Experimental investigation of unsteady flow in axial skewed fans according to flow rates

Experimental Thermal and Fluid Science 48 (2013) 81–96

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Experimental investigation of unsteady flow in axial skewed fans according to flow rates Guangyuan Jin a,⇑, Hua Ouyang b, Zhaohui Du b,c a

School of Mechanical Engineering, Jiangnan University, China School of Mechanical Engineering, Shanghai Jiao Tong University, China c School of Aeronautics and Astronautics, Shanghai Jiao Tong University, China b

a r t i c l e

i n f o

Article history: Received 27 August 2012 Received in revised form 15 January 2013 Accepted 18 February 2013 Available online 27 February 2013 Keywords: Unsteady flow Axial fan Skewed blade Off-design condition Hot-wire anemometry

a b s t r a c t Unsteady flow in axial fans with circumferential skewed blades were measured at off-design conditions using a hot-wire test, to investigate the effect of sweep on aerodynamic limit and aeroacoustic sound source. Two circumferential skewed fans, with the blade skew angles at 8.3° forward and backward, respectively, were investigated in this study. The instantaneous flow field in the upstream and downstream of these fans was measured at off-design operations, using a two-dimensional (2D) probe based on a Constant Temperature Anemometry (CTA) system, and a NI PXI data collecting system. From the measured results, the characteristics of time domain and frequency domain were presented. The three-dimensional (3D) structure and turbulent characteristics of inlet and outlet flow in circumferential skewed fans were analyzed according to flow rate. Unsteady flow instability and its relation with circumferential skewed blades, to expand stall-free operation range and suppress noise of axial fans, were discussed. The sweep blade is found to be effective in controlling unsteady flow to improve aerodynamic limit and reduce noise source in these rotors at off-design operations. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction In the case of fan noise, skewed/swept blade as a passive noise reduction technique plays a very important role in aerodynamic and aeroacoustic performance improvement of axial compressors/ fans [1–4]. These rotors could have lower noise, wider stable operation range, and higher efficiency, etc. Skewed/swept blade alleviates the dominant source of noise, such as trailing-edge noise, turbulence-ingestion noise, and tip-clearance noise, which alters the spatial distribution of passage flow to modify the acoustic level so that the elementary noises are not generated simultaneously on one blade radius. As fluid measurement technique and computational fluid dynamics improvement, the detail flow information can be obtained to investigate the relation between the flow and noise to reveal mechanism of noise reduction in such turbomachines. The earlier works of Wright and Simmons [1], Cummings et al. [5], Hayden [6] and Mikkelson et al. [7] have shown that reasonable amounts of sweep may be very beneficial in improving aerodynamic performance and reducing the noise. Hanson [8] studied the problem primarily in terms of reduction of blade tonal noise through phase-shift cancellation of the noise generated at different radial locations. His work shows that very large angles of blade

⇑ Corresponding author. E-mail address: [email protected] (G. Jin). 0894-1777/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2013.02.011

sweep may be required, particularly for low-speed rotors. Fukano et al. [9] performed a flow measurement in the rotational frame. They measured the periodic velocity fluctuation in the downstream of the trailing edge of a rotating flat-plate blade using a hot-wire sensor attached to the originally designed measuring system. The result showed the important role of the periodic velocity fluctuation due to Karman vortex street in the generation of broadband noise. Kergourlay et al. [10] investigated experimentally the effect of different types of sweep on 3D structure of the downstream unsteady flow. They reported that an anisotropic character of the flow field was observed especially at the tip of the fans, and in addition to axial and tangential velocity, radial component must be collected for such turbomachines. Hurault et al. [11] investigated experimentally and numerically the sweep effect on three-dimensional flow downstream of axial flow fans. They reported that a second-moment closure turbulence model (RSM) is successfully used to obtain turbulence information generated by such fans, and radial velocity plays an important role, and cannot be negligible. Currently, experimental evidence and numerical analysis suggest that skewed and/ or swept blade may produce significant reduction of fan noise over a wide range of operating conditions. Unsteady flow in skewed and/ or swept rotors is responsible for a substantial proportion of the total losses and noise in turbomachines, particularly at off-design operations. However, no single mechanism can explain the noise reduction in skewed or swept rotors, and skewed or swept blades seem to affect many known aeroacoustic sound sources [12].

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Nomenclature C D E Fr LA k n N p Pt Q Qmin Q1 r R tk Tu U, V, W

velocity in fan coordinates endwall diameter (m) hot wire voltage (V) radial blade force (N) overall sound pressure level (dBA) yaw factor rotor speed (rpm) number of repeated sampling pressure (Pa) total pressure rise (Pa) volume flow rate (m3/s) minimum flow rate before flow separation (m3/s) flow rate at peak efficiency point (m3/s) radius (m) relative blade height turbulent kinetic energy turbulence intensity velocity components with respect to x, y, z coordinates (m/s) U1, U2; U 01 ; U 02 wire coordinates Ut tip rotation speed (m/s) Z number of blade

Therefore, additional investigation is still need, especially of the detail flow information at off-design conditions, such as the threedimensional unsteady flow in these rotors. In prior works [13–15], the unsteady flow and noise in tip clearance in circumferential skewed axial fans was investigated by PIV technique and wall pressure measurement at off-design operations. The emphasis of present experimental investigation is to acquire

a

angle between hot wire and x axis (°) blade stagger angle (°) circumferential skew angle (°) tip clearance (m) circumferential coordinate (°) hub–tip ratio m = rh/rt fluid density (kg/m3) reynolds stress tensor (m2/s2) flow deflection angle flow rate coefficient total pressure coefficient stable operating range

b dSK

e h

m q s r u wt D

Subscripts a, r, t axial, radial, and tangential direction cal calibration mean averaged value m meridian direction n normal direction 1 wire 1 of X hot wire 2 wire 2 of X hot wire

the effect of circumferential skewed blades on three-dimensional flow in the upstream and downstream of axial fans at off-design conditions, and to clarify improvement mechanism of stable operating range and reduced noise in these circumferential skewed rotors. A circumferential forward-skewed 8.3° rotor, a backwardskewed 8.3° rotor, and a Basic rotor were tested, where the circumferential skew is composited of sweep and dihedral. These fans have

Dihedral γ

rotation

Chord line Circumferential skew δ

θ Sweep λ

(a) Definition of skew, sweep and dihedral

C

A

δ sk

H

O

(b) Circumferential skew direction and angle Fig. 1. Circumferential skew definition.

Flow

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G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96 Table 1 Parameters of base blade profile. Span (%)

Blade stagger angle (°)

Chord (mm)

Pith of cascade (mm)

0 20 40 61.5 70 80 100

38.5

109 106 100 96 95 92 89

110 150.8 191.6 235.6 252.9 273.3 314.2

25

18.5

Forward-skewed blade

Blade tip

(a) CAT equipment setup

Backward-skewed blade Base blade

NI board Fig. 2. Circumferential skewed blade geometries.

