on the performance analysis of savonius rotor with twisted blades (1)

8/3/2019 On the Performance Analysis of Savonius Rotor With Twisted Blades (1)

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Renewable Energy 31 (2006) 17761788

www.elsevi er.com/locate/renene

On the performance analysis of Savonius rotor withtwisted blades

U.K. Saha

, M. Jaya Rajkumar

Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati-781 039, India

Received 1 March 2004; accepted 6 August 2005

Available online 21 October2005

Abstract

The present investigation is aimed at exploring the feasibility of twisted bladed Savonius rotor for

power generation. The twisted blade in a three-bladed rotor system hasbeen tested in a low speed

wind tunnel, and its performance has been compared with conventional semicircular blades (with

twist angle of01). Performance analysis has been made on the basis of starting characteristics, static

torque and rotational speed. Experimental evidence shows the potential of the twisted bladed rotor in

terms of smooth running, higher efficiency and self-starting capability as compared to that of the

conventional bladed rotor. Further experiments have been conducted in the same setup to optimize

the twist angle.

r2005 Elsevier Ltd. All rights reserved.

Keywords: Savonius rotor; Twisted blade; Starting characteristics; Static torque; Coefficient ofperforman ce

1. Introduction

Savonius rotor is a unique fluid-mechanical device that has been studied by numerous

investigators since 1920s. Applications for the Savonius rotor have included pumping

water, driving an electrical generator, providing ventilation, and agitating water to keep

stock ponds ice-free during the winter [14]. Savonius rotor has a high starting torque and

a reasonable peak power output per given rotor size, weight and cost, thereby making it

less efficient [5]; the coefficient of performance is of the order of 15% [6,7]. From the

point of aerodynamic efficiency, it cannot compete with high-speed propeller and the

Corresponding author. Tel.: +91 361 2691085; fax: +91 361 2690762.E-mail address: [email protected] r net.in (U.K. Saha).0960-1481/$ - see front matter r2005 Elsevier Ltd. All rights

reserved. doi:10.1016/j.renene .2005.08.030
http://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenemailto:[email protected]:[email protected]://www.elsevier.com/locate/renene


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U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 17761788 1777

Nomenclature

A projected area of rotor, m2

AR aspect ratio, H/dCp coefficient of performance, P1/(1/2rAU

3)

d blade chord (2r), mm

H blade height, mm

N rotational speed of rotor, RPM

P1 shaft power (2pNTB/60), W

R tip radius of semicircular bladed rotor, mm

R1 top tip radius of twisted bladed rotor, mm

R2 bottom tip radius of twisted bladed rotor, mm

r blade arc radius, radius of brake wheel, mm

S gap width, mm

TB brake torque, Nm

U mean stream velocity in x-direction, m/s

r density of atmospheric air, kg/m3

a twist angle (deg)

y Orientation angle (deg)

Z efficiency, P1/(0.593 1/2rAU3)

Darrieus-type wind turbines. Various types of blades like semicircular, bach type [810],

Lebost type [11,12] have been used in vertical axis wind turbine to extract energy from the

air, however, no attempt so far has been made to reduce the negative torque, and increase

the starting characteristics and efficiency with the changes in the air direction. The use of

deflecting plates [8,13] and shielding to increase the efficiency has not only made the system

structurally complex, but also dependent of air direction. In view of this, a distinct blade

shape with a twist for the Savonius rotor has been designed, developed and tested in the

labora tory [14,15]. Preliminary investigation has shown good starting characteristics of the

twisted blades.

