Optimization And Reduction Of Combustion Reaction Mechanisms

The combustion of fuels provides necessary energy for human life and promotes the development of human society.Combustion reaction kinetic mechanisms can describe detailed combustion reaction processes,which plays a key role in the effective use of fuel and reducing pollution.However,combustion is a complex physical and chemical process,and detailed reaction kinetic mechanisms usually contain a large number of species and reactions.Reliability of results from numerical combustion simulations is closely related to the integrity of reaction network and accuracy of thermal,kinetic and transport data.It is difficult to measure all the thermodynamic and kinetic data in a detailed mechanism experimentally,and theoretical chemistry methods with high accuracy are generally adopted to determine these data.In addition,if the mechanism is complex and huge,and the time scale of each reaction is very different,it will be difficult to directly couple the mechanism with computational fluid dynamics（CFD）in three-dimensional numerical engine simulation.Therefore,studying key reaction paths use high-precision theoretical chemistry methods and calculating the corresponding thermodynamic and kinetic data are important to optimize combustion reaction mechanisms.In addition,developing new mechanism reduction methods and constructing reduced combustion mechanism for real fuels are important in engine design.This dissertation is divided into three parts:The first part is to study reaction paths and reaction rate constants of combustion of ethanol and propanol using high precision quantum chemistry methods for optimization of the core mechanism.The second part is to develop a new mechanism reduction method to reduce computational cost of mechanism reduction and to be applicable to large-size mechanisms.The third part is the development of a reduced mechanism for combustion of RP-3 aviation fuel surrogate model and its application to combustor numerical simulation of engineering scale.The main content of this dissertation includes the following chapters:In chapter 1,the research background,current status and problems of combustion reaction mechanisms are introduced briefly.In chapter 2,the quantum chemistry methods and reaction rate theory are introduced.The quantum chemistry methods described here include Hartree-Fock method,configuration interaction,coupled-cluster theory and density functional theory.For the reaction rate theory,the traditional transition state theory（TST）for reactions with an energy barrier,variational transition state theory（VTST）for barrierless reactions and the Rice-Ramsperger-Kassel-Marcus/master equation（RRKM/ME）for unimolecular reactions with rate constants dependent on temperature and pressure are discussed.The reduction methods of combustion reaction mechanism and related analysis methods are provided in chapter 3.Three types of mechanism reduction methods including skeletal reduction method,lumping and time scale analysis are presented.As for mechanism analysis,the popular sensitivity analysis method and element flow analysis method are described.In chapter 4,the reaction paths and rate constants of the reactions ofα-hydroxyethyl radical andα-hydroxypropyl radical with oxygen are studied using high-precision theoretical chemistry methods.α-hydroxyethyl radical andα-hydroxypropyl radical are products from H-abstraction reactions of ethanol and propanol,respectively.On the one hand,ethanol and propanol are important intermediates in the combustion process of many hydrocarbon fuels.It is usually necessary to include detailed combustion mechanisms of ethanol and propanol in constructing the core mechanism.Therefore,it is important to study the chemical kinetic mechanism for combustion of alcohol in the development of combustion mechanism of hydrocarbon fuels.On the other hand,due to the increasing environmental pollution and energy crisis,ethanol and propanol as renewable fuels have attracted more and more attention.However,their combustion process will also produce other harmful substances.This requires a clearer understanding of alcohol combustion and its microscopic mechanism.The reaction ofαsite alcohol radical with oxygen is important in the low temperature combustion of alcohols.Reliable reaction paths and reaction rate constants still lack and most of the involved rate constants are estimated from those of other similar reactions.In this chapter,we have studied the reactions ofα-hydroxyethyl radical with oxygen andα-hydroxypropyl radical with oxygen,clarified the corresponding reaction path,and obtained rate constants suitable for a wide range of temperature and pressure.At the same time,these results are added to the Aramco Mech3.0 mechanism and the simulation results of acetone concentration in 2-propanol combustion are improved.In addition,we also give the reaction channel of enol formation during the combustion of propanol.Main innovations:The reaction channels and rate constants obtained in this work can be used to construct the detailed combustion reaction mechanisms of ethanol and propanol,which are helpful for the optimization of the core mechanism.In chapter 5,we propose an improved sensitivity analysis（SA）reduction method.In the traditional SA reduction method,the species with the lowest sensitivity coefficient is deleted in a one-by-one manner in each step.The sensitivity analysis reduction method can thus provide more compact skeletal mechanism compared with other skeletal mechanism reduction methods.If the mechanism contains a large number of species,the traditional SA reduction method will be very expensive.In order to improve the efficiency of mechanism reduction and to be able to reduce mechanisms with a large number of species,we have proposed an improved SA reduction method.In this method,species with strong coupling are identified firstly,and these paired species are regarded as one species in the mechanism reduction.In addition,several species are deleted at the same time in each step of reduction based on their sensitivity coefficients and they are deleted in an incremental manner in the improved SA method.In order to further reduce the calculation time of SA method,ignition delay times under different conditions are simulated in a parallel way using MPI.Compared with the traditional SA method,the computational cost of this method is reduced significantly.It can be applied to reduction of large-scale mechanisms containing many species and reactions.Using the improved SA method,we have reduced the detailed combustion mechanism of C₈-C₁₆alkanes with 2115 species and 8157 reactions developed by Lawrence Livermore National Laboratory（LLNL）.The n-decane skeletal mechanism with 126 species and n-dodecane skeletal mechanism with 162species are obtained respectively.Compared with the detailed mechanism,the skeletal mechanisms can provide reasonable results on some important combustion characteristics.This indicates that the improved SA method is applicable to reduction of large-scale mechanisms and reliable and compact skeletal mechanisms can be obtained.Main innovations:The improved SA method has been developed,which improves the efficiency of mechanism reduction and can reduce huge and complex mechanisms.In chapter 6,the reduction and validation of a combustion mechanism for surrogate model of RP-3aviation kerosene are carried out.Some combustion mechanisms for RP-3 fuel have been developed.However,some mechanisms with small size are only applicable to high-temperature combustion behaviors of RP-3.Other mechanisms that can describe the combustion characteristics at both high and low temperatures contain a large number of species and reactions,and they are difficult to be used in numerical combustion simulation.Therefore,developing a reduced mechanism that can describe combustion characteristics of RP-3 fuel under wide conditions is still the focus of research.Based on the physical and chemical properties of the real aviation kerosene,we select n-dodecane,1,3,5-trimethylcyclohexane and n-propylbenzene（0.73/0.147/0.123 by mass fraction）as surrogate components.A RP-3 high temperature combustion mechanism is reduced by using the newly-developed sensitivity analysis reduction method,and low-temperature combustion reaction paths are added to this skeletal mechanism.A 44-species mechanism of RP-3 aviation kerosene which can describe combustion characteristics of both low and high temperature in a wide range of conditions is obtained.Ignition delay times and laminar flame speeds are simulated with the 44-species mechanism and other aviation kerosene mechanisms,and these results are compared with experimental values.In addition,sensitivity analysis is carried out to analyze these mechanisms.Simulation and analysis results show that the 44-species mechanism can give reasonable results.Moreover,the44-species mechanism is further employed in numerical simulations for scramjet and ramjet engines,and the results are in good agreement with experimental data.By using the 44-species mechanism in CFD simulation,practical problems of aviation fuel application in engine can be further explored.Main innovations:The 44-species mechanism of RP-3 aviation kerosene is developed and applied to combustor numerical simulation of engineering scale.