1、Fluent 6.0 Staff Training,Graham Goldin October 25 2001,Combustion and DPM,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,2,Summary,Laminar flames General finite rate chemistry Premixed laminar flames (flame sheet model) Non-premixed laminar flames (equilibrium f model)Turbulen
2、t flames Enhancement of v5 models Partially premixed model EDC modelDiscrete Phase Model Enhancement of v5 models Spray models Multiple surface reactions,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,3,Laminar Flames,Chemistry invariably stiff Reaction time/length scales flow
3、time/length scales Special numerical methods required (stiff solvers)Non-premixed (diffusion flames) Fuel and oxidizer diffuse into the reaction zone, then burnPremixed Fuel and oxidizer mixed molecularly, then burn Moving reaction front usually thin and difficult to model Deflagrations Subsonic: ve
4、ry difficult to model since the flame speed depends on the chemistry as well as the molecular diffusion parameters, and the flame zone must be resolved. Detonations Supersonic: ignition due to heat release behind shock. Simpler to model than deflagrations since the shock is not resolved, and detaile
5、d molecular transport is not essential.,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,4,General Finite-Rate Chemistry,Fluent v6 can import a CHEMKIN IIdetailed chemical mechanism file File - Import - ChemkinReactions v5: Arrhenius with reversible reactions and third body effic
6、iencies v6: Pressure dependent reactions (Lindemann, Troe and SRI) Low pressure and high pressure rates, with blending functionsMolecular transport Critical in subsonic laminar flames since it determines mixing and flame speeds Recommend using kinetic theory Can get the Leonard-Jones parameters from
7、 the CHEMKIN transport database (TRAN.DB),Laminar flames,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,5,Numerical methods,Need special numerics since stiff reaction mechanismCoupled solver Advance species and temperature simultaneously over time step v6: stiff solver option U
8、se Implicit for subsonic flames Use Explicit for supersonic flames (detonations=explosions)Segregated solver Default steady, segregated algorithm will diverge Can use unsteady, segregated algorithm, but time step must be near chemistry time-scale (typical 10-9s): not practical! v6: has a fractional
9、step scheme (hidden from the user),Laminar flames: General Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,6,Stiff solver,Coupled solverPreconditioned NS: G = preconditioning matrix Q = r, ui, T, Yi F = inviscid and viscous fluxes S = source termsImplicit s
10、patial discretization: J = Jacobian of S = d S/d Q A = Jacobian of F = d F/d Q Rn = Residual at previous time step = d F/d xi Sn,Laminar flames: General Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,7,Implicit stiff coupled solver Default time step (stiff
11、 solver inactive)where lmax is the maximum eigenvalue of the matrix G 1A stiff solver activewhere lmax is the maximum eigenvalue of the matrix G 1J,and e1 is a the max time-step parameter (default = 0.9)In addition, steady Implicit/Explicit stiff coupled solver Limit updates when solution changing q
12、uicklyQn+1 = Qn + s DQwhere e3 = positivity rate (default = 0.2)e2 = temp. redux (default = 0.25),Laminar flames: General Finite-Rate Chemistry,Stiff solver,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,8,Example: Mitchell flame,Subsonic, methane-air, diffusion flameSmooke mec
13、hanism 16 reactive species, 46 reaction stepsMolecular transport with kinetic theoryAxi-symmetricCoupled, implicit solverThanks to Amish Thaker,Laminar flames: General Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,9,Example: Mitchell flame,Laminar flames:
14、 General Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,10,Example: Mitchell flame,Laminar flames: General Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,11,Convergence tricks,Stiff chemistry simulations are very
