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UNIVERSITY OF CATANIA
Department of Industrial and Mechanical
Engineering
Author: M. ALECCI
Co-Authors: G. CAMMARATA, G. PETRONE
NUMERICAL SIMULATION OF A
“SWIRLING JET” EXPANDING INSIDE
A COMBUSTION REACTOR

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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:
““SSwirlwirl” is defined as the spiral rotational motion imparted to a fluid” is defined as the spiral rotational motion imparted to a fluid
upstream of an orifice. This spiral develops in a direction parallelupstream of an orifice. This spiral develops in a direction parallel
to the injection one.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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CFD SOFTWARE : Comsol Multiphysics

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MODELLING STEPS:
Construction of the
geometrical model
Comsol Multiphysics module
choice and physics settings.
Meshing the model
Plotting e post-processing of the
results.
Problem solving

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EQUATIONS AND MODULE
CHOICE:
FLOW HYPOTHESES :
INCOMPRESSIBLE
(Ma<0.3)
TURBULENT
(Re>2000)
NEWTONIAN FLUID
(homogeneous gases mixture)
( ) ( )T
Fu u p uρ ν ν ρ
 ∇ = −∇ + ∇ + ∇ + 
r r r
g g
0u−∇ =
r
g
( ) i T
ij
j k
u
u k k
x
ν
τ ε ν
σ
  ∂
∇ = − +∇ + ∇  ÷
∂    
r
g g
( ) 2
1 1/ /i T
ij
j
u
u c k c k
x
ε ε
ε
ν
ε ε τ ε ν ε
σ
  ∂
∇ = × − + ∇ + ∇  ÷
∂    
r
g g
Momentum balance
Mass balance
(continuity)
Turbulent Kinetic
energy (K)
equationDissipative turbulent
(e) energy equation
K-e Turbulence module

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PHYSICS SETTINGS:
•Density: 1 kg/m3
•Kinematic viscosity: 1 E-5 m2
/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
RESULTSCross 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
12
tan
3 1
h
x h
R RG
S
G R R R
ϑ
α
 −
=  
−  
;“Swirl number”:
S<0.6S<0.6 WeakWeak
swirlswirl
0.6<S<10.6<S<1 MediumMedium
swirlswirl
S>1S>1 StrongStrong
SwirlSwirl
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. A complete thermal characterization of heat exchanges will
complete the entire model.

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ACNOWLEDGEMENTS:
This work has been developed at theThis work has been developed at the
DepartmentDepartment
of Industrial and Mechanical Engineering ofof Industrial and Mechanical Engineering of
thethe
University of CataniaUniversity of Catania
AUTHOR
REFERENCES:
marco.alecci@libero.it