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:
FEMLAB Conference Stockholm 2005
The
anterior side is characterized by the following devices:
Holes for the fuel injection
Duct for the flame
revelation probe
Axial swirler
FEMLAB Conference Stockholm 2005
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
FEMLAB Conference Stockholm 2005
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
FEMLAB Conference Stockholm 2005
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
FEMLAB Conference Stockholm 2005
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.
FEMLAB Conference Stockholm 2005
Longitudinal section:
When the fluid enters the reactor, it expands with the classical
cone course, up to velocity of 1

2 m/s.
FEMLAB Conference Stockholm 2005
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
FEMLAB Conference Stockholm 2005
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)
FEMLAB Conference Stockholm 2005
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.
FEMLAB Conference Stockholm 2005
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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