15、 difficult to converge Start with a very coarse grid (1000 cells) Multiple adaptions after convergence to add resolution I use region adaption to minimize cell volume changes Start with a small CFL (0.01) and ramp up (100) For premixed and partially premixed flames: Patch unburnt ahead of stabilizer
16、, burnt behind, or Set premixed inlets to equilibrium (burnt) species and temperature Disable reactions and solve for mixing. Enable reactions flame should propagate back to flame stabilizer. For non-premixed flames: For low temperature inlets and walls, an ignition source is required Patch high tem
17、perature zone in mixing layer. Or, temporarily set an inlet temperature above the ignition temperature,Laminar flames: General Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,12,Detonation,Physics Premixed fuel and oxidizer Ignition (spark) Slow (subsonic)
18、deflagration transitions to detonation (supersonic) Mixture ignited by heat increase behind shock Front moves at Rankine-Hugoniot speedNumerics Spark details difficult to capture (small time/length scales) Deflagration to detonation difficult to capture Solution: Skip these and start simulation at d
19、etonation Patch a high pressure in spark zone to initiate shock Acceptable since spark kernel usually small, and simulation not sensitive to initial conditions Explicit solver for shock capturing: not robust for stiff chemisty Solution: 1 step chemistry with tuned kinetics Acceptable since detonatio
20、n speed determined only by heat release.,Laminar flames: General Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,13,Example: Detonation,Stochiometric methane-air in an open pipe CH4 + 2O2 - CO2 + 2H2O R=Ae-E/RT CH4O22 A = 1013, E = 1.25*108,Laminar flames:
21、General Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,14,Numerical methods,Segregated solver Fractional time stepping: over a time step Dt Advance solution with no chemical source terms(only convection and diffusion) for DtThen, advance chemistry in each
22、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 Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,15,Numerical methods,Ch
23、emistry integrated with stiff ODE solver CVODERequires unsteady solution, even for steady state!Final solution depends on time step!Hence, only use for unsteady reacting flows Fractional step scheme is first order accurate in timeHidden from gui/tui: activate with scheme commands (rpsetvar stiff-che
24、m-seg? #t) (models-changed),Laminar flames: General Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,16,Example: Rapid Compression Machine,Single, driven piston compresses hydrogen-oxygen-argon mixture which ignites due to heat of compressionExperiments by L
25、ee, D., and Hochgreb, S., “Rapid Compression Machines: Heat Transfer and Suppression of Corner Vortex”, Combustion and Flame 114:531-545, 1998H2/O2/Ar 8 reacting species, 19 step mechanismMoving mesh, segregated solver, fractional step stiff chemistry solverThanks to Dan Lee,Laminar flames: General
26、Finite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,17,Example: Rapid Compression Machine,Validation: comparison of adiabatic, constant volume ignition delay (solid line) vs results from stand alone CHEMKIN code Senkin (square symbols),Laminar flames: General F
27、inite-Rate Chemistry,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,18,Example: Rapid Compression Machine,Mesh,Laminar flames: General Finite-Rate Chemistry,Temperature,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,19,Example: Rapid Compression Machine,Pe
28、ak pressures,Laminar flames: General Finite-Rate Chemistry,Peak temperatures,Ignition delay,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,20,Non-premixed flames,Under the assumptions of chemical equilibrium constant diffusivities for all species and enthalpy (Le=1) constant pr
29、essure single, distinct fuel and oxidizer streams (diffusion flame)the chemistry can be reduced to a single, conserved scalar, the mixture fraction, denoted fIn Fluent, the non-premixed model is only available for turbulent flows, so we have to trick the solverRapid solution Minutes, compared to day
