Numerical investigation of the non-reacting unsteady flow behind a disk stabilized burner with large blockage

118 Pages·2002·2.907 MB·English

by  Del Taglia C.

Diss.ETH no 14875
NUMERICALINVESTIGATION OFTHE NON-REACTINGUNSTEADY
FLOW BEHINDA DISK STABILIZED BURNERWITHLARGE BLOCKAGE
A dissertationsubmittedtothe
SWISSFEDERALINSTITUTEOFTECHNOLOGYZURICH
forthedegree of
DoctorofTechnical Sciences
presented by
ChristianDelTaglia
MechanicalEngineer
University ofRome “LaSapienza”(Italy)
born July23rd,1971
CitizenofGermany
acceptedon therecommendation of
Prof.Dr.D.Poulikakos,examiner
Prof.Dr.P.Koumoutsakos,co-examiner
2002
“Muore prima chiè piùcaroaglidei.”
Amiopadre.
Acknowledgements
I performed the research work of this thesis at the Laboratory for Thermodynamics in Emerging
Technologies (LTNT) of the Swiss Federal Institute of Technology (ETH), Zurich, between Feb-
ruary 1999and April 2002.
IwouldliketothankverymuchmyreferentProf.Dr.D.Poulikakosforthesupervisionduringthe
work. I thank Prof. Dr. P. Koumoutsakos for the agreement to become correferent, for the severe
review and for the fruitful suggestions. For the supervision and administrative management I
wouldlike to thankDr.J.Gass.Thanks alsoto Dr.Y.Ventikos,who constantlyprovided precious
tipsandsuggestions.
Iwanttothank Dr.A.Hintermann,FederalOffice ofEnergy,Switzerland forthe projectsponsor-
ing.
Iowe thequality ofthis work also to theexperimentalworkofDipl.-Ing.L.Blum.
I would like to thank Dr. T.Steiger, ETH Computer Services for the computing systems adminis-
trationandforthecooperation.Thanks tothecolleaguesatLTNTfortheirkindness.Especiallyto
Dr.A.Obieglo,Dr. D.Attinger, Dipl.-Ing.S.Arcidiacono, Dr.P.Bajaj,Dr.L.Demiraydin,Dipl.-
Phys. A. Prospero and Dipl.-Ing. S. Baykal for the fruitful discussions. I would like to thank Dr.
A. Moser, Dr. A. Schälin, Dr. D. Gubler, Dr. S. Barp, Dr. P. Rosemann, Dr. P. Lengweiler, Dr. Y.
Liu and Mrs. L. Eggels for the friendly and open work environment during the stay at the LOW
offices.
Thanks to myfriends andrelatives for theirsupportduringmyresearchwork.
ChristianDel Taglia
Zürich,March 2003
Abstract
This work deals with numerical simulations of annular jets, with particular emphasis on
highblockageratiojets.Annularjetsareofpracticalinterestbecauseoftheiraxisymmetric
geometry and their strong recirculating flow due to flow separation. When combustion is
included, recirculation guarantees high levels of mixing, leading to stable flames and re-
duced pollutants emission.
The investigation is performed using numerical simulations, as annular jets at high block-
ageratioshaveneverbeenstudiednumericallybefore.Moreover,neitherthree-dimensional
simulations norunsteadysimulations have everbeen performedbeforeon annularjets,the
natureofwhichcanbecharacterizedbyintense mixing,recirculationand vortexshedding.
Therefore,the results ofthis workserve as a theoretical basis forthe design ofhigh block-
age/high recirculation axisymmetricbluffbodygasburners.
Thetechniqueusedforthesimulations isthe solutionofthe steadyandunsteadyReynolds
AveragedNavier-Stokes(RANS)equationswiththeflowsolverCFX-TASCflow.Theaxi-
symmetricsteadysimulationsatseveralblockageratiosshowthatthepredictionsofthere-
circulation zone length is accurate at low blockage ratios. In the high blockage ratio range
thesimulationsareinaccurate,astheflowisasymmetric.Thevelocityfluctuationsobtained
witha Reynolds Stress model aresignificantly belowthe measured fluctuations.
FlowasymmetryatthehighblockageratioisobservedwithLDAmeasurementsandthree-
dimensionalsteadysimulations.Theasymmetrydevelopsafterthejetnozzleandischarac-
terized by apreferential direction from one partof the annularjet to the other.The stagna-
tionpointis dislocated andshiftedfromthesymmetryaxis.Asymmetric flows comingout
fromsymmetricgeometriesandboundaryconditionshasbeenalreadyinvestigatedbyother
researchers and are possible solutions of the non-linear problem expressed by the Navier-
Stokes equations.
