an improved radial impulse turbine for owc

Renewable Energy 36 (2011) 1477e1484

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An improved radial impulse turbine for OWCBruno Pereiras a, *, Francisco Castro a, Abdelatif el Marjani b, Miguel A. Rodrguez aa b

Energy Engineering and Fluid Mechanics Department, University of Valladolid, Paseo del cauce 59, 47011, Valladolid, Spain Labo. de Turbomachines, Ecole Mohammadia dIngnieurs (EMI), University of Mohammed V Agdal. Av Ibn Sina, B.P. 765 Agdal Rabat, Morocco

a r t i c l e i n f oArticle history: Received 15 April 2010 Accepted 17 October 2010 Available online 18 November 2010 Keywords: Wave energy OWC Radial impulse turbine CFD Flow analysis

a b s t r a c tTraditionally, wells turbines have been widely used in OWC plants. However, an alternative has been studied over recent years: a self-rectifying turbine known as an impulse turbine. We are interested in the radial version of the impulse turbine, which was initially proposed by M. McCormick. Previous research was carried out using CFD (FLUENT), which aimed to improve knowledge of the local ow behavior and the prediction of the performance for this kind of turbine. This previous work was developed with a geometry taken from the literature, but now our goal is to develop a new geometry design with a better performance. To achieve this, we have redesigned the blade and vane proles and improved the interaction between them by means of a new relation between their setting angles. Under sinusoidal ow conditions the new design improves the turbine efciency by up to 5% more than the geometry proposed by Professor Setoguchi, in 2002. In this paper, the design criteria we have used is described, and the ow behavior and the performance of this new design are compared with the previous one. 2010 Elsevier Ltd. All rights reserved.

1. Current status of air turbines Wave energy power plants based on oscillating water columns (OWC) convert wave energy into low-pressure pneumatic power. An OWC plant consists mainly of a submerged air chamber connected to the atmosphere through a duct where a turbine is installed. The successive sea water waves come into contact with the chamber, compressing and decompressing the air in it by the periodic motion of the oscillating sea water free surface. This periodic motion creates a bidirectional periodic ow through the turbine. Under these particular operational conditions, and although both self-rectifying turbines and conventional ones are characterized by unidirectional rotation, the rst ones show a better behavior. Here, it must be said that there have been proposals on wave energy devices using a system of non-return valves for rectifying the air ow, together with conventional turbines [1], but they are complex and difcult to maintain. The development of self-rectifying turbines has been problematic for two reasons: the geometrical design itself is complicated, and the designer must nd the best possible solution to achieve the highest efciency of the entire system. The global efciency of the wave energy plant depends not only on the turbine efciency but also on the performance of the chamber. This means that the

* Corresponding author. Tel.: 34983184536; fax: 34983423363. E-mail addresses: [email protected] (B. Pereiras), [email protected] (F. Castro), [email protected] (A.el Marjani), [email protected] (M.A. Rodrguez). 0960-1481/$ e see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.10.013

turbine must provide the optimal pneumatic damping (pressure difference across the turbine) so that the capture efciency of the OWC chamber is maximized. In this study this point is not considered, the objective is only the turbine performance. Different types of self-rectifying turbines have been proposed for use in OWC plants, the wells turbine being the rst one in 1976. Subsequently, impulse turbines were suggested as an alternative. There are two kinds of impulse turbine: axial and radial (Fig. 1). The performance of the Wells turbine has been described in many articles and reports [2e4]. All the research agreed about the main disadvantages of this turbine: narrow range of ow rates with good efciencies, poor starting characteristics, high speed operation, high noise level and high periodical axial thrust. In order to overcome the drawbacks of the Wells turbine certain modications have been tested: self-pitch-controlled guide vanes [5], variablepitch angle blades [6], contra-rotating rotors [7,8], using different chord blades [9] and geometry ratios [10]. One of the alternatives to the Wells turbine is the axial impulse turbine with self-pitching linked guide vanes proposed in [11]. The site trials conrmed the superiority of this type of turbine over the Wells turbine [12]. However, the moving guide vanes lead to maintenance and operating life problems [13]. Therefore, an axial impulse turbine with xed guide vanes was also studied. There are reports which compare the Wells and axial impulse turbines with xed guide vanes. A comparison between turbine performances under irregular wave conditions is made in [14e16], and they show that axial impulse turbines are superior in running and starting characteristics under irregular ow conditions.

