Transcript Combustion Modeling in FLUENT/UNS V4 and FLUENT V4.4

Fluent Software Training
MTG-97-183
Combustion Modeling
in FLUENT
1
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Outline







Applications
Overview of Combustion Modeling Capabilities
Chemical Kinetics
Gas Phase Combustion Models
Discrete Phase Models
Pollutant Models
Combustion Simulation Guidelines
2
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Applications

Wide range of homogeneous
and heterogeneous reacting
flows






Furnaces
Boilers
Process heaters
Gas turbines
Rocket engines
Temperature in a gas furnace
Predictions of:




CO2 mass fraction
Flow field and mixing
characteristics
Temperature field
Species concentrations
Particulates and pollutants
Stream function
3
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Aspects of Combustion Modeling
Combustion Models
Dispersed Phase Models
Premixed
Partially premixed
Nonpremixed
Droplet/particle dynamics
Heterogeneous reaction
Devolatilization
Evaporation
Governing Transport Equations
Mass
Momentum (turbulence)
Energy
Chemical Species
Radiative Heat Transfer Models
Pollutant Models
4
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Combustion Models Available in FLUENT

Gas phase combustion





Generalized finite rate formulation (Magnussen model)
Conserved scalar PDF model (one and two mixture fractions)
Laminar flamelet model (V5)
Zimont model (V5)
Discrete phase model

Turbulent particle dispersion






Stochastic tracking
Particle cloud model (V5)
Pulverized coal and oil spray combustion submodels
Radiation models: DTRM, P-1, Rosseland and Discrete Ordinates (V5)
Turbulence models: k-, RNG k-, RSM, Realizable k- (V5) and LES (V5)
Pollutant models: NOx with reburn chemistry (V5) and soot
5
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Modeling Chemical Kinetics in Combustion

Challenging




Most practical combustion processes are turbulent
Rate expressions are highly nonlinear; turbulence-chemistry interactions
are important
Realistic chemical mechanisms have tens of species, hundreds of reactions
and stiff kinetics (widely disparate time scales)
Practical approaches

Reduced chemical mechanisms


Finite rate combustion model
Decouple reaction chemistry from turbulent flow and mixing

Mixture fraction approaches



Equilibrium chemistry PDF model
Laminar flamelet
Progress variable

Zimont model
6
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Generalized Finite Rate Model


Chemical reaction process described using global mechanism.
Transport equations for species are solved.


These equations predict local time-averaged mass fraction, mj , of each
species.
Source term (production or consumption) for species j is net reaction
rate over all k reactions in mechanism:
R j   R jk
k


Rjk (rate of production/consumption of species j in reaction k) is
computed to be the smaller of the Arrhenius rate and the mixing or
“eddy breakup” rate.
Mixing rate related to eddy lifetime, k /.

Physical meaning is that reaction is limited by the rate at which turbulence
can mix species (nonpremixed) and heat (premixed).
7
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Setup of Finite Rate Chemistry Models

Requires:


List of species and their properties
List of reactions and reaction rates

FLUENT V5 provides this info in a mixture material database.

Chemical mechanisms and physical properties for the most common
fuels are provided in database.
If you have different chemistry, you can:



Create new mixtures.
Modify properties/reactions of existing mixtures.
8
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Generalized Finite Rate Model: Summary

Advantages:




Applicable to nonpremixed, partially premixed, and premixed combustion
Simple and intuitive
Widely used
Disadvantages:




Unreliable when mixing and kinetic time scales are comparable (requires
Da >>1).
No rigorous accounting for turbulence-chemistry interactions
Difficulty in predicting intermediate species and accounting for
dissociation effects.
Uncertainty in model constants, especially when applied to multiple
reactions.
9
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Conserved Scalar (Mixture Fraction)
Approach: The PDF Model


Applies to nonpremixed (diffusion) flames only
Assumes that reaction is mixing-limited



Reaction mechanism is not explicitly defined by you.



