Journal of Thermal Science Vol.24, No.2 (2015) 123130
DOI: 10.1007/s11630-015-0764-4
Article ID: 1003-2169(2015)02-0123-08
Effects of Perforation Number of Blade on Aerodynamic Performance of Dual-rotor Small Axial Flow Fans HU Yongjun1, WANG Yanping1, LI Guoqi1, JIN Yingzi 1, Toshiaki Setoguchi2, Heuy Dong Kim3 1. The Zhejiang Provincial Key Laboratory of Fluid Transmission Technology, Zhejiang Sci-Tech University, Hangzhou 310018, China 2. Department of Mechanical Engineering, Saga University, Honjo-machi, Saga, 840-8502, Japan 3. School of Mechanical Engineering, Andong National University, Seongcheon-Dong1375, Gyeongdong-RD, Andong, Gyeongsangnuk-DO, 760-749, Korea © Science Press and Institute of Engineering Thermophysics, CAS and Springer-Verlag Berlin Heidelberg 2015
Compared with single rotor small axial flow fans, dual-rotor small axial flow fans is better regarding the static characteristics. But the aerodynamic noise of dual-rotor small axial flow fans is worse than that of single rotor small axial flow fans. In order to improve aerodynamic noise of dual-rotor small axial flow fans, the pre-stage blades with different perforation numbers are designed in this research. The RANS equations and the standard k-ε turbulence model as well as the FW-H noise model are used to simulate the flow field within the fan. Then, the aerodynamic performance of the fans with different perforation number is compared and analyzed. The results show that: (1) Compared to the prototype fan, the noise of fans with perforation blades is reduced. Additionally, the noise of the fans decreases with the increase of the number of perforations. (2) The vorticity value in the trailing edge of the pre-stage blades of perforated fans is reduced. It is found that the vorticity value in the trailing edge of the pre-stage blades decreases with the increase of the number of perforations. (3) Compared to the prototype fan, the total pressure rising and efficiency of the fans with perforation blades drop slightly.
Keywords: dual-rotor small axial flow fans, the blade with perforation, perforation number, aerodynamic performance
Introduction The static characteristics of dual-rotor small axial flow fans are better than that of single rotor small axial flow fans, but its noise increases by much. So it is important to reduce the noise of dual-rotor small axial flow fans. Akaike et al [1] showed that forward bent blades could reduce the noise of the fans. But in many cases, due to the blades of fans are fixed, only adopt the methods of controlling the boundary layer to reduce noise, such as
set the groove on the suction surface, design serrated leading edge blade and so on. Tzuoo et al [2] conducted a series of optimized study on Wennerstrom's rotor, the results show that the extensive flow separation was observed in Wennerstrom's rotor and it can be completely eliminated by redesigning the main blade and splitter vane. Fang et al [3] studied the effects of blade with perforation on aerodynamic noise performance of fans. The inclination angle of the perforation fan is 45°. The perforation coefficient is 0.08. According to the national stan-
Received: October 2014 JIN Yingzi: Professor This work was supported by National Natural Science Foundation of China (No. 51276172) www.springerlink.com
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dard of measurement (the test point is one meter in front of the fan), the total sound pressure level of the fans noise reduce 8.2dB. Kazuo et al [4] used two methods to control the noise. One is the mode replacing method that changes the traveling mode with blade passing frequency into a damping mode in the stage. The other is the sound absorbing method by which the energy of pressure fluctuation is absorbed by loudspeakers. It is found that both noise reduction systems are effective in controlling the noise. According to the fact that the long-eared owl wing with non-smooth leading edge demonstrates a remarkable reduction of the flight noise, Sun et al [5] designed a kind of bionic axial fan blade with non-smooth leading edge. The experiments show that the noise level of the bionic non-smooth blade in the 50-2000 Hz frequency range is remarkable lower than that of the prototype blade, and the maximal noise reduction rate is 2.52%. In this paper, pre-stage perforated blades are designed for the dual-rotor small axial flow fans. It means that perforate on the pre-stage blades of the prototype fan (Model A). But the number of the perforations on blades is different. Then, the total pressure rising, efficiency, total sound pressure level (SPL) and power spectral density of the fans with different number of perforations on blades and the prototype fan are obtained from the numerical simulation, It is found that the blade with perforation design could decrease the aerodynamic noise of the dual-rotor small axial flow fans. What’s more, the noise of the fans decreases with the increase of the number of perforations.
