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Experimental studies on effect of guide vane shape on performance of impulse turbine for wave energy conversion
Renewable Energy 30 (2005) 2203–2219 www.elsevier.com/locate/renene
Experimental studies on effect of guide vane shape on performance of impulse turbine for wave energy conversion A. Thakker*, T.S. Dhanasekaran, J. Ryan Wave Energy Research Team, Department of Mechanical and Aeronautical Engineering, University of Limerick, Limerick, Ireland Received 16 January 2005; accepted 4 February 2005 Available online 23 March 2005
Abstract This paper presents the experimental results of effect of guide vane shape on performance of an impulse turbine for wave energy conversion. Two types of guide vanes are considered in the present study: two-dimensional (2D) guide vanes and three-dimensional (3D) guide vanes. The previous investigations by the authors revealed that the 2D guide vanes cause large recirculation zones at leading edge of downstream guide vanes, which affect the performance of turbine considerably. In order to improve the performance of turbine, three-dimensional guide vanes are designed based on free-vortex theory. Detailed aerodynamic and performance tests have been conducted on impulse turbine with the two types of guide vanes. The experiments have been conducted under various inlet conditions such as steady, sinusoidal and random (real Sea) flows. From the results, it was proved that the efficiency of impulse turbine has been improved for 4.5% points due to 3D guide vanes. The hysteric characteristic has been noticed from the experimental results of impulse turbine with sinusoidal and random flow inlet conditions. Furthermore, it was investigated that the performance of turbine is considerably more during deceleration of inlet flow than the acceleration in a half cycle of sinusoidal wave. q 2005 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: C353 61 202223; fax: C353 61 202944. E-mail address: [email protected] (A. Thakker).
0960-1481/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.02.002
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Nomenclature b: Caxial: Crad: Ctan: H s: k: l r: m: r R: T s: U R: vaxial: vrad: vtan: z:
height of blade axial velocity coefficientZ vUaxial R radial velocity coefficientZ vUradR tangential velocity coefficient Z vUtanR significant wave height nondimensional periodZ(rRm)/Hs chord length of rotor blade turbine to OWC open area ratio mid span radius mean period of incident wave circumferential velocity at rR axial flow velocity radial flow velocity tangential flow velocity number of rotor blades
Greek Symbols f: flow coefficient r: density of air Subscripts 2: station at downstream of inlet guide vane 3: station at downstream of rotor 4: station at downstream of downstream guide vane 1. Introduction For the last two decades, scientists have been investigating and defining different methods for power extraction from wave motion. Some of these devices utilize the principle of an oscillating water column (OWC) The OWC based wave energy power plants convert wave energy into low-pressure pneumatic power in the form of bi-directional airflow. Selfrectifying air turbines are used to extract mechanical shaft power, which is further converted into electrical power by a generator. Two different turbines are currently in use around the world for wave energy power generation, the Wells turbine, introduced by Dr A. A. Wells (Raghunathan [1]) in 1976 and the impulse type turbine by some authors (Kim et al. [2]; Setoguchi et al. [3]). Both these turbines are currently in operation in different power plants in Europe, Canada, Australia and Asia. Currently, the research around the world is focused on improving the performance of both these turbines under different operating conditions. The present investigation deals with the impulse type turbine. The impulse turbine was initially designed to operate with self-pitch controlled guide vanes, i.e. the guide vanes pitch at the wave frequency. But such moving parts lead to maintenance and operating life problems and more cost. Hence, Setoguchi et al. [4] have investigated the performance of impulse turbine with fixed guide vanes. The extensive experimental study on impulse
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turbine with fixed guide vanes have been made with outer diameter of 0.3 m diameters. They have concluded that the most favourable pitch chord ratio is 0.5 and hub to tip ratio is 0.7. On the other hand experimental results of 0.6 m diameter impulse turbine (Thakker et al. [5]) showed that the hub to tip ratio of 0.6 was better than 0.7. The detailed aerodynamic measurements made at various stations of the impulse turbine revealed that the downstream guide vanes are less efficient than the upstream guide vanes (Thakker and Dhanasekaran [6]). In the present investigation in order to improve the performance of impulse turbine, three-dimensional guide vanes have been designed to reduce the losses at guide vanes. Experiments have been conducted on impulse turbine with 2D guide vanes and 3D guide vanes to prove the improvement in the performance. Experiments have been conducted for various inlet conditions such as steady flow, sinusoidal and real sea conditions. Furthermore, aerodynamic measurements have been made to investigate the flow physics responsible for improvement in the efficiency.
2. Experimental set-up A schematic layout of the experimental set-up of Wave Energy Research Team at University of Limerick is shown in Fig. 1. It consists of a bell mouth entry, test section, drive and transmission section, a plenum chamber with honeycomb section, a calibrated nozzle and a centrifugal fan. Air is drawn into the bell mouth shaped open end; it passes through the turbine and then enters the plenum chamber. In the chamber, the flow is conditioned and all swirls/vortices are removed prior to passing through a calibrated nozzle and finally exhausting at the fan outlet. A valve at fan exit controls the flow rate. The turbine test section had an internal diameter of 600 mm and fabricated rotor had a diameter of 598 mm, leaving tip clearance of 1 mm. The hub diameter selected as 358.8 mm, providing hub to tip ratio of 0.6. The geometry of the impulse turbine has been arrived based on the investigations by Setoguchi et al. [4], who conducted large number of experiments on impulse turbine with different pitch chord ratio, blade profile and hub to tip ratio. The turbine was mounted on a shaft in a cylindrical annular duct, with a blade tip clearance of 1 mm. The shaft is coupled to motor/generator via a torque meter.
Fig. 1. Schematic diagram of test rig.