Local chemical equilibrium conditions prevail.
Composition and properties in each cell defined by extent of turbulent
mixing of fuel and oxidizer streams.
Reacting system treated using chemical equilibrium calculations (prePDF).
Solves transport equations for mixture fraction and its variance, rather
than species transport equations.
Rigorous accounting of turbulence-chemistry interactions.
10
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Mixture Fraction Definition

The mixture fraction, f, can be written in terms of elemental mass
fractions as:
f 



Z k  Z k ,O
Z k , F  Z k ,O
where Zk is the elemental mass fraction of some element, k. Subscripts F
and O denote fuel and oxidizer inlet stream values, respectively.
For simple fuel/oxidizer systems, the mixture fraction represents the fuel
mass fraction in a computational cell.
Mixture fraction is a conserved scalar:

Reaction source terms are eliminated from governing transport equations.
11
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Systems That Can be Modeled Using a Single
Mixture Fraction

Fuel/air diffusion flame:
60% CH4
40% CO
21% O2
79% N2
f=1
f=0
35% O2
65% N2


Diffusion flame with oxygenenriched inlets:
System using multiple fuel
inlets:
12
f=0
60% CH4
40% CO
f=1
35% O2
65% N2
f=0
60% CH4 20% CO
10% C3H8 10% CO2
f=1
21% O2 79%
N2
60% CH4 20% CO
10% C3H8 10% CO2
f=0
f=1
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Equilibrium Approximation of System
Chemistry


Chemistry is assumed to be fast enough to achieve equilibrium.
Intermediate species are included.
13
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
PDF Modeling of Turbulence-Chemistry Interaction

Fluctuating mixture fraction is completely defined by its probability
density function (PDF).
1
T  T
p(V )V  lim



i
i
p(V), the PDF, represents fraction of sampling time when variable, V,
takes a value between V and V + V.
p(f) can be used to compute time-averaged values of variables that
 i  01 p( f ) i ( f )df
depend on the mixture fraction, f:


Species mole fractions
Temperature, density
14
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
PDF Model Flexibility

Nonadiabatic systems:



In real problems, with heat loss or gain, local thermo-chemical state must
be related to mixture fraction, f, and enthalpy, h.
Average quantities now evaluated as a function of mixture fraction,
enthalpy (normalized heat loss/gain), and the PDF, p(f).
Second conserved scalar:

With second scalar in FLUENT, you can model:




Two fuel streams with different compositions and single oxidizer stream
(visa versa)
Nonreacting stream in addition to a fuel and an oxidizer
Co-firing a gaseous fuel with another gaseous, liquid, or coal fuel
Firing single coal with two off-gases (volatiles and char burnout products)
tracked separately
15
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Mixture Fraction/PDF Model: Summary

Advantages:






Predicts formation of intermediate species.
Accounts for dissociation effects.
Accounts for coupling between turbulence and chemistry.
Does not require the solution of a large number of species transport
equations
Robust and economical
Disadvantages:



System must be near chemical equilibrium locally.
Cannot be used for compressible or non-turbulent flows.
Not applicable to premixed systems.
16
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
The Laminar Flamelet Model



Extension of the mixture fraction PDF model to
moderate chemical nonequilibrium
Turbulent flame modeled as an ensemble of
stretched laminar, opposed flow diffusion flames
Temperature, density and species (for adiabatic)
specified by two parameters, the mixture
fraction and scalar dissipation rate
 Recall that for the mixture fraction PDF
model (adiabatic), thermo-chemical state is
function of f only
i  i ( f , c )

c  (f / x)2
c can be related to the local rate of strain
17
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Laminar Flamelet Model (2)

Statistical distribution of flamelet ensemble is specified by the PDF
P(f,c), which is modeled as Pf (f) Pc (c), with a Beta function for Pf (f)
and a Dirac-delta distribution for Pc (c)
1
i   i ( f , c )  Pf ( f ) Pc ( c )dc df
00


