Single-Ended Sensor for Thermometry and Speciation in Shock Tubes Using Native Surfaces

A single-ended laser absorption sensor for time-resolved measurements of temperature and H₂O, CO₂, and CO concentrations was designed and deployed on a cylindrical shock tube operating at combustion-relevant conditions. The sensor transmitted four monochromatic laser beams probing strong species-specific mid-infrared absorption transitions through a single optical port and measured the backscattered laser radiation from the native surface across the 13.97-cm-diameter shock tube. Despite the non-ideal concave reflective surface and the long optical path relative to the port diameter (1.37 cm), an optimal optical configuration that maximized signal-to-noise ratio and was free of physical blockage was found and implemented using a numerical ray-tracing optimization algorithm. Wavelength-modulation spectroscopy was employed to further compensate for interference sources in the shock tube environment. The four lasers were sinusoidally injection-current modulated at various frequencies near 100 kHz to yield a measurement rate of 44 kS/s. Demonstration measurements involving non-reacting mixtures of the target species and reacting mixtures of ethane and oxygen in a N₂ bath were performed in a shock tube facility at temperatures between 1100 and 1800 K and pressures between 2 and 9 atm, with measured quantities demonstrating excellent agreement with the expected values. Despite experiencing strong vibrations induced by the dynamic shock tube environment, the sensor resolved sub-millisecond transients during ethane oxidation and detected H₂O, CO₂, and CO mole fractions as low as 0.025%, 0.012%, and 0.005%, respectively, representing highly sensitive detection limits suitable for single-port low-concentration multi-species sensing in shock tubes and other flow channels of comparable dimensions.