Open Journal of Fluid Dynamics, 2013, 3, 75-80
http://dx.doi.org/10.4236/ojfd.2013.32A012 Published Online July 2013 (http://www.scirp.org/journal/ojfd)
Numerical Study on Internal Flow of Small Axial Flow
Fan with Splitter Blades
Lifu Zhu, Yingzi Jin*, Yuzhen Jin, Yanping Wang, Li Zhang
The Province Key Laboratory of Fluid Transmission Technology, Zhejiang Sci-Tech University, Hangzhou, China
Received May 30, 2013; revised June 7, 2013; accepted June 14, 2013
Copyright © 2013 Lifu Zhu et al. This is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
The splitter blades are widely used in axial compressors and play an active role in the improvement of the overall per-
formance of compressors. However, little research on the application of splitter blades to small axial flow fans is con-
ducted. This paper designs a splitter blade small axial flow fan (model B) with a small axial flow fan as the prototype
fan (model A) by adding short blades at the second half part of the passageway among long blades of model A. The
steady simulation for the two models was conducted with the help of RNG k-ε turbulence model provided by software
Fluent, and static characteristics and internal flow characteristics of the two models were compared and analyzed. Re-
sults show that splitter blades can improve the unsteady flow in the small flow rate region and also have a positive role
to increase static pressure rise and efficiency in the higher flow rate region. The variation of static pressure gradient on
the meridian plane in model B is well-distributed. The static pressure on the blade surface of model B distributes more
uniformly. Splitter blades can suppress the secondary flow from pressure side to suction side in the leading edge be-
cause the pressure difference between suction side and pressure side in model B is generally lower than that of model A.
And it also can restrain the vortex shedding and flow separation, and further it may be able to get the aerodynamic noise
lower because static pressure gradient on the blade surface is well-distributed and the vortex shedding is not developed.
Therefore, the performance of the fan with splitter blades is better than that of the prototype fan. The findings of this
paper can be a basis for the design of high performance small axial flow fans.
Keywords: Small Axial Flow Fan; Splitter Blade; Internal Flow; Performance
1. Introduction
The technology of splitter blade is adding short blades at
the passageway among the original long blades of impel-
lers, and the splitter blade is known as short blade or
small blade. The short blades and long blades are alterna-
tive arrangements, which can improve the distribution of
the internal flow field of impellers; what’s more, it also
can increase the load of blades and pressure ratio of im-
pellers, it is an effective method to improve the overall
performance of impellers [1]. The technology of splitter
blade is widely used in centrifugal impellers, and then
the research and application in the axial impellers are
starting lately. In 1974, Wennerstrom [2,3] applied the
technology of splitter blade in the axial compressors, he
decreased the deviation angle of outlet airflow in the high
load rotor by changing the aerodynamic arrangement.
Because of being limited by the methods of numerical
simulation and experiment at that moment, the applica-
tion of splitter blades to the axial impellers was not suc-
cessful and this technology was stalled. Since 1980s,
with the development of computer and methods of full
three-dimensional numerical simulation, the technology
of splitter blade was applied again and it had obtained the
significant achievements [4-6]. Tzuoo et al. [7] reviewed
the results of a detailed analytical study performed on
Wennerstrom’s rotor, followed by the details of a redes-
ign effort using advanced design methodology, and the
results showed that the extensive flow separation ob-
served in Wennerstrom’s rotor can be completely elimi-
nated by redesigning the main blade and splitter vane.
Yongxin Zhang et al. [8] found that splitter could recon-
struct the balance of pressure in rotor passage, and con-
trolled the flow near rotor blade. Ming Yan et al. [9]
studied flow characteristics of one axial compressor rotor
with splitter, they concluded that the rotor with splitter
could be operated with higher total pressure ratio, higher
efficiency and larger mass flow than normal designed
rotors, under the high loaded condition and with the same
*Corresponding author.
C
opyright © 2013 SciRes. OJFD

L. F. ZHU ET AL. 76
surge margin. Besides, many scholars paid their attention
to the influence of the chord length of splitter blades and
circumferential position in impeller to the performance of
rotors [10,11].
