fluent ansys- advanced combustion systems 1 intro

7/29/2019 fluent Ansys- Advanced Combustion systems 1 Intro

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Fluent Software Training

Combustion Apr 2005

Advanced Combustion

Modelingin FLUENT


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Combustion Apr 2005

Course Agenda8:00- 8:30 Introduction to Combustion Modeling

8:30-10:00 Combustion Models I

10:00-11:00 Hands-on Exercise Session I

11:00-12:00 Combustion Models II

12:00-1:00 Lunch

1:00-1:45 Additional Physical ModelsDiscrete Phase Modeling and Spray Models

1:45-2:30 Additional Physical Models

Radiation Modeling

Pollutant Modeling

2:30-3:00 Combustion Modeling

Case Studies

Strategies

3:00-5:00 Hands-on Exercise Session II


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Combustion Apr 2005

Introduction to Combustion Modeling

Applications of Combustion Modeling

Overview of Capabilities in FLUENT 6

Meshes for Combustion Simulations

Kinetics and Turbulence-Chemistry Interaction

Scaling Analysis


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Combustion Apr 2005

Applications of Combustion Modeling Wide range of homogeneous

and heterogeneous reacting

flows:z Furnaces

z Boilers

z Process heaters

z Gas turbines

z Rocket engines

Predictions of:

z Flow field and mixingcharacteristics

z Temperature field

z Species concentrations

z Particulates and pollutants

Temperature in a Gas Furnace

CO2 Mass Fraction

Stream Function


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Combustion Apr 2005

Overview of Combustion Modeling FLUENT 6provides an extensive array of physical models for combustion

simulations.

Zone-based definition of volumetric and surface reaction mechanisms

z Reactions can be turned off/on in different fluid zones

z Allow different reaction mechanisms in different zones

FLUENT 6provides maximum mesh flexibility, and GAMBIT 2

makes it easy to generate hybrid meshes.

Additional distinctive capabilities include:

z Materials database

z Robust and accurate solver

z Solution-adaptive mesh refinement (conformal and hanging-node)

z Industry-leading parallel performance

z User-friendly GUI, post-processing and reporting

z Highly customizable through user defined functions


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Combustion Apr 2005

Aspects of Reaction Modeling

Dispersed Phase Models

Droplet/particle dynamics

Heterogeneous reaction

Devolatilization

Evaporation Governing Transport

Equations

Mass

Momentum (turbulence)

Energy

Chemical Species

Pollutant Models Radiative Heat

Transfer Models

Reaction Models

Combustion

Premixed, Partially premixed

andNon-premixed

Infinitely Fast Chemistry

Finite Rate Chemistry

Surface Reactions


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Combustion Apr 2005

Reaction Models in Fluent

Laminar Finite-Rate Model

Eddy-Dissipation Concept (EDC) Model

Composition PDF transport Model

Non-Premixed

Laminar Flamelet

ModelFinite-RateChemistry

Eddy Dissipation Model

Partially

Premixed Model

(Reaction Progress

Variable + Mixture

Fraction)

Non-Premixed

Equilibrium

Model

(Mixture Fraction)

Premixed

Combustion

Model

(Reaction Progress

Variable)*

Infinitely Fast

Chemistry

Partially

Premixed

Combustion

Non-Premixed

Combustion

Premixed

Combustion

* Rate classification not truly applicable since species mass fraction is not determined.

Flow config.

Chemistry


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Combustion Apr 2005

Surface Combustion

Discrete phase modelz Turbulent particle dispersion

Stochastic tracking

Particle cloud model

z Pulverized coal and spray models

Radiation models: DTRM, P-1, Rosseland, S2S and Discrete Ordinates

Turbulence models: k-, RNG k-, Realizable k-, , RSM and LES and

DES

Pollutant models: NOx with reburn chemistry and soot

Other Models in FLUENT 6


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Combustion Apr 2005

Meshes for Combustion Simulations For convergence and accuracy, a quality mesh is critical ...

z Low skew (


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Combustion Apr 2005

Complicated Geometry-Tetrahedral Mesh

Burner has several

complicated parts Flow is not aligned

in any particular

direction

High gradients atsonic inlets

Use a tetrahedral

mesh


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Combustion Apr 2005

Complicated Geometry-Tetrahedral Mesh

Tetrahedral mesh

allows for a finemesh on the small

inlet holes with

larger cells in the

furnace domain.


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Combustion Apr 2005

Hybrid Mesh - Boiler

hexes

pyramids

tets

Conical section at bottom

favors a tetrahedral mesh

Heat exchanger plates at top

are suited for a hex mesh

Prisms can be extruded off

the triangular surface at the

corner inlet planes to model

the windbox - get better jet

penetration


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Combustion Apr 2005

Semi-Automatic Hex/Hybrid Meshing

Nuclear Reactor HeadTypical Burner


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Combustion Apr 2005

FLUENT 6: Arbitrary Mesh Interfaces Mesh flexibility,

parts-based meshing

and model building


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Combustion Apr 2005

Mesh Adaption Dynamic hanging node adaption to resolve temperature gradients more accurately.