2. Brief overview of past work

Numerous investigations have been carried out in the past to study theperformance

characteristics of two and three bucket Savonius rotor. These investigations included wind

tunnel tests, field experiments and numerical studies. Blade configurations were studied in

wind tunnels to evaluate the effect of aspect ratio, blades overlap and gap, effect ofadding

end extensions, end plates and shielding [8,10,1618]. Vishawakarma [4] attempts todiscover an alternate energy option for water pumping, which can be cost-efficient,

environment friendly and sustainable. Two types of installations viz., low-speed wind

turbines operating piston pumps, and high speed wind turbines driving rotary pumps have

been studied. Kumar and Grover [6 ] have investigated a case study of a Savonius rotor forwind power generation. Mojola has investigated field tests of Savonius rotor where data

were collected for speed, torque, and power of the rotor at a large numbers of wind speeds

at different overlap ratio [12]. Detailed experiments have been conducted by some


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1778U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 17761788

investigators to increase the output of a Savonius rotor by using a flow deflecting plate

[13,20]. The aerodynamic performance was also studied by Fujisaw and Gotoh [19] from

the blade surface pressure distribu tions at various rotor angles and tip-speed ratios.

Fujisaw and Gotoh [21] studied the power mechanism of Savonius rotor by pressure

measurements on the blade surface and by flow visualization experiments. Modi

and Fernando [18] have described a mathematical model based on the discrete vortex

method to predict the performance of a stationary and a rototary Savonius configuration.

Table 1 shows the details of the experiments carried out with varying tunnel dimensio ns,

Reynolds number and tip speed ratio. The data obtained from the recent investigations

[14,15] have been included in the table along with the data available in the published

literature [13].

3. The present study

In the present investigation, the twist angle of the blade was varied from a 01 to 251

and the performance of the rotor was studied in a low speed wind tunnel to find the

optimum twist angle. It is worth mentioning here that the blade with a twist of a 01

corresponds a semicircular blade. All the tests were carried out in a three-bladed system

with blade aspect ratio of 1.83. Performance studies of the rotor system have been made on

the basis of starting characteristics, static torque, rotational speed and coefficient of

performance.

3.1. Blade manufacture

The blades of Savonius rotor fabricated from galvanized iron sheets are attached to a

central shaft held between the two bearings in framework. The schematic diagram of

developed blades is shown in Fig 1. In either case, the blades are having an aspect ratio (H/d) of 1.83, where H and d are the height and the blade chord, respectively. The maingeometric parameters are the blade chord ( 120 mm), blade height ( 220 mm) and the

twist angle (a). The semicircular (a 01) shape of the blade has been made on a rollingmachine. The radius of the rotation R is measured from axis of rotation to the outer edge

of the blades. Twisted blade (a 1012251) under present investigation has a tip radius R1

measured from the tip of the blade to the axis of rotation, whereas root radius R2 ismeasured from the root of the twisted blade (Fig. 2). Each blade has a mass of 126.5 g.

4. Experimental setup

The experiments were carried out in an open circuit wind tunnel (Fig. 3) with the exit

section of 0.375 m 0.375 m in cross section [15,28,29]. The air speed at the tunnel exit

(wind speed) could be varied from 6 to 12 m/s. A single block dynamometer was used to

measure the static torque, while a digital tachometer (with an accuracy of 71RPM)measured the rotational speed (RPM) of the rotor. A thermal velocity probe anemometer

(with an accuracy of70.1 m/s) was used to measure the air velocity. The rotor consisted of blades rolled from sheet metal and attached to a central vertical shaft held between two

bearings in the framework. The rotor axis was kept at a distance of 200 mm from the

tunnel exit (Fig. 3).


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Table 1

Performance of Savonius /S-shaped rotor

Authors Yearof

study

Typeof

rotor

Rotor dia

(m)

Rotor

height

Wind tunnel

dimensions

Free stream

velocity(m/s)

Reynolds

number 105

Tipspeed

ratio

Correctedmax.

