Experimental Study on Sectional Performance of Horizontal Axis Wind Turbine at Optimum Operation by...

International Conference and Utility Exhibition 2014 on Green Energy for Sustainable Development (ICUE 2014) Jomtien Palm Beach Hotel and Resort, Pattaya City, Thailand, 19-21 March 2014

Abstract-- In the present, we are trying to find alternative

energy to replace fossil fuels. Wind power is a good choice as alternative energy. Wind power produces the electricity without producing greenhouse gases or other pollutants. Wind power is converted to electricity by using wind turbines. The performance of wind turbine generator depends on the rotor blade shape. Generally, sectional performance in axial and tangential flow directions is used to design rotor blades. The designed sectional flow is different from actual one with span-wise flow. For this reason, this study would like to study sectional performance in three dimensional directions for the wind turbine rotor blades, designed by including span-wise flow effect in the future. A model wind turbine with 2.4 m diameter is tested in the large wind tunnel. The velocity filed, count number of local velocity, and standard deviation of velocity in three dimensions are discussed.

Index Terms-- Horizontal Axis Wind Turbine, Wind energy, Sectional performance, Aerodynamics, Velocity distribution, LDV measurement method, Boundary layer, Span-wise flow, Velocity field on airfoil section.

I. NOMENCLATURE r:Radial position [m] CP:Power coefficient [-] R :Radius of rotor=1.2 [m] U0 :Main stream velocity= 7 [m/s] u1,u2,u3:Velocity components from LDV system [m/s] u:Velocity component in axial direction [m/s] v:Velocity component in tangential direction [m/s] w:Velocity component in radial direction [m/s] :Tip speed ratio=r/U0 U2D:Two-dimensional resultant relative velocity [m/s] Uref:Geometrical inflow velocity [m/s] w0:Span-wise velocity perpendicular with airfoil section [m/s] Countu1,Countu2,Countu3:Count number in LDV direction[-] 1,2,3:The standard deviation of three components [m/s] :Azimuth angle [deg] :Angular velocity [1/s]

II. INTRODUCTION n the present, many researchers have been interesting in using renewable energy to reduce the amount of fossil fuel

uses.One of the interesting energy is the energy that comes fromwind.

The Division of Mechanical Engineering, Graduate School of Mie University, Tsu, Mie, Japan. (e-mail: [email protected]).

In order to convertwind to electrical energy, wind turbines have to be applied to change the dynamic power of the wind into electrical energy. The performance of a wind turbine depends on the wind turbine rotor blades. In the present, the design of wind turbine blades uses BEM theory [1]. With this method the wind turbine blade is divided into a number of independent sections along the blade radial position and no interaction between sectional elements. At each section element, a force balance is applied, including 2D section lift and a balance of axial and angular momentum [2]. It is obvious that the BEM theory can find sectional performance from two dimensional directions of sectional elements [3]. The theory does not find effects in three dimensional directions such as 3D flow velocities. A wind turbine in rotating blades have centrifugal force which can make flow in the span-wise direction [4]. The velocity field can change boundary layer around the blade surface and can affect the sectional performance of sectional elements [5],[6]. Therefore, the sectional performance can be changed by the span-wise flow direction [7], [8]. This research would like to study sectional performance in radial positionr/R =0.5 of the blade at the tip speed ratio optimum operation compare with stall operation. In this experimental research, Laser Doppler Velocimetry method was used to measure flow around the rotating blade.

The future prospects when the final goal of the study is completed. The wind turbine new blade design from three-dimensional geometry has highly power coefficient and performance more than commercial wind turbines.

III. EXPERIMENTAL SETUP AND METHODOLOGY

A. Experimental Devices The experimental devices are depicted in Fig. 1.

Ahorizontal axis wind turbine (HAWT) is tested in awind tunnel. The wind turbine rotorhas three blades with 1.2 m rotor radius. The wind tunnel is a single return type with an opened test section. The outlet diameter of the wind tunnel is 3.6m.The wind tunnel can make maximum wind speed at 30m/s. The main flow velocity is used at a constant speed of 7 m/s. The wind turbine is installed downstream at a distance of 1 diameter from the wind tunnel outlet. The azimuth angle is used to detect rotational angle of the blade. The origin of azimuth is a position that the target blade is set vertically upward. The wind turbine rotor can be rotated by a variable

Experimental Study on Sectional Performance of Horizontal Axis Wind Turbine at Optimum

Operation by Using LDV System

T. Phengpom, Y. Kamada, T. Maeda, J. Murata, Y. Kagisaki and S. Nishimura

I


rotational speed, which depends on the operating conditions. The velocity flow around rotatingblades is measured by a two-dimensional Laser Doppler Velocimetry.

