Shock Tube/Laser Absorption Measurements of the High-Temperature Spectra and Decomposition of Propyl Ethers

This work presents measurements of temperature-dependent absorption spectra and thermal decomposition rates of propyl ethers, specifically di-n-propyl ether (DnPE) and diisopropyl ether (DiPE), which are two renewable fuel candidates. We employed a broadband rapid-tuning MIRcat-QT laser, operating in the scan/fixed-wavelength mode in combination with a shock tube. Spectral measurements were performed over the wavelength range of 8.4-11 μm (909.1-1190.5 cm^-1), covering the strongest infrared absorption bands of the studied ethers, at temperatures of 559-853 K and pressure near 1 bar. These high-temperature spectra help in selecting the optimum wavelength for sensitive and selective measurements of the target ethers. Based on the criteria of high sensitivity, minimum interference, and insensitivity to temperature and pressure variations, we selected a wavelength of 1121.82 cm^-1 for high-temperature diagnostics of DnPE and DiPE. Absorption cross sections at the selected wavelength of 1121.82 cm^-1 were measured over 550-1500 K, and pressures ranging from 0.3-1.4 bar. This diagnostic was then applied to study the high-temperature pyrolysis of these ethers by measuring their time histories behind the reflected shock waves. Our experimentally measured overall decomposition rate coefficients for DnPE and DiPE are given as (unit of s^-1) k_DnPE = 1.25 × 10²⁷ × T^-3.483 × exp(-37620 K/T) and k_DiPE = 5.26 × 10²³ × T^-2.857 × exp(-32360 K/T).