Author: Chi, Yicheng
Title: Ab initio chemical kinetics and flickering flame dynamics of n-alkane fuel combustion
Advisors: Zhang, Peng (ME)
Wen, Chih-yung (AAE)
Degree: Ph.D.
Year: 2022
Subject: Combustion
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xxii, 166 pages : color illustrations
Language: English
Abstract: The thesis consists of two parts. In Part 1, the ab initio chemical thermodynamics and chemical kinetics of typical combustion reactions of large straight-chain alkanes were investigated computationally. In Part 2, the dynamical behaviors of coupled flickering buoyant diffusion flames of alkane fuels were investigated experimentally. Four main studies and achievements are summarized as follows.
1. Accurate thermochemical data are of great importance in developing quantitatively predictive reaction mechanisms for transportation fuels, which are primarily composed of large hydrocarbon molecules, especially of long-chain alkanes containing more than 10 carbon atoms. The thesis presents an ONIOM[QCISD(T)/CBS:DFT]-based theoretical thermochemistry study on the hydrogen abstraction reactions of straight-chain alkanes, n-CnH2n+2 + R (n = 1-16, R = H, OH, HO2). These reactions (n ≥ 10) are computationally intractable for the prevalent high-level ab initio methods but are readily dealt with by the ONIOM-based method. The calculated results are in very good agreement with those obtained by using the widely accepted high-level QCISD(T)/CBS method, and their discrepancies are generally less than 0.10 kcal/mol. The present results demonstrate that the ONIOM-based method provides an accurate and efficient approach for the computational thermochemistry of large straight-chain hydrocarbon molecules in transportation fuels.
2. Besides the single-point energy, the partition function is another crucial factor in the calculation of thermochemistry and chemical kinetics of large straight-chain alkanes. In the present thesis, we aimed to propose a systematic method to assess and explain the performance of variants of the MS-T (the multi-structural approximation with torsional anharmonicity) method which have widely used for large molecules. First, we proposed the simplest variant MS-2NN (two nearest neighborhood torsions are coupled) and systematically validated it for large alkanes n-CnH2n+2 (n=6-10) and their transition states of hydrogen abstraction reactions. Second, we proposed a metric-based method to explain the underlying reason for the good performance of MS-2NN ⎯ it includes the torsional conformers that have dominant contributions to the partition function calculations. These conformers are closer to the lowest-energy conformer in the space of dihedral and energy metrics. Third, the same observation and explanation apply to the other two variants, MS-2DT (any two torsions are coupled) and MS-3DT (any three torsional are coupled), which contain increasingly more torsional conformers than MS-2NN but are subsets of the complete set of torsional conformers considered by the MS-T method. Overall, the present method provides a mathematically rigorous and computationally effective diagnosis tool to assess various MS-T methods dealing with the torsional anharmonicity of large molecules in partition function calculation.
3. Torsional modes within a complex molecule containing various groups are often strongly coupled so that the harmonic approximation and one-dimensional torsional treatment are inaccurate to evaluate their partition functions. Although the MS-T method has been proposed to deal with the torsional anharmonicity, it approximates the exact "almost periodic" potential energy as a summation of local periodic functions with symmetric barrier positions and heights. In the present thesis, we illustrated that the approximation is inaccurate when the torsional modes present non-uniformly distributed local minima. Therefore, we proposed an improved method (MS-ASB) to reconstruct the approximate potential to replace the periodic potential by using the information of the local minima and their Voronoi tessellation.
4. Regardless of the noteworthy progress in experimental progress in discovering dynamical behaviors of coupled multiple flickering diffusion flames, there are two major deficiencies in the existing experimental studies to be solved and the understanding of the multiple coupled flickering diffusion flames is still in the infant stage. Consequently, we established a well-controlled gaseous n-alkane diffusion flame experiment which well remedies the deficiencies of prevalent candle-flame experiments, and we developed a Wasserstein-space-based methodology for dynamical mode recognition, which is validated in the present triple-flame systems but can be readily generalized to the dynamical systems consisting of an arbitrary finite number of flames. By use of the present experiment and methodology, seven distinct stable dynamical modes were recognized, such as the in-phase mode, the flickering death mode, the partial flickering death mode, the partial in-phase mode, the rotation mode, the partial decoupled mode, and the decoupled mode. These modes unify the literature results for triple flickering flame system in the straight-line and equal-lateral triangle arrangements. Compared with the mode recognitions in physical space and phase space, the Wasserstein-space-based methodology avoids personal subjectivity and is more applicable in high-dimensional systems, as it is based on the concept of distance between distribution functions of phase points. Consequently, the identification or discrimination of two dynamical modes can be quantified as the small or large Wasserstein distance, respectively.
Rights: All rights reserved
Access: open access

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