Document7

International Journal of Advance Foundation and Research in Science & Engineering (IJAFRSE)

Volume 1, Issue 5, October 2014. Impact Factor: 1.036, Science Central Value: 10.33

46 | 2014, IJAFRSE All Rights Reserved www.ijafrse.org

Simulations of Small Scale Straight Blade Darrieus Wind

Turbine Using Latest CAE Techniques to get Optimum

Power Output. Hiren Tala (B.E.Mechanical),

Sandip patel (Assistant Professor in Mechanical Department ,GEC Valsad)

Government Engineering College Valsad.

[email protected] , [email protected]

A B S T R A C T

We are going to design & simulate small scale darrieus windmill by using CAE techniques. After

Referring previous Literatures Out of Various 4-digit Profiles available for the Vertical axis

turbine, we have chosen symmetrical axis NACA profile 0012, 0015, 0018 and, out of those

profiles, after various CFD simulations at different azimuth angles 00, 300, 600, 900 by using

FLUENT in ANSYS (Workbench 14), optimum profile (NACA0012) have been optimized (according

to FLUENT analysis data) for our design of model, and then again by various FLUENT analysis of

varying pitch angle of blades ,out of -80, -40, 00, +40, +80 angles at different azimuth angles 00, 300,

600, 900 and our favorable outcome is at -80 fix pitch of blade, by using those data we made

prototype of our model which gives same result as expected according to CAE analysis results. Our

main conclusion for our work is for small scale profiles NACA0012 gives better result than other

two profiles and we gets high torque at -80 fix pitch angle then other pitch angles.

I. INTRODUCTION

In current era power availability is costly but necessary, so it's vital to produce it within our workplace

with minimal coast and which resulted small scale power generation is widely accepted. VAWTs come in

a wide and interesting variety of physical configurations and they involve a range of complex

aerodynamic characteristics. VAWTs in principle can attain coefficients of performance, Cp max, that are

comparable to those for horizontal-axis wind turbines (HAWTs) and they have several potentially

significant advantages over the HAWTs [1]. These advantages include the fact that VAWTs are cross-flow

devices and therefore accept wind from any direction. The Darrieus Wind Turbine resembles a gigantic

eggbeater and has two main advantages. The first main advantage is that the equipment including the

gear box and the generator can be placed close to the ground. The second advantage to this type of wind

turbine is that you dont need a new mechanism to turn the rotor against the wind. The Darrieus Wind

Turbine is a lift-type vertical axis turbine and can function effectively no matter which way the wind is

blowing. This wind turbine is powered by the lift forces that are created by a set of airfoils, which are the

actual wing-shaped blades of the turbine. These allow the turbine to reach speeds that are higher than

the actual speed of the wind, which makes them well suited to generating electricity.

II. LITERATURE REVIEW

The Darrieus wind turbine was patented by the U.S. Patent Office in the name of G.J.M. Darrieus in 1931

[2]. The Darrieus patent states that each blade should have a streamline outline curved in the form of

skipping rope. In other words, the Darrieus rotor has curved blades that approximate the shape of a

perfectly flexible cable, of uniform density and cross-section, hanging freely from two fixed points; under

the action of centripetal forces such a shape minimizes inherent bending stresses.




For many years no significant research in field of Darrieus VAWT is carried out. Finally in 1970s at

Sandia National Laboratories, the study of VAWT technology started and concluded in the 1990s [3].

These studies concentrated on the Darrieus configurations because of their high inherent efficiency. The

Sandia VAWT program culminated with the design of the 34-m Test Bed Darrieus VAWT. This turbine

was designed and built to test various VAWT design concepts and to provide the necessary databases to

validate analytical design codes and algorithms.

The straight blade Darrieus wind turbine is patented by [4] in 1987 (US4247252). It has simple

construction than conventional egg-beater shaped Darrieus VAWT.