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The two guide vanes were mounted on the up-stream and down-stream hubs of the rig. The apparatus is fully equipped with instrumentation for measuring the essential parameters like torque, speed and pressure. A vibrometer torque transducer (Model TM 204-208) measures the torque and speed. The mechanical losses due to bearing friction and windage losses were tested before commissioning the rig and a mean torque curve was deduced to correct the measured torque reading. Measurement of pressure was made by means of pneumatic pressure probes. A miniaturized five hole probe was used for aerodynamic measurements at 40 mm ahead of inlet guide vane (IGV) inlet, 10 mm behind IGV, 10 mm behind the turbine and 80 mm behind downstream guide vane (DGV). The probe was mounted on traverse mechanism fixed to the pipe wall of the test rig. Furness Control micro-manometer (Model FC012, Furness Controls Ltd, UK) through a 60 channel scanning box (Model FCS421, Furness Controls Ltd, UK) with a least count of 0.01 mm of water column were used to read the pressure data. Pressure measurements were carried out at 11 equi-spaced radial locations. The probe data obtained was reduced using the calibration charts to evaluate all necessary parameters, including the flow rates. 2.1. Data acquisition system In order to increase the accuracy and to avoid the direct human interaction in the measurements, a Data Acquisition System (DAQ) was developed for the test rig. The idea behind the work was to automate the tedious process of acquiring data and minimize the probability of errors by eliminating direct human interaction in the process. The target was achieved using a National Instruments Data Acquisition card and LabVIEW, graphical programming software for writing instrumentation programs, was used to write program for Data Acquisition. After implementation of DAQ system, it was commissioned by carrying out a number of tests on 0.6 m, 0.6 hub to tip ratio impulse turbine with fixed guide vanes. Overall, the new system was found to be much more accurate, easier, efficient and reliable. 2.2. Experimental procedure 2.2.1. steady flow inlet condition The overall performance of the turbine was evaluated by the turbine angular velocity u, torque generated T, flow rate Q and total pressure drop dp across the rotor The results are expressed in the form of torque coefficient CT, input power coefficient CA and efficiency h in terms of flow coefficient f The definitions are given below: CT Z T=frðv2a C UR2 Þblr zrR =2g
(1)
CA Z dpQ=frðVa2 C UR2 Þblr zva =2g
(2)
f Z va =UR
(3)
h Z Tu=ðdpQÞ Z CT =ðCA fÞ
(4)
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As flow coefficient is the ratio of two velocities va and UR, experimental range can be achieved in two ways. One, by keeping the rotational speed constant, thus, keeping UR constant and changing the flow rate Q, with the help of flow control valve, and therefore, changing axial velocity va. This is the methodology used for the experimental analysis of 0.6 m Wells Turbine at the University of Limerick, as used by Instituto Superior Technico (IST), Portugal for experimental analysis of scaled models for the European Wave Energy Pilot Power Plant. Second, by keeping the flow rate Q, constant, thus keeping axial velocity va constant, the required range of flow coefficient can be achieved by changing the rotational speed of the rotor using speed controller, and therefore changing UR. This is the method used by Saga University, Japan for the analysis of 0.3 m impulse turbines. Therefore, for the experimental analysis of 0.6 m impulse turbine the method used by Saga University, Japan was used. 2.2.2. Regular sinusoidal and Irish Sea inlet flow condition The sinusoidal and random (Irish Sea climate) wave inlet condition to the turbine have been generated by controlling the open area of centrifugal fan outlet of test rig using a valve actuator Initially, the valve actuator was calibrated to find the correlation between the open area of fan exit and pressure drop across the nozzle (Dpn) at inlet to the fan. Later, the relation between the valve displacement and flow rate through the test section was arrived using the nozzle calibration curve. In order to generate a given regular or random wave, time history of valve displacement was calculated using the above mentioned method. The valve actuator controller was utilized to operate the valve according to the time history. For this purpose, a computer program has been written and the controller was interfaced with a computer.
3. Results and discussion 3.1. Performance of the impulse turbine under steady flow inlet condition In order to bring out the effect of guide vane shape, experiments have been conducted on impulse turbine with 3D guide vanes and 2D guide vanes for various flow coefficients under unidirectional and steady flow inlet condition. The performance data for both the guide vanes are collected at the flow rate, Q of 1.52 m3/s. The range offlow rate possible for the test rig with 0.6 m diameter and 0.6 hub to tip ratio turbine was from 0.33 to 2.11 m3/s. Beyond the value of 2.11 m3/s, the turbine generated a torque exceeding the maximum holding torque capacity (20 Nm) of the drive motor. Hence, the experiments have been conducted at a safe flow rate of 1.52 m3/s. Also the flow rate was corresponding to the critical Reynolds number of 0.87!105. Various flow coefficients have been arrived by keeping the axial velocity at inlet constant (8.4 m/s) and increasing the rotational speed of turbine from 100 to 1200 r.p.m to get the flow coefficients from 3.5 to 0.3. Fig. 2 shows the variations of torque coefficient, input coefficient and efficiency of impulse turbine with 3D and 2D guide vanes with respect to flow coefficients. From the figure it can be seen that the turbine extracts comparatively higher torque with 3D guide vanes than the 2D guide vanes in the range of flow coefficient after 1.5. In the lower flow coefficients the torque produced by the turbine with 2D and 3D guide vanes are almost the same. It can also be
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Fig. 2. Performance curves of turbine with 2D and 3D guide vanes.
observed that the improvement in torque using 3D guide vanes increases as flow coefficient increases beyond the flow coefficients of 1.5. The input coefficient for both the guide vanes are almost equal for the flow coefficients from 0 to1.2. In the range of flow coefficient from 1.2 to 2.5 considerable reductions in input flow coefficient can be seen using 3D guide vanes. Beyond the value of 2.5, the input flow coefficient of 3D guide vanes is almost constant. But in case of 2D guide vanes drastic increase in input flow coefficient can be observed after the flow coefficient of 2.5. As far as the improvement in efficiency of the turbine is concern, the peak efficiency was attained at flow coefficient of 0.94 with 3D guide vanes and at the flow coefficient of 1.02 with 2D guide vanes. It evidences that the turbine reaches its maximum efficiency at slightly earlier flow coefficient with 3D guide vanes than that of 2D guide vanes. It can be seen from the figure that there is considerable increase in efficiency of the turbine for wide range of flow coefficient. The peak efficiency of 46.15 and 44.61% are observed for the cases of 3D and 2D guide vanes respectively. The average improvement in efficiency is calculated as 4.5% points. In general, it is found that the turbine with 3D guide vanes gives stable performance than that of 2D guides vanes. The Reynolds numbers based on the chord length at the peak efficiency points were 0.87!105 and 0.97!105 respectively for the turbine with 3D guide vanes and 2D guide vanes. 3.2. Performance of the impulse turbine under regular sinusoidal inlet flow conditions As the turbine operates under irregular and random flow of real sea conditions, the experiments have been conducted on impulse turbine with 3D and 2D guide vanes under such flow conditions. Initially the performance test of the turbine has been conducted with the inlet flow varies as a pattern of sinusoidal wave with various time periods. As mentioned earlier, the test rig used for this purpose was unidirectional and hence only half upper portion of the sine wave was generated. Later, the performance of turbine has been conducted with irregular and random flow of two different Irish Sea wave conditions (the results are presented in the next subsection). Electricity Supply Board International (ESBI), Ireland, provided the Irish wave data.
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Fig. 3. Variation of inlet velocity with time for sine waves with half period of 3.25 and 4.6 s.