Numerical simulation Computational model and mesh Model A is selected as the prototype fan, as shown in Fig.1 (a). The parameters of the single rotor model are shown in Table 1. Where, the two rotors are counterrotating. Then, the pre-stage perforated blades are designed on the basis of prototype fan. According to the blade width and height of the prototype fan and other geometric size, different arrangements of perforations are designed, and they are at the distance of the leading edge of blade 7/40 of arc length, at the distance of the trailing edge of blade 1/8 of arc length, 2/8 of arc length, 3/8 of
(a) Model A
(b) Model B
arc length, 4/8 of arc length, respectively. And each row is designed three or four perforations. The inclination angle α of perforations facing the stream is 55°. The aperture of perforations is 1mm. Four models are created, namely the model B, model C, model D and model E, as shown in Fig.1 (b), (c), (d) and (e). And the schematic view of perforation is shown in Fig.2. Where, the first row of perforations must be in front of the separation point to ensure that the generation of vortex shedding is inhibited effectively. Then the separation point moves backward and the other rows of perforations are in the separation region which moves backward. It prevents the emergence and expansion of the separation region effectively. The middle part of the blade is not perforated with the reason of that it provides the most of the lift. So it can guarantee the lift coefficient decrease minimally. The computational domain is divided into five parts: the extension of inlet and outlet, casing region, pre-stage rotating fluid region and post-stage rotating fluid region. Simultaneously, eight noise monitoring points are set in the computational domain. The computational domain and the coordinates of the monitoring points are shown in Fig.3. The parameters of the computational domain are also shown in Table 1. Additionally, structural grids (hexahedral cooper mesh) are applied in the extension of inlet and outlet. Non-structural grids (tetrahedral T-grid) are applied in casing region, pre-stage rotating fluid region and post-stage rotating fluid region. The total grid is about 2.98×107 to 3.2×107. The mesh of computational domain and the rotor are shown in Fig.4. Numerical calculation methods The FW-H (Flowcs Williams-Hawkings) acoustic model is used to do the noise prediction. The FW-H equation is essentially an inhomogeneous wave equation which can be derived from the continuity equation and the Navier-Stokes equations. The FW-H equation can be written as [6,7]:
(c) Model C
Fig. 1 Fan models
1 2 p' 2 2 ' p {Tij H ( f )} a02 t 2 xi x j
{[ Pij n j ui (un vn )] ( f )} xi
{[ 0 vn (un vn )] ( f )} t
(d) Model D
(e) Model E
(1)
HU Yongjun et al.
Effects of Perforation Number of Blade on Aerodynamic Performance of Dual-rotor Small Axial Flow Fans 125
where ui is the fluid velocity component in the xi direction, un is the fluid velocity component normal to the surface f=0, vi is the surface velocity component in the xi direction, vn is the surface velocity component normal to the surface, δ(f) is the Dirac delta function, H(f) is the Heaviside function, p’ is the sound pressure at the far field (p’=p-p0), a0 is the far-field sound speed, and Tij is the Lighthill stress tensor, defined as Tij ui u j Pij a02 ( 0 ) ij (2)
Fig. 2 Schematic view of perforation Table 1 Parameters of the single rotor model and the computational domain Parameters External diameter Hub diameter
Value
Unit
85
mm
61
mm
Hub ratio
0.72
Rated rotating speed
3000 1.5
mm
Stagger angle of blade
27.4
degree
Number of blades
Pij p ij [
