Open Journal of Fluid Dynamics, 2013, 3, 81-84
http://dx.doi.org/10.4236/ojfd.2013.32A013 Published Online July 2013 (http://www.scirp.org/journal/ojfd)
Effect of Hub-Ratio on Performance of Asymmetric
Dual-Rotor Small Axial Fan
Yongmin Wu, Yingzi Jin, Yuzhen Jin, Yanping Wang, Li Zhang
Faculty of Mechanical Engineering & Automation, Zhejiang Sci-Tech University, Hangzhou, China
Received May 30, 2013; revised June 7, 2013; accepted June 14, 2013
Copyright © 2013 Yongmin Wu 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
Currently, domestic and abroad scholars put more attention on contra-rotating dual-rotor axial fan. But there is less
scholars study on asymmetric dual-rotor small axial fan, which is one of the contra-rotating dual-rotor axial fans. Like
axial fan, many factors have influence on the performance of the asymmetric dual-rotor small axial flow fan, such as the
wheel hub ratio, blade shape, blade number, stagger angle and the tip clearance. Because wheel hub ratio has great im-
pact on the performance of the fan, we choose the size of wheel hub ratio as a variable factor to study the model change.
There is a different wheel hub ratio between front stage impeller and rear stage of asymmetric dual-rotor small axial fan,
so it is very beneficial to select the greater wind area that the fan area of external diameter minuses the area occupied by
the blades and the hub as front stage impeller. In this paper, the hub-ratio of front stage impeller is 0.72, and that of rear
stage is 0.72, 0.67 and 0.62 respectively along with the front stage impeller. Three kinds of models with different hub
ratio of rear stage are simulated using the CFD software and the static characteristics are obtained. Based on the ex-
perimental test results, the internal flow field of the asymmetric dual-rotor small axial fan is analyzed in detail, the im-
pact trends of different hub-ratio on the performance of asymmetric dual-rotor small axial fan are obtained and the ar-
gument of structure optimization for dual-rotor small axial fan is provided.
Keywords: Asymmetric Dual-Rotor; Axial Flow Fan; Hub Ratio; Numerical Simulation; Internal Flow Field
1. Introduction
With the sustained development of China’s economy,
electronic devices, such as computers are widely used by
people. And with all kinds of computers’ speed im-
provement, the computers’ calorific value is also in-
creased, then it comes up with higher performance of the
cooling fan in the computers. There are many factors that
can affect axial fan’s aerodynamic performance. Like
axial fan, many factors have influence on the perform-
ance of the asymmetric dual-rotor small axial flow fan,
such as the wheel hub ratio, blade figure, blade number,
stagger angle and the tip clearance. In the references
[1,2], the dual-rotor inversion structure was applied to a
small axial fan and internal flow characteristics were
analyzed. The reference [3] indicated that the dual-rotor
fan had a higher pressure rise than traditional. The refer-
ence [4] found a best distance between the front stage
impeller and the rear. And it indicated that the counter-
rotating fan could achieve higher pressure than tradi-
tional axial fan. The reference [5] analyzed the aerody-
namic performance by the hub ratio of ordinary axial fan.
It optimized the hub ratio of counter-rotating axial fan.
As a result, it ensured a best hub ratio. The reference [6]
optimized the flex and sweep parameters between the
front stage impeller and the rear of mining counter-ro-
tating fan. Consequently, it improved the total pressure
efficiency and aerodynamic performance of the fan. In
the reference [7], it calculated unsteady flow field of
seven different axial spacing of mining counter-rotating
fans. Therefore, it indicated that 0.7b was the better axial
spacing for the combination property of the fan.
In this paper, we put attention on the influence of aero-
dynamic performance and internal flow characteristics of
different hub-ratios, which have the same front stage
impeller and the different rear on the performance of
asymmetric dual-rotor small axial fan. It will provide
some theoretical basis for quantitative analysis and
structure optimization of asymmetric dual-rotor small
axial fan.
2. Models and Numerical Simulation
In this paper, asymmetric dual-rotor small axial fan is
C
opyright © 2013 SciRes. OJFD

Y. M. WU ET AL. 82
based on the small axial fan. The parameters of the small
axial fan: the impeller diameter is 85 mm, the tip clear-
ance is 1.5 mm, the number of blades is 5, and the rotat-
ing speed is 3000 r/min. Three fan models are shown in
Figure 1. The hub-ratio of both impeller of Type1 is all
0.72. In Type2 and Type3, the hub-ratio of front stage
impeller is 0.72, and that of rear stage is 0.67, 0.62 re-
spectively along with the front stage impeller. Figure 2
is the sketch map of computational domain. Origin of
coordinates is in the centre of two fans. In order to ensure
the reliability of flow numerical simulation, we extend
the inlet and outlet of the fan. The distance between
computational inlet and fan’s inlet is 120 mm and the
diameter of inlet channel is 120 mm. The distance be-
tween computational outlet and fan’s outlet is 550 mm
and the diameter of outlet channel is 240 mm. There are
different kinds of gridding to divide fluid mass because
of different structures in the computational domain. Tet-
rahedral TGrid is used in rotating fluid area and sur-
rounding channel. Hexahedral cooper is used in the inlet
and outlet channel.
Because, in the fan, the Mach number of airflow is less
than 0.3, the fluid can be regarded as incompressible. K-
ε turbulence model and segregated equation solver are
used in the calculation model. N-S equation is used in
control equation. The problem of heat conduction is ne-
glected in simulation. In order to improve the accuracy of
numerical simulation, the SIMPLE algorithms are used
Type1 Type2 Type3
Figure 1. Fan models.
computational inlet
550
fan’s inlet
fan’s inlet computational exit
120
120
240
Figure 2. Computational domain.
to solve pressure and velocity coupling and numerical
discretization, method of the control equation employ
second-order upwind scheme.
3. Static Characteristic of Models
Figure 3 represents p-Q performance curve which got
from numerical simulation of Type1, Type2 and Type3.
Figure 4 shows η-Q efficiency curves which were ob-
tained by numerical simulation of Type1, Type2 and
Type3. From the two figures, some conclusions can be
acquired which will be discussed in the following pas-
sage. From Figures 3 and 4, we can obtain that Type2
and Type3 have different p-Q and η-Q curves with Type1.
When the flow rate Q is in the range of 0.004 kg/s < Q <
0.01 kg/s, the pressure rise and efficiency of Type1 is
higher than that of another two types. When the flow rate
Q increases to 0.01 < Q < 0.014, the pressure rise and
efficiency of Type2 and Type3 are higher than the model
Type1
Type2
Type3
p/Pa
100
90
80
70
60
50
40
30
20
10 0.0040.0060.008 0.010 0.0120.014
Q/kg/s
Figure 3. p-Q performance curve.
Type1
Type2
T
yp
e3
45
40
30
20
0.0040.0060.008 0.010 0.0120.014
Q/kg/s
25
30
Efficiency η/%
Figure 4. η-Q efficiency curve.

