Experimental Analysis of a 20° Twist Helical Savonius Rotor at Different

Overlap Conditions

Bachu Deb1, a, Rajat Gupta 2, b and R.D. Misra3, c 1Ph.D. Scholar, Department of Mechanical Engineering, NIT Silchar

Silchar - 788010, Assam, India

2Professor & Director, Department of Mechanical Engineering, NIT Srinagar, India

3Professor, Department of Mechanical Engineering, NIT Silchar - 788010, Assam, India

abachudeb@gmail.com,brguptanitsri@gmail.com, crdmisra@gmail.com

Keywords: Helical Savonius rotor, power coefficient, tip speed ratio, overlap ratio.

Abstract: Wind power is a major source of sustainable energy and can be harvested using both

horizontal and vertical axis wind turbines. Vertical axis wind turbines (VAWTs) accrue more

popularity due to its self starting characteristics and Omni directional in nature. Out of which

Savonius rotor is the most popular drag-based VAWT which is having lower efficiencies but having

good self starting characteristics. In order to improve the performance, helix in the tip of the blade

is targeted which reduces the negative torque coefficient of the rotor thereby could improve the

performance of the rotor. Therefore, in this paper the power coefficients of a two-bucket helical

Savonius rotor at different overlap ratios (from 0.0% to 19.76%) with helix twist angle of 20° are

investigated experimentally. The investigations mainly concentrate to find out the optimum overlap

ratio which is responsible for generation of maximum aerodynamic power. It is seen from the

results that the power coefficient of the rotor increases with the increase in overlap ratio up to a

certain limit, and further increase of the same decreases the power coefficients. The maximum

power coefficient Cp of 0.289 is obtained at an optimum overlap ratio of 12.76 %.

Introduction

Wind is the world’s fastest growing energy today and it has significantly retaining the position

consecutively for the last five years. The major advantages of this growing technology are to reduce

carbon dioxide emission to the environment. The five leading countries shown in fig.1, China,

USA, Germany, Spain, and India, represent together a total share of 74% of the global wind

capacity. The top ten countries in terms of installed wind power capacity are china (67,774 MW),

USA (49,802 MW), Germany (30,016 MW), Spain (22,087 MW), India (17,351 MW), France

(7,182 MW), Italy (7,280 MW), France (7,182 MW), U.K (6,840 MW), Canada (5,511 MW),

Portugal (4,398 MW), Rest of the World (35,500 MW).

Figure 1: Top ten wind power installed capacity (2011-2012 MW) Source WWEA 2012

Thus in the past few decade many researcher had worked on the different design of conventional

Savonius rotor to improved the performance of the rotor such as Fujisawa [1] and Fujisawa &

Applied Mechanics and Materials Vols. 592-594 (2014) pp 1060-1064 Submitted: 19.05.2014Online available since 2014/Jul/15 at www.scientific.net Accepted: 20.05.2014© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.592-594.1060

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.207.50.37, Georgia Tech Library, Atlanta, USA-16/11/14,05:45:18)

Gotoh [2] mainly studied the influence of overlap ratio on the aerodynamic characteristics of the

rotors and reported that the static torque performance is improved by increasing the overlap ratio on

the returning blade due to the pressure recovery effect through the overlaps. They also observed that

conda like flow pattern contributes largely for the generation of torque on the convex side of the

advancing blade. Further again, Fujisawa [3] studied the aerodynamics performance of a Savonius

rotor with and without rotation for measuring the pressure distributions on the blade surfaces at

various rotor angles and tip-speed ratios and reported that drastic change in pressure distribution

occurs on the convex side of the advancing blade of a rotating rotor compare with without rotation.

Saha and Jaya Rajkumar [4] tested three bladed twisted rotor performances over conventional

Savonius rotor in a low speed wind tunnel and obtained, a twist angle of 15° has a maximum power

coefficient of 0.14 at a tip speed ratio of 0.65. Further investigation made by Kamoji et.al [5] on two

bucket helical Savonius rotor with and without shaft with a bucket twist angle of 90° at different

overlap ratios of 0.0, 0.1 and 0.16 and compared their results with conventional Savonius rotor.

They conducted their experiments on an open jet wind tunnel, and reported that helical Savonius

rotor with and without shaft having positive coefficient of static torque at all the rotor angles and

their aerodynamic performance in terms of power coefficient and torque coefficient are also high.

Damak et.al [6] conducted experiments on two bucket helical Savonius rotor at bucket twist angle

of 180° and compared their results with conventional Savonius rotors particularly in terms of

overlap ratios and Reynolds number. Further, Bhaumik & Gupta [7] studied experimentally the

performance of helical Savonius rotor at 45º twist angle in a centrifugal blower. They consider the

provision of different overlap ratio from 0.106 to 0.186. It is concluded from their result that

maximum Cp is obtained as 0.421 at an overlap ratio of 0.147. Gupta & Deb [8-9] studied the CFD

analysis of a two bucket helical Savonius rotor with shaft at 45º twist angle. From their study they

concluded that the highest values of dynamic pressure and velocity magnitude were obtained at the

chord ends with 450

bucket twist and 900 rotor angle, which would ensure improved performance of

the rotor as a whole by increasing the aerodynamic torque production of the rotor. Later on Deb

et.al [10] studied the performance of helical roror at 10° twist angle. They obtained maximum Cp of

0.191 at overlap ratio of 0.127.