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Nomenclature Characteristic area AR b Rotor blade height CA DP=1rv2 u2 Input coefcient. R R 2

Characteristic dynamic pressure PDIN 0:5rv2 element DP Total pressure drop DPo Total pressure drop between settling chamberatmosphere q Flow rate Mean radius rR T Period Output mechanical torque To

CT TO =1rv2 u2 AR rR Torque coefcient R R 2

UR urR Circumferential velocity at rR velement Characteristic element velocity vR q/2prRb Mean radial velocity a Absolute ow angle b Relative ow angle h Efciency hrotor R Rotor efciency R 1 T 1 T h T 0 T0 u dt=T 0 DPq dt Mean efciency z DP0 =PDIN Loss coefcient r Air density u Rotational speed 4 vR/uR Flow coefcient F Flow coefcient amplitude; 4 F sin(2pt/T)

In [17] an experimental work of the impulse radial turbine, proposed by [18], is made. The study shows that this kind of turbine has an acceptable efciency. The main advantages of the radial turbines are, according to [19], their low manufacturing cost, the high torque obtained due to the radial conguration and their ruggedness. Another advantage with regard to axial turbines for OWC is the lack of bidirectional axial trust which reduces the fatigue loads on the bearings. However, the radial turbine causes a high damping on the OWC. In order to develop a high performance radial turbine, in [20] a turbine with pitch-controlled guide vanes was proposed, which in terms of efciency is better, but its manufacturing and maintenance is quite expensive. The aim of this work is to improve the performance of a radial impulse turbine with xed guide vanes for OWC using CFD. A previously validated numerical model [21] and improved in [22,23], allows us to study in detail the ow through the machine and its sources of energy loss. Using these data, the blades and the guide vane prole have been modied by means using the one-dimensional analysis and the Eulers equation for turbomachines. A new relation between the setting angle of the guide vanes and the rotor has been introduced to reduce incidence losses, a new blade prole has been designed to reduce the ow instabilities and to increase the torque obtained by means of increasing the ow deection. This new geometry achieves a remarkable advance in turbine performance.

2. Numerical model In order to validate the numerical model, the turbine geometry used was that considered as Case 1 in [17]; hence forward, this turbine geometry will be denominated M8 turbine, Fig. 2a. This is equipped with a single rotor of symmetrical blades (R), one row of outer guide vanes (OGV), one row of inner guide vanes (IGV) and an elbow (Fig. 3). The main geometrical characteristics are briey indicated in Table 1. Details of the turbine geometry characteristics and dimensions can be found in Ref. [17]. The guide vane prole used in this M8 model consists of a straight line and circular arc, Fig. 2a. The blade prole consists of a circular arc on the pressure side and an ellipse on the suction side. The ow simulation is solved with FLUENT v6.3, which uses the nite volume numerical method for solving the Navier-Stokes equations. Since the computational volume includes rotating components (u 234 rpm), the sliding mesh technique was used in order to manage the relative movement between the rotor and the stator of the turbine. A hexaedrical unstructured grid of 500.000 cells is used. The ow model solves the incompressible uid conservation equations by using a segregated solver. The realizable kee turbulence model was used with the standard wall function. The time dependent term is approximated with a second order implicit scheme. The pressureevelocity coupling was recreated through the SIMPLE algorithm. The highest order MUSCL scheme has been used for convection terms discretization and the classical central differences approximations for diffusion terms. The ow characteristics description is conducted by solving equations in the threedimensional turbine geometry. However, in order to reduce variable storage and to improve numerical accuracy, we have reduced the 3D calculation domain to a small angular sector with periodic boundaries (Fig. 4).

