Activity Report from the Division of Combustion Physics ... · ment of Fire and Safety Engineering. The Combustion Centre was also base for the initiation of the Centre of Competence

Activity Reportfrom theDivision of CombustionPhysics

1999– 2000Lund Instituteof Technology

Lund,Sweden

LRCP-62Lund Reportson CombustionPhysics2000

LundReportsonCombustionPhysics,LRCP-62Divisionof CombustionPhysicsLund Instituteof TechnologyBox 118SE-22100 LundSweden

Telephone + 46(0) 46 2229790Fax + 46(0) 46 2224542www http://www.forbrf.lth.se

Printedat Universitetstryckeriet,Lund,SwedenFebruary2001

Cover illustration: Simultaneousmeasurementof flamefrontstructureandflowfield usingplanarlaser-inducedfluorescence(PLIF) of hydroxylradicals(OH) andparticle imagevelocimetry(PIV)in a turbulentnon-premixedflame

TABLE OFCONTENTS I

Tableof Contents

1 Intr oduction 1

2 General Inf ormation 32.1 Staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Visitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 AcademicDegrees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.4 Seminars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.5 InternationalProjects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.6 Budget. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.7 Courses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Laser Diagnostics 73.1 TechniqueDevelopment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1.1 PicosecondLaser-InducedFluorescencefromPolycyclic AromaticHydro-carbonsat ElevatedTemperatures. . . . . . . . . . . . . . . . . . . . . . 7

3.1.2 Two-dimensionalTemperatureMeasurementsUsingTLAF in SootingEn-vironments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.3 RotationalCoherentAnti-StokesRamanSpectroscopy . . . . . . . . . . . 123.1.4 SootDiagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.1.5 PolarisationSpectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 183.1.6 SurfaceTemperatureMeasurementUsingThermographicPhosphors. . . . 20

3.2 TechniqueApplication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.2.1 HCCI Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.2.2 High Speed2-D and3-D Visualisationof TurbulentFlamePhenomena . . 243.2.3 TemporallyResolved SingleCycle Measurementsof Fuel- andOH-Dis-

tributionsin aSparkIgnition EngineUsingHigh SpeedLaserSpectroscopy 273.2.4 SpectroscopicInvestigationson Biofuel Pyrolysis. . . . . . . . . . . . . . 283.2.5 Investigationon DME Sprays . . . . . . . . . . . . . . . . . . . . . . . . 303.2.6 Activities in aFull Scale80MW PowderfueledBoiler . . . . . . . . . . . 32

4 Electrical Phenomenain Gases 354.1 IonisationSensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.1.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354.1.2 Studyof theBasicProperties. . . . . . . . . . . . . . . . . . . . . . . . . 364.1.3 Applicationof theTechniquein aLean-BurningNaturalGasEngine. . . . 37

4.2 Measurementof SurfaceCharge . . . . . . . . . . . . . . . . . . . . . . . . . . . 384.3 LightningandAircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

5 Pulsating Combustion 40

II TABLE OFCONTENTS

6 ChemicalKinetics 426.1 FundamentalKinetic Investigations . . . . . . . . . . . . . . . . . . . . . . . . . 42

6.1.1 A ComputationalStudyof SootParticleThermalIonisation . . . . . . . . 426.1.2 SootFormationin TurbulentReactingFlows — A PDF-BasedApproach

Applied to CarbonBlackProduction. . . . . . . . . . . . . . . . . . . . . 446.1.3 MechanismReduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

SkeletalMechanismGeneration . . . . . . . . . . . . . . . . . . . . . . . 46AutomaticReductionProcedurefor ChemicalMechanisms. . . . . . . . . 48

6.2 AppliedKinetic Calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506.2.1 ImplementationandValidationof a DetailedSootModel andApplication

to AeroengineCombustors . . . . . . . . . . . . . . . . . . . . . . . . . . 506.2.2 Knock in SI Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526.2.3 Calculationsof HCCI-EngineCombustion . . . . . . . . . . . . . . . . . 53

1

1 Intr oduction

The Division of CombustionPhysicsis since1991a separatedivision within the DepartmentofPhysicsat Lund Instituteof Technology. TraditionallytheDivisionhasa largeactivity in thefieldof developmentandapplicationof lasertechniquesfor studiesof combustionprocesses,alsoworkaimingatadeeperunderstandingof sparkignition phenomenahasbeenmadeduringseveralyears.During the last yearsa very strongexpansionhasbeenmadein the areaof theoreticalchemicalkinetics. In thefield of combustionmodelling,thereis alsowork directedtowardspulsatingcom-bustion.

The activities within the Division have strongconnectionsto the Combustion Centrethat wasinitiated at LTH in 1985. The aim with this Centrewas to createinterdisciplinarycombustionactivities involving variousSchoolsof the Institute,e.g. EngineeringPhysics,MechanicalEngi-neering,ChemicalEngineeringandCivil Engineering.Thanksto this CentretheDivision hasgotvery stronglinks, especiallywith theDepartmentof HeatandPower EngineeringandtheDepart-mentof Fire andSafetyEngineering.The CombustionCentrewasalsobasefor the initiation oftheCentreof Competencein CombustionProcesses,in which the laserdiagnosticsandchemicalkineticsfrom theDivisionaremajoractivities.

TheDivision is alsoheavily involvedin theNationalCentrefor CombustionScienceandTechnol-ogy, CECOST, which besidesLund Instituteof TechnologyalsoinvolvesChalmersUniversityofTechnology, Royal Instituteof TechnologyandGothenburg University.

AmongtheEU projectsthattheDivision is participatingin, theMarie CurieTrainingSiteshouldbe mentioned.This Training site, which wasappliedfor togetherwith the Departmentsof HeatandPower EngineeringandFire andSafetyEngineering,givesgraduatestudentsfrom EU thepossibilityto applyandspendsomeof their time at theseDepartments.

TheDivision hasalso,togetherwith representativesfrom theDepartmentof HeatandPower En-gineering,successfullyappliedfrom DESS(Delegationenfor SydsverigesEnergiforsorjning) forresourcesto set-upa High-pressureBurnerFacility. Theburnerwill beconstructedfor a pressureup to 16bar, preheatingof thefuel to 1000K andwith amaximumairflow of 1.3kg/s.Theburnerwill beequippedfor optimumopticalaccess.

In order to host the expandedactivities as describedabove, the Lund University hastaken thedecisionto set-upanew building ( � 2000m

�) within theDepartmentof Physics.Theconstruction

work for thenew building wasinitiatedin August2000andit is plannedto bereadyby May 2001.Thenew building will hostthewholeDivisionof CombustionPhysicsaswell aspersonsfrom theDepartmentsof HeatandPowerEngineeringandFire andSafetyEngineering.

The Division hasnot yet beenstrongly involved in the undergraduateeducation(MSc.). TheDivision is, however, involved in the processof trying to createa specialprogramwithin theMSc educationdirectedtowardsenvironmentallyfriendly andefficient energy conversion. The

2 CHAPTER1. INTRODUCTION

educationaleffort doesalsoincludework in theframework of GronaBilen, aimingatanoptimisedMSceducationfor providing theengineindustrywith better-preparedengineers.

MarcusAlden,professor

Headof Division

3

2 General Inf ormation

2.1 Staff

Thestaff includedthefollowing members:

Mar cusAlden, professor, Headof Division

Per-Erik Bengtsson,PhD, Docent

Edward Blurock, PhD

Shahrokh Hajir eza, PhD (000601–)

ClemensKaminski, PhD

Philip Klaus, PhD (–990630)

Anders Larsson, PhD (990901–)

ZhongshanLi, PhD (001101–)

Fabian Mauss, PhD, Docent

Frederik Ossler, PhD

Hakan Soyhan, PhD

Christer Lofstrom, reseng

Mikael Afzelius, gradstud(000501–)

Per Amneus, gradstud

Oivind Andersson, gradstud

BomanAxelsson, gradstud

Xiao Bai, holderof ascholarship(000201–)

Michael Balthasar, gradstud

Raffaella Bellanca, gradstud(001001–)

Joakim Bood, gradstud

Christian Brackmann, gradstud

Robert Collin, gradstud

Anne Dederichs, gradstud

JohanEngstrom, gradstud

Axel Franke, gradstud

JohanHult, gradstud

TereseLovas, gradstud

ThomasMetz, gradstud

Sven-IngeMoller, gradstud

Daniel Nilsson, gradstud(–000830)

JennyNygren, gradstud

Alaa Omrane, gradstud

Mattias Richter, gradstud

JoachimWalewski, gradstud

Elna Brodin, secretary

Bir gitta Olofsson, secretaryCECOST

Marie Larsson, secretary

Rutger Lor ensson, instrumentmaker

Robert Ternemo, el techn(991001–)

2.2 Visitors

Thierry Baritaud, IFP, Paris,France

Heidi Bohm, PhysicalChemistry, Universityof Bielefeld,Germany

Marina Braun-Unkhoff , DLR, Instof CombustionTechnology, Stuttgart,Germany

AndreasBrockhinke, PhysikalischeChemieI, Universityof Bielefeld,Germany

Robert Cattolica, Dept of MechanicalandAerospaceEngineering,University of California atSanDiego,USA

4 CHAPTER2. GENERALINFORMATION

Tieying Ding, Michel Versluis,Theo van der Meer, Delft Universityof Technology, TheNether-lands

Michael C. Drake, GeneralMotors,PhysChemDept,Warren,MI, USA

RogerFarr ow, Combustion ResearchFacility, SandiaNational Laboratories,Livermore,CA,USA

Michael Frenklach, MechanicalEngineering,Universityof CaliforniaatBerkeley, USA

SusiGardiner, ChemicalEngineering,Universityof Cambridge,UK

Xavier Georges, ENSMA, FuturoscopeChasseneuil,France(TRIDENT)

Fredrik Herman, ABB Stal,Finspang

Markus Kraft, WeierstrassInstitutefor AppliedAnalysisandStochastics,Berlin,Germany (presentpostion:ChemicalEngineering,Universityof Cambridge,UK)

Alfr edKurtz, Universityof AppliedSciences,Cologne,Germany

Mark Linne, ColoradoSchoolof Mines,Div of Engineering,Golden,CO,USA

Ulrich Maas, TechnicalCombustion,Universityof Stuttgart,Germany

Patrizio Massoli,RaffaelaCalabria, IstitutoMotori, Naples,Italy

Christoph Pels-Leusden,TechnicalMechanics,RWTH Aachen,Germany

Michael Pfitzner, BMW-RR,(presentpostion:Universityof Munchen,Germany)

Heinz Pitsch, TechnicalMechanics,RWTH Aachen,Germany (presentpostion:StanfordUniver-sity, USA)

ThomasSeeger, LTT Erlangen,Germany

Hai Wang, MechanicalEngineering,Universityof Delaware,USA

2.3 AcademicDegreesBasedon Work Carried out at the Divi-sion During 1999–2000

Doctorates

Frederik Ossler, “Laser Diagnosticsin Combustion– ElasticScatteringandPicosecondLaser-InducedFluorescence”,990604,LRCP-47

Joakim Bood, “Developmentof Dual-broadbandRotationalCARSfor CombustionDiagnostics”,000616,LRCP-56

Michael Balthasar, “DetailedSootModellingin LaminarandTurbulentReactingFlows”, 000925,LRCP-59

Oivind Andersson, “DevelopmentandApplication of LaserTechniquesfor StudyingFuel Dy-namicsandNO Formationin Engines”,001215,LRCP-61

Licentiates

JohanEngstrom, “Developmentof a 2D TemperatureMeasurementTechniquefor CombustionApplicationsusing2-Line Atomic Fluorescence”,990126,LRCP-46

Michael Balthasar, “DetailedSootModellingin TurbulentDif fusionFlames”,990614,LRCP-49

2.4. SEMINARS 5

BomanAxelsson, “LaserTechniquesfor combustionEngineDiagnostics”,000218,LRCP-53

Daniel Nilsson, “Analysis andReductionof Fuel andPollutantChemistryin CombustionSys-tems”,000303,LRCP-54

Axel Franke, “Diagnosticsof ElectricalPhenomenain Gasesfor theMonitoringof Spark-IgnitedCombustion”,000606,LRCP-57

Per Amneus, “DetailedChemicalKinetic StudiesonHomogeneousIC-EngineIgnition”, 000925,LRCP-60

JohanHult, “Developmentof time resolved laserimagingtechniquesfor thestudyof turbulentreactingflows”, 001219,LRCP-64

2.4 Seminars

JohanEngstrom, “Developmentof a 2D TemperatureMeasurementTechniquefor CombustionApplicationsusing2-LineAtomic Fluorescence”,990126

Michel Versluis, Universtyof Twente,“The Delft turbulentpilotedjet-diffusionflame”990518

David Ottesen, SandiaNat Labs, “Generalover view of laser-optical applicationsin industrialcontrol”, 990610

Ernst Winklhofer, KTH ochAVL, List, Graz,“In-cylinder visualisation:whereandhow doesitsupportcombustionenginedevelompent?”,990611

Michael Balthasar, “DetailedSootModelling in TurbulentDif fusionFlames”,990614

BomanAxelsson, “LaserTechniquesfor CombustionEngineDiagnostics”,000218

Daniel Nilsson, “Analysis andReductionof Fuel andPollutantChemistryin CombustionSys-tems”,000303

Axel Franke, “Diagnosticsof ElectricalPhenomenain Gasesfor theMonitoringof Spark-IgnitedCombustion”,000606

RogerL Farr ow, SandiaNatLabs,“Energy-TransferStudiesUsingTunablePicosecondLasers”,000615

Alfr ed Kurtz, “Dif ferentialInterferometryandMultipoint VibrationalCARS”, 001012

Luigi Biasi, Univ of Pavia andAntonio Tazzi,Matecs.r.l., “Industrial ProcessSimulationEnvi-ronment”,001024

Per Amneus, “Detailedchemicalkinetic studiesonhomogeneousIC-engineignition” 001027

Michael C Drake , “LaserDiagnosticsfor GasolineDirect InjectionEngines”001214

2.5 Inter national Projects

EU Program SubjectPLANET Platform on Auto-Ignition NumericalEngineSimulation

Tools/ ExperimentaldevelopmentsG-LEVEL GasolineDirect InjectionLow EmissionLevelsby Engine

ModellingD-LEVEL DieselLow EmissionLevelsby EngineModelling

6 CHAPTER2. GENERALINFORMATION

InternationalProjects(continued)

EU Program SubjectCFD4C ComputationalFluid Dynamicsfor CombustionFLAMESEEK FlameSensorsfor EfficientGasTurbineEngineCyclesMarie CurieTrainingSite Training of PhD students(in the framework of the Com-

bustionCentre)

2.6 Budget

TheDivision’sbudgetfor 2000totalled14.3MSEK of which � 80% camefrom externalsources.

2.7 Courses

During thelasttwo-yearperiodthefollowing undergraduatecourseshavebeengiven:

� FundamentalCombustion

Optionalcoursefor studentson their lasttwo yearson theprogramsof EngineeringPhysics,MechanicalEngineeringandChemicalEngineering.Thecoursehasbeengivenby Per-ErikBengtsson.Thecourseliteraturehasmainlybeenthetext bookGriffithsandBarnard,FlameandCombustion,3� � edition, 1995. The coursehasbeenextendedandnow 5 creditsaregivento thestudentaftercourseexamination.Theextensionis theresultof an inclusionofa projectthatshouldsynthesisethetopicslearnedfrom thecourse.Theprojectis presentedbothorally andwith awritten report.� Laser-basedCombustion Diagnostics

Optionalcoursefor studentson their fourth yearon the programof EngineeringPhysics.Thecoursethatcorrespondsto threecreditshasbeengivenby MarcusAlden,andthecourseliteraturehasbeenEckbreth,LaserDiagnosticsfor CombustionTemperatureandSpecies,GordonandBreach,1996.Thestudentsalsohaveapossibilityto extendthecourseby mak-ing an additionalprojectin connectionwith the researchin laserdiagnostics.This specialprojectgivestwo additionalcreditsto thestudent.

� Molecular Physics

Optional coursefor studentson their fourth year of study on the programEngineeringPhysics. The courseis given for the first time the year 2000with ClemensKaminski asteacher. The coursegivesfive credits,and the courseliteratureis Banwell and McCash,Fundamentalsof molecularspectroscopy, 4� � edition,McGraw-Hill, 1994.

7

3 Laser Diagnostics

3.1 TechniqueDevelopment

3.1.1 PicosecondLaser-InducedFluorescencefr omPolycyclicAr omaticHy-drocarbonsat ElevatedTemperatures

F. Ossler, T. MetzandM. Alden

Aromatic hydrocarbonsareknown to be carcinogenicor mutagenicagents.Polycyclic aromatichydrocarbons(PAH) arenaturallypresentin fossil fuels,they areformedduringhydrocarboncom-bustionandareconsideredto playanimportantrole in theformationof soot.They canbefoundasproductsfrom incompletecombustionof gasphaseandliquid phasehydrocarbons,e.g. in burn-offproducts,exhaustfrom biphasecombustionin enginesandsprays,andfumesfrom heatedmineraloils. Concentrationmeasurementsof aromaticsubstancesareimportantsincethey provide infor-mation for the study and understandingof hydrocarbonchemistryin flames. Monitoring PAHconcentrationsis necessaryto maintainsafetylevelsin theemissionof thesesubstances.

