In axial fan, the static pressure difference between the suction and the pressure side of the impeller produces a leakage flow through the blade and the casing. This secondary flow occurs in the opposite direction of the working flow and has a negative impact on the overall performances. It tends to reduce the pressure coefficient, the efficiency and the fan operating range while increasing the noise level. That is why many studies dealt with ways to reduce this leakage flow. In this paper, the study focusses on the control of the secondary flow by air injection. Two ways to control this flow are compared. In a first case, the air is ejected from the fixed casing and in a second case the air exit from the rotating shrouds. In both configurations, the ejected air has a direction to counter the leakage flow. To realize the second configuration, a new method to build fan with hollow blades was developed. This new kind of fan allows having internal flows which could exit by any area of the fan. The results obtained by the active controls on the fan characteristic and the efficiency are presented in this article

Azzam, Tarik,

Bakir, Farid

Khelladi, Sofiane

Mekadem, Mahmoud

Oualli, Hamid

Paridaens, Richard

Ravelet, Florent

I-Revues

22ème Congrès Français de Mécanique Lyon, 24 au 28 Août 2015
Active control of the leakage flow by air injection
into the rotational shroud or the fixed carter of an
axial fan composed of hollow blades
T. Azzama, b, R. Paridaensa, F. Raveleta, M. Mekademb, S. Khelladia, H.
Ouallib, F. Bakira
a. Laboratoire DynFluid, Arts et Métiers ParisTech, Paris, France
b. Laboratoire de Mécanique des Fluides, Ecole Militaire Polytechnique, Algiers, Algeria
Abstract :
In axial fan, the static pressure difference between the suction and the pressure side of the impeller
produces a leakage flow through the blade and the casing. This secondary flow occurs in the opposite
direction of the working flow and has a negative impact on the overall performances. It tends to reduce
the pressure coefficient, the efficiency and the fan operating range while increasing the noise level. That
is why many studies dealt with ways to reduce this leakage flow. In this paper, the study focusses on the
control of the secondary flow by air injection. Two ways to control this flow are compared. In a first
case, the air is ejected from the fixed casing and in a second case the air exit from the rotating shrouds.
In both configurations, the ejected air has a direction to counter the leakage flow. To realize the second
configuration, a new method to build fan with hollow blades was developed. This new kind of fan allows
having internal flows which could exit by any area of the fan. The results obtained by the active controls
on the fan characteristic and the efficiency are presented in this article.
Keywords : active control ; air injection ; leakage flow ; axial fan ; hollow
blades.
1 Introduction
In recent years, more and more investigations on turbomachinery deal with the control by tip injection.
In fact, this type control could improve different aspects of the device which could not be enhanced by
the geometry. By using injection upstream of the rotor, Weigl et al. [1] succeeded to stabilize rotating
stall and surge in a transonic compressor which provided an enlargement of the operating range. Rhee
et al. [2] increased the lift of a hydrofoil by injection control. Eberlinc et al. [3, 4, 5] experimentally rose
of approximatively 6% the pressure of an axial fan composed of hollow blades. This gain was due to
a free jet at the trailing edge of the blades. In fact, the free jet increased the velocity and consequently
reduced the adverse pressure gradient in the boundary layer. Wang and Zhao [6] experimentally and
numerically investigated a transonic turbine cascade with different trailing edge ejection. As Eberlinc et
al., the authors successfully reduced the vortex at the trailing edge of the blades.
This paper focuses on the injection used to reduce the phenomenon of leakage flow occurring at the blade
22ème Congrès Français de Mécanique Lyon, 24 au 28 Août 2015
tip. This flow driven by the pressure difference between the pressure side and suction side is responsible
for important energy dissipations.That is why many authors have sought to reduce this phenomenon.
Neuhaus and Neise [7] attempted to reduce the leakage flow of an axial fan by applying active flow
control in the tip region of the impeller. They injected air in the opposite direction of the flow by slit
nozzles flush mounted in the inner casing wall. By this process, the authors succeeded to shift the stall
point towards lower flow rates and to enlarge by 62% the usable range of the fan characteristic. They also
increased the pressure of 28% and the efficiency by about 10%. Hamik and Willinger [8] analytically
and numerically investigated the concept patented by Auxier [9] in 1995. The idea is to connect the blade
leading edge to the blade tip by an internal channel. Due to the pressure difference, a jet is injected to
the tip gap and blocks the leakage flow. With an injection mass flow rate of 0.5% of the mass flow, they
predicted a performance improvement which is independent of the tip gap width. Hu et al. [10] applied
the concept proposed by Auxier to an axial fan and experimentally investigated the interaction between