3. Global performances 3.1. M8 turbine analysis Turbine performance under steady ow conditions was obtained by CFD and was evaluated as in [17]. Fig. 5 shows the CTe4 and CAe4 characteristics for the M8 turbine. Fig. 6 shows the steady efciency. The curves show a big difference between the performances in exhalation and inhalation. A detailed ow analysis inside the M8 turbine is made in [22,23] and taking into account these data we can state that: Inner guide vanes (IGV): the IGV guide the ow towards the rotor adequately during the exhalation. However, during the

Fig. 1. Radial impulse turbine.


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Fig. 2. Radial turbine geometry. (a) M8 geometry [17]. (b) M16 geometry.

Fig. 3. Turbine sketch and reference surfaces.

inhalation the IGV do not guide the ow towards the elbow very well, which increases the secondary ow loss in that element. The guiding could be improved with longer vanes. Rotor: in exhalation, the blades do not guide the ow in a correct way because of the divergent passage between them. Moreover, the torque produced during exhalation is not as high as inhalation due to the high incidence losses and because inow is the natural ow direction for a radial turbine, which corresponds to inhalation. In inhalation, the inter-blade passage is convergent and the guiding is good. Outer Guide Vanes (OGV): during exhalation, the ow approach velocity vectors are mismatched with the leading edge angle of the OGV vanes. This mismatch causes additional energy loss which is referred to as incidence or incidence losses. During the inhalation phase there are no problems.

Fig. 4. Periodic calculation domain and boundary conditions.

It can be concluded that important energy loss exists in the elbow and in the guide vanes, mainly in exhalation. Therefore, the modications made in the M8 turbine geometry aim to diminish this loss and increase the rotor torque. In the new geometry (henceforward, M16 turbine, Fig. 2b) the rotor blade and guide vane proles, both inner and outer, have been modied. The IGV length has also been increased. These changes are based in results given by a onedimensional model which was used to look for a better alignment of the ow with the blades and vanes. The deviation angle of the ow in the rotor was changed to improve the rotor performance in exhalation, as a result the inter-blade passage is almost uniform.

Table 1 Main geometrical characteristics. M8, Case 1 [17] Blade number IGV Rotor OGV 52 51 73 Chord length 50 mm 54 mm 50 mm Solidity 2.29 2.02 2.28 Setting angle 25 19.8 /35.8 25 M16 (improved geometry) Blade number 34 51 85 Chord length 71 47 45 Solidity 2.54 1.78 2.42 Setting angle 20 20 /25 20

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Fig. 5. (a)Torque coefcient CT and (b) input coefcient CA.

3.2. M16 turbine performance Fig. 2 shows the differences between both geometries. The number and geometry of IGV have changed. The number of IGV has been reduced to avoid loss, mainly during inhalation. The new inner guide vanes prole used in geometry M16 consists of straight lines and circular arcs, Fig. 2b, the vanes are longer and the outer angle is 20 in order to increase the rotor torque during exhalation. The blade prole consists of circular arcs on the pressure side and ellipse arcs on the suction side. Furthermore, the inner and outer angles are different. The geometry of the OGV is very similar to that of the M8. It consists of a straight line and a circular arc, but the inner angle is 20 so that the rotor performance improves during inhalation. The number of OGV is increased to improve the guidance. Figs. 5 and 6 show the effect of the geometry on turbine performances under steady ow conditions. The rotor torque has increased, as expected, due to the new angles of the elements, mainly during exhalation. The input coefcient, CA, has also increased but the efciency attained by M16 is higher in exhalation and it does not drop in inhalation. This way, turbine performance between the inhalation and the exhalation is more balanced. This fact is very important because the turbine works alternatively between exhalation and inhalation.

Fig. 6. Steady efciency.

Fig. 7. Flow angles (b, a) and geometry angles (b*, a*).


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Fig. 8. Flow angles in the sections E, D, C and B.