Laser-inducedfluorescence(LIF) is a sensitive tool for detectionof aromaticspecies.However,thespectralprofilesof aromaticsubstancesin gasphasearein generalbroadandalmoststructure-lessandmaychangewith temperature.This makesit difficult to selectdifferentspeciesby theirspectra.Measurementsof the temporalevolution of thefluorescencewould in principle increasethe possibility to discriminatebetweendifferentPAH by temporalselection. Moreover, the de-tectionsensitivity dependson the fluorescencequantumyields of the substances,which dependon the temperature.This dependencecanbe studiedusing time-resolved laser-inducedfluores-cence.Suchmeasurementshave beenperformedby othergroups,however, they wereconductedat relatively low temperaturescomparedto realflameconditions.

DuringthelastyearswehaveinvestigatedLIF from somePAHs spectrallyandtemporallyresolvedattemperaturesover400K. Thework wasafirst stepto investigatethepossibilityof usingtemporalselectionasa complementto spectralmeasurementsin order to distinguishdifferentPAHs in acombustionenvironment.Theaim wasa characterizationof thefluorescencepropertiesof PAHswith respectto temperatureat conditionsreachingthosevalid for combustion.

For this purposemostof the measurementswereperformedon gasphasePAHs in a quartzcellat varioustemperaturesbetween400and1200K andatmosphericpressure.In additiontheeffectfrom oxygenquenchingwas investigatedby addingoxygenwith mole fractionsup to 10% tothe buffer gas[Ossleret al., 2001a]. In anotherwork the behavior of the PAHs at even highertemperatureswas studiedby measurementsin the post-flamezoneof a flat, premixed CH� /airflameusingaMcKennatypeburner[Ossleret al., 2001b].

8 CHAPTER3. LASER DIAGNOSTICS

ThefluorescencewasinducedusingapicosecondNd:YAG lasersystemproviding laserpulsesatawavelengthof 266nmwith adurationof lessthan35ps.Thelaserradiationwasfocusedinto aflowcell madeof quartz.Thecell wasplacedinsideanoven,whichhadopticalaccessfor theincomingandoutgoinglaserbeam,aswell as,for thefluorescencemeasuredat90 with respectto thelaserbeam. The time-resolved measurementswere donewith a photomultiplierandan oscilloscopeor a streakcameraandspectraweretaken a with spectrographanda gatedoptical multichannelanalyzer(OMA). In theflameseedingexperimentsthequartzcell wasreplacedby theMcKennaburnerandtheOMA by aCCDcamerato makespatiallyresolvedspectralmeasurementspossible.

Cell measurements

Thespeciesinvestigatedin thecell werefluorene,naphthalene,anthraceneandpyrene.Thetime-resolved fluorescencemeasurementsrevealedtwo components,the decaylifetimes of which de-creasedcontinuouslywith increasingtemperature.At 540K and0% oxygenconcentrationthelong lifetime componentslay aroundsometensof nanosecondswhile theshortonesamountedtoseveralnanosecondsdependingon thespecies(Figure3.1 for naphthalene).Only the lifetime ofthelongcomponentof pyrenelay over100ns.Thelifetimesat1200K wereapproximatelyoneor-derof magnitudeshortercomparedto theresultsat lower temperatures.For fluorene,naphthaleneandpyreneat lower temperaturestherelative intensityof thelongcomponentwasclearlystrongercomparedto the shorter, whereasthe shortcomponentbecamedominantat highertemperatures.Thustheeffective lifetime, which is theintensity-weightedmeanof thetwo lifetimes,approachedcontinuouslytheshortonewith increasingtemperature.Moredetailsabouttheresultscanbefoundin [Ossleret al., 2001a].

400 500 600 700 800 900 1000 1100 12000.1

1

10

100τ1

τ2

τeff

Life

time

(ns)

Temperature (K)

Figure3.1: Lifetimesevaluatedfor naphthalenein N atdif-ferenttemperatures.Thefluorescencewasexcitedat266nmandobservedat350nm. � � : Shortlifetime, � : Longlifetime,� � : Intensity-weightedmeanof � � and � .

At temperaturesabove 800K thelifetimes showed a nearly expo-nentialdependenceon temperature.Fitting the dependencewith an ex-ponentialmodel function and ex-trapolationto higher temperaturesindicatesthat lifetime componentsof tensto hundredsof picosecondsshould be expectedat flame tem-peratures.Thereforea high tempo-ral resolutionwould be necessaryto discriminate betweendifferentPAHs with time-resolvedmeasure-ments.Fastdetectorssuchasstreakcamerasor picosecondpump-probetechniqueswould thereforebe re-quired.

The decreaseof the lifetimes withincreasingtemperaturecan be ex-plainedby radiationlesstransitions(internal quenching)of the PAH moleculeto its electronicgroundstate(internalconversion)or atriplet state(intersystemcrossing),wherethemoleculedoes

3.1. TECHNIQUE DEVELOPMENT 9

notcontributeto thefluorescence.Duringsucha transition,theenergy of themoleculecanhardlychangeso that the vibrationalstatesof the two electronicsystemsbetweenwhich the transitionoccursmust have nearly the sameenergy. Suchcoincidencesbecomemore probablethe morevibrationalstatescanbe found per energy interval, i. e. the higher the densityof vibrationalen-ergy statesis. Thus the probability of radiationlesstransitionsis proportionalto the densityofvibrationalstates(Fermi’s goldenrule). In largemoleculessuchasPAHs thestructureof thevi-brationallevelsis quitecomplex leadingto highdensitiesof statesmakingradiationlesstransitionsimportant.Whenthetemperatureis increased,highervibrationallevelsarepopulated,wherethedensityof statesis higher. Hencetheratesof radiationlesstransitionsincreaseandthefluorescencelifetimesareshortened.

The spectralprofilesof fluorene,naphthaleneandanthraceneshowed broadeningandred shiftswhenthetemperaturewasincreased.Thespectrumof pyrenealsobroadenedanda new emissionbandappearedon theUV sideknown asdualfluorescence.

Mixtures with oxygenshowed ratherhigh thermalstability for all substances.Naphthalenewasstableup to 980K, whereasfluorenehadthe loweststability not revealingany fluorescenceanymoreabove770K andoxygenmolefractionslargerthan2%. Thequenchingcausedby oxygenatlow temperaturesshowedaStern-Vollmer behaviour, i. e. thefluorescencedecayrates(theinverselifetimes) increasedlinearly with oxygenconcentration.Quenchingcrosssectionslay typicallybetween�� and �� m� for the slow componentsand between � �� and �� m� for the fastones.The fastcomponentsweremoresensitive to temperatureasthe slowonesandmostof thesubstancesshowedconsiderablyhighercrosssectionsabove500K. In somecases,whenthespectrawereunstable,thedependenceof thedecayratesonoxygenconcentrationappearednon-linear.

FlameSeedingMeasurements

Theflame-relatedmeasurementswereperformedwith naphthaleneandpyrene. Thecell wasre-placedby the McKennatype burneranda mixture of the correspondingPAH andnitrogenwasseededinto thepost-flamezone.By changingthestoichiometryof thepremixedmethane/airflame,differentconcentrationsof oxygenwereobtainedaroundtheinjectedgas.ThePAH mixtureburnedlikeacandleshapedflameabove thereactionzoneof thepremixedflame.

Thevalidity of anextrapolationof theresultsfrom thecell measurementsto highertemperatureswaschecked. For this purpose,measurementswereconductednearthe injection zone,whereitcould be assumedthat the seededPAHs hadhardly reactedchemically. The measuredlifetimesagreedconsiderablywith thoseexpectedfrom the extrapolation. Thereforewe tested,if the ex-trapolatedtemperaturedependenceof thelifetimescouldbeusedto determinethetemperatureinthe flamefrom lifetimes measuredin the flame. The temperaturesobtainedfrom different life-time componentsanddifferentseededPAHs this way agreedwell amongoneanother, but weresignificantly lower thanthat determinedfrom an elasticscatteringmeasurement,wherenon gaswasseededinto theflame. However, it cannotbe ruledout that this differenceis dueto coolingfrom theseedinggasflow ratherthanasystematicerrorfrom theextrapolation.Thesametestwasmadewith the temperaturedependenceof certainfeaturesin thefluorescencespectrasuchasthepositionsof the maximumand the edgeswherethe intensitywasat half maximum. Even heredeviationsto lower temperatureswereobtainedin somecases,however, notassignificantaswhen


usingthelifetimes.

At largerdistancesfrom the injectorspectralredshiftsandlongerlifetimeswereobserved. Thiswasprobablydueto speciesformedfromtheseededPAHs. Thuschangesof spectralfeaturesmightindicatethe transitionfrom the seededPAHs via intermediatespeciesto soot. In an attempttomakeaqualitative,spatialmapof thesespeciesone-dimensionalspectrawererecordedatdifferentheightsover the burner. An indicator for molecularor particlegrowth was the compositionofthespectralemissionin termsof UV, blueandgreen-yellow contributionsandtheratio of elasticscatteringto total emission.

3.1.2 Two-dimensionalTemperatureMeasurementsUsingTLAF in SootingEnvir onments

J. Engstrom,J. Nygren,C. KaminskiandM. Alden

Measurementsof the temperaturefield arecrucial for theunderstandingof combustionsincethetemperaturehasa large influenceon chemicalreactionrates. Dataon the temperaturefield dis-tributions in combustion processesprovide important input for the improvementof theoreticalmodelsand the optimisationof practicalcombustiondevices. Most laserdiagnostictechniqueshave beenperformedin leancombustionsituationsandhave not beenoptimisedfor applicationunderfuel-rich andsootingconditions.Heretemporallyresolvedsoot/temperaturemeasurementsareessentialfor improving theoverall understandingof sootformationandoxidation. In general,lasertechniquesarenot easilyappliedin stronglysootingflames,owing to thestrongabsorption,spectralinterferencefrom particulatescatteringand fluorescencefrom large moleculessuchasPAH. Therefore,developmentof novel temperaturetechniquesfor applicationsin sootingenviron-mentis important.Thepresentgrouphasrecentlyshown thatTLAF is avaluabletool for practicalapplications[Engstrom et al., 1999b]andholdspromisefor temperaturemeasurementsin highlysootingflames [Engstromet al., 2000].

TLAF involvessequentiallymeasuringthe Stokes and anti-Stokes direct-line fluorescencepro-ducedby optical excitation in a threelevel system.The basicprinciple of TLAF is that suitablemetalatoms(in our caseindium), having two optically accessibleandtemperaturesensitive en-ergy states0 and1, areseededinto thecombustionenvironment. Dependingon the temperaturetheatomicenergy levelsarepopulatedaccordingto theBoltzmanndistributionunderconditionsoflocal thermodynamicequilibrium.Temperatureis thenevaluatedfrom theratioof thecorrespond-ing fluorescencesignals,seeEquation3.1. As the excitation is to the sameupperstatefor eachfluorescenceprocess,theeffectsof quenchingareexactlythesameandcancelout in theexpressionfor T. !#" $&% ' (&)�*+-, .0/ 1 '/ 1 %32 , .04 ' 14 % 1 2 , .0561 '561 %32 7 (3.1)

Theexperimentswereperformedin a slot burnerfitted with a standardanalyticalflameatomiserassembly. The flamewasan atmosphericpressureacetylene/airflameseededwith aqueousso-lutionsof indium chloride(InCl 8 ). Dif ferentstoichiometries( 9 ) wereinvestigated,rangingfromlean( 9;: 1) to fuel-richcases( 9 "

3.5). The 9 -valuewasvariedby increasingtheC1 H 1 flow.


φ = 1.0 φ = 2.5 φ = 3.5

5 0 5

5101520

5 0 5 5 0 5

H eig ht ab o veb urner [ m m]

1 0 0 0 K

1 5 0 0 K

2 0 0 0 K

2 5 0 0 K

3 0 0 0 K

Figure3.2: Temperatureimageobtainedfrom TLAF for different < -values.

Thetemperaturedistributionswereevaluatedfor stoichiometriesrangingfrom < =1.0 to 3.5. Fig-ure 3.2 shows the result for < =1.0, 2.5 and3.5. Temperaturesshown correspondto an averageof 50 singleshotimagesfor eachfluorescencesignal. Eachindividual imagewasnormalisedbyits laserprofile to accountfor intensityvariationsacrossthelasersheet.To obtainthecalibrationconstantC, rotationalCARS temperaturemeasurementswhereperformedat differentheightsintheflame.Regionsappearingblackcorrespondto pixelswith too low SNRfor temperaturesto beevaluated

Theresultsshow thatTLAF thermometryperformswell overa largerangeof < -valuesdespitethedrasticdropof temperaturesandthehigh concentrationof sootparticlesassociatedwith large < -values.Indiumis anattractivecandidatesinceboththeexcitationanddetectionwavelengthsareinthevisible range(410nm and451nm), whereabsorptionby hydrocarbonsandothernative com-bustionspecieswerefoundto benegligible. Thelargeoscillatorstrengthof indiummeansthatverylow laserenergy is required,which is a tremendousadvantagecomparedto otherlasertechniques,sincehighlaserpowersgiveriseto LII from sootparticlesor LIF from PAH. Suchinterferencesaredifficult to accountfor by backgroundsubtraction,particularlyin fluctuatingenvironmentssuchasturbulentflames,wherethis causesuncertaintiesin evaluatedtemperatures.

Furthermore,in a fuel-rich flamethe seedingefficiency increased,becausethe oxidationrateoftheactive speciesis lowered.This is a majoradvantage,sincea lower seedingconcentrationcanbe usedto reachsufficient signalto noiseratios,minimising effectsof the seederspecieson thecombustionchemistry.


3.1.3 Rotational Coherent Anti-StokesRamanSpectroscopy

J. Bood,C.Brackmann,M. Afzelius,T. Seeger1, M. Schenk1, A.Kurtz2 , T. Dreier3 andP.-E.Bengts-son

Thetemperatureis animportantparameterin thecharacterisationof combustionprocesses,andthetemperaturedistribution hasa strongconnectionto the emissionof pollutantsandthe efficiencyof suchprocesses.It is a difficult taskto performaccuratein-situ temperaturemeasurementsinreal practicalsystems,andthe developmentof lasertechniqueshasbeenvery importantfor theprogressin this area. The variantof CARS namedrotationalCARS hasbeenfurther developedandinvestigatedin differentprojects.Mostof thiswork hasbeenpresentedin aPhD-thesis[Bood,2000].

Nd :YAG laser

Dye laser

1-m spectrograph

CCD

Engine

M

M M

BS

M

DM L

f=300 mm

L SP

ND =A CL

f=500 mm DM

M 10%

A >

PC

Figure3.3: Experimentalset-upfor dual-broadbandrotationalCARS.

In Figure3.3, a typical experimentalset-upis shown for dual-broadbandrotationalCARS (DB-RCARS). In DB-RCARS,threelaserbeamsare focussedto a commonintersectionpoint fromwhicha signalis generatedif moleculeswith suitableresonancesarepresent.Thesignalis gener-atedasa laser-like beamandis directedto a spectrographanda detector. Thespectrallyresolvedsignal is analysedby fitting its shapeto a library of theoreticallycalculatedspectraat differenttemperatures.Thetemperatureis evaluatedfrom aninterpolationbetweenthetheoreticalspectra.

In many combustionsituationsit is desirableto have temperatureinformationin many pointsatthesametime. For this reason,we have workedwith differentprojectsto try to solve theseprob-lems. In oneprojecta novel multi-point methodfor rotationalCARSwasdeveloped[Bood et al.,2000e].Usinga systemof cylindrical lenses,onelensfor focusingin thehorizontaldirectionanda packageof small lensesfor focusingin the vertical direction, eachlaserbeamwas split intoseveralfocusedbeamsresultingin separateplanarBOXCARS configurations.TheCARSsignalswereimagedontotheentranceslit of aspectrographusingasphericallens,andaCCDcamerade-tectedCARSspectrafrom differentspatialpositions.By usingdual-broadbandrotationalCARStheset-upwasdemonstratedfor quantitativemeasurementsof profilesof temperaturesandoxygen

1LTT Erlangen,Germany2Universityof AppliedSciences,Cologne,Germany3Universityof Heidelberg, Germany


concentrations.Thetechniquewasdemonstratedfor threepointsonly, but canbeextendedto morepointsusingspecialoptics,wherethis choicemustbebasedonasufficient signal-to-noiseratio inall pointsfor theactualmeasurementcondition.

Developmentof multi-point vibrationalCARSwasmadetogetherwith Prof. A. Kurtz (Universityof Applied Sciences,Cologne,Germany) in a Lund LaserCentreproject. A multi-passcell wasusedto achieve temperaturemeasurementsin several points along a line. The multi-passcellconsistedof two apochromaticlensesandtwo dichroicmirrorsreflectingthepumpandtheStokesbeamwhile transmittingthe CARS signal. The reflectedpumpandStokesbeamswere thusateachdichroicmirror surfacereflectedbackandcreatedanew CARSsignalin thenew focalpoint.Two branchesof CARSsignals,onefrom eachendof themulti-passcell, werethencombinedtoacommondetectionpathandimagedontotheentranceslit of thespectrograph.All CARSsignalscould thenbe registeredon differentheightsof thedetector. Themeasurementswereperformedduringfall 2000,andevaluationis in progress.

The phenomenonof engineknock in spark-ignitionengineshasbeenstudiedin a collaborationprojectconsistingof thefollowing sub-projects:evaluationof knockdetectionmethods,modellingof flow andflamepropagation,modellingof chemicalkinetics,thermalanalysisandheattransfer,andourcontributionon laserdiagnostictemperaturemeasurementsusingDB-RCARS.

