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Investigation on CFD Analysis Practices for Predicting Flow in Centrifugal Fan
Using Steady State and Transient Analysis
Conference Paper · December 2018
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Proceedings of the 7th International and 45th National Conference on Fluid Mechanics and Fluid Power (FMFP)
December 10-12, 2018, IIT Bombay, Mumbai, India
FMFP2018 PAPER NO. -127
Investigation on CFD Analysis Practices for Predicting Flow in Centrifugal Fan Using Steady State
and Transient Analysis
Yogesh Chande
John Deere India Pvt Ltd.
Pune, India.
Email: chande.yogesh.n@gmail.com
Abstract
The rotating systems like fans, turbines, blowers etc. can
be modelled in CFD using either less accurate steady-state
assumption which is computationally cheaper or using
transient analysis which is computationally expensive but
generally more accurate. In this paper, the centrifugal fan
is considered for study and the difference between steady
and transient computational analysis is reported. In this
study, the effects of parameters like the number of blades,
blade type, fan speed, and blade positions were considered.
The CFD results suggest that steady-state analysis is less
accurate for less fan blade compared to more number of
blades. Steady-state CFD results are closer to transient
when the blades are flat as compared to curved fan blades.
The study also reports a high dependency of blade position
in steady CFD, whereas transient analysis is independent
of blade position. The present study, therefore, compares
the steady and transient analysis under various parameters.
In this paper, the study is done to simulate the centrifugal
fan and flow parameters like the mass flow rate and
pressure are reported using two different CFD techniques
i.e. Steady-steady and transient analysis. Various
geometric parameters considered in the study, the details
are given in Table 1.
Transient CFD simulation is performed using the Moving
Mesh method where the mesh essentially rotates at every
time step resulting in replication of actual flow condition.
Steady-state simulation is performed in CFD using Moving
Reference Frame (MRF) technique, where the fan remains
still but the effect of fan rotation is provided by calculating
the impact of blades to the fluid in the vicinity, the frame
of reference rotates giving rise to an effect of fan flow
rather than actual fan rotation. The MRF technique is
computationally faster, however, it may not be highly
accurate [6], over a wide range of circumstances.
The available literature suggests no clear guideline for
selection of the type of simulation for a trade-off between
simulation time and computational accuracy. The current
study, therefore, is to identify the usage of two mentioned
CFD methods for various fan parameters. The results
suggest the situations where the precautions need to be
taken while deciding the analysis technique for simulating
centrifugal fan.
II. METHODOLOGY
Commercial CFD code Star-CCM+ is used for simulating
centrifugal fan flow. For modelling steady-state in CFD the
Moving Reference Frame (MRF) is used while for
modelling transient phenomenon in CFD, the Moving
Mesh simulation is used. In both the approaches, SST kturbulence model is used since it is more accurate
compared to kworking fluid with a constant density of 1.18 kg/m3. Flow
is assumed to be isothermal and therefore variation in
temperature is not considered.
A. Details of the fan geometry and CFD domain
Generic centrifugal fan considered for study with fan blade
varying from 6 to 30. Fan blades are varied in large range
to cover all the applications of centrifugal fans. Analysis of
Keywords: Centrifugal fan; CFD, Fan blade; Steady state
analysis; Transient analysis; Moving Mesh; Moving
Reference Frame.
I.INTRODUCTION
Computational Fluid Dynamics (CFD) has been widely
used for simulating fans, blowers, turbines etc. [1-4]. For
higher accuracy in CFD, more accurate physics like
transient analysis is required, which leads to high
computational time and cost [6]. The physics with faster
computational time like steady-state CFD may not be
highly accurate [7]. The challenge, therefore, is to get the
balance between computational time and accuracy desired.
The literature review suggests that the studies have been
conducted to understand the impact of various parameters
but the focus was on fan performance [1-6] and little focus
is given to the detailed study of differences in the
techniques being used which is transient vs steady state
CFD analysis for predicting fan flow under various
parametric changes.
1
fewer fan blades become particularly important in CFD
since, with less fan blade, the blade position may have
significant impact on flow parameters. The fan blade
number is, therefore, one of the factors in determining
which CFD analysis should be used. Ambient pressure of
101.3 kPa is considered.
Flat and curved are the two different blade types
investigated. Three different fan speeds of a large range are
chosen for study i.e. 800 rpm, 1600 rpm and 2400 rpm to
cover the extremes in fan flow behaviour.
