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.
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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.
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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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