A single-cylinder spark-ignitionenginewasmodifiedfor the experiments.The enginehasbeendesignedto givezeroswirl andwasequippedwith two spark-plugsto provideasplaneflamefrontaspossibleandtherebyawell-definedend-gasregioncloseto aplanarwall. Thisanticipatedend-gasregionwasprobedusingtheDB-RCARStechnique.An exampleof anexperimentalspectrumrecordedin an engineis shown in Figure 3.4a [Grandin et al., 2000a]. The conditionswere apressureof 2.0MPa,a distancefrom thewall of 0.7mm,andthetime position4 CAD aTDC.Thebest-fitspectrumshown in Figure3.4bgaveanevaluatedtemperatureof 902K.

?@ ACB D DFE G HJI K KJL M NJO P PQ R QS T UV W XY Z [\ ] ^_ ` a b0c

d e�fge�hji k l m n&o p�frq st uv wuxyv z{ |}~ ��uyv�

� �� J� � �J� � �J� � �� ¡ ¢ £3¤

¥ ¦§ ¨¦©ª§ «¬ ®¯ °±¦ª§²

³ ´�µg´�¶6· ¸ ¹ º »&¼ ½�µg¾ ¿ ÀFigure3.4: a) ExperimentalrotationalCARSspectrumin IC-engineprior to ignition at a pressureof 2.0MPa. b) Best-fitspectrumthatgaveanevaluatedtemperatureof 902K.

A comprehensive measurementprogramwas performedwith simultaneoustime-resolved mea-surementsof the cylinder pressureat threedifferent locations,the heatflux to the wall, andthetemperatureat differentspatialpositionsfrom thewall. A resultis shown in Figure3.5,whereatemperatureprofileis shown from two casesusingdifferentfuel mixtures,bothpureiso-octaneanda fuel mixtureconsistingof 75% iso-octaneand25% n-heptane.Therearesignificantdifferencesbetweenthe curves. The temperatureis higherfor the PRF75 fuel thanfor the iso-octanefuel.


This is asexpectedsincechemicalreactionsstartsata lower temperaturefor this fuel. Therebytheauto-ignitionoccursat earliertime positionsandtheknockonsetwill alsooccurearlier.

0 1 2 3 4 5 400

500

600

700

800

900

1000

PRF 75

Tem

pera

ture

(K

)

Distance from the wall (mm)

0 1 2 3 4 5 400

500

600

700

800

900

1000

iso -octane

Distance from the wall (mm)

Tem

pera

ture

(K

)

Figure3.5: Temperatureprofilesin spark-ignitionengineusingtwo differentfuels recordedat atimepositionof 4 CAD aTDC.Eachmeanvalueis basedonmorethan100singleshots.

A fuel moleculethat givesrise to a rotationalCARS spectrumwith distinct peaksis acetylene.RotationalCARS measurementswere performedfor temperaturesrangingfrom 294 to 582K,andin mixturesof acetyleneandnitrogenin themolefractionrangeof 0.06– 0.32for acetyleneatroomtemperature[Boodetal.,2000b].Theexperimentalspectrawereevaluatedby aleast-squaresfitting to librariesof theoreticallycalculatedspectrausingtwo differentRamanlinewidth models,onewith andonewithout dependenceon the rotationalquantumnumberJ. It wasfound that aJ-dependentmodel is favourable,both regardingtemperaturemeasurementsin pure acetylene,andsimultaneousacetyleneconcentrationandtemperaturemeasurementsin differentmixturesofacetyleneandnitrogen.

In a Lund LaserCentreprojecttogetherwith T. SeegerandM. Schenk(LTT Erlangen,Germany)anapproachfor simultaneousvibrationalandrotationalCARSwasdeveloped[Brackmannet al.,2000b].Thiswasachievedusingonly asingleNd:YAGlaserandadyelasersystemtogetherwith asinglespectrographandCCD-camera.UsingaRhodaminemixturein thedyelaserit waspossibleto performdual broadbandrotationalCARS on several molecules,mainly nitrogenandoxygen,aswell asQ-branchvibrationalCARSon nitrogenandcarbonmonoxide.Frommeasurementsinpremixedsootingethene/airflamesit waspossibleto evaluatetemperaturesfrom bothvibrationalCARSandrotationalCARSspectra,oxygenconcentrationfrom therotationalCARSspectra,andcarbonmonoxideconcentrationfrom thevibrationalCARSspectra.


0 10 20 30 40 50 265

270

275

280

285

290

295

300

305

310

315

320

325

ECS MEG SCL T=295 K

Tem

pera

ture

(K)

Pressure (MPa)

Figure3.6: Evaluatedtemperaturesversuspressureusingthescalefactorsobtainedfor threedif-ferentlinewidth models.

In a fundamentalresearchproject in collaborationwith T. Dreier (Heidelberg, Germany), purerotationalCARSwasfor thefirst time investigatedfor pressuresup to 44MPa in nitrogengasatroom temperature[Bood et al., 2000d]. An atomicfilter consistingof a heatedcell with sodiumvapourwassuccessfullyappliedfor suppressionof stray light originating from the narrowbandlaserbeam.ResultsobtainedusingRamanlinewidthscalculatedwith threedifferentmodels;theenergy correctedsuddenscalinglaw (ECS), the modifiedexponentialgapmodel (MEG), andasemiclassicalabinitio model,werecompared.By usingdatafor Ramanlinewidthsatatmosphericpressureandscalethemlinearlywith pressure,all threelinewidth modelsresultedin poorspectralfits whenfitting experimentaldata. By usingadditionalscalefactorsfor the Ramanlinewidths,uniformly scaledfor thewholeS-branch,thequality of thespectralfits, aswell asthetemperatureaccuracy wassignificantly improved. The resultingscalefactorsindicatea non-linearpressuredependenceof the linewidths, andvisualisea narrowing of the spectralenvelopeat the highestpressure,i. e.44MPa. Theresults,of which part is shown in Figure3.6, indicateshortcomingsinthepresentmodel,assumingisolatedlinesandtheimpactapproximation,andemphasisetheneedof furthertheoreticaldevelopmentconcerningRamanlinewidthsandtheirpressuredependence.


3.1.4 SootDiagnostics

B. Axelsson,R.Collin, X. Georges4 andP.-E.Bengtsson

For morethana century, emissionfrom automotive vehicleshasbeenan issuefor mankind. Anenormousincreaseof vehiclescombinedwith an intensifiedenvironmentaldebatehas lead toharderlegislationon emissionsfrom carsetc.For anefficient developmentandevaluationof newmethodsfor particulatereduction,diagnostictechniquesaswell ascomputermodellingneedstobedevelopedandcombined.

ÁÃÂ Á ÁÅÄ Á ÁÇÆ Á ÁÅÈ Á ÁÅÉ Á ÁÂ Á Á

Â Á Á Á

Â Á Á Á ÁÊ Ë Ì ÍÏÎ

ÐÑÒ ÓÔ ÕÖ×Õ×ØÕ ÙÒ ÚÑ×Û ÙÒ ÚÜ ÝÞ ßÝàáÞ âã ä åæ å

çÏè�é�êFigure3.7: Temporaldecayof theLII signal

Laserdiagnosticsoffer thepossibilityofin-situ two-dimensionalmeasurementsof, e.g. sootvolumefractionwith anex-ceptionaltemporalresolution.Laserdi-agnostictechniquesare often more ex-pensive and complex than post com-bustion diagnosticssuch as filters orGC/MS-systemsbut to answertheques-tion of where, when, and to what ex-tentsootis formed,laserdiagnostictech-niquesare superior. This text presentsa brief overview of the soot diagnosticactivities at theDivision of CombustionPhysics,Lund University.

Thesootdiagnosticsat thedivisionhadarestartin November1997whenBomanAxelssonstartedasPhD studentandyet anotherboostwhenRobertCollin started6 monthslater. The first in-vestigationsaimedat a betterunderstandingof the laser-inducedincandescence(LII) technique.Theseinvestigationswereperformedin premixedlaminarflamesfor optimalcontrolandlatercol-lectedandsubmittedto AppliedOptics[Axelssonetal.,2000a].Thecurvepresentedin Figure3.7displaysthe two interestingfeaturesof the LII process.Thecurve is a measurementof the radi-ation intensityasa functionof time for laser-heatedsootparticles.Theintegratedareabelow thecurve,especiallyfor thefirst tensof nanosecondsis proportionalto thesootvolumefraction,whichmakestwo-dimensionalmeasurementsof sootvolumefractionpossible,seee.g. Figure3.8. Theparticlesizecanbededucedfrom theslopeof thecurve,sincethecoolingbehaviour is coupledtotheparticlesize.Theparticlesizecanalsobededucedfrom thequotientbetweentwo consecutivemeasurementswherethefirst measurementis performedduringtheearlystagesof thecoolingpro-cessandthesecondsome300nanosecondslater. Theresultfrom suchameasurementis presentedin Figure3.9. It is evident from Figure3.7 that thequotientbetweenthe integral at two differenttimingswill bedependentof theparticlesize.

MeasurementsusingLII werealsoperformedat the departmentof fire safetyengineering,LundInstituteof technology(not yet published).Thestudiedflamewasa modelfire burningpropyleneasadiffusionflame.

Whentheprimaryparticlesizeis deducedfrom theparticlesize,theambientgastemperatureis avery importantparameter. Therearea limited numberof techniquessuitablefor two-dimensional

4Xavier Georges,guestdiplomaworker from ENSMA


−200

200

10

200

0.5

1

HAB / mmRadial position / mm

Soo

t vol

ume

frac

tion

/ ppm

−200

200

10

205

10

15

20

25

30

HAB / mm

Radial position / mm

Par

ticle

siz

e / n

m

Figure3.8: Sootvolumefractiondistributioninanethylene-airflame.

Figure3.9: Primaryparticlesizedistribution inanethylene-airflame.

measurementsof temperaturein sootingflames.Oneof thesemightbefilteredRayleighscatteringandasa partof his diplomawork Xavier Georgesmadesomeinitial feasibility studiesduringthesummerof 2000.

An LII measurementcanprovide informationof thetwo-dimensionalsootdistribution. However,unlessthereis a calibrationsourcethis informationis only qualitative. Thereforethenext inves-tigationwasthedevelopmentof a novel techniquefor combined,simultaneousmeasurementsofsootvolumefraction andabsorption.The techniqueenablesquantitative two-dimensionalmea-surementsof sootvolumefraction, usingonly a singleNd:YAG laseranda singledetector. Si-multaneousinformationof the sootvolumefraction andthe absorptionis collectedon the samedetectorimage.In thatway theabsorptionmeasurementcanbeusedto calibratetheLII measure-ment.If asecondimageof theLII intensitycouldbecollected,some300nsbeforethefirst one,theinformationof theparticlecoolingcouldbeusedto deducetheparticlesize. Unfortunatelytherearestill no image-intensifiedCCD detectorsthatarethis fast.Thearticle,containingthedetailsofthetechniquehasbeenacceptedfor publicationin AppliedPhysics[Axelssonet al., 2000b].

During thespringof 2000sootmeasurementswereperformedin cooperationwith JohanHult andClemensKaminskiat thedivision. TheYAG laserclusterwasusedfor three-dimensionalimagingof the sootvolumefraction in laminarandturbulent flames. The resultshave beensubmittedtoExperimentsin Fluidsandarecurrently(24/10/2000)underreview. An exampleof theresultscanbefoundin Figure3.10.

As a validation for the laser-basedparticlesizemeasurements,preparationsfor thermophoreticsampling(TS) of soot particlesfollowed by transmissionelectronmicroscopy (TEM) are nowbeingdone. Measurementswill be performedduring autumn2000. The resultswill hopefullyconfirmthelaser-basedmeasurementsandalsoprovide informationof thephysicalappearanceofthesootparticles.


ëìíî

ïðñòó&ôõ

ö�÷3÷3ø ùûú3ú3ü ýÿþ3þ��

Figure3.10: Resultsfrom a 3-D measurementin a laminarethylenediffusionflame(lower right).Eight, quantitative, two-dimensionalimagesof the soot volume fraction (top) are combinedtoiso-concentrationsurfaces(lower left).

3.1.5 Polarisation Spectroscopy

J. Walewski,C. F. KaminskiandT. Metz

Polarisationspectroscopy (PS) is an experimentallyfairly simplenon-lineartechniquewith thepotentialof selective detectionof moleculeswhich arenot accessiblewith LIF. Anyhow, to turnPSinto aworkingtool for combustiondiagnosticsthedemonstrationof thistechniquefor anumberof combustionspeciesis not enough.In thenext steponehasto realisethequantificationof thePSsignal in termsof numberdensity, temperatureetc. Oneneedsin otherwordsa quantitativetheorywhich - in order to turn PSinto an applicabletool - hasto be analytically fairly simple.A realistic theory for PSwaspresentedby ReichardtandLucht5 andGiancolaet al.6, but theirapproachis basedonatime-consumingintegrationof thequantum-mechanicalLiouville equation.To comparethe predictionsof their PStheoryandto derive an approximative descriptionof thedependenceof PSsignalsonparameterssuchaslaserirradiance,collisionrateandnumberdensity,aseriesof experimentswereconducted.

TheexperimentscarriedoutwerePSandLIF measurementsin stoichiometriclaminarlow-pressureflameswith methaneandoxygenasthefuel mixture.ThechosenspecieswasOH, sinceit inheritsa simpleandwell-understoodspectrum.Low-pressureflameswerechosenin orderto beabletovary thecollision ratein the probevolumewithin a large orderof magnitude,sincethis ratehasa pronouncedeffect on thePSsignalstrength.5 � 6 Signalsweremeasuredfor pressuresbetween30

5T.A. ReichardtandR.P. Lucht. Theoreticalcalculationof line shapesandsaturationeffectsin polarisationspec-troscopy. J. Chem.Phys.,109(14):5830–5843,1998.

6W.C. Giancola, T.A. Reichardtand R.P. Lucht, Multi-axial-mode laser effects in polarisationspectroscopy,J.Opt.Soc.Am. B, 17(10):1781–1793,2000.


and900mbarafterexcitationin the �� branch.

Threekindsof measurementswerecarriedout:

A. Powerdependenceof PSsignalsfor the �� line.

A simplealgebraicequationremindingof theAbrams-Lindequation7 for degeneratedfour-wavemixing was proposedfor the first time and fitted with greatsuccessto the power dependencecurves[Walewski et al., 2000a].

While evaluatingthesemeasurementswewerefacedwith theproblemthatfluctuationsof thelaserintensityduring the measuremententailedskew signaldistributions. The latter is causedby thenon-linearrelationbetweenlaserintensityandPSsignal.Theskewnessof thesignaldistributionsleadsto biasedestimatesof modelparameterswhenstandardfitting methodslike theleast-squaresandthe �� fitting schemesareused.We developedthe �� fitting schemeto take this skewnessofthesignaldistribution into account[Metz etal., 2000].Thisnew fitting methodwasappliedto ourmeasurementsandyieldedsignificantlyincreasedprecisionandaccuracy of thefitting parameterscomparedwith theleast-squaresfitting scheme.

Simulationsof themeasuredpowerdependenciesfor thesameconditionsasin our measurementsarerecentlycarriedout by thegroupof R.P. Lucht8 andwill becomparedwith our experimentalresults.

0 5 10 15 200

20

40

60

80

100

h/mm

Sig

nal/a

.u.

PS0.5

LIF

Figure 3.11: Relative LIF and � � � signalsas a function ofheightover thelow-pressureburnerfor p = 300mbar.

B. Recording of partially sat-urated PS and LIF signalsalong the centreline of theflame.

In Figure3.11therelativeLIFsignal and the squareroot ofthePSsignalfor p = 300mbaris shown. The LIF sig-nal waspartially saturatedandhenceonly weakly dependingon quenchingrate and colli-sion rate. Neglecting the de-pendenceof the PS signal onthecollision ratethePSsignaldependson the squareof thenumberdensity.9 As onecanseein Figure3.11 differ bothsignalsonly slightly from eachother. The quenchingrate is

7R.L. AbramsandR.C.Lind, Degeneratefour-wavemixing in absorbingmedia.Opt.Lett., (3):94–96,1978.8Departmentof MechanicalEngineering,TexasA&M University, CollegeStation,TX 77843-3123,USA.9R. E. TeetsandF. V. KowalskiandW. T. Hill andN. CarlsonandT. W. Hansch,LaserPolarizationSpectroscopy.

Proc. Soc.Photo-Opt.Instrum.Eng., 113:80–87,1977.


expectedto changewith approximately20% whenscanningthroughthe flamefront10 and thismeasurementindicateshencethat the dependenceof the PSsignalon the collision rateandthequenchingis only very weak [Walewski et al., 2000a]. This is a very promisingfeatureof PSregardingtheapplicationof thismethodfor concentrationmeasurements.

C. PSspectra of the !�" branch for differentexcitationintensities.

Therecordedspectrahavenot beenevaluated,yet.

Theresultsof measurementA andB will bepublishedin 2001.

3.1.6 SurfaceTemperatureMeasurementUsingThermographic Phosphors

A. Omrane, F. Ossler, M. AldenandU. Goransson11

During the last tenyearsa new techniquehighly sensitive to temperaturehasbeendevelopedforremotesurfacetemperaturemeasurementusing thermographicphosphors.This techniqueis ofa high interestfor industrial andscientific applications. In this work we investigatethe useofphosphorsin combustionapplications.Surfacetemperaturesof burning materialsaremeasuredandcomparedto thermocoupleresults. The investigatedmaterialsareboardsof low-, medium-densityfiber, andPMMA.