In this modern time, there is resurgence of interests regarding VAWTs as numerous universities and

research institutions have carried out extensive research activities and developed numerous designs

based on several aerodynamic computational models. These models are crucial for deducing optimum

design parameters and also for predicting the performance before fabricating the VAWT. [5] (2006) have

compile the main aerodynamic models that have been used for performance prediction and design of

straight-bladed Darrieus type VAWT. At present the most widely used models are the double-multiple

streamtube model, free-Vortex model and the Cascade model. It has been found that, each of these three

models has their strengths and weaknesses. Though among these three models, the Vortex models are

considered to be the most accurate models according to several researchers, but they are

computationally very expensive and in some cases they suffer from convergence problem. It has also

been found that the double-multiple streamtube model is not suitable for high tip speed ratios and high-

solidity VAWT. On the other hand, the Cascade model gives smooth convergence even in high tip speed

ratios and high solidity VAWT with quite reasonable accuracy.

[06] Studied the (NACA 0024, NACA 4420 and NACA 4520) and effect of changing the design parameters

on the performance of the Giromill vertical axis wind turbine with fixed pitch angle variation (coefficient

of performance, tip speed ratio and torque coefficient). Also, to determine the variation which will result

in the best performance based on the different performance parameters.

A number of cases were studied by [07] with different numbers of blades, various tip speed ratios and

pitch angles. Their results shown that turbines with varied pitch angles perform better than those with a

fixed pitch angle.

III. WIND MEASUREMENT

Wind speed data has been collected by using anemometer at 1 m above the sea level (at ground level)

and at 12 m above the sea level (at rooftop of boy's hostel building, GEC Valsad). The readings were taken

almost three times a day from October-2012 to April-2013. To evaluate the wind resource and the wind

power production potential on site the meteorological data is treated with statistical methods. The

method used separates the data into wind speed intervals or bins in which it occurs. A series of N wind

speed observations is assumed. The data are separated into NB bins of width wj, with midpoints mj and

with the number of occurrences in each bin (the frequency) fj such that

=

[01]

With this technique the mean wind speed is calculated using equation,




U = 1Nm

f[02]

From above equation average wind speed at ground level is found 1m/s and at 12 meter altitude average

wind speed is 1.5 m/s.

A. Wind speed frequency distribution

Figure 1: Wind speed frequency distribution for ground level

Figure 1 shows the wind speed frequency distribution at ground level and figure 2 shows the wind speed

frequency distribution at 12 m height. Every bar in the diagram represents the frequency of occurrence

for that special wind speed interval. Wind speeds between four and five meters per second are the least

common. Wind speed 1 m/s & 2 m/s the most common and represent around 90 percent of the time. The

highest wind speed ever measured is 5.2 meters per second.

Figure 2: Wind speed frequency distribution for 12m height

IV. ROTOR EFFICIENCY ESTIMATION

A. Preliminary Estimation of Power

Maximum Power of wind energy has been estimated as,

Availablepower = Kineticenergytime =12 AV

* [3] = 12 1.22557 0.3600 4

*[4] = 14.1185Watt[5]

For wind power, the maximum power coefficient for free flow is 16/27 according to the Betz limit.[08]




so, maximum power can be extracted from any kind of wind turbine is 59.29 %

Power567 = 14.1185 0.5929[6] = 8.37089:;;[7]

But, According to the graph shown in figure 3 , Maximum Power can be extracted by darrieus wind

turbine is 35.00 %

Power567 = 14.1185 0.3500Watt[8] = 4.94149:;;[9]

Figure 3 : Maximum power can be extracted from any type of wind turbine

V. ROTOR SIZE ESTIMATION

Windmill blades are designed to move in response to wind force, and it can extract a substantial portion

of the energy and power available. The wind energy available in a unit volume (one cubic foot or one

cubic meter) of air depends only upon the air density and the instantaneous wind speed V.

There are two principle ways to determine the frontal area of wind machine rotor. first is to decide size

that is How large a machine be required, and Calculate the power it produces. According to second

method average power needs is determined and the wind resources at wind resource site and then

equate the two to determine the rotor area . The first method is one most often used, The second is more

complex but results in a much closer match between your power needs and the wind power available.