(a)
Where, is density and
is dynamic viscosity, and
the external force,
is the velocity vector and g the
acceleration due to gravity. The equations contain more
unknown variables compared with the number of equations
to solve and therefore is not closed. For closure of the
above equations the Boussinesq hypothesis for Reynolds
stress in terms of average velocity is given by equation (3)
as below,
(3)
Where i and j indicates vectors along X and Y direction
is the eddy viscosity and k is the turbulent
respectively,
kinetic energy.
The energy equation is not solved due to the assumption
of isothermal flow and change of temperature is not
considered in the study.
(b)
Validation of results
The CFD code for rotating systems has been validated by
many [7-8], thus in the present study the validation of the
CFD code for the turbulence model is not done explicitly.
The results obtained from transient simulation is
considered as more accurate compared to steady-state
simulation [3], it is therefore assumed as the reference
while comparing results from steady-state analysis.
(c)
Figure 1: (a) Fan geometry with duct and fan housing,
(b) Straight fan blades and (c) Curved blade fan
Due to numerical and geometric symmetry, only half
model of fan is considered assuming a symmetry boundary
condition at the plane of symmetry to save computational
time. Table 1 indicates the details of variation in the
parameters undertaken for the present study.
III. RESULTS AND DISCUSSION
The mesh independent study is carried out to make the
results free from numerical error arising due to the mesh
count.
Table 1: Parameters considered in present study
Parameter
Value
Number of fan Blades
6 to 30
Normalized Mass Flow
Fan Blade type
Fan Speeds
Blade angle
Fan diameter
Fan blade width
Fan Inlet diameter
Fan Outlet duct size
Number of blade position (for
steady state analysis-MRF)
1.00
Flat and Curved
800 - 2400 rpm
90o
200 mm
100 mm
180 mm
108x108 mm
4
0.98
0.96
0.94
0.92
0.90
0
1
2
3
4
5
Mesh Count in Millions
6
7
Figure 2: Mesh independent study for CFD analysis.
Mesh count around 2 million gives reasonable accuracy
it is, therefore, selected in the presented study.
B. Numerical details and Governing equations
The governing equations are given in this section, the flow
assumption is incompressible the continuity given by
equation (1) and momentum equation given by equation
(2), Reynolds averaged Navier-stokes equations (RANS)
in its absolute form is written as,
(1)
Figure 3: Mesh used for CFD analysis.
In the transient CFD analysis, the results may fluctuate
initially. Thus, the fan is allowed to rotate for sufficient
time until the flow at the exit is stable. The variation of
pressure with time is shown in Figure 4.
(2)
2
B. Impact of Fan blade type, Flat vs Curved
The transient CFD results are considered as reference and
results from curved and flat blade steady-state CFD
analysis is plotted as shown.
300
250
Pressure Pa
200
Stable Flow
150
100
Normalized mass flow rate
50
0
0
0.2
0.4
0.6
Normalized Time
1
0.8
Figure 4: Time evolution of Pressure in transient
analysis at fan outlet.
1
(a)
Transient
0.98
Flat Steady
0.96
Curved
Steady
0.94
0.92
0.9
0
5
10
15
20
25
Number of blades
30
40
35
Normalized pressure
1
Figure 5: CFD results for transient and steady
0.8
(b)
0.6
0.4
Transient Pressure (normalized)
Flat Steady
Curved Steady
0.2
0
0
Normalized flow rate
A. Impact of Number of blades
The number of blades varied from 6 to 30, the difference
between transient and steady-state CFD are plotted by
normalizing the graph for transient CFD data.
(a)
0.96
Steady
Transient
0.94
0.92
0.9
5
10
15
20
Number of blades
25
30
35
15
20
Number of blades
25
30
35
1
Normalized Mass flow
0
10
Figure 7: Impact of fan blade type on mass flow (a) and
pressure (b).
For the curved fan blade, the mass flow prediction using
steady-state analysis is not close to transient. Steady-state
CFD predicts the flow better with a flat blade.
The difference in the transient and steady-state analysis
for pressure prediction is more with the curved blade as
compared to the flat blade.
C. Impact of Fan Speed
Fan speed changed in steady-state and transient analysis
and mass flow rate along with pressure is predicted.
1
0.98
5
(b)
(a)
0.8
0.6
Steady
0.4
Transient
0.2
0
0
400
800
1200
1600
2000
2400
2800
Fan Speed - rpm
1
Normalized Pressure
Figure 6: Impact of number of blades on mass flow (a)
and pressure (b).