CTA Computer

Table 2 Key design parameters of circumferential skewed rotors. Rotor speed n (r min1) Tip radius rt (m) Hub–tip ratio m Number of blade Z Blade stagger angle b (°) Stacking line Blade profile Blade thickness Tip clearance eR/rt Circumferential skewed angle of blade dsk (°) Endwall diameter D (m)

1440 0.2475 0.35 5 25 Straight line + arc Cambered plate 2 mm 1% ±8.3 0.5

2D hotwire probe Outlet side Measure area Inlet side

Cr Ca

Ct

Test rotor Phase locking Trigger

been observed with lower noise and wider stable operation range. The instantaneous velocity was measured with a hot wire Constant Temperature Anemometry (CTA) system at different flow rates (u = 0.235, u = 0.18, and u = 0.168, respectively). The three-dimensional flow structure and turbulence characteristic in blade passage were analyzed as flow rate decreases, especially for radial flow. The characteristics of boundary layer separation and blade loading was discussed based on radial equilibrium equation. The data analysis leading to averaged and turbulent velocities, the components of the Reynolds’ stress tensor and the turbulent kinetic energy was presented in order to illustrate the effect of these different

Electro-motor

(b) Schematic diagram of CTA experimental setup Fig. 4. Schematic diagram of CTA measurement.

circumferential skewed blades on the spatial instantaneous flow field under off-design operations. Velocity spectral analysis was also performed. The unsteady flow and its relation with circumferential skewed blades to expand stable operating range and to de-

Manometer

Manometer

Sound level meter

1m

Throttle screen 3D

0.75D

45 º axis Test rotor

Electro-motor

Honeycomb

Fig. 3. Schematic of experimental facility.

90 º Arc inlet

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G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

0.10

Peak pressure point

0.09 Design point

ψt

0.08 0.07

Basic_experiment Forward_experiment Backward_experiment

0.06

Fig. 5. Probe coordinates and wire coordinates.

0.05

crease noise source in axial fans were discussed. The analysis attempts to reveal the unsteady flow according to flow rate in axial fans with circumferential skewed blades, and could be helpful to further argument of improvement mechanism of the stable operation range and reduced noise by skewed and swept blades.

0.04 0.08

0.12

0.16

0.20

0.24

ϕ Fig. 8. Aerodynamic performance curves.

(a) Probe position 1

(b) Probe position 2

Fig. 6. Velocity decomposition in fan system.

2.0

2.4

E1 E2 2.2

1.0 0.5 Error(%)

2.0

E(V)

Error1 Error2

1.5

1.8

0.0 -0.5 -1.0

1.6

-1.5 1.4 0

5

10

15

20

25

30

-2.0 0

5

10

15 Ueff (m/s)

Ueff (m/s)

(a) Velocity calibration curve of X-probet

25

30

(b) Error distribution of curve fit 3

2.10 2.05

20

E1.E2 VS Angle

k1,k2,VS Angle

2 2.00 1

1.90 1.85 1.80

k1,k2

Voltage(V)

1.95

E1 E2

0

k1

-1

k2

1.75

-2

1.70 1.65 -50 -40 -30 -20 -10 0 10 Angle(deg)

20

30

40

50

-3 -40

-30

-20

-10

0

10

20

30

Angle(deg)

(c) E1 and E2 varieties with yaw angle

(d) k1 and 2 varieties with yaw angle

Fig. 7. Calibration of 55P62 hot wire probe.

40

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G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

3. Experimental setup and procedures

86

Basic rotor Forward-skewed rotor Backward-skewed rotor

84 82

Design point

Peak pressure point

LA (dB)

80 78 76 74 72 70 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28

ϕ Fig. 9. Overall sound pressure level of three rotors.

80 70 60 50

L h (dB)

85

40 Basic rotor Forward-skewed rotor Backward-skewed rotor

30 20 10 0 100

1000

10000

f (HZ) Fig. 10. 1/3 noise spectrum at the design point.

2. Circumferential skewed blades The skew/sweep blade is usually described as Fig. 1a. In this study, the circumferential skewed angle of the blade is defined as followings. Fig. 1b illustrates the stacking line HAC of the blade which is observed in the axial direction. It is composed of a straight-line segment and an arc segment, marked by HA and AC, respectively. Point ‘‘H’’ is located on the hub and point ‘‘C’’ is located at the blade tip. Point ‘‘A’’ is the intersection of the arc and the straight-line segment of the stacking line. Point ‘‘O’’ is located on the axis of the blade. The angle dSK between line OA and line OC is called the circumferential skew angle of the blade. If the angle dSK is positive, it is called a forward-skewed (FSK) blade. If it is negative, it is called a backward-skewed (BSK) blade. The base blade is a low pressure industrial fan blade with a straight stacking line and a uniform thickness profile. Parameters of the base blade profile are shown in Table 1. The circumferential skewed blades retain some design parameters, such as the chord and blade inlet angle. The same tip gap height is also retained to evaluate the skewing influences. The stacking line of the base blade is skewed in the circumferential direction at an angle of dSK = ±8.3° and from 40% span (point A in Fig. 1) to the blade tip. The blade geometries are presented in Fig. 2. These blades with 2 mm thickness are so thin that the skewing effect on the gap width is small. Table 2 summarizes the key design parameters of the skewed rotors.

3.1. Test rig description The low-speed axial flow fan facility used in this investigation is shown schematically in Fig. 3. The test apparatus consist of a circuit tunnel as well as velocity and pressure measurement equipment. The inlet flow can be kept steady and uniform. The flow rate could be adjusted easily from 0 to 2.5 m3/s. The rotating speed of the motor was set at 1440 rpm using a frequency converter. Both the aerodynamic and aeroacoustic experiments were carried out for each of the test rotors separately. The pressure rise and flow rate were measured in the bellmouth and the duct using conventional techniques (static pressure tubes). The measurement is taken at an ambient temperature of 300 K and about a standard atmospheric pressure. The static pressure is measured by a micro-manometer with a resolution of 0.5 Pa. The pressure difference between and inlet and outlet section is less than 300 Pa, so the density and temperature of the air can be considered as constant values. Aeroacoustic experiments were carried out in a semi-anechoic aerodynamic chamber, whose ambient noise level is below 20 dB and the cut-off frequency is 125 Hz. The instruments of sound pressure level measurements included precision integrating sound level meter type B&K 2230, pre-polarized condenser microphone cartridge type B&K 4155, and 1/3–1/1 octave filter set type B&K 1625. Sound pressure level meter type B&K 2230 accords with Chinese National Standard GB 3785–83, which is equivalent to IEC 651 type accuracy. The 1/3–1/1-octave filter set type B&K 1625 accords with standard IEC R 225-1966. The sound level meter was located at the rotor outlet side. The position was set at a horizontal distance of 1.0 m to the fan exit, an angle of 45° to axis, and at a height of 1.5 m above the ground. Performance parameters of the test rotors, which included the pressure rise, the flow rate, and the sound pressure level, were collected. In this study, three pair supporting struts were located at upstream of rotor with a distance of 100 mm, whose wakes are hardly enter the inlet region of rotor. Furthermore, there’s no discrete noise spectrum which may be produced by the supporting struts in velocity spectrum. So the effect of supporting struts of electric motor on the aerodynamic and acoustic performance due to rotor–stator interaction can be neglected.