Experiments have been conducted on impulse turbine with 3D and 2D guide vanes with varying inlet flow conditions of two sine waves with half period of 3.25 and 4.6 s as shown in Fig. 3. In both the sine waves the peak velocity was kept the constant as 8.4 m/s. Fig. 4 shows the mean efficiency of turbine with 3D guide vanes under regular upper half sine wave with half time periods of 3.25 and 4.6 s. The mean efficiency of the turbine with 2D guide vanes is also plotted for the sake of comparison. The mean efficiencies were arrived from the timeaveraged instantaneous input and torque flow coefficients. The figure shows the similar kind of trend in improvement of efficiency with 3D guide vanes as seen in the case of steady flow inlet (previous section), i.e. improved efficiency in higher flow coefficients. In case of 3.25 s sine wave, the improvement in efficiency is observed in the flow coefficient in the range of 1.2 to higher flow coefficients. The average efficiency of 2.8% points improvement is
Fig. 4. Variations of mean turbine efficiency.
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Fig. 5. Variation of inlet velocity with time of Irish Sea condition.
calculated due to 3D guide vanes beyond the flow coefficient of 1.2. In the range of low coefficient up to 1.2, the efficiency of turbine with 2D guide vanes seems slightly higher than 3D guide vanes. In case of sinusoidal inlet flow with half time period of 4.6 s (Fig. 4), the trend is the same as the case of half time periodZ3.25 s. The average improvement in efficiency of turbine is about 1.2% points. The improvement in efficiency of the turbine with 3D guide vanes is higher in case of half time periodZ3.25 s than the case of 4.6 s. The comparison between the efficiency curves of both the half period of sinusoidal waves is also shown in Fig. 4. It can be seen that the turbine efficiency with 3D guide vanes is higher in the sinusoidal wave with half period of 3.25 s by 5.15% points than the half period of 4.6 s. 3.3. Performance of the turbine under real Sea inlet flow condition Experiments have been conducted on impulse turbine with 2D and 3D guide vanes under irregular and random inlet flow conditions based on Irish Sea climate from two different sites named 1 and 2. The variations of inlet velocity with time for sites 1 is shown in Fig. 5. The average half-time periods of site 1 and 2 are 4.78 and 4.6 s respectively.
Fig. 6. Variations of mean turbine efficiency under Irish Sea condition.
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The improvement in efficiency of turbine with 3D guide vanes for site 1 and site 2 are shown in Fig. 6a and b respectively. Unlike the cases of sinusoidal inlet flow conditions, in the random and irregular flow conditions the efficiency of turbine with 3D guide vanes are values, i.e. in the superior than 2D guide vanes in the range of low to medium 1=ðkuÞ range of higher rotational speeds of the turbine (greater than approximately 500 r/min). The non-dimensional term 1=ðkuÞ represents the flow coefficient for ordinary fluid includes characteristic parameters of the irregular waves machines. The parameter ku (Hs and Ts), turbine speed (u) and dimensions of the turbine and air chamber (rR and m).
Fig. 7. Variations of instantaneous torque coefficient, input coefficient and efficiency at 1=ðkuÞZ 0:7.
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The average improvements in efficiency of the turbine with 3D guide vanes are 5.6 and 2% points respectively. In order to analyze the difference in behavior of impulse turbine with 3D and 2D guide vanes in lower and higher 1=ðkuÞ values, instantaneous torque coefficient and input coefficient have been plotted for two half periods of site 1 condition. Fig. 7a–c show the instantaneous torque coefficient, input coefficient and efficiency at the value of 1=ðkuÞZ 0:7. The Fig. 7a shows the torque coefficient, input coefficient and efficiency follow two different paths during acceleration of inlet flow from zero to maximum velocity and deceleration from maximum to zero velocity and it forms a counter clockwise hysteretic loop per half period of wave. The hysteretic characteristic of the turbine was found in Wells turbine also by few authors (Setoguchi et al.[4]). Fig. 7b confirms the torque developed by the turbine with 3D guide vanes are more than 2D guide vanes during deceleration of the inlet flow though the torque developed seems almost constant during acceleration condition. Fig. 7c shows the instantaneous efficiency of turbine with 3D and 2D guide vanes. In general, the efficiency of turbine, with 2D or 3D guide vanes, gradually
Fig. 8. Variations of instantaneous torque coefficient, input coefficient and efficiency at 1=ðkuÞZ 1:7.
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increases during its acceleration process. During the deceleration process the efficiency seems constant for more than half of the flow coefficient range, beyond that, it abruptly decreases near to zero. The region of constant efficiency during deceleration is increased due to 3D guide vanes. It can be noted that the constant region of efficiency was not seen in case of Wells turbine reported by Setoguchi et al. [4]. Hence, the area of hysteretic loop of impulse turbine seems higher than that of Wells turbine. The torque coefficient, input coefficient and efficiency of turbine with 3D and 2D guide vanes are shown in Fig. 8a–c respectively at 1=ðkuÞZ 1:7. The figures show that the values of torque and efficiency of the turbine are almost the same with 3D and 2D guide vanes during acceleration condition and during deceleration the values are considerably less with 3D guide vanes than that of 2D guide vanes. 3.4. Studies on annulus flow with 2D and 3D guide vanes In order to understand the aerodynamics of the turbine with 3D guide vanes and 2D guide vanes, flow investigations have been made in the annulus using five-hole probe at various stations such as upstream rotor, behind the rotor and downstream of DGV and presented in the following subsections. 3.4.1. Flow measurements at upstream of the rotor Fig. 9a and b show the distribution of axial velocity from hub to tip region for the cases of 2D guide vanes and 3D guide vanes respectively for various flow coefficients from 0.45 to 1.68. In 2D guide vanes, axial velocity at hub region is higher than the tip region and the difference in velocity at hub and tip region increases as the flow coefficient increases. In 3D guide vanes variations of axial velocity from hub to tip is considerably reduced than that of 2D guide vanes. Furthermore, the magnitude of axial velocity is reduced considerably in higher flow coefficients by using 3D guide vanes as it produces more tangential components than 2D guide vanes. Also, it can be noted that the variation of axial velocity from hub to tip is reduced and it is almost constant from hub to tip region by using 3D guide vanes.
Fig. 9. Distribution of axial velocity coefficient at downstream of inlet guide vanes.
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Fig. 10. Distribution of tangential velocity coefficient at downstream of inlet guide vanes.
Fig. 10a and b show the increase in tangential velocity components using 3D guide vanes from hub to tip region. In lower flow coefficient of fZ0.45, the tangential velocity is observed as negative for the case of 2D guide vanes (Fig. 10a) near hub region in the range of radius ratio RZ0.6–0.68. Using 3D guide vanes eliminated the negative region as shown in Fig. 10b. From the Fig. 11a and b, it is seen that the radial components are almost close to zero. This evidence the flow is nearly two-dimensional for both the cases of 2D and 3D guide vanes. The negative sign indicates the flow is directed towards hub section. The close observation near the tip region, i.e. RZ0.97, the radial components are positive in case of 2D guide vanes and in case of 3D guide vanes there is no region of positive sign is appeared for all the flow coefficients. 3.4.2. Flow measurements at downstream of the rotor The Fig. 12a and b show the distribution of axial velocity coefficient behind the rotor for the cases of 2D guide vanes and 3D guide vanes respectively. The figure shows the significant difference in the distribution of axial velocity with 2D and 3D guide vanes
Fig. 11. Distribution of radial velocity coefficient at downstream of inlet guide vanes.
