High-temperature measurements of methane and acetylene using quantum cascade laser absorption near 8 μm

- Methane measured at the peak of Q(12) transition in the ν4 band.
- Acetylene measured at the peak of P(23) transition in the ν4+ν5 band.
- Differential absorption strategy employed to eliminate broadband interference absorption.
- Absorption cross-sections measured over 1200-2200 K and 1-4 atm.
- Methane and acetylene time-histories measured during the pyrolysis of n-pentane.

The mid-infrared wavelength region near 8 μm contains absorption bands of several molecules such as water vapor, hydrogen peroxide, nitrous oxide, methane and acetylene. A new laser absorption sensor based on the ν4 band of methane and the ν4+ν5 band of acetylene is reported for interference-free, time-resolved measurements under combustion-relevant conditions. A detailed line-selection procedure was used to identify optimum transitions. Methane and acetylene were measured at the line centers of Q12 (1303.5 cm−1) and P23 (1275.5 cm−1) transitions, respectively. High-temperature absorption cross sections of methane and acetylene were measured at peaks (on-line) and valleys (off-line) of the selected absorption transitions. The differential absorption strategy was employed to eliminate interference absorption from large hydrocarbons. Experiments were performed behind reflected shock waves over a temperature range of 1200-2200 K, between pressures of 1-4 atm. The diagnostics were then applied to measure the respective species time-history profiles during the shock-heated pyrolysis of n-pentane.