3.2. CTA system According to the radial equilibrium hypothesis, a swept/skewed fan should present a 3D flow field: the radial component of absolute velocity should not be negligible, so that a 3D-description of flow field in circumferential skewed fan is needed. Axial component Ca, tangential component Ct and also radial component Cr of velocity vector must be measured. In this study, the unsteady flow field in the upstream and downstream of axial fan was quantified locally by using a hot wire Constant Temperature Anemometry (CTA) for its simplicity and rapidity, its ability to deliver accurate time series and broadband spectral signals. The complete measuring system is composed of a 55P62 hot wire probe, a 5H24 probe support, a Dantec streamline CTA anemometer, a National Instrument 6143 A/D converter board, a tachometer trigger (ONOSOKKI FS-540 and FG-1200) and LabVIEW software toolkit which enables sampling of the signals up to 250 kHz, shown schematically in Fig. 4. The hot wire probe (Dantec 55P62) was used to measure the components of instantaneous velocity. The sensor was made out of Pt-plated tungsten wire, having length and diameter of 1.25 mm and £5 lm, respectively. Measuring at two different angular positions (a rotation at a 90° angle is performed) enables measurement of the three

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G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

velocity components. Fig. 4 shows the experimental facility. In this experiment, the Dantec streamline CTA is only used as electric bridge output unit without considering the temperature correction. The CTA system was configured with a sampling frequency of 25,000 Hz (leading to 1060 samples per rotation). The number of samples taken was 106,000 (corresponding to 100 fan rotation periods), and 10 measurements were performed at each point.

in Fig. 5, so that the whole equation system (four equation) can be expressed in terms of components (U, V, W). 2

2

U 21cal  ð1 þ k1 Þ  cosð90  a1 Þ ¼ k1  U 21 þ U 22

ð1Þ

  2 2 U 22cal  1 þ k2  cosð90  a2 Þ ¼ U 21 þ k2  U 22

ð2Þ

The velocity component (U, V) under probe coordination:

U ¼ U 1 cos a1 þ U 2 cos a2 3.3. Measurement procedures

V ¼ U 1 sin a1  U 2 sin a2

The detailed methodology to get the complete 3D structure of the flow field using a 2D-probe is described in this Section. A Dantec 55P62 hot wire probe was generally used to measure the 2D components of the instantaneous velocity [16]. The 3D flow in probe coordinate system is characterized by its velocity components (U, V, W). Measuring at two different angular positions (a rotation at a 90° angle is performed) enables measurement of the three velocity components (U, V, W). Measurements are performed to get the wire coordinates (U1, U2) and  binormal component W in position 1. Wire coordinates U 01 ; U 02 and binormal component V are measured in position 2. The wire coordinates (U1, U2, U 01 ; U 02 ) are linearly dependent of the probe coordinates (U, V, W), as shown

Components U and U0 (velocity components in position 2) are supposed to be close so that the components of instantaneous velocity (Ca, Cr, Ct) in the fan coordinate system are expressed from (U, V, W) as following formulation.

C a ¼ U;

C t ¼ W;

ð4Þ

The components (U , W) can be obtained by rotating the probe coordination

U 0 ¼ U 01 cos a1 þ U 02 cos a2 W ¼ U 01 sin a1  U 02 sin a2

ð5Þ

1 Cr/Ut

Ca/Ut

0.8

0.3 0.29 0.28 0.27 0.26 0.25 0.24 0.23 0.22 0.21 0.2 0.19 0.18

0.7 0.6 0.5 0.4 0.3

0.9

0.7 0.6 0.5 0.4 0.3

0.2

0.2

0.1

0.1 0

0

100

200

0.14 0.13 0.12 0.11 0.1 0.09 0.08 0.07 0.06 0.05

0.8

R

0.9

R

C r ¼ V;

0

1

0

ð3Þ

300

0

100

200

300

phase (deg)

phase (deg)

(a) Axial velocity

(b) Radial velocity

1 Ct/Ut

0.15 0.14 0.13 0.12 0.11 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02

0.8 0.7

R

0.6 0.5 0.4 0.3 0.2

0.4

Backward rotor ϕ=0.235

R=0.65 outlet Ca

0.3

ps

Cmean /Ut

0.9

ss

0.2

wake Ct

0.1

Cr 0.0

tailing vortex

0.1 0 0

100

200

300

-0.1 -40

0

40

80

120 160 200 240 280 320 360 400

phase (deg)

phase (deg)

(c) Tangential velocity

(d)

Fig. 11. Phase-averaged velocity distribution at fan outlet (u = 0.235, BSK rotor).

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G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

where a1 = a2 = 45°. Velocity decomposition in fan system and two measurement positions are shown in Fig. 6.

The measurement area was limited to fan inlet and outlet. The hot-wire probe is positioned spanwise at 33 uniformly spaced

1

1 Ca/Ut

R

0.6

0.4

Ca/Ut

0.28 0.27 0.26 0.25 0.24 0.23 0.22 0.21 0.2 0.19 0.18 0.17 0.16

0.8

0.6

R

0.28 0.27 0.26 0.25 0.24 0.23 0.22 0.21 0.2 0.19 0.18 0.17 0.16

0.8

0.4

0.2

0.2

0 0

100

200

0

300

0

100

phase (deg)

200

300

phase (deg)

ϕ =0.168

(a) Axial velocity ϕ =0.18 1

1 Cr/Ut

R

0.6

0.4

0.6

0.4

0.2

0.2

0

0.14 0.13 0.12 0.11 0.1 0.09 0.08 0.07 0.06 0.05

0.8

R

0.14 0.13 0.12 0.11 0.1 0.09 0.08 0.07 0.06 0.05

0.8

Cr/Ut

0

100

200

0

300

0

100

phase (deg)

200

300

phase (deg)

ϕ =0.168

(b) Radial velocity ϕ =0.18 1

1 Ct/Ut

0.8

R

0.6

0.4

0.2

Ct/Ut

0.6

0.4

0.2

0

0 0

100

200

300

phase (deg)

(c) Tangential velocityϕ =0.18

0.15 0.14 0.13 0.12 0.11 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02

0.8

R

0.15 0.14 0.13 0.12 0.11 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02

0

100

200

phase (deg)

ϕ =0.168

Fig. 12. Phase-averaged velocity distribution at fan outlet (BSK rotor).