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Fig. 12. Distribution of axial velocity coefficient at downstream of the rotor.
for the flow coefficients of 1.0, 1.35 and 1.68. In the low coefficients (fZ0.45 and 0.67), the distribution is almost the same for the 2D and 3D guide vane cases. Considering the Fig. 12a for the case of 3D guide vanes, the axial velocity coefficient is almost constant in the hub region of RZ0.6–0.675 and in the tip region of RZ0.875–1. Very sharp variation in axial velocity can be noted in the mid region of annulus. On the other hand, in case of 3D guide vanes (Fig. 12b), the axial velocity coefficient is constant from RZ0.6–0.65 and it increases from RZ0.65–0.7, keeping constant from RZ0.7–0.8 and reduces gradually from 0.8 to tip region. It may be due to the reduction in flow separation region from suction side of the blade along the blade height. Hence, it indicates that the flow passes through the turbine passage effectively due to favorable blade inlet flow from hub to tip using 3D guide vanes (Fig. 12b). Fig. 13a–e show the distribution of tangential velocity coefficients for the flow coefficients of 1.68, 1.35, 1.00, 0.67 and 0.45 respectively for the cases of 3D guide vanes and 2D guide vanes. In all the flow coefficients considered here, the tangential components are reduced considerably in case of 3D guide vanes while comparing with the case of 2D guide vanes. The reduction in tangential velocity components indicates the enhancement of momentum transfer from fluid to the rotor. Hence, it is seen that the power developed by the turbine with 3D guide vanes is enhanced considerably at higher flow coefficients (fZ1.68 and 1.35) than the low flow coefficients (fZ0.67 and 0.45). The trend of reduction in tangential velocity components for various flow coefficients follows the trend of torque developed by the turbine with 2D and 3D guide vanes as shown in Fig. 2. The radial velocity distribution from hub to tip region for 2D and 3D guide vanes are shown in Fig. 14a and b respectively. There is no significant effect on radial velocity component is seen from the figures due to 3D guide vanes. 3.4.3. Flow measurements behind the downstream guide vanes The distribution of axial velocity and tangential velocity at the station of downstream of downstream guide vanes are shown in Figs. 15 and 16 respectively for the cases of 2D and 3D guide vanes. As far as the axial velocity coefficients are concern, the difference is not that noticeable between the cases of 2D and 3D guide vanes (Fig. 15a and b).
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Fig. 13. Distribution of tangential velocity coefficient at rotor exit.
But, the tangential velocity coefficients are much reduced with 3D guide vanes reveals that the kinetic energy and swirl energy recovery have improved while compare with the case of 2D guide vanes. This is quite noticeable at higher flow coefficients of 1.68 and 1.35, Fig. 16a and b. The mean tangential velocity coefficients from hub to tip have been measured for various flow coefficients and shown in Fig. 17. The figure shows the reduction in tangential velocity components at exit of downstream guide vanes using 3D guide vanes. It can be observed that the reduction in tangential velocity component increases as the flow coefficient increases.
4. Conclusions It was observed from the experimental results on the performance of impulse turbine with 2D and 3D guide vanes that the 3D guide vanes are superior than the 2D guide vanes
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Fig. 14. Distribution of radial velocity coefficient at rotor exit.
under various inlet conditions of steady, sinusoidal and random real sea conditions. The experimental investigation revealed that the impulse turbine behaves significantly different under various inlet conditions. For example, the steady flow test showed that the turbine efficiency with 3D guide vanes has been increased by average of 4.5% points than the turbine efficiency with 2D guide vanes in the higher flow coefficients in the range after 1.0. In the lower range of flow coefficients the improvement in efficiency was very mild. In case of sinusoidal inlet flow condition the improvement in efficiency due to 3D guide vanes are 2.8 and 1.2% points for the half periods of 3.25 and 4.6 s respectively. The improvement in efficiency was observed at higher flow coefficients as in the case of sinusoidal inlet condition. But in case of random real sea conditions the improvement in efficiency of the turbine was about 5.6 and 2% points for site number 1 and 2 respectively. The improvement in efficiency was noticed in the lower flow coefficients unlike the cases of steady and sinusoidal inlet conditions. Hence, the experimental investigation proves
Fig. 15. Distribution of axial velocity coefficient at DGV exit.
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Fig. 16. Distribution of tangential velocity coefficient at DGV exit.
that the impulse turbine has to be tested under real sea condition in the laboratory in order to obtain the realistic behavior of the turbine. Hysteretic characteristics of impulse turbine with 2D and 3D guide vanes have been analyzed from the instantaneous performance test under real sea condition. It was found that the improvement in performance of turbine with 3D guide vanes was achieved during deceleration of inlet flow and the performance was almost the same during acceleration of inlet flow. From the aerodynamic measurements it was proved that the generation of tangential velocity components ahead of the turbine has improved using 3D guide vanes. Flow measurements behind the rotor showed the increase in momentum transfer from fluid to the rotor using 3D guide vanes for various flow coefficients. Distribution of tangential velocity coefficient at downstream of downstream guide vanes has reduced considerably using 3D guide vanes showed the improved kinetic energy recovery using 3D guide vanes than 2D guide vanes.
Fig. 17. Distribution of tangential velocity coefficient.
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Acknowledgements The authors would like to acknowledge the financial support given by ESBI, Ireland and also by the Irish Research Council for Science, Engineering and Technology: funded by the National Development Plan.
References [1] Raghunathan S. Performance of the Wells self-rectifying turbine. Aeronaut J 1985;89:369–79 paper number 1368. [2] Kim TW, Kaneko K, Setoguchi T, Inoue M. Aerodynamic performance of an impulse turbine with self-pitchcontrolled guide vanes for wave power conversion. In: The proceedings of first KSME-JSME Thermal and Fluid Engineering Conference, 1988;2:133–137. [3] Setoguchi T, Takao M, Kinoue Y, Kaneko K, Santhakumar S, Inoue M. Study on an impulse turbine for wave energy conversion. Int J Offshore and Polar Eng 2000;10(2):355–62. [4] Setoguchi T, Kinoue Y, Kim TH, Kaneko K, Inoue M. Hysterestic characteristics of Wells turbine for wave power conversion. Renew Energ 2003;28(13):2113–27. [5] Thakker A, Frawley P, Khaleeq HB, Abughalia Y, Setoguchi T. Experimental and CFD analysis of 0.6 m impulse turbine with fixed guide vanes. In: Proceedings of the 11th (ISOPE 2001) International Offshore and Polar Engineering Conference, June 17–22, 2001, Stavanger, Norway, 2001;1:625–629. [6] Thakker A, Dhanasekaran TS. Experimental and computational analysis on guide vane losses of impulse turbine for wave energy conversion. Renew Energ 2004;30:1359–72.
