Assessment of CN Non-Boltzmann Kinetics Against Low Density Shock Tube Data

This paper presents experimental characterization of shock layer radiative heating relevant to Titan atmospheric entry, with a focus on the CN Red and Violet systems, measured in the Electric Arc Shock Tube (EAST) at NASA Ames Research Center. Tests were conducted in the Low Density Shock Tube (LDST), and included incident shocks from 3.8 to 7.6 km/s and fill pressures of 30 and 90 mTorr in a mixture of 2.2% CH4 in N2 by mole. The measured radiance indicates strong chemical and thermal non-equilibrium, thus requiring accurate chemical kinetics and non-Boltzmann modeling of the state populations. Numerical simulations using DPLR and NEQAIR were employed to reproduce the experimental results. The main kinetic processes are highlighted, and different sets of non Boltzmann rates are evaluated against emission and absorption spectroscopy data. Results reveal limitations in current models, with increasing deviations towards lower velocity and pressure. Selection of faster heavy particle rates improves agreement with the CN Violet emission, while updates to the spectroscopic database show improved agreement over some wavelength range but overpredict CN Red at lower wavelengths. Comparisons with measured spectra, inferred state densities, and complementary TDLAS measurements indicate the need for further model refinement to reconcile these differences.