Ijmet 06 10_001

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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 6, Issue 10, Oct 2015, pp. 01-09, Article ID: IJMET_06_10_001
Available online at
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=6&IType=10
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
NUMERICAL INVESTIGATION OF
TURBULENT FLOW USING RANS
MODELING APPROACH
Imed Miraoui
Aljouf University, Mechanical Engineering Department,
College of Engineering, KSA
Mouna Zaied
University of Gafsa, College of Sciences, Research Unit MEER,
Gafsa, Tunisia
Mouldi Chrigui
Technical University of Darmstadt, Mechanical Engineering Department, Germany
ABSTRACT
In this paper, the air turbulent flow in coaxial jet burner is investigated
numerically using the Reynolds Averaged Navier Stokes (RANS) modeling
approach. 3D computational fluid dynamics code (CFD) has been used and
the gas phase equations are solved using the finite volume method. The
transport equations have been solved for the components of velocity, mass
conservation, turbulent kinetic energy, and dissipation rate of turbulent kinetic
energy. The predicted results have been compared to the experimental
measurements. The results include the profiles of swirl air velocity, axial air
velocity, and turbulent kinetic energy at different axial positions. The
simulation results show good agreement with the experimental data, except at
the side regions in which the air velocity is under estimated.
Key words: RANS, Turbulent flow, Air velocity, Kinetic energy.
Cite this Article: Imed Miraoui, Mouna Zaied and Mouldi Chrigui,
Numerical Investigation of Turbulent Flow Using RANS Modeling Approach,
International Journal of Mechanical Engineering and Technology, 6(10),
2015, pp. 01-09.
http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=6&IType=10
1. INTRODUCTION
Turbulent flow is one of the most complex area of engineering. Most fluid flows are
turbulent in many industrial applications such as gas turbine, combustor, furnaces, and
burner. In order to enhance the efficiency and reduce the harmful exhaust gases,
deeper detailed analysis of involved phenomena is essential.

2. Numerical Investigation of Turbulent Flow Using RANS Modeling Approach
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Turbulent flow can be modeled using several methods such as Reynolds Averaged
Navier Stokes (RANS), Large Eddy Simulation (LES), and Direct Numerical
Simulation (DNS). RANS modeling approach is the most widely used method for
industrial flows, it is reasonably accurate and robust.
Turbulent flow and spray combustion have been investigated numerically and
experimentally by several authors [1-3]. Speziale et al. [4] have studied the turbulent
flow in a circular pipe. They indicated that three dimensional models are needed to
predict the mean swirl velocity.
Ushijima et al. [5] presented a detailed study of non-isothermal coaxial jets with a
second-order closure model. They indicated that the calculated time-averaged velocity
generally agreed with the experimental results.
Chrigui et al. [6] have studied the interaction in spray between evaporating
droplets and turbulence using second order turbulence RANS Model and a
Lagrangian approach.
Ströhera et al. [7] have been applying the realizable k-ε turbulence model for a
free incompressible isothermal turbulent coaxial jet problem. They have found good
agreement between numerical and experimental results.
Xie and Castro [8] have applied Large Eddy Simulation (LES) and RANS to
calculate the turbulent flow over wall-mounted obstacles. Zhou et al. [9] have
calculated the steady free jet flow using the k–ε turbulent model.
Ranga Dinesh et al. [10] have simulated a turbulent swirling coaxial swirl jet with
high inlet axial velocities using LES model. They have indicated that the simulations
produced both instantaneous and time averaged quantities to describe the flow and
mixing fields. Cho and Chung [11] have developed a economical model by
incorporating an intermittency transport equation in an existing k-ε turbulence model.
The objective of the present work is to investigate the turbulent flow in coaxial jet
burner by comparing numerical and experimental results of air axial velocity and air
swirl velocity at different axial positions.
2. EXPERIMENTAL SETUP
In order to compare the numeric results to the experimental measurements, we have
used the configuration investigated by Marchione et al. [12].
Figure 1 shows the geometry of the configuration used in this study. The burner
consists of two concentric circular ducts of length 350 mm, equipped with a conical
bluff body of diameter 25mm. The inner diameter of the outer duct is 35 mm. The
internal diameter of the inner duct is 10 mm. The flame area was enclosed using an
80-mm-long fused silica quartz cylinder of inner diameter 70 mm, which provided
optical access for the imaging and also avoided air entrainment from the
surroundings.
The air entered from the annular open area between the outer duct wall and the
bluff body. Swirl was granted by static swirl vanes at 60◦ with respect to the flow axis
along the flow passage between the inner and outer ducts. The air mass flow rate is
0.42 kg/min.

