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Westbrook CHEMKINWorkshop2006

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    Modeling Strategiesfor Advanced Automotive Engine

    and Fuels Research

    Workshop onChemkin in Combustion

    Charles K. WestbrookAugust 6, 2006

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    This is a very exciting period for

    combustion modeling Computer power is growing rapidly

    Combustion models on supercomputers Single CPU capabilities are growing

    Multi-dimensional combustion codes areincluding more detail in more submodels

    Chemical kinetic mechanisms are beingpublished for fuels of much greatercomplexity and size than ever

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    Practical combustion problems are complex

    Engines are 3D, complicated 3D CFD calculations are expensive Chemical kinetics calculations are expensive Radiation transport calculations are expensive Liquid spray, multiphase problems are expensive Particulate, soot calculations are expensive

    Expensive means computer time, computer size,model development time

    Usually significant model simplifications are made

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    Combustion Modeling Challenges

    Many past models have avoided a full 3D, multiphase,detailed kinetics, radiation transport, complex

    geometry, treatment We have emphasized detailed kinetics of fuel

    reactions, with 0D (ignition) or 1D (laminar ame)geometry.

    Clever formulation of these 0D and 1D problems hasanswered some very challenging engine questions

    We have extended chemical models to mechanismsmany times more complex than in the past

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    Causes and implications of ammability limits

    Law and Egolfopoulos for atmospheric pressure ames Basic concept is competition between 2 reactions

    H + O 2 = O + OH R1H + O 2 + M = HO 2 + M R2

    Rates of these reactions have different temperature and

    pressure dependence, and for atmospheric pressure,lean limit occurs at adiabatic ame temperature whereR2 becomes faster than R1

    Currently a topic of considerable attention at high pressure

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    Chemical classes being modeled in combustion

    HydrocarbonsMethane, ethane, paraffins through decaneNatural gas

    Alcohols (e.g., methanol, ethanol, propanol

    Other oxygenates ( e.g., dimethyl ether, MTBE, aldehydes ) Automotive primary reference fuels for octane and cetane ratings Aromatics (e.g., benzene, toluene, xylenes, naphthalene )

    OthersOxides of nitrogen and sulfur (NOx, SOx)Metals (Aluminum, Sodium, Potassium, Lead)Chlorinated, brominated, fluorinated speciesSilane

    Air toxic speciesChemical warfare nerve agents

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    Kinetic modeling covers a wide range of systems

    Types of systems studied

    Flames Waste incinerationShock tubes Kerogen evolutionDetonations Oxidative couplingPulse combustion Heat transfer to surfacesFlow reactors Static reactorsStirred reactors IgnitionSupercritical water oxidation Soot formationEngine knock and octane sensitivity Pollutant emissionsFlame extinction Cetane number

    Diesel engine combustion Liquid fuel spraysCombustion of metals HE & propellant combustionCW agent chemistry Gasoline, diesel, aviation fuelsCatalytic combustion CVD and coatingsMaterial synthesis Chemical process controlmany others . . . .

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    Laminar ames in quenching problems

    Bulk quenching in direct injection stratied charge(DISC) engine

    Bulk quenching due to volume expansion in lean

    mixtures Flame quenching at lean and rich ammability limits

    Flame quenching on cold walls and unburnedhydrocarbon emissions from internal combustion

    engines Flame inhibition

    Soot production and reduction

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    Combustion chemistry modeling needsChemistry models for transportation fuels

    Extend chemistry models for these fuels by adding

    complexity and realism with new chemicalcomponents

    Our chemistry modeling needs are somewhatincremental, and we must validate each newcomponent species as it is added to the overallmodel

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    Chemical Kinetic ModelContains a large database of:

    Thermodynamic properties of species

    Reaction rate parameters

    Number of species:

    Number of reactions:

    7 30 100 450

    25 200 400 1500

    Fuel: H 2 CH4 C3H8(Propane)

    C6H14(Hexane)

    C16H34(Cetane)

    1200

    7000

    Size of mechanism grows with molecular size:

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    Most fuels of interest consist of complexmixtures of many chemical species

    Natural gasGasoline

    DieselJet fuelRocket fuel

    These fuels contain many componentsthat do not have detailed mechanismsGasoline, diesel and jet fuel have hundredsof components (even natural gas)

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    There are 10more pages of speciesconcentrations

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    Gasoline has many components

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    Gasolinehas many

    branchedalkanes

    Gasoline is lower incycloalkanes

    Jet fuel has the

    highestn-alkane

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    Natural gas is the easiest case

    For natural gas, simplified fuels are generallyaccepted (e.g. 95% methane, 4 % ethane, 1

    % n-butane)For liquid fuels, choosing a substitute is moredifficult:

    GasolineDieselJet fuel

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    Development of a Simplified Fuel

    Mechanism for SimulationsClasses of compounds in gasoline and diesel fuel:

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    Approaches to Surrogate fuelsHave one or more fuel components torepresent each chemical class of componentsSurrogate fuel should be able to predictdesired combustion and physicalproperties, e.g.:

    Ignition properties

    Flame speedsSooting tendencyOthers ???

