Wells Turbine for Wave Energy Conversion -Effect of Trailing Edge Shape-

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1 International Journal of Fluid Machinery and Systems DOI: Vol. 9, No. 4, October-December 2016 ISSN (Online): Original Paper Wells Turbine for Wave Energy Conversion -Effect of Trailing Edge Shape- Katsuya Takasaki 1, Tomohiro Tsunematsu 2, Manabu Takao 3, M M Ashraful Alam 3, Toshiaki Setoguchi 4 1 Graduate School of Science and Engineering, Saga University 1 Honjo-machi, Saga , Japan, takasaki@agl-inc.co.jp 2 Advanced Engineering Faculty, National Institute of Technology, Matsue College 14-4 Nishiikuma-cho, Matsue, , Japan, p1404@matsue-ct.jp 3 Department of Mechanical Engineering, National Institute of Technology, Matsue College 14-4 Nishiikuma-cho, Matsue, , Japan, alam@matsue-ct.ac.jp, takao@matsue-ct.jp 4 Institute of Ocean Energy, Saga University 1 Honjo-machi, Saga, , Japan, setoguchi@me.saga-u.ac.jp Abstract The present study reported of the use of special shaped blade to reduce the difference in pressure across the Wells turbine for wave energy conversion. The blade profile was composed of NACA0020 airfoils and trailing edge was notched like chevron. Experiments were performed investigating the influence of trailing edge shape on the turbine performance. Four notch depths were used to investigate the effect of depth of cut on the turbine performance. As results, by placing a notch-cut at the trailing edge of the blade, it was possible to reduce the pressure difference across the turbine without lowering the efficiency. In addition, the pressure difference substantially reduced at a constant rate with the increase of the cut ratio. 1. Introduction Keywords: Chevron, Fluid machinery, Oscillating water column, Wave energy conversion, Wells turbine Wave energy is an abundant source of energy and the development of an effective technology for wave energy utilization is expected. The wave power generation is one of the technologies for harnessing the wave energy. In terms of primary conversion, the wave power generation system can roughly be divided into three types: moveable body, wave overtopping and oscillating water column (OWC) (as shown in Fig. 1). Although the moveable body type has high energy absorption efficiency, it has serious problem of device strength during abnormal sea conditions. Wave overtopping type, which has extremely simple structure using the conventional water wheel technology, has low energy conversion efficiency against the large-scaled construction. Meanwhile, the OWC is exceedingly simple in construction and the influence of abnormal sea conditions on the device is relatively small. Hence, the development of OWC is progressing in many countries. In the OWC, a reciprocating airflow is generated in an air chamber by the wave motion of water surface. This airflow is then converted into the mechanical energy by being rotated a generator through an air turbine. Wells turbine [1, 2] which is one of selfrectifying air turbines, is widely used as the secondary conversion device of wave energy conversion with OWC (Fig. 2). This special type of air turbine is designed to rotate in the same direction by the reciprocating airflow. However, the pressure difference between the upstream and downstream of the turbine is large because of the peripheral velocity and this leads to lowering the conversion efficiency. Moreover, Wells turbine has few disadvantages compared with other conventional turbines such as low efficiency, stall [3] and poorer starting characteristics. In this study, the authors proposed a special Wells turbine to reduce the pressure different across the turbine. The trailing edge of the turbine blade was cut into notched shape like a chevron nozzle. In general, the chevron nozzle [4] is used to the rear end of the jet engine nacelle for noise reduction by mixing the exhaust gas with the outside air. In order to obtain the suitable shape of trailing edge, authors produced turbines changing the depth of cut and compared their performances with the turbine that has no cut and same blade area. Received October ; accepted for publication November : Review conducted by Prof. Tadashi Tanuma. (Paper number O15062S) Corresponding author: M M Ashraful Alam, Associate Professor, alam@matsue-ct.ac.jp This paper was presented at 13th Asian International Conference on Fluid Machinery (AICFM), September 7-10, 2015, Tokyo, Japan 307

2 Fig. 1 Wave energy conversion with OWC Fig. 2 Outline of Wells turbine 2. Tested Turbine The turbine rotor which was composed of NACA0020 airfoil has the following configuration: the casing inside diameter, D=300mm; blade tip diameter, 299mm; mean radius, r = mm; hub diameter, D h =210mm; tip clearance, 0.5mm; chord length, l=90mm; number of blades, z=6; solidity, s=lz/(2pr)=0.67; hub-to-tip ratio, u=d/d h =0.7. In order to investigate the effect of depth of the trailing edge cut a, the experiments were conducted for the case of a=0, 6, 12, 18 mm. The corresponding ratios of the depth of cut to chord length a/l was 0, 0.067, 0.13, 0.2. The performance of the turbine of a=12mm was compared with the nocut turbine of same blade area (a=0, l=84 mm, z=6, s=0.63). The detail configuration of the tested turbines are illustrated and presented in Figs. 3~5 and in Table 1. (a) Fig. 3 Blade shape of the tested turbine: (a) airfoil, (b) configuration of trailing edge cut (b) Fig. 4 Detail of the tested turbine 308