However, only few works on Helical Savonius rotor were reported in the literature. There is

hardly any reported literature on the detailed performance investigations at different designed

overlap conditions for the helical Savonius rotor. Keeping this in view, a two bucket helical

Savonius rotor with a bucket twist of 20° was designed and fabricated and tested in a centrifugal

blower test rig at different gate openings. Power coefficients were evaluated at six different overlap

ratios ranging from 0.00% to 19.76%.

Experimental Setup

The experiment was carried out at the exit of a centrifugal blower test rig shown in fig. 1 in open

condition. The air is sucked from the atmosphere into the suction side and the slightly compressed

air passes through the spiral casing before it comes out through the outlet. The motor of the blower

has rated r.p.m of 2880 and 10 HP. The model was kept at the exit side and the gate opening which

can be adjusted to achieve a wide range of velocities. RPM of the rotor was recorded with the help

of a non-contacting type digital tachometer having an accuracy level of ± (0.05%+1 digit) and

measuring range of (2.5 to 99,999 RPM) with sampling time of 0.8 sec(over 60 RPM). The free

stream velocity can be measured with the help of a digital anemometer with a range of 0.4-30 m/s at

an accuracy level of ± (2% + 0.2 m/s).

Applied Mechanics and Materials Vols. 592-594 1061

Figure 1: Schematic diagram of the experimental setup

Design and Fabrication of the Model

The two-bucket helical Savonius rotors at 20° twist angle as shown in Fig.1 were designed and then

fabricated in the departmental workshop. The buckets were semi cylinder in cross section which

was attached to the rotor shaft using flange at both ends. In order to minimize friction, the rotor was

supported with bearings of very low friction and the whole assembly is mounted on the base. The

model is fabricated with an aluminium sheet of having the gauge thickness of 2mm with chord

length of 9cm and bucket height of 18cm. Rotor shaft is made up of mild steel rods with 1.5 cm in

diameter and 25 cm in length. The base where the rotor shaft is mounted is made up of cast iron

with a base width of 7cm and 2.4 cm in thick. The buckets had the provision to set six overlap

ratios, namely 0.00%, 10.56%, 12.76%, 15.0%, 17.2% and 19.5% with the help of nuts and washer

arrangements between the buckets and rotor shaft which causes the change in the overall diameter

of the rotors.

Figure 2: (a) Two bucket helical Savonius rotor at 20° twist angle (b) At without overlap condition

(c) At overlap condition

Analysis of Results

The performance of a wind rotor is characterized by the various design parameters which is usually

display in terms of its power coefficient (Cp) versus tip speed ratio (λ). In this paper all the

measured experimental data is calculated from the standard relationship given by the following

relations:

= =12

=

12

( − )

12

[1]

= [2]

= [3]

= [4]

Figure 3 to 8 illustrates the enhancement of power coefficient of 20° twist helical Savonius rotor

over all expected tip speed ratio. It is recommended that the rotor performance can be improved by

making an overlap gaps between the two-blade [1, 2]. As a consequence of which six different

overlap ratios (0.00%, 10.56%, 12.76%, 15.0%, 17.2% and 19.5 %) are considered and tested. It is

observed from the tested result shown in Fig. 3 that at no overlap condition (0.00%), the maximum

Cp enriched 0.213 at the TSR of 0.585. Likewise at 10.56% overlap ratio shown in Fig. 4, maximum

1062 Dynamics of Machines and Mechanisms, Industrial Research

Cp of 0.250 is obtained at a TSR of 0.679. Fig. 5, at 12.76 % overlap ratio, the maximum Cp of

0.289 is obtained at the TSR of 0.760. For 15% overlap ratio the maximum Cp of 0.222 is obtained

at a TSR of 0.627 shown in Fig. 6. Thus it is perceive from the previous overlap condition that at

this overlap ratio, the performance of the rotor decreases. Therefore, to verify the above statement,

two more overlap ratio beyond 15.0% is tested. At 17.2% overlap ratio, the maximum Cp of 0.187 at

a TSR of 0.543 is obtained shown in Fig. 7. Likewise at 19.5% overlap ratio, the maximum Cp of

0.161 at a TSR of 0.417 is obtained shown in Fig. 8. Thus it is clear that, no subsequent overlap

ratio is required to be test beyond 19.5% overlap ratio as the performance decreases consecutively.