    Produce reduced kinetic model as needed

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    Need to add new species for specific

    applications and conditionsLarger hydrocarbon molecules, with theirsignificantly larger reaction mechanisms

    New species for liquid fuels, use existing techniques

    Mechanisms for individual species must each bevalidated thoroughly (Comprehensive mechanism)

    Purely kinetic tests, including shock tubes, flowreactors, flame speeds, stirred reactors

    Applied tests in application environments

    Relevant pressure and temperature rangesidentified for each type of application

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    Prior simplified versions of diesel/gasoline

    n-heptane: frequently used to representdiesel fuel. Has similar cetane no. (55) todiesel fuel

    n-heptane/iso-octane: primaryreference fuels for gasoline. Some successas a substitute for gasoline under HCCIconditions and engine knock, someproblems too

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    Work in progressTeams of kinetics modeling researchers are working to

    produce these surrogate fuel models

    Recent example:Violi et al., CST 174, 399 (2002), surrogates for JP-8

    a. iso-octane, MCH, m-xylene, dodecane, tetralin andtetradecane

    b. iso-octane, MCH, toluene, decane, dodecane,tetradecane

    Used semi-detailed mechanisms from Ranzi et al.Included boiling point and other physical properties

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    n-Heptane mechanism validationShock tube ignition

    Stirred reactor

    Flow reactor

    Rapid compression machine

    Laminar flame

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    Shock tube ignition, higher

    pressure, lower temperatures

    660K - 1300K, = 0.5, 1.0, 2.0 P = 6, 13.5, 40 bar

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    Reference fuels for cetane number in Diesel combustion

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    We are ready to provide additional complexity

    to our simplified diesel fuel We have used n-heptane due to its ignition

    properties and cetane number n-heptane has no aromatic characteristics Aromatics ignite more slowly than n-heptane To simulate ignition timing of diesel fuel with

    aromatic components, we will have to include acomponent more reactive than n-heptane

    Solution is to combine aromatics and dodecaneor hexadecane

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    Diesel surrogate fuel in the future

    Made up of straight-chain alkanes, branched-chain alkanes,

    cyclic alkanes, simple aromatics, alkylated aromatics,

    polycyclic aromatics and others

    Example test: Surrogate diesel:

    n-alkane: n-hexadecane, n-dodecane or n-decane

    branched chain component: iso-octane or branched heptane

    cyclic alkane component: cyclohexane or methyl cyclohexane

    aromatic component: toluene or mixture of xylenes

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    In some classes, we have many examples

    of fuels with reaction mechanisms

    n-paraffinsCH4 (methane) through nC 16 H34 (n-hexadecane)

    iso-paraffinsall isomers through octanes, selected larger iso-paraffins

    Large variety of olefins through C8 andselected larger species

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    We can have mechanisms for many

    oxygenated componentsMethanol, ethanoldimethyl ether,dimethoxymethaneMethyl butanoate(surrogate forbiodiesel)TPGME (tripropyleneglycol monomethylether)DBM (di-butyl

    maleate)DGE (diethyleneglycol diethyl ether)

    Under developmentO

    O

    O

    CH 3 OHC

    O

    CH

    O

    O

    CH 3

    O

    O

    OH

    O

    O

    O

    O OO

    OH

    OH

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    We now have more components to

    represent classes of hydrocarbonstoluene (aromatics)

    methylcyclohexane(cycloalkanes)

    diisobutylene(alkenes)

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    New componentsdiisobutylene

    low temperaturechemistry

    o-xylene,m-xylene,p-xylene

    -methylnaphthalene

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    A five component surrogate to

    represent gasolinen-heptane (straightchain alkanes)

    iso-octane (branchedalkanes)

    1-pentene (alkenes)

    toluene (aromatics)

    methylcyclohexane(cycloalkanes)

