High-temperature laser absorption diagnostics for CH2O and CH3CHO and their application to shock tube kinetic studies

Laser absorption diagnostic methods were developed for the quantitative measurement of formaldehyde (CH2O) and acetaldehyde (CH3CHO) at high temperatures in shock tube kinetic studies. Investigation of the high-temperature CH2O spectrum has shown that the optimal wavelength for CH2O detection using commercially available lasers is near 2896 cm−1. By exploiting the structural difference between the absorption spectra of CH2O and that of broadband interfering species, a two-color (2895.92 cm−1 and 2895.60 cm−1) interference-free detection scheme for CH2O sensing in a combustion environment was developed. A third color (32601.10 cm−1) has also been added to develop a UV/IR detection scheme for combined CH3CHO/CH2O measurements. To implement these schemes, aldehyde cross-sections at all three colors were measured behind reflected shock waves over a wide span of temperatures (600–1800 K) and pressures (0.8–3.6 atm), with an uncertainty of ±5%, and the diagnostic schemes were validated using two controlled experiments with well-established chemistry. Applications of these diagnostics were also demonstrated in shock tube pyrolysis experiments of 1,3,5-trioxane, CH2O and CH3CHO. The unimolecular decomposition rate of 1,3,5-trioxane was determined over 869–1037 K at an average pressure of 2.1 atm: kI = 3.58 × 1012 exp (−18,590 K/T) s−1, with an overall uncertainty of less than 20%.