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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