Faculty of Mechanical Engineering

Institute for power, process and environmental engineering

Laboratory for combustion and environmental enrgineering

Combustion simulation in

Combustion simulation in

laboratory scale starved air

laboratory scale starved air

incinerator

incinerator

F F.. K Ko ok ka alljj

Vransko, September 2007Outline

• Governing equations of turbulent reacting flow

• The closure problem of turbulent combustion

• Turbulent combustion models

• Practical case of turbulent combustion simulation

applying CFX programme in second stage of pilot

scale starved air incinerator

• Numerical control of “3T” combustion conditions

(Temperature, Turbulence and Time)

• ConclusionsSumary of RANS equations for reacting flow

Continuity:

∂ρ ∂

+ (ρυ )= 0

j

∂t ∂x

j

Momentum:

∂ ∂ ∂p ∂

′ ′

(ρυ )+ (ρυυ )=− + f − (τ +ρυυ )

j j i υi ij j i

∂t ∂x ∂x ∂x

j i j

Bussinesq approximation of Reynolds streeeses

∂υ

2 ∂υ ∂υ

j

k i

′ ′

ρυ h = δ ρk+η −η +

j ij t t

3 ∂x ∂x ∂x

k j i

where

2

k

η = ρC

t η

εSumary of RANS equations for reacting flow

Transport equations for k andε :

∂υ

∂ ∂ ∂ η ∂k ∂υ ∂υ

j

t i i

( )

(ρk)+ υ k − η+ =η + −ρε

j t

σ

∂t ∂x ∂x ∂x ∂x ∂x ∂x

j j k j j i j

2

∂υ

∂ ∂ ∂ η ∂ε ε ∂υ ∂υ ε

j

t i i

( )

(ρε)+ υ ε − η+ = C η + −C ρ

j 1 t 2

σ

∂t ∂x ∂x ∂x k ∂x ∂x ∂x k

j j ε j j i j

Energy:

∂ ∂ ∂p ∂

′ ′

(ρh)+ (ρυ h)− + (q +ρυ h)=I

j j j T

∂t ∂x ∂t ∂x

j j

Turbulent heat flux:

η ∂T

t

′ ′

ρυ h =− c

j p

Pr ∂x

t jSumary of RANS equations for reacting flow

Mass species transport equation:

∂ ∂ ∂ ∂ξ

k

(ρξ )+ (ρυ ξ )− Γ =I

k j k k ,eff ξ

k

∂t ∂x ∂x ∂x

j j j

Effective diffusivity:

η η

t t

Γ =ρD + =Γ +

k,eff km k

Sc Sc

t t

Equation of state:

N

ξ

k

p= ρR T

m ∑

M

k=1

kSource terms

• Source term for energy equation:

N

0

I =− ΔH ω

T ∑ f ,k k

k=1

• Sorce term for mass species transport equation:

I =M ω

ξ k k

k

• Formation/consumption rate of reacting flow components

Combustion

d[X ]

k

′′ ′

ω = =(ν −ν )R

k k k k

modells

dt

• General form of chemical reaction

N N

k

f

→

ν X ν X

′ ′′

∑ ∑

k k ← k k

k

b

k=1 k=1Closure problem of turbulent

combustion

( )

R ≠ R T ,ξ k = 1,...,N

k k k

Mean reaction rate term is not equal to the term based on

mean temperature and mean species concentration.

R

k

TTurbulent combustion models

• Kinetic rates by modified Arrhenius expressions or Finite Chemical

Reaction rates – FCR

• Eddy Break-up modell – EBUM

• Eddy Dissipation Combustion Modell – EDCM

• PDF Mixture Fraction based models

• Multi step reaction models based on EDCM or EBUM enables the

calculation of PIC (CO, H , NO, ...) applying additional reactions

2

• Combined models (ie. EDCM/FCR), etc. Finite Chemical Reaction rates – FCR

N N

′ ′′

ν ν

k k

R =k [X ] −k [X ]

k f∏ k b∏ k

k=1. k=1

Modified Arrhenius equation:

′

k =k +k

f ,b f ,b f ,b

2

′

E E T

a a

k =k (T) 1+ −1 + L

f ,b f ,b

2

2R T R T T

m m

2

2 2 3

′ ′ ′ ′

E T E E T −T E E T

a a a a a

′

k = k (T) + −1 + −1 +L .

f ,b f ,b

2 3

R T T 2R T R T T 6R T R T T

m m m m m

depends only on temperature and activation energy.Eddy Break-up Modell – EBUM, 1

Chemical reaction rate prpotional to mixing frequency

1

R ≈

k

t

t

Mixing is controled by turbulence

t

t

1 ε

=

t k

t

Eddy life time, or time for an eddy to break-up

Reaction rate proportional to fluctuation of species

2

′

R ≈ξ

k kEddy Break-up Modell – EBUM, 2

Final form of EBUM reaction rate

ε

2

′

R =−C ρ ξ

k EBU k

k

C is an empirical constant which has to be fitted to the experimental data

EBU

• Model for fast chemistry

• Derived for single step chemistry (Fuel + oxidizer → Products)

• Provides reasonable results for heat release and main species

• Simplified chemistry required

• Empirical constant changes from case to caseEddy Dissipation Combustion Modell - EDCM

Chemical reaction rate proportional to minimum

concentration of participating species

[X ]

k

R ≈ min

k

ν '

k

Different expressions are used for reaction rate of reactants

(fuel and oxidizer) consumptionn

ε [X ]

k

R = A min

k

k ν '

k

and products formation

[X ]M

∑ k k

ε

P

R = AB

k

k ν '' M

∑

k k

P Eddy Dissipation Combustion Modell – EDCM

• Model for fast chemistry.

