Problems in Reaction Networks and Low Temperature Combustion

Abstract

This dissertation consists of two parts, with Part I focusing on aspects of chemical reaction networks, including the construction of kinetic models for hydrocarbon oxidation and the development of a computational algorithm based on the concept of betweenness centrality for numerical combustion diagnostics and reduction of complex reaction networks, and Part II on low temperature combustion, specifically on the characteristics of low-temperature chemistry (LTC) in homogeneous combustion environments and its coupled effect with transport processes and flows under nonpremixed and premixed conditions.

In Part I of this dissertation research, a comprehensive chemical kinetic model for butene isomers in the intermediate-to-high temperature range is developed based on the understanding of C0 to C4 chemistry. High-fidelity nonpremixed counterflow ignition experiments as well as laminar flame speed measurements under various pressures are acquired as the validation targets. Numerical simulations with detailed chemistry and transport under the experimental conditions were conducted, with the results compared with measurements to assess the adequacy of the kinetic models. Satisfactory performance of the model prediction is achieved for the current experimental results as well as the literature data such as those of flat flame speciation, shock tube autoignition delay, and jet-stirred reactor species measurements.

Recognizing the complex nature of chemical kinetics, the prospect of using network science to analyze and simplify complex chemistry is explored. A numerical tool based on the concepts of the shortest path and betweenness centrality (BC) in network science is developed for computational diagnostics and model reduction, and has been applied to various combustion scenarios such methane oxidation, NOx formation, n-heptane LTC, and soot formation to yield useful reaction path information. Furthermore, with the index of importance of species assigned as the global BC from the reactants to the major products, skeletal mechanisms of flexible sizes have been generated by keeping species with higher BC values, showing good capability in reproducing autoignition delays and S-curves in perfectly-stirred reactors (PSR). Comparison of the BC and other methods such as directed relation graph (DRG), directed relation graph with error propagation (DRGEP) and sensitivity analysis (SA), demonstrates its satisfactory performance in generating accurate skeletal mechanisms.

In Part II of the dissertation research, the characteristics of LTC in homogeneous systems as well as the coupled effects with transport processes are investigated, recognizing that LTC has been extensively studied in homogeneous systems, such as shock tubes (ST), rapid compression machines (RCM), jet-stirred reactors (JSR) and flow reactors (FR), and has been demonstrated to be closely related to important combustion features such as the negative-temperature coefficient (NTC) in large hydrocarbon ignition and engine knocks. In this work, the NTC behavior of the first stage autoignition delay and the associated temperature increment are demonstrated and analyzed through Chemical Explosive Mode Analysis (CEMA) and mechanism reduction. The roles of the controlling detailed and global kinetics are revealed, leading to significant simplification in the modeling and description of the entire regime of autoignition.

The study of NTC chemistry in homogeneous systems is then extended to inhomogeneous systems since inhomogeneity through temperature and concentration stratifications invariably exists in realistic situations, resulting in more complexities in the coupled effect of LTC with transport and flows. It is subsequently demonstrated that, contrary to the previous notion that LTC is below and thereby irrelevant to the elevated transport-affected ignition temperature, LTC could become strongly coupled to the diffusive-convective transport processes with high enough pressure and/or low enough residence time, leading to distinctive ignition and extinction events. New classes of flames, namely nonpremixed and premixed cool flames, are observed for a typical fuel, dimethylether (DME), exhibiting LTC in the counterflow system and hence supporting the detailed numerical predictions and providing new targets for the validation of low temperature kinetics.