300 kW BERL Combustor


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Combustion Apr 2005

Gas Phase Combustion Spatio-temporal conservation equations (Navier-Stokes) for

z Mass ()

z Momentum ()z Energy (h)

z Chemical Species (Yk)

The conservation equations have the general form

rate of change convection diffusion source

It is useful to quantify energy in terms of enthalpy, defined as .

chemical thermal

( ) ( ) +

=

+

Sx

Dx

uxt ii

i

i

+=species

T

T

pk

o

kko

)dTch(Yh


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Combustion Apr 2005

Chemical Kinetics The kth species mass fraction transport equation is:

Nomenclature: chemical species, denoted Sk, react as:

Example:

( ) ( ) ki

kk

i

ki

i

k RxYD

xYu

xY

t+

=

+

==

N

1k

kk

N

1k

kk S"S'

OH2COO2CH 2224 ++

2"1"0"0"

0'0'2'1'

OHSCOSOSCHS

4321

4321

24232241

====

====

====


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Combustion Apr 2005

Chemical Kinetics The calculated reaction rate is proportional to the products of the

reactant concentrations raised to the power of their respective

stoichiometric coefficients.

kth species reaction rate (for a single reaction):

where A = pre-exponential factor

Cj = molar concentration =Yj/ MjMk = molecular weight of species k

E= activation energy

R = universal gas constant = 8313 J / kgmol K

= temperature exponent

Note that for global reactions, , and may be noninteger

=

=

N

1j

'

jRT

E

kkkk

*kCeAT)'"(MR

k*k ''


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Combustion Apr 2005

Flames Length

scale (m)

Velocity

scale (m/s)

Reynolds

number

Gas turbine combustor 0.1 50 250,000

Fire 5 2 500,000

After-burner 0.5 100 2,500,000

Utility Furnace 10 10 5,000,000

Practical Combustion Processes are Turbulent

Smallest length scale in turbulent flow (called the Kolmogorov scale)

L / Re3/4

, whereL is the combustor characteristic dimension

Number of grid points required for Direct Numerical Simulation (DNS)(resolving all flow scales) ~ (L/)

3= Re

9/4

Example:Re ~ 10 4, number of grid points ~ 10 9

DNS is computationally intractable, and will remain so indefinitely


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Combustion Apr 2005

Necessity for Combustion Modeling Governing reacting Navier-Stokes equations are accurate,

but DNS is prohibitive ...

Turbulence

z Large range of time and length scales

z Model by time (Reynolds) averaging

Imagine a long exposure photograph of the

visualized flow

Introduces terms (the Reynolds stresses) which must be modeled

Chemistryz Realistic chemical mechanisms have tens of species, hundreds of

reactions, and stiff kinetics (widely disparate time scales)

Determined for a limited number of fuels


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Combustion Apr 2005

Reynolds (Time) Averaged Species Equation

{unsteady term} convection convection molecular mean

(zero for by mean by turbulent diffusion chemical

steady flows) velocity velocity fluctuations source term

are the kth species mass fraction, diffusion coefficient and

chemical source term respectively

Turbulent flux term modeled by mean gradient diffusion as,, which is consistent in the k- context

Gas phase combustion modeling focuses on

z Arguably more difficult to model than the Reynolds stresses (turbulence)

( ) ( ) ( ) ki

kk

i

ki

i

ki

i

k Rx

YD

x"Y"u

xYu

xY

t+

=

+

+

kkk R,D,Y

kR

ikttki x/Y/ScY"u" =


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Combustion Apr 2005

Turbulence Chemistry Coupling in Flames

( )RTEexpCATRj

jkj =

Arrhenius reaction rate terms are highly nonlinear

Cannot neglect the effects of turbulence fluctuations on chemical

production rates

)T(RR kk


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Combustion Apr 2005

Turbulence-Chemistry InteractionDemonstration: single step methane reaction (A=2*1011, E=2*108)

Assume turbulent fluid at a point has constant species concentration at all

times, but spends one third its time at T=300K, T=1000K and T=1700K

3.0

2

2.0

4OH21

COO21

CH

2224

]O[]CH[)RT/Eexp(ARRRR

OH2COO2CH

2224====

++

t

T1700

1000

300

time trace

T

P(T)

PDF

300 1000 1700

T [K] 300 1000 1700

R [kgm-3s-1] 10-25 1 105 134

13

smkg103R

smkg1)T(R

=

=


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Combustion Apr 2005

Modeling Chemical Kinetics in Combustion

Practical Approaches:

Reduced chemical mechanismsz Finite rate/Eddy Dissipation model

Decouple chemistry from turbulent flow and mixing

z Mixture fraction approaches Equilibrium chemistry PDF model

Laminar flamelet model

z Progress variable

Zimont model

z Mixture fraction and progress variable

Partially premixed combustion model


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Combustion Apr 2005

Scaling Analysis

forceviscous

forceinertial~

ULRe

=

, U, L, are characteristic (e.g. inlet) density, velocity, length and

dynamic viscosity, respectively

Turbulence models valid at highRe

scaletimechemical

scaletimemixing~

R/

/k~

R/

U/LDa

slowadslowad

=

adiabatic flame density

slowest reaction rate at and stoichiometric concentrations

ad

adTslowR

Gas phase turbulent combustion models valid at highDa

Reynolds number

Damkohler Number


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Combustion Apr 2005

Mach number

Boltzman number

speedacoustic

speedconvection~

c

UMa =

fluxheatradiation

fluxheatconvection~

T

)TUc(Bo

4

ad

inletp

=

Stefan-Boltzman constant (5.672 10-8 W/m2K4)

Radiation important atBo < 10

Mixture fraction model valid atMa < 0.3 (incompressible)

(assumes convection overwhelms conduction)

Scaling Analysis

fluent ansys- advanced combustion systems 1 intro

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