the tip leakage flow and the tip injection flow by a 2D-PIV. As expected, the authors observed a reduction
of the leakage flow. However, the tip injection generated a flow phenomenon similar to Karman vortex
street in the wake which exacerbate the complexity and non-stationarity of the flow.
In this study, active control by air injection in the tip clearance gap of an axial fan is experimentally
investigated. A new methodology was developed to build hollow fan in which an internal flow could oc-
cur. The internal flow, generated by the compressed air system of the laboratory, exited the fan at the tip
clearance in purpose to reduce the leakage flow. The results of this control by air injection are presented
in this paper.
2 Experimental setup
The fan (figure 1a) used in this study was developed for automotive engine cooling system application
[11]. The fan has six blades with a hub-to-tip radius ratio (Rint/Rmax) equal to 0.337 and a the tip radius
(Rmax) equal to 179 mm. It has the particularity to be built in polyethylene by a rotomolding process in
order to make it hollow. This characteristic allows a fluid circulation into the fan. With internal flows
into the fan different applications could be developed. The authors thought to use a heat-transfer fluid
into the fan so that the fan could also operate as a heat exchanger. The authors also planned to study the
impact of air ejected from the trailing edges. According to Eberlinc et al. [3], ejected air from the trailing
edges increases locally the velocity flow and tends to reduce the boundary layer stalls. In addition to flow
ejection, the authors have also planned to experiment suction flows for fan control. In this study, air is
injected into the fan and exits at the shroud of the fan. The shroud is composed 8 holes of 4mm regularly
spaced on the circumference. The holes are oriented in the direction (r, θ, z) = (1,−1, 1) in order to
counter the leakage flow. The directions have a component in θ to compensate the fan rotation.
The leakage flow control is also realized by ejecting air from the fixed casing (figure 1b). To compare the
better method to control this phenomenon between air injection into the rotational shroud and the fixed
casing, the injection holes was realized in the same configuration as for the shroud : 8 holes of 4mm
regularly spaced. Yet as the casing does not rotate, the holes have all the direction (r, θ, z) = (−1, 0, 1).
To not damage the fan, the casing was made of rigid foam.
To supply air to the fan and the casing, the compressed air system of the laboratory is used. A flowmeter
and a valve are installed upstream the fan and the casing in order to control the injection. A specific drive
system showed on figure 2 was built to connect the compressed air system to the hollow fan. The drive
22ème Congrès Français de Mécanique Lyon, 24 au 28 Août 2015
a) b) 
Figure 1 – The hollow fan (a) and the casing (b)
system has two functions. It transmits the rotation from the motor to the fan and allows the injection
from the compressed air system into the fan. To fulfill its functions, the drive system is composed of
two shafts connected through a pulley and belt system. The first shaft connects a 4kW motor to the
pulley and belt system. On this shaft a HBM strain gauge transducer is installed. It measures the torque
with an uncertainty corresponding to 0.1% of the full scale. In this case the full scale is equal to 1N.m.
The second shaft connects the pulley and belt system to the fan. It also connects the compressed air
system to the fan. This connection is possible because the shaft has the particularity to be hollow. The
fan angular velocity is measured with a tachymeter of relative precision±0.2%. The power absorbed on
the fan shaft is then estimated through the productCω. To determine the global performances of the fan,
an air suction test bench was used. This experimental facility was designed and built at the laboratory
according to the ISO 5801 standard [10]. The air flow rate is set and measured according to the ISO-5167
norm [11] by setting the hydraulic impedance of the bench through diaphragms of various sizes. The
elevation pressures are measured with an absolute precision of ±0.1 Pa.
Axial fan Pulley and belt system 4-kW motor HBM strain gauge 
transducer 
Compressed air 
system 
Flowmeter 
Ejected 
holes 
Figure 2 – The fan drive system
22ème Congrès Français de Mécanique Lyon, 24 au 28 Août 2015
3 Results
Only the aerodynamic characteristic is represented in this extended abstract. The figure 3 shows the
results obtained at three rotation speeds 1020, 1560, 2000 rpm for two configurations with and without
air injection into the fan. The injection control was realized with an internal flow equal to 13.3% of the
external volume flow. Each curve is the result of five measurement points. The air injection induced
an increase in pressure difference regardless of the rotation speed. As the injection was realized in the
purpose of controlling the leakage flow, the pressure increase is assumed to be a consequence of the
secondary flow reduction. For φ = 0.065, the increase varies from 18% to 25% depending of the rotation
speed, whereas at higher flow rate of about 0.1, it varies from 41% to 98%. These results could be
explained by a better control of the leakage flow at φ = 0.065 than at 0.1 as this phenomenon intensifies
at higher flow rate [7].
0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Φ
Ψ
 
