A mid-IR laser absorption diagnostic for measuring formaldehyde at high pressures and its demonstration in shock tubes

This work presents a new diagnostic for quantitative measurements of formaldehyde (CH2O) in the temperature range of 700–1500 K and pressures from 10 to 60 atm targeted for combustion applications. A two-color, online-offline strategy using laser absorption spectroscopy allows CH2O to be measured without interference from heavy carbonyls, hydrocarbons, water, and other combustion intermediates. The online and offline wavenumbers were chosen as 1745.2 cm−1 and 1738.38 cm−1 (near the Q-branch of formaldehyde's CO stretch), respectively, after a careful line selection process to maximize sensitivity over the temperature (T), pressure (P) range of interest. Absorption cross sections of CH2O diluted in Ar and O2 at these wavelengths were determined by shock-heating 1,3,5 trioxane (C3H6O3) as a formaldehyde precursor in the Stanford high-pressure shock tube between 13 and 52 atm. Analytical expressions for the cross sections at the two colors as a function of temperature and pressure are provided, with an uncertainty of ±5%. Applications of the diagnostic have been demonstrated in three different classes of experiments of interest in combustion studies: 1) Pyrolysis of dimethyl ether (DME)/Ar; 2) pyrolysis of ethanol/Ar; and 3) oxidation of heavy hydrocarbons, including a three-component gasoline surrogate (TPRF-60), two high-performance gasolines, and a jet fuel (Jet A). The measured CH2O time histories were compared with predictions of their respective chemical kinetic models, which provided insights into the deficiencies in these models. To the best of the authors’ knowledge, this work presents the first quantitative measurement of formaldehyde during the oxidation of these fuels at engine-relevant conditions.