0. Equation (1) can be conducted the comprehensive analysis under the assumptions of the absence of obstructions between the sound sources and the receivers，and the free-space flow. The complete solution includes the surface integrals and the volume integrals. The surface integrals represent the contribution from monopole and dipole acoustic sources and partially from the quadrupole sources, whereas the volume integrals represent the quadrupole sources in the region outside the source surface. The contribution of the volume integrals becomes small when the source surface encloses the source region and the flow is low subsonic. For this reason, the volume integrals are ignored. Thus, we have (4) p ' ( x , t ) pT' ( x , t ) p L' ( x , t ) where
5
Inlet diameter
120
mm
Outlet diameter
340
mm
Length of inlet extension
120
Mm
Length of outlet extension
500
Mm
Fundamental frequency
250
Hz
ui u j 2 u k ij ] x j xi 3 xk
(3) The free-stream quantities are denoted by the subscript
r/min
Tip clearance
Here，Pij is the compressive stress tensor. For the Stokesian fluid, this is given by
4 p ( x , t ) ' T
[
0 (U n U )
f 0
r (1 M r )
n 2
]dS
Fig. 3 Computational domain and the coordinates of the monitoring points
[
0U n {r M r a0 ( M r M )} r 2 (1 M r )3
f 0
1 4 p ( x , t ) a0 ' L
[r
1 a0
]dS
L f 0 [ r (1 Mr r )2 ]dS 2
f 0
(5) 2
Lr LM ]dS (1 M r )2
(6)
[
f 0
Lr {r M r a0 ( M r M 2 )} ]dS r 2 (1 M r )3
where U i vi
(u v ) 0 i i
(7)
Li Pij n j ui (un vn )
(8) When the integration surface coincides with an impenetrable wall, the two terms on the right in equation (4), pT' ( x , t ) and pL' ( x , t ) are often referred to thickness and Fig. 4 Mesh of computational domain and the rotor
loading terms, respectively. What’s more, t is the receiver
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time and r is the distance to the receiver. The Mach number Mi in equation (5) and equation (6) relates to the motion of the integration surface: Mi=vi/a0. The finite volume method is adopted for numerical calculation. Mass flow inlet is set as inlet boundary condition of the computational domain. The outlet boundary condition is pressure outlet. In steady calculations, the RANS method and the standard k-ε turbulence model is taken as the governing equations. Meanwhile, the secondorder upwind difference scheme is applied for the convective term and second central scheme is used for the diffusion term. In the unsteady calculations, the large eddy simulation (LES) is applied to solve the flow field. The results of steady calculation are applied as the initial field of unsteady calculation. Also, the LES is used to calculate the sound field until the pressure field becomes stable. Finally, the Fast Fourier Transform (FFT) method is applied to process the sound pressure signals to obtain the characteristics of noise spectral distribution.
slightly. They decrease with the increase of the number of perforations. It means that the total pressure rising for model E decreases the most. This is because the fluid leaks through the perforations of blades from the pressure side to the suction side, which results in fluid leakage and the decrease of total pressure rising. The total leakage increases with the increase of the number of perforations. As such, the total pressure rising of the four perforated fans decrease with the increase of the number of perforations.
Results and Discussions Static characteristics of fans The qualities of the static characteristics of small axial flow fans are mainly depended on the P-Q and the η-Q performance curves. Where, P is the static pressure, Q is the mass flow rate of inlet and η is the efficiency. In this paper, five models are simulated to obtain the condition of flow at the sixteen operating points from Q=0.002 kg/s to Q=0.017 kg/s. Fig.5 and Fig.6 are the P-Q and η-Q dimensionless curves of numerical simulation of five models respectively.