The small axial fans are influenced by sizes and appli-
cations, and the internal flow characteristics of small
axial fans are different from the large scale fans. So, the
researches about small axial flow fans have attracted
many scholars in recent years. The application of splitter
blades in axial machineries is mainly concentrated on
high-loaded compressors currently. However, little re-
search on the application of splitter blades to small axial
flow fans is conducted. This paper designs a splitter
blade small axial flow fan (model B) with a small axial
flow fan as the prototype fan (model A) by adding short
blades at the second half part of the passageway among
long blades of model A. The steady simulation for the
two models is conducted with the help of RNG k-ε tur-
bulence model provided by software Fluent, and static
characteristics and internal flow characteristics of the two
models are compared and analyzed.
2. Geometrical Models
In this paper, model A is the prototype fan, as shown in
Figure 1(a).The diameter of fan is 85 mm, hub ratio is
0.72, number of blades is 5, the rated rotating speed is
3000 r/min, and the tip clearance is 1.5 mm. On the basis
of model A, this paper designs a small axial flow fan
with splitter blades (model B), i.e. adding short blades at
the second half part of the passageway among long
blades of model A, as shown in Figure 1(b). Meanwhile,
the location of short blades is in the middle of passage-
ways, the geometric similarity ratio between long blades
and short blades is 5:2.
Figure 2 shows the blade cascade of small axial fan
with splitter blades. It represents the distribution of short
blades in the passageway clearly. Meanwhile, it also can
be found that the aerodynamic arrangement of model B
must be changed by splitter blades, and differ from
model A.
(a) (b)
Figure 1. Fan models. (a) Model A; (b) Model B.
3. Meshing and Numerical Simulation
3.1. Computational Domain and Grid
In this study, the software Gambit is applied to divide
grids, and the center of hub is set as the coordinate origin.
To ensure the reliability of numerical calculation, the
inlet and outlet of fan should be extended. The computa-
tional domain is divided into 4 parts: the extension region
of inlet and outlet, rotating fluid region and pipeline re-
gion, as shown in Figure 3. Meanwhile, non-structural
grids (tetrahedral T-grid) are used in rotating fluid region
and pipeline region, and the Figure 4(a) represents the
grids of fan. The length of the extension of inlet is 85
mm and its diameter is 120 mm while the extensions of
outlet are 510 mm and 340 mm respectively. The struc-
tural grids (hexahedral cooper mesh) are used in the re-
gions of front channel and back channel, and the interval
of grids is three, as shown in Figure 4(b). additionally,,
the degree of twist of grids is predominantly between 0.1 -
0.5.
3.2. Boundary Conditions and Numerical
Calculation
In this paper, mass flow inlet is set as inlet boundary
Pressure side
Suctio
n
side
Rotation directio
n
Flow
Figure 2. Blade cascade of model B.
Pipeline region
Rotating fluid region
Front channel
Inlet Outlet
Back channel
340
510
85
85
120
Figure 3. Computational domain.

L. F. ZHU ET AL. 77
(a)
(b)
Figure 4. Computational grids. (a) Girds of fan; (b) Girds of
the extension of inlet and outlet.
condition, while, the outlet boundary condition is pres-
sure outlet. The solid walls such as vane surfaces and hub
satisfy the no-slip condition in the computational do-
main.
The finite volume method is carried out in numerical
calculation. It is assumed that the flow field of the im-
peller is incompressible and inviscid. The steady simula-
tion for the two models is conducted with the help of
RNG k-ε turbulence model provided by software Fluent.
Meanwhile, second order upwind difference scheme is
adopted as numerical discretization method of governing
equation. The residuals are equal or less than the given
standard (10−3), and relative error of flow rate at the inlet
as well as outlet is less than 0.5%, then the calculation is
convergence.