Experimental studies on effect of guide vane shape on performance of impulse turbine for wave energy conversion A. Thakker*, T.S. Dhanasekaran, J. Ryan Wave Energy Research Team, Department of Mechanical and Aeronautical Engineering, University of Limerick, Limerick, Ireland Received 16 January 2005; accepted 4 February 2005 Available online 23 March 2005
Abstract This paper presents the experimental results of effect of guide vane shape on performance of an impulse turbine for wave energy conversion. Two types of guide vanes are considered in the present study: two-dimensional (2D) guide vanes and three-dimensional (3D) guide vanes. The previous investigations by the authors revealed that the 2D guide vanes cause large recirculation zones at leading edge of downstream guide vanes, which affect the performance of turbine considerably. In order to improve the performance of turbine, three-dimensional guide vanes are designed based on free-vortex theory. Detailed aerodynamic and performance tests have been conducted on impulse turbine with the two types of guide vanes. The experiments have been conducted under various inlet conditions such as steady, sinusoidal and random (real Sea) flows. From the results, it was proved that the efficiency of impulse turbine has been improved for 4.5% points due to 3D guide vanes. The hysteric characteristic has been noticed from the experimental results of impulse turbine with sinusoidal and random flow inlet conditions. Furthermore, it was investigated that the performance of turbine is considerably more during deceleration of inlet flow than the acceleration in a half cycle of sinusoidal wave. q 2005 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: C353 61 202223; fax: C353 61 202944. E-mail address: [email protected] (A. Thakker).
0960-1481/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.02.002
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Nomenclature b: Caxial: Crad: Ctan: H s: k: l r: m: r R: T s: U R: vaxial: vrad: vtan: z:
height of blade axial velocity coefficientZ vUaxial R radial velocity coefficientZ vUradR tangential velocity coefficient Z vUtanR significant wave height nondimensional periodZ(rRm)/Hs chord length of rotor blade turbine to OWC open area ratio mid span radius mean period of incident wave circumferential velocity at rR axial flow velocity radial flow velocity tangential flow velocity number of rotor blades
Greek Symbols f: flow coefficient r: density of air Subscripts 2: station at downstream of inlet guide vane 3: station at downstream of rotor 4: station at downstream of downstream guide vane 1. Introduction For the last two decades, scientists have been investigating and defining different methods for power extraction from wave motion. Some of these devices utilize the principle of an oscillating water column (OWC) The OWC based wave energy power plants convert wave energy into low-pressure pneumatic power in the form of bi-directional airflow. Selfrectifying air turbines are used to extract mechanical shaft power, which is further converted into electrical power by a generator. Two different turbines are currently in use around the world for wave energy power generation, the Wells turbine, introduced by Dr A. A. Wells (Raghunathan [1]) in 1976 and the impulse type turbine by some authors (Kim et al. [2]; Setoguchi et al. [3]). Both these turbines are currently in operation in different power plants in Europe, Canada, Australia and Asia. Currently, the research around the world is focused on improving the performance of both these turbines under different operating conditions. The present investigation deals with the impulse type turbine. The impulse turbine was initially designed to operate with self-pitch controlled guide vanes, i.e. the guide vanes pitch at the wave frequency. But such moving parts lead to maintenance and operating life problems and more cost. Hence, Setoguchi et al. [4] have investigated the performance of impulse turbine with fixed guide vanes. The extensive experimental study on impulse
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turbine with fixed guide vanes have been made with outer diameter of 0.3 m diameters. They have concluded that the most favourable pitch chord ratio is 0.5 and hub to tip ratio is 0.7. On the other hand experimental results of 0.6 m diameter impulse turbine (Thakker et al. [5]) showed that the hub to tip ratio of 0.6 was better than 0.7. The detailed aerodynamic measurements made at various stations of the impulse turbine revealed that the downstream guide vanes are less efficient than the upstream guide vanes (Thakker and Dhanasekaran [6]). In the present investigation in order to improve the performance of impulse turbine, three-dimensional guide vanes have been designed to reduce the losses at guide vanes. Experiments have been conducted on impulse turbine with 2D guide vanes and 3D guide vanes to prove the improvement in the performance. Experiments have been conducted for various inlet conditions such as steady flow, sinusoidal and real sea conditions. Furthermore, aerodynamic measurements have been made to investigate the flow physics responsible for improvement in the efficiency.
2. Experimental set-up A schematic layout of the experimental set-up of Wave Energy Research Team at University of Limerick is shown in Fig. 1. It consists of a bell mouth entry, test section, drive and transmission section, a plenum chamber with honeycomb section, a calibrated nozzle and a centrifugal fan. Air is drawn into the bell mouth shaped open end; it passes through the turbine and then enters the plenum chamber. In the chamber, the flow is conditioned and all swirls/vortices are removed prior to passing through a calibrated nozzle and finally exhausting at the fan outlet. A valve at fan exit controls the flow rate. The turbine test section had an internal diameter of 600 mm and fabricated rotor had a diameter of 598 mm, leaving tip clearance of 1 mm. The hub diameter selected as 358.8 mm, providing hub to tip ratio of 0.6. The geometry of the impulse turbine has been arrived based on the investigations by Setoguchi et al. [4], who conducted large number of experiments on impulse turbine with different pitch chord ratio, blade profile and hub to tip ratio. The turbine was mounted on a shaft in a cylindrical annular duct, with a blade tip clearance of 1 mm. The shaft is coupled to motor/generator via a torque meter.
Fig. 1. Schematic diagram of test rig.