• Derived for single step chemistry (Fuel + oxidizer → Products).

• Can be extended to multi-step chemistry either 2 or 5 steps

enabling, the claculation of some PIC’s.

• Simplified chemisty required.

• Two constants which must be fitted by experimental data from case

to case.

• Applicable to non-premixed, partially premixed and premixed

combustion.

• Not very sensitive for non-premixed combustion.

• Provides reasonable results for heat release and main species.

• Can be linked to multiphase reacting flows.Practical case of combustion simulation in pilot scale

starved air incinerator applying CFX 5

• (CH + CO)/air diffusion flame

4

• 3-D bounded geometry

• Reduced number of species considering single step

reaction (Fuel + oxidizer → Products).

• Different combustion models offered by CFX 5 have

been tested to find the most suitable one for our case

– MIB (involved just in CFX 4)

– EBUM

– EDCM (1,2,5 step)

– Combined model EDCM/FCR (1,2,5 step)

• Numerical control of “3T” combustion parameters

applying EDCMPilot scale two stage starved air icinerator

750

1500Secondary chamber -

thermoreactorBoundary conditions

Parameters that were changed:

Volatile gases from

• secondary and tertiary air

primary chamber:

amount and their inlet direction

- amount

- composition

Secondary air inlets:

• speed and composition of

- temperature

- amount

combustible gases

- direction

- temperature

Parameters that were not changed

but may influence the combustion:

Tertiary air inlets:

- amount

• geometry

- direction

- temperature

• capacity

Wall: dQ/dt = 0

• additional air inlets

• additional fuel

Combustion

• insulation

products outlet

• feedingThermoreactor discretization

• Main geometry: 2.4 m x 0.75 m

• Inlets and outlet

• Computational grid with 275.674 control

volumesAverage velocity comparison Average temperature comparison

2100

EBUM

MIB

EDCM (1 step)

1900

EDCM (2 step)

EDCM (5 step)

1700

EDCM/FCR (1 step)

EDCM/FCR (2 step)

EDCM/FCR (5 step)

1500

X

X

1300

X

X

1100

900

0 -0,5 -1 -1,5 -2 -2,5

Longitudinal axle of thermoreactor [m]

Temperature [K]Experimental and numerical data comparison-EDCM

1280 K 1360 K

measured

temperature

in secondary

1230 K 1310 K

chamberAverage fuel mass fraction comparison

0,6

EBUM

MIB

EDCM (1 step)

0,5

EDCM (2 step)

EDCM (5 step)

0,4

EDCM/FCR (1 step)

EDCM/FCR (2 step)

EDCM/FCR (5 step)

0,3

0,2

0,1

0

0 -0,5 -1 -1,5 -2 -2,5

Longitudinal axle of thermoreactor [m]

Fuel mass fraction [CH4]Carbon monoxide mass fraction comparison

0,02

EDCM (2 step)

EDCM (5 step)

0,018

EDCM/FCR (2 step)

EDCM/FCR (5 step)

0,016

0,014

0,012

0,01

0,008

0,006

0,004

0,002

0

0 -0,5 -1 -1,5 -2 -2,5

Longitudinal axle of thermoreactor [m]

Carbon monoxide mass fraction [CO](“3T”) combustion conditions control

(using EDCM)

• Temperature: set by legislation, influences on

the speed of destruction of organic pollutants,

depends of calorific value of gas

• Residence time: set by legislation, influences

on the speed of destruction of organic pollutants

together with temperature, depends on amount

of gases (at the same geometry)

• Turbulence: ensures proper mixing of volatile

gases with air throughout the whole

thermoreactorInfluence of temperature

on combustion quality

Calorific value - temperature of combustion:

35 % CH 55 % CH 75 % CH

4 4 4Influence of residence time

on combustion quality - 1

Amount of volatile gases - residence time:

v =0,25 m/s v =0,38 m/s v =0,5 m/s

1 1 1Influence of residence time

on combustion quality - 2

Amount of volatile gases - residence time:

v =0,25 m/s v =0,38 m/s v =0,5 m/s

1 1 1Influence of turbulent mixing

on combustion quality

Mixing - turbulence:

secondary air tertiary air

• velocity: projected 6 m/s projected 4 m/s

4 m/s 6 m/s

4 m/s 8 m/s

• direction: projected projected

o o

projected + 10 projected + 10

o o

projected + 15 projected + 15o o

Projected direction Projected + 10 Projected + 15

Projected

velocity:

s: 6 m/s

t: 4 m/s

Velocity

Velocity

Changed

velocity:

s: 4 m/s

t: 6 m/s

field

field

Changed

velocity:

s: 4 m/s

t: 8 m/s

.o o

Projected direction Projected + 10 Projected + 15

Projected

velocity:

s: 6 m/s

t: 4 m/s

Temperature

Temperature

Changed

velocity:

s: 4 m/s

t: 6 m/s

field

field

Changed

velocity:

s: 4 m/s

t: 8 m/s

.Conclusion

• EDCM and EBUM are well established and simple to

implement

• Single step combustion models (EDCM, EBUM) are

convenient just for combustion macro-parameters

calculation (temperature, velocity and combustion products

field)

• Require empirical input which has to be fitted from case to

case

• They can be used for fast reactions only

• For slow chemistry reaction rates are not mixing controled

• For some PIC’s concentration calculation (i.e. CO, NO, etc.)

multi step combustion models should be applied but there

are still limitations for other minor species (OH, HC, soot,...)

or complex chemical processes

• It is possible to improve EDCM extending it by chemical

kinetics

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