fluent燃烧教程讲义.ppt
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1、Fluent 6.0 Staff TrainingGraham GoldinOctober 25 2001SummaryLaminar flamesuGeneral finite rate chemistryuPremixed laminar flames(flame sheet model)uNon-premixed laminar flames(equilibrium f model)Turbulent flamesuEnhancement of v5 modelsuPartially premixed modeluEDC modelDiscrete Phase ModeluEnhance
2、ment of v5 modelsuSpray modelsuMultiple surface reactions2Company Confidential Copyright 2001 Fluent Inc.All rights reserved.Laminar FlamesChemistry invariably stiffuReaction time/length scales Import-ChemkinReactionsuv5:Arrhenius with reversible reactions and third body efficienciesuv6:Pressure dep
3、endent reactions(Lindemann,Troe and SRI)tLow pressure and high pressure rates,with blending functionsMolecular transportuCritical in subsonic laminar flames since it determines mixing and flame speedsuRecommend using kinetic theorytCan get the Leonard-Jones parameters from the CHEMKIN transport data
4、base(TRAN.DB)Laminar flames4Company Confidential Copyright 2001 Fluent Inc.All rights reserved.Numerical methodsNeed special numerics since stiff reaction mechanismCoupled solveruAdvance species and temperature simultaneously over time steptv6:stiff solver optionuUse Implicit for subsonic flamesuUse
5、 Explicit for supersonic flames(detonations=explosions)Segregated solveruDefault steady,segregated algorithm will divergeuCan use unsteady,segregated algorithm,but time step must be near chemistry time-scale(typical 10-9s):not practical!uv6:has a fractional step scheme(hidden from the user)Laminar f
6、lames:General Finite-Rate Chemistry5Company Confidential Copyright 2001 Fluent Inc.All rights reserved.Stiff solverCoupled solveruPreconditioned NS:tG G=preconditioning matrixtQ=r,ui,T,YitF=inviscid and viscous fluxestS=source termsuImplicit spatial discretization:tJ=Jacobian of S=d S/d QtA=Jacobian
7、 of F=d F/d QtRn=Residual at previous time step=d F/d xi SnLaminar flames:General Finite-Rate Chemistry6Company Confidential Copyright 2001 Fluent Inc.All rights reserved.uImplicit stiff coupled solvertDefault time step(stiff solver inactive)where lmax is the maximum eigenvalue of the matrix G G 1At
8、stiff solver activewhere lmax is the maximum eigenvalue of the matrix G G 1J,and e1 is a the max time-step parameter(default=0.9)uIn addition,steady Implicit/Explicit stiff coupled solvertLimit updates when solution changing quicklyQn+1=Qn+s DQwheree3=positivity rate(default=0.2)e2=temp.redux(defaul
9、t=0.25)Laminar flames:General Finite-Rate ChemistryStiff solver7Company Confidential Copyright 2001 Fluent Inc.All rights reserved.Example:Mitchell flameSubsonic,methane-air,diffusion flameSmooke mechanismu16 reactive species,46 reaction stepsMolecular transport with kinetic theoryAxi-symmetricCoupl
10、ed,implicit solverThanks to Amish ThakerLaminar flames:General Finite-Rate Chemistry8Company Confidential Copyright 2001 Fluent Inc.All rights reserved.Example:Mitchell flameLaminar flames:General Finite-Rate Chemistry9Company Confidential Copyright 2001 Fluent Inc.All rights reserved.Example:Mitche
11、ll flameLaminar flames:General Finite-Rate Chemistry10Company Confidential Copyright 2001 Fluent Inc.All rights reserved.Convergence tricksStiff chemistry simulations are very difficult to convergeStart with a very coarse grid(1000 cells)uMultiple adaptions after convergence to add resolutionuI use
12、region adaption to minimize cell volume changesStart with a small CFL(0.01)and ramp up(100)For premixed and partially premixed flames:uPatch unburnt ahead of stabilizer,burnt behind,oruSet premixed inlets to equilibrium(burnt)species and temperaturetDisable reactions and solve for mixing.tEnable rea
13、ctions flame should propagate back to flame stabilizer.For non-premixed flames:uFor low temperature inlets and walls,an ignition source is requiredtPatch high temperature zone in mixing layer.tOr,temporarily set an inlet temperature above the ignition temperatureLaminar flames:General Finite-Rate Ch
14、emistry11Company Confidential Copyright 2001 Fluent Inc.All rights reserved.DetonationPhysicsuPremixed fuel and oxidizeruIgnition(spark)uSlow(subsonic)deflagration transitions to detonation(supersonic)uMixture ignited by heat increase behind shockuFront moves at Rankine-Hugoniot speedNumericsuSpark
15、details difficult to capture(small time/length scales)uDeflagration to detonation difficult to captureuSolution:Skip these and start simulation at detonationtPatch a high pressure in spark zone to initiate shocktAcceptable since spark kernel usually small,and simulation not sensitive to initial cond
16、itionsuExplicit solver for shock capturing:not robust for stiff chemistyuSolution:1 step chemistry with tuned kineticstAcceptable since detonation speed determined only by heat release.Laminar flames:General Finite-Rate Chemistry12Company Confidential Copyright 2001 Fluent Inc.All rights reserved.Ex
17、ample:DetonationuStochiometric methane-air in an open pipeuCH4+2O2-CO2+2H2OuR=Ae-E/RT CH4O22A=1013,E=1.25*108Laminar flames:General Finite-Rate Chemistry13Company Confidential Copyright 2001 Fluent Inc.All rights reserved.Numerical methodsSegregated solveruFractional time stepping:over a time step D
18、ttAdvance solution with no chemical source terms(only convection and diffusion)for DttThen,advance chemistry in each cell for Dt as a constant pressure reactorwhere the chemical source term S=wk Wk/r,wk is the reaction rate,Wk is the molecular weight,and r is the density Laminar flames:General Finit
19、e-Rate Chemistry14Company Confidential Copyright 2001 Fluent Inc.All rights reserved.Numerical methodsuChemistry integrated with stiff ODE solver CVODEuRequires unsteady solution,even for steady state!uFinal solution depends on time step!uHence,only use for unsteady reacting flowstFractional step sc
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