300

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G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

(a)

A Base rotor

B FSK rotor

1

C BSK rotor

1

1

Ca/Ut

Ca/Ut Ca/Ut

R

0.6

0.4

0.8

0.6

0.4

0.2

0 120

0.29 0.28 0.27 0.26 0.25 0.24 0.22 0.21 0.2 0.19 0.18 0.17 0.16

0.8

0.6

160

180

200

0 120

220

0.22 0.21 0.2 0.19 0.18 0.17 0.16

0.4

0.2

140

0.29 0.28 0.27 0.26 0.25 0.24 0.23

R

0.8

R

0.29 0.28 0.27 0.26 0.25 0.24 0.23 0.22 0.21 0.2 0.19 0.18 0.17 0.16

0.2

140

160

180

200

0 120

220

140

160

180

200

220

phase (deg)

phase (deg)

phase (deg)

(a) Axial velocity

(b) 1

1

1

Cr/Ut

0.4

0.8

0.05 0.04 0.03 0.02 0.01 0 -0.01 -0.02 -0.03 -0.04 -0.05

0.6

0.4

0.8

0.6

0.4

0.2

0.2

0.2

0 120

140

160

180

200

0 120

220

phase (deg)

0.13 0.12 0.11 0.1 0.09 0.08 0.07 0.06 0.05

R

R

0.6

Cr/Ut

Cr/Ut

0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0

R

0.8

140

160

180

200

0 120

220

140

160

180

200

220

phase (deg)

phase (deg)

(b) Radial velocity 1

1

1

Ct/Ut

0.18 0.17 0.16 0.15 0.14 0.13 0.12 0.11 0.1 0.09 0.08

R

0.6

0.4

0.2

0 120

Ct/Ut 0.18 0.17 0.16 0.15 0.14 0.13 0.12 0.11 0.1 0.09 0.08

0.6

0.4

160

180

phase (deg)

200

220

0 120

0.18 0.17 0.16 0.15 0.14 0.13 0.12 0.11 0.1 0.09 0.08

0.6

0.4

0.2

0.2

140

Ct/Ut

0.8

0.8

R

0.8

R

(c)

140

160

180

200

phase (deg)

220

0 120

140

160

180

200

220

phase (deg)

(c) Tangential velocity Fig. 13. Phase-averaged velocity distribution in single passage of three sweeps rotors outlet (u = 0.18).

points (relative blade height R 2 {0.014–0.97}, interval 5 mm) in the upstream and downstream of the fans, 10 mm away from the leading and trailing edge of the airfoil (about 10% of blade tip

chord), parallel to the fan plane, in the alignment of mean flow direction which corresponds to absolute angle at the midspan of leading and trailing edge.

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G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

0.6

R =0.1 R =0.25 R =0.43 R =0.65 R =0.80 R =0.9

0.4

0.6

TuCa

TuCa

0.5

0.3

0.7

0.7

Basic rotor outlet ϕ =0.235

R=0.9

Basic rotor outlet ϕ =0.18

R=0.1

0.5

0.5

0.4

0.4

0.3

R=0.65 R=0.25

R=0.43

R=0.1

R=0.8

0.0

40

0.0 0

80 120 160 200 240 280 320 360

R =0.9 R=0.8

0.1

0.0 0

R=0.65

R=0.9

0.1

0.1

R =0.25

0.3 0.2 R =0.43

0.2

0.2

Basic rotor outlet ϕ =0.168

R=0.1

0.6

TuCa

0.7

40

80 120 160 200 240 280 320 360

0

40

80 120 160 200 240 280 320 360

phase (deg)

phase (deg)

phase (deg)

(a) Base rotor (ϕ =0.235, ϕ =0.18, and ϕ =0.168) 0.7

Forward rotor outlet ϕ =0.18

0.6

0.6

0.5

0.5

0.4 R=0.1

R=0.8

TuCa

TuCa

0.7

R=0.9

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

Backward rotor outlet ϕ =0.18

R=0.1

R=0.9

0.0 0

40

80 120 160 200 240 280 320 360

0

40

80 120 160 200 240 280 320 360

phase (deg)

phase (deg)

(b) FSK rotor (ϕ =0.18)

(c) BSK rotor (ϕ =0.18)

Fig. 14. Turbulence intensity of axial velocity at fan outlet.

R=0.8

Tu (Ca)

R=0.1

0.2 0.1 0

40

80

120

160

200

240

280

320

0.2 0.1

R=0.9

0.3

0.3

0.0

0.3

0.0 0.4

Basic rotor Forward rotor Backward rotor

R=0.25

Tu (Ca)

Tu (Ca)

Tu (Ca)

0.4 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.4

360

0.2 0.1 0.0 0

40

80

120

160

200

phase (deg)

phase (deg)

(a) near hub

(b) near tip

240

280

320

360

Fig. 15. Turbulence intensity distribution of axial velocity at fan outlet (u = 0.18).

The probe was calibrated in a free wind jet before and after the test in order to control its drift and to accurately measure velocities up to 30 m/s. The Velocity calibration curve and Error distribution of curve fit are shown in Fig. 7a and b. The yaw factor k1, k2 for wire 1 and wire 2 are obtained by Eq. (1). The characteristic of wire voltage E1 & E2, k1 and k2 under different yaw angle is shown in Fig. 7c and d.

N¼9 X  ¼1 k k1N ; 1 N N¼1

N¼9 X  ¼1 k k2N 2 N N¼1

ð6Þ

3.4. Measurement uncertainties Uncertainties in the hot-wire measurements have several origins that can globally be classified as precision errors of the probe (probe sensitivity and anemometer drift), systematic errors of the

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2.5

2.5 Backward rotor

ϕ =0.235 ϕ =0.18 ϕ =0.168

τ13 (m /s )

2.0 1.5

2

1.5

2

2

2

τ12 (m /s )

2.0

ϕ =0.235 ϕ =0.18 ϕ =0.168

Backward rotor

1.0

1.0

0.5 0.0

0.5

0.4

0.5

0.6

0.7

0.8

0.9

0.0

1.0

0.4

0.5

0.6

0.7

R

R

(a) τ12

(b) τ13

0.8

0.9

1.0

Fig. 16. Comparison of s12 and s13 for BSK rotor according to flow rate.