The unsteady RANS simulations are generally able to capture the large vortex dynamics
andtheassociatedvelocityfluctuations.So,thetotalfluctuationscanbecomputedfromthe
coherent or deterministic (large eddy) fluctuations and the modelled (small eddy) fluctua-
tions.Ourthree-dimensionalunsteadysimulationsofthehighblockageannularjetareper-
formedusingdifferentapproachesforthemodeledvelocityfluctuations.Theapproachwith
the Standard k-ε model shows damping of the coherent fluctuations, due to the excessive
dissipation introduced by the turbulent viscosity. Instead, the no-model approach and the
approach witha Reynolds Stress modelpresentstablevelocity oscillations.
The time averaged solution of the three-dimensional unsteady simulations is asymmetric,
withthesamefeaturesobtainedinthethree-dimensionalsteadysimulations.Thisindicates
thatthe asymmetrypersists also iflarge vortexfluctuations areintroduced.
Whencomparedtothe experimentalresults,boththe no-modelapproachandthe approach
witha Reynolds Stress modelshow good agreement for the velocity fluctuations.The two
approaches result in more accurate values of the fluctuations than a steady computation,
whichignores the largescaleunsteadinessofthe flow.So,the contribution ofthe coherent
fluctuations iscrucial.Moreover,both approachesrevealanoscillationfrequencywhich is
of the same order of magnitude of the frequency previously measured in the combusting
flow.Thisfrequencyisassociatedtotheperiodicmovementoflargevortexstructures,e.g.,
vortex shedding andconvection.
Ingeneral,thereisnosuperiorperformanceoftheapproachwiththeReynoldsStressmodel
when compared to the no-model approach. This latter approach is a kind of LES, as the
small scale dissipative effects are reproduced through numerical dissipation. Inside the re-
circulation zone the no-model approach give very good levels of fluctuations already with
a medium resolution grid. The approach with the turbulence model behaves slightly better
intheregionswherethehighfrequencyfluctuationcontributionislargerandthegridislo-
callycoarse(e.g.,the downstream region).
Withanumericalstudyitisobserved,thatbreakofsymmetryofhighblockageratioannular
jetsisprecededbyoscillationsinthenearwallregion.Theseperturbationspropagatetothe
axialstagnationpointandcausesymmetrybreaking.Indeed,boththeimbalanceofpressure
andinertiaforceat the stagnation point,and thesmallthickness ofthejet,representanun-
stable conditionforsymmetry.Therefore,asmallperturbation is able to distortthe jetsuf-
ficiently tolose symmetry.
Sommario
Questo lavoro si occupa delle simulazioni numeriche di getti anulari e,con particolare en-
fasi, di quelli ad alto blockage ratio. I getti anulari sono di interesse per la loro geometria
assialsimmetricaeperillorointensoflussoricircolantegeneratodaldistaccodivenafluida.
Quando si prende in considerazione la combustione,la ricircolazione garantisce alti livelli
di miscelamento, da cui ne derivano fiamme stabili ed emissioni ridotte di sostanze in-
quinanti.
Lostudiosibasasusimulazioninumeriche,poichéigettianulariadaltoblockagerationon
sonomaistatianalizzatinumericamentenelpassato.Inoltre,nésimulazionitridimensiona-
li, né simulazioni instazionarie sono mai state effettuate in precedenza su getti anulari, la
natura dei quali è caratterizzata da un intenso miscelamento, ricircolazione e vortex shed-
ding.Pertanto irisultatidiquestolavoroservonocome base teoricaperla progettazionedi
bruciatori a gas conostacoloadaltoblockageratioe con intensa ricircolazione.
La tecnica utilizzata nelle simulazioni è la risoluzione delle equazioni RANS stazionarie e
instazionariemedianteilcodiceCFX-TASCflow.Lesimulazioniassialsimmetrichestazio-
narieeffettuatecondiversiblockageratiomostranochelalunghezzadiricircolazioneècal-
colata in maniera accurata per bassi valori di questo parametro. Per alti blockage ratio le
simulazioninonsonoaccurate,poichéilflussoèasimmetrico.Lefluttuazionidellevelocità
ottenute con un modello degli Stress di Reynolds sono significativamente al di sotto delle
fluttuazionimisurate.
MediantemisureLDAesimulazionitridimensionalistazionariediungettoadaltoblockage
ratiosi osserva unflusso asimmetrico.L’asimmetriasi sviluppadopol’ugelloanulare edè
caratterizzata da una direzione laterale preferenziale. Il punto di stagnazione è al di fuori
dell’asse di simmetria. Getti asimmetrici che si sviluppano a partire da geometrie simmet-
riche e condizioni al contorno simmetriche sono già stati analizzati da altri ricercatori in
precedenza.Questitipi diflussisonosoluzionipossibili delproblemanon lineare espresso
dalle equazionidi Navier-Stokes.