Cp (%)

(m) (m m)

Sheldahl et al. [16] (two-

bladed rotor)

1978 Savonius 1.000 1.500 4.9 6.1 closed

sec

14 9.3 0.85 19.5

Sheldahl et al. [16]

(three-bladed rotor)

1978 Savonius 1.000 1.500 4.9 6.1 closed

sec

14 8.67 0.65 15 including

frictional power

Alexander and 1978 Savonius 0.383 0.460 Closed sec 69 1.532.32 0.49 12.5

Holownia [17]

Baird and Pender[23] 1980 Savonius 0.076 0.060 0.305 0.305

closed sec

29.224.6 1.041.25 0.78 18.118.5

Bergless and

Athanassiadis [24]

1982 Savonius 0.700 1.400 3.5 2.5 closed

sec

8 2.83.7 0.70 12.512.8

Sivasegaram and

Sivapalan [25]

1983 0.120 0.150 0.46 0.46 open

jet

18 1.44 0.75 20

Bowden andMc-Aleese 1984 Savonius 0.164 0.162 0.76 m dia open 10 0.871.09 0.680.72 1415

[26] jet

Ogawa and Yoshida

[27] withoutdeflector

1986 S-shaped 0.175 0.300 0.8 0.6 openjet 7 0.81 0.86 17

Ogawa and Yoshida

[27] with deflector

1986 Savonius 0.175 0.300 0.8 0.6 openjet 7 0.81 0.86 21.2

Huda et al. [13] without 1992 S-shaped 0.185 0.320 0.5 m dia open jet 6.512.25 0.081.5 0.680.71 15.217.5

deflector

Huda et al. [13] with 1992 S-shaped 0.185 0.320 0.5 dia open jet 12.25 1.5 0.650.72 1721

deflector

Grinspan [15] (twistof

101)

2002 Twisted

Savonius

0.280 0.22 0.375 0.375

open sec

8.22 1.327 0.669 11.59 excluding

frictional power

Raj Kumar [22] (twist

of 12.51)

2004 Twisted

Savonius

0.250 0.220 0.375 0.375

open sec

8.23 1.327 0.6523 13.99 excluding

frictional power


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Height(H)

=220

mm


R1

R

R2

120

r

S S

Top view of semicircular

bladed rotorTop view of twisted bladed

rotor

Fig. 1. Schematic diagram of the developed blades.

Y-axis

yaxis

=10.28

x x

=10.28

60 mm

Chord = 120 mm

Section at XX

x x

zaxis

xaxis

60mm

Chord = 120mm

Section atX-X

Z-axis

X-axis

Fig. 2. Schematic diagrams of semicircular and twisted blades.

5. Results and discussion

A series of experiments have been carried out with semicircular and twisted types of

Savonius wind turbine rotor in a three-bladed system. All the tests were conducted at a

room temperatu re of 25 1C. Performance studies of the rotor system in both the cases have

been made on the basis of starting characteristics, No load speeds, static torque, torque

coefficient, coefficient of performance and efficiency. The difference of experimental

condition such as the tunnel blockage effect, the Reynolds number, the rotor conditionsand experimental uncertainty makes difficult to compare quantitat ively all the researchers

works. Frictional losses should be taken into account as they may affect performance of

small models substantially. Hence, series of experiments have been conducted in the set upto compare the results of semicircular and twisted blades.


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508mm

RPM

8H

H


450 mm 769 mm 750 mm

Coarse screenHoney comb

2.42H

Fan section

Diffuser Fine screenSetting chamber

Contraction cone (8:1) 240 mm

Bearing

Housing

Twisted bladed

Savonius Rotor

Fan

A.C. Motor20-deg

3500 mm920 mm

Fig. 3. Schematic diagram of the wind tunnel with Savonius rotor.

500

450

400

350

300

250

200

150

100

50

0 deg

10 deg

12.5 deg

15 deg20 deg

25 deg

00 5 10 15 20 25

Time - Sec

Fig. 4. Starting characteristics at wind speed, U 10 m=s.

5.1. Starting characteri stics

The starting characteristics of the twisted bladed rotor at various twist angles (a) at awind speed of U 10 m=s is shown in Fig. 4. The rotor with semicircular blade (a 01)

attains RPM of N 232 in 5 s, while all other twisted bladed rotor goes beyond 350 RPM,thereby indicating a better starting characteristics of twisted bladed rotor. The rotor with


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RPM


a 12:51 shows a maximum value of N 365 in 5 s. It can also be seen from the plot that

after 20 s, the difference in RPM between the twisted bladed and semicircular bladed rotors

is more than 20. Thus, at a wind speed of U 10 m=s, twist angle of 12.51 is preferable. It

stands to reason that for the semicircular blade, the maximum force acts centrally

(curvature center) and vertically. Whereas for the twisted blade, the maximum force moves

towards to the tip of the blade because of the twist in the blade. Due to these changes, a

twisted blade gets a longer moment arm, and hence a higher value of net positive torque.

