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Due to the hierarchical nature of high-temperature hydrocarbon oxidation, modeling the combustion chemistry of higher hydrocarbon fuels typically requires a fuel-specific reaction model that describes the fragmentation of the fuel to small species, and a foundational fuel chemistry model that describes the oxidation of these species. Shared by the combustion of large hydrocarbons, the foundational fuel chemistry is also the rate-limiting step and therefore a crucial part for constructing reliable combustion models for any higher hydrocarbons. The dissertation studies the aforementioned problems from both ends. A detailed reaction kinetic model of the foundational fuel combustion is comprised of elementary chemical reactions and their associated rate coefficients. Each of the rate coefficients comes with some uncertainty. The inherent uncertainties of the rate parameters propagate into model predictions and need to be properly quantified. Without characterizing the uncertainty, a reaction model merely represents a feasible combination of rate parameters within a high-dimensional uncertainty space. The Foundational Fuel Chemistry Model (FFCM) is an effort directed at developing a reliable combustion model for the foundational fuels with rate parameters optimized and uncertainty minimized. The first version, FFCM-1, optimized for H2, H2/CO, CH2O and CH4 combustion was constrained with carefully evaluated fundamental combustion data that includes laminar flame speeds, shock tube ignition delay times, shock tube species profiles, and flow reactor species profiles. It has been shown that the model reconciles all the experimental targets chosen and has significantly reduced prediction uncertainties. The remaining kinetic uncertainties in FFCM-1 were further analyzed with extinction and ignition residence time predictions in perfectly-stirred reactor conditions. The reactions that were responsible for the remaining prediction uncertainties were studied with an impact factor analysis over a wide range of temperature, pressure and equivalence ratio. The optimization and uncertainty quantification procedure was then extended to also include temperature dependency by considering the joint optimization of pre-exponential factors and activation energies. An effective temperature was defined for every target and utilized in the response surface derivation. The resulting temperature-dependent uncertainties of key reactions in H2/CO flames in a test problem were discussed. JP10 was studied as a single-component large-fuel example, using the Hybrid Chemistry (HyChem) approach. The HyChem approach assumes a decoupled fuel pyrolysis and oxidation of pyrolysis products. The pyrolysis model is described with highly-lumped steps and optimized against experimental data from shock tube and flow reactor species profiles and shock tube ignition delay. Special attention was paid to the unique molecular structure of the fuel in the lumped pyrolysis reactions, and the overall performance of the model is shown to be satisfactory.
Real fuels usually contain hundreds to thousands of hydrocarbon components. Over a wide range of combustion conditions, large hydrocarbon molecules undergo thermal decomposition first to form a small set (usually less than 10 species) of low molecular weight products, followed by the oxidation of those products, which is usually rate limiting. Hence, the composition of the decomposed products determines the overall global combustion properties. For conventional distillate fuels, the pyrolysis products comprise ethylene (C2H4), hydrogen (H2), methane (CH4), propene (C3H6), 1-butene (1-C4H8), iso-butene (i-C4H8), benzene (C6H6) and toluene (C7H8). From a joint consideration of thermodynamics and chemical kinetics, it is shown that the composition of the thermal decomposition products is a weak function of the thermodynamic condition, the equivalence ratio and the fuel composition within the range of temperatures relevant to high temperature combustion phenomena. In this dissertation study, I demonstrate a hybrid chemistry (HyChem) approach to modeling the high-temperature oxidation of real, liquid fuels. In this approach, the kinetics of fuel pyrolysis is modeled using experimentally derived, lumped reaction steps, while the oxidation of the pyrolysis fragments is described by a detailed foundational fuel chemistry model. A wide range of modeling results are provided to support the approach, including three conventional aviation fuels (JP-8 POSF10264, Jet A POSF10325, JP-5 POSF10289), two rocket fuels (RP2-1 POSF7688, RP2-2 POSF5433), and a bio-derived alternative jet fuel (Gevo alcohol-to-jet fuel, C1 POSF11498). The HyChem models of those fuels were developed using advanced speciation data obtained from shock tubes and a flow reactor, and the models were subsequently tested against global combustion properties, including ignition delay time, laminar flame speed, and flame extinction strain rates across a wide range of pressure, temperature and reactant mixture conditions. Sensitivity analysis of the model predictions with respect to the measurement uncertainties and rate parameter uncertainties of foundational fuel chemistry model is assessed. In this dissertation, the HyChem modeling approach was also extended to three key aspects critical to modeling fuel combustion over an even wider range of condition. First, a modified HyChem model was formulated for capturing the physics in negative temperature coefficient (NTC) and low-temperature oxidation regimes. Sensitivity test and suggestions on future NTC enabled HyChem model development are presented. Second, the HyChem approach was applied to modeling the blend of a conventional Jet A fuel and an alternative, alcohol-to-jet synthetic fuel. The pyrolysis as well as the combustion properties of several blended fuels were predicted by a simple combination of the HyChem models of the two individual fuels, thus demonstrating that the HyChem models for two jet fuels of very different compositions can be "additive" as far as high-temperature properties are concerned. Lastly, I will discuss a case study in which the HyChem model of Jet A is extended to NOx prediction after combining it with a recently updated reaction model of nitrogen chemistry. The combined reaction model is shown to predict NOx formation in premixed stretched-stabilized Jet A flames satisfactorily.