3. Imed Miraoui, Mouna Zaied and Mouldi Chrigui
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Figure 1 Configuration geometry of the burner
3. MODELLING AND NUMERICAL APPROACHES
The RANS approach is used to model the turbulent fluid phase. The continuity and
momentum equations are given as follow
  0i
i
u
x



(1)
 
  ji i
i j i j
j i j j i
uu up
u u u u
t x x x x x

  
     
                  (2)
Where iu is the mean value of velocity, p is the mean pressure, ρ is the fluid
density, µ is the turbulent viscosity and ''
i juu is the Reynolds Stresses.
In order to close the momentum equations, The Reynolds Stresses must be
modeled. The RANS equations are closed in Standard k-ε Model by assuming that the
turbulent stresses are proportional to the mean velocity gradients and that the constant
of proportionality is the turbulent viscosity (Boussinesq Hypothesis 1977).
2
3
ji
i j ij t
j i
uu
u u k
x x
   
 
         (3)
The turbulent viscosity(µt) is correlated with the turbulent kinetic energy (k) and
the dissipation rate of turbulent kinetic energy (ε), as indicated in the following
relation
2
t
k
C 

 (4)

4. Numerical Investigation of Turbulent Flow Using RANS Modeling Approach
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The turbulent kinetic energy equation can be derived directly from the transport
equations for the velocity fluctuations.
( )j ji
i t t k
i i j i i i Dissipation
Convection DiffusionGeneration
U UUk k
U
x x x x x x
    
      
              
(5)
The exact equation for the dissipation rate is very complex and virtually all
turbulence models that use the dissipation rate use a transport equation that is
“modeled” and has the same types of terms as the TKE transport equation.
2
1 2( )j ji
i t t
i i j i i i
Convection DestructionDiffusionGeneration
U UU
U C C
x k x x x x x k
  
   
    
         
                     
(6)
Where  21,, CandCk are empirical constants given by Launder and
Spalding (1974).
The simulation has been performed using 3D computational fluid dynamics code
(CFD code). The flow is solved by CFD-FASTEST as numerical code using a 3D
axisymmetric block structured grid. The code uses a finite volume method with co-
located cell arrangement. The time integration is achieved explicitly for time
dependent problems while the diffusion terms are discredited with central schemes on
a non orthogonal block structured grid using the standard k-ϵ model.
Figure 2 shows the geometry and the CFD grid used in this study. The
computational grid consists of 1500000 cells.
Figure 2 Geometry and mesh
4. RESULTS AND DISCUSSION
Figure 3 and figure 4 show the contours plot of the mean axial air velocity and the
swirl air velocity, respectively, of cold flow in absence of spray. It was observed that
the highest air velocity is at the sudden expansion due to the jet formed by the
geometry contraction.

5. Imed Miraoui, Mouna Zaied and Mouldi Chrigui
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In contrast, the lowest air velocity is at the side and mainly at the central zone
(near the bluff body) due to the recirculation zone created by the bluff body and the
swirl vanes.
Figure 3 Contours plot of the mean axial air velocity of cold flow in absence of spray
Figure 4 Contours plot of the swirl air velocity of cold flow in absence of spray
The Radial profiles of turbulent kinetic energy of the mean axial and swirl air
velocity, at different axial positions z, are presented in figure 5 and figure 6,
respectively. It can be seen that the highest turbulent kinetic energies are at the radial
position between 0 and about 12.5 mm (the radius of the bluff body). At the zone near
the bluff body, the turbulent kinetic energy is maximal. After that, the turbulent
kinetic energy decreases as the radial position increases due to the recirculation zone
created by the swirl vanes at the side area.

6. Numerical Investigation of Turbulent Flow Using RANS Modeling Approach
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Figure 5 Radial profiles of turbulent kinetic energy of the mean axial air velocity in absence
of spray at different axial positions
Figure 6 Radial profiles of turbulent kinetic energy of the swirl air velocity in absence of
spray at different axial positions
Figure 7 and figure 8 show a comparison between predicted and experimental
measurements of the mean axial and swirl air velocity at different axial positions,
respectively. One observes a plausible agreement between numerical and
experimental results, especially at the regions close to the bluff body (at small axial
positions).
It is also to observe that the highest air velocity is at a radius between about 14
and 20 mm corresponding to the annular inlet of air at the burner face. The lowest air

7. Imed Miraoui, Mouna Zaied and Mouldi Chrigui
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velocity is at a radial position less than 10 mm (at the central zone, near the bluff
body) and more than 20 mm (at the side zone) which corresponding to the
recirculation zone. One remarks an under estimation of the air velocity in the
recirculation zone.
Figure 7 Predicted and experimental radial profiles of the mean axial air velocity at
different axial positions
Figure 8 Predicted and experimental radial profiles of the mean swirl air velocity at
different axial positions

8. Numerical Investigation of Turbulent Flow Using RANS Modeling Approach
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5. CONCLUSION
The turbulent flow in coaxial jet burner was investigated numerically using the
Reynolds Averaged Navier Stokes modeling approach. The standard k–ε turbulent
model was used for the RANS approach. The simulation was implemented using a
three-dimensional CFD code. The numerical results was compared to experimental
measurements.
The results show that the turbulent kinetic energy is maximal near the bluff body
and minimal at the side zone. The lowest axial and swirl air velocity are at the central
zone (near the bluff body) and at the side zone which corresponding to the
recirculation zone. The mean axial and swirl air velocity predicted numerically are in
reasonable agreement with the experimental measurements, especially at the regions
close to the bluff body and to the annular inlet of air at the burner face. In the
recirculation zone, the air velocity is under estimated. It is interesting to promote the
simulation to large eddy simulation (LES).
6. ACKNOWLEDGEMENTS
The authors would like to acknowledge Aljouf University, KSA, for the financial
support of this project. Grant no. 34/244.
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