| Deep understanding on the reactions of aviation fuel combustion and soot formation at molecular level is vital for improvement of fuel combustion efficiency,particulate emission control and for design of aviation engines.Since the components of aviation fuel and the molecular structures of soot particles are very complicated,it is challenging to fully understand the chemical reaction mechanism therein.ReaxFF MD is a computational approach combining the novel reactive force field ReaxFF with molecular dynamics(MD)simulations.The ReaxFF force field was developed on the basis of bond order,which allows for the descriptions of breaking and formation of chemical bonds for chemical reactions in a reactive molecular model.The accuracy of ReaxFF force field is close to DFT method with much less computational cost than DFT.Molecular models of?1,000-10,000 atoms or even larger models have been used in ReaxFF MD simulations for varied applications.Driven by the reactive potential,pre-defined reaction pathways are not required in ReaxFF MD simulations.We believe that ReaxFF MD simulation is a potential new method to reveal the underlying complex reaction pathways occurred in the combustion of aviation fuels and the generation of soot particles.This thesis attempts a new approach to reveal chemical reaction pathways occurred in the combustion of aviation fuel and the formation of soot particles by ReaxFF MD simulations of complex molecular models of RP-1.By using the leading codes of GMD-Reax for high performance computing and VARxMD for reaction analysis,the comprehensive chemical reaction details were revealed for the combustion of aviation fuel and the soot formation process.A complex RP-1 molecular model containing 24 fuel components and a 3-component surrogate model were constructed.The components of the RP-1 models were determined on account of experimental data of RP-1 fuel reported in literatures.By a series of ReaxFF MD simulations for RP-1 fuel in pyrolysis and oxidation conditions,reactivity differences were discussed between the two RP-1 fuel models,especially for the heat-up simulations in the pyrolysis condition.It can be obtained that the weight fraction evolution of fuel molecules and the important product ethylene are similar between the two RP-1 models.However,RP-1 component consumption is slightly slower in the 3-component surrogate model than that of the 24-component model,and the weight fraction difference can be as much as 20%between the two RP-1 models in pyrolysis conditions.Then RP-1 fuel components in the two RP-1 models are classified into three categories as normal paraffins,branched paraffins and cyclic hydrocarbons.Cyclic hydrocarbons contribute mostly to the RP-1 evolution differences,because the cyclic hydrocarbons in the 3-component surrogate model are stable and their weight fraction is up to 43.6%that is higher than that of the other two components in the surrogate.By investigating the overall chemical reaction pathways of RP-1 fuel pyrolysis of the two RP-1 models with the aid of VARxMD,the differences of RP-1 pyrolysis reaction pathways between the two RP-1 models were revealed.For the branched paraffins,the heptamethylnonane in the 3-component surrogate contains multi-branched structure of quaternary carbon,the branched alkene of 2-methylpropene will be formed in the pyrolysis,which is not a major pyrolysis product for the 24-component model.Most of the branched paraffins in the 24-component model are characterized by tertiary carbons,their alkene pyrolysis products are linear alkenes.For the cyclic hydrocarbons,the methylcyclohexane in the 3-component model is mono-ring structure containing very short side chain,its ring-opening reaction pathways are relatively simple.However,the cyclic hydrocarbon fuel components in the 24-component model consist of double-ring structures and mono-ring structures with complex branched side chains.More versatile ring-opening pathways can be observed.This work can provide clues for further optimizing the fuel components in the 3-component surrogate model.More importantly,this work proposes a potential computational approach for evaluating chemical properties of exising surrogate fuel models.Reaction pathways for the nucleation of soot nanoparticles from polycyclic aromatic hydrocarbon(PAH)precursors were investigated by ReaxFF MD simualtions in pyrolysis conditions.A model of coronene was constructed and used as a representative PAH precursor in soot formation process.The 1 ns simulations indicate that the growth of the maximal nanoparticles observed can be promoted at 2600-3000 K with temperature increasing,but its C number decreases slightly at higher temperature conditions.In addition,the increasing of temperature contributes to the growth of maturity of the nanoparticles in terms of H/C ratio and morphology.At 3400 K,chemical reaction pathways of soot nanoparticle formation from PAHs can be observed as follows.The C-C bonds formed between PAH molecules will result in the formation of PAH dimers or even larger oligomers,which suggests the nucleation of incipient soot nanoparticles.The size of incipient nanoparticles will increase by reactions with oligomers,PAHs and acyclic molecules.The continuous size increase of the nanoparticle will result in the formation of amorphous nanoparticles with long side chains and large rings.The size of PAH sheets in the nanoparticle will increase by inter-bridging of C-C atoms inside the large rings.The decrease of length and number of side chains will occur in the nanoparticle.The continuous size decrease of these side chains will lead to well-formed soot nanoparticles.These simulation results uncover the chemical structure evolution of soot nanoparticle formation from PAHs at all-atom level,which help for enriching the knowledge of the reaction pathways for soot formation.By adding oxygen molecules to the same 24-component RP-1 fuel models,the overall scenario of reaction pathways of soot nanoparticle formation was investigated by performing the long time 5-10 ns ReaxFF MD simualtions of RP-1 oxidation at fuel-rich conditions.The soot nanoparticle formation obtained from the simulations can be divided into three stages.The first stage is characterized with incipient ring formation from fuel molecules and the ring growth.The incipient ring formation was initiated by ring closure reactions of long acetylene-like hydrocarbon chains generated from dehydrogenation of fuel species.Then PAH-like molecules with side chains can be formed by bridged C-C bonds of the incipient ring structures,leading to the ring condensation and resulting in the increase of ring number.The reaction pathways for the formation of PAH-like molecules from fuel molecules obtained in this work may suggest new ring formation pathways beyond the well-known HACA mechanism of PAH generation.In the second stage,nucleation can be observed through the C-C bond formation reactions of side chains between two adjacent PAH-like molecules.Then large rings form by further C-C bonding between side chains of the two PAH-like molecules.The continuous ring condensation through the internal bridging between carbon atoms of the formed large rings leads to the born of new and large PAH-like core.This observation indicates that the side chains of PAH-like structures play significant role in the nucleation of soot nanoparticles.The third stage corresponds to the graphitization of the soot nanoparticles.Slow reorganization occurring in the whole carbon skeleton of the soot nanoparticles can be observed in this stage from irregular shaped nanoparticles to well-formed fullerenic nanoparticles,through the long time transformation reactions from C5 and C7 rings into C6 rings via the intermediates of C3 rings.Systematic investigations on the simulation condition effects of temperature(1500-4000 K),equivalence ratio(0.5-10),and density of 0.01-0.5 g/cm3 were also carried out by performing long time ReaxFF MD simulations up to 5 ns.The work reveals the overall scenario of molecular structures and chemical reactions in soot nanoparticle formation from fuel molecules in fuel rich RP-1 oxidation condition at full atom simulations,which is of help for enriching the knowledge of reaction details of soot formation mechanism.This work demonstrates that ReaxFF MD simulations can provide a promising alternative approach for enriching the available knowledge of complex reaction pathway details in RP-1 combustion and soot nanoparticle formation at molecular levels. |
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