The results suggest that the difference between
transient analysis and steady-state CFD is more when the
number of fan blades are less. With less fan blades the
blade position becomes critical and it impacts the flow
prediction using the steady-state analysis.
The steady-state CFD analysis predicts the total
pressure more accurately as compared to static pressure.
For both the pressures, as the number of blades increased
the prediction becomes gets closer to transient analysis.
0.8
(b)
0.6
Steady
Transient
0.4
0.2
0
0
400
800
1200
1600
Fan Speed - rpm
2000
2400
2800
Figure 8: Impact of fan speed on mass flow (a) and
pressure (b).
3
Steady-state analysis prediction is better with low fan
speed as compared with high fan speed. With the high fan
speed using steady-state analysis, the deviation of flow
prediction is more.
large angle between two consecutive blades. The high
number of blades causes the smaller angle between them,
this results into the less dependence since the blade
position can repeat itself after a very small angle.
D. Impact of Fan blade position
In the steady-state CFD, the fan does not essentially
rotate, rather the effect of fan is simulated for a fixed blade
position. The results, therefore, are position dependent.
IV.
CONCLUSIONS
CFD results suggest that steady-state results for fewer fan
blades deviate more with transient as compared more fan
blades.
Flat fan blades results using steady-state analysis are
closer to transient when compared with the curved fan
blades.
For low fan speed, the prediction is better with steady as
compared with high fan speed.
The number of blades in fan varied from 6 to 30 suggests
that for steady-state analysis (MRF) less fan blade is highly
depended on its position whereas transient analysis
(Moving Mesh) simulation is independent of initial blade
position.
REFERENCES
[1] W.K. Ng and M. Damodaran, Computational Flow
Modeling For Optimizing Industrial Fan Performance
Characteristics,
European
Conference
on
Computational Fluid Dynamics, The Netherlands, 2006
[2] R. Elder, A. Tourlidakis and M. Yates, Advances of
CFD in Fluid Machinery Design , Wiley & Sons, 2003.
[3] Zhang De-s, Shi W-d, Bin C. and Guan X-f., Unsteady
flow analysis and experimental investigation of axialflow pump, Journal of Hydrodynamics, Ser. B, Vol. 22,
No. 1, 35-43, 2010.
[4] Pathak, Y.R., Baloni, B.D. and Channiwala, D.S.,
Numerical simulation of centrifugal blower using CFX,
International Journal of Electronics, Communication &
Soft Computing Science and Engineering, 242-247,
2012.
[5] Fan-nian Menga, Liang-wen Wangb, Gui-zhong Xiec,
Feng Zhaod, De-hai Zhange And Wen-liao Duf Effects
of Blade Inlet Angle on Flow Field of Centrifugal Fan,
International Conference on Mechanical Engineering
and Control Automation (ICMECA), 2017.
[6] Eric Lofgren Accuracy of transient versus steady state
forces on a rudder operating in a propeller slipstream,
(b)
(a)
Figure 9: Picture showing difference between two fan
blade positions.
Four different fan blade positions are analysed using
steady-state simulation and its relative results are plotted
against the transient simulation data. The data from
transient simulation data is considered as the reference and
the results were analysed by normalizing.
Normalized flow rate
1
(a)
0.95
Transient
Steady Position 1
Steady Position 2
Steady Position 3
Steady Position 4
0.9
0.85
0.8
0
Normalized pressure
1
5
10
15
20
Number of blades
25
30
35
(b)
0.8
0.6
Transient
Steady Position 1
0.4
Steady Position 2
Steady Position 3
0.2
Steady Position 4
Department of Physics, Umeå University, June 2015.
[7] N. Rajabi, R. Rafee, S. Frazam-Alipour, Effect of Blade
Design Parameters on Air Flow through an Axial Fan,
International Journal of Engineering, Vol. 30, No. 10 ,
1583-1591, October 2017.
[8] Javad Alinejad and Farhad Hosseinnejad, Aerodynamic
Optimization In the Rotor of Centrifugal Fan Using
Combined Laser Doppler Anemometry and CFD
Modeling, World Applied Sciences Journal 17 (10):
1316-1323, 2012.
0
0
5
10
15
20
Number of blades
25
30
35
Figure 10: Variation of results by varying position of
fan blade position, impact on mass flow (a) and
pressure (b).
The blade position plays a critical role in the steady-state
analysis for determining the flow parameters like pressure
and mass flow rate. The dependence of flow parameters on
blade position is higher for less number of blades. The
difference is more widespread in the pressure prediction.
The reason for more variation with less blade is due to the
4
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