Shock tube study of nitrogen-containing fuels [electronic resource]
Keywords: Mechanical Engineering.

The combustion chemistry of nitrogen-containing fuels is important in the study of bio-derived fuels and nitrogen-based propellants. However, little high-quality shock tube kinetics data exists for these systems. The primary objective of the research presented in this dissertation is to augment the experimental database and to improve understanding of the chemical kinetics for four nitrogen-containing fuels: morpholine, dimethylamine, ethylamine and monomethylhydrazine. Morpholine (C4H9NO, 1-oxa-4-aza-cyclohexane) is a good representative candidate of a nitrogen-containing fuel because of its cyclic structure and wide industrial applications. Morpholine ignition delay times were measured behind reflected shock waves. A morpholine mechanism was developed based on this shock tube study and previous works in the literature. The simulations from this morpholine mechanism were in good agreement with the current morpholine experiments as well as previous morpholine flame data. Refinement of this morpholine mechanism required improvements in the sub-mechanisms of two major intermediate species dimethylamine and ethylamine, as discussed in a progressive manner in this dissertation. The overall rate constants of hydroxyl radicals (OH) with dimethylamine (DMA: CH3NHCH3) and ethylamine (EA: CH3CH2NH2) were measured behind reflected shock waves using UV laser absorption of OH radicals near 306.7 nm. The overall rate constants were determined by fitting the measured OH time-histories with the computed profiles using the detailed dimethylamine and ethylamine sub-mechanisms contained in the morpholine mechanism. Variational transition state theory was used to compute the H-abstraction rates by OH for dimethylamine and ethylamine. The calculated reaction rate constants are in good agreement with the experiment. The calculated reaction rate constants were used to update the morpholine mechanism for simulations in the following sections. Dimethylamine (DMA) ignition delay times and OH time-histories were investigated behind reflected shock waves. The dimethylamine ignition delay time measurements were carried out in 4% oxygen/argon. OH time-histories were measured in stoichiometric mixtures of 500 ppm DMA/O2/argon. The morpholine mechanism was then updated by adding the DMA unimolecular decomposition channel: DMA = CH3NH + CH3. With this modification, the simulation results are in excellent agreement with both the dimethylamine ignition delay times and OH time-history data. Ethylamine (CH3CH2NH2) pyrolysis and oxidation were studied behind reflected shock waves. For ethylamine pyrolysis, NH2 time-histories were measured in 2000 ppm ethylamine/argon mixtures. For ethylamine oxidation, ignition delay times, NH2 and OH time-histories were measured in ethylamine/O2/argon mixtures. By fitting the simulations to the early time-histories of NH2 and OH, the rate constants for the two major ethylamine decomposition pathways in the morpholine mechanism were updated for better agreement with the experiment. In addition, recommendations from recent theoretical studies of ethylamine radical reactions were implemented. With these modifications, the final updated morpholine mechanism provides significantly improved agreement with the species time-history measurements and the ignition delay times of ethylamine. The morpholine mechanism, after implementing the aformentioned updates based on the dimethylamine and ethylamine data, was compared with the morpholine ignition delay time data again. It was shown that those modifications improve the agreements of the mechanism with the morpholine data. Amine groups are common structural features for rocket propellants as well, and using the same approach as above, the pyrolysis of an important propellant monomethylhydrazine (MMH) was studied using NH2 time-histories in MMH/argon mixtures. The MMH pyrolysis mechanism developed by Sun et al. (2009), with the updates by Cook et al. (2011), was used to compare with the experiment. The rate constant of the reaction: MMH = CH3N.H + NH2 was determined based on early time of the NH2 time-histories. Pressure dependence of this reaction was observed at 0.3-5 atm. The measured reaction rate constants follow a pressure dependence trend close to the theoretical results by Zhang et al. (2011) based on transition state theory master equation analysis. Using the high and low-pressure limit expressions by Zhang et al., a new Troe's expression in the fall-off region was proposed based on the current experimental data. Utilizing the later times of the NH2 time-histories, a new reaction rate expression was recommended for the reaction: NHNH2 + H = NH2 + NH2.