3 l = 90 a/l = 0 a/l = l = 84 a/l = 0.13 a/l = 0.2 (a) s = 0.67 (b) s = 0.63 Fig. 5 Trailing edge shapes Table 1 Specification of turbines Profile l mm s a mm a/l NACA Experimental Apparatus A schematic view of the test apparatus is shown in Fig. 6. The test apparatus consists of a large piston-cylinder (diameter: 1.4m, length: 1.7m), one end of which is followed by a settling chamber. Turbine testing is done in 300-mm-diameter test section with bell-mouthed entry/exit at both its ends. The piston can be driven back and forth inside the cylinder by means of three ball-screws through three nuts fixed to the piston. All three screws are driven together by a D.C. servo-motor through chain and sprockets. A computer controls the motor, and hence the piston velocity to produce the targeted flow velocity. The test turbine is coupled to a servo-motor/generator through a torque transducer. The motor/generator is electrically controlled such that the turbine shaft angular velocity is held constant at any set value. The overall performance was evaluated by the turbine output torque T o, flow rate Q, total pressure drop across the turbine Dp, and the turbine angular velocity w. The flow rate through the turbine Q, whether it is inhalation (i.e., flow from atmosphere into the settling chamber) or exhalation (i.e., flow from settling chamber to atmosphere), is calculated by measuring the motion of piston, where the value of Q agrees with that obtained by a Pitot tube survey. In the present study, experiments were conducted with the flow rates up to 0.32m 3 /s and with the angular velocities up to 471 rad/s. The performance of Wells turbine is evaluated by a torque coefficient C T, an input efficient C A, and an efficiency η against the flow coefficient f. Equations are as follows: C T = T o /{r(v 2 +u 2 )AR/2} (1) C A =DpQ/{r(v 2 +u 2 )Av/2}=Dp/{r(v 2 +u 2 )/2} (2) h =T o w/(dpq) = C T /(C A f) (3) f =v/u (4) where A, u, v and r denote the flow passage area {= pd 2 (1-n 2 )/4}, circumferential velocity at mean radius {= rw}, axial flow velocity {=Q/A} and density of air, respectively. 309

4 PC PC Wind tunnel 2 Piston 3 Ball-screw 4 Servomotor 5 D/A converter 6 Servo-pack 7 Settling chamber 8 Turbine 9 Torque transducer 10 Servomotor-generator 11 Pressure transducer 12A/D converter Fig. 6 Experimental apparatus 4. Experimental Results The experimental results are shown in Fig. 7 through 9, and Table 2. According to Fig. 8 (a), in the case of s=0.67, the torque coefficient decreases with the increase of ratio of depth of cut to chord length a/l in the order of a/l=0, 0.067, 0.13, 0.2. It implies that the torque decreases as the blade area becomes small. Moreover, the torque coefficient of the turbine with a/l=0 and s=0.63 is approximately equal to that of the turbine with same blade area (s=0.67 and a/l=0.13). The stall point becomes small with the solidity, and the stall point in the case of s=0.63 becomes small compared with that of s=0.67. In Fig. 8 (b), the input coefficient is shown a similar tendency to the torque coefficient. The input coefficient of turbine with a/l=0 and s=0.63 is approximately equal to that of the turbine with the same blade area having a/l=0.13 and s=0.67. In addition, from Fig. 9, the pressure difference decreases at a constant rate as the ratio of the depth of cut increases. According to the abovementioned discussion, it can be mentioned that the decrease of pressure difference across the turbine is not occurred by the depth of cut but for the effect of blade area. Moreover, the pressure difference does not affected by the solidity of blade. From Fig. 8 and Table 2, the peak efficiency in the case of s=0.67 is approximately equal in the range of 0 a/l 0.1. The peak efficiency decreases in the range of a/l³0.2. In addition to that, the peak efficiency in case of s=0.63 is high in comparison with that of cases with s=0.67. (a) Torque coefficient (b) Input coefficient Fig. 7 Turbine characteristics 310

5 Fig. 8 Efficiency Fig. 9 Decrease of input coefficient Table 2 Peak efficiency and stall point of each turbine s a/l h p f s Conclusion In the present study, the performance of Wells turbine with chevron-shaped notch-cut trailing edge was investigated experimentally by the wind-tunnel test. As a result, it seems that it is possible to decrease the pressure difference across the turbine without being decreased the peak efficiency by introducing the chevron-shaped notch-cut at trailing edge. The peak efficiency in the range of ratio of depth of cut to chord 0 a/l 0.13 is almost equal to the turbine of s=0.67. However, the peak efficiency is decreased beyond a/l³0.2. The torque coefficient C T and input coefficient C A are affected by the blade area. The peak efficiency and stall point are influenced by the solidity of blade. From the present study, it seems that the turbine with s=0.67 and a/l=0.13 is the best trailing edge shape. 6. Acknowledgment This investigation was supported by Institute of Ocean Energy, Saga University (Project No.14004D). The third author wishes to thank them for their financial help in conducting this study. Nomenclature a Depth of cut [mm] A Flow passage area [m 2 ] C A Input coefficient C T Torque efficient D Casing inside diameter [mm] D h Hub diameter [mm] l Chord length [mm] Q Flow rate [m 3 /s] r Mean radius of turbine [mm] T o Output torque [N m] u Peripheral velocity [m/s] v Mean flow velocity [m/s] z Number of blades h Efficiency h p Peak efficiency u Hub to tip ratio r Density of air [kg/m 3 ] s Solidity f Flow coefficient f s Flow coefficient at stall point w Angular velocity [rad/s] p Pressure different between the front and back of turbine [Pa] 311

6 References [1] Raghunathan, S., 1995, The Wells Air Turbine for Wave Energy Conversion, Progress in Aerospace Sciences, Vol. 31, No. 4, pp [2] Setoguchi, T., Manabu, M., 2006 Current Status of Self-rectifying Air Turbine for Wave Energy Conversion, Energy Conversion and Management, Vol. 47, No , pp [3] Suzuki, M., Arakawa, C., 2008, Influence of Blade Profiles on Flow around Wells Turbine, International Journal of Fluid Machinery and Systems, Vol. 1, No. 1, pp [4] Pierre, L., Jacques, J., Alain, D., 2004, CFM56 Jet Noise Reduction with the Chevron Nozzle, 10th AIAA/CEAS Aeroacoustics Conference, Manchester, UK, May,