Fig. 5 P-Q dimensionless curve
As can be seen from Fig.5, the total pressure rising of five models remain decreasing tendency with the increase of mass flow Q. Compared to the prototype fan, the total pressure rising of the four perforated fans all decrease
Fig. 6 η-Q dimensionless curve
From Fig.6, it can be seen that the efficiency of five models first increase and then decrease with the increase of mass flow Q. The efficiency achieves maximum when Q=0.012 kg/s (Φ=0.12943). The efficiency of four perforated fans are all lower than that of the prototype fan (Model A) at each operating point. What’s more, the efficiency of the fans decreases with the increase of the number of perforations. It also means model E decrease the most. It is also because the fluid flows through the perforations of blades from the pressure side to the suction side, resulting in fluid leakage and the decrease of efficiency. Moreover, the more number of perforations, the more fluid leaks, and the more efficiency drops. So the efficiency of four perforated fans decrease more and more in turn. Internal flow analysis Fig.7 and Fig.8 represent the distribution of static pressure on pressure side and suction side of pre-stage blades respectively of each model at the optimum operating point Q=0.012 kg/s. It is found that the static pressure of pressure side is higher than that of the suction side. Static pressure around the perforations of pressure side decrease slightly, but it increases on the suction side. Moreover, with the increasing of the number of the perforations, the region of static pressure decreases on the pressure side and the region of static pressure increases on the suction side. It indicates that the perforation de-
HU Yongjun et al. Effects of Perforation Number of Blade on Aerodynamic Performance of Dual-rotor Small Axial Flow Fans 127
(a) Model A
(b) Model B
Fig. 7
(a) Model A
(c) Model C
(d) Model D
(e) Model E
Distribution of static pressure on pressure side of pre-stage blades
(b) Model B
(c) Model C
(d) Model D
(e) Model E
Fig. 8 Distribution of static pressure on suction side of pre-stage blades
sign results the fluid flows through the perforations of blades from the pressure side to the suction side. Also it results the fluid leaks and the total pressure rising of fans decreases. Where, the more number of perforations, the more fluid leaks, and the more total pressure rising decreases. So the number of perforations on blades must be reasonable. Simultaneously, the suction sides of blades of model A have pressure rising along the swept direction. So the reflux and swirl may be formed on the surface of the blades when boundary layer develops to a certain extent. But due to perforation design, parts of fluid flows from the pressure side to the suction side, which makes the distribution of static pressure and velocity within the boundary layer of suction side change. Then the fluid within the boundary layer obtains new kinetic energy to overcome the frictional resistance. Thus, the generation of reflux is suppressed and reduced, and the occurrence of vortex shedding is inhibited, too. Except above mentioned, it also can be seen that the leading edge and top on the suction side of blades have low pressure region. With the increase of the number of perforations, the static pressure of low pressure region of the leading edge and top on the suction side of blades increase slightly. And the region of low pressure gradually decreases. Thus on the leading edge and top of blades, the pressure difference of pressure side and suction side is reduced. Thereby, the reflux on the leading edge and top of blades which flows from the pressure side to the suction side is reduced. Fig.9 shows the geometrical position of the rotative
surface in the blade tip clearance. The rotative surface is S1 stream surface. And its radius is 43.5mm. Fig.10 represents the distribution of vorticity of the rotative surface in the blade tip clearance (R=43.5mm) of small axial flow fans. As can be seen from Fig.10, the vorticity values in the blade tip are higher than that in other regions, the vorticity values at the trailing edge of blades are relatively high. Meanwhile, the vorticity values at the trailing edge of pre-stage blades of four perforated fans are all lower than that of model A. The centralized regions of them are also less than that of model A. Additionally, the vorticity value in the trailing edge of the pre-stage blades decreases with the increase of the number of perforations. While the centralized regions at the trailing edge of post-stage blades of four perforated fans are all higher slightly than that of model A. It indicates that the flow condition of pre-stage blades is improved by the pre-stage perforation, but the flow condition of post-stage blades becomes worse.
Fig. 9 Schematic view of geometrical position of the rotative surface in the blade tip clearance(R=43.5mm)
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number of perforations of model D is relatively good. At the monitoring point 8, the total SPL of model D lower than that of model A is about 2.7dB.