Frontal area of the rotor is given by following equation.

A = D H[10]

Figure 4 : Frontal area of straight blade Darrieus rotor

This kinetic energy of the air in motion is given by the formula:

Kineticenergyunitvolume =

12 V

? [11]




Wind power is the amount of energy which flows through the surface per unit time, and is calculated the

wind energy by the elapsed time t,

Availablepower = Kineticenergytime =12 AV

* [12] Here from the equation no.12, we can say that both energy and power are proportional to the cube of

wind speed.

VI. AERODYNAMIC DESIGN

Though the straight-bladed darrieus type VAWT is the simplest type of wind turbine, its aerodynamic

analysis is quite complex. Before comparative analysis of the main aerodynamic models, the general

mathematical expressions, which are common to most of the aerodynamic models, are described in this

section.

A. Variation of local angle of attack

The flow velocities in the upstream and downstream of the Darrieus-type VAWTs are not constant. One

can observe that the flow is considered to occur in the axial direction. The chordal velocity component Vn

are respectively, obtained from the following expressions:

V@ = R + V6cos[13] VD = V6sin[14]

Where Va is the axial flow velocity (i.e. induced velocity) through the rotor, is the rotational velocity, R

is the radius of the turbine, and is azimuth angle. The attack can be expressed as

= tanE FVDV@G [15]

= tanE F V6 sinR +V6 cosG = tanE F sinR Va +cosG [16]

Figure 5: Flow velocities of straight-bladed Darrieus type VAWT

Substituting the values of Vn and Vc and non-dimensionalizing,


Volume 1, Issue 5, October 2014.

51 | 2014, IJAFRSE All Rights Reserved

= tanIf we consider blade pitching then,

tan

B. Variation of local relative flow velocity

The relative flow velocity (W) can be obtained as

W IInserting the values of Vc and Vn and non

WV WV6 , V6V

Figure 6 : Force diagram of a blade airfoil.

C. Variation of tangential and normal forces

The directions of the lift and drag forces and

force coefficients (Ct) is basically the difference between the tangential components of lift and drag

forces. Similarly, the normal force coefficients (C

lift and drag forces. The expressions of CCK CCD CThe net tangential and normal forces can be defined as

FK FD

Where is the air density, C is the blade chord and H is the height of the turbine.

D. Calculation of total torque

Journal of Advance Foundation and Research in Science & Engineering (IJAFRSE)


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tanE M sinNOPQ NP6PQR BcosS17

tanE M sinNOPQ NP6PQR BcosS T 18 Variation of local relative flow velocity

The relative flow velocity (W) can be obtained as

IV@? B VD?19 and non-dimensionalizing, One can find velocity ratio as,

V6V , UVFRV V6WVX G B cosY? B sin

Figure 6 : Force diagram of a blade airfoil.

Variation of tangential and normal forces

The directions of the lift and drag forces and their normal and tangential components. The tangential

) is basically the difference between the tangential components of lift and drag

forces. Similarly, the normal force coefficients (Cn) is the difference between the normal compon

lift and drag forces. The expressions of Ct and Cn can be written as C sin T C[ cos ,21 C cos B C[ sin .22 The net tangential and normal forces can be defined as

CK 12 CHW?,23 CD 12 CHW?,24 is the blade chord and H is the height of the turbine.


Impact Factor: 1.036, Science Central Value: 10.33

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dimensionalizing, One can find velocity ratio as,

Y sin? 20

their normal and tangential components. The tangential

) is basically the difference between the tangential components of lift and drag

) is the difference between the normal components of




Since, the tangential and normal forces represented are for any azimuthal position, so , they are

considered as a function of azimuth angle

as

FK6 The total torque (Q) for the number of blades (N) is obtained asQ Power Output

The total power (P) can be obtained as P Table 1

Parameter

No. of stages

Shape of blade

Chord length

No. of blade

Aspect ratio

Tip Speed Ratio

Frontal area

Pitch Angle

VII. AERODYNAMIC DESIGN USING CFD TECHNIQUES.

Workbench FLUENT analysis is 5 steps procedure, in 1st step geometry is defined which is made in CREO

and exploited to ANSYS. The geometry has two domains called fluid

is square prism in shape and approximately 10 times bigger than rotor, i.e. 8m 8m 10m long. This

fluid region mimics environment. The reason behind bigger size of fluid domain compared to rotor size is

that effect of external wall can be minimized so that flow of air will be same as environmental conditions.