In generale, le simulazioni RANS instazionarie sono capaci di riprodurre la dinamica dei
vortici grandi e le conseguenti fluttuazioni delle velocità. Quindi le fluttuazioni totali pos-
sonoessere calcolatepartendodallefluttuazionicoerenti/deterministichedeivorticigrandi
e dalle fluttuazioni stocastiche dei vortici piccoli ottenute con modelli di turbolenza. Le
simulazioni tridimensionali instazionarie del getto ad alto blockage ratio qui presentate
sono realizzate utilizzando diversi approcci per modellare le fluttuazioni stocastiche delle
velocità.L’approccio col modellok-ε standardmostra uno smorzamento dellefluttuazioni
coerenti, a causa della dissipazione eccessiva introdotta dalla viscosità turbolenta. Al con-
trario, l’approccio senza modello e l’approccio col modello degli Stress di Reynolds pre-
sentano oscillazioni persistenti dellevelocità.
La soluzione mediata nel tempo delle simulazioni instazionarie tridimensionali è asimme-
tricaehalestessecaratteristicheottenutenellesimulazionistazionarietridimensionali.Ciò
indicachela asimmetriapersiste anche introducendo lefluttuazionidei vortici grandi.
Nelconfrontocon irisultatisperimentalisia l’approcciosenza modello,siaquello colmo-
dellodegliStressdiReynoldspresentanodellefluttuazioniaccuratedellevelocità.Idueap-
procci danno valori delle fluttuazioni migliori dei calcoli stazionari, i quali non
comprendono le fluttuazioni instazionarie di grande scala. Quindi, il contributo delle flut-
tuazioni coerenti è determinante in questi flussi. Inoltre, i due approcci rivelano una fre-
quenza di oscillazione dello stesso ordine di grandezza della frequenza misurata
precedentemente in presenza di combustione. Questa frequenza è associata al movimento
periodico delle strutture di vortici grandi, come ad esempio il distacco e la convezione di
vortici.
Unconfronto mostrache l’approccio turbolentocon gliStressdi Reynolds nonè più accu-
ratodell’approcciosenzamodello.Infatti,anchel’approcciosenzamodellosimulal’effetto
dissipativodeivorticipiccoli,attraversounadissipazioneditiponumerico.All’internodel-
lazona di ricircolazione l’approccio senzamodello dàdegli ottimi valori dellefluttuazioni
divelocitàancheconunagrigliagrossolana.L’approcciocolmodelloditurbolenzasicom-
porta leggermente meglio nelle zone in cui le fluttuazioni stocastiche hanno un contributo
maggiore e la griglia è localmente grossolana (ad esempio, nella regione più lontana dalla
parete).
Con uno studio numerico si osserva che la rottura di simmetria nei getti anulari ad alto
blockageratioèprecedutadaoscillazioninellaregionevicinaallaparete.Questeperturba-
zionisispostanoversoilpuntodistagnazioneassialeecausanolarotturadisimmetria.In-
fatti,sia lo sbilanciamentofra forze dipressione e diinerzia al punto di stagnazione,sia lo
spessoresottiledelgetto,rappresentanounacondizionediinstabilitàperlasimmetria.Una
piccola perturbazione è in quindigrado dideformare il gettoin modo sufficiente a perdere
lasimmetria.