Only available for adiabatic systems in V5
Import strained flame calculations


prePDF or Sandia’s OPPDIF code
Single or multiple flamelets


Single:
Multiple:



user specified strain, a
strained flamelet library, 0 < a < aextinction
a=0 equilibrium
a= aextinction is the maximum strain rate before flame extinguishes
Possible to model local extinction pockets (e.g. lifted flames)
18
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
The Zimont Model for Premixed Combustion

Thermo-chemistry described by a single progress variable, c  Yp / Ypad
   t c 

 


c



u
c





i
  Sc x   Rc

t

x

x


i
i 
t
i 


p
0 c 1
Mean reaction rate, Rc   unburnt U t c
Turbulent flame speed, Ut, derived for lean premixed combustion and
accounts for





p
Equivalence ratio of the premixed fuel
Flame front wrinkling and thickening by turbulence
Flame front quenching by turbulent stretching
Differential molecular diffusion
For adiabatic combustion,
T  (1  c )Tunburnt  c Tad

The enthalpy equation must be solved for nonadiabatic combustion
19
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Discrete Phase Model

Trajectories of particles/droplets/bubbles are
computed in a Lagrangian frame.






Although the mass loading can be large
No particle-particle interaction or break up
Turbulent dispersion modeled by


Continuous phase
flow field calculation
Discrete phase volume fraction must < 10%


Exchange (couple) heat, mass, and
momentum with Eulerian frame gas phase
Stochastic tracking
Particle cloud (V5)
Rosin-Rammler or linear size distribution
Particle tracking in unsteady flows (V5)
Model particle separation, spray drying,
liquid fuel or coal combustion, etc.
20
Particle trajectory
calculation
Update continuous
phase source terms
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Particle Dispersion: The Stochastic Tracking Model

Turbulent dispersion is modeled by an ensemble of
Monte-Carlo realizations (discrete random walks)

Particles convected by the mean velocity plus a random
direction turbulent velocity fluctuation

Each trajectory represents a group of particles with the
same properties (initial diameter, density etc.)

Turbulent dispersion is important because


Physically realistic (but computationally more expensive)
Enhances stability by smoothing source terms and
Coal particle tracks in an
eliminating local spikes in coupling to the gas phase
industrial boiler
21
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Particle Dispersion: The Particle Cloud Model






Track mean particle trajectory along mean velocity
Assuming a 3D multi-variate Gaussian distribution about this mean
track, calculate particle loading within three standard deviations
Rigorously accounts for inertial and drift velocities
A particle cloud is required for each particle type (e.g. initial d, etc.)
Particles can escape, reflect or trap (release volatiles) at walls
Eliminates (single cloud) or reduces (few clouds) stochastic tracking


Decreased computational expense
Increased stability since distributed source terms in gas phase
BUT decreased accuracy since


Gas phase properties (e.g. temperature) are averaged within cloud
Poor prediction of large recirculation zones
22
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Particle Tracking in Unsteady Flows



Each particle advanced in time along with the flow
For coupled flows using implicit time stepping, sub-iterations for the particle
tracking are performed within each time step
For non-coupled flows or coupled flows with explicit time stepping, particles
are advanced at the end of each time step
23
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Coal/Oil Combustion Models

Coal or oil combustion modeled by changing the modeled particle to


Droplet - for oil combustion
Combusting particle - for coal combustion
Particle Type
Inert
Droplet (oil)
Combusting (coal)

Description
inert/heating or cooling
heating/evaporation/boiling
heating;
evolution of volatiles/swelling;
heterogeneous surface reaction
Several devolatilization and char burnout models provided.

Note: These models control the rate of evolution of the fuel off-gas from
coal/oil particles. Reactions in the gas (continuous) phase are modeled
with the PDF or finite rate combustion model.
24
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
NOx Models

NOx consists of mostly nitric oxide (NO).




Precursor for smog
Contributes to acid rain
Causes ozone depletion
Three mechanisms included in FLUENT for NOx production:

Thermal NOx - Zeldovich mechanism (oxidation of atmospheric N)


Prompt NOx - empirical mechanisms by De Soete, Williams, etc.