Fig.1. Experimental device

B. Wind turbine blade The wind turbine blade is twisted and tapered as same as

commercial wind turbine blades. Fig.2 shows the chord length and twist distributions along radial position of the blades. The wind turbine blade is set maximum angle of twist 18.33[deg] atblade position 0.24m. The pitch angle of blade is set -2[deg]. The wind turbine blade cross sections are composed of four kinds of airfoil: DU91-W2-250, DU93-W-210, NACA63-618 and NACA63-215. Airfoil sections are installed at locations starting from the blade root.

Fig.2. Chord length and twist angle distribution in blade position

C. Experimental methods The surface flow measurements are performed by a LDV

system.The LDV detects the flow velocity at laser intersection. The laserbeams, green and blue are used in this experiment. The focal length 1000mm is set to target blade position. The measuring volume size of green beams is 0.17mm in diameter and 4.3mm in length, and the measuring volume size of blue beams is 0.15mm in diameter and 4.5mm in length. The measuring point is set on the horizontal plane at height of rotor axis, whereazimuth angle is90 [deg]. To get velocity field in three dimensional directions, two probe settings are used. The reference LDV probe setting in case of measuring point at Y=0and Z=r is shown in Fig.3(a) and (b).

Fig.3(a). Placement of LDV probe for first probe setting

Fig.3 (b). Placement of LDV probe for second probe setting

1 is yaw angle between LDV axisand rotor plane.It is used to prevent interference by outer blade. 2 is tilt angle between LDV axisand horizontal plane. First probe setting, yaw and tilt angles are set 1=5[deg],2 = 0[deg]. The velocity components u1 in almost axial direction and u2 in rotational direction are detected. After that, second probe setting, yaw angle is the same and tilt angle is set 2=30[deg]. The velocity component u3can detect resultant components of radial and rotational directions. The u1, u2 and u3 areconverted tovelocity onrotor blade coordinate by using vectorcalculation,(1), (2) and (3) as below.

= 1 cos 1 + (2

tan 2+ 3

sin 2) sin1 (1)

= 2 (2)

= 1 sin1 + (

2tan 2

+ 3sin 2

) cos1(3)

Wind tunnel outlet

Base

r=1.

2m

Pitot tube

=3.

6m

Rotor

Tower

Nacell

Inlet

4.5m

1.85m

Z

Y

X

LDV probe

Laser 1=5.0[] Focus

Optical axis

u2 u1

X Z

Y

LDV probe

Laser

1=5.0[]

2=30[]

Wind turbine

u3

Optical axis

Blade position [m]

Chord length [mm]

Twist angle [mm]

0

50

100

150

200

0

0.25

0.50

0.75

1.00

1.25

0

5

10

15

20 Chord length

Twist angle

Wind turbine X

Z

Y


Here u, vandw are velocity components in axial, tangential and radial directions onrotor blade coordinate. The velocity components in axial, tangential and radial directions are shown. The number of sample is 106 for 1 measuring point. One local velocity in the flow map is derived from phase-locked averaged velocity of seeded particles passing through the measuring point. Number of seeded particles pass through the measuring point is called as Count number.To average the velocity data, 0.1 degree azimuth BIN is used. The LDV probe is set on the traversing device, the measuring point can be moved in axial direction for detecting velocity field around blade. The measuring point on blade coordinate is detected by azimuth signal and axial traverse position.

IV. RESULTS AND DISSCUSSION

A. Wind turbine performance The performance of windturbine is shown in Fig.4. The test

result shows the highest power coefficient Cp = 0.43 at tipspeed ratio of = 5.2. When the tip speed ratio decreases, stall phenomenon occurs. Investigation of the sectional flow around the rotatingblade was studied atoptimum tip speed ratioand stall conditions. In case of stalling conditions, flow measurements were performed at tip speed ratio of =4.0 and 4.2. The detail flow measurements were performed at radial position (r/R = 0.5).

