Presentation is loading. Please wait.

Presentation is loading. Please wait.

Fluent 6.0 Staff Training Graham Goldin October 25 2001.

Similar presentations


Presentation on theme: "Fluent 6.0 Staff Training Graham Goldin October 25 2001."— Presentation transcript:

1 Fluent 6.0 Staff Training Graham Goldin October

2 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)  Turbulent flames  Enhancement of v5 models  Partially premixed model  EDC model  Discrete Phase Model  Enhancement of v5 models  Spray models  Multiple surface reactions

3 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 3 Laminar Flames  Chemistry invariably stiff  Reaction time/length scales << flow time/length scales  Special numerical methods required (stiff solvers)  Non-premixed (diffusion flames)  Fuel and oxidizer diffuse into the reaction zone, then burn  Premixed  Fuel and oxidizer mixed molecularly, then burn  Moving reaction front – usually thin and difficult to model  Deflagrations  Subsonic: very 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 detailed molecular transport is not essential.

4 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 4 General Finite-Rate Chemistry  Fluent v6 can import a CHEMKIN II detailed chemical mechanism file  File -> Import -> Chemkin…  Reactions  v5: Arrhenius with reversible reactions and third body efficiencies  v6: Pressure dependent reactions (Lindemann, Troe and SRI)  Low pressure and high pressure rates, with blending functions  Molecular transport  Critical in subsonic laminar flames since it determines mixing and flame speeds  Recommend using kinetic theory  Can get the Leonard-Jones parameters from the CHEMKIN transport database (TRAN.DB) Laminar flames

5 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 5 Numerical methods  Need special numerics since stiff reaction mechanism  Coupled solver  Advance species and temperature simultaneously over time step  v6: stiff solver option  Use 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 s): not practical!  v6: has a fractional step scheme (hidden from the user) Laminar flames: General Finite-Rate Chemistry

6 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 6 Stiff solver  Coupled solver  Preconditioned NS:   = preconditioning matrix  Q = [ , u i, T, Y i ]  F = inviscid and viscous fluxes  S = source terms  Implicit spatial discretization:  J = Jacobian of S =  S/  Q  A = Jacobian of F =  F/  Q  R n = Residual at previous time step = [  F/  x i – S] n Laminar flames: General Finite-Rate Chemistry

7 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 7  Implicit stiff coupled solver  Default time step (stiff solver inactive) where max is the maximum eigenvalue of the matrix  –1 A  stiff solver active where max is the maximum eigenvalue of the matrix  –1 J, and  1 is a the max time-step parameter (default = 0.9)  In addition, steady Implicit/Explicit stiff coupled solver  Limit updates when solution changing quickly Q n+1 = Q n +   Q where  3 = positivity rate (default = 0.2)  2 = temp. redux (default = 0.25) Laminar flames: General Finite-Rate Chemistry Stiff solver

8 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 8 Example: Mitchell flame  Subsonic, methane-air, diffusion flame  Smooke mechanism  16 reactive species, 46 reaction steps  Molecular transport with kinetic theory  Axi-symmetric  Coupled, implicit solver  Thanks to Amish Thaker Laminar flames: General Finite-Rate Chemistry

9 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 9 Example: Mitchell flame Laminar flames: General Finite-Rate Chemistry

10 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 10 Example: Mitchell flame Laminar flames: General Finite-Rate Chemistry

11 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 11 Convergence tricks  Stiff chemistry simulations are very 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, 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 temperature zone in mixing layer.  Or, temporarily set an inlet temperature above the ignition temperature Laminar flames: General Finite-Rate Chemistry

12 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 12 Detonation  Physics  Premixed fuel and oxidizer  Ignition (spark)  Slow (subsonic) deflagration transitions to detonation (supersonic)  Mixture ignited by heat increase behind shock  Front moves at Rankine-Hugoniot speed  Numerics  Spark details difficult to capture (small time/length scales)  Deflagration to detonation difficult to capture  Solution: Skip these and start simulation at detonation  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 detonation speed determined only by heat release. Laminar flames: General Finite-Rate Chemistry

13 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 13 Example: Detonation  Stochiometric methane-air in an open pipe  CH 4 + 2O 2 -> CO 2 + 2H 2 O  R=Ae -E/RT [CH 4 ][O 2 ] 2 A = 10 13, E = 1.25*10 8 Laminar flames: General Finite-Rate Chemistry