Contribution is in general small
Significant at fuel rich zones
Fuel NOx - Empirical mechanisms by De Soete, Williams, etc.


Most significant at high temperatures
Predominant in coal flames where fuel-bound nitrogen is high and
temperature is generally low.
NOx reburn chemistry (V5)

NO can be reduced in fuel rich zones by reaction with hydrocarbons
25
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Soot modeling in FLUENT

Two soot formation models are available:

One-step model (Khan and Greeves)


Two-Step model (Tesner)




Single transport equation for soot mass fraction
Transport equations for radical nuclei and soot mass fraction
concentrations
Soot formation modeled by empirical rate constants
R formation  C p f Fne E / RT
where, C, pf, and F are a model constant, fuel partial pressure and
equivalence ratio, respectively
Soot combustion (destruction) modeled by Magnussen model
Soot affects the radiation absorption

Enable Soot-Radiation option in the Soot panel
26
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Combustion Guidelines and Solution Strategies

Start in 2D




Determine applicability of model physics
Mesh resolution requirements (resolve shear layers)
Solution parameters and convergence settings
Boundary conditions

Combustion is often very sensitive to inlet boundary conditions



Correct velocity and scalar profiles can be critical
Wall heat transfer is challenging to predict; if known, specify wall
temperature instead of external convection/radiation BC
Initial conditions



While steady-state solution is independent of the IC, poor IC may cause
divergence due to the number and nonlinearity of the transport equations
Cold flow solution, then gas combustion, then particles, then radiation
For strongly swirling flows, increase the swirl gradually
27
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Combustion Guidelines and Solution Strategies (2)

Underrelaxation Factors

The effect of under-relaxation is highly nonlinear






Once solution is stable, attempt to increase all URFs to as close to defaults as possible
(and at least 0.9 for T, P-1, swirl and species (or mixture fraction statistics))
Discretization



Decrease the diverging residual URF in increments of 0.1
Underrelax density when using the mixture fraction PDF model (0.5)
Underrelax velocity for high bouyancy flows
Underrelax pressure for high speed flows
Start with first order accuracy, then converge with second order to improve accuracy
Second order discretization especially important for tri/tet meshes
Discrete Phase Model - to increase stability,


Increase number of stochastic tracks (or use particle cloud model)
Decrease DPM URF and increase number of gas phase iterations per DPM
28
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Combustion Guidelines and Solution Strategies (3)

Magnussen model

Defaults to finite rate/eddy-dissipation (Arrhenius/Magnussen)





For nonpremixed (diffusion) flames turn off finite rate
Premixed flames require Arrhenius term so that reactants don’t burn
prematurely
May require a high temperature initialization/patch
Use temperature dependent Cp’s to reduce unrealistically high temperatures
Mixture fraction PDF model



Model of choice if underlying assumptions are valid
Use adequate numbers of discrete points in look up tables to ensure
accurate interpolation (no affect on run-time expense)
Use beta PDF shape
29
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Combustion Guidelines and Solution Strategies (4)

Turbulence



Start with standard k- model
Switch to RNG k- , Realizable k- or RSM to obtain better agreement
with data and/or to analyze sensitivity to the turbulence model
Judging Convergence




Residuals should be less than 10-3 except for T, P-1 and species, which
should be less than 10-6
The mass and energy flux reports must balance
Monitor variables of interest (e.g. mean temperature at the outlet)
Ensure contour plots of field variables are smooth, realistic and steady
30
© Fluent Inc. 7/16/2015
Fluent Software Training
MTG-97-183
Concluding Remarks

FLUENT V5 is the code of choice for combustion modeling.
 Outstanding set of physical models

Maximum convenience and ease of use




Built-in database of mechanisms and physical properties
Grid flexibility and solution adaption
A wide range of reacting flow applications can be addressed by the
combustion models in FLUENT.
Make sure the physical models you are using are appropriate for your
application.
31
© Fluent Inc. 7/16/2015