ANALYSIS AND MODELLING OF A

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FEMLAB Conference Stockholm 2005




UNIVERSITY OF CATANIA



Department of Industrial and Mechanical Engineering






Authors
:
M.

ALECCI, G. CAMMARATA,
G. PETRONE

ANALYSIS AND MODELLING OF A
LOW NOx SWIRL BURNER

FEMLAB Conference Stockholm 2005

PROBLEM FACED

:

CFD

COMPUTATIONAL FLUID DYNAMIC

ADVANTAGES:


Reduction of planning
time and costs.


Availability to study
systems for which the
experimentation is difficult
and expensive.


Availability to study
systems

in conditions of
extreme safety .

DISADVANTAGES:


Discretized models
present inevitable PDE
approximation .


In the linear systems
solution iterative methods
are used. These allow to
obtain only solutions
close to the exact ones.

FEMLAB Conference Stockholm 2005

OBJECTIVES OF THE STUDY:


FEM modelling of the “cold” fluid
-
dynamics

of a swirl burner.



Evaluation and analysis of the velocity

and pressure fields.

Comparison of the obtained


results with those coming from literature.

FEMLAB Conference Stockholm 2005

SWIRL EFFECT:

“S
wirl
” is defined as the spiral rotational motion imparted to a fluid

upstream of an orifice. This spiral develops in a direction parallel

to the injection one.



Then, a tangential velocity component and high


pressure gradients (axial and radial) develop.

The low pressure zone inside the spiral core is

characterized by toroidal vortexes:

(Precessing Vortex Core phenomenon PVC)

This results (for strong degree of swirl) in the setting up of a


Reverse Flow Zone (RFZ)

where the fluid is recirculated towards the burner’s outlet.


1) Good mixing of reactants.

2) A decrease

in

flame temperature.

3) Flame stabilization.

4) High performance combustion for

several carboneous materials.



NOx


REDUCTION



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THE SWIRL BURNER:


The modelled burner is used in several industrial applications:

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The

anterior side is characterized by the following devices:

Holes for the fuel injection

Duct for the flame


revelation probe

Axial swirler

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MODELLING STEPS:

Construction of the


geometrical model

Femlab module choice and

physics settings.

Meshing the model

Plotting e post
-
processing of the

results.

Problem solving

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GEOMETRICAL MODEL

The swirler has been realized by a CAD


software, due to its complex shape,

and further imported into

the Femlab drawing grid.

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EQUATIONS AND MODULE CHOICE:

FLOW HYPOTHESES :

INCOMPRESSIBLE

(Ma<0.3)

TURBULENT

(Re>2000)

NEWTONIAN FLUID

(homogeneous gases mixture)





T
F
u u p u
  

 
      
 
0
u
 


i
T
ij
j k
u
u k k
x

  

 
 

     
 
 

 
 
 


2
1 1
//
i
T
ij
j
u
u c k c k
x
 


     

 
 

      
 
 

 
 
 
Momentum balance

Mass balance (continuity)

Turbulent Kinetic energy (K)

equation

Dissipative turbulent (
e
)
energy equation

K
-
e

Turbulence module

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PHYSICS SETTINGS:


Density: 1 kg/m
3


Kinematic viscosity: 1 E
-
5

m
2
/s


Volume forces neglected

Inlet flow with axial velocity:
u=20 m/s.

No slip conditions:
U
=0.

Pressure: p=3 bar

SUBDOMAIN

SETTINGS:

BOUNDARY

CONDITIONS:

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COMPUTATIONAL GRID AND USED SOLVER


Used solver:

DIRECT (UMFPACK), NON LINEAR

Finer mesh close to

the swirler zone

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PLOTTING E POST
-
PROCESSING OF THE RESULTS

Cross sections: velocity field

It is possible to observe how in the first duct the fluid accelerates when

it goes through the swirler.

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Longitudinal section:

When the fluid enters the reactor, it expands with the classical

cone course, up to velocity of 1
-
2 m/s.

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Streamlines of the fluid:

Spiral motion inside the “core”, typical of

“swirling jets”.

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“SWIRL NUMBER” AND LITERATURE RESULTS





3
2
1
2
tan
3
1
h
x
h
R R
G
S
G R
R R


 


 

 
 
“Swirl number”:

S<0.6

Weak swirl

0.6<S<1
Medium swirl

S>1
Strong Swirl

LDV

(Laser Doppler

Velocimetry)

Swirl number of the analyzed

system:

S=0.77

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Radial distribution of the axial velocity close to the
burner’s outlet:

The negative values correspond to the RFZ development

according to the literature results.

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Iso
-
surfaces of axial velocity:

The bulb, located in the central core, corresponds to negative


values of axial velocity. That means the fluid is recirculated


towards the burner outlet section. (RFZ development)


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Radial distribution of the axial velocity close to the
burner’s outlet and 10 cm and 20 cm from it:


RFZ results stronger close to the burner’s outlet and it decreases as soon as

the fluid reaches a certain distance from it.



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CONCLUSIONS AND FURTHER
DEVELOPMENTS:

1.
A

three
-
dimensional

simulation

of

a

low

NOx

“swirl

burner”

is

reported

in

this

study
.

The

analysis

has

been

focused

on

the

swirl

device

by

the

evaluation

of

the

velocity

and

pressure

fields

of

the

jet

entering

the

combustion

reactor
.

2.
The

model

reflects,

with

good

approximation,

the

real

behaviour

of

the

system,

and

finds

a

good

correspondence

with

literature
.

Thus,

it

may

be

used

to

simulate

different

operative

conditions

(such

as

other

fluids

or

other

inlet

velocities),

avoiding

expensive

experimentation
.

3.
In

a

further

development

the

combustion

reaction

will

be

introduced

into

the

model,

analyzing

how

it

may

influence

the

velocity

and

pressure

fields
.


4.
The

thermal

characterization

of

heat

exchanges

will

complete

the

entire

model
.

FEMLAB Conference Stockholm 2005

ACNOWLEDGEMENTS:

This work has been developed at the Department



of Industrial and Mechanical Engineering of the



University of Catania with the precious collaboration of


ITEA S.p.A, SOFINTER Group




www.iteaspa.com

AUTHORS’ REFERENCES:

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