2.5

1.5

2

2

1.5

2

2

Basic rotor Forward rotor Backward rotor

ϕ =0.18

2.0

τ13 (m /s )

2.0

τ12 (m /s )

2.5

Basic rotor Forward rotor Backward rotor

ϕ =0.18

1.0

0.5

0.5

0.0

1.0

0.0 0.4

0.5

0.6

0.7

0.8

0.9

0.4

1.0

0.5

0.6

0.7

R

R

(a) τ12

(b) τ13

0.8

0.9

1.0

60

25 20 15 10 5 0 30 25 20 15 10 5 0 40

Basic rotor Forward rotor Backward rotor

ϕ=0.235

ϕ=0.18

ϕ=0.18

Backward rotor R=0.65

20 0 -20

ϕ=0.168

30

tk

40

Veloctiy (dB)

tk

tk

Fig. 17. Comparison of s12 and s13 for three rotors at u = 0.18.

-40

20 -60

10 0

0.4

0.5

0.6

0.7

0.8

0.9

100

1.0

R

1000

10000

frequency (Hz) Fig. 19. Spectrum of instantaneous velocity Ca downstream.

Fig. 18. Turbulent kinetic energy tk.

this measurement is analyzed by root sum square method [10], and estimated at 95% confidence level. procedure (near-wall effects, misalignment, conversion errors during calibration and measurement, fan and probe support vibrations) and fortuitous variations on the facility (changes in fluid properties between calibration and measurement), as well as uncertainty on the flow direction. The ambient temperature was considered as a constant, so uncertainty of a fluctuating temperature in the facility was not taken into account. The uncertainty in

4. Results and discussion In the case of turbulent flow through the turbomachine, instantaneous velocity C can be separated in two parts: the periodic fluctuation due to the blade passage, the so-called phase averaged velocity C, and the random fluctuation due to turbulence C0 , whose

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G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

range D. As shown in Fig. 8, the total pressure coefficient wt of circumferential skewed rotors is low, compared with the base rotor, especially for that of BSK rotor. The stable operation range of FSK rotor and BSK rotor is 42% and 36% respectively. A shift of the stall margin towards a lower flow rate is found in FSK rotor. In these experiments, when the drastic fluctuation of measured fan pressure rise occurred, the volume flow rate Q was considered to be the limit of stable operation range, Qmin.

40

Backward rotor R=0.65

averaged instant

Veloctiy (dB)

30

20

10

Q

u¼ 0

-10

wt ¼ 0

100

200

300

400

500

frequency (Hz)

D¼

Fig. 20. Spectrum of instantaneous and averaged Cr velocity downstream.

average equals zero. The periodic part is the discrete frequency component specific to the blade passage frequency and its harmonics. It originates from the periodic unsteady forces due to the interaction between the rotor blades and their environment. The random component is the broadband signal which is mainly due to turbulence phenomena around blade airfoil. Flow mean velocity Cmean, turbulence intensity Tu, Reynolds tensor sij, and turbulent kinetic energy tk can be obtained based on Eq. (7). All the analysis is in the fixed frame. All CTA measurements were conducted at operations u = 0.235, u = 0.18, and u = 0.168.

ð8Þ

put r2t Pt

ð9Þ

qu2t

Q 1  Q min Q min

ð10Þ

Fig. 9 shows the overall sound pressure level (SPL) LA of circumferential skewed rotors. Within the stable operation range, the SPL for BSK rotor tends to have a relative minimum at design condition, increase from design point to peak pressure point, and then decrease after peak pressure point. However, for BSK rotor, it tends to monotonously decrease. Fig. 10 shows the 1/3 octave spectrum of circumferential skewed rotors at the design condition. The noise frequency domain is from 100 Hz to 10,000 Hz. This indicates that the broadband noise is dominant in the measured spectrums. The SPL of FSK rotor is low compared with BSK rotor except in the frequency domain of [6000 Hz, 10,000 Hz]. 4.2. Unsteady flow structure in blade passage

N 1X Ci N 1 vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N u 1 X Tu ¼ t ðC i  C mean Þ2 =C mean N1 1

C mean ¼

tk ¼

 1  02 q C a þ C 02r þ C 02t 2

4.1. Fans characteristics Fig. 8 represents the non-dimensional aerodynamic performance curves of the circumferential skewed rotors. Where flow rate coefficient u, total pressure coefficient wt and stable operation

R=0.97

R=0.97

40

R=0.9

R=0.8 R=0.65 R=0.43

Forward rotor 30

Velocity (dB)

Basic rotor

R=0.9

R=0.31 R=0.25 R=0.1

20 10

Backward rotor

R=0.65

R=0.65

30

R=0.31 R=0.25 R=0.1

R=0.9

40

R=0.8

R=0.8 R=0.43

20 10

R=0.43

30

R=0.31 R=0.25 R=0.1

20 10

0 0.97

0 0.97

0 0.97

R

R

R

0.1

0.1 200 400 600 800 1000

R=0.97

40

200 400 600 800 1000

0.1 200 400 600 800 1000

frequency (Hz)

frequency (Hz)

frequency (Hz)

(a) Base rotor

(b) FSK rotor

(c) BSK rotor

Fig. 21. Spectrum of averaged axial velocity Ca downstream.

Velocity (dB)

sij ¼ qC 0i C 0j

Velocity (dB)

ð7Þ

In attempts to understand the evolution of three dimensional flow within circumferential skewed blade passage at off-design operations, the detail flow structures in passage of the three sweeps at rotor downstream are captured, as shown as Figs. 11– 13. In this section, The instantaneous velocity (Ca, Cr, Ct) represents phase averaged velocity, and is dimensionless by blade tip velocity Ut, measured at various phase angles. Figs. 11 and 12 show three dimensional velocity distribution at various phase angles at the whole blade passages in BSK rotor at flow rate u = 0.235, u = 0.18, and u = 0.168. The passages present a 72° periodicity observed in three components. For the backward-skewed blades, the five blade passages give velocity deficit laws which vary slightly from blade to blade. This can be related to the geometric non-uniformity of the blades and casing. The same observation has been