30、s for the finite rate solver,Laminar flames,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,21,Strategy,Activate k-e model, but disable their solutionInitialize k to 10-10 and e to 10+10 Turbulent diffusivity 0Activate Non-premixed model Read in PDF fileForce variance to zero by
31、 zeroing production and dissipation constants via scheme (rpsetvar cdvar 0) (rpsetvar cgvar 0)Set appropriate (or tuned) molecular diffusivity,Laminar flames: Non-premixed flames,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,22,Example : Mitchell flame,Laminar flames: Non-prem
32、ixed flames,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,23,Premixed flames,Fuel and oxidizer mixed together at molecular level prior to burning (reactants) Radicals and heat diffuse from burnt products into unburnt reactants and igniteFlame moves as a front with laminar flam
33、e speed,Laminar flames,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,24,Theory,Laminar flame speed, sl, determined by internal flame structure balance between heat /radical production in inner layer and conduction/diffusion to preheat zone Requires complex chemistry and transp
34、ort properties not feasible to resolve in industrial 3D simulationsLaminar flame thickness, lF D / sl, O(0.1mm) D is the thermal diffusivity = l / r cpLaminar flame speed is a function of reactant temperature, pressure and species composition measured or computed from 1D complex chemistry simulation
35、s determine flammability limits: typically between f=0.5 and f=1.5, where f is the equivalence ratio = (XF/XO) / (XF/XO)sto,Laminar flames: Premixed flames,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,25,Strategy,Not feasible to resolve the small reaction zone,as well as the
36、detailed chemistry and moleculartransport propertiesModel flame as a sheet propagating with a specified velocity, with heat release at the frontUse the VOF model, with UDFs for propagating speed and heat releaseThanks Boris Makarov and Andrey Troshko,Laminar flames: Premixed flames,Company Confident
37、ial Copyright 2001 Fluent Inc. All rights reserved.,26,Flame sheet UDF (1),Laminar flames: Premixed flames,#include “udf.h“ #include “sg.h“ #include “sg_mphase.h“ #include “flow.h“ #include “mem.h“#define flame_speed 2.;DEFINE_ADJUST(area_density, domain) Thread *t;Thread *pt;cell_t c;Domain *pDomai
38、n = DOMAIN_SUB_DOMAIN(domain,P_PHASE);real voidx, voidy, voidz=0;Alloc_Storage_Vars(pDomain,SV_VOF_RG,SV_VOF_G,SV_NULL);Scalar_Reconstruction(pDomain, SV_VOF,-1,SV_VOF_RG,NULL);Scalar_Derivatives(pDomain,SV_VOF,-1,SV_VOF_G,SV_VOF_RG,Vof_Deriv_Accumulate);mp_thread_loop_c (t,domain,pt)if (FLUID_THREA
39、D_P(t),Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,27,Flame sheet UDF (2),Laminar flames: Premixed flames,Thread *tp = ptP_PHASE;begin_c_loop (c,t)voidx = C_VOF_G(c,tp)0;voidy = C_VOF_G(c,tp)1; #if RP_3Dvoidz = C_VOF_G(c,tp)2; #endif/* calculation of the interfacial area den
40、sity */C_UDMI(c,t,0)= sqrt( SQR(voidx) + SQR(voidy) + SQR(voidz) );end_c_loop (c,t)Free_Storage_Vars(pDomain,SV_VOF_RG,SV_VOF_G,SV_NULL); ,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,28,Flame sheet UDF (3),Laminar flames: Premixed flames,DEFINE_SOURCE(reactants, cell, thread
41、, dS, eqn) real source;Thread *tm = THREAD_SUPER_THREAD(thread);Thread *pt = THREAD_SUB_THREADS(tm);source = - C_UDMI(cell, tm, 0)*C_R(cell,pt0);source *= flame_speed;dSeqn = 0;return source; DEFINE_SOURCE(product, cell, thread, dS, eqn) real source;Thread *tm = THREAD_SUPER_THREAD(thread);Thread *p
42、t = THREAD_SUB_THREADS(tm);source = C_UDMI(cell, tm, 0)*C_R(cell,pt0);source *= flame_speed;dSeqn = 0;return source; ,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,29,Example: Deflagration,Stochiometric methane-air in an open pipe VOF model with UDF,Laminar flames: Premixed,Company Confidential Copyright 2001 Fluent Inc. All rights reserved.,30,Competitors capabilities,CFX Fractional step scheme (pressure based solver) STAR Offer a link to CHEMKIN Fractional step scheme GASP/FASTRAN Equivalent coupled, density based solver,Laminar flames,