The rotor volume is subtracted from the fluid region.



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and normal forces represented are for any azimuthal position, so , they are

considered as a function of azimuth angle . Average tangential force Fta on one blade can be expressed

12^ FK_`. d.?b 25 rque (Q) for the number of blades (N) is obtained as NFK6R26

The total power (P) can be obtained as Q27

Table 1 Preliminary Design Parameters

Parameter Value

No. of stages 1

Shape of blade To be selected out of

NACA0012, NACA0015,

NACA0018

Chord length 100 mm

No. of blade 3

Aspect ratio 1 : 1

Tip Speed Ratio 1

Frontal area 360000 mm2

Pitch Angle To be estimated

AERODYNAMIC DESIGN USING CFD TECHNIQUES.

Figure 7: Fluid Domain


and exploited to ANSYS. The geometry has two domains called fluid domain and rotor. The fluid domain



external wall can be minimized so that flow of air will be same as environmental conditions.

The rotor volume is subtracted from the fluid region.



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and normal forces represented are for any azimuthal position, so , they are

on one blade can be expressed


domain and rotor. The fluid domain



external wall can be minimized so that flow of air will be same as environmental conditions.




Figure 8: Name selection of Surfaces

Figure 9 Meshed model of fluid domain

In fluid domain name selection are defined in order to apply load as shown in (figure 8) Surface A

indicates "INLET", Surface B indicates "OUTLET" Surface C indicates "WING 1" Surface D indicates "WING

2" & Surface E indicates "WING 3".

In mesh step element type & its size is defined

tetrahedral elements are used and refined meshing is applied near rotor in 5 layers. The minimum size of

elements is 10 mm.

Figure 10 Enlarged meshed region showing rotor.

In setup step boundary condition and load are applied. Steady air of 10 m/s is applied. Normal to "INLET"

face is applied. Which exits from "OUTLET" face.

In Problem setup stage, K-epsilon realizable (Standard Wall Functions) model has been used. Material

used in this stage is air and its initial speed is 10 m/s. then boundary conditions has been selected.

As observed by [09] the most important parameters to

conditions, blade geometry and airfoil lift and drag coefficients

For design velocity range of 711 m/s, the solidity values for darrieus VAWTs should be chosen in the

range of 0.20.25 [10]



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Figure 8: Name selection of Surfaces

Figure 9 Meshed model of fluid domain

n are defined in order to apply load as shown in (figure 8) Surface A


In mesh step element type & its size is defined and meshing of body is performed (Fig 9) for meshing


Figure 10 Enlarged meshed region showing rotor.

tion and load are applied. Steady air of 10 m/s is applied. Normal to "INLET"

face is applied. Which exits from "OUTLET" face.

epsilon realizable (Standard Wall Functions) model has been used. Material

nd its initial speed is 10 m/s. then boundary conditions has been selected.

the most important parameters to be provided to the simulation code are operating

geometry and airfoil lift and drag coefficients

11 m/s, the solidity values for darrieus VAWTs should be chosen in the



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n are defined in order to apply load as shown in (figure 8) Surface A


and meshing of body is performed (Fig 9) for meshing


tion and load are applied. Steady air of 10 m/s is applied. Normal to "INLET"

epsilon realizable (Standard Wall Functions) model has been used. Material

nd its initial speed is 10 m/s. then boundary conditions has been selected.

be provided to the simulation code are operating

11 m/s, the solidity values for darrieus VAWTs should be chosen in the




In solution setup stage Momentum, turbulent kinetic energy & Turbulent Dissipation Rate has been set

as second order Upwind. And solution is Hybrid initialization has been selected. Solution for 1000

iterations is applied for different analyses & results are obtained in final step. In result stage, Lift Force &

Drag Force has been carried out from reports for each model separately.

