Shock tube measurements of oxygenated fuel combustion using laser absorption spectroscopy [electronic resource]
Keywords: Mechanical Engineering.

In the current engine development, fuel reformulation is considered as one of the potential strategies to improve fuel efficiency, reduce petroleum consumption, and minimize pollutant formation. Oxygenated fuels can be used as neat fuels or additives in spark-ignition and diesel engines to allow for more complete combustion. To understand the influence of oxygenated fuels on engine performance, accurate comprehensive kinetic mechanisms, which can consist of hundreds to thousands of elementary reactions, are needed to describe the chemistry of the combustion events, such as autoignition and pollutant formation. The primary objective of the research presented in this dissertation is to provide reliable experimental kinetic targets, such as ignition delay times, species time histories, and direct reaction rate constant measurements, using shock tube and laser absorption techniques in order to evaluate and refine the existing kinetic mechanisms for two different types of oxygenated fuels (i.e., ketones and methyl esters) and to reexamine the kinetics of the H2 + OH reaction. The topics of this work are mainly divided into three sections: (1) H2 + OH kinetics, (2) ketone combustion chemistry, and (3) methyl ester + OH kinetics. The reaction of OH with molecular hydrogen (H2) H2 + OH → H2O + H (1) is an important chain-propagating reaction in all combustion systems, particularly in hydrogen combustion, and its direct rate constant measurements are discussed in the first part of this dissertation. The rate constant for reaction (1) was measured behind reflected shock waves over the temperature range of 902-1518 K at pressures of 1.15-1.52 atm. OH radicals were produced by rapid thermal decomposition of tert-butyl hydroperoxide (TBHP) at high temperatures, and were monitored using the narrow-linewidth ring dye laser absorption of the well-characterized R1(5) line in the OH A--X (0, 0) band near 306.69 nm. Consequently, this work aims to report the rate constant for reaction (1) with a much lower experimental scatter and overall uncertainty (as compared to the data available in the literature). Ketones are important to a variety of modern combustion processes. They are widely used as fuel tracers in planar laser-induced fluorescence (PLIF) imaging of combustion processes due to their physical similarity to gasoline surrogate components. Additionally, they are often formed as intermediate products during oxidation of large oxygenated fuels, such as alcohols and methyl esters. In the second part of this dissertation, the combustion characteristics of acetone (CH3COCH3), 2-butanone (C2H5COCH3), and 3-pentanone (C2H5COC2H5) are discussed in the context of the reflected shock wave experiments. These experiments were performed using different laser absorption methods to monitor species concentration time histories (i.e., ketones, CH3, CO, C2H4, CH4, OH, and H2O) over the temperature range of 1100-1650 K at pressures near 1.6 atm. These speciation data were then compared with the simulations from the detailed mechanisms of Pichon et al. (2009) and Serinyel et al. (2010). Consequently, the overall rate constants for the thermal decomposition reactions of acetone, 2-butanone, and 3-pentanone CH3COCH3 (+ M) → CH3 + CH3CO (+ M) (2) C2H5COCH3 (+ M) → Products (+ M) (3) C2H5COC2H5 (+ M) → Products (+ M) (4) were inferred by matching the species profiles with the simulations from the detailed mechanisms at pressures near 1.6 atm. In addition, an O-atom balance analysis from the speciation data revealed the absence of a methyl ketene removal pathway in the original models. Furthermore, the overall rate constants for the reactions of OH with a series of ketones CH3COCH3 + OH → CH3COCH2 + H2O (5) C2H5COCH3 + OH → Products (6) C2H5COC2H5 + OH → Products (7) C3H7COCH3 + OH → Products (8) were determined using UV laser absorption of OH over the temperature range of 870-1360 K at pressures of 1-2 atm. These measurements included the first direct high-temperature measurements of the overall rate constants for reactions (6)-(8), and were compared with the theoretical calculations from Zhou et al. (2011) and the estimates using the structure-activity relationship (SAR) (1995). Biodiesel, which consists of fatty acid methyl esters (FAMES), is a promising alternative to fossil fuels. The four simplest methyl esters include methyl formate (CH3OCHO), methyl acetate (CH3OC(O)CH3), methyl propanoate (CH3OC(O)C2H5), and methyl butanoate (CH3OC(O)C3H7), and their combustion chemistry is a building block for the chemistry of large methyl esters. In the third part of this dissertation, the rate constant measurements for the reactions of OH with four small methyl esters are discussed: CH3OCHO + OH → Products (9) CH3OC(O)CH3 + OH → Products (10) CH3OC(O)C2H5 + OH → Products (11) CH3OC(O)C3H7 + OH → Products (12) These reactions were studied behind reflected shock waves using UV laser absorption of OH over 876-1371 K at pressures near 1.5 atm. This study presented the first direct high-temperature rate constant measurements of reactions (9)-(12). These measurements were also compared with the estimated values from different detailed mechanisms and from the structure-activity relationship (SAR).