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The two guide vanes were mounted on the up-stream and down-stream hubs of the rig. The apparatus is fully equipped with instrumentation for measuring the essential parameters like torque, speed and pressure. A vibrometer torque transducer (Model TM 204-208) measures the torque and speed. The mechanical losses due to bearing friction and windage losses were tested before commissioning the rig and a mean torque curve was deduced to correct the measured torque reading. Measurement of pressure was made by means of pneumatic pressure probes. A miniaturized five hole probe was used for aerodynamic measurements at 40 mm ahead of inlet guide vane (IGV) inlet, 10 mm behind IGV, 10 mm behind the turbine and 80 mm behind downstream guide vane (DGV). The probe was mounted on traverse mechanism fixed to the pipe wall of the test rig. Furness Control micro-manometer (Model FC012, Furness Controls Ltd, UK) through a 60 channel scanning box (Model FCS421, Furness Controls Ltd, UK) with a least count of 0.01 mm of water column were used to read the pressure data. Pressure measurements were carried out at 11 equi-spaced radial locations. The probe data obtained was reduced using the calibration charts to evaluate all necessary parameters, including the flow rates. 2.1. Data acquisition system In order to increase the accuracy and to avoid the direct human interaction in the measurements, a Data Acquisition System (DAQ) was developed for the test rig. The idea behind the work was to automate the tedious process of acquiring data and minimize the probability of errors by eliminating direct human interaction in the process. The target was achieved using a National Instruments Data Acquisition card and LabVIEW, graphical programming software for writing instrumentation programs, was used to write program for Data Acquisition. After implementation of DAQ system, it was commissioned by carrying out a number of tests on 0.6 m, 0.6 hub to tip ratio impulse turbine with fixed guide vanes. Overall, the new system was found to be much more accurate, easier, efficient and reliable. 2.2. Experimental procedure 2.2.1. steady flow inlet condition The overall performance of the turbine was evaluated by the turbine angular velocity u, torque generated T, flow rate Q and total pressure drop dp across the rotor The results are expressed in the form of torque coefficient CT, input power coefficient CA and efficiency h in terms of flow coefficient f The definitions are given below: CT Z T=frðv2a C UR2 Þblr zrR =2g
(1)
CA Z dpQ=frðVa2 C UR2 Þblr zva =2g
(2)
f Z va =UR
(3)
h Z Tu=ðdpQÞ Z CT =ðCA fÞ
(4)
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As flow coefficient is the ratio of two velocities va and UR, experimental range can be achieved in two ways. One, by keeping the rotational speed constant, thus, keeping UR constant and changing the flow rate Q, with the help of flow control valve, and therefore, changing axial velocity va. This is the methodology used for the experimental analysis of 0.6 m Wells Turbine at the University of Limerick, as used by Instituto Superior Technico (IST), Portugal for experimental analysis of scaled models for the European Wave Energy Pilot Power Plant. Second, by keeping the flow rate Q, constant, thus keeping axial velocity va constant, the required range of flow coefficient can be achieved by changing the rotational speed of the rotor using speed controller, and therefore changing UR. This is the method used by Saga University, Japan for the analysis of 0.3 m impulse turbines. Therefore, for the experimental analysis of 0.6 m impulse turbine the method used by Saga University, Japan was used. 2.2.2. Regular sinusoidal and Irish Sea inlet flow condition The sinusoidal and random (Irish Sea climate) wave inlet condition to the turbine have been generated by controlling the open area of centrifugal fan outlet of test rig using a valve actuator Initially, the valve actuator was calibrated to find the correlation between the open area of fan exit and pressure drop across the nozzle (Dpn) at inlet to the fan. Later, the relation between the valve displacement and flow rate through the test section was arrived using the nozzle calibration curve. In order to generate a given regular or random wave, time history of valve displacement was calculated using the above mentioned method. The valve actuator controller was utilized to operate the valve according to the time history. For this purpose, a computer program has been written and the controller was interfaced with a computer.
3. Results and discussion 3.1. Performance of the impulse turbine under steady flow inlet condition In order to bring out the effect of guide vane shape, experiments have been conducted on impulse turbine with 3D guide vanes and 2D guide vanes for various flow coefficients under unidirectional and steady flow inlet condition. The performance data for both the guide vanes are collected at the flow rate, Q of 1.52 m3/s. The range offlow rate possible for the test rig with 0.6 m diameter and 0.6 hub to tip ratio turbine was from 0.33 to 2.11 m3/s. Beyond the value of 2.11 m3/s, the turbine generated a torque exceeding the maximum holding torque capacity (20 Nm) of the drive motor. Hence, the experiments have been conducted at a safe flow rate of 1.52 m3/s. Also the flow rate was corresponding to the critical Reynolds number of 0.87!105. Various flow coefficients have been arrived by keeping the axial velocity at inlet constant (8.4 m/s) and increasing the rotational speed of turbine from 100 to 1200 r.p.m to get the flow coefficients from 3.5 to 0.3. Fig. 2 shows the variations of torque coefficient, input coefficient and efficiency of impulse turbine with 3D and 2D guide vanes with respect to flow coefficients. From the figure it can be seen that the turbine extracts comparatively higher torque with 3D guide vanes than the 2D guide vanes in the range of flow coefficient after 1.5. In the lower flow coefficients the torque produced by the turbine with 2D and 3D guide vanes are almost the same. It can also be
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Fig. 2. Performance curves of turbine with 2D and 3D guide vanes.
observed that the improvement in torque using 3D guide vanes increases as flow coefficient increases beyond the flow coefficients of 1.5. The input coefficient for both the guide vanes are almost equal for the flow coefficients from 0 to1.2. In the range of flow coefficient from 1.2 to 2.5 considerable reductions in input flow coefficient can be seen using 3D guide vanes. Beyond the value of 2.5, the input flow coefficient of 3D guide vanes is almost constant. But in case of 2D guide vanes drastic increase in input flow coefficient can be observed after the flow coefficient of 2.5. As far as the improvement in efficiency of the turbine is concern, the peak efficiency was attained at flow coefficient of 0.94 with 3D guide vanes and at the flow coefficient of 1.02 with 2D guide vanes. It evidences that the turbine reaches its maximum efficiency at slightly earlier flow coefficient with 3D guide vanes than that of 2D guide vanes. It can be seen from the figure that there is considerable increase in efficiency of the turbine for wide range of flow coefficient. The peak efficiency of 46.15 and 44.61% are observed for the cases of 3D and 2D guide vanes respectively. The average improvement in efficiency is calculated as 4.5% points. In general, it is found that the turbine with 3D guide vanes gives stable performance than that of 2D guides vanes. The Reynolds numbers based on the chord length at the peak efficiency points were 0.87!105 and 0.97!105 respectively for the turbine with 3D guide vanes and 2D guide vanes. 3.2. Performance of the impulse turbine under regular sinusoidal inlet flow conditions As the turbine operates under irregular and random flow of real sea conditions, the experiments have been conducted on impulse turbine with 3D and 2D guide vanes under such flow conditions. Initially the performance test of the turbine has been conducted with the inlet flow varies as a pattern of sinusoidal wave with various time periods. As mentioned earlier, the test rig used for this purpose was unidirectional and hence only half upper portion of the sine wave was generated. Later, the performance of turbine has been conducted with irregular and random flow of two different Irish Sea wave conditions (the results are presented in the next subsection). Electricity Supply Board International (ESBI), Ireland, provided the Irish wave data.
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Fig. 3. Variation of inlet velocity with time for sine waves with half period of 3.25 and 4.6 s.
Experiments have been conducted on impulse turbine with 3D and 2D guide vanes with varying inlet flow conditions of two sine waves with half period of 3.25 and 4.6 s as shown in Fig. 3. In both the sine waves the peak velocity was kept the constant as 8.4 m/s. Fig. 4 shows the mean efficiency of turbine with 3D guide vanes under regular upper half sine wave with half time periods of 3.25 and 4.6 s. The mean efficiency of the turbine with 2D guide vanes is also plotted for the sake of comparison. The mean efficiencies were arrived from the timeaveraged instantaneous input and torque flow coefficients. The figure shows the similar kind of trend in improvement of efficiency with 3D guide vanes as seen in the case of steady flow inlet (previous section), i.e. improved efficiency in higher flow coefficients. In case of 3.25 s sine wave, the improvement in efficiency is observed in the flow coefficient in the range of 1.2 to higher flow coefficients. The average efficiency of 2.8% points improvement is
Fig. 4. Variations of mean turbine efficiency.
