Experimental investigation of helical Savonius rotor with a twist of 180°

Experimental investigation of helical Savonius rotor with a twist of 180°

Renewable Energy 52 (2013) 136e142 Contents lists available at SciVerse ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/ren...

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Renewable Energy 52 (2013) 136e142

Contents lists available at SciVerse ScienceDirect

Renewable Energy journal homepage: www.elsevier.com/locate/renene

Technical note

Experimental investigation of helical Savonius rotor with a twist of 180 A. Damak*, Z. Driss, M.S. Abid National Engineering School of Sfax (ENIS), Laboratory of Electro-Mechanic Systems (LASEM), University of Sfax, B.P. 1173, km 3.5 Soukra, 3038 Sfax, Tunisia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 April 2012 Accepted 31 October 2012 Available online 27 November 2012

Conventional Savonius rotors have low performance such as low coefficient of power and low coefficient of torque. In order to increase this performance, a helical Savonius rotor with a twist of 180 is proposed. In this paper, we are interested in studying the aerodynamic behavior of the helical Savonius rotors installed in an open jet wind tunnel. Particularly we are interested in studying the influence of variation of Reynolds number and the overlap ratio on the performance of a modified Savonius rotor with aspect ratio of 1.57 at a Reynolds numbers equal to Re ¼ 79,794, Re ¼ 99,578, Re ¼ 116,064 and Re ¼ 147,059. Results conclude that the variation of Reynolds number and overlap ratio has an effect on the global characteristics of the helical Savonius rotor. A comparison between the helical one and the conventional one shows that the maximum power coefficient of the Savonius wind rotor is higher. This work is developed at Laboratory of Electro-Mechanical System (LASEM) of the National School of Engineers of Sfax (ENIS). Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Helical Savonius rotor Aerodynamic performance Experimental results Wind tunnel

1. Introduction Wind energy is an energy source having a low impact on the environment, low effects on health, negligible safety issues and is completely renewable. Currently, wind power is the fastest growing source of energy. There are several types of converters of this energy, such as the wind turbine. It’s a device to change the wind into mechanical energy. These turbines are generally classified as two families, named horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs). This classification refers to the position of rotor axis relatively to wind. The Savonius rotor is a vertical axis wind turbine (VAWT) developed by the Finnish engineer, Sigurd Savonius, in 1925 [1]. Savonius rotor is considered as a drag wind turbine whose principle of operation is based mainly on the difference of drag between the convex and the concave parts of the blades. Even if the performance of these rotors is lower than the other conventional wind rotors, they have more advantages. Such us, the design of such rotors is simple and cheap. They start to run on their own and they are independent of the direction of the wind [2]. They also have good starting characteristics. 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 [3,4]. Numerous investigations have been carried out to study the performance

* Corresponding author. Tel.: þ216 74 274 088; fax: þ216 74 275 595. E-mail address: [email protected] (A. Damak). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.10.043

characteristics of the Savonius rotor. A number of scientists have tested many models of Savonius rotor and they have experimentally and numerically examined the effects of various design parameters of Savonius wind rotor such us the rotor aspect ratio, the overlap ratio, the profile change of the bucket cross-section, the number of buckets, the presence or absence of rotor end plates, the influence of bucket stacking, and the influence of Reynolds numbers [5e9]. Saha and Jaya Rajkumar [10] tested a twisted blade in a three-bladed rotor system in a low-speed wind tunnel, and they compared the performance with conventional semi-circular blades. Altan et al. [11] placed a curtain in front of the rotor in order to increase the performance of the Savonius wind rotor. The geometrical parameters of the curtain arrangement were optimized to generate an optimum performance. The rotor with different curtain arrangements was tested, and its performance was compared with that of the conventional rotor. Saha et al. [12] are conducted wind tunnel tests to assess the aerodynamic performance of single, two- and three-stage Savonius rotor systems. Both semi-circular and twisted blades have been used in either case. They report that the twisted tow bladed Savonius rotor with twostage has a maximum coefficient of power of 0.32. Kamoji et al. [13] studied a modified Savonius rotor without central shaft between the two end plates, tested in an open jet wind tunnel. Investigation is undertaken to study the effect of geometrical parameters on the performance of the rotors in terms of coefficient of static torque, coefficient of torque and coefficient of power. The parameters studied are overlap ratio, blade arc angle, aspect ratio and Reynolds number. They found that the modified Savonius rotor