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G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

made in the flow field measurements in sweep fans by Kergourlay et al. [10]. The evolution of three-dimensional flow within the blade passage of BSK rotor is observed to investigate the fluid pattern close to shroud and hub according to flow rate. As the flow decreasing, the main flow of axial velocity shrinks and dominates in mid span, and the lower energy region enlarges in the vicinity of shroud and hub, and moves towards blade pressure surface side, which indicates the unstable flow occurs; velocity in wake region decreases significantly, and wake region is broadened; a reduction in the secondary flow region on blade the suction surface side is expected. As shown as radial velocity distribution, velocity increases in wake region, which appears that the strong radial flow weakens the thickness of blade boundary layer on blade suction surface side, and lessens the accumulated flow near shroud. As shown as tangential

velocity distribution, velocity increases from blade rear to mid span; lower energy flow occurs near hub, and wake region is broadened. To compare the effect of circumferential skewed blades, the detailed structures of single passage flow at fan outlet at u = 0.18 are shown by Fig. 13. The phase angles ranges from 120° to 220°, which includes a whole single passage. Compared with Basic rotor, the blockings of axial velocity in circumferential skewed rotors expand slightly near shroud and hub, especially in FSK rotor, which appears to improve aerodynamic properties. The radial velocity distribution is controlled by circumferential skewed blades. In FSK rotor, the radial flow moves towards the hub on side of blade pressure surface, but towards the hub on side of blade suction surface, and varies slightly with flow rate decreasing. But in BSK rotor, the radial flow only moves towards the shroud and increases sig-

40

40

Basic rotor FSK rotor BSK rotor

Basic rotor FSK rotor BSK rotor

R=0.94

30

Velocity (dB)

Velocity (dB)

R=0.25

20

10

30

20

10 100

200

300

400

500

0

100

frequency (Hz)

200

300

400

500

frequency (Hz)

(a) Axial velocity spectrum 40

40

Basic rotor FSK rotor BSK rotor

R=0.1

30

Velocity (dB)

Velocity (dB)

30

Basic rotor FSK rotor BSK rotor

R=0.97

20

20

10

10

0

0 0

100

200

300

400

500

0

100

frequency (Hz)

200

300

400

500

frequency (Hz)

(b) Radial velocity spectrum 40

40

30

Velocity (dB)

Velocity (dB)

30

20

20

10

10

0

Basic rotor FSK rotor BSK rotor

R=0.97

Basic rotor FSK rotor BSK rotor

R=0.1

0

100

200

300

400

500

0

0

100

frequency (Hz)

200

300

frequency (Hz)

(c) Tangential velocity spectrum Fig. 22. Distribution of averaged velocity spectrum near blade rear and tip.

400

500

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G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

Basic rotor

0.7

Backward rotor

Forward rotor

R=0.97

R=0.1 R=0.25 R=0.31 R=0.43 R=0.65 R=0.8 R=0.9 R=0.97

R=0.1

0.6 0.5

R=0.97

R=0.1

Tu(Ca )

nificantly with flow rate decreasing. For tangential velocity, a higher absolute velocity region exists near hub, especially in Basic rotor, which moves towards mid blade span. The above discussion shows that, the passage flow is controlled by circumferential skewed blades at off-design conditions. The aerodynamic performance in the vicinity of shroud and hub is improved by lessening the flow blocking, restraining the unstable flow, and reducing inner flow losses, where the boundary layer on the side of blade suction surface are controlled by the circumferential skewed blades, and moves towards where is benefit to lower the loss.

0.4

R=0.97 R=0.9 R=0.1 R=0.9

0.3 0.2

R=0.9

0.1

4.3. Effect of skewed blades on boundary layer

0.0

2

1 @p C 2t W 2m dW m sin r cos r  ¼ þ þ Fr q @r r rm dm 2

120

240

360 0

120

240

3600

120 240 360

phase (deg)

(a) Axial turbulence intensity Basic rotor

0.4

0.3

Tu(Ct )

In above discussions, as the flow rate decreases, the three dimension flow is controlled by circumferential skewed rotors, especially for radial flow, which works on the boundary layer intensively at off-design operations. Though the boundary layer has not been determined explicitly in this work, the effect of skewed blades on its thickness and influence would be discussed based on radial equilibrium theory, considering the radial component (Fr) of the force F acting on the inner flow by the circumferential skewed blades. The radial equilibrium equation includes the radial component (Fr) of this force F as the following equation:

0

R=0.1 R=0.97 R=0.9

Forward rotor

R=0.1 R=0.97 R=0.9

Backward rotor

R=0.1 R=0.97

R=0.9

0.2

ð11Þ 0.1

where op/or is radial pressure gradient, C 2t =r is centrifugal force, W 2m =rm cos r is radial component of centrifugal force produced by the variation streamline curvature in meridian plane, 2 0:5 sin rdW m =dm is the radial component of inertia force produced by accelerated flow in meridian plane. For axial fan/compressor with the low pressure rise and small flow angle, the equation can be expressed as the following equation:

0.0

0

120

240

360 0

120

240

3600

120 240 360

phase (deg)

(b) Tangential turbulence intensity Fig. 23. Turbulence intensity distribution of axial velocity (u = 0.18).

ð12Þ

By the Eq. (12), op/or in circumferential skewed blade passage is decided by centrifugal force C 2t =r and radial force Fr. C 2t =r is always positive and point towards shroud. For the specified rotor, Fr and tangential velocity are specific. The magnitude of Fr is the direct ratio of circumferential skewed angle, and the direction of Fr is decided by the circumferential direction. For circumferential forward-skewed blade, Fr points to rational axis, while for circumferential backward-skewed blade, it points to shroud. Therefore for FSK rotor, Fr is contrary to centrifugal force, which might cause opposite or negative radial pressure gradient. As flow rate decreasing, as shown in Fig. 12, C 2t =r increases significantly in most of blade span (R > 0.2), which suggests the increasing centrifugal force caused by tangential flow. When the Fr is not enough large to overcome this force, the radial pressure gradient is positive, and the low energy flow in boundary layer still moves towards the blade tip and accumulates near the shroud. When Fr is equal to this centrifugal force, the effect on the radial movement of boundary layer is slight. When Fr is enough large, the radial pressure gradient is negative, the low energy flow in boundary layer moves towards the blade rear and the loss rearranges along the blade span. For BSK rotor, Fr points to blade tip, same to centrifugal force, which only causes positive radial pressure gradient. Whatever the magnitude of Fr is, the boundary layer always moves towards shroud. At off-design conditions, this rapid movement accelerates the transfer of low energy flow, and thins the boundary layer. Though accumulated flow near shroud is worsen, but the higher

60

Forward rotor ϕ=0.18

40

Velocity (dB)

1 @p C 2t ¼ þ Fr q @r r

R=0.65

20 0 -20 -40 -60

100

1000

10000

frequency (Hz) Fig. 24. Spectrum of instantaneous Ca at inlet (R = 0.65, u = 0.18).

velocity main flow can take the accumulated flow into the main flow. In this study, the skew feature of the circumferential skewed blades occurs from 40% of blade span to blade tip. Therefore, there is only the centrifugal force acting on the boundary layer flow up to 40% of blade span, which forces the low energy fluid on blade suction surface side moving towards blade tip. At flow rate u = 0.18, as shown in Fig. 13b, for FSK rotor, though the centrifugal force in-

G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

Forward rotor

20 10

Velocity (dB)