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Fig. 5. Variation of inlet velocity with time of Irish Sea condition.
calculated due to 3D guide vanes beyond the flow coefficient of 1.2. In the range of low coefficient up to 1.2, the efficiency of turbine with 2D guide vanes seems slightly higher than 3D guide vanes. In case of sinusoidal inlet flow with half time period of 4.6 s (Fig. 4), the trend is the same as the case of half time periodZ3.25 s. The average improvement in efficiency of turbine is about 1.2% points. The improvement in efficiency of the turbine with 3D guide vanes is higher in case of half time periodZ3.25 s than the case of 4.6 s. The comparison between the efficiency curves of both the half period of sinusoidal waves is also shown in Fig. 4. It can be seen that the turbine efficiency with 3D guide vanes is higher in the sinusoidal wave with half period of 3.25 s by 5.15% points than the half period of 4.6 s. 3.3. Performance of the turbine under real Sea inlet flow condition Experiments have been conducted on impulse turbine with 2D and 3D guide vanes under irregular and random inlet flow conditions based on Irish Sea climate from two different sites named 1 and 2. The variations of inlet velocity with time for sites 1 is shown in Fig. 5. The average half-time periods of site 1 and 2 are 4.78 and 4.6 s respectively.
Fig. 6. Variations of mean turbine efficiency under Irish Sea condition.
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The improvement in efficiency of turbine with 3D guide vanes for site 1 and site 2 are shown in Fig. 6a and b respectively. Unlike the cases of sinusoidal inlet flow conditions, in the random and irregular flow conditions the efficiency of turbine with 3D guide vanes are values, i.e. in the superior than 2D guide vanes in the range of low to medium 1=ðkuÞ range of higher rotational speeds of the turbine (greater than approximately 500 r/min). The non-dimensional term 1=ðkuÞ represents the flow coefficient for ordinary fluid includes characteristic parameters of the irregular waves machines. The parameter ku (Hs and Ts), turbine speed (u) and dimensions of the turbine and air chamber (rR and m).
Fig. 7. Variations of instantaneous torque coefficient, input coefficient and efficiency at 1=ðkuÞZ 0:7.
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The average improvements in efficiency of the turbine with 3D guide vanes are 5.6 and 2% points respectively. In order to analyze the difference in behavior of impulse turbine with 3D and 2D guide vanes in lower and higher 1=ðkuÞ values, instantaneous torque coefficient and input coefficient have been plotted for two half periods of site 1 condition. Fig. 7a–c show the instantaneous torque coefficient, input coefficient and efficiency at the value of 1=ðkuÞZ 0:7. The Fig. 7a shows the torque coefficient, input coefficient and efficiency follow two different paths during acceleration of inlet flow from zero to maximum velocity and deceleration from maximum to zero velocity and it forms a counter clockwise hysteretic loop per half period of wave. The hysteretic characteristic of the turbine was found in Wells turbine also by few authors (Setoguchi et al.[4]). Fig. 7b confirms the torque developed by the turbine with 3D guide vanes are more than 2D guide vanes during deceleration of the inlet flow though the torque developed seems almost constant during acceleration condition. Fig. 7c shows the instantaneous efficiency of turbine with 3D and 2D guide vanes. In general, the efficiency of turbine, with 2D or 3D guide vanes, gradually
Fig. 8. Variations of instantaneous torque coefficient, input coefficient and efficiency at 1=ðkuÞZ 1:7.
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increases during its acceleration process. During the deceleration process the efficiency seems constant for more than half of the flow coefficient range, beyond that, it abruptly decreases near to zero. The region of constant efficiency during deceleration is increased due to 3D guide vanes. It can be noted that the constant region of efficiency was not seen in case of Wells turbine reported by Setoguchi et al. [4]. Hence, the area of hysteretic loop of impulse turbine seems higher than that of Wells turbine. The torque coefficient, input coefficient and efficiency of turbine with 3D and 2D guide vanes are shown in Fig. 8a–c respectively at 1=ðkuÞZ 1:7. The figures show that the values of torque and efficiency of the turbine are almost the same with 3D and 2D guide vanes during acceleration condition and during deceleration the values are considerably less with 3D guide vanes than that of 2D guide vanes. 3.4. Studies on annulus flow with 2D and 3D guide vanes In order to understand the aerodynamics of the turbine with 3D guide vanes and 2D guide vanes, flow investigations have been made in the annulus using five-hole probe at various stations such as upstream rotor, behind the rotor and downstream of DGV and presented in the following subsections. 3.4.1. Flow measurements at upstream of the rotor Fig. 9a and b show the distribution of axial velocity from hub to tip region for the cases of 2D guide vanes and 3D guide vanes respectively for various flow coefficients from 0.45 to 1.68. In 2D guide vanes, axial velocity at hub region is higher than the tip region and the difference in velocity at hub and tip region increases as the flow coefficient increases. In 3D guide vanes variations of axial velocity from hub to tip is considerably reduced than that of 2D guide vanes. Furthermore, the magnitude of axial velocity is reduced considerably in higher flow coefficients by using 3D guide vanes as it produces more tangential components than 2D guide vanes. Also, it can be noted that the variation of axial velocity from hub to tip is reduced and it is almost constant from hub to tip region by using 3D guide vanes.
Fig. 9. Distribution of axial velocity coefficient at downstream of inlet guide vanes.
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Fig. 10. Distribution of tangential velocity coefficient at downstream of inlet guide vanes.
Fig. 10a and b show the increase in tangential velocity components using 3D guide vanes from hub to tip region. In lower flow coefficient of fZ0.45, the tangential velocity is observed as negative for the case of 2D guide vanes (Fig. 10a) near hub region in the range of radius ratio RZ0.6–0.68. Using 3D guide vanes eliminated the negative region as shown in Fig. 10b. From the Fig. 11a and b, it is seen that the radial components are almost close to zero. This evidence the flow is nearly two-dimensional for both the cases of 2D and 3D guide vanes. The negative sign indicates the flow is directed towards hub section. The close observation near the tip region, i.e. RZ0.97, the radial components are positive in case of 2D guide vanes and in case of 3D guide vanes there is no region of positive sign is appeared for all the flow coefficients. 3.4.2. Flow measurements at downstream of the rotor The Fig. 12a and b show the distribution of axial velocity coefficient behind the rotor for the cases of 2D guide vanes and 3D guide vanes respectively. The figure shows the significant difference in the distribution of axial velocity with 2D and 3D guide vanes
Fig. 11. Distribution of radial velocity coefficient at downstream of inlet guide vanes.