4. Results and Discussions
4.1. Static Characteristics of Models
The static characteristics are an important factor to ana-
lyze the performance of small axial fan. Meanwhile, the
static characteristics are reflected by the P-Q and the η-
Q performance curves in general, where P is static pres-
sure and η is efficiency. In this research, the different
inlet flow rates are set, and the steady flow fields of fans
are got in 21 flow-rate conditions from Q = 0.002 kg/s to
Q = 0.012 kg/s at the flow-rate interval of Q = 0.0005
kg/s. Figures 5 and 6 represent P-Q and η-Q performance
curves obtained from steady simulation of two models.
Figure 5. Performance curve of flow rate and static pres-
sure.
Figure 6. Performance curve of flow rate and efficiency.
From Figure 5, it can be seen that the static pressure
rise at the inlet and outlet of the two models decrease
with the increase of inlet flow rate Q on the whole. In
addition to this, when the inlet flow rate Q is less than
0.045 kg/s, fans are working under the small flow rate at
the moment, so the flow is unsteady. For this reason, the
curve of model A have a concave region, nevertheless,
this phenomenon does not appear in model B. Thus it can
be concluded that splitter blades can improve the un-
steady flow in the small flow rate region. When the inlet
flow rate Q > 0.045 kg/s, the static pressure rise of model
B is always higher than that of model A. Hence, splitter
blades have a positive role to increase the static pressure
rise.
Figure 6 shows the η-Q curves of two models. It is
found that the efficiency of the two models are closely
similar while 0.002 kg/s < Q < 0.006 kg/s, when Q >
0.006 kg/s, the efficiency of model B is notably higher

L. F. ZHU ET AL. 78
than that of model A.
4.2. Pressure Distribution
Figures 7(a) and (b) illustrate the contour distribution of
the static pressure on the meridian plane of two models
respectively when the mass flow rate is 0.007 kg/s. It
indicates that the pressure at the outlet is higher than inlet
pressure because of the influence of fans. And beyond
this, it also shows that the static pressure rise at the inlet
and outlet of model B is higher than that of model A.
From Figure 7(a), it represents that there is a low pres-
sure region in front of the fan and this region is sur-
rounded by the higher pressure region, then the variation
of pressure gradient in model B is well-distributed in
Figure 7(b). Furthermore, the distribution of pressure
gradient is symmetrical distributed along the axis of hub
in model B, while model A is deflected sideways. For
these reasons, the internal flow of model A is very com-
plicated, so it may be easy to cause secondary flow and
vortex.
Figure 8 represents the distribution of the static pres-
sure along with the axial direction of two models. The
abscissa is axial position in this figure. After analysis, it
can be seen that the pressure at the outlet is higher than
inlet pressure, and the maximum pressure of model A is
higher than that of model B in the rotating fluid region or
Low pressure region
(a)
(b)
Figure 7. Distribution of static pressure on the meridian
plane. (a) Model A; (b) Model B.
5.00e+00
0.00e+00
-5.00e+00
Static Pressure/Pa
-1.00e+01
-1.50e+01
-2.00e+01
-2.50e+01
-3.00e+01
-3.50e+01
-4.00e+01
-100 100200 300 4005000
Position (mm)
(a)
5.00e+00
0.00e+00
Static Pressure/Pa
-1.00 e+01
-1.50 e+01
-2.00 e+01
-2.50 e+01
-3.00 e+01
-3.50 e+01
-4.00 e+01
-100 100200 300 400 5000
Position (mm)
-4.50 e+01
-5.00 e+00
(b)
Figure 8. Distribution of thwith the
ipeline region. It also concludes that the static pressure
ribution of static pressure on suction side and
pr
e static pressure along
axial direction. (a) Model A; (b) Model B.
p
rise at the inlet and outlet of model B is higher than that
of model A, which is the same as the conclusions of
Figure 7.
The dist
essure side in two models is shown in Figure 9. It can
be seen from the figure that the static pressure of pres-
sure side is generally higher than that of suction side.
