Fluent 6.0 Staff Training Graham Goldin October 25 2001.
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Transcript of Fluent 6.0 Staff Training Graham Goldin October 25 2001.
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
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.
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
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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 10-9s): not practical! v6: has a fractional step scheme (hidden from the user)
Laminar flames: General Finite-Rate Chemistry
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Stiff solver Coupled solver
Preconditioned NS: = preconditioning matrix Q = [, ui, T, Yi]
F = inviscid and viscous fluxes S = source terms
Implicit spatial discretization: J = Jacobian of S = S/Q A = Jacobian of F = F/Q Rn = Residual at previous time step = [F/xi – S]n
Laminar flames: General Finite-Rate Chemistry
Sx
F
t
Q
i
Γ
n
i
tRQx
t
A
JΓ
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Implicit stiff coupled solver Default time step (stiff solver inactive)
where max is the maximum eigenvalue of the matrix –1A stiff solver active
where max is the maximum eigenvalue of the matrix –1J,
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
Qn+1 = Qn + Q
where 3 = positivity rate (default = 0.2)
2 = temp. redux (default = 0.25)
Laminar flames: General Finite-Rate Chemistry
maxxCFL
t
max
1
t
otherwise
TT
132
Stiff solver
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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
Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 9
Example: Mitchell flameLaminar flames: General Finite-Rate Chemistry
Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 10
Example: Mitchell flameLaminar flames: General Finite-Rate Chemistry
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
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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
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Example: Detonation Stochiometric methane-air in an open pipe CH4 + 2O2 -> CO2 + 2H2O
R=Ae-E/RT [CH4][O2]2 A = 1013, E = 1.25*108
Laminar flames: General Finite-Rate Chemistry
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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 = wk Wk / wk is the reaction rate, Wk is the molecular weight, and is the density
Laminar flames: General Finite-Rate Chemistry
Sdt
dQ
ix
F
t
Q
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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
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 114:531-545, 1998
H2/O2/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
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
0.01
0.1
1
10
100
1000
850 900 950 1000 1050 1100 1150 1200
Temperature (K)
Ign
itio
n D
elay
(m
s)
0.10
1.00
10.00
0.01 0.10 1.00 10.00
Pressure (MPa)
Ign
itio
n D
ela
y (
ms
)
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Example: Rapid Compression Machine
Mesh
Laminar flames: General Finite-Rate Chemistry
Temperature
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Example: Rapid Compression Machine
Peak pressures
Laminar flames: General Finite-Rate Chemistry
0
1
2
3
4
5
6
0 1 2 3 4 5 6
Experiment (MPa)
Flu
ent
(MP
a)
950
1000
1050
1100
950 1000 1050 1100
Experiment (K)F
luen
t (K
)
Peak temperatures
0
10
20
30
40
50
60
0 10 20 30 40 50 60
Experiment (ms)
Flu
ent
(ms)
Ignition delay
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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
Company Confidential Copyright 2001 Fluent Inc. All rights reserved. 21
Strategy Activate k- model, but disable their solution
Initialize k to 10-10 and to 10+10
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
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Example : Mitchell flameLaminar flames: Non-premixed 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 ignite
Flame moves as a front with laminar flame speed
Laminar flames
Flame thickness = lF
sl
Intermediate specie
Temperature
preheat zone oxidation zoneinner layer
Laminar flame speed = sl
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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 transport properties not feasible to resolve in industrial 3D simulations
Laminar flame thickness, lF ~ D / sl, ~ O(0.1mm) D is the thermal diffusivity = cp
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 = (XF/XO) / (XF/XO)sto
Laminar flames: Premixed flames
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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
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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))
{
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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);
}
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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;
}
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Example: Deflagration Stochiometric methane-air in an open pipe VOF model with UDF
Laminar flames: Premixed