Figure3.12:Lifetime variationversustemperatureof thephosphorMg# (F)GeO$ :Mn

10P. MonkhouseandS.Selke,Energy transferin the %�& ')( stateof OHfollowing * +-,�. excitationin alow-pressureCH/ /O& -flame.Appl. Phys.B, 66:641–651,1998.

11Fire engineeringdepartment)


Figure3.13:Experimentalset-up

Calibration investigationhas beenachievedspectrally and temporally on the selectedphosphor(Mg0 (F)GeO1 :Mn). Sincelifetimecalibration has shown higher sensitivity totemperatureFigure3.12we have chosenit tobe thebasefor our measurement.Thedecayof the resultingemission(phosphorescence)signal was fitted to an iterative exponentialfit 2�3 4 57682 9;: <�=�3 >?4 @ A)5 , so the lifetime isdeducedandplottedagainsttemperatureFig-ure3.12.

Radiationfrom a nsNd:YAG laserat 532nmwas frequency doubled to 266nm a wave-lengthsuitablefor exciting thephosphors.Aholewasdrilled on thematerial( BC6CD mm)andfilled with thephosphor. Theboardwaswetby aheptaneor ethanolat thebottomandthenignited.Thelaserlight wasdirectedontothefilled holeandthesubsequentemissionwascollectedby afibercloseto themeasuredsurfaceandtransmittedto aphotomultiplier. Thephosphorescencewasstoredin thecomputerfor subsequentprocessing.

E F G H I I J K L M N N O P Q R S STU VW X XY Z [\ ] ]^ _ à b bc d ef g gh i jk l l

m�n o?p q r s

t uvwuxyz {xu|}~� � � � � �

� � � � �

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Figure3.14:SurfacetemperaturetestedonLow densityfiberboard

The surfacetemperaturewasmeasuredusingphosphorescenceanda thermocouple,which wasmountedonto the surfacecloseto the phosphor. Figure3.14 shows that surfacetemperatureoflow-density-fiberboardswhile burning is around450 � C, anotherteston PMMA materialshowsthatlowersurfacetemperatures��-�� C arereached.


Thermographicphosphorstechniquehasshown to beeffective undernonsootingconditionsandtwo dimensionalmeasurementof temperaturein nonsootyenvironmentwill beperformed.

3.2 TechniqueApplication

3.2.1 HCCI Activity

M. Richter, A. Franke, J. Engstrom,A. Hultqvist12 andB. Johansson12

In a HomogeneousChargeCompressionIgnition (HCCI) engine,thefuel is premixedto createahomogeneouscharge. During thecompressionstroke, thechargeis heatedto obtainautoignition,preferablycloseto TDC.Thesetwo traitscarrysimilaritiesto theconventionalsparkignitedengineandthedirect injecteddieselengine.Onecouldthink of theHCCI engineashybrid,however, thecombustionmodeis completelydifferent.In SparkIgnition (SI) engines,largecycle-to-cyclevari-ationsoccursincetheearlyflamedevelopmentvariessignificantly. Thecycle-to-cycle variationsof theHCCI combustionprocessareverysmallsincecombustioninitiation occursin many placessimultaneously. HCCI hasno conventionalflamepropagation,insteadthe whole mixture burnscloseto homogeneousduring combustion. As the whole bulk burnsalmostsimultaneously, thecombustionratebecomesveryhigh. Thereforehighly dilutedmixtureshave to beusedin ordertolimit therateof combustion.Themajoradvantagesof HCCI comparedto thedieselengineis lowNOx emissionsand,dependingonthefuel, lessproblemwith smoke. In dieselenginesit is difficultto reducebothNOx andsootsimultaneouslythroughcombustionimprovement.ThebenefitwithHCCI comparedto the(SI) engineis themuchhigherpart loadefficiency. Themajorproblemisto controltheignition timing overa wide loadandspeedrangeandto obtainanacceptablepowerdensity. Powerdensityis limited by combustionnoiseandhighpeakpressures.Anotherdrawbackcomparedto SI anddieselenginesis thehigheremissionsof unburnedhydrocarbons.

HomogeneousCharge CompressionIgnition (HCCI) enginehave beeninvestigatedin order tocharacterisethecombustionprocess.

Absorptionmeasurements

Theabsorptionof light propagatingthroughthecombustionchamberhasbeenspectrallyresolvedfor differentfuels.Significantdifferencesbetweenthefuelscouldbedetected.A Deuteriumlampwasusedasbroadbandlight sourcein the200–350nmspectralregion. In orderto coverthevisibleregion a high pressureXenon lamp was also usedin a similar setup. Collimating lenseswereusedto createa thin sheetof light that could transversethe combustionchamberof the engineunobstructedindependentof crank angle. The light sheetwas then focusedonto the slit of aspectrometer(Acton ResearchSpectraPro-150). The spectrallyresolvedsignalwasdetectedbyan imageintensifiedCCD camera.A crankangleencoderwasusedto trig the camera.Hence,crankangleresolvedmeasurementsof theabsorptioncouldbeperformed.For moreinformation,see[Richteret al., 1999b].

12Departmentof CombustionEngines

3.2. TECHNIQUE APPLICATION 23

254� 256� 258� 260� 262� 264� 266� 268� 270�λ / nm

0.0

0.2

0.4

0.6

0.8

1.0

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measured datafit

O�

2

N�

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Fuel

Figure3.15:Single-shotRamanspectrumfrom HCCI engine.

Chemiluminescence

An experimentalstudyof the HCCI combustionprocesshasbeenconductedby usingchemilu-minescenceimaging. The major intent was to characterisethe flamestructureand its transientbehaviour. To achievethis,timeresolvedimagesof thenaturallyemittedlight weretaken.Emittedlight wasstudiedby recordingits spectralcontentandapplyingdifferentfilters to isolatespecieslikeOH andCH. Theresultsfrom thisactivity arereportedin [Hultqvist etal.,1999;Richteretal.,1999b].

Ramanmeasurements

For an absolutedeterminationof the air-to-fuel ratio, Ramanscatteringof laserlight wasused.Similar experimentshave beencarriedout beforeby Grunefeldet al.13 and Richteret al.14. AKrF excimer laserwasusedfor excitation. Theexperimentsresultedin images,holdingspectralinformationin onedimensionandspatialinformationin theother. Thosetwo-dimensionalimageswere binnedto obtain a spectrumwith high signal-to-noiseratio. An examplecan be seeninFigure3.15. The areaundereachpeakis proportionalto the concentrationof molecules.It wasrevealedthatsinglepulseRamanmeasurementscouldprovide, cycle resolved,absolutevaluesoftheequivalenceratio with a precisionbetterthan5%. For moreinformation,see[Richteret al.,1999b].

13G. Grunefeldt,V. Beushausen,P. AndresenandW. Hentschel,“Spatially resolvedRamanscatteringfrom multi-speciesandtemperatureanalysisin technicallyappliedcombustionsystems:Sprayflameandfour-cylinder in-lineengine”,Appl. Phys.B 58, 333–342(1994).

14M. Richter, B. Axelsson,K. Nyholm andM. Alden, “Real-time calibrationof laser-inducedfluorescenceair-fuel ratio measurementsin combustionenvironmentsusingin-situ Ramanscattering”,in Twenty-SeventhSymposium(International)on Combustion, pages51–57,TheCombustionInstitute(1998).


Figure3.16:Experimentalsetupfor PLIF measurements.

PLIF measurements

In-cylinder crank-angleresolvedimagingof fuel andOH distributionswasobtainedusingplanarlaserinducedfluorescence(PLIF) in an HCCI engine. The schematicview of the experimentalsetupcanbeseenin Figure3.16.Investigationswerecarriedoutto ascertaintheextentto whichthecombustionprocessin anHCCI engineis affectedby thechargehomogeneity. In theexperiments,theheterogeneityof thechargewasvariedandtheeffectonthecombustionprocesswasmonitored.Theresultshows a heterogeneousonsetof combustionwith largespatialandtemporalvariations,evenwith a homogeneouslypremixedcharge,seeFigure3.17. It is thereforeconcludedthat thecharge inhomogeneityhasa modesteffect on the combustion process. For more information,pleasereferto [Richteret al., 2000a].

Figure3.17: Exampleof OH distribution at anearlystageof combustionin anHCCI enginewithhomogeneousfuel distribution.

3.2.2 High Speed2-D and 3-D Visualisationof Turbulent FlamePhenomena

J. Hult andC. F. Kaminski

Time resolved2-D measurementscanbeusedto quantitatively reveal theevolution of large-scaleturbulentstructures.Turbulence-chemistryinteractionsin flames,suchasflameextinction andre-ignition, which area challengeto moderncombustionresearch,canbestudiedaswell asthedy-namicsof flameignition andpropagation.As turbulenceis anintrinsically three-dimensionalphe-nomenon,three-dimensionalmeasurementsof scalarsarehighly desirable.Thethree-dimensionalinformationis necessaryto reveal thetopologyof turbulentflamesandmixing layers,andalsotomeasureinstantaneous3-D gradients.


Figure3.18: Top row: High speedOH PLIF in a non-premixednatural-gas/airflame, ¡ t=500 ¢ s.Bottomrow: Simultaneousemissionimages.

At thedivisionof combustionphysicsin Lundalaseranddetectorsystemfor highspeedimagingofreactive andnon-reactive flows hasbeensetup. Thesystemcanbeusedfor acquiringa sequenceof up to 8 imageswith repetitionratesrangingfrom 100Hz up to 100MHz. The goodspatialresolutionof thedetectorandthehigh outputpower of thelaserenablesthesystemto beusedfora variety of laserdiagnosticmethods.The high repetitionrateof the systemalsoenablesthree-dimensionalmeasurementsby rapidly sweepingthe lasersheetthroughthe measurementobject.The lasersourceis a Nd:YAG lasercluster(BMI, France),consistingof four individual Nd:YAGlaserswhich arecombinedinto oneunit, eachlasercanbeoperatedin doublepulsemode,firingtwo laserpulseswithin a shorttime interval. Theunit alsocontainsopticsto combinethebeamsfrom thefour lasersinto onebeampathwith minimal energy losses.Thedetectoris a high-speedcamerabasedon eight individual CCD cameras(DRS Hadland,UK). The imageis collectedbya commonoptical systemand is then split into eight identical copies; theseare relayedto theindividualcameras,whicharethenexposedin series.

Time resolved OH PLIF measurementshave beenperformedin several turbulent non-premixedflames. Figure3.18shows measurementsperformedin a piloted natural-gas/airflame(the Delftflame),£�¤¦¥¨§�© ª�ª . An OH PLIF sequenceis shown in theupperrow andsimultaneouslyrecordedflameemissionin thebottomrow. Theimagedareacorrespondsto «�«�¬¦ ª mmtakenjustabovetheburnernozzle.In theimagesflamevortex interactionin thenearnozzlefield is illustrated,vorticesinducedby the innershearlayer interactstronglywith theflamefront, stretchingandthinning it.This leadsto asmallerheatreleaserelativeto theheatlosses,whichdecreasesthetemperatureandextinguishestheflame.Theimagesequenceindicatesthatthis vortex-flameinteractionprocessisperiodic. From the emissionimages,which give line-of-sightinformation,it is clearly seenthattheextinction is completeall aroundtheflameaswholestructuresdetachesfrom theflamebase.


This studywasperformedaspartof a LLC projecttogetherwith T. Ding, Th. VanderMeerandM. Versluisfrom Delft Universityof Technology, Netherlands[Ding etal., 2000].

Figure3.19:High speedOH PLIF of theflamestabilisationregion in a lifted non-premixedH ® /airflame, ¯ t=30 ° s.

Figure3.19shows an OH PLIF imagesequencetaken in a lifted H ® /air flame, ±�²´³¶µ ·�¸-¸�¸ , thetimeseparationbetweenconsecutiveimagesis 25 ° s. Thetimeresolved2-D measurementsof OHconcentrationspresentedhereallowedstudiesof the temporalevolution of large-scalestructures,whichwill provideadditionalinsightsinto theimportanceof thesestructuresfor flamestabilisationandlift-of f. The imagedareacorrespondsto ¹»º7¼;½ ¾ mm; oneof the flamefronts at the lift-of fpositionis seen.In theimagesequenceentrainmentof fuel into theflameis seen,astheflametipcurls inwards. This studywasperformedaspart of a LLC project togetherwith A. Brockhinkefrom UniversitatBielefeld,Germany.

Figure3.20: Simultaneousmeasurementof flamefront structureandflow field, usingOH PLIFandPIV, in a turbulentnon-premixedflame, ¯ t=125° s.

Figure 3.20 shows simultaneoustime resolved OH PLIF and PIV in a CH¿ /H ® /N ® /air flame(TECFLAM burner), ±¦²ÁÀÃÂ ¸�¸-¸�¸ . Extinctionphenomenawerestudiedby simultaneouslymap-ping theflow field by usingPIV andstudyingthetimeevolutionof OH structuresby usingLIF. InFigure3.20a localextinctioneventis seen,theflow field correspondingto thesecondimagein theOH PLIF sequenceis plottedon topof this image.Thetimeseparationbetweenimagesis 125 ° s,andthe imagedregion is µ ¼ÁºÄµ · mm. In theflow field a vortex from the fuel side(to theright)is seenimpinging on the flamefront andextinguishingthe flamefront locally. This studywasperformedtogetherwith G. Josefssonfrom Volvo TechnologicalDevelopmentCorporation[Hultet al., 2000b].

3-D imaging is performedby scanningthe lasersheetsfrom the Nd:YAG laserclusterusingafast rotating mirror, thus creating8 equally spacedparallel lasersheetswithin the flame. Theacquisitionof the 8 planarimageswasfasterthan the time scaleof the flow, thusproviding aninstantaneous3-D imageof theflame.In thetop of Figure3.21eightslicesfrom a turbulentnon-premixedC® H ¿ /air flameareshown, theimagesshow sootvolumefractionandwereacquiredusing


Figure3.21: 3-D imagingof sootvolumefraction in a turbulentdiffusionflame. Top: eight2-Dcuts throughthe flame. Bottom left: 1ppm iso-concentrationsurface. Bottom right: 3-D sootconcentrationgradient.

Laserinducedincandescence(LII). Theimagedregionsare Å ÆÈÇ7Å-É mm largeandtheimagesareseparatedby 0.4mm from eachother. To the bottom left the 3-D iso-concentrationsurfacefor1ppm is shown, to the bottomright the 3-D sootconcentrationgradientcorrespondingto slicenumber5 is shown. This studywasperformedtogetherwith A. Omrane,B. Axelsson,R. CollinandJ.Nygren[Hult et al., 2000a].

3.2.3 Temporally Resolved Single Cycle Measurementsof Fuel- and OH-Distrib utions in aSpark Ignition EngineUsingHigh SpeedLaserSpec-tr oscopy

J. Nygren,M. Richter, J. Hult, C. F. KaminskiandM. Alden

Laserspectroscopy is a powerful tool for performingenginediagnosticswith high temporalandspatial resolution. The conventional techniquefor generatingcycle-resolved measurementsofspeciesconcentrationis to collectonedatasetfrom independentenginecyclesatsubsequentcrankangles.However, dueto thecycle-to-cyclevariationsin combustionenginesthisstrategy will onlyproduceaveragedresults.Ideally all datashouldberecordedwithin a singlecycle. Until now thishasnotbeenpossibledueto thelackof high-speedtuneablelasersourcesanddetectors.

True singlecycle resolved measurementsof fuel distribution, OH-distribution andchemi-lumin-escencein a laboratorysparkignition (SI)-enginehasnow beenperformed.A uniquelaseranddetectionsystemfor high-speedimagingwasusedfor this purpose.The lasersourceconsistsoffour individual Nd:YAG lasers,whosebeamswerecombinedinto oneoutputbeam. Eachlasercanbeoperatedin doublepulsemode,which givesa maximumnumberof eightpulses.Thetimedelaybetweenthepulsescanbevariedfrom a few nanosecondsto 100milliseconds.A framingcamera,capableof recordingeightimageswith a maximumrepetitionrateof 100MHz, wasusedasdetector.


Figure3.22: Imagesequenceshowing theevolution of OH concentrationfield distribution duringonesinglecycle in theengine.Thetimeseparationbetweenimagescorrespondsto 100 Ê s.

PlanarLaserInducedFluorescence(PLIF) was usedboth for the fuel visualisationand for theOH measurements.3-pentanonemoleculeswereusedastracerspeciesfor the fuel. The funda-mentalYAG-wavelengthwasquadrupledfor excitationof 3-pentanoneat 266nm. After onsetofcombustion,the3-pentanonemoleculesactasmarkersfor theunburnedregions.To investigatethechangesof thereactionzoneduringasinglecycle,OH fluorescencewasused;seeFigure3.22.Forexcitationat 283nm a dye-laser, pumpedby theNd:YAG-cluster, wasused.Theframingcamerarecordedthecorrespondingfluorescenceat309nm. Theseimagesvisualisetheflamefront aswellaspostflameregions.Integralaspectsof flamepropagationcouldbeobservedon theline-of-sightchemi-luminescenceimages,alsorecordedby thefastframingcamera.

3.2.4 SpectroscopicInvestigationson Biofuel Pyrolysis

C. Brackmann,P.-E.BengtssonandM. Alden

The pyrolysis of wood (biomass)particlesis a complex processthat dependson both particlepropertiesandthe surroundings.During the heatingof a solid particle,free moistureevaporatesandbreakdown of themoreunstablepolymersbegins. Thevolatile materialflowing out from thesolid is thenableto participatein secondaryreactions.Dependingonthesurroundingenvironmenttheprocessesof particledrying,pyrolysis,andgasificationmayoverlapin acomplicatedway.

