Thermometry and speciation for high-temperature and -pressure methane pyrolysis using shock tubes and dual-comb spectroscopy

Abstract

Quantum-cascade-laser dual-comb spectroscopy (QCL-DCS) is a promising technology with ultra-fast time resolution capabilities for chemical kinetics, atmospheric gas sensing, and combustion applications. A pair of quantum-cascade frequency combs were used to measure absorbance from methane’s band between 1270 and 1315 cm−1 at high-temperature and -pressure conditions that were generated using a high-pressure shock tube. Results here mark a major improvement over previous QCL-DCS measurements in shock tubes. Improvements came from a unique spectral-filtering strategy to correct for a bimodal power-spectral density of QCL frequency combs and careful optimization of the laser setup and experimental conditions. Our modified QCL-DCS was ultimately used to measure temperature within 2% and methane mole fraction within 5% by fitting HITEMP spectral simulations to spectra recorded at 4 μs temporal resolution. We measure temperature and species time-histories during methane pyrolysis at conditions between 1212–1980 K, and 12–17 atm, all at 4 μs resolution. Good agreement is observed with kinetic models, illustrating the potential of future applications of DCS in kinetics and combustion research.

Multiwavelength Speciation in Pyrolysis of n-Pentane and Experimental Determination of the Rate Coefficient of nC5H12 = nC3H7 + C2H5 in a Shock Tube

“…Cross section information on relevant species at these selected wavelengths have been adopted from work done previously at Stanford . Further details on the benefits of a multiwavelength strategy can be found in refs and − . true[ .25ex2ex α ν 1 ⋮ α ν m true] = n L true[ .25ex2ex σ 1 , ν 1 false( italicT , italicP false) ⋯ σ n , ν 1 false( italicT , italicP false) ⋮ ⋱ ⋮ σ 1 , ν m false( italicT , italicP false) ⋯ σ n , ν m false( italicT , italicP false) true] true[ .25ex2ex x 1 ⋮…”

“…Cross section information on relevant species at these selected wavelengths have been adopted from work done previously at Stanford . Further details on the benefits of a multiwavelength strategy can be found in refs and − . …”

Mid-infrared dual-comb polarimetry of anisotropic samples

“…For a given polarizer configuration, measuring the difference between a set-up without and one with a sample gives the complex transmission t xy , the angle of which yields the absolute phase. The theoretical time resolution is 1/Δf rep 35,36 however, for data-handling and signal-to-noise reasons, the spectrometer is typically operated at a time resolution of 4 μs 35,36 . In the present study, the integration time was 65 μs.…”

Broadband 1-GHz mid-infrared frequency comb

“…Such ~100 MHz repetition rate MIR frequency combs have been used in a DCS configuration for atmospheric sensing 7 , trace gas detection [7][8][9][10][11] , and studies related to wildfire spread 12 , where the time scale of the events under study is on the order of seconds. However, a wide range of scientific studies, such as those in combustion [13][14][15] and biological reactions 16 , would benefit from MIR DCS systems with increased speed (μs time resolution), while maintaining broad spectral coverage and high spectral resolution.…”