Studies were conducted to finalize a fundamental and predictive reaction model for the combustion of hydrogen and carbon monoxide through a multi-parameter optimization. These studies showed that reliable data of hydrogen and carbon monoxide oxidation at high temperatures can be reconciled be a single kinetic model. An advanced approach to solve the master equation of collision energy transfer was developed to predict the rate constants of combustion reactions of arbitrary complexities. Studies were also conducted on a class of unique combustion reactions that involve spin state crossing-one of the last few unresolved theoretical problems in combustion reaction kinetics.
The goal of this research program was to develop a comprehensive, predictive, and detailed kinetic model of hydrocarbon combustion for aero propulsion simulations. Sensitivity analyses were performed to examine the influences of the uncertainty in binary diffusion coefficients on flame simulations. First-principles calculations were carried out to determine the molecular binary diffusion coefficients of H-He, H2-He, H-H2, and H-Ar gas mixtures. This study resulted in an updated transport property library commonly used in combustion simulations. A new class of radical-chain initiation reactions was discovered for the homogeneous oxidation of unsaturated hydrocarbons. This element of the study utilized advanced quantum chemistry tools, reaction rate theory, and the method of detailed kinetic modeling. This new class of initiation reactions was found to be critical to reaction model development. A detailed reaction model of C1-C4 fuel combustion was updated. The foundation of this model, namely the H2/CO sub-model, was revised completely and optimized. A new method, termed the Sensitivity Analysis Based (SAB) method, was developed for rapid model optimization. The method was shown to be far more efficient than the factorial design method used in previous kinetic model optimization efforts.
Studies were conducted in several relevant areas, including (1) validation of the chemistry and transport models against the extinction of ultra-lean premixed hydrogen-air mixtures, (2) a comprehensive theoretical analysis of the reaction kinetics of carbon monoxide and the hydroxyl radical, (3) a theoretical kinetic study of the decomposition of ethylene oxide; (4) a gas-kinetic analysis for the transport properties of long chain molecules in dilute gases, (5) quantum-chemistry, master equation modeling of the unimolecular decomposition of ortho-benzyne, (6) extension of the previously developed hydrogenicarbon model to combustion pressures as high as 600 atm, (7) an updated kinetic mechanism of small-hydrocarbon fuel combustion for use as a kinetic foundation of higher hydrocarbon combustion, and (8) a methodology for kinetic uncertainty propagation. These projects represent the two key ingredients for meeting the overall project objectives: (a) an accurate physico-chemical property database for combustion kinetics, and (b) a unified and optimized kinetic model for liquid aliphatic and aromatic fuel combustion with quantifiable uncertainties.
Aimed at understanding practical combustion environments, present modeling efforts have been hampered by difficulties related to coupling combustion chemistry to the complex fluid mechanics present. In an attempt to circumvent such difficulties the present research program is aimed at the development of simplified chemical kinetic models (usually termed global models) to represent the combustion chemistry. Initially aimed at simple hydrocarbon fuels the program is progressing to studies of more complex aliphatics as well as important alternative fuels. The objective of this research is multifold: (a) to determine mechanistic oxidation routes of hydrocarbons derived from crudes and alternate sources, so that efficient and environmentally clean power plants based on internal and external combustion processes can be designed; (b) to develop and validate actual simplified (global) reaction rates for these hydrocarbons so that these power plants can be modelled; and (c) to develop an understanding of particulate (soot) formation to permit the rapid and successful introduction of the inexpensive, heavy, highly aromatic fuels. Studies of paraffin, olefin and alcohol hydrocarbons are reviewed. Appropriate global models are presented and compared with experimental data. The results clearly demonstrate that the turbulent flow reactor facility can be used to develop accurate global models for a variety of important fuels.
Lists citations with abstracts for aerospace related reports obtained from world wide sources and announces documents that have recently been entered into the NASA Scientific and Technical Information Database.
The 26th International Symposium on Shock Waves in Göttingen, Germany was jointly organised by the German Aerospace Centre DLR and the French-German Research Institute of Saint Louis ISL. The year 2007 marked the 50th anniversary of the Symposium, which first took place in 1957 in Boston and has since become an internationally acclaimed series of meetings for the wider Shock Wave Community. The ISSW26 focused on the following areas: Shock Propagation and Reflection, Detonation and Combustion, Hypersonic Flow, Shock Boundary Layer Interaction, Numerical Methods, Medical, Biological and Industrial Applications, Richtmyer Meshkov Instability, Blast Waves, Chemically Reacting Flows, Diagnostics, Facilities, Flow Visualisation, Ignition, Impact and Compaction, Multiphase Flow, Nozzles Flows, Plasmas and Propulsion. The two Volumes contain the papers presented at the symposium and serve as a reference for the participants of the ISSW 26 and individuals interested in these fields.
Rocket and air-breathing propulsion systems are the foundation on which planning for future aerospace systems rests. A Review of United States Air Force and Department of Defense Aerospace Propulsion Needs assesses the existing technical base in these areas and examines the future Air Force capabilities the base will be expected to support. This report also defines gaps and recommends where future warfighter capabilities not yet fully defined could be met by current science and technology development plans.