Y. M. WU ET AL. 83
of Type1. Type3 compared with Type2, two facts of
pressure rise and efficiency both are higher than that of
Type2. For this reason, it can be deduced, contrasted
with Type1 which has the same hub ratio of the front
stage impeller and the rear, that asymmetric dual-rotor
small axial fan which has a smaller hub ratio of the rear
stage impeller is valuable for the fan’s performance. Be-
cause of the greater wind area of small hub ratio of the
rear stage impeller, air can flow front to back more easily.
Focus on the flow rate Q equal to 0.011 kg/s in Figure 4,
there is a peak in efficiency curve of Type3 which is a
highest efficiency than the efficiency of Type2. As a re-
sult, according to the pressure rise and the efficiency
curve that the fan model of Type3 is better than the oth-
ers.
4. Effect of Hub Ratio on Performance of
Models
Figure 5 is the distributions of axial velocity on meridian
plane of three models. As can be seen from the figure,
the axial velocity on meridian planes of the models of
Type2 and Type3 is higher than the model of Type1.
Therefore, the models of Type2 and Type3, which have
small hub ratio of the rear impeller, have a high capacity
to do work on air than the model of Type1, which has the
same hub ratio of two impellers. There are negative ve-
locities which can generate refluxes in impeller passage
way closed to blade tip of three models. The refluxes
showed in the Figure 6 have a disadvantage for air flow.
The contour distribution of the static pressure on the
meridian plane of three models is shown in Figure 7
when the mass flow rate is 0.011 kg/s. It illustrates that
the pressure at the outlet is higher than inlet pressure. In
addition to this, the absolute value of static pressure rise
at the inlet and exit of Type2 and Type3 are higher than
that of Type1. This result is the same as p-Q performance
curve showed.
Figure 8 indicates the distribution of static pressure on
-6 -5 -4 -3 -2 -1 0 12 3 4 5 6 7
Type1 Type2 Type3
Figure 5. Distributions of axial velocity on meridian plane.
Type1 Type2 Type3
Figure 6. Distributions of streamline on meridian plane.
Type1
Type2
Type3
Figure 7. Contour distribution of the static pressure on the
meridian plane.
suction side and pressure side of three models. The left
suction sides are the front stage impellers’ and the right
pressure sides are the rear stage impellers’. It is observed
that the static pressure of pressure side is generally
higher than that of suction side from the figure. Com-
pared Type2, Type3 with Type1, we can find that the
absolute value of static pressure on suction side of Type2
and Type3 is higher than Type1. As a result therefore,

Y. M. WU ET AL. 84
Type1
Type2
Type3
Figure 8. Distribution of static pr essure on suction side and
pressure side.
the fans of Type2 and Type3 have a high capacity to do
work on air than the fan of Type1. And it also shows that
the static pressure on pressure side of Type2 and Type3
distribute more homogeneously, which means air can be
transmitted smoothly and the fans have a higher effi-
ciency to do work because of lower energy consumption
to balance the pressure gradient. This result is the same
as η-Q efficiency curve showed.
5. Conclusions
The performance of asymmetric dual-rotor small axial
fans with different hub ratios of the rear stage impeller
was investigated by numerical analysis. The conclusions
are as follows:
1) When the flow rate Q is in the range of 0.01 kg/s <
Q < 0.014 kg/s, the fan which has a 0.62 hub ratio of the
rear stage impeller can obtain higher static pressure rise
than the other fans discussed in the paper.
2) With the decrease of hub ratio of the rear stage im-
peller, the flow rate Q of the maximum efficiency in-
creases. When the flow rate Q equals to 0.011 kg/s, the
model of Type3 has a highest efficiency to do work than
the others.
3) The axial velocity on meridian planes of the models
and the absolute value of static pressure on suction side
of Type3 is higher than the others. Beyond that, the static
pressure on pressure side of Type3 distributes more ho-
mogeneously, which means the fan has a higher capacity
to do work on air.
4) Considering static performance and internal flow of
the fans, the fan with a hub ratio of 0.62 of the rear stage
impeller is best of all.
6. Acknowledgements
This work was supported by grants from the National
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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