Then by contrasting Figures 9(a) and (b), it can be found
that the static pressure on pressure side of model B dis-
tributes more uniformly and the static pressure of pres-
sure side in model A is obviously higher than that of
model B, which means fluid can be transmitted smoothly
in model B because of lower energy consumption to bal-
ance the pressure gradient. Moreover, pressure pulsation
amplitude of turbulent boundary layer may be reduced,
which controls the generation of separation vortices. And
combining with the figures of suction side of two models,
it concludes that splitter blades maybe can restrain reflux
and vortex to some extent. Besides these, the figure also
shows that the pressure difference between suction side
and pressure side in model B is on the whole lower than

L. F. ZHU ET AL. 79
(a)
(b)
(c)
(d)
Figure 9. Distribution of staticessure on suction side and
surfaces of two models
ar
ve sur-
fa
pr
pressure side in two models. (a) Pressure side of model A;
(b) P r es s ur e s i de of model B; (c) Suc tion side of model A ; ( d)
Suction side of model B.
that of model A, especially in the leading edge. So, the
secondary flow from pressure side to suction side can be
suppressed.
4.3. Vorticity Distribution
Vorticity derives from the velocity gradient exist in the
flow field, and it is an important physical quantity to de-
scribe the internal flow of fluid, because the vorticity
relates to flow separation, aerodynamic noise and other
phenomena in the flowing fluid. Therefore, in order to
further understand the internal flow characteristics, the
vorticity distribution of small axial fan with splitter
blades should be discussed.
In this research, the rotative
e selected as object of study, where the rotative surface
is also known as S1 stream surface. Figure 10 shows the
geometrical position of the rotative surface at 1/3 blade
height, the diameter of rotative surface is 69 mm.
The vorticity contour distribution on the rotati
ce at 1/3 blade height of two models is shown in Fig-
ure 11. From Figures 11(a) and (b), it can be seen that
the vortex shedding is existed in the trailing edge of
model A because the highly centralized region of vortic-
ity is usually regarded as vortex. However, the vortex
shedding is not developed in model B. The existence of
the vortex may increase energy consumption and degrade
performance of fans. Meanwhile, aerodynamic noise of
model A is higher than that of model B according to the
vortex-sound theory. Therefore, it can be concluded that
Figure 10. Geometrical position of the rotative surface at
1/3 blade height.
Vortex shedding
(a)
(b)
Figure 11. The vorticity con distribution on the rotative tour
surface at 1/3 blade height. (a) Model A; (b) Model B.

L. F. ZHU ET AL. 80
hed-
all axial fan with splitter blades.
e unsteady flow of
sm
transmitted smoothly in the small axial
fa
shedding and
flo
ants from the National
splitter blades may be able to suppress the vortex s
ding and flow separation, and then improve the perform-
ance of fans to some extent, such as static characteristics
and aerodynamic noise.
5. Conclusions
The paper designs a sm
With the help of the numerical simulation, the influence
of splitter blades on the performance of small axial fan is
investigated. The static characteristics of models are rep-
resented, and the internal flow characteristics are dis-
cussed from two aspects, i.e. pressure and vorticity. The
conclusions are shown as followed:
1) Splitter blades can improve th
[4]
all axial flow fan in the small flow rate region. When
the inlet flow rate Q > 0.045 kg/s, the static pressure rise
of model B is obviously higher than that of model A, and
the efficiencies of the two models are closely similar
while 0.002 kg/s < Q < 0.006 kg/s; when Q > 0.006 kg/s,
the efficiency of model B is notably higher than that of
model A. So, splitter blades have a positive role to in-
crease the static pressure rise and efficiency in the higher
flow rate region.
2) Fluid can be
[7]
n with splitter blades, because the static pressure dis-
tributes more uniformly. And pressure pulsation ampli-
tude of turbulent boundary layer may be reduced, which
controls the generation of separation vortices. Splitter
blades can suppress the secondary flow from pressure
side to suction side in the leading edge.
3) Splitter blades can restrain the vortex
w separation, and further it may be able to get the
aerodynamic noise lower because static pressure gradient
on the blade surface is well-distributed and the vortex
shedding is not developed.
6. Acknowledgements
This work was supported by gr
[11]
Natural Science Foundation of China (No. 51006090)
and the Major Special Project of Technology Office in
Zhejiang Province (No. 2011C11073, No. 2011C16038).
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