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with an overlap ratio of 0.0, blade arc angle of 124 and an aspect ratio of 0.7 has a maximum coefficient of power of 0.21 at a Reynolds number of 150,000, which is higher than the conventional Savonius rotor. Irabu and Roy [14] improved and adjusted the output power of Savonius rotor under various wind power. They found that the maximum rotor rotational speed was achieved in the range of the guide-box area ratio between 0.3 and 0.7 and the value of the output power coefficient of the rotor with guide-box tunnel of the area ratio 0.43 increases about 1.5 times with three blades and 1.23 times with two blades greater than that without guidebox tunnel, respectively. It seems that the performance of Savonius rotor within the guide-box tunnel is comparable enough with other methods for augmentation and control of the output. Golecha et al. [15] studied the influence of the location of the deflector plate on the performance of a modified Savonius rotor with water as the working medium at a Reynolds number of 1.32  105. They attempted eight different positions of the deflector plate. They found that deflector plate placed at its optimal position increases the coefficient of power by 50%. Two stages and three stages modified Savonius rotors are tested to study the influence of deflector plate at the optimal position. Maximum coefficient of power improves by 42%, 31% and 17% with deflector plate for two stages 0 phase shift, 90 phase shift and three stages modified Savonius rotor respectively. Kamoji et al. [16] tested a helical Savonius rotors with a twist of 90 conducted in an open jet wind tunnel, comparing the performance of a conventional Savonius wind rotor and that of a helical Savonius wind rotor. Akwa et al. [17] numerically investigated the influence of the buckets overlap ratio of a Savonius wind rotor on the averaged moment and power coefficients. They have used a software which is based on Finite Volume Method and they compared their results with experiments of other authors. Altan and Atılgan [18] arranged a curtain design so that to improve the low performance levels of the Savonius wind rotors. This curtain has been placed in front of the rotor. Performance experiments have been carried out when the rotor is with and without curtain. The results obtained experimentally have been supported with numerical analysis. Dobrev and Massouh [19] used CFD to study the behavior of a Savonius wind turbine under flow field conditions and to determine its performance and the evolution of wake geometry. The flow analysis helps for the design of the wind turbine. They are making an experimental investigation in wind tunnel using PIV to validate simulations. There is investigation which permits to determine the structure of the real flow and the access of numerical simulations quality. Afungchui et al. [20] numerically explored the non-linear two-dimensional unsteady potential flow over a Savonius rotor and develop a code for predicting its aerodynamics performances. The torque distribution of the stationary rotor and the unsteady pressure field on the blades of the rotating rotor, predicted by the code developed, have been compared and validated by some experimental data.

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Altan and Atılgan [21] analyzed both experimentally and numerically the effect of presence of the curtain on the static rotor performance. This curtain has been placed in front of the rotor to prevent the torque that occurs on the convex blade of the rotor in the negative direction. They noted that the static torque values with the long curtain dimensions were increased. According to these studies, we can confirm that the performance of the conventional Savonius rotor can be ameliorated. In the present paper, we propose to use helical Savonius rotor with a twist of 180 . This rotor could provide positive coefficient of static torque. Helix can be defined as a curve generated by a marker moving vertically at a constant velocity on a rotating cylinder. The blade retains its semi-circular cross-section from the bottom (0 ) to the top (180 ). In this study, we are experimentally investigating the effect of overlap ratio (0.0 and 0.24), and the effect of the Reynolds number (Re ¼ 79,794, Re ¼ 99,578, Re ¼ 116,064 and Re ¼ 147,059) on the performance of the rotor. These experimental results are compared with the conventional Savonius rotor. 2. Material and methods 2.1. Test facility To study the wind turbine performance, a wind tunnel is designed and constructed [22]. The experimental device is used to predict the aerodynamic behavior and investigate the conditions experienced by a wind turbine placed in the air flow. In our case a low-speed wind tunnel with an open test section facility has been developed as shown in Fig. 1. The rotor axis is placed in the middle of the test section (cuboid shape), having a cross-section area of 400 mm  400 mm. By changing the rotation frequency of the vacuum cleaner with variable speed, the wind tunnel exit air velocity can be changed. The entire tests have been conducted with a maximum value of air velocity equal to 12.7 m/s. A thermal velocity probe anemometer was used to measure the air velocity with an accuracy of 0.1 m/s, while the rotational speed (RPM) of the rotor was measured with a digital tachometer. Furthermore, it’s important to indicate that the obtained results are difficult to be evaluated in the fully external flow. In fact, in these conditions, a stable external flow could not be obtained. So, the wind tunnel was used to obtain a uniform flow which allows deducing the Cp curve by changing the electrical load. 2.2. Torque measurement To measure the dynamic torque on the rotor shaft, a DC generator was used, which transforms the torque on its axis to an electrical current. At first, we must couple the generator to the electric motor that can display simultaneously the speed and the torque. The electric motor is used to provide mechanical power to the

