Broadband ultrafast-laser-absorption spectroscopy for multi-hydrocarbon measurements near 3.3 µm in flames.

This paper presents the development and application of a broadband ultrafast-laser-absorption-spectroscopy (ULAS) technique operating in the mid-infrared for simultaneous measurements of temperature, methane (C H 4), and propane (C 3 H 8) mole fractions. Single-shot measurements targeting the C-H stretch fundamental vibration bands of C H 4 and C 3 H 8 near 3.3 µm were acquired in both a heated gas cell up to ≈650K and laminar diffusion flames at 5 kHz. The average temperature error is 0.6%. The average species mole fraction errors are 5.4% for C H 4 and 9.9% for C 3 H 8. This demonstrates that ULAS is capable of providing high-fidelity hydrocarbon-based thermometry and simultaneous measurements of both large and small hydrocarbons in combustion gases.

Characterization of non-Boltzmann CN X2Σ+ behind shock waves in CH4-N2 via broadband ultraviolet femtosecond absorption spectroscopy.

This article describes the temporal evolution of rotationally and vibrationally non-Boltzmann CN X2Σ+ formed behind reflected shock waves in N2-CH4 mixtures at conditions relevant to atmospheric entry into Titan. A novel ultrafast (i.e., femtosecond) laser absorption spectroscopy diagnostic was developed to provide broadband (≈400 cm-1) spectrally resolved (0.02 nm resolution) measurements of CN absorbance spectra belonging to its B2Σ+ ← X2Σ+ electronic system and its first four Δv = 0 vibrational bands (v″ = 0, 1, 2, 3). Measurements were acquired behind reflected shock waves in a mixture with 5.65% CH4 and 94.35% N2 at initial chemically and vibrationally frozen temperatures and pressures of 4400-5900 K and 0.55-0.75 bar, respectively. A six-temperature line-by-line absorption spectroscopy model for CN was developed to determine the rotational temperature of CN in v″ = 0, 1, 2, and 3, as well as two vibrational temperatures via least-squares fitting. The measured CN spectra revealed rotationally and vibrationally non-Boltzmann population distributions that strengthened with increasing shock speed and persisted for over 100 µs. The measured vibrational temperatures of CN initially increase in time with the increasing CN mole fraction and eventually exceed the expected post-shock rotational temperature of N2. The results suggest that strong chemical pumping is ultimately responsible for these trends and that, at the conditions studied, CN is primarily formed in high vibrational states within the A2Π or B2Σ+ state at characteristic rates, which are comparable to or exceed those of key vibrational equilibration processes.