A specialreactorfor fundamentalexperimentalstudiesof thepyrolysisof singleparticleshasbeendesignedandconstructedat the Division of PhysicalChemistryat Gothenburg University. Themassof theparticleis registeredwith thermogravimetric analysis(TGA) andthecompositionofthe emittedgashasbeenstudiedwith massspectrometry. Furtherstudieswith molecularbeammassspectrometryareplanned.This reactoris illustratedin Figure3.23

As apartof thebiofuelprogramwithin theCECOSTgraduateschooltheDivisionof CombustionPhysicshasparticipatedin thesestudiesby usingdifferentspectroscopictechniquesto obtainmoreinformationaboutthepyrolysisprocess.

Preparationsfor the measurementsin the speciallyconstructedreactorin Gothenburg wereper-formedby studiesof pyrolysis in a simplerexperimentalarrangement(regardingthepyrolysisofwood)in Lund.

Measurementsweredoneusingabsorptionspectroscopy andspectrallyresolved aswell astwo-dimensionalimaginglaser-inducedfluorescence(LIF) to retrieve informationaboutgascomposi-tion andgasflow in thevicinity of theparticle.

The studieswere performedon cylindrical birch particleshaving a massof about70mg. The


2

4

4

h ori zo ntal v i ew v erti cal v i ew

3 0 0 m m6 6

Figure3.23:Horizontalandverticalviewsof thepyrolysisreactor. 1) massspectrometry-capillary,2) connectionto theTGA balance,3) insertiontubefor particles,4) heater, 5) flow of heatedgas,6) platefor thesample.

pyrolysis processwasstudiedat different temperatureswith the different techniques.A specialstudy with imaging laser-inducedfluorescencewas performedon particleswith a well-defineddirection of the wood fibres. The aim of this study was to try to relatethe gasflow from theparticleto thewoodfibreorientation.

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(a.u

.)Í

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a) b)

Figure3.24: Examplesof a) transmissioncurve andb) LIF spectrumof emittedgasfrom a woodparticleduringthepyrolysisprocess

The resultsfrom thesemeasurementsare to be evaluated,however, preliminary studiesto getan overview of the materialhave beendone. Figure3.24 illustratesexamplesof oneabsorptionspectrumandoneLIF spectrum.


Figure 3.25: LIF imageshowing an exampleofthedistributionof pyrolysisgasin thevicinity of aparticle.Theparticlehasbeenaddedto thepicturefor clarity

The absorptionis broad in the UV regionwithout any distinct structures,which illus-tratesthecomplex multi-speciescompositionof thepyrolysis gas. TheLIF spectrumcon-sistsof a broadstructure,which makesiden-tification of specific speciesdifficult. Thissignal, however most likely arisesfrom hy-drocarbons,so it might be possibleto inter-pret someinformationfrom thesespectrabycombiningpreviouslaserspectroscopicstud-iesof hydrocarbonswith massspectroscopicdatafor relevant masses.The interpretationof the imagesfrom the two-dimensionalLIFmeasurements,suchasthe oneillustratedinFigure3.25,will besupportedby furtherex-perimentsin Lund.

3.2.5 Investigationon DME Sprays

O. Andersson,R.Collin andM. Alden

Dimethyl Ether(DME) hasprovedto bea promisingfuel for dieselengines.Without negativeef-fectson efficiency, DME eliminatesparticulateemissionsandsubstantiallyreducestheformationof oxidesof nitrogen. Thus,a deeperunderstandingof themechanismsmakingDME a superiordieselfuel is highly desirable.Suchknowledgehasmajor implicationsalsofor dieselcombus-tion sinceit couldcontributeto a reductionof emissionsfrom dieselenginesrunningon standard

Figure3.26: Schematicdescriptionof the experimentalset-up. To the left; laser-sheetformingoptics.Abovethevessel;narrowbandmirror andCCDcamera.

or slightly modified fuels. The investigationsdeal with DME spraycombustion. Quantitativeequivalence-ratiomeasurementsperformedwith laser-Rayleighimaginghave beenmadeaspre-sentedin [Anderssonet al., 2000]. Themeasurementswerecarriedout in a chemicallypreheatedcombustionvesselof constantvolume.Thegoalwasto studytheconditionsat thetimeof ignition


of thespray. An investigationhasalsobeenperformedutilising thesametechniquein anopticallyaccessibleDI Dieselengine.The result is presentedin [Anderssonet al., 2001]. In this applica-tion therelative fuel concentrationswereobtainedfrom thestartof injection(SOI) up to thepointwhereall fuel hadbeenconsumed.Theaim wasto studythespraydevelopmentandto comparetheresultsto thoseobtainedin thevessel.Thetwo investigationsarepresentedbelow.

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Figure3.27:Equivalenceratioandtemperaturefield measuredwith Rayleighscatteringfor anon-reactingspray. Taken1.1msafterstartof injection.

Thevesselusedin [Anderssonet al., 2000]wasa cylindrical steeltank,252mm in diameter. Tosimulatethehot atmospherein a dieselengineat thetime of injection,thevesselatmospherewaschemicallypreheatedby igniting alean,homogeneousmixtureof COandoxygen-enrichedairwithasparkplug. After thispremixedcombustiontheatmospherecontainedabout18% oxygen,alevelof dilution correspondingto adieselenginewith 15% exhaustgasrecirculation(EGR).Whenthetemperaturehaddroppedto 700–800K andthepressurehasreachedthe25barregime,DME wasinjectedinto the vesselusinga common-raildieselinjector. Figure3.26shows the arrangementof theexperimentalset-up.Thelasersheetfrom a KrF excimerlaser(248nm) illuminateda crosssectionthroughthe DME sprayat different timesafter the startof injection. Figure3.27 is anexampleof evaluatedequivalenceratioandtemperaturefor anon-reactingspray. After processing,thequantitativeimageswereanalysedusingastatisticalapproach.It couldbeseenthatzoneswithequivalenceratio Î�ÏCÐ;Ñ Ò andleanerwerelesscommonin theburningsprays.Thus,combustioncommenceswhenmixing hastransferredfuel to zoneswith Ð;Ñ ÒÔÓÃÎÄÓÃÕ�Ñ Ð . Whenstudyingthe


imagesit couldbeconcludedthattheflamewasinitiatedat theperipheryof thespray.

Theengineusedfor theinvestigationsin [Anderssonet al., 2001]wasanopticallyaccessibleone-cylinder versionof a six-cylinder 12-l diesel truck engine. Sprayswere studiedfrom the SOIuntil therewasno fuel left in the imagesfor differenttemperaturesandpressures.In theengine,autoignitionoccurredvolumetricallythroughouttheleadingportionof thespray, asopposedto thecombustionvesselwhereautoignitionoccurredat thesprayperiphery.

Theresultsshow thatfuel sprayscanbehavequitedifferentlyin differentenvironments.Althoughstudyof theeffectsof temperature,density, andEGRon autoignitionwould yield valuableinfor-mation,sincecurrentmodelsdo not seemto give a generaldescriptionof this phenomenon.Thesprayshapeandstructurevariedduringacycle. Theearlyspraywascharacterisedby awidevortexregion. Combustioneventuallycommencedmoreor lesssimultaneouslythroughoutthis region.Later, during the slow heat-releasephase,the sprayhada narrow appearance.The imageareaoccupiedby fuel wasevaluatedandplottedasfunctionof crank-angleposition.Whencomparingthecurve to therateof heatrelease,a strongqualitativecorrelationwasnoticedbetweenthetwo.This resultsuggeststhattheheat-releaseratecanbecontrolledby meansof thefuel injectionrate,providing a tool for controllingtheNOx formation.

3.2.6 Activities in a Full Scale80MW PowderfueledBoiler

C. Lofstrom

Onekey factorfor optimumperformanceof SNCR-baseddeNOx systemsis that theammoniaisintroducedin acorrecttemperaturewindow around1220K.

Traditionaltemperaturemeasurementprinciplesusingstandalonethermocouplesor in acombina-tion with gassuctiondevicesareknown to beeffectedby severalhundreddegreesby thesurround-ing environment.Thiswork demonstratedtheuseof CARSasagastemperaturediagnostictool atthepositionof theammoniainjectionin a full scale80MW PFboiler.

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� � "! � # $ % & # �('*)

+ ,-. /.010 2 345

Figure3.28: TemperaturePDF from CARS measurements,singleshot(10ns), constantthermalload72MW

Thework wasdividedinto a laboratoryphaseanda full-scaletrial. Theaim of thelaboratorytrialwasto increasetheknowledgeof theoptical-particleinteraction.Temperaturemeasurementswerepossibleto performin the50kW laboratoryPFburner. With bio-powderasfuel,10% of theCARS


spectraweredistorteddueto particlesin the probevolume,andcould not be used. In the pointusedfor measurementtheaveragetemperaturewas1136K with astandarddeviation of 215K.

In the full-scaletrial, temperaturemeasurementswereperformedin the boiler whereoneof theammoniainjectionprobesis situated.SeeFigure3.29.MeasurementswereperformedatdifferentpositionsusingbothCARSandasuctionpyrometerduringdifferentrunningconditions.Changingtheplant loadandtemperatureshowedcomparableresultsfor thepyrometerandCARSbetween1170and1320K. However, thepyrometerwasnotableto capturetheturbulentphenomenatakingplacein theboiler. TheCARStechniqueshowedclearly temperatureslying well out of theopti-mumammoniainjectionregion. Figure3.28shows anexampleof a temperaturePDF. This workwaspresentedin [Lofstromet al., 2000a].

6 7 8 9 : 8 ;

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� � � � �� *� � � � �d� � � �

� � � � � � � � � � � � � � �d�

Figure 3.29: Visualisation set-up at the80MW boiler

Figure3.30:CARSsystemandsuctionpyrom-eterset-upat the80MW boiler

During thework it wasfound thataddinga dye to waterandusingLaserInducedFluorescence,LIF, couldbea way to visualisetheammonia/waterinjectionto theboiler. SeeFigure3.30. Thework alsoshowedthataddingsmall micron sizedparticlesto air andusingMie scatteringcouldbea way to visualiseair supplyto hot combustiongases.Sucha visualisationgivesan ideain atwo-dimensionalrepresentation“how it looks”. It doesnotgivevalueswith aknown accuracy andprecision.It can,carefullydone,howeverbeusedto comparedifferentsituations.


Thework doneby Vattenfall on theboiler including theactivities with the laserdiagnostictoolssubstantiallyloweredtheNOx emissionsfrom the80MW boiler.

35

4 Electrical Phenomenain Gases

4.1 Ionisation Sensor

A. Franke, P. Einewall1, B. Johansson1, A. LarssonandR.Reinmann2

4.1.1 Intr oduction

Theformationof ionsduringcombustionis stronglydependenton thecompositionof thegas,itstemperature,andthe underlyingchemistry. This offers the prospectof usingthe abundanceandtypesof ions asa measurefor the quality of combustion. By applyinga low voltageacrosstheelectrodegapof a sparkplug during combustion,a currentcanbe registeredwhich reflectsthepresenceof ionsaswell asotherlocal conditionsin thevicinity of theelectrodegap.Thecurrentsignalwhich canberecordedduringcombustionoftenconsistsof two moreor lessdistinctpeaks.Thefirst peakis usuallyexplainedasbeingaresultof chemi-ionisation, andoccurswhentheflamefront passestheelectrodes.Whentheflamekernelgrows further, pressureandtemperaturein theburnedgasaroundtheelectrodesincreaserapidly, andtheprocessof thermalionisationbecomesmoreprobable.Whenthepressurereachesa maximumvalue,a secondcurrentpeakcanoftenbeobserved.This techniqueis calledionisationsensingandhasbeenappliedfor combustionsensingin spark-ignitionengines;detectionof misfire andknock beingexamplesof currentlyavailableimplementations.

The researchperformedwithin this areaat our division is primarily aimedat the understandingof thebasicprocessesgoverningthe formationof chargedparticlesin theburninggasaswell astheir transportin theelectricfield in theelectrodegap. During thepastyears,both theoretical3 � 4andexperimental5 � 6 investigationshave beencarriedout, which resultedin the developmentofmodelsdescribingtheformationof thecurrentthroughtheelectrodegap.Accordingto themodeldescribedin oneof theseworks3, thelocal air-fuel ratio in thevicinity of theelectrodegapshouldaffect theformationof chargedparticlesin theelectrodegap,andthus,theamplitudeof thecurrentthroughthegap. In orderto checkthis, an experiment7 wasdesignedenablingtheestimationofthelocalmixturecompositionbyanindependentmethod,namelylaser-inducedfluorescence(LIF).Theresultsconfirmedthetheorythatthereexistsa relationshipbetweenthelocalair-fuel ratioandthe amplitudeof the first currentpeakin caseof quiescentmixtures. However, in particularforturbulent inhomogeneousmixtures,therestill remaineda considerableamountof variationin the

1Division of CombustionEngines,Lund Instituteof Technology2SAAB AutomobileAB, Sodertalje

36 CHAPTER4. ELECTRICAL PHENOMENA IN GASES

currentdatawhichcouldnot beexplainedby variationsof thelocalair-fuel ratio.

4.1.2 Study of the BasicProperties

A moresophisticatedevaluationof thedataobtainedin theexperimentmentionedabove7 revealedthat,evenin caseof turbulentinhomogeneousair-fuel mixtures,a strongrelationshipbetweenthelocal equivalenceratio andthecurrentsignalcanbefoundin certainregions,cf. Figure4.1. Thedarkshadedregionwherethecorrelationsarefoundespeciallyreliablehasasizecorrespondingtothatof theelectrodesused,andits shaperesemblesa ring. Although thecorrelationbetweenthetwo propertiescannotbedenied,thevariationin thecurrentdatais only to a minor partexplainedby the variationof the local air-fuel ratio. That indicatesthe existenceof other processes,i. e.turbulencewhich canbeassumedto beresponsiblefor a considerablepartof thevariationsin thecurrentdata.Experimentsinvestigatingtheroleof turbulencein theformationof thecurrentsignalhavebeencarriedout recently, andarecurrentlybeingevaluated.

1 cm

−lo

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Figure4.1: The region aroundthe sparkplug electrodescanbe seenin the figure. White boxesshow thelocationsof thesparkplugelectrodes.Darkshadingmarksregionswith highcorrelationbetweenlocal equivalenceratio andcurrentsignal. See[Franke, 2000] for a detaileddescriptionof theprocedureemployedto obtainthatgraph.

In ordertoextendourknowledgeabouttheoriginof thefirst currentpeak,directflamephotographywasappliedto determinethepositionof theflameat anarbitrarytime after ignition. It wasfound

3R. Reinmann,A. Saitzkoff andF. Mauss,“Local air-fuel ratiomeasurementsusingthesparkplugasanionizationsensor”,SAETechnicalPaper, 970856(1997).

4A. Saitzkoff, R. Reinmann,T. Berglind andM. Glavmo, “An ionizationequilibriumanalysisof thesparkplugasanionizationsensor”,SAETechnicalPaper, 960337,(1996).

5A. Saitzkoff, R. Reinmann,F. MaussandM. Glavmo,“In-cylinderpressuremeasurementsusingthesparkplugasanionizationsensor”,SAETechnicalPaper, 970857(1997).

6R. Reinmann,A. Saitzkoff, B. LassessonandP. Strandh,“Fuel andadditive influenceon the ion current”,SAETechnicalPaper, 980161(1998).

7H. Neij, A. Saitzkoff, R. Reinmann,A. FrankeandM. Alden,“Applicationof two-dimensionallaser-inducedfueltracerfluorescencefor ion currentevaluation”,CombustionScienceandTechnology, 140:295–314(1998).

4.1. IONISATION SENSOR 37

0 2 4 6 8 10 12time / ms

Cur

rent

/ a.

u.

Figure4.2: Thecurrentmeasuredduringcombustionis shown for eightdifferentcycles,alongwithflamephotographstakenatvarioustimesafterignition. Theobservationangleaswell astheshapeof theelectrodesusedhereareshown in thesmall imageabove. Thepositionof theelectrodesintheflamephotographscanbeseenin thelastflamephotograph.

thatthecurrentreachesa localmaximumwhile theflamefront is tangentto theelectrodes.This isillustratedin Figure4.2,wheretheshapeof theelectrodesis shown aswell.

Whenlooking for anexplanationconsistentwith theobservationsmadein theLIF experimentandin theflameimaginginvestigations,thefollowing scenariocanbeconstructed:

1. Thetotal resistanceof thegapis smallestwhentheflamefront is tangentto theelectrodes.

2. Thetotal resistanceof thegapis furthermoreinfluencedby thelocal equivalenceratio. Theequivalenceratio in a ring-shapedregionaroundtheelectrodesexhibitsaparticularlystrongcorrelationwith themaximumcurrentin thefirst peak.

3. Thetotal resistancecanbeseenasthesumof thecontactresistance(betweenelectrodesandgas)anda bulk resistance(in theflamefront), � �¡�£¢ ¤ ¥ ¦ § ¢ ¦ ¨ª©«�£¬ ® ¯°©«�£¢ ¤ ¥ ¦ § ¢ ¦ ± . Thattotalresistancewill behighwheneveroneof its threeconstituentsis high.

4. From(1)-(3), it canbeconcludedthat thecurrentis transportedalongtheshellof theflamekernel.

Dedicatedexperimentscheckingthecorrectnessof thathypothesisarecurrentlybeingconducted.