14 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 14 Numerical methods  Segregated solver  Fractional time stepping: over a time step  t  Advance solution with no chemical source terms (only convection and diffusion) for  t  Then, advance chemistry in each cell for  t as a constant pressure reactor where the chemical source term S = w k W k /  w k is the reaction rate, W k is the molecular weight, and  is the density Laminar flames: General Finite-Rate Chemistry

15 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 15 Numerical methods  Chemistry integrated with stiff ODE solver CVODE  Requires 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 time  Hidden from gui/tui: activate with scheme commands… (rpsetvar ‘stiff-chem-seg? #t) (models-changed) Laminar flames: General Finite-Rate Chemistry

16 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 compression  Experiments by Lee, D., and Hochgreb, S., “Rapid Compression Machines: Heat Transfer and Suppression of Corner Vortex”, Combustion and Flame 114: , 1998  H 2 /O 2 /Ar 8 reacting species, 19 step mechanism  Moving mesh, segregated solver, fractional step stiff chemistry solver  Thanks to Dan Lee Laminar flames: General Finite-Rate Chemistry

17 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 Finite-Rate Chemistry

18 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 18 Example: Rapid Compression Machine Mesh Laminar flames: General Finite-Rate Chemistry Temperature

19 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 19 Example: Rapid Compression Machine Peak pressures Laminar flames: General Finite-Rate Chemistry Peak temperatures Ignition delay

20 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 pressure  single, distinct fuel and oxidizer streams (diffusion flame) the chemistry can be reduced to a single, conserved scalar, the mixture fraction, denoted f  In Fluent, the non-premixed model is only available for turbulent flows, so we have to trick the solver  Rapid solution  Minutes, compared to days for the finite rate solver Laminar flames

21 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 21 Strategy  Activate k -  model, but disable their solution  Initialize k to and  to  Turbulent diffusivity ~ 0  Activate Non-premixed model  Read in PDF file  Force variance to zero by zeroing production and dissipation constants via scheme… (rpsetvar ‘cdvar 0) (rpsetvar ‘cgvar 0)  Set appropriate (or tuned) molecular diffusivity Laminar flames: Non-premixed flames

22 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 22 Example : Mitchell flame Laminar flames: Non-premixed flames

23 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 ignite  Flame moves as a front with laminar flame speed Laminar flames Flame thickness = l F slsl Intermediate specie Temperature preheat zone oxidation zone inner layer Laminar flame speed = s l

24 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 24 Theory  Laminar flame speed, s l, determined by internal flame structure  balance between heat /radical production in inner layer and conduction/diffusion to preheat zone  Requires complex chemistry and transport properties  not feasible to resolve in industrial 3D simulations Laminar flame thickness, l F ~ D / s l, ~ O(0.1mm)  D is the thermal diffusivity =  c p  Laminar flame speed is a function of reactant temperature, pressure and species composition  measured or computed from 1D complex chemistry simulations  determine flammability limits: typically between  =0.5 and  =1.5, where  is the equivalence ratio = (X F /X O ) / (X F /X O ) sto Laminar flames: Premixed flames

25 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 25 Strategy  Not feasible to resolve the small reaction zone, as well as the detailed chemistry and molecular transport properties  Model flame as a sheet propagating with a specified velocity, with heat release at the front  Use the VOF model, with UDFs for propagating speed and heat release  Thanks Boris Makarov and Andrey Troshko Laminar flames: Premixed flames

26 Company Confidential 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 *pDomain = 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_THREAD_P(t)) {

27 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 27 Flame sheet UDF (2) Laminar flames: Premixed flames Thread *tp = pt[P_PHASE]; begin_c_loop (c,t) { voidx = C_VOF_G(c,tp)[0]; voidy = C_VOF_G(c,tp)[1]; #if RP_3D voidz = C_VOF_G(c,tp)[2]; #endif /* calculation of the interfacial area density */ 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); }

28 Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 28 Flame sheet UDF (3) Laminar flames: Premixed flames DEFINE_SOURCE(reactants, cell, thread, 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,pt[0]); source *= flame_speed; dS[eqn] = 0; return source; } DEFINE_SOURCE(product, cell, thread, 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,pt[0]); source *= flame_speed; dS[eqn] = 0; return source; }

29 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

30 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


Download ppt "Fluent 6.0 Staff Training Graham Goldin October 25 2001."

Similar presentations


Ads by Google