Near-Infrared Measurements of Cyano Radical Number Density and Temperature using Rapidly Scanned Tunable Diode Laser Spectroscopy

Studies on the Titan aerothermal environment have determined the cyano radical (CN) to be dominant in radiation intensity. While previous experiments at the NASA Ames Electric Arc Shock Tube (EAST) have successfully deployed emission- and absorption-based diagnostics to capture number density and single-mode temperature at higher pressures, there remains an unmet need for measurements at low pressures and additional investigations of the Titan kinetics mechanism. Therefore, a fast-scanning TDLAS-based diagnostic targeting CN in the near-infrared (near-IR) near 926 nm and 1128 nm is employed in mixtures of 0.5% CH4/ 2% N2 in Ar by mole for a sweep of temperatures ranging 4800-6800 K at pressures from 0.3-0.5 atm. Multiple CN absorption features in the near-IR encompassing a range of lower vibrational states (v" = 0-2) and lower rotational states (J" = 14.5-68.5) are probed at 50 kHz scan rates to provide quantitative measurements of temperature and species number density profiles behind a reflected shock. In all instances, the measured rotational and vibrational temperatures are found to differ by no more than 3% and both temperatures agree with the Stanford Gökçen model to within 5%. The larger discrepancies exist at early test times, which suggest transient nonequilibrium. However, the disagreement between measurements and model diminishes with test time as equilibrium between the rotational and vibrational temperatures is established. A similar behavior is observed in number density profiles, with measurements of the CN(X) state trending with simulations for the bulk of the test time. At early times, measurements suggest a more rapid production of CN than the model predicts, with late times exhibiting slightly faster depletion of CN than the model predicts. These discrepancies motivate ongoing and future work which will consider additional lines in the near-IR for reduced fitting uncertainty, increased (>1 MHz) scan rates for the resolution of transient nonequilibrium, and tailored gas mixtures in order to isolate key reactions in the CN kinetics mechanism to resolve the accuracy of modeling assumptions.