4.1.3 Application of the Techniquein a Lean-Burning Natural GasEngine

An ionisationsensorhasbeenusedto studythe combustionprocessin a six-cylinder leanburn,truck-sizedSI enginefuelled with naturalgasandoptimisedfor low pollutant(NOx) emissions.

38 CHAPTER4. ELECTRICAL PHENOMENA IN GASES

The final goal of the investigationswasto studythe prospectsof usingthe ionisationsensorforfinding theoptimaloperatingpositionwith respectto low NOx emissionandstableengineopera-tion. Thiswork hasbeenperformedin collaborationwith theDivisionof CombustionEngines.

Several factors,suchasair-fuel ratio, ignition positionandengineload, canbe expectedto in-fluencethe amplitudeof the currentsignal. Thesethreefactorshave beenstudiedon two levelsin a ² ³ factorial designexperiment. It was found that theseparametersaffect the currentbothindependentlyandin combinationwith eachother.

Theresults,which arecurrentlybeingpublished[Franke et al., 2001],indicatethatunstablecom-bustioncanbedetectedby analysingthecoefficient of variationof thedetectorcurrentamplitude.Closerelationshipsbetweenthis propertyandthecoefficient of variationin indicatedmeaneffec-tivepressurehavebeenfound.

4.2 Measurementof SurfaceCharge

M. Bengtsson8, A. LarssonandS.Kroll8

All electricalsystemsrely on thedielectricstrengthof their electricalinsulation.Theelectricallyweakest point of insulatorsystemsis normally the surfaceof the insulators. One propertyofthesurfacethatdeterminestheability of a surfacedischarge to propagateis the ability to boundelectricchargeat thesurface.Theobjective of this projectis to developa non-invasive techniquefor remotesensingof thechargedensityondielectricmaterials.Theprimaryareaof applicationisthedevelopmentof new polymericmaterialsfor high-voltageinsulators.Furtheron, thetechniqueis intendedbeusedalsofor measuringsurfacechargeon theceramichousingof sparkplugs.

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Figure4.3: Experimentalset-upfor themeasurementof thesecondharmonicgeneratedat a sur-face.

Theobjectivewill beachievedby utilising non-linearopticalprocesses.Whena high-power laserbeamis reflectedor scatteredataninterface,it maybesubjectedto anon-linearprocesswheretwophotonsof onefrequency aredestroyed andonephotonof twice the frequency is emitted. This

8Division of Atomic Physics,Lund Instituteof Technology

4.3. LIGHTNING AND AIRCRAFT 39

processis calledsecondharmonicgeneration(SHG). SHG is greatlyenhancedin the presenceof a spatialasymmetry. Surfacesandsurfacecharge areexamplesof suchspatialasymmetries.Theprojectis in its initial stageandthepresentstatusof theprojectis that theSHGsignalfromanaluminiumplatehasbeenmeasured.Currentactivities regardthe theoreticalanalysisandtheconstructionof anexperimentalset-upfor thestudyof chargeon metalsurfaces.

Theprojectreceivesfundingfrom theELEKTRA programmesupportedby Elforsk andtheELISprogrammesupportedby theSwedishFoundationfor Strategic Research(SSF).

4.3 Lightning and Air craft

A. Larsson,A. Bondiou-Clergerie9, Ph.Lalande9 andP. Laroche9

A lightning strike is a naturalelectricaldischargeandlightning strikesto aircraftarefrequentandunavoidableevents.On average,every civilian airliner is struckby lightning aboutonceperyear.Thus, the issueof lightning protectionof aircraft is of utmost importance. In responseto thisneed,theEuropeanCommissionsupportstheresearchprogrammeMethodsandTechnologiesforAircraft SafetyandProtectionagainstElectromagneticHazards(EM-Haz)within their Fifth RTDFramework Programme. Office Nationald’Etudeset de RecherchesAerospatiales(ONERA) isoneof thepartnersin EM-Haz. ONERA andtheDivision of CombustionPhysicscollaboratesina sub-projectwithin EM-Haz,namelyin the analysisof the lightning sweptstoke. This work isa continuationof the postdoctoralvisit by AndersLarssonat ONERA during 1998–99[Larssonet al., 2000a,b].

Figure4.4: An extract from a videofilm showinga lightning strike to anaircraftin flight.

During the lightning strike to an aircraft inflight, thelightning channelgetsdeformedintheaerodynamicflow anddisplacedalongtheaircraft,a so-calledsweptstroke. Thedefor-mation and the displacementare causedbytheinteractionbetweentheaerodynamicflowandthe plasmapropertiesof the channelto-getherwith the propertiesof the surface. Amain part of the lightning currentcomprisesof a continuouscurrentwith a magnitudeofhundredsof amperesanda durationof hun-dredsof milliseconds. High-amplitudecur-rent impulsesaresuperimposedon this con-tinuouscurrent. Eventually, the continuouscurrentstopsandonly high-amplitudecurrentimpulsesoccurseparatedby zero-currentintervals.During theflight of theaircraft,theattachmentpoint of the lightning channelwill sweepalong the body of the aircraft. The lightning currentandthe locationof the attachmentpoint arepotentialhazardsfor flight safety, both in termsofdirecteffects(thermalor mechanicaldamage)andof indirecteffects(electromagneticinterferencewith on-boardequipment).To beableto correctlydeterminethedifferentlevelsof hazardson anaircraft(zoning),thephysicalbehaviour of thelightning sweptstrokemustbeunderstood.

9Office Nationald’Etudeset deRecherchesAerospatiales,France

40 CHAPTER5. PULSATING COMBUSTION

5 PulsatingCombustion

S.-I.Moller

Theactivities reportedhereareincludedin a joint projectaboutpulsatingcombustionat Lund In-stituteof Technology. Thisprojectis acollaborationbetweentheDivisionof CombustionPhysics,theDivision of MechanicsandtheDivision of Energy EconomicsandPlanning.Pulsatingcom-bustionis a spontaneouslyoscillatingcombustionprocessi. e. velocity, pressureandheatreleaseetc.vary periodicallyin time dueto interactionbetweenthe flow dynamicsandthe chemicalre-actionsin thecombustionprocess.Pulsatingcombustionhasbeensuccessfullycommercialisedinapplicationssuchasdomesticwaterheaters.It hasbeenobserved that this techniqueis advanta-geousto traditionaltechniquese.g. in termsof lower emissionlevelsandhigherfuel efficiency.However, the spontaneousoscillationsappearonly whenthe flow dynamicsandthe combustionprocessform a positive feedbackloop. This implies restrictionson how thepulsecombustorcanbe designedfor a given setof operatingparameters.Traditionally, pulsecombustorshave beendesignedfrom experienceor developedwith trial anderrormethods.

The aim of the project is to increasethe fundamentalknowledgeof this combustionprocessbymeansof experimentalandtheoreticalinvestigations,which arecarriedout in closecollaborationwith eachother. Particularly theprocessesin thecombustionchamberarestudiedin detail. Theexperimentalpulsecombustorshave optical accessin order to allow for non-intrusive measure-mentsof velocity, heatreleaseandtemperatureetc.Thechemicalreactionsarestronglycontrolledby the enhancedmixing in the turbulent flow, thereforethe velocity field andheatreleasefieldhavebeeninvestigatedfor variousoperatingconditions.Similar theoreticalinvestigationsin formof numericalsimulationshave beencarriedout. The possibility of usingnumericalsimulationsasa designtool have beenstudiedin termsof validationsbetweensimulationsandexperiments.Anotherobjectivewith theprojectis to suggestandstudyotherpossibleapplicationsof pulsatingcombustion.

The major contribution from the Division of CombustionPhysicsto this project is detailednu-mericalsimulationsof non-steadyturbulentchemicallyreactingflow with particularemphasisonsimulationof pulsatingcombustion.Thenumericalsimulationmodelusedhereis ageneralmodelfor time dependentturbulentflow of chemicallyreactingmixtures. Themodelis baseduponthetheory of mixtureswithin continuummechanicsandutilises the Large Eddy Simulation(LES)concept.TheSmagorinsky modelis usedfor modellingof thesub-gridscalestressesandfluxes.The chemicalreactionratesareestimatedfrom the kinetically controlledreactionratesand themixing rates.Velocity field, pressure,temperatureandspeciesconcentrationsareoutputparame-tersfrom themodel.A submodelfor theformationof nitrousoxidesis includedin themodel.Thesimulationmodelis implementedinto theCFD-codePhoenics,CHAM Ltd., London.

For validationof themodel,simulationsof pulsecombustorsof Helmholtztypecorrespondingtoexperimentalpulsecombustorsdesignedandoperatedat the division of Energy Economicsand

41

planninghave beencarriedout. Naturalgasis usedasfuel. In thestudy, [Moller andLindholm,1999],theability of themodelto predicttheoperatingcharacteristicsfor differentinlet geometrieswas investigated.Here the pressureandheatreleasecycle was investigatedaswell as tail pipetemperatureandOL andNO concentrationsat theoutletof thepulsecombustor. It wasfoundthatthetiming betweentheheatreleaseandthepressurewasaffectedby thedifferentinlet geometries.Also the emissionof NO waschanged.The modelwasfound ableto predictthe differentcom-bustioncharacteristicsbothin termsof thetiming of thecombustioncycleaswell asregardingtheemissionlevels.

TheDivision of CombustionPhysicshasalsoparticipatedin anappliedproject(cofiring in largerfurnaceswith pulsecombustors)wherethreepulsecombustorsof Helmholtz-typehave beenin-stalledin a wastefurnaceoperatedby SYSAV, Spillepengen,Malmo. Theaim of this experimentis to usethe pulsatingjets to changethe flow field in the large furnaceandtherebyincreasethemixing in orderto breakup regionswith high CO contents.Thepulsecombustorsweretestedin1999duringtwo separateperiods.TheresultsshowsthattheCOemissionswereloweredby arateup to 20%. Especiallythepeaksof COwerereduced.

42 CHAPTER6. CHEMICAL KINETICS

6 ChemicalKinetics

6.1 FundamentalKinetic Investigations

6.1.1 A Computational Study of SootParticle Thermal Ionisation

M. Balthasar, F. MaussandH. Wang1

It is well known that a significantfraction of soot particlesformed in flamesare positively ornegatively charged. In purehydrocarbonflamestheseparticlesusuallyhave no more than twocharges,andareoften presentasa resultof thermalequilibrium with neutralparticles,electronsandchargedspecies.This processis known asthermalionisation. For example,positively uni-chargedparticlesmaybeproducedfrom theequilibriumprocessM NPORQ NTSVUwhere

M NandQ N

areneutralanduni-chargedparticles,respectively.

Early studiesshowedthatchargedparticles( WYX Z\[ Z]Z�Z amu)andthenumberof chargeson theseparticlescouldbe satisfactorily accountedfor with theSahaequation.Dependingon flametem-peratureandparticlesize,asmany as30% of flamesootmaybethermallyionised.Thenumberdensityof chargedparticlesincreaseswith anincreasein flametemperature.In addition,thepoten-tial energy thresholdof particleionisationdecreaseswith anincreasein particlesize.As a result,chargedparticlestendto populatein high temperatureflamesandin large-sizeregion of thesizedistribution function.

Thermalionisationcan produceat leasttwo effects that are potentially importantto soot-massgrowth. First, thecoagulationratesbetweenchargedandneutralparticlesandparticlesof oppositecharge tendto be larger thanthatof neutral-neutralparticles.This effect is causedby collisionalenhancementrelatedto theinteractionbetweenthechargeandacharge-induceddipoleandduetocoulombforces.Second,theratesof gas-surfacereactionsandPAH surfacecondensationreactionsmay also be enhanceddue to the sameeffect. Further it is possiblethat the reactionbetweenchargedsurfacesandgaseoushydrocarbonspeciesdoesnot requiretheactivationof thesurfacebytheH-abstractionreactionif chargedparticlescontainradicalions.

Previous studiesshowed that thermalionisationplaysa minor role in the primary zoneof sootformation in laminar premixed flames. Theseanalyseswere basedon assumedparticle size-distribution functions(PSDF),suchas monodispersedor self-preservingdistributions. A moredetailedanalysisis yet to beconductedwith size-resolvedtreatmentof thermalionisationwithoutassumingtheform of PSDFsof sootformedin flames.

1Departmentof MechanicalEngineering,Universityof Delaware,USA

6.1. FUNDAMENTAL KINETIC INVESTIGATIONS 43

Therecentprogressesin thedevelopmentof detailedkinetic modelsof sootformationin laminarflamesoffer the possibility that particle ionisationcanbe analysedin finer details. It hasbeenshown that detailednumericalsimulationis able to predict, to within a few factors,the volumefraction and meandiameterof soot particlesformed in laminar premixed and diffusion flamesburningsimplefuels.Thesesimulationstudies,however, assumedthatthecoagulationandsurfacereactionkineticsaregovernedentirelyby neutral-neutralinteractions,without consideringeffectsinducedby particle ionisation. This assumptionis yet to be verified from the considerationsofsize-dependentthermalionisationkinetics.In particular, it is not known to whatextentneglectingparticle ionisationaffects the numericalpredictionof particlepropertiesasnumberdensityandparticlesizes.

Under the combustionconditionsof engineswherethe peaktemperaturecould reachvaluesashigh as 2500 K, thermal ionisationmay be even more prevalent than in laboratoryflames. Itcan be estimatedbasedon thermal ionisationthat more than half of the particlesare charged.Thus the collision enhancementof particles in enginesmay be governed to a greater ex-tent by interactionsbetweenneutral and charged particlesand betweenparticlesof oppositecharges, than neutral-neutralinteractions. The influenceof ionisation on surface reactionsisnot known. Regardlessit is prudentto develop a rigourousmathematicaltreatmentof particleionisationso that the more relevant physicalprocessesareaccountedfor underthe combustioncondition of engines. The work describedin the presentpaperis an effort in that direction.

0

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0 5 10 15 20 25 30 35 40

Rel

ativ

e so

ot v

olum

e fr

actio

n %

T [K

]

x [mm]

Figure 6.1: Comparisonof computed(lines) and exper-imental (symbols) relative soot volume fraction: neu-tral particles: line and dots; positively charged parti-cles:dashedline andsquares;negatively chargedparticles:dashed-dottedline anddiamonds.

Specifically, amathematicalmethod-ology was formulated, which ex-tendsthe methodof moments,pre-viously developedfor neutral parti-cles, to the combined neutral andchargedsystem.In thismathematicalformulation the PSDFsof both uni-chargedandneutralparticlesarecal-culatedand coupledby a reversibleionisation reaction. The collisionenhancementfactors for charged-neutral and charged-charged inter-actions were derived as a functionof particle sizes,and were incorpo-rated in the mathematicalformula-tion of theSmoluchowski equationofparticle-particlecoagulation.Numer-ical simulationswerethenconductedfor soot formation in a low-pressurelaminar premixed acetylene flameandcomparedto measurements.Us-ing sensitivity tests,we analysetheextentof thermalionisationandits effectsonsootgrowth in laminarflames.

Theeffect of thermalionisationon sootparticlegrowth is analysedby detailedkinetic modellingof a low-pressurepremixedacetyleneflame.Thedetailedkinetic modelconsidersfuel oxidation,theformationandgrowth of polycyclic aromatichydrocarbons,andparticleinception,coagulation,andmassgrowth via surfacereactions.A numericalmethodis developed,whichconsidersthepro-


ductionof chargedparticlesby thermalionisationaswell ascoagulationandsurfacereactionsoftheseparticles.Thecollision enhancementof charged-chargedandcharged-neutralparticlecoag-ulationis rigorouslyaccountedfor in thenumericalmodel.Theparticlesizedistribution functionsfor bothneutralandchargedparticlesweresolvedusingthemethodof moments.Thecomputedrelativesootvolumefractionfor neutralandchargedsootparticleswascomparedto measurementsandfound to be in goodagreement(seeFigure6.1). The resultsshow alsothat the omissionofparticlethermalionisationdoesnot leadto significanterrorsin the simulationof sootformationin the acetyleneflame,as long as the natureof the surfacereactionsbetweenchargedparticlesandgaseousmoleculesremainsto be the sameas that for neutralparticles. This result can begeneralisedto mostlaboratorylaminarpremixedandcounterflow diffusionflameswith flametem-peraturenot exceeding2100K. Insofar asthesimulationof sootformationin laboratorylaminarpremixedanddiffusion flamesis concerned,the omissionof thermalionisationdoesnot leadtosignificantdifferencesin thepredictedsootmassgrowth. In mostof theseflames,thermalionisa-tion leadsto only asmallnumberof chargedparticles,whichcoagulatewith theneutralparticlesatapproximatelythesamerateasneutral-neutralpairs.Only if thesurfacereactionbetweenchargedparticlesandgasmoleculesis enhancedshouldthe thermalionisationpotentiallyplay a role inparticlemassgrowth. Regardless,theuncertaintyassociatedwith theomissionof thermalionisa-tion is significantlysmallerthantheuncertaintiesin thekineticsof particleinceptionandsurfacegrowth in mostlaboratoryflames.

