Chemical Kinetic Mechanism Development and Optimization for Conventional and Alternative Fuels
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Beschreibung
The increasing demand for cleaner combustion and reduced greenhouse gas emissions motivates research on the combustion of hydrocarbon fuels and their surrogates. Accurate detailed chemical kinetic models are an important prerequisite for high fidelity reacting flow simulations capable of improving combustor design and operation. The development of such models for many new fuel components and/or surrogate molecules is greatly facilitated by the application of reaction classes and rate rules. Accurate and versatile rate rules are desirable to improve the predictive accuracy of kinetic models. This thesis focuses on the development of accurate chemical reaction schemes. For this, kinetic models are constructed based on the concept of reaction classes and rate rules for fuels of interest, an automatic model optimization approach is developed by calibrating rate rules, and optimized rate rules are proposed for future development of reaction models. In the first part of this thesis, the methodology of reaction classes and rate rules is introduced. The methodology categorizes all possible fuel-specific reactions as classes of reactions with prescribed rules for the rate constants. This ensures consistency in the chemical mechanism. Based on this approach, detailed kinetic models are derived for two promising alternative fuels, di-n-butyl ether (DBE) and n-octanol. The developed models show good agreement with experimental data and provide a basis for the investigation of long chain ethers and alcohols. It is found that, for the linear DBE, the weak α C-H bonds enhance the hydrogen migration reactions across the central oxygen atom and thus promote the low temperature chain branching routes in conjunction with the low barrier of ketohydroperoxide decomposition. For n-octanol, the α radicals react with molecular oxygen yielding aldehydes instead of peroxy radicals, which retards the fuel reactivity. Next, a more appropriate method for automatic calibration of chemical kinetic models, which can be used when specific information of particular elementary reactions is lacking, is developed by performing optimization of reaction rate rules instead of elementary reactions. The methodology is demonstrated first by optimizing an n-pentane combustion mechanism and further applied to calibrate the rate rules, which have been employed to construct the multi-component chemical mechanism that is in this study the mechanism of n-heptane and iso-octane. The calibration applied only to rate rules provides equally good performance in comparison with the optimization of the single elementary reactions, yet it leads to a chemically consistent mechanism. The methodology reduces the number of parameters that need to be considered and therefore enables also optimization of cases, where a large number of reactions appears as important, as is the case for low temperature auto-ignition of larger aliphatic fuels. The computational effort is further advanced for the case of calibrating the multi-component mechanism. As the oxidation chemistry of n-heptane and iso-octane consists essentially of identical reaction steps at similar experimental conditions, the number of uncertain rate rules remains unchanged, when the oxidation of binary fuel blends is investigated instead of neat fuel ignition. Following this, the possibility to develop optimized universal rate rules, with which accurate chemical mechanisms can be derived for a set of fuels rather than for only one particular fuel, is examined. It is demonstrated that rate rules can be used and consistently optimized for a set of normal alkanes including n-heptane, n-octane, n-nonane, n-decane, and n-undecane, thereby improving the predictive accuracy for all the considered fuels. The optimized rate rules are subsequently applied to generate a mechanism for n-dodecane, which was not part of the training set for the optimized rate rules. The developed mechanism shows accurate predictions compared with published well-validated mechanisms for a wide range of conditions.
Autor*in
Liming Cai
Themen in »Chemical Kinetic Mechanism Development and Optimization for Conventional and Alternative Fuels«
Alternative Fuels Chemical kinetics Conventional Fuels