Near-surface carbon-dioxide tunable diode laser absorption spectroscopy concentration measurements in hypervelocity flow

Measurements of carbon-dioxide concentration are made in the LENS-XX expansion tunnel at the Calspan - University at Buffalo Research Center to investigate the effects of surface catalysis in hypersonic flows. A tunable diode laser absorption spectroscopy setup probes the P36e CO₂ absorption line of the ν₁ + ν₃ combination band at 2.7153 μm. Numerical simulations are computed with data-parallel line relaxation usingthe specified reaction efficiency surface catalysis model. Absorption measurements adjacent to the surface of an aluminum cylinder correlate well with simulations indicating low catalytic efficiency at the surface. The velocity, density, freestream temperature, specific enthalpy, and stagnation point pitot pressure of the run are 4.6 km/s, 1.53 g/m³, 1328.4 K, 11.75 MJ/kg, and 31.6 kPa, respectively. Simulation of the absorption spectrum is performed using a nonhomogeneous line-by-line code using the high-resolution transmission molecular absorption database for spectroscopic parameters. Input data for the code are obtained from computational fluid dynamics simulations of the cylinder and facility. The specified reaction efficiency for the mechanism CO + O → CO₂ is determined inversely using detailed computational fluid dynamics and comparisons to experimental measurements. The best agreement was found for γ_CO = 0.00025 to 0.0005, indicating that the coupled line-by-line code and computational fluid dynamics approach is useful for assessing near-surface composition using tunable diode laser absorption measurements of hypersonic flows.