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Fig. 12. Distribution of axial velocity coefficient at downstream of the rotor.
for the flow coefficients of 1.0, 1.35 and 1.68. In the low coefficients (fZ0.45 and 0.67), the distribution is almost the same for the 2D and 3D guide vane cases. Considering the Fig. 12a for the case of 3D guide vanes, the axial velocity coefficient is almost constant in the hub region of RZ0.6–0.675 and in the tip region of RZ0.875–1. Very sharp variation in axial velocity can be noted in the mid region of annulus. On the other hand, in case of 3D guide vanes (Fig. 12b), the axial velocity coefficient is constant from RZ0.6–0.65 and it increases from RZ0.65–0.7, keeping constant from RZ0.7–0.8 and reduces gradually from 0.8 to tip region. It may be due to the reduction in flow separation region from suction side of the blade along the blade height. Hence, it indicates that the flow passes through the turbine passage effectively due to favorable blade inlet flow from hub to tip using 3D guide vanes (Fig. 12b). Fig. 13a–e show the distribution of tangential velocity coefficients for the flow coefficients of 1.68, 1.35, 1.00, 0.67 and 0.45 respectively for the cases of 3D guide vanes and 2D guide vanes. In all the flow coefficients considered here, the tangential components are reduced considerably in case of 3D guide vanes while comparing with the case of 2D guide vanes. The reduction in tangential velocity components indicates the enhancement of momentum transfer from fluid to the rotor. Hence, it is seen that the power developed by the turbine with 3D guide vanes is enhanced considerably at higher flow coefficients (fZ1.68 and 1.35) than the low flow coefficients (fZ0.67 and 0.45). The trend of reduction in tangential velocity components for various flow coefficients follows the trend of torque developed by the turbine with 2D and 3D guide vanes as shown in Fig. 2. The radial velocity distribution from hub to tip region for 2D and 3D guide vanes are shown in Fig. 14a and b respectively. There is no significant effect on radial velocity component is seen from the figures due to 3D guide vanes. 3.4.3. Flow measurements behind the downstream guide vanes The distribution of axial velocity and tangential velocity at the station of downstream of downstream guide vanes are shown in Figs. 15 and 16 respectively for the cases of 2D and 3D guide vanes. As far as the axial velocity coefficients are concern, the difference is not that noticeable between the cases of 2D and 3D guide vanes (Fig. 15a and b).
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Fig. 13. Distribution of tangential velocity coefficient at rotor exit.
But, the tangential velocity coefficients are much reduced with 3D guide vanes reveals that the kinetic energy and swirl energy recovery have improved while compare with the case of 2D guide vanes. This is quite noticeable at higher flow coefficients of 1.68 and 1.35, Fig. 16a and b. The mean tangential velocity coefficients from hub to tip have been measured for various flow coefficients and shown in Fig. 17. The figure shows the reduction in tangential velocity components at exit of downstream guide vanes using 3D guide vanes. It can be observed that the reduction in tangential velocity component increases as the flow coefficient increases.
4. Conclusions It was observed from the experimental results on the performance of impulse turbine with 2D and 3D guide vanes that the 3D guide vanes are superior than the 2D guide vanes
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Fig. 14. Distribution of radial velocity coefficient at rotor exit.
under various inlet conditions of steady, sinusoidal and random real sea conditions. The experimental investigation revealed that the impulse turbine behaves significantly different under various inlet conditions. For example, the steady flow test showed that the turbine efficiency with 3D guide vanes has been increased by average of 4.5% points than the turbine efficiency with 2D guide vanes in the higher flow coefficients in the range after 1.0. In the lower range of flow coefficients the improvement in efficiency was very mild. In case of sinusoidal inlet flow condition the improvement in efficiency due to 3D guide vanes are 2.8 and 1.2% points for the half periods of 3.25 and 4.6 s respectively. The improvement in efficiency was observed at higher flow coefficients as in the case of sinusoidal inlet condition. But in case of random real sea conditions the improvement in efficiency of the turbine was about 5.6 and 2% points for site number 1 and 2 respectively. The improvement in efficiency was noticed in the lower flow coefficients unlike the cases of steady and sinusoidal inlet conditions. Hence, the experimental investigation proves
Fig. 15. Distribution of axial velocity coefficient at DGV exit.
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Fig. 16. Distribution of tangential velocity coefficient at DGV exit.
that the impulse turbine has to be tested under real sea condition in the laboratory in order to obtain the realistic behavior of the turbine. Hysteretic characteristics of impulse turbine with 2D and 3D guide vanes have been analyzed from the instantaneous performance test under real sea condition. It was found that the improvement in performance of turbine with 3D guide vanes was achieved during deceleration of inlet flow and the performance was almost the same during acceleration of inlet flow. From the aerodynamic measurements it was proved that the generation of tangential velocity components ahead of the turbine has improved using 3D guide vanes. Flow measurements behind the rotor showed the increase in momentum transfer from fluid to the rotor using 3D guide vanes for various flow coefficients. Distribution of tangential velocity coefficient at downstream of downstream guide vanes has reduced considerably using 3D guide vanes showed the improved kinetic energy recovery using 3D guide vanes than 2D guide vanes.
Fig. 17. Distribution of tangential velocity coefficient.
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Acknowledgements The authors would like to acknowledge the financial support given by ESBI, Ireland and also by the Irish Research Council for Science, Engineering and Technology: funded by the National Development Plan.
References [1] Raghunathan S. Performance of the Wells self-rectifying turbine. Aeronaut J 1985;89:369–79 paper number 1368. [2] Kim TW, Kaneko K, Setoguchi T, Inoue M. Aerodynamic performance of an impulse turbine with self-pitchcontrolled guide vanes for wave power conversion. In: The proceedings of first KSME-JSME Thermal and Fluid Engineering Conference, 1988;2:133–137. [3] Setoguchi T, Takao M, Kinoue Y, Kaneko K, Santhakumar S, Inoue M. Study on an impulse turbine for wave energy conversion. Int J Offshore and Polar Eng 2000;10(2):355–62. [4] Setoguchi T, Kinoue Y, Kim TH, Kaneko K, Inoue M. Hysterestic characteristics of Wells turbine for wave power conversion. Renew Energ 2003;28(13):2113–27. [5] Thakker A, Frawley P, Khaleeq HB, Abughalia Y, Setoguchi T. Experimental and CFD analysis of 0.6 m impulse turbine with fixed guide vanes. In: Proceedings of the 11th (ISOPE 2001) International Offshore and Polar Engineering Conference, June 17–22, 2001, Stavanger, Norway, 2001;1:625–629. [6] Thakker A, Dhanasekaran TS. Experimental and computational analysis on guide vane losses of impulse turbine for wave energy conversion. Renew Energ 2004;30:1359–72.