Basic rotor

30

R=0.97 R=0.9 R=0.8 R=0.65 R=0.43 R=0.31 R=0.25 R=0.1

30 20 10

40

Backward rotor 30 20 10

0

0

0

-10 0.97

-10 0.97

-10 0.97

R

R 200 400 600 800 1000

40

R

0.1

0.1

0.1 200 400 600 800 1000

200 400 600 800 1000

frequency (Hz)

frequency (Hz)

Velocity (dB)

40

Velocity (dB)

94

frequency (Hz)

(a) Axial velocity

20 10

30 20 10

0

0

-10 0.97

-10 0.97

40

Backward rotor

10 0 -10

0.97 R

0.1

0.1

200 400 600 800 1000

frequency (Hz)

frequency (Hz)

30 20

R

R 200 400 600 800 1000

40

Velocity (dB)

30

Velocity (dB)

Forward rotor

Velocity (dB)

R=0.97 R=0.9 R=0.8 R=0.65 R=0.43 R=0.31 R=0.25 R=0.1

40

Basic rotor

200 400 600 800 1000

0.1

frequency (Hz)

(b) Tangential velocity Fig. 25. Spectrum of instantaneous velocity at inlet.

creases, the radial flow on the blade suction surface side is slight, which contributes to reduce accumulated flow near shroud; for BSK rotor, as shown in Fig. 13c, Fr always accelerates the boundary layer flow towards shroud, and the accumulated flow near shroud is taken away by main flow. 4.4. Effect of skewed blades on outlet turbulent flow In this section, the influence of circumferential skewed blades on the turbulent flow in fan downstream at off-design conditions is discussed. Fig. 14a shows evolution of turbulence intensity for Basic rotor according to flow rates. The intensity increases significantly in wake region and is larger than that of other phases, and its distribution appears to strong periodic fluctuation in whole passage. When flow rate decreases, the intensity increases near shroud and hub, especially for blade rear, and the periodic fluctuation decreases. This gives an evidence that the unsteady behavior is the main characteristic near hub and shroud of Basic rotor at off-design conditions. Compared with Basic rotor, the intensity of circumferential skewed fans at u = 0.18 is shown in Fig. 14b and c. The results indicate the magnitude of periodic fluctuating at peak pressure operation is lessened, and the instantaneous characteristic in hub and tip region of the skewed rotor is stronger than that of other blade height. This can be observed clearly in Fig. 15, which shows turbulence intensity distributions of axial velocity R = 0.1, 0.25, 0.8, and 0.9 in three rotors. The FSK and BSK design appears to be is especially effective in reducing the turbulent fluctuating in hub region. In order to quantify the 3D unsteady flow field in the time and frequency domains, Reynolds’ stress tensor sij and the turbulent kinetic energy tk are evaluated to represent its variations in

spatial domain. In this experiment, Reynolds’ stress tensor of axial and radical fluctuation velocity, and axial and circumferential fluctuation velocity, separately, s12 and s13, are obtained, as shown in Figs. 16 and 17. Fig. 16 shows a comparison of Reynolds’ stress tensor s12 and s13 for BSK rotor according to flow rate. s12 is regular over the entire radius, and increases up to 1.25 m2/s2 as flow rate decreases. s13 is predominant near the hub and the tip and increases significantly according flow rate. Fig. 17 shows that Reynolds’ stress tensor s12 and s13 of circumferential skewed rotors at flow rate u = 0.18. Compared with Basic rotor, s12 significantly increases in all radiuses, but s13 decreases near the hub, which indicates that radical component is not negligible and flow field is anisotropy. FSK rotor has the lower turbulent velocity Reynolds’ stress tensor in all radiuses. The turbulent kinetic energy tk according to flow rate is present in Fig. 18. As flow rate decreasing, the turbulent character is predominant near the hub and the tip, which of Basic rotor increases significantly near the hub. The low and regular character is found in FSK rotor in all radiuses. 4.5. Outlet flow velocity spectral analysis In this section, the outlet flow velocity spectra obtained by processed data for circumferential skewed rotors at flow rate u = 0.18 is presented. The sampling frequency of fs = 25,000 Hz enables a spectral analysis of unsteady velocity measurement up to 12,500 Hz. All the spectra are expressed in decibels (dB) using the formula: 20 log 10|C/C0| with the reference velocity C0 = 1 ms1. Fig. 19 resents the spectrum of the instantaneous velocity Ca downstream of BSK rotor at R = 0.65 in the frequency range [10 Hz, 10 kHz]. It shows that the broadband signal occurs

G. Jin et al. / Experimental Thermal and Fluid Science 48 (2013) 81–96

at all frequencies whereas the discrete signal appears in the form of fundamental frequency (24 Hz) and its harmonics up to 1000 Hz beyond which their amplitude is less than the broadband signal. Fig. 20 shows the radical instantaneous and averaged Cr velocity spectrum on a frequency band ranging from 0 to 500 Hz in the downstream of BSK rotor at R = 0.65 at flow rate u = 0.18. The fundamental frequency and its harmonics become more visible when approaching the tip (R = 0.8–0.9), because of the shroud and the higher fan momentum, and the geometrical differences between the blades. The broadband amplitude is around 10 dB. Fig. 21 presents the spectrum spanwise evolution for ensemble average axial velocity of three rotors. The signal spectrum intensity increases approaching the hub and tip, which indicates a growing turbulent signal level, as shown in Fig. 21. The spectrum difference between three rotors is observed near blade rear and tip, whereas a spectrum comparison of Ca, Cr, Ct is made at R = 0.1, 0.25, 0.94, and 0.97 as shown in Fig. 22. In circumferential skewed rotors compared with Basic rotor, spectrum intensity of Ca decreases at R = 0.25, especially in FSK rotor; it increases at R = 0.1 for Cr and Ct, especially in FSK; near blade tip at R = 0.94 and R = 0.97, it increases for Ca and Ct, but decreases for Cr. This indicates that the effect of circumferential skewed blades focuses on increasing broadband spectrum near blade rear and tip at off-design condition. The phase shift thus produced results in destructive or constructive interferences of spanwise components and consequently in a modification of the radiated noise. 4.6. Effect of skewed blades on inlet turbulent flow When flow rate decreases, the inlet unsteady flow in skewed rotors is more turbulent. In this section, the effect of skewed blades on inlet turbulent flow under off-design condition (u = 0.18) is discussed based on turbulence intensity and velocity spectrum. The spanwise distributions of turbulence intensities of Ca and Ct for circumferential skewed rotors at flow rate u = 0.18 are shown in Fig. 23. It appears that the inlet flow is the most turbulent in BSK rotor, and its turbulence intensity increases up to 0.3 near blade rear and tip at phase positions corresponding to the blade trailing edges. Whereas its magnitudes at phase positions for the main passage are lower than that of Basic rotor and FSK rotor. The inlet flow in FSK rotor appears to be slight turbulent at off-design condition, which indicates that forward skewed blade is efficient to control inlet turbulent flow. Fig. 24 presents a spectrum of the instantaneous velocity Ca at the inlet of FSK rotor at R = 0.65 in the frequency range [10 Hz, 10 kHz]. It shows that the broadband signal occurs at all frequencies whereas the discrete signal appears in the form of blade passing frequency (120 Hz) and its harmonics up to 1000 Hz, beyond which harmonics amplitude is less than the broadband signal. The spectrum of instantaneous velocity Ca and Ct at the inlet of circumferential skewed rotors in frequency range [10 Hz, 1000 Hz] at flow rate u = 0.18 are shown in Fig. 25. The higher intensity for the peaks of the discrete signal is observed in Basic rotor. The higher intensity for broadband signal is observed near blade rear and tip, especially for Basic rotor, which indicates that the circumferential skewed blades are benefit to restrain the turbulent inlet flow, especially for FSK blade in shroud and hub region. 5. Conclusions This work presents some measurement results to investigate the unsteady velocity fluctuations in axial flow fans with circumferential skewed blades at off-design operations. The