Fig. 1. Schematic of the open jet wind tunnel.

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twisted Savonius rotor which is exactly the top view of conventional Savonius rotor. The power coefficient Cp of a wind turbine is defined by:

  Cp ¼ 2P= ArV 3

(1)

In case of twisted Savonius rotor the swept area is equal to:

A ¼ HD

(2)

The coefficient of torque Cm is given by:

  Cm ¼ 4 M= rV 2 D2 H

(3)

The tip peripheral velocity of the rotor is equal to:

U ¼ ur

(4)

The tip speed ratio of the turbine is defined as:

Fig. 2. Calibration curve currentetorque.

l ¼ U=V generator which delivers an electric current in a resistive load. Torque measurement integrated into the electric motor, allows tracing the calibration curve that connects the electric current supplied by the generator to the dynamic torque. This calibration curve will be used to determine the dynamic torque by referring to the value of the electric current supplied by the generator. Fig. 2 shows the calibration curve obtained with the generator.

(5)

The aspect ratio represents the height of the rotor relatively to its diameter.

AR ¼ H=D

(6)

The overlap ratio is represented by:

b ¼ e=d

(7)

The Blockage ratio (B) is given by:

2.3. Data reduction In the present investigation, a helical Savonius rotor has been manufactured to improve the low aerodynamic performance of Savonius wind rotor. The rotor was studied in a low-speed wind tunnel. It should be noted that the blade with a twist of a ¼ 0 corresponds a semi-circular blade. The helical rotor consists of two parts, each part can be defined as a curve generated by a marker moving vertically at a constant velocity on a rotating cylinder (at a constant angular velocity). Fig. 3 shows a single helical rotor with two blades. The blade retains its semi-circular cross-section from the bottom (0 ) to the top (180 ). Fig. 1 shows the top view of the

B ¼

HD Hw W

(8)

3. Experimental results In this investigation, the power and the torque of the helical Savonius rotor for different air flow conditions were measured. From these measures, the power and the torque coefficients could be deduced in function of the specific speed l. These global characteristics were made with helical rotor with an aspect ratio of 1.57.

Fig. 3. Helical Savonius rotor.

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3.1. Power coefficient Helical Savonius rotor with an overlap ratio of 0.0 is tested at different Reynolds numbers corresponding to wind velocities of 6 m/s, 7.5 m/s, 8.8 m/s and 11.1 m/s. These velocities correspond respectively to Reynolds numbers equal to Re ¼ 79,794, Re ¼ 99,578, Re ¼ 116,064 and Re ¼ 147,059. Figs. 4 and 5 show the variation of coefficients of power Cp for a helical rotor with an aspect ratio of 1.57 and overlap ratio of 0.0 at different Reynolds numbers. According to these results, it is observed that the maximum coefficient of power increases with the increase of the Reynolds number. These observations are also reported by Kamoji [16] and Akwa et al. [17] for conventional Savonius rotors. Also, tip speed ratio at which the maximum coefficient of power, increases with the increase of the Reynolds number from 79,794 to 147,059. Particularly, it is noted that the curves present the same pace. At a Reynolds number of 147,059, maximum coefficient of power of 0.25 occurs at a tip speed ratio in the range of 0.4e0.45. This fact is also reported by Kamoji et al. [16] for the helical Savonius rotors with a twist of 90 . 3.2. Torque coefficient Figs. 6 and 7 show the evolution of the torque coefficients of the helical Savonius rotor with a twist of 180 depending on the speed ratio l at different Reynolds number. Coefficient of torque is almost dependent of Reynolds numbers Re ¼ 79,794, Re ¼ 99,578, Re ¼ 116,064 and Re ¼ 147,059. These results show that the Reynolds numbers have an effect on the torque coefficient values.

