Shock tube / laser absorption study of aldehydes kinetics [electronic resource]
Keywords: Mechanical Engineering.

The world's ever-increasing needs for energy, currently fulfilled primarily by the combustion of hydrocarbon fuels, are demanding development of more efficient and cleaner combustion processes, which ultimately requires improved knowledge of fundamental combustion kinetics. Present in the combustion of most hydrocarbon fuels, aldehydes are important intermediate species that hold key information to such knowledge. Recognizing the critical role of aldehydes in combustion research, the current study presents (1) an advanced aldehydes diagnostic system for use in combustion environments, (2) a set of improved rate constant measurements for several key reactions of aldehydes, and (3) a toolbox that will facilitate future kinetics studies of aldehydes. A system of continuous-wavelength (CW) laser absorption diagnostic methods was developed for the quantitative measurement of formaldehyde (CH2O) and acetaldehyde (CH3CHO) in shock tube kinetic studies. Investigation of the high-temperature CH2O spectrum showed that the optimal wavelength for CH2O detection using commercially available lasers was near 2896 cm−1. By exploiting the structural difference between the absorption spectra of CH2O and that of broadband interfering species, the current study proposed a two-color (2895.92 and 2895.60 cm−1) interference-free detection scheme for CH2O sensing in combustion environments. A third color (32601.10 cm−1) was also added to develop a UV/IR detection scheme for combined CH3CHO/CH2O measurements. Aldehyde absorption cross-sections at all three colors were measured behind reflected shock waves over a wide range of temperatures (600--1800 K) and pressures (0.8--3.6 atm), with an uncertainty of ±5%. The diagnostic system was then validated in two well-controlled experiments, and demonstrated in shock tube pyrolysis experiments of 1,3,5-trioxane, CH2O and CH3CHO. The rate constant of acetaldehyde thermal dissociation, CH3CHO (+M) = CH3 + HCO (+M), was measured behind reflected shock waves at temperatures of 1273 - 1618 K using a sensitive CO diagnostic. Example simulations of existing reaction mechanisms updated with the current rate constant values demonstrated substantial improvements with regard to the acetaldehyde pyrolysis chemistry. The rate constant of the H-abstraction reaction of formaldehyde (CH2O) by hydrogen atoms (H), CH2O + H = H2 + HCO, was also studied behind reflected shock waves using the same CO absorption diagnostic, over temperatures of 1304--2006 K. These experiments were carefully designed to maintain relatively constant H radical concentrations, which significantly boosted the measurement sensitivity of the target reaction and suppressed the influence of interfering reactions. Compared to previous studies, the current work has significantly reduced the measurement uncertainty. The overall rate constants of the H-abstraction reactions of 10 different aldehydes, namely formaldehyde (CH2O), acetaldehyde (CH3CHO), propionaldehyde (C2H5CHO) and n-butyraldehyde (n-C3H7CHO), isobutyraldehyde (i-C3H7CHO), n-valeraldehyde (n-C4H9CHO), isovaleraldehyde (i-C4H9CHO), trans-2-pentenal (C2H5CHCHCHO), trimethylacetaldehyde ((CH3)3CCHO) and benzaldehyde (C6H5CHO), by hydroxyl radicals (OH), were studied behind reflected shock waves at temperatures of 958 -- 1391 K, using UV laser absorption at 306.69 nm. The current study reported the first direct rate constant measurement for C2 and higher aldehydes + OH at temperatures above 1000 K. To aid future kinetics research in shock tubes, a novel toolset of advanced laser absorption diagnostics, namely shock-tube-integrated cavity-enhanced absorption spectroscopy (CEAS), was also developed in the current study. This CEAS technique utilized high reflectivity mirrors directly mounted on the shock tube to enhance the effective optical pathlength in shock tube/laser absorption measurements by factors of about 50 - 90, thereby greatly improving the species detection limits in shock tube kinetics studies. A CW laser CEAS method was explored and applied to ultra-sensitive CO detection in rate constant measurements for the acetone thermal dissociation reaction, CH3COCH3 (+ M) = CH3 + CH3CO (+ M), over 1004-1094 K. A pulsed-laser CEAS was also explored in the current study to develop an improved laser absorption diagnostic for CH3 at 216.62 nm. Example application of this diagnostic was demonstrated in rate constant measurements for the thermal dissociation reaction of methane, CH4 + M = CH3 + H + M over 1487 - 1866 K. This CEAS toolset should prove very useful in future shock tube kinetics studies, including but not limited to, the studies of aldehydes.