95

instantaneous flow field in the upstream and downstream of these rotors at different conditions was measured using a 2D probe based on a Constant Temperature Anemometry (CTA) system, and a NI PXI data collecting system. The time domain and frequency domain characteristics were presented. These results clearly indicate the influence of blade sweep on the 3D unsteady flow, which contributes to aerodynamic and aeroacoustic characteristics. A 3D flow structure in circumferential skewed fans was observed according to flow rates. The flow structure and its relation with circumferential skewed blades to extend the stable operating range of fans were discussed. Effect of skewed blades on boundary layer at lower flow rate was discussed using blade radial force Fr based on radial equilibrium equation. The aerodynamic performance in the vicinity of shroud and hub in circumferential skewed rotors is improved by lessening flow blocking, restraining stable flow, and reducing inner flow losses; the radial flow cannot be neglected in these rotors, which forces boundary layer moving towards where is benefit to lower the loss. As flow rate decreases, though centrifugal force increases, the effect of blade radial force Fr on radial pressure gradient is different in circumferential skewed rotors, which controls the migration pattern of boundary layer. The forward-skewed blade is found to be effective in controlling low energy flow in the vicinity of shroud and hub to expand stall-free operation range. The skewed blade has a great influence on the downstream turbulence intensity and velocity spectrum, which is directly related to acoustic signature. Whereas the forward skewed blade appears to be the best adapted to decrease the discrete and broadband signal. The time and frequency results indicate that the flow is random and less organized in the case of BSK rotor. The spanwise variations of turbulent velocity Reynolds’ tensor for FSK rotor are the lowest, the most regular and smoothest. This clearly indicates the advantage of using this kind of sweep to get a less noisy 3D structure in rotor wake under off-design conditions; and the effect of circumferential skewed blades focuses on increasing broadband spectrum near blade rear and tip at off-design operations. The phase shift thus produced results in destructive or constructive interferences of spanwise components and consequently in a modification of the radiated noise. The inlet turbulent flow is observed in circumferential rotors, and the intensity of broadband velocity spectrum is lessened near blade rear and tip, which shows that the circumferential skewed blades are benefit to restrain the turbulent inlet flow, especially for FSK blade in shroud and hub region. Acknowledgement The authors would like to acknowledge the support of the Fundamental Science Special Foundation for Central Universities of China (Grant No. JUSRP111A17). References [1] T. Wright, W.E. Simmons, Blade sweep for low-speed axial fans, J. Turbomachinery 112 (1990) 151–158. [2] M.G. Beiler, T.H. Carolus, Computation and measurement of the flow in axial flow fans with skewed blades, J. Turbomachinery 121 (1999) 59–66. [3] A. Corsini, F. Rispoli, Using sweep to extend the stall-free operational range in axial fan rotors, IMechE, Part A: J. Power Energy 218 (2004) 129–139. [4] J. Vad, A.R.A. Kwedikha, H. Jaberg, Effects of blade sweep on the performance characteristics of axial flow turbomachinery blades, IMechE, Part A: J. Power Energy 220 (2006) 737–751. [5] R.A. Cumming, W.B. Morgan, R.J. Boswell, Highly skewed propellers, Trans. SNAME 80 (1972) 98–135. [6] R.E. Hayden, Some advances in design techniques for low noise operation of propellers and fans, paper, In Noise-Con77 Proceedings, NASA Langley Research Center, Hampton, 1977.

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[7] D.C. Mikkelson, G.A. Mitchell, L.J. Bober, Summary of Recent NASA Propeller Research, NASA, Tech Pap, TM83733, 1984. [8] D.B. Hanson, Near-field frequency-domain theory for propeller noise, AIAA J. 23 (1984) 499–504. [9] T. Fukano, H. Saruwatari, H. Hayashi, H. Isobe, M. Fukuhara, Periodic velocity fluctuation in the near wake of a rotatingflat-plate blade and their role in the generation of broadband noise, J. Sound Vib. 181 (1995) 53–70. [10] G. Kergourlaya, S. Kouidri, G.W. Rankina, R. Rey, Experimental investigation of the 3D unsteady flow field downstream of axial fans, Flow Meas. Instrum. 17 (5) (2006) 303–314. [11] J. Hurault, S. Kouidri, F. Bakir, R. Rey, Experimental and numerical study of the sweep effect on three-dimensional flow downstream of axial flow fans, Flow Meas. Instrum. 21 (2010) 155–165.

[12] Th. Carolus, M. Beiler, Skewed Blades in Low Pressure Fans: A Survey of Noise Reduction Mechanisms, paper, in: AIAA Conference, 1997–1591. [13] H. Ouyang, Y. Li, Z.H. Du, Computational and experimental study on tip leakage vortex of circumferential skewed blades, J. Chin. J. Mech. Eng 20 (2007) 579– 586. [14] G.Y. Jin, H. Ouyang, B.B. Hu, Y.D. Wu, Z.H. Du, An experimental study of the unsteady characteristics of tip leakage flow of axial fans with circumferential skewed blades at off-design conditions, Proc. Inst. Mech. Eng., Part A, J. Power Energy 225 (2011) 802–816. [15] G.Y. Jin, H. Ouyang, B.B. Hu, Y.D. Wu, Z.H. Du, Effect of Skewed Blades on Tip Clearance Noise According to Flow Rate in Axial Fans, Noise Control Eng. J. 59 (2011) 320–332. [16] DANTEC — Air probe manufacturers. Dantec Dynamics A/S.