Hypersonic Scramjet Testing via Diode Laser Absorption in a Reflected Shock Tunnel

A wavelength-multiplexed two-color tunable diode laser absorption spectroscopy sensor probing transitions near 1.4 mu m was developed to measure H2O temperature and column density simultaneously across three lines of sight in a ground-based model scramjet combustor. High-enthalpy scramjet conditions equivalent to Mach 10 flight were generated with a reflected shock tunnel. The sensor hardware development and optical engineering to overcome noisy combustor conditions, including short test time, beam steering, and mechanical vibration, are discussed. A new scanned-wavelength-modulation spectroscopy technique was used to acquire the complete spectral lineshape while maintaining the high signal-to-noise ratio characteristic of wavelength-modulation spectroscopy measurements. Two combusting flow experiments were conducted, and the results compared with steady-state computational fluid dynamics calculations, with both the simulations and the measurements finding formation of H2O from H-2 combustion during the test time. This work represents the first use of wavelength-modulation spectroscopy in a reflected shock tunnel and the first use of scanned-wavelength-modulation spectroscopy in a scramjet combustor.