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    Surrogate mixtures for gasoline based

    on composition and octane number

    Based on http://www.atsdr.cdc.gov/toxprofiles/tp72-c3.pdf

    Iso-alkanes

    Aromatics

    Miscellaneous n-Alkanes

    Alkenes

    CycloalkanesCycloalkenes

    Based on http://www.atsdr.cdc.gov/toxprofiles/tp72-c3.pdf

    I s o - a l k a n e s

    A r o m a t i c s

    M i s c e l l a n e o u s n - A l k a n e s

    A l k e n e s

    C y c l o a l k a n e s

    C y c l o a l k e n e s

    % Composition Mixture 1 Mixture 2 Mixture 3iso-Octane 60 40 40n-Heptane 8 10 20Toluene 20 10 10Methyl cyclohexane 8 40 301-Pentene 4 0 0

    RON (linear mixing) 93.7 81.7 83.7MON (linear mixing) 90.6 79.3 79.8RON (blend*) 99.2 94 87.6MON (blend*) 94.5 84.8 82

    Typical gasoline: RON = 90.8, MON = 83.4

    Iso-alkanes

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    Add new fuel components as needed to

    model important features of the overall fuelNew sources of diesel fuels has led tomuch greater levels of cycloalkanes, forwhich detailed mechanisms did not exist

    Methyl cyclohexane chosen asrepresentative sample for this class

    Additional experiments needed forvalidation of this new component

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    Cycloalkanes: methyl cyclohexane

    Cycloalkanes are becoming of muchinterest due to oil sands

    methylcyclohexane

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    Canadas oil sands

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    Canadian oil sands

    Second only to Saudi Arabia in proven oil reserves- Saudi Arabia 262 billion barrels- Canada oil sands 175 billion barrels

    - Arctic National Wildlife Refuge 10 billion barrels (est) Currently largely strip mined Production is a serious source of greenhouse gases

    - 2 tons of sand produce one barrel of oil- production of one barrel of oil =

    daily emissions from 4 cars- huge usage of natural gas for extraction

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    Strip mining oil sands, using 400 ton capacity trucks

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    Diesel fuels derived from oil sands presentcombustion challenges that require research

    Derived diesel fuel is rich in cyclic alkanes- e.g., methyl cyclohexane

    Most of these are rather large, complex cyclic alkanes

    Very little scientic research has been done on any cyclic alkanes

    Preliminary practical experience suggests that these species areimportant in determining ignition and soot production in diesels

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    Asphaltene moleculetypical of oil sands

    Interest in cycloalkanes has increaseddue to oil sands

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    A mechanism for diisobutylene to further represent alkenes in gasoline (17 newspecies, 83 new reactions)

    Diisobutylene molecular structure issimilar to iso-octane

    2,4,4-trimethyl-1-pentene(jc8h16)

    2,4,4-trimethyl-2-pentene(ic8h16)

    (2,2,4-trimethyl-pentane)

    Iso-octane:

    Diisobutylene is comprised of two isomers:

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    Biodiesel fuels

    Alternative fuel from vegetable oils and animal fat

    Methyl esters with 16-18 Carbon atoms

    Low sulfur allows use of catalysts for NO x removal

    10% oxygen content in fuel lowers soot emissions

    Liquid fuel at room temperature

    Renewable diesel fuel

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    Note the methyl ester group at the end of each long hydrocarbon chain

    figure from C. Mueller, Sandia

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    Methyl butanoate, a biodiesel surrogate fuel

    MB has the essential structure characteristic of biodiesel fuels

    MB has basic chemical features of larger methyl esters

    Does not have the higher molecular weight of biodiesel fuels

    Molecule is long enough to display alkylperoxy isomerization kineticscharacteristic of biodiesel fuels

    Computationally much easier to model than true biodiesel fuels

    Optimal vehicle to learn about modeling methyl ester kinetics

    Paper in current symposium examines the strengths and limits ofmethyl butanoate as a biodiesel surrogate

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    Fischer-Tropsch fuel can be treated as a mixtureof n-paraffin and iso-paraffin components

    Recent advances in catalysts for Fischer-Tropsch production from CO and H 2 helpeconomics

    Extremely clean fuel, with virtually nosulfur or other atoms

    We have kinetic models for many n-paraffin and iso-paraffin molecules

    - isomers of heptane on LLNL webpage

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    Next steps - 2

    Mechanism reduction to intermediatelevels useful for efficient modelingcalculations

    Mechanism reduction to very small modelsuseful for CFD applications

    CARM or other techniques, automaticoperation is highly desirable

    Note that reduction can be application-dependent and environment-dependent

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    The End

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