Table of contents I
1 Introduction................................................................................................................. 1
1.1 Gascombustionand bluff-bodyflames 1
1.2 Annular Jets,Definitions 3
1.3 Literature reviewon AnnularJets 3
1.3.1 Review ofexperiments on annularjets 3
1.3.2 Review ofsimulations onannular flows 10
1.4 Openissues and objectives ofthis work 12
1.5 Annular jetunderinvestigation 13
1.6 Summaryofthe following chapters 14
2 Numerical models forthe flow................................................................................. 15
2.1 Navier-Stokes equations 15
2.2 Reynolds andensembleaverages ofa fluctuating quantity 15
2.3 Unsteady Reynolds Averaged Navier-Stokes equations 17
2.4 Approachesforthe descriptionofstochastic fluctuations 18
2.4.1 No-model approach 18
2.4.2 Turbulentapproach withthe k-ε model 18
2.4.3 Turbulentapproach withthe Reynolds Stress model 20
2.5 Steady Reynolds Averaged Navier-Stokes equations 21
3 Steadyaxisymmetricsimulations............................................................................. 23
3.1 Introduction 23
3.2 Sensitivity studyof the annularjetsimulations atBR=0.89 23
3.2.1 Sensitivity oftheboundary conditions 24
3.2.2 Sensitivity ofthecomputationalgrid 31
3.2.3 Sensitivity oftheturbulence model 32
3.3 Velocity fluctuations 33
3.4 Investigation oftheannularjetat differentblockageratios 35
3.4.1 Comparisonwith experiments 37
3.4.2 Relationbetweenrecirculationlength andblockageratio 39
3.5 Discussion 39
4 Visualizationof flow asymmetry inhighblockageratioannularjets.................. 42
4.1 Introduction 42
4.2 Description ofthe annularjets simulations 42
4.3 Results 45
4.3.1 Profiles on thesymmetry axis 45
4.3.2 Descriptionofthe flow asymmetry 47
4.3.3 Characterization oftheflowatthe annularrim 51
4.3.4 Externalairentrainment 56
4.3.5 Far field results 56
4.4 Discussion 57
5 Analysisof flow fluctuations..................................................................................... 60
5.1 Introduction 60
5.2 Description ofthe annularjetsimulations 60
5.2.1 Approaches forthe turbulentstresses 60
5.2.2 Grids 61
5.2.3 Boundaryconditions 61
5.2.4 Summaryofthe unsteadycomputations 62
5.3 Computation oftheaveragedradial velocityand itsfluctuation 65
II Table of contents
5.4 Results 66
5.4.1 Turbulent approachwith theStandardk-εmodel 66
5.4.2 No-modelapproach 67
5.4.3 Turbulent approachwith theSSG-Reynolds Stress model 75
5.4.4 Comparison ofthe no-model andthe SSG-RSM approaches 81
5.5 Discussion 85
6 Breakofsymmetry:Mechanism and condition...................................................... 87
6.1 Introduction 87
6.2 Description oftheannularjetsimulations 87
6.3 Results 88
6.3.1 Pastinitialtransient asymmetryvisualization 88
6.3.2 Break ofsymmetrymechanism 88
6.3.3 Condition forbreak ofsymmetry 90
6.4 Discussion 97
7 Conclusions................................................................................................................. 99
References................................................................................................................. 101
AAccuracyofthesimulations.................................................................................... 106
A.1Discretization 106
A.2Time statistics 106
A.3Model 107
B Simulationcode........................................................................................................ 108
1.1 Gas combustion and bluff-body flames 1
1 Introduction
1.1 Gas combustionandbluff-body flames
Fossil fuel is currently the most used energy source worldwide. It has a significant impact
inmany sectors ofenergytechnology: heatproduction,transportation,electric energypro-
duction. Coal is the most used fossil fuel, followed by oil and gas. It is likely to maintain
this leadingpositionasthe availabilityofoilis limited.Inthecase ofgascombustion,me-
thane, propane and buthane are common fossil gases. They find practical application in
powerplants(withgasturbines),transportvehiclesandburnersusedinindustryandhouse-
hold. It is worthy to cite also hydrogen as a non-fossil artificial gas fuel, which is having
growingimpact inthe automobile industryforits extremelylow pollutant emissions.
Sustainability has indeed grown in importance in engineering design besides the criterion
ofefficiency.Designandproductionofdeviceswhichleadtolimitedpollutantemissionis
critical nowadays. Soot, unburnt hydrocarbons (UHC), nitrogen oxides (NO ), sulfur ox-
x
ides and carbonoxides (CO andCO)are pollutants commonly emittedin fossilfuel com-
2
bustion.Theyaredangerous forhealthandenvironment.Soot particles penetrate the lungs
and cause breathing problems and lung diseases. UHC may contain carcinogens, which
cause cancer.NO contributes tothe productionof substances causingacidrain,to thede-
x
struction of ozone in the stratosphere and to global warming (greenhouse effect)
[Bowman92].CO isthe majorgas involved inthe greenhouse effect.
2
In gas combustion, CO , CO and NO are the only pollutants of concern. While CO is
2 x 2
physically impossible to avoid during all kind of fossil fuel combustion, there are tech-
niques for diminish the emissions of the two latter. CO production can be efficiently sup-
pressed through its complete oxidation. Techniques for reducing the NO are discussed in
x
Refs.[Turns92],[Bowman92].Underthesetechniques,fluegasrecirculation(FGR)isone
ofthemostusedduetoitsefficiencyandsimplicity.InFGR,theinertcombustionproducts
are re-introduced directly in the combustion region after having lost part of their enthalpy.
So,theheatcapacityoftheburningfluidisincreasedwhentheburnedgasesaremixedwith
the unburned gases, yielding to reduced peak temperatures and, consequently, to reduced
thermalNO emission.
x
Therecirculationofportionsofgaseousproductscanbeinducedeitheraerodynamicallyor
throughexternalpumpsandparticularcombustionchamberdesigns.Intheaerodynamicre-