Total 12 no. of CFD analysis were performed for different blade profiles including NACA0012, NACA0015

& NACA0018, on different these graphs flow behavior around the rotor can be studied azimuth positions,

keeping pitch angle 0. Pressure gradient and velocity gradient around the rotor at various azimuths for

different blade profiles and pitch angles are shown in subsequent figures.

Figure 11: NACA0012_AZ(00)-Pitch(0)_PRESSURE

Figure 12: NACA0012_AZ(00)-Pitch(0)_VELOCITY

Figure 13: NACA0012_AZ(30)-Pitch(0)_PRESSURE


Figure 15: NACA0012_AZ(60)-Pitch(0)_ PRESSURE





Figure 17: NACA0012_AZ(90)-Pitch(0)_ PRESSURE


Figure 19: Program showing Torque of various

profiles.

Figure 20: Torque V/S Azimuth Angle for NACA

profile selection

From the numerical analysis, forces acting on blades of rotor are derived and mean torque has been

calculated. For each profile at different azimuth angle, torque data has been plotted in graph From the

results conclusion can be made that among all considered NACA profiles NACA0012 has maximum

torque output for wind speed 10 m/s and same frontal area, which is 360000 mm2.

Our result of selecting NACA0012 Profile for getting optimum power is matching with various previous

literatures.[12,13]

Optimum power output is our prerequisite and we did more work after selecting NACA0012 profile on

the varying its pitch angle.

To improve our Power output, variable pitch of blades the answer?

From the previous designs in 1980s and 1990s, VAWT designs based on the straight blade Darrieus

pattern but with variable pitch blades, known as Gyromills or Cycloturbines, have been used since 1970s.

Variable pitch mechanism can achieve high starting torque, high efficiency and can reduce or eliminate

stall. However variable pitch VAWT never reached commercial production, because of (1) Mechanical

complexity in turbines with active pitch control, where blade pitch is forced to follow a predetermined

regime (2) dubious effectiveness of designs with passive pitch control in which pitch is not

predetermined but is driven by an interaction of fluid dynamic and inertial or other forces. [11]

While for NACA0012 other 16 no. of CFD analysis were performed with the same fluid conditions in the

fluid domain and on different azimuth positions, keeping pitch angle -4, +4,-8 & +8.




Figure 21: NACA0012-AZ(00)-Pitch(-8) PRESSURE

Figure 22: NACA0012-AZ(00) Pitch(-8) VELOCITY

Figure 23: NACA0012_AZ(30)_pitch(-8)-PRESSURE

Figure 24: NACA0012_AZ(30)_pitch(-8)-VELOCITY

Figure 25: NACA0012-AZ(60)Pitch(-8)PRESSURE

Figure 26: NACA0012-AZ(60)Pitch(-8)VELOCITY

Figure 27: NACA0012-AZ(90) Pitch(-8) PRESSURE

Figure 28: NACA0012-AZ(90) Pitch(-8) VELOCITY




Figure 29: Program to select pitch angle

Figure 30: Torque V/S Azimuth Angle for Pitch angle

selection

From the numerical analysis of blades at pitch angle 0, -4, -8, +4, -4 & +8 has been plotted, the Lift

force and Drag force of blades has been listed form final result of analysis and the and average force of all

the blades at a positions has been individually calculated graph of pitch angle and torque has been made

which shown optimum output mean torque has been concluded, and the maximum mean torque has

been found at -8 pitch angle position of blade, so it has been selected as a model blade position for rotor.

VIII. CONCLUSION

From our work to calculate optimum mean torque out of changing various blade positions we concluded

as the NACA0012 shows the better results than the other NACA 00XX profiles which relates to the

previously available literatures, and after determining NACA profile our further works to optimize torque

output has been carried out, we have changed the positions of blades at various pitch angle positions at

various azimuth angle and concluded as the optimal power output with fixed pitch angle can be

determined at -80 pitch angle of blades.