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Fig. 9. Loss coefcient, z. (a) Elbow, (b) IGV, (c) OGV.

4. Flow analysis The next topic studied is the ow pattern inside the M8 and M16 turbines, which will be performed via the analysis of the ow angles in sections B, C, D and E (Fig. 3). The ow angle with the geometrical angle of the turbine elements, vanes and blades, will be compared. In sections which involve xed elements (sections B and E) the ow angle under study is the absolute one (a). In sections delimiting the rotor (C and D) the relative ow angle (b) is also considered. Angles a* and b* are the geometrical angles of vanes and blades, respectively (Fig. 7). 4.1. Inhalation Section E: in this section the vanes are radial, so they do not have any inuence. Section D: Fig. 8a shows that the OGV guiding is better in the M16 turbine than in the M8 as expected, due to the higher number of vanes. Moreover, the geometrical angle is lower for the M16 where a*D 20 . Fig. 8b shows that the relative ow going into the rotor is well adapted to the setting angle at high ow coefcient. Section C: Fig. 8c shows that the ow going into the inner guide vanes from the rotor shares the same angle in both geometries. However, as the geometrical angle of the inner guide vanes is different, incidence losses is more important in the M16. Section B: it is important to point out that, in turbine M16, section B is out of the inner guide vanes, too. Therefore, Section B in M16 and M8 is in different places, although in both cases it is equivalent. The guidance made by the IGV of the M16 in inhalation is far better than the one made by the M8. This can be veried in Fig. 8d, since for the M16 there is a difference of 5 between the ow angle and the setting angle. In the M8 the difference is 20 , which corroborates that the IGV guidance was poor. 4.2. Exhalation Section B: in this section the vanes are radial, so they do not have any inuence. Section C: the IGV guide the ow in an adequate way in both turbines, though efciency is higher in the M16 than in the M8, Fig. 8h. This is because in the M16 the ow direction is better adapted to the leading edge angle of the rotor blade, Fig. 8g, and it causes smaller incidence losses and an increase in the rotor efciency. Section D: In Fig. 8f it appears that the absolute rotor outlet ow angle and the setting angle of the downstream outer guide vanes are similar in the M16 geometry. However, for the M8 there are larger differences (25 ) which means a rise in incidence losses. Section E: the OGV guide the ow more efciently in the M16 than in the M8, Fig. 8e.

5. Loss analysis In order to study the energy loss in xed elements we have used use the loss coefcient z. This coefcient is the relationship between the total pressure drop in an element and the representative dynamic pressure in it. The loss coefcient (z) of the elbow can be seen in Fig. 9a, they are very similar. The ow is better guided by the IGV in the M16 than in the M8. However, as the number of guide vanes is lower in M16, there are important secondary ows. In Fig. 9b there appears the loss coefcient of the IGV. In the M16 the guiding vanes were designed with the main goal of beneting to the rotor. During the exhalation phase, the IGV work appropriately because of the low number of guide vanes and the convergent evolution of the transversal section. However, during the inhalation there are several drawbacks, the main one being incidence losses in its entrance. From the analyses of Fig. 9c it can be said that the loss in OGV are more important in the M16 than in the M8. This is the result of the different number of guide vanes, since in the M16 there are 85, whereas in the M8 there are only 73. This increases both friction and incidence losses, since increasing the number of OGV does not avoid the ow detachment at the leading edge of the OGV. Efciency of the energy exchange in the rotor is calculated by using the relation:

hrotor r

Uvu Inlet Uvu Outlet P0 Inlet P0 Outlet

Rotor efciency is depicted in Fig. 10. It can be observed that rotor efciency drops during inhalation due to the stronger tip and secondary ows. However, the turbine efciency is maintained

Fig. 10. Rotor efciency.


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Fig. 11. Mean efciency and energy per cycle.

(Fig. 6). The main point is the efciency increase during exhalation caused by the improved angles relation.

