Rotor blade sectional performance under yawed inflow conditions

1
Rotor blade sectional performance under yawed
inflow conditions
Mie University, JAPAN
Takao MAEDA Hideyasu FUJIOKA
Yasunari KAMADA Jun SUZUKI
2
Introduction
Global warming issue
Renewable energy is remarked Wind energy is one
of this
Wind turbine
Aerodynamic efficiency greatly depends on the
performance of the rotor blade, the airfoil
section and the plan form.
Pressure measurement plays a particularly big
role to grasp aerodynamic performance
Some studies using pressure distribution
measurements is done in field experiments But
There are still many unanswered questions
3
Introduction
In the field..
Wind direction changes with time
But the yaw mechanism can not change the nacelle
direction continually.
wind turbines are operated in some yaw
miss-arraignment most of the time
Wind speed changes with time
The flow around blades become complexly
Grasp the performance for yawed rotor with
pressure distribution in wind tunnel
4
Contents

• Experimental setup
• Power curve
• Pressure measurement
• Sectional aerodynamic force
• Conclusions

5
Experimental setup
Inlet
Outlet
z
2800
x
y
2400
3600
4500
3 Blades
Mie University wind tunnel
6
Definition of parameter
Rotational direction
wind
Pitch Angle
Rotational direction
Azimuth angle ? Clock wise as plus Yaw Angle
Fyaw See from upward Clock wise as plus Pitch
Angle Direction that the leading edge learn the
upstream as plus
Fyaw
wind
7
Test Blade
200 150 100 50 0
25 20 15 10 5 0
Chord length
Twist angle, deg
Chord length , c mm
Twist angle
0 0.2 0.4 0.6 0.8 1.0
Radius position , r/R -
32 Pressure taps
8
Pressure sensor
Pressure transducer Range 7.65kPa number of
channels 32 Sampling interval 0.1msec/ch
9
Power Curve with various yaw
No difference in Cpower between plus yaw angle
and minus
10
Maximum Power in yawed rotor
1.0 0.8 0.6 0.4 0.2 0
Cpower /Cpower0
Experiment cos2F cos3F
-60 -30 0 30 60
Yaw Angle F
cos2fVortex theory cos3fMomentum theory
The measurement is placed between cos2f and cos3f
11
Dynamic response of pressure system
Section blade pressure changes dynamically
Calibrate the amplitude ratio and phase-lag of
pressure tube and pipes
Rotational Frequency ? 5 Hz
0.94
Amplitude -
Amplitude drop 6 Phase lag 25 degree
Consider the scanner conversion time
Phase-lag deg
Scanning is being done 30 degree before
25
Frequency Hz
12
Aerodynamic forces
n
a
n
Ca
Axial force
Rotational force
Chord line
Cr
t
Rotational direction
r
ß?
a
Ct
Inflow direction
c chord length m ? pitch angle deg pt
surface pressure Pa ? twisted angledeg
pboss boss pressure Pa U relative wind speed
m/s r radial position m ? air density
kg/m3 ? rotational speed
1/s
13
Pressure distribution
-4 -2 0 2
F0 F15 F30 F45
Pressure Coefficient Cp
0 0.2 0.4 0.6 0.8
1.0
Chord Station x/c
Comparison of the pressure distribution of
?0for various yaw angles (r/R0.7, ?4.7)
14
Curves of Cr and Ca (?0)
0.4 0.2 0.0 -0.2
r/R0.7
0.5
0.4
0.3
Fyaw0
Fyaw
15

Rotational Force Coefficient Cr
Rotational Force Coefficient Cr
Fyaw
-
15
0.2
Fyaw
30

Fyaw
30
-
Fyaw

45
0.1
F0
F30
Fyaw
-
45
F15
F45
0
0
2
4
6
8
0 2 4 6
8

• The angle of attack is large so that flow
  separates from suction side of blade.

2.0 1.5 1.0 0.5

• The ? that shows a sudden drop in Cr, results in
  a low tip speed ratio as ? increases.

Axial Force Coefficient Ca

• The relationship between the angle of attack
  and ? changes according to F.
• The relationship between the angle of attack and
  the torque produced by blade element does not
  change.

0 2 4 6
8
Tip Speed Ratio ?
15
Curves of Cr and Ca (?180)
0.4 0.2 0.0 -0.2
Rotational Force Coefficient Cr
F0
F15
F30
F45
0 2 4 6
8
3.0 2.5 2.0 1.5 1.0 0.5
Even if F increases , the angle of attack changes
little.
Axial Force Coefficient Ca
0 2 4 6
8
Tip Speed Ratio ?
16
Curves of Cr and Ca (?180)
0.4 0.2 0.0 -0.2
Geometrical attack angle
Rotational Force Coefficient Cr
F0
F15
F30
F45
0 2 4 6
8
Azimuth angle
3.0 2.5 2.0 1.5 1.0 0.5
Even if F increases , the angle of attack changes
little.
Axial Force Coefficient Ca
Agreement with the ? that shows a sudden drop in
Cr,Ca is approximately same.
0 2 4 6
8
Tip Speed Ratio ?
17
Curves of Cr and Ca (?90,270)
0.4 0.2 0.0 -0.2
0.4 0.2 0.0 -0.2
?90?upstream side
?270? downstream side
Rotational Force Coefficient Cr
Rotational Force Coefficient Cr
F0
F15
F0
F30
F30
F15
F45
F45
0 2 4 6
8
0 2 4 6
8
2.0 1.5 1.0 0.5
2.0 1.5 1.0 0.5
F0
F0
F15
F15
F30
F30
F45
F45
Axial Force Coefficient Ca
Axial Force Coefficient Ca
0 2 4 6
8
0 2 4 6
8
Tip Speed Ratio ?
Tip Speed Ratio ?
18
3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5
?270
Hysteresis curve
Lift Coefficient Cl
?90
Static
U0
Dynamic
Span-wise velocity component of an expanded
flow Span-wise velocity component of main
stream Total Span-wise velocity
-2015 10 5 0 5 10 15 20 25 30
Angle of Attack a deg
19
Conclusions (1/2)

• Increasing the yaw angle, the maximum power
  coefficient decreases while the optimum tip speed
  ratio is lower.
• The power coefficient of the rotor becomes higher
  when the yaw angle is larger during low tip speed
  operation. The reason is that there is an optimal
  value of angle of attack when the blade is moving
  forward for main flow under yawed conditions so
  that the blade indicates a high lift coefficient.
  
• Under the yawed condition in r/R 0.7, the
  forward blade placed in the horizontal position
  greatly contributes to the rotor torque. However
  the blade placed opposite has a smaller
  contribution.

20
Conclusions (2/2)

• Under the yawed condition in r/R 0.7, Ca and Cr
  are determined by a change in the angle of attack
  at ?0 and 180. However, at ?90 and 270,
  these coefficients are largely affected by the
  velocity component of span-wise flow and wake
  induced velocity in addition to a change in the
  angle of attack.
• In the low ? region, when Cr is dropping sharply,
  the backward blade contributes less to rotor
  torque. It is thought that the separation of flow
  causes a drop in Cr, and when the flow separates,
  the angle of attack of the backward blade is
  moving to be decreased by rotating, but there is
  delay in flow reattachment and there are wake
  induced reduction.