6.1.2 Soot Formation in Turbulent Reacting Flows — A PDF-BasedAp-proachApplied to Carbon Black Production

M. Balthasar, F. Mauss,A. Knobel2 andM. Kraft2

The formation of soot is of interestform two points of view: On the one hand, it is a majorpollutant,formedduringmany combustionprocesses,suchasaDieselengine.On theotherhand,it is an importantindustrialproduct;mainly usedfor improving the structureof materials,(e.g.reinforcementfillers in tirestreads),but alsousedasablackpigmentdueto its goodpigmentationproperties,suchaslight stabilityandinsolubility. In this projectwe focussedon theproductionofsootasa desirableindustrialproduct,i. e. carbonblack. World-widecarbonblackis producedona large scale(4748Megatons,1983)mainly via the furnaceblack process.In this process,asitis shown in Figure6.2, fuel (oil or naturalgas)is burnt underfuel leanconditionsin theprimarystage.In a secondarystagea feedstock,usuallyoil, is injectedthroughan atomiserinto the hotexhaustfrom theprimarystage.After thereactionmixture is quenchedwith waterandcooledinheatexchangers,thecarbonblack is collectedfrom the tail gasusinga filter system.Dependingon the operatingconditionsdifferentgradesof carbonblack with differentproperties(e.g. sizeandsurfacearea)areproduced.Specificapplicationsrequirea certaingradeof carbonblack. It isthereforeof importanceto know which furnacesettingsleadto whatgradeof carbonblack.

In orderto improve productquality andyield, reactorgeometry, andotherparametershave to beoptimised. This is usuallydonethrougha large setof trial experiments,which areusuallyveryexpensive. A precisephysicalunderstandingof thesootformationprocessandits interactionwiththe turbulentfluid flow is necessaryto find thesensitivity of thevariousparameters.This is turncanleadto a shorteningof thedesigncycle. This, andthefact that it is possibleto studyfurnace

2Departmentof ChemicalEngineering,Universityof Cambridge,Cambridge,UK


Figure6.2: Schematicdiagramof acarbonblackfurnace.

Variable Predicted(IEM) Predicted(Curl) ActualExit gassootmassfraction(kg/kg) 0.084 0.086 0.127Yield (kg carbon/kgfeedstock) 0.398 0.407 0.605

conditionswhereexperimentaldataareeithernot availableor experimentscannotbe performedmotivatetheuseof reliablemodels.

For thepurposeof simulatinga carbonblackfurnaceit is necessaryto combinethemodellingofthe turbulent fluid flow, with a modeldescribingthe formationof soot. The two processesarestrongly coupled. In this work we useda detailedmodel basedon the work of FrenklachandWang. This modelhasto becoupledto a modelthatdescribesturbulent reactingflow basedon ajoint scalarPDFtransportequation.

Theinteractionof turbulentflow andsootformation,andtheincorporationof thesootmodelintothe probability approachleadto physicalandmathematicalquestions.Firstly, turbulencemightincreasethecollision frequency of sootparticlesdueto turbulentdiffusionandinertiaeffects.Sec-ondly, it is importantto ensurethatthemixing of themomentsof thesootparticlesizedistribution(PSDF)is equivalentto themixing singlesootparticles.

The influenceof turbulenceon soot particles’ collision frequency is found to be of minor im-portancefor the conditionsof this study. The investigationof the consistency conditionsof thecombinedscalarandsootmomentapproachshowsthattheIEM andCurl modelcanbeusedto de-scribethemixing of thestochasticmomentsof thesootPSDF. A furnaceblackprocessis simulatedandtheresultscomparedto dataof anindustrialreactor(seeTable6.1.2).ResultsobtainedusingtheIEM andCurl modeldiffer only slightly. Themodelpredictionsarein reasonableagreementtothemeasureddata.This is remarkablesincenoadjustmentsto thesootmodelparametersobtainedfrom laminarpremixedflameshavebeendone.

However the model needsfurther extensionsto becomeapplicableas a predictive tool for theproductionof carbonblack. To achieve a morerealisticdescriptionof sootformationin a carbonblackreactoragglomerationof sootparticleshasto betakeninto account.Theevaporationof thefeedstockcouldbe includedusinga spraymodel. Futurework alsoincludesa betterdescriptionof the turbulent flow andthe intensityof mixing by usinga time dependentmixing time, which


couldbeobtainedfrom CFD-simulations.Finally thepredictive capabilitiesof themodelhave tobevalidatedfor differentoperatingconditionsandit hasto betestedif changesof thesootparticlepropertiescausedby varyingsingleoperationparameterscanbepredictedaccurately.

6.1.3 MechanismReduction

SkeletalMechanismGeneration

H. S.Soyhan,P. AmneusandF. Mauss

This researchconcentratesontheconstructionof theskeletalmechanism,i.e. theautomaticdetec-tion of theredundantspecies.Theclassificationin redundantor non-redundantspeciesdependsonthecombustionsystembeinginvestigated.

Sincetheenginegeometryis verycomplex, theperformanceof commercialsoftwareproductsforcombustiondevicesoptimisationdecreasesconsiderablyif the numberof speciesexceedsabout10. Consequently, a variety of methodsin chemicalkinetic modellingareneededto constructareactionmechanismfor acomplex fuelasnaturalgasor PRF-fuels,andto reduceit toalow numberof capablespecieswithout a lossof informationthatmight be of importancefor theaccuracy ofthecalculations.Oneplausiblemethodis thefollowing:^ Thegenerationof adetailedreactionmechanism.^ Theconstructionof theskeletalmechanism.^ The final reductionof the reactionmechanismusing QuasiSteadyStateApproximations

(QSSA)or otherreductiontechniques.

A methodfor automaticreductionof detailedkinetic to skeletalmechanismsfor complex fuelsis proposedhere. The methodis basedon the simultaneoususeof sensitivity andreaction-flowanalysis.Thesensitivity analysesdetectsspecies,theoverall combustionprocessis sensitive on.Here,the taskis to identify thespeciesthataremostsensitive on theentirecombustionprocess,i.e. hasthehighestimpacton theheatreleaseandthepre-setnon-redundantspecies.Thereforeadevelopedform of thesensitivity analysisis performed:_aàb ced fhgi j kal mmmm n]oàb jqp�rp�s j mmmmSab c

is thesensitivity of thespeciesB ontemperatureT, n oàb j is thestoichiometriccoefficientof thespeciesB in reactiont , s j is thereactionvelocity of reactiont andN u is thenumberof reactions(Figure6.3).

The automaticdetectionof redundantspeciesis doneby meansof reactionflow analysesfromandtowardsthe mostsensitive species,the fuel, the oxidiserandthefinal products(Figure6.4).


v w x y z{v w x y |}v w x y ~}v w x y ��v w x y y

� � ��y �\v �� z � y� � � |� � � z�� ~� | x x x| x xv x x x

� � �\�q� �\� � � ��

� � � � ¡ ¢ � � �h£ � ¤ ¡ � ¥ ¤ �§¦a¨ x x�©ª¡ «h¬ �\# ¥ �§® � ¤ ¦ª� y

¯ °q± ² ³ ´²q± ² µ°�± ² ³ ´±§± ² ³ ¶± · °q± ² ³ ¶¸ ± ² ³ ¶

27 27.5 28 28.5 29 29.5 30

¹ º » ¼ ½ ¾ º ¹° ¿± ² ¿± ¸ ¿± ° ¿¸ ² ¿ÀÁ ÂÃÄ ÅÆ ÇÈÉÊË ÃÌ initial temperature = 800 K

crank angle (cad) Figure6.3: Sensitivity of radicalson tempera-tureasa functionof time.

Figure 6.4: Mole-fraction profiles of the hy-droxyl radical in the end-gasof an SI-enginefor detailedanddifferentskeletalmechanisms.

The procedurefor the reactionflow analysismaybe formulatedmathematicallyasfollows: Therelative importanceof speciesÍ , Î Ï , is definedas

Î ÏªÐTÑÓÒ ÔÖÕe×hØÙ Ú ÛaÜ Î Ú Ý Ú Ï Þ#×hØÙ Ú ÛaÜ Î Ú ß Ú Ï Þ�à Ï á âÑÓÒ Ôäã à Ï á â å æ Þwheref

Ú Ï is the formationof speciesj from speciesi, andc

Ú Ï is the consumptionof speciesj tospeciesi. Ý Ú á Ï Ð ×hçèâ ÛhÜ�é â ê]ëÏ á â ê]ë ëÚ á â× çèâ ÛaÜ é â ê]ì ìÚ á â

ß Ú á Ï Ð ×hçèâ ÛhÜ�é â ê]ë ëÏ á â ê]ëÚ á â× çèâ ÛaÜ�é â ê ëÚ á âTheproposedreductionmethodis exemplifiedon adetailedreactionmechanismfor iso-octane/n-heptanemixtures. The gas-phasechemistryis analysedin the endgasof an SI-engine,usingatwo-zonemodel. Theconditionschosenarerelevant for engineknock. Redundantspeciesin themechanismareidentifiedandeliminatedfor differentpre-setlevelsof minimumreactionflow.

Comparingresultsobtainedfrom the skeletalmechanismswith resultsfrom the detailedmecha-nismshowstheaccuracy of theresultingmechanisms(Figure6.5). It is shown thattheerrorin theskeletalmechanismsincreasewith increasingpre-setlevelsof reduction.This is visualisedby thehelpof thepredictedcrankangledegreeat which autoignitionin theendgasof theengineoccurs(Figure6.6).

The proceduresuggestedis fully automaticand provides a fast techniquefor finding problemorientedskeletalmechanisms.


íí î ïí î ðí î ñí î òó î íó î ïó î ðó î ñ

ô í íõò í í{ö í í÷ó í í íøó ó í íøó ï í íùó ú í í

û üó í üó ï üó û üï í üýþþÿþ� �� ÿ�� ÿ�� ý� ��

� � � � � � � � ��

Figure6.5: Calculatedtemperatureprofiles inthe end-gasof an SI-engine. Dif ferent skele-tal mechanismsarecomparedwith thedetailedmechanism.

Figure6.6: Thedifferencein theoccurrenceofmaximumheatreleasedependontheinitial gastemperatureat 42.3 CAD BTDC for differentskeletalmechanisms.

Automatic ReductionProcedure for ChemicalMechanisms

T. Lovas,D. NilssonandF. Mauss

Taking the computationaldemandsof turbulent flow into account,it is difficult to seehow fulldetailedkinetic mechanismscould be directly coupledto ComputationalFluid Dynamicscalcu-lationsof practicalcombustiondeviceswithin a foreseeabletime. A compromisemustbe madebetweena time-consuminggeneraldescriptionof for examplepollutantsandautoignitionon onehand,andcomputertime limits andphysicalcomplexity on theother.

By useof areducedchemicalmechanisminsteadof adetailedone,asubstantialreductionin com-putationaleffort is possiblein termsof both CPU time andmemoryrequirements.A commonreductionstrategy is the introductionof quasisteady-stateapproximations(QSSA)for chemicalspecies.Until now, this techniquehasrequiredaskilled operatorto decide,accordingto chemicalexperience,which of thespeciesin theoriginal detailedmechanismarein quasisteady-state.Fol-lowing properselectionof steady-statespecies,the reductionprocedureis strictly mathematical.Thespeciesin steadystatearealgebraicallyeliminatedfrom thesystemof elementaryreactions,usuallyresultingin asmallsetof explicit globalreactions.As aresultit is commonto useacoupleof previously publishedandwell known reducedmechanismsfor CFD calculations.Our investi-gationon this matteris to developamethodwherethereducedmechanismis automaticallyfoundby usingasetof a few physicalselectionparametersthatselectsthespeciesin steadystate.

In [Nilsson et al., 1999]a purelifetime measureof thechemicalspecieswasusedasa selectionparameteralong with a restrictionon the elementmassfraction a speciescan containin orderto not violate the conservation laws andappliedto a systemsimulatingcombustionin an HCCIengine.Theoriginal mechanismcontaining52 speciesinteractsin 590reactions.After applyingtheQSSAthrougha codedevelopingautomaticallyreducedmechanismthechemicalsystemwasreducedto containingonly 17 species.A seriesof reducedmechanismwasproducedin ordertoinvestigatehow theerrorin thedesiredresultwasdependingon degreeof reduction.It wasfoundthattheaccuracy of theresultdependsmonotonouslyon thedegreeof reduction.In Figure6.7thetemperatureprofilesasfunctionof crankangleis shown resultingfrom computationwith anumber


of reducedmechanisms.

In this investigationsusingthe purelifetime quantityanda mass-fractionweightedvariety, fullyautomaticQSSAreductionwaspossibleto a certainextentonly, resultingin mechanismsfor ig-nition of 4-componentnaturalgaswith thenumberof non-steady-statespeciesstill amountingto20 to achieve a goodresult. If a specieshasa low sensitivity on the desiredresult, for exampleflamespeedor exhaustgasemissions,a largererror is acceptablein theequationfor this species.Hence,specieswith longlifetimesandlow sensitivity couldbeeliminatedandthereducedreactionmechanismwouldbemoresuitablefor usein CFD calculations.

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Detailed (52 sp.)1e-16 sec. (42 sp.)1e-14 sec. (37 sp.)1e-12 sec. (32 sp.)1e-10 sec. (27 sp.)1e-8 sec. (23 sp.)1e-6 sec. (17 sp.)

Tem

pera

ture

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)

Crank angle (degrees)

Figure6.7: Temperatureasfunctionsof crankanglefor differentdegreesof reductioncomparedto thedetailedcase.

In [Løvaset al., 2000] an existing skeletalmechanismfor laminarpremixed methane-airflameswasusedasa startingpoint for further automaticreductionby quasi-steady-stateapproximation(QSSA)for specieswith shortchemicallifetimesand/orminor influenceon thechemicalsystem.In thiscasewedevelopedannew selectionparameterwith respectto staticanddynamiccharacter-isticsaccordingto a Level Of Importance(LOI) measureobtainedfrom their chemicallifetimes,diffusionvelocitiesandflame-zoneresidencetimesin combinationwith aspeciessensitivity mea-sure. Specieswith low LOI were selectedfor QSSA,and their concentrationswere calculatediteratively by solving thecoupledalgebraicsystem.Kinetic modelswith a varyingdegreeof re-ductionwerethenautomaticallygeneratedandimplementedasFORTRAN sourcecodeby settingdifferentlowerLOI andelementmassfractionlimits.

It was found that the previous lifetime and new LOI measurediffered due to the inclusion ofsensitivity counteractingthe rise in lifetime at low temperatures.The laminarburning velocitiespredictedby the moststrongly reducedmechanismwith five global reactionstepsshowed verygoodagreementwith detailedcalculationsasseenin Figure6.8. (Seealso[Løvaset al., 1999;Soyhanet al., 2000]).


0

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x Exp. Law+ Exp. Warnatz

Figure 6.8: Burning velocity of laminar premixed methane-airflamesas a function of equiva-lenceratio � , calculatedusingmechanismswith increasingdegreeof reduction.Comparisonwithexperimentaldatafrom Law andWarnatz.

6.2 Applied Kinetic Calculations

6.2.1 Implementation and Validation of a DetailedSootModel and Applica-tion to AeroengineCombustors

M. Balthasar, F. Mauss,M. Pfitzner3 andA. Mack3

Figure6.9: BRR stagedcombus-tor configuration.

The soot model, which has been validated in laboratoryflames,hasbeenimplementedinto a commercialCFD codeusinguser-definedsubroutines.The sootmodel is basedonsolvinga singletransportequationfor thesootmassfractionwith a sourceterm usingfits to a comprehensive flameletli-brary. Thesootmodeloperatingwith thefull flameletlibrarycompareswell with sootmeasurementsfrom a laboratoryjetflame.Themodelhasalsobeenappliedsuccessfullyto com-bustionin a reciprocatingengineapplication.Thesootsourcetermsin the current formulation are linear functionsof thesootvolumefraction(surfacegrowth, fragmentationandoxi-dation). A formulationusingsurfaceareafor thegrowth andoxidationtermsis currently tested. The effect of turbulenceis implementedvia a probability densityfunction of mixturefractionandscalardissipationrate.Radiationeffectsarenot yet included,but work is in progressto incorporatetheseby solvinganadditionalequationfor enthalpy.

3BMW Rolls-RoyceAeroEngines,Dahlewitz, Germany

6.2. APPLIED KINETIC CALCULATIONS 51

The sootmodel is operatedin postprocessingmodeusingconvergedcombustingflow fields. Itconvergesquickly andthe numericaleffort is negligible comparedto the generationof the flowfield CFD solution. Theapplicationof thesootmodelto the3-D stagedcombustorconfigurationindicatesthatthesootvolumefractionattheexit of thecombustoris overpredictedby an1-2ordersof magnitude,which is consistentwith resultsreportedelsewherein theliterature.Thesootmodelis extremelysensitive to thesootsourcetermsandmorework is neededto optimisetheseterms.It is expectedthattheagreementbetweenmodelandexperimentwill becomebetterif thesurfacereactionsarecalculatedto bedependenton thesootsurfaceinsteadof thesootvolume.

Figure6.10:BRRstagedcombustor(mainzoneplane):Contoursof sootvolumefraction( � �� ! ).

Thereis experimentalevidencefrom combustordevelopmentteststhatsootemissionscanbeverysensitiveto detailsof thefuel injectorconfiguration.Theextremesensitivity of thecombustorexitfield to smallchangesin hardwareconfigurationcanalsobeseenin full annulartests,wherethereis oftena quite largevariationin temperatureandsootemissionexit traversesevenfor nominallyidenticalupstreamgeometry.

This indicatesthat the CFD solutionhasto be very accurateto capturesucheffects. In partic-ular, the fuel injector hasto be modelledvery accuratelyandthe effect liquid fuel 2-phaseflowhasto be taken into account.Thenumberof grid pointsin a typical combustorCFD calculation(300000 - 500000 nodes)is still far from sufficient to achieve even a grid independentsolutionof thecombustingflow field. Thusa quantitativepredictionof sootemissionsin aeroenginecom-bustorsawaitsthedevelopmentof computerswhich areat leastoneorderof magnitudefasterandhave a tenfold increasedmemory. Nevertheless,theknowledgeof the locationof regionsof highsootconcentrationandhigh productionandconsumptionratesenablesthecombustordesignertooptimisethesootemissionsandpredictthecorrecttrendsevenwith suchanimperfecttool. Further


researchis requiredto improve thesootmodelandto remove thecritical sourcesof inaccuracy intheflow field simulation.

