High-Speed Laser Absorption Measurements of Carbon Oxides in Linear Detonation Channels

A mid-infrared laser absorption sensing strategy for carbon monoxide (CO) and carbon dioxide (CO2) has been developed for application to detonation flows. Bias-tee circuitry integrated with a distributed feedback quantum cascade laser near 4.94 μm and an interband cascade laser near 4.19 μm has enabled MHz-rate wavelength scanning, capturing a cluster of P-branch transitions of CO and a cluster of R-branch transitions of CO2 in their respective fundamental vibrationalbands. The mid-infrared light sources were both aligned onto a single detector in a compact optical arrangement, and diplexed in time to reduce the integration time of the measurements to 500 ns while maintaining sufficient wavelength scan depth to resolve the absorption features of interest. Post-detonation CO mole fraction values were inferred from a multi-line spectral fitting routine and CO2 mole fraction values were inferred using a collisional line mixing model developed previously. Quantitative mole fraction measurements were obtained behind vaporized n-heptane/oxygen detonation waves in a linear, optically-accessible detonation tube at the Air Force Research Laboratory (AFRL) at Edwards Air Force Base, reflective of rotating detonation engine geometries relevant to hydrocarbon detonation propulsion research projects at AFRL. Results reveal delayed oxidation of CO to CO2 behind the detonation waves, which has not previously been observed in laser absorption measurements behind detonation waves involving typical gaseous hydrocarbon fuels.