Fig.4.Performance of windturbine

B. Comparison of the velocity field The velocity field around the rotating blade at r/R=0.5 is

discussed. Generally, the sectional performance is discussed with two-dimensional flows. The scalar velocity around bladecan be calculated as the following (4).

U2D= 2 + ( + rev )2(4)

Here U2D is two-dimensional relative velocity to blade section. Vrev means tangential velocity at r/R=0.5. u and v are axial and tangential velocity components. In the flow map, U2Dis normalizedby using geometrical inflow velocity.Uref is defined in (5).

Uref= 02 + 2 (5)

Fig.5 shows the flow map around the rotating blade in radial position (r/R=0.5) at optimum operation. The velocity contour on suction side of airfoil is shown. The highly accelerated area is seen at 0


C. The velocity field in span-wise direction From previous section, the velocity field in two-

dimensional directions is presented.Our previous study was shown effect from three dimensionflow can change boundary layer around airfoil section which affects the sectional performance [4]. The three-dimensional resultant and radialdirection velocities for radial position r/R=0.3 was studied. The results is shown, the three-dimensional resultant had highly accelerated region at leading part. The velocity in radial direction is shown high accelerated area towardinside blade at leading part 0


standard deviation 3 shows high variance compare to 1 and 2 case. The reasons are because in u3 case has an effect of measuring volume layout. The measuring volume for u1 and u2 cases is set radially and chord-wise length is less than 0.2mm. The measuring volume foru3 case has chord-wiselength of 2mmbecause the measuring volume has inclined angle of 30 degrees to the radial direction. The chord-wise length of measuring volume of u3 case is larger than other cases. Thus, in u3 case the measured velocity has the poor resolution of azimuthal position.The wider measuring volume includes high variance of velocity due to position change.

Fig.11 (a).Standard deviation 1 of velocity u1(r/R=0.5, =5.2)

Fig.11 (b).Standard deviation 2 of velocity u2(r/R=0.5, =5.2)

Fig.11 (c).Standard deviation 3 of velocity u3(r/R=0.5, =5.2)

F. The pressure distribution of rotating blade From the velocity measurements,the result show surface

flow is depends on the tip speed ratio.The surface flow affects the pressure distribution on airfoil surface.This section pressure distribution on airfoil surface is discussed. The aerodynamic performance can be studied by referencing to the distribution of pressure on the airfoil. Fig.12 shows pressure distribution on airfoil suction surface. This distribution is usually expressed in terms of the pressure coefficient.When the air flow attacks to stagnation point of airfoil section, Cp = 1.0, then the air flowwill beseparated to upper and lower surface. The negative pressure rises rapidly on the suction surface because of highly accelerated flow in

the leading edge. The negative pressure is decreased as close to the trailing edge. The velocity and pressure are dependent on each other. From Fig.12, the pressure distribution at r/R =0.5 shows large negative pressure on leading part. The negative pressure on leading edge at the tip speed =4.2is higher than that at=4.0 and optimum operation. These pressure distributions are correspond to the surface velocity distribution shown in Figs 5-7.

Fig.12.Pressure distribution along chord station (=5.2)

G. Velocity profile on blade surface at optimum condition The velocity profile on blade surface is discussed in this

section. Velocity profile is important to study the boundary layeron airfoil section.The velocity component is the relative to blade section. Fig.13 (a) and (b) shows the velocity profile at optimum operation forx/c = 0.1 and x/c = 0.8, respectively.

Fig.13(a) shows parallel and radial velocity components. Blue color shows parallel velocity and red color shows radial velocity.Positive parallel velocity means velocity toward to trailing edge. Positive radial velocity means velocity toward to blade tip. From Fig.13(a) parallel velocity at x/c=0.1 shows tangentially induced velocity correspond to circulation on blade section. The highest parallel velocity reaches 33.24m/s at 1.27mm from blade surface. Radial velocity is almost uniform at x/c=0.1. On the suction surface of leading edge shows small negative value.