Moreover, with the increase of twist angles, the energy capture in the lower part of the

blade reduces drastically as compared to the upper part, and hence the net positive torque

reduces.

When tested at a wind speed of U 8 m=s, blades with a 12:51 and 151 show similar

starting characteristics over the entire range of time (Fig. 5), and thus found to be superiorthan the semicircular bladed rotor. The starting characteristics at a wind speed of U

7 m=s shows an optimal twist angle of151 as seen from Fig. 6. The effect of twist angleat

various airspeeds can be studied from Fig. 7. It has been observed that higher twist angle

captures more energy at lower airspeeds and vice versa. Furthermore, the starting

characteristics are better at higher airspeeds than at lower airspeeds for all the twist angles.Three-bladed semicircular Savonius rotor is well known for its self-starting character-

istics and it has been improved by providing a twist to these blades. Semicircular blades are

taken as zero angle of twist, and by increasing the angle, the performance of the Savonius

rotor is increased in its starting characteristics and static toque.

5.2. No-load speeds

Variation of no-load RPM with the wind speed is shown in Fig. 8. There is a sharp rise

in speed at U 6:528 m=s. Blade with a 151 shows maximum rise in RPM than a

450

400

350

300

250

200

150

100

50

0 deg

10 deg

12.5 deg

15 deg

20 deg

25 deg

00 5 10 15 20 25

Time - Sec



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RPM

RPM


300

250

200

150

100

50

0 deg

10 deg

12.5 deg

15 deg

20 deg

25 deg

0

0 5 10 15 20 25

Time - Sec


500

450

400

350

300

250

200

150

100

50

10 m/s

8 m/s

7 m/s

0

0 5 10 15 20 25Time - sec

Fig. 7. Starting characteristics at wind speed, U 7; 8; 10 m=s.

12:51 in the range of U 6:528 m=s. However, a 12:51 gives a better performance thana 151 in the range of U 8210 m=s. It is evident that larger twist angle is preferable in

the lower range of wind speed for producing maximum power and better starting


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0 deg 10 deg 12.5deg

15 deg 20 deg 25 deg

RPM

Torque

Nm

040

80

120

160

200

240

280

320


600

500

400

300

200

100

0

6 6.5 7 7.5 8 8.5 9 9.5 10 10.5

Wind Speed, m/s

Fig. 8. Variation of RPM with velocity for twisted bladed rotor at various twistangles.

0340

320

300

280

260

240

2040

60

80

100

120

0.07

0.06

0.05

0.04

0.03

0.02

12.5 deg 0 deg

220200

180

140160

0.01

0

12.5 deg 0deg

Angle deg

Fig. 9. Static torque vs. orientation angle diagram at U 10:17 m=s.

characteristics. Thus, from starting acceleration and maximum no load speed character-

istics, a 151 becomes the optimal angle at low velocity of 6.5 m/s. Further, with the

increase of twist angles (from a 151 to 251), the energy capture in the lower part of the

blade reduces drastically.

5.3. Static torque diagram comparisons

The static torque of the rotors has been measured at 201 intervals for one complete

revolution as shown in Fig. 9. The area under T y diagram for twisted blade shows a

larger area as compared to the semicircular bladed rotor. The static torque coefficient of


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Co-effofTo

rque

Co-effofTorque


0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0

0 20 40 60 80 100 120 140

Angle,deg

0 deg Twist 10 deg Twist 12.5 deg Twist 15 deg Twist

Fig. 10. Static torque coefficient for various twisted bladed Savonius rotor at U 10 m=s.