IX. REFERENCES

[1] Tong W., Wind power generation & wind turbine design., WIT press, ISBN : 978-1-84564-205-1,

2010

[2] US1835018, G.J.M. Darrieus , U.S. Patent.,1931

[3] Sutherland H, Dale E. Bergn and Thomas D. Ashwill, "SANDIA National Laboratories" A

Retrospective of VAWT Technology, January 2012

[4] US4247252, Seki Kazauchi, Ishehara, Shimizu Yoshio, Sagamihara, Kata Yoshio, U.S.Patent,1981

[5] Islam. M. , "Department of Mechanical , Automotive & Material Engineering " , University of

Windsor , doi : 10.1016/j.rser.2006.10.023, 2006

[6] Marco Torresia,, Bernardo Fortunatoa, Sergio M. Camporealea "Numerical investigation of a

Darrieus rotor for low-head hydropower generation",2013




[7] ZHANG Liang, SUN Ke and LI Feng-lai. "Hydro- dynamic experimental study on new type of

vertical- axis variable-pitch turbine[J]". Journal of Harbin Engineering University, 27(Suppl. 2):

346-352, 2006

[8] Betz A. "Das maximum der theoretisch mglichen ausnutzung des windes durch windmotoren.

Zeitschr. f. das gesamte Turbinenwesen" 1920;26:307e9.

[9] Bak C, Fuglsang P, Srensen NN, Madsen HA, Shen WZ, Srensen JN. "Airfoil characteristics for

wind turbines". Ris-R-1065(EN). Roskilde: Ris National Laboratory; March 1999.

[10] Jain, Pramod,2011. "Wind Energy Engineering". McGraw Hill Companies, ISBN978-0-07171478-

5 MHID:007-171478-2, /http://www.tatamcgrawhill.com/ html/9780071714778.htmlS.

[11] B.K. Kirke and L. Lazauskas "Limitations of fixed pitch Darrieus hydrokinetic turbines and the

challenge of variable pitch", doi:10.1016/j.renene.2010.08.027),2010

[12] M. Saqib Hameed, S. Kamran Afaq, "Design and analysis of a straight bladed vertical axis wind

turbine blade using analytical and numerical techniques"

http://dx.doi.org/10.1016/j.oceaneng.2012.09.007, 2012

[13] In Seong Hwang, Yun Han Lee, Seung Jo Kim, "Optimization of cycloidal water turbine and the

Performance improvement by individual blade control" doi:10.1016/j.apenergy.2008.11.009,

2008

[14] Bertagnolio F, Niels sorensen Jeppy johansen and Peter Fuglsang , 2001,"Wind Turbine Airfoil

Catalogue" , Riso National Laboratory, Roskilde, Denmark, ISBN 87-550-2910-8

[15] Hau , E., " Wind Turbines - Fundamentals, Technologies, Application, Economics", Springer-Verlag

Berlin Heidelberg , ISBN-103-540-24240-6, 2005

[16] Paraschivoiu.I., "wind turbine design-with emphasis on darrieus concept" , Presses

internationales Polytechnique , ISBN 978-2-553-00931-0, 2009

[17] Qasim A.Y.,R. Usubamatov and Z.M. Zain, "Design of Vertical Axis Wind Turbine with Movable

Vanes", Australian Journal of Basic and Applied Sciences, 5(11): 896-902, 2011 , ISSN 1991-8178,

2011

[18] Paraschivoiu I, Trifu O, Saeed F. H-Darrieus wind turbine with blade pitch control. Int J Rotat;

Article ID 505343, Mach 2009

[19] Staelens Yann, Saeed F, Paraschivoiu I. "A straight-bladed variable-pitch VAWT concept for

improved power generation". 41st Aerospace sciences meeting, AIAA-2003-0524, 2003

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