Fig. 5. Effect of Reynolds numbers on the power coefficient.

Coefficient of torque increases nearly linear with the decrease in the tip speed ratio. Globally, for a fixed value of tip speed ratio l, it’s noted that the helical Savonius rotor torque coefficient reaches the most important values for the Reynolds numbers equal to 147,059. 3.3. Effect of the overlap ratio In this section, the influence of the overlap ratio b given in the equation (7) was studied. The best efficiencies are obtained for values of b between 20 and 30% [1,22]. Particularly, Menet [23,24]

Fig. 4. Variation of power coefficient at different Reynolds numbers.

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Fig. 6. Variation of torque coefficient at different Reynolds numbers.

proved that the optimum coefficient of overlap ratio b ¼ e/d is equal to 0.242. This ratio was maintained to take account of the blocking effect introduced by the shaft. Fig. 8 shows the evolution of the power coefficients depending on the speed ratio l at different Reynolds numbers equal to Re ¼ 79,794, Re ¼ 99,578,

Re ¼ 116,064 and Re ¼ 147,059. Particularly, recoveries equal to (e  e0 )/d ¼ 0 and (e  e0 )/d ¼ 0.242 was examined. According to these results, it is found that the value of power coefficient for the overlap ratio of 0.242 is the maximum whatever the value of the Reynolds number. 4. Comparison of helical Savonius with the conventional Savonius

Fig. 7. Effect of Reynolds numbers on the torque coefficient.

Fig. 9 shows the superposition of the experimental results of the conventional Savonius rotor (with the twist of 0 ) and the helical rotor (with the twist of 180 ) at Reynolds number equal to 116,000 and an overlap ratio of 0.0. These results show the effect of the geometrical parameters on the power coefficient. In these conditions, the values of the power coefficients Cp reach largest values in the case of a helical Savonius rotor. Indeed, the maximum of power coefficient of the helical Savonius rotor is 0.2 at a tip speed ratio of 0.33. However, the maximum of power coefficient of the conventional Savonius rotor is equal to 0.16 at the same speed ratio. In fact, in the case of conventional turbine, the torque developed by the rotor is alternative and sometimes takes positive values and sometimes negative values. In contrast, in the case of helical turbine, the torque is also alternative but always positive. Consequently, the average torque developed by the helical rotor is greater than that developed by the conventional rotor. So, the performance increases of the helical Savonius.

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141

Fig. 8. Effect of the aspect ratio (e  e0 )/d on the power coefficient of the helical Savonius rotor.

0.3 Helical Savonius

0.25

Conventional Savonius

0.2 0.15

gives better performances than the conventional one. Also, the effect of the Reynolds number and the overlap ratio on the performance of the helical Savonius rotor was determined. It is observed that the helical Savonius rotor is sensitive to the Reynolds number. The overlap ratio of 0.242 is better than the overlap of 0.0. The experimental results presented will be used to validate the numerical results developed in our Laboratory of Electro-Mechanic Systems at National School of Engineers of Sfax.

0.1

Nomenclature 0.05 0 0

0.2

0.4

0.6

0.8

Fig. 9. Comparison of helical Savonius rotor with the conventional Savonius rotor.

D d H Hw W r A

5. Conclusion

u

In this paper, the performance of the helical Savonius wind rotor was investigated. A helical geometry has been developed and experimently investigated that presented the global characteristics of the helical Savonius rotor. The comparison of these results with the conventional Savonius rotor shows that the helical geometry

U V e e0 P M

diameter of the rotor (m) diameter of the blade(m) height of the rotor (m) height of the test chamber (m) width of the test chamber (m) radius of the rotor (m) swept area of the rotor (m2) rotating speed of the rotor (s1) tip peripheral velocity of the rotor (m/s) speed of air (m/s) overlap portion of the two buckets (m) shaft diameter (m) power (W) torque (N m)

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Greek letters r density of fluid (kg m3) m dynamic viscosity of the fluid (Pa s) l speed ratio q angular coordinate (rad)

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