 
Qinj/Q=0
Qinj/Q=13.32%
1020 rpm
1560 rpm
2000 rpm
Figure 3 – Comparison of aerodynamic characteristic of the axial fan for the case with and without
internal flow and for 3 rotation speeds : 1020, 1560 and 2000 rpm
Références
[1] H. J. Weigl, J. Paduano, L. Frechette, A. Epstein, E. Greitzer, M. Bright, and A. Strazisar, “Active
stabilization of rotating stall and surge in a transonic single stage axial compressor,” in ASME 1997
International Gas Turbine and Aeroengine Congress and Exhibition, pp. 1–15, American Society
of Mechanical Engineers, 1997.
[2] S. Rhee, S.-E. Kim, H. Ahn, J. Oh, and H. Kim, “Analysis of a jet-controlled high-lift hydrofoil
with a flap,” Ocean engineering, vol. 30, no. 16, pp. 2117–2136, 2003.
[3] M. Eberlinc, B. Širok, and M. Hočevar, “Experimental investigation of the interaction of two flows
on the axial fan hollow blades by flow visualization and hot-wire anemometry,” Experimental ther-
mal and fluid science, vol. 33, no. 5, pp. 929–937, 2009.
22ème Congrès Français de Mécanique Lyon, 24 au 28 Août 2015
[4] M. Eberlinc, B. Širok, M. Dular, and M. Hočevar, “Modification of axial fan flow by trailing edge
self-induced blowing,” Journal of Fluids Engineering, vol. 131, no. 11, p. 111104, 2009.
[5] M. Eberlinc, B. Širok, M. Hočevar, and M. Dular, “Numerical and experimental investigation of
axial fan with trailing edge self-induced blowing,” Forschung im Ingenieurwesen, vol. 73, no. 3,
pp. 129–138, 2009.
[6] Y. Wang and L. Zhao, “Investigation on the effect of trailing edge ejection on a turbine cascade,”
Applied Mathematical Modelling, vol. 37, no. 9, pp. 6254–6265, 2013.
[7] L. Neuhaus and W. Neise, “Active control to improve the aerodynamic performance and reduce
the tip clearance noise of axial turbomachines with steady air injection into the tip clearance gap,”
in Active Flow Control, pp. 293–306, Springer, 2007.
[8] M. Hamik and R. Willinger, “An innovative passive tip-leakage control method for axial turbines :
Basic concept and performance potential,” Journal of Thermal Science, vol. 16, no. 3, pp. 215–222,
2007.
[9] T. Auxier, “Aerodynamic tip sealing for rotor blades,” 1995, No. 5.403.158.
[10] J. Hu, X. Kong, Z. Li, Y. Zhang, and J. Xu, “Experimental investigation of aerodynamic interaction
between tip leakage flow and spontaneous tip injection flow using 2d-piv,” Experimental Thermal
and Fluid Science, vol. 54, pp. 127–135, 2014.
[11] C. Sarraf, H. Nouri, F. Ravelet, and F. Bakir, “Experimental study of blade thickness effects on
the overall and local performances of a controlled vortex designed axial-flow fan,” Experimental
Thermal and Fluid Science, vol. 35, no. 4, pp. 684–693, 2011.


Active control of the leakage flow by air injection into the rotational shroud or the fixed carter of an axial fan composed of hollow blades

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Active control of the leakage flow by air injection into the rotational shroud or the fixed carter of an axial fan composed of hollow blades

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