6.2.2 Knock in SI Engines

S.HajirezaandF. Mauss

In SI engines,if thetemperaturein someregionsof theendgas,onthecylinderwall, exhaustvalvesor sparkplugsis higherthantherestof thegas,autoignitionwill occur. The inhomogeneitiesinthe pressureor speciesfield of the endgasmay also lead to autoignition. This meansthat theendgasin oneor somepointsmay autoignitedue to the pre-flamereactionsin the gas,beforethe flamecanpropagateto thesepoints. Autoignition in oneor somepoints,i.e., hot spots,canlead to autoignitionin the restof the gastoo, if thereis enoughtime for occurrenceof furtherpre-flamereactions.Autoignition in theendgasmayleadto engineknock.Knock is anabnormalcombustion modeandan undesirablephenomenonin SI engines. It may causedamageand itis a sourceof noisein engines.The occurrenceof autoignitionis accompaniedby an extremelyrapidreleaseof chemicalenergy andthegaswill beburnedcompletelyin theauto-ignitedpoints.Afterwardsthereactionfrontswill propagatefrom thehotspotsto thesurroundingpointsandwillignite therestof thegas.Thevelocity of thesereactionfronts is comparablewith thevelocity ofsound.A very fastenergy releasein theendgascausesa high local pressureandpropagationofpressurewaveswith highamplitudeacrossthecombustionchamber. In knockingcombustionboththermo-chemicalandgasdynamiceffectsareinvolved. Sincetheoccurrenceof inhomogeneitiesin thegasis stochastic,thenatureof theexothermiccentresis stochastic.After ignition a reactionfront startsto propagatefrom the hot spotsinto the surroundinggases.This leadsto spaceandtime-dependentprocessesgovernedby thesuperpositionof chemicalkinetics,gasdynamicsandtransport.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08t = 4.8 ms

t = 6.0 ms

t = 6.3 ms

t = 6.4 ms

t = 6.6 ms

t = 6.8 ms

t = 7.0 ms

t = 7.1 ms

0.0 2.0 4.0 6.0 8.0 10.0

CO

Distance x [mm]

Figure6.11: Calculatedprofilesof themole fractionsof CO asfunctionof thedistancefrom thewall andthetime.


0

0.002

0.004

0.006

0.008

0.01

0.0 2.0 4.0 6.0 8.0 10.0C

H2O

Distance x [mm]

Figure 6.12: Calculatedprofiles of the mole fractions ofCH" O asfunctionof thedistancefrom thewall andthetime.Thesymbolsareidenticalwith thesymbolsin Figure6.11.

A three-zonemodel[Hajirezaetal.,1999] was developed in order tounderstandthe effect of thermalboundarylayeron theautoignition.In thatmodel,theeffectof thether-malboundarylayerthicknessontheautoignitionphenomenongenerallyand the autoignition delay timein the mixture of n-heptaneandiso-octanewas investigated. Fur-thermodeldevelopmentsarefoundin [Hajireza, 2000]. In order toinvestigatethe effect of inhomo-geneitiesof the temperaturefield,the temperatureand flow fields ofthe end gas were solved togetherwith a reduced four-step globalreaction mechanismin [Hajireza,2000] for a simplified 2-d geometryanda detailedreactionmechanismfor C# -C$ hydrocarbonscombinedwith skeletalmechanismfor PRFsn-heptaneandiso-octanein [Hajirezaetal.,2000a,b].In thesespublications,theoccurrenceof exothermiccentreshasbeensimulatedby developingamulti-speciestransportmodel in the endgas. The calculationsshow that the exothermiccentres(ETC) aredevelopeddue to the gasinhomogeneityin the temperature,oxidiseror fuel. Whenexothermiccentresaredeveloped,apparentreactionfrontswill propagatefrom thesecentresto thesurroundings.Profilesof theintermediateproductsCO andCH" O areshown in Figures6.11and6.12. The productionof CO during the inductionphaseis low. During the excitation phasetheCOconcentrationrisessuddenly. At theendof theexcitationphaseCOis consumeduntil it finallyreachesequilibriumconcentrations.Dueto thisbehaviour COmarksthepositionof currentexcita-tion sharply. It is evidentfrom theFigure6.12thatCH" O resultsfrom bothkinetic pathways,lowandhigh temperaturekinetic. HenceCH" O cannottracethehistoryof theignition process.CH" Ois constantlyproducedduringtheinductionphaseandsuddenlyconsumedduringexcitation.

In addition,by meansof the three-zonemodel[Hajirezaet al., 1999], theheatflux to thewall iscalculatedandtheeffectof anextra thermalresistanceon thewall andthewall temperatureon theautoignitionprocessis investigatedin [HajirezaandSunden,2000].

6.2.3 Calculationsof HCCI-Engine Combustion

P. Amneus,H. S.SoyhanandF. Mauss

Thehomogeneouschargecompressionignition (HCCI) engineis a promisingalternative to exist-ing engineconcepts.Its function shows similaritiesto both theotto (SI) engineandthe conven-tionaldiesel(CI) engine.Justlike in adiesel,thefuel is auto-ignitedby compressionof thepistonuntil sufficient high temperatureandpressureis reached,but a premixedair/fuel mixture is used,like in anotto-engine.

Fromacombustionmodeler’s viewpoint, it is neitheranotto-enginenor a diesel.In anSI-engine,


Figure6.13: HCCI combustion;intake of premixedair/fuel mixture; compression;auto-ignition;followedcombustion

the premixed air-fuel mixture is ignited by a spark,startinga flamethat propagatesthroughthecombustionchamber. In adieselengine,thefuel is injectedinto thecompressedair in thecombus-tion chamber. Combustionoccursin a thick surfaceof closeto stoichiometricconditions,betweenregionsof fuel-richandfuel leanconditions.In bothof theseenginetypes,heat,massandimpulsetransferareimportantparametersfor thedevelopmentof thecombustion.

What differs thesetwo conceptsfrom the HCCI engineis the high dependenceon masstransferandswirl for thecombustion. In anHCCI engine,thepremixed fuel/air mixture is ignitedwhensufficienthightemperatureandpressureis reached.As thephysicalconditionsaresimilar through-out thecombustionchamber, theentirebulk will auto-igniteat almostthesametime. Thereforeitis a justifiedassumptionto considerthechemicalreactionsto betherate-determiningstepfor theignition process.

This givesustheopportunityto formulatea simplezero-dimensionalmodel4 with detailedchem-ical kinetics for the calculationsof the ignition processillustratedin Figure6.13. Ignition cal-culationsusing this model have predicteda high sensitivity to fluctuationsin temperature(seeFigure6.14)andfuel compositions.Thesepredictionshave laterbeenconfirmedby experiments.

4M. Christensen,B. Johansson,P. AmneusandF. Mauss,“Superchargedhomogeneouschargecompressionigni-tion engine”,SAETechnicalPaper0787(1998).P. Amneus,D. Nilsson,F. Mauss,M. Christensen,andB. Johansson,“Homogeneouscharge compressionignition engine:Experimentsanddetailedkinetic calculations”,in COMODIA98, pages567–572,JapanSocietyof MechanicalEngineers(JSME),(1998).


800

1000

1200

1400

1600

1800

2000

2200

-30 -20 -10 0 10 20 30

case 10, expcase 11, expcase 12, expcase 10, simcase 11, simcase 12, sim

T (

K)

Time (CAD)

Figure 6.14: Comparisonof calculated(broken line)and 100 cycles meanexperimentalvalues(solid line)temperatureprofiles within the combustion chamberduring HCCI combustion, using the simplified math-ematicalmodel.

A problemwith thesimplemodelis thatit doesnot take small inhomogeneitiesin mixtureandtemperatureinto account.Neither does it account for the lowertemperaturesin theclose-to-wall region.A way to deal with theseproblemsisto model the problem in the form ofa Probability Density Function (PDF),where the combustion chamberis ex-pressedasa spectrumof stochasticpar-ticles of a certaintemperaturedistribu-tion [Maigaard et al., 2000]. The in-homogeneitieswill result in transportof heat,massand impulsebetweenthecells. The PDF-methodis supposedto be moreaccuratethan the simplifiedmodel,andallowsthecalculationof pol-lutant formation during engine opera-tion [Kraft et al., 2000]. A drawbackis the requiredcalculationcost for themethod,which emphasisestheneedfor valid mechanismreductions[Løvaset al., 1999;Soyhanet al., 1999a,b,2000].

1000

1200

1400

1600

1800

2000

2200

-15 -10 -5 0 5 10 15

+20 K+10 K+ 5 K+/- 0- 5 K-10 K-20 KT

(K

)

Time (CAD)

2 106

3 106

4 106

5 106

6 106

7 106

8 106

-20 -15 -10 -5 0 5 10 15 20

Experiment

Previous model

This model

P [P

a]

CAD

Figure 6.15: Sensitivity of ignition point tofluctuationsin temperature

Figure 6.16: Comparisonof experimentsandcalculations,usingthePDF-model

56 REFERENCES

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T. Løvas,D. Nilsson,andF. Mauss. Developmentof reducedchemicalmechanismsfor nitro-gencontainingfuels. In CleanAir V, volumeI, pages139–143,Lisbon, Portugal,1999.TheCombustionInstitute,PortugalSection.

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T. Løvas,D. Nilsson,andF. Mauss. Automatic reductionprocedurefor chemicalmechanismsappliedto premixed methane-airflames. In 28th Symp.Combust., pages23–45,Edinburgh,Scotland,2000.TheCombustionInstitute.

C. Lynga, F. Ossler, T. Metz, and J. Larsson. A lasersystemproviding tunable,narrow-bandradiationfrom 35nmto 2 % m. Appl.Phys.B, 2000.Submitted.

P. Maigaard,F. Mauss,andM. Kraft. Homogeneouschargecompressionignition engine:A simu-lationstudyon theeffectsof inhomogeneities.ASME, Apr 2000.SanAntonio,Texas,USA.

H. Malm, G. Sparr, J.Hult, andC. Kaminski. Nonlineardiffusionfiltering of imagesobtainedbyplanarlaserinducedfluorescencespectroscopy. J. Opt.Soc.Am.A, 17:2148–2156,2000.

T. Metz, F. Ossler, andM. Alden. Spectroscopicandtime–resolvedinvestigationsof picosecondlaserinducedfluorescencefrom polycyclic aromatichydrocarbonsat elevatedtemperatures.In63.Physikertagung(DeutschePhysikalischeGesellschaft), Heidelberg, Germany, 1999.

T. Metz, J. Walewski, andC. F. Kaminski. Parameterestimationfrom datafitting in non-linearspectroscopy. In CLEO/Europe–IQEC2000, pageIPD2.1,Nice,France,Sep2000.InstituteofElectricalandElectronicsEngineers.

S.-I. Moller andA. Lindholm. Theoreticalandexperimentalinvestigationof theoperatingchar-acteristicsof a Helmholtztypepulsecombustordueto changesin theinlet geometry. Combust.Sci.Tech., 149:389–406,1999.

D. Nilsson,T. Løvas,P. Amneus,andF. Mauss.Reductionof complex fuelschemistryfor simula-tion of combustionin anHCCI engine.VDI–Berichte, 1492:511–516,1999.

J. Nygren,J. Engstrom, C. F. Kaminski,andM. Alden. Two–dimensionaltemperaturemeasure-mentsusingtwo–lineatomicfluorescencein highly sootingenvironments.In 28thInternationalSymposiumonCombustion, Edinburgh,Scotland,2000a.

J. Nygren,J. Engstrom, C. F. Kaminski,andM. Alden. Two–dimensionaltemperaturemeasure-mentsusingtwo–lineatomicfluorescence.In IEA konf, Stockholm,Sweden,2000b.

F. Ossler, T. Metz, andM. Alden. Picosecondlaser–inducedfluorescencefrom gas–phasepoly-cyclic aromatichydrocarbonsatelevatedtemperatures:Implicationsto combustiondiagnostics.In GordonResConf“Laser Diagnosticsfor CombRes”, Pisa,Italy, 1999a.

F. Ossler, T. Metz, andM. Alden. Picosecondlaser–inducedfluorescencefrom gas–phasepoly-cyclic aromatichydrocarbonsat elevatedtemperatures.In IEA–TLM, Ohtsu,Japan,1999b.

F. Ossler, T. Metz,andM. Alden.Short–pulsedlaser–inducedfluorescenceappliedto combustion.In 3rd Nat Meetingon FemtosecondSpectrandDynamics, KTH, Stockholm,Sweden,1999c.

F. Ossler, T. Metz,andM. Alden.Opticalin–situcharacterizationof gasphasepolycyclic aromatichydrocarbonsat elevatedtemperaturesby picosecondlaser–inducedemission.In 28thInterna-tional Symposiumon Combustion, Edinburgh,Scotland,2000.Work–in–progressposter.

F. Ossler, T. Metz, andM. Alden. Picosecondlaser-inducedfluorescencefrom gas-phasepoly-cyclic aromatichydrocarbonsat elevatedtemperatures.I. Cell measurements.Appl. Phys.B,2001a.Acceptedfor publication.

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M. Richter, B. Axelsson,and M. Alden. Investigationof the fuel distribution and the in-cylinder flow field in a stratifiedcharge engineusing laser techniquesand comparisonwithcfd-modelling.SAE, 1999-01-3540,1999a.

M. Richter, J. Engstrom, A. Franke, M. Alden,A. Hultqvist, andB. Johansson.Theinfluenceofchargeinhomogeneityon theHCCI combustionprocess.SAE, 2000-01-2868,2000a.

M. Richter, A. Franke, M. Alden,A. Hultqvist, andB. Johansson.Opticaldiagnosticsappliedtoa naturallyaspiratedhomogeneouscharge compressionignition engine. SAE, 1999-01-3649,1999b.

M. Richter, A. Franke,J.Engstrom,M. Alden,A. Hultqvist,andB. Johansson.Opticaldiagnostictechniquesfor characterizationof homogeneouscharge compressionignition (HCCI) engines.In WIPP–sessionat the 28th InternationalSymposiumon Combustion, Edinburgh, Scotland,2000b.

H. Soyhan,P. Amneus,F. Mauss,andC. Sorusbay. A skeletalmechanismfor naturalgasfuelledSI-engines.In ULIBTK ’99–144, 1999a.

H. Soyhan,P. Amneus,F. Mauss,andC. Sorusbay. A skeletalmechanismfor the oxidationofiso-octaneandn-heptanevalidatedunderengineknockconditions.SAETransactions,Journalof FuelsandLubricants, 108,Sec.4:1402–1409,Jan1999b.SAE1999-01-3484.

H. S.Soyhan,P. Amneus,T. Løvas,D. Nilsson,P. Maigaard,F. Mauss,andC.Sorusbay. Automaticreductionof detailedchemicalreactionmechanismsfor autoignitionunderSI engineconditions.In CEC/SAESpringFuels& LubricantsMeeting, Jan2000.SAE2000-01-1895.

J. Walewski, C. F. Kaminski, andM. Alden. Pressuredependenceof polarizationspectroscopysignals.In LACEA2000, pages120–122,SantaFe,New Mexico, Feb2000a.OpticalSocietyofAmerica.

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J.Walewski, J.Larjo, andH. R. Self-sustainedsecondarydischargein inductively coupledplasmareactor. Appl.Phys.Lett., 76(18):2508–2510,2000c.

Invited Talks

M. Alden, Developmentand Application of Laser SpectroscopicTechniquesfor TemperatureMeasurementsin aCombustionEnvironment, ESF-NANO workshop,Res.CenterKarlsruhe,Germany, May 21–22,1999

M. Alden,Developmentof LaserTechniquesfor Applicationsin CombustionEngines, HausderTechnik,Essen,Germany, Sep26,2000

64 REFERENCES

M. Alden,Experiencewith,andLessonsLearnedfromUniversity-Government-IndustryCollabo-rationonFundamentalResearch in CombustionProcesses, IEA, RD&D, Paris,France,Oct23,2000

M. Alden, Developmentand Applicationof LaserTechniquesfor CombustionDiagnostics, In-vited paperat theWorkshopon Monitoring of Air Pollutiondueto CombustionProcesses,Univ. of CapeCoast,Ghana,Nov 1–3,2000

C. F. Kaminski, Developmentand Applicationsof an Ultra High SpeedLaser/DetectorSystemfor CombustionDiagnostics, 10& ' GordonResearchConferenceon LaserDiagnosticsforCombustionResearch,Il Ciocco,Italy, Jun1999

F. Mauss,PopulationBalanceModellingof ParticulateSystems, A EngineeringFoundationCon-ference,Kailua-Kona,Hawaii, Jan23–28,2000

F. Mauss,SootModelling in Diesel Engines, 3( ) Symp.TowardsCleanDiesel Engines,IFP,Rueil-Malmaison,Paris,France,Jun15–16,2000

F. Ossler, Emissionfrom PAH at High Temperatures Inducedby PicosecondLaserRadiation,Dipartimentadi IngegneriaChimica,Universitadi Napoli, “FredericoII” P.le V Tecchio80,Naples,Italy, Dec18,2000

Documents

Activity Report from the Division of Combustion Physics ... · ment of Fire and Safety Engineering. The Combustion Centre was also base for the initiation of the Centre of Competence