Abstract
A dual-frequency-comb spectrometer based on two quantum-cascade lasers is applied to kinetics studies of formaldehyde (HCHO) in a shock tube. Multispectral absorption measurements are carried out in a broad spectral range of 1740–1790 cm–1 at temperatures of 800–1500 K and pressures of 2–3 bar. The formation of HCHO from thermal decomposition of 1,3,5-trioxane (C3H6O3, 0.9% diluted in argon) and the subsequent oxidation of formaldehyde is monitored with a time resolution of 4 µs. The rate coefficient of the decomposition of C3H6O3 (i.e., HCHO formation) is found to be k1 = 6.0 × 1015 exp(− 205.58 kJ mol−1/RT) s–1. For the oxidation studies, mixtures of 0.36% C3H6O3 and 1% O2 in argon are used. The information of all laser lines, along with the consideration of individual signal variance of each line, is utilized for kinetic and spectral analysis. The experimental kinetic profiles of HCHO are compared with simulations based on the mechanisms of Zhou et al. (Combust Flame, 197:423–438, 2018) and Cai and Pitsch (Combust Flame, 162:1623–1637, 2015).
Highlights
Time-resolved diagnostics of combustion processes are a prerequisite for the development and validation of chemical reaction mechanisms that are required for the development and optimization of combustion systems
The pyrolysis of 1,3,5-trioxane and the subsequent formation of formaldehyde has been studied in the shock tube for the temperature range of 800–1300 K and pressures between 2 and 3 bar
A quantum-cascade dual-frequency-comb spectrometer emitting in the spectral range of 1740–1790 cm−1 is successfully applied for spectroscopic and kinetic studies of HCHO in the temperature and pressure ranges of 800–1500 K and 2–3 bar
Summary
Time-resolved diagnostics of combustion processes are a prerequisite for the development and validation of chemical reaction mechanisms that are required for the development and optimization of combustion systems. Shock tubes are routinely used for ultra-fast investigations of compressible flow phenomena, gas-phase (combustion) reactions, and explosions. To unravel the complex dynamics of the involved physico-chemical processes, time-resolved monitoring of temperature, total pressure, and concentrations of key species is required. The low repetition rate of experiments and the high costs involved in operating shock tubes require the simultaneous determination of multiple quantities as a function of time in a single experiment. Most of the established spectroscopic techniques for measuring these quantities, such as, e.g., tunable diode laser absorption spectroscopy (TDLAS) [1], are based on narrowband laser sources.
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