Fig.13 (a) Velocity profile on the blade surface (r/R= 0.5, x/c=0.1)

6 0

x/c= 0 0.2

0.4 0.6 0.8 1

1 2 3 1= 4 5 7 8 [m/s]

6 0

x/c= 0 0.2

0.4 0.6 0.8 1

1 2 3 2= 4 5 7 8 [m/s]

6 0

x/c= 0 0.2

0.4 0.6 0.8 1

1 2 3 3= 4 5 7 8 [m/s]

Power coefficient (Cp)

1

0

-1

-2

-3

-4

-5

-1

-6

-1

0

0.2 0.4 0.6 0.8 1

=5.2

=4.2

=4.0

Chord station (x/c)

Parallel velocity Radial velocity

Height from blade surface (mm)

Parallel and Radial velocity (m/s)


Fig.13 (b) shows velocity profile at trailing part x/c = 0.8.

Parallel and radial velocity show fluctuation on the blade surface. This velocity fluctuation seems to be caused by the flow separation.

Fig.13 (b) Velocity Profile on the Blade Surface (r/R= 0.5, x/c=0.8)

V. CONCLUSIONS The sectional performance of wind turbine blade at radial

position r/R = 0.5 is explained in this paper. The surfaceflow around airfoil is important to design wind turbine blades. The results can be summarized next.

The flow map in two-dimensional directions at optimum operation shows that the velocity flow is attached with airfoil suctionsurface except trailing edge.. The highly accelerated area is occurred at leading part area. At the tip speed ratio =4.2 and =4.0 are shown stall phenomenon. It is occurred due to high angle of attack. The velocity flow is not attached with airfoil suction surface. In the experiments proposed the effect of span-wise flow which will be considered in future designs. The result shows high span-wise velocity is occurred at above the leading part inward inside to airfoil section.

The measuring volume of u3 is larger in azimuthal direction which can make high velocity variance in measuring volume. Thus, in future experimental studytwo probes are used for synchronized measurement of three velocity components. The local velocity is detected for one particle, so the measuring volume is limited in the point where three pairs of laser beams cross.

VI. REFERENCES [1] Grant, I.(2011,Oct.) Wind Turbine Blade Analysis using the Blade

Element Momentum Method: Wind Turbine Blade Analysis Technical Report from Durham University. Durham, UK.[Online]. Available http://www.dur.ac.uk/g.l.ingram/download/wind_turbine_design.pdf.

[2]Johansen, J., Madsen, H., Gaunaa, M., Bak , C., Design of a WindTurbine Rotor for Maximum Aerodynamic Efficiency,Wind Energy, vol.12, pp. 261-273,2009

[3] C. Sicot, P. Devinant, S. Loyer, J. Hureau., Rotational and turbulenceeffects on a wind turbine blade. Investigation of the stall mechanisms,Journal of Wind Engineering and Industrial Aerodynamics,vol. 96, pp. 1320-1331, Jan. 1988.

[4] Suzuki, D., Kamada, Y., Maeda, T., Murata, J. , Kagisaki, Y., Nishimura, S., Experimental Study on Flow Over Surface of Horizontal Axis Wind Turbine Blades, Proceeding of The 2ndInternational Symposium for Sustainability by Engineering at MIU,vol.1, 1,pp.163-166,2012.

[5] Hernandez, G.G., Laminar-Turbulent Transition on Wind Turbines, in DTU Thesis,2011.

[6] Dua, Z., Seligb, M.S., The Effect of Rotation on the Boundary Layer of a Wind Turbine Blade, Renewable Energy, vol.20, pp.167-181,2000.

[7] Martnez, G.G.,Srensen, H. J. N., Shen, W. Z., 3D Boundary Layer Study on A Rotating Wind Turbine Blade, Journal of Physics, 37, pp.12-32, 2007.

[8] T. Burton, N. Jenkins, D. Sharpe, E. Bossanyi, Wind Energy Handbook 2nd edition, vol. 1, New York: Wiley, p. 16-20, 2011.

VII. BIOGRAPHY TinnapobPhengpom was born in Uttaradit province in the Thailand. He graduated Bachelor's degree in the Industrial Engineering, and Master's degree in Mechanical engineering from Chiang Mai University. His employment experience included new product development engineer at Fujikura Electronics (Thailand) ltd., research assistance at Energy and Environment for Fluid Engineering Mie University Japan. His is interested in wind energy and biomass gasification fields. Now he is Ph.D. student in System Engineering Mie University.

Parallel velocity Radial velocity

Height from blade surface (mm]

Parallel and Radial velocity [m/s]

Experimental Study on Sectional Performance of Horizontal Axis Wind Turbine at Optimum Operation by...

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