Fig. 11 Total sound pressure level of monitoring points of each model
Fig. 10 The distribution of vorticity of the rotative surface in the blade tip clearance
Based on the above analysis and combining with the formation mechanism of aerodynamic noise, it is concluded that the perforation design could suppress and reduce the generation of reflux, thereby suppressing the occurrence of vortex shedding which is the main factor of formation of broadband noise. So the perforation design could reduce aerodynamic noise of small axial flow fans. Aerodynamic noise The total sound pressure level (SPL) of eight monitoring points of each model at the optimum operating point Q=0.012 kg/s are shown in Fig.11. The horizontal axis is the axial position. From the figure, it is found that the total SPL at monitoring point 2 and 4 are higher than that at other monitoring points. It indicates that the noise at the blade tip clearance of rotor is higher than that of other positions. At the monitoring point 1, 2, 5, 6, 7 and 8, the total SPL of each perforated model are lower than that of model A. Moreover, the total SPL of the fans decreases with the increase of the number of perforations. But at the monitoring point 3 and 4, the total SPL of each perforated model are higher than that of model A. The total SPL of the fans increases with the increase of the number of perforations. Where, the total SPL of model B and C decline slightly. The total SPL of model D and E reduce relatively much. However, the static characteristics of model E decrease much. Combined with the static characteristics and aerodynamic noise into account, the
Fig.12 shows the power spectral density of eight monitoring points of each model at the optimum operating point Q=0.012 kg/s. It can be seen from the figure, the power spectral density at monitoring point 2 and 4 are higher than that of other monitoring points. It also indicates that the noise at the blade tip clearance of rotor is higher than that of other positions. At the monitoring point 1, 2, 5, 6, 7 and 8, the power spectral density of each perforated model are all lower than that of model A. And the power spectral density of the fans decreases with the increase of the number of perforations. However, at the monitoring point 3 and 4, the power spectral density of each perforated model are higher than that of model A, the power spectral density of the fans increases with the increase of the number of perforations. Therefore, the number of perforations can’t be too little, such as model B and C. Their noise reduction effect is not obvious. On the other hand, the number of perforations can’t be too much, such as model E. Although its noise reduction effect is better than that of other models, the static characteristics decrease more than other models. Moreover the noise at the blade tip clearance between two rotors (monitoring point 3) and at the blade tip clearance of post-stage rotor (monitoring point 4) increases relatively more. Due to perforation design of pre-stage blades, the fluid leaks through the perforation of blades from the pressure side to the suction side. It makes the flow in the middle of two rotors and at the post-stage rotor become more complex and the pressure pulsation increase. Thereby the noise of this part increases. So combined with the static characteristics and aerodynamic noise into account, the number of perforations of model D is relatively good. To sum up, the perforation design on blades reduces the aerodynamic noise of small axial flow fans. It is be-
HU Yongjun et al. Effects of Perforation Number of Blade on Aerodynamic Performance of Dual-rotor Small Axial Flow Fans 129
Fig. 12 Power spectral density of monitoring points of each model
130
cause that the perforated blades could suppress and reduce the generation of reflux, moreover suppress the occurrence of vortex shedding. Thus the broadband noise is reduced and the effect of noise reduction is better. However the number of perforations should be reasonable. In this paper, the number of perforations of model D is relatively good.
Conclusions In this paper, dual-rotor small axial flow fans with perforated blades are designed, and the effects of the number of perforations on blades on static characteristics and aerodynamic performance of dual-rotor small axial flow fans are studied by numerical simulation. The conclusions are summarized as follows: (1) The perforations on blades reduce the aerodynamic noise of the dual-rotor small axial flow fans. The noise of the fans decreases with the increase of the number of perforations. Additionally, the number of perforations can’t be too little. Otherwise, the effect of noise reduction is not obvious. On the other hand, the number of perforations can’t be too much. It is because too much number of perforations makes the static characteristics decrease too much and the noise in the middle of two rotors and at the post-stage rotor increase too much. In this paper, the number of perforations of model D is relatively good. At the monitoring point 8, the total SPL of model D lower than that of model A about 2.7dB. (2) The perforations on blades suppress and reduce the generation of reflux and the occurrence of vortex shedding on the suction side. Meanwhile, the reflux on the leading edge and top of blades which flows from the pressure side to the suction side is suppressed and reduced. It also reduces the vorticity value and the centralized regions of vorticity at the trailing edge of blades. Then the flow condition at the trailing edge of blades is improved and the effect of noise reduction is good. What’s more, the vorticity value at the trailing edge of pre-stage blades decreases with the increase of the number of perforations. (3) The perforations on blades decrease the static characteristics of dual-rotor small axial flow fans. Moreover the static characteristics of the fans decrease with the
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increase of the number of perforations. Therefore the number of perforations should be reasonable.
Acknowledgment This work was supported by National Natural Science Foundation of China (No. 51276172) and National Science and Technology Support Project (No.2013BAF05B01). Simultaneously, Natural Science Foundation of Zhejiang Province (No.LY14E060003), Zhejiang Province Key Science and Technology Innovation Team (2013TD18), and the Fluid Engineering Innovation Team of Zhejiang Sci-Tech University (No. 11132932611309) were also offering support to this research.
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