature is determined by the rotational Raman technique by comparing measured rotational spectra to simulated spectra based on the measured chemical composition. Pressure is calculated from density and temperature measurements through the ideal gas law. The independent assessment of density and temperature allows for measurements in environments where the pressure is not known a priori. In the present study we applied the diagnostics to laboratory scale supersonic air and vitiation jets, and examine the feasibility of such measurements in reacting supersonic flames. Results of full thermochemistry were obtained for the air and vitiation jets that reveal the expected structure of an under-expanded jet. Centerline traces of density, temperature, and pressure of the air jet agree well with computations, while measurements of chemical composition for the vitiation flow also agree well with predicted equilibrium values. Finally, we apply the new diagnostics to the exhaust of the developed burner, and show the first ever results for density, temperature, and pressure, as well as chemical composition in a supersonic flame.
Bayeh, Alexander C (2013). Rotational and Vibrational Raman Spectroscopy for Thermochemistry Measurements in Supersonic Flames. Doctoral dissertation, Texas A & M University. Available electronically from