The test results of the 20° twisted helical Savonius rotor demonstrate how the maximum power

performance of the rotor depends on tip speed ratio and overlap ratio. Accordingly, it is observed

from the result that for all overlap conditions the same trend persist i.e. both power and torque

coefficient of the rotor increases with the increase in tip speed ratio up to a certain limit which is

also reported by Kamoji et al. [5]. It is also observed that at an optimum overlap ratio of 12.76%,

the energy transfer is most efficient and thus the power coefficient is the maximum (Cpmax= 0.289).

The tip speed ratio that executes best performance for this rotor lies within the range of 0.51-0.90.

Figure 3: Variation of Cp with TSR at

without overlap

Figure 4: Variation of Cp with TSR at 10.56 %

overlap

Figure 5: Variation of Cp with TSR at 12.76

% overlap

Figure 6: Variation of Cp with TSR at 15.0 %

overlap

Figure 7: Variation of Cp with TSR at 17.2

% overlap

Figure 8: Variation of Cp with TSR at 19.5 %

overlap

0.00

0.10

0.20

0.30

0.20 0.45 0.70 0.95

Cp

Tip Speed Ratio

Without Overlap

0.00

0.10

0.20

0.30

0.20 0.45 0.70 0.95

Cp

Tip Speed Ratio

Overlap = 10.56 %

0.00

0.20

0.40

0.20 0.45 0.70 0.95

Cp

Tip Speed Ratio

Overlap = 12.76 %

0.00

0.10

0.20

0.30

0.20 0.45 0.70 0.95

Cp

Tip Speed Ratio

Overlap = 15.0 %

0.00

0.10

0.20

0.20 0.40 0.60 0.80

Cp

Tip Speed Ratio

Overlap = 17.2 %

0.00

0.10

0.20

0.20 0.35 0.50 0.65

Cp

Tip speed Ratio

Overlap = 19.5 %

Applied Mechanics and Materials Vols. 592-594 1063

Conclusion

In this work, the performance of helical Savonius rotor at 20° twist angle with overlap ratio of

0.00%, 10.56%, 12.76%, 15.0%, 17.26% and 19.56% are investigated experimentally to determine

the optimum overlap ratio. The result show that performance of the rotor increase with the increase

of overlap ratio upto a certain limit and then decrease further increase in tip speed ratio. The

maximum power coefficient of 0.289 was obtained for an optimum overlap ratio of 12.76% and tip

speed ratio that executes best performance for this rotor lies within the range of 0.51-0.90.

Reference

[1]Fujisawa N. On the torque mechanism of Savonius rotors. Journal of Wind Engineering and

Industrial Aerodynamics 1992; 40: 277–292.

[2]Fujisawa N, Gotoh F. Pressure measurements and flow visualization study of a Savonius rotor.

Journal of Wind Engineering and Industrial Aerodynamics 1992; 39: 51-60.

[3] Fujisawa N, Gotoh F. Experimental Study on the Aerodynamics performance of a Savonius

rotors. Journal of Solar Energy Engineering, Transactions of the ASME 1994;116: 148-152.

[4] Saha UK, Jaya Rajkumar M. On the performance analysis of Savonius rotor with a twisted

blades. Renewable Energy 2006; 31: 1776-1788.

[5] Kamoji MA, Kedare SB, Prabhu SV. Performance tests on helical Savonius helical Savonius

rotors. Renewable Energy 2009; 34: 521-529.

[6] Damak A, Driss Z, Abid MS, Experimental Investigation of helical Savonius rotor with a twist

of 180°. Renewable Energy 2013; 52: 136-142

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[10] Deb B, Gupta R, & Misra R.D. “Experimental Analysis of a 10° Helical Savonius Rotor at

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Dynamics of Machines and Mechanisms, Industrial Research 10.4028/www.scientific.net/AMM.592-594 Experimental Analysis of a 20° Twist Helical Savonius Rotor at Different Overlap Conditions 10.4028/www.scientific.net/AMM.592-594.1060

DOI References

[1] Fujisawa N. On the torque mechanism of Savonius rotors. Journal of Wind Engineering and Industrial

Aerodynamics 1992; 40: 277-292.

http://dx.doi.org/10.1016/0167-6105(92)90380-S [2] Fujisawa N, Gotoh F. Pressure measurements and flow visualization study of a Savonius rotor. Journal of

Wind Engineering and Industrial Aerodynamics 1992; 39: 51-60.

http://dx.doi.org/10.1016/0167-6105(92)90532-F [3] Fujisawa N, Gotoh F. Experimental Study on the Aerodynamics performance of a Savonius rotors. Journal

of Solar Energy Engineering, Transactions of the ASME 1994; 116: 148-152.

http://dx.doi.org/10.1115/1.2930074 [4] Saha UK, Jaya Rajkumar M. On the performance analysis of Savonius rotor with a twisted blades.

Renewable Energy 2006; 31: 1776-1788.

http://dx.doi.org/10.1016/j.renene.2005.08.030 [5] Kamoji MA, Kedare SB, Prabhu SV. Performance tests on helical Savonius helical Savonius rotors.

Renewable Energy 2009; 34: 521-529.

http://dx.doi.org/10.1016/j.renene.2008.06.002