Laser absorption diagnostic for measuring acetylene concentrations in shock tubes

- Acetylene is an important combustion intermediate and soot precursor.
- We report a laser diagnostic for measuring acetylene concentrations in shock tubes.
- Direct absorption laser diagnostic is developed at the peak of the 3300 cm−1 band.
- High-temperature absorption spectrum of acetylene was studied in great detail.
- Proposed diagnostic shows excellent utility for studying the pyrolysis of alkenes.

A fixed-wavelength direct absorption laser diagnostic for high-temperature measurements of acetylene concentration was developed. The diagnostic, based on a tunable continuous wave distributed feedback diode laser, was optimized primarily for studying chemical kinetics behind reflected shock waves. The center wavelength (3335.55 cm−1) of the tunable diagnostic was typically set at the peak of the 3300 cm−1 absorption band of acetylene at high temperatures. The absorption spectrum of acetylene diluted in argon was characterized using scanned-wavelength direct absorption measurements from 1070 to 1720 K and 0.8 to 4.0 atm. Line fitting of the measured absorption spectra was not possible due to the large number of transitions overlapped by pressure broadening that contribute to the spectrum. Instead, empirical fits for the peak absorption coefficient and its corresponding wavelength as a function of temperature and pressure were generated. Furthermore, in order to allow for characterization of interference absorption in kinetic studies, empirical fits for the acetylene absorption coefficient in the region around the primary absorption feature were developed. Absorption coefficient measurements of propyne and 1-butyne, which may be the primary interference candidates, reveal that their absorption coefficients are constant in the wavelength range of interest, and are much smaller than those of acetylene. Therefore, the acetylene concentration in the presence of these interfering species can be inferred using two-color techniques. The utility of the acetylene diagnostic was demonstrated by measuring acetylene mole fraction time-histories during the pyrolysis of propene and 1-butene.