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

0

75 80 85 90 95 100 105 110 115 120

Angle, deg

0 deg 10 deg 12.5 deg 15 deg

Fig. 11. Shipment of stall angle for various twisted bladed rotor at wind speed U 10 m=s.

semicircular and twisted blades (a 02151) in a three-bladed rotor system is shown for

1201orientat ion (Fig. 10). The stalling angle of twisted blade is found to be shifted by 251

with the increase in angle of twist from a 0 to 12.51 (Fig. 9). It can also been seen from

Fig. 11 that with the increase of twist angles, the stalling angle shifts further. Moreover, the

twisted blade shows a maximum peak torque and a lesser falling slope, and hence a greater

area than the semicircular blades (Fig. 9). It is clear that the Savonius rotor is not self-

starting at three specific positions. Due to friction, these models are not developing

sufficient powers to start rotation. However, by measuring frictional tare torque with an

air motor, it is possible that at every angle oforientation the rotor will develop some static

torque as observed by Sheldahl et al. [16]. This stalling problem can be avoided by making

two stages of rotor one above the other with a stagger of 601. Due to this, the starting

capability would be higher, thus giving a higher torque and efficiency as compared to thesemicircular bladed rotor. There is a wide variation of static torque coefficient with angular


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Cp


0.16

0.12

0.08

0 deg

10 deg

12.5 deg15 deg

0.04

0

0 2 4 6 8 10 12

Wind Speed,m/s

Fig. 12. Variation of coefficient ofperformance with velocity for various twisted bladed rotors.

position of rotor. Thus, to initiate rotation, the aerodynamic torque must exceed combined

load and friction torques for a rotor from any angular position. This implies that the

minimum value of static torque coefficient may be the deciding factor controlling the size

and stacks of the Savonius rotor [30].

5.4. Coefficient of performance comparison

Fig. 12 compares the performance of the Savonius rotor with different twist angles at

various airspeeds. From the performance viewpoint, a 151 is superior at lower windvelocities, whereas a 12:51 is suitable at higher velocities. Maximum coefficient of

performance, Cp 13:99 is found at tip speed ratio of l 0:65 (U 8:23 m=s) and

forsemicircular bladed rotor is giving Cp 11:04 at the same velocity.

6. Conclusions

In summary, wind tunnel studies show the potential of the Savonius rotor with twisted

blades in terms of smooth running, higher efficiency and self-starting capability as

compared to that of the semicircular bladed rotor. The principal observations of the

present findings can be briefly stated as under:

For the twisted blade, the maximum force moves towards to the tip of the blade

because of the twist in the blade. Due to these changes, a twisted blade gets

a longer moment arm, and hence a higher value of net positive torque. Moreover,

with the increase of twist angles, the energy capture in the lower part of the

blade reduces drastically as compared to the upper part, and hence the netpositive

torque reduces.

Three-bladed semicircular Savonius rotor is well known for its self-star ting

characteristics and it has been improved by providing a twist to these blades.Semicircular blades are taken as zero angle of twist, and by increasing the angle, the

performance of the Savonius rotor is increased in itsperformance.

Larger twist angle is preferable in the lower wind velocity for producing maximum

power and better starting characteristics. The twist angle a 151 gives optimum


12/13


performance at low airspeeds of U 6:5 m=s in terms of starting acceleration and

maximum no load speed.

The stalling angle of twisted blade is found to be shifted by 251 with the increase in angle

oftwist from a 01 to 12.51, and it has been found that the stalling angle shifts further

with the increase of twist angle.

This stalling problem can be avoided by making two stages (stacking) of rotor one

above the other with a stagger of 601. Due to this, the starting capability would be

higher, and hence a higher torque and efficiency as compared to the semicircularbladed

rotor.

Twisted blade with a 151 shows a maximum of Cp 13:99 and Z 23:6 at tip speed

ratio ofl 0:65 (i.e., at U 8:23 m=s), whereas the semicircular blade (a 01) shows aCp 11:04 and Z 18:67 at the airspeed. This significant raise ofCp and efficiency are

inevitable to further proceeding in this area.

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