A 100 kHz TDLAS Diagnostic for Characterizing Non-Equilibrium CN Formed Behind Shock Waves in N2-CH4Mixtures

Abstract

A tunable diode laser absorption spectroscopy (TDLAS) diagnostic was developed to characterize the thermochemical evolution of non-equilibrium CN formed behind shock waves at conditions relevant to entry into Titan’s atmosphere. The diagnostic utilizes two time-multiplexed tunable diode lasers (TDLs) emitting near 1156.3 nm and 1180.3 nm to measure four absorption lines of CN belonging to the Δv = 0 band of the A2Π ← X2Σ+ electronic system. Scanned-wavelength direct absorption spectroscopy was used to provide measurements of CN’s translational temperature, three internal temperatures (two rotational, one vibrational), and mole fraction at 100 kHz. Measurements of a single rotational temperature were also demonstrated at 200 kHz. The diagnostic was demonstrated behind reflected shock waves in CH4/N2 mixtures at two test conditions in the Purdue High-Pressure Shock Tube where the initial post-reflected-shock temperature was near 5000 K. The measurements clearly reveal that CN exhibits pronounced non-equilibrium rotational and vibrational state populations which is believed to result from CN being produced in high vibrational states of A2Π or B2Σ+.