6. Turbine performances under sinusoidal ow conditions In order to clarify the turbine suitable for wave energy conversion, it is necessary to evaluate the turbine performance in connection with an OWC under irregular ow conditions [16]. In order to evaluate the behavior under unsteady ow we use the mean efciency, h, [14]. The running characteristics of the turbine under periodic ow conditions can be simulated by using the steady ow characteristics of a turbine at a constant rotational speed by assuming quasi-steady ow conditions, [24]. Fig. 11 shows the mean efciency and the energy per cycle when the turbine works under sinusoidal ow (period T 10 s). It can be observed that in the M16 mean efciency is better than in the M8. For F 1 where efciency is highest, it is 5% better. The energy per cycle is higher in M16.

Above all, the loss is important when the vanes are downstream. The curvature and variation of the channel passage section are also important in this case. Nevertheless, the determining factor of the loss, in this case, is the ow detachment which takes place in the leading edge of the vanes. This problem has been reduced by xing a new relation between the setting angle of the guide vanes and the rotor angles. Increasing the length of the IGV in the radial direction has proved to be quite positive, since it reduces the loss in the elbow. This reduction is based on the fact that the ow circulates in a more ordered way, and consequently the loss associated with secondary ows is reduced. With these results, we can deduce that from the point of view of the global behavior of the turbine, it is better to improve the rotor performance than that of the guide vanes. Due to this, in the guide vanes of the proposed geometry, the loss increases with respect to the initial geometry. One of the main problems of the initial geometry was the inequality of its performance between inhalation and exhalation, the difference in efciency reaches 10%. This is because the torque obtained by the rotor in both modes is very different. The proposed blade prole is superior to the previous one because the torque during inhalation stays the same and increases notably in exhalation. Thanks to this, the efciency in exhalation increases by up to 9%, whereas in inhalation it remains approximately equal. Consequently, the mean efciency under unsteady ow increases sensitively in the new geometry by around 4e5%. Acknowledgements Here we want to mention that these researches are conducted as part of the common project between the Fluid Mechanic and Turbomachinery research teams of both the University of Valladolid (Spain) and the University of Mohammed V-Agdal (Morocco) for the development of a project based on an OWC converter plant. This project is an AI of the Agencia Espaola de Cooperacin Internacional. This team would like to thank to group GR57 for the support provided. References[1] Ueki K, Ishizawa K, Nakagawa H. Output of electrical power from pneumatic wave power generation system with water valve rectier. Proceedings of 10th ISOPE conference, Seattle, vol. I, pp. 339e404; 2000. [2] Raghunathan S. The wells turbine for wave energy conversion. Prog Aerospace Sci 1995;31:335e86. [3] Govardhan M. Numerical studies on performance improvement of a selfrectifying air turbine. Eng Appl Comput Fluids Mech 2007;1(1):57e70. [4] Inoue M, Kaneko K, Setoguchi T, Raghunathan S. Simulation of starting characteristics of the wells turbine. ASME 4th uids mechanics, plasma dynamics and lasers conference, Atlanta; 1986. [5] Kim T, Setoguchi T, Kaneko K. The optimization of blade pitch settings of an air turbine using self-pitch-controlled blades for wave power conversion. Journal of Solar Engineering 2001;123:382e6. [6] Setoguchi T, Santhakumar S, Takao M, Kim T, Kaneko K. A modied Wells turbine for wave energy conversion. Renewable Energy 2003;28:79e91. [7] Raghunathan S, Eves A, Whittaker T, Long A. The biplane Wells turbine. Proceedings of the OMAE conference, Houston; 1987. p. 475e9. [8] Gato L, Curran R. Performance of the biplane Wells turbine. Journal of Offshore Mechanics and Arctic Engineering (OMAE) 1996;118:210e5. [9] Thakker A, Abdulhadi R. Effect of blade prole on the performance of the wells turbine under unidirectional sinusoidal and real sea ow conditions. International Journal of Rotating Machinery; 2007; doi:10.1155/2007/51598. [10] Torresi M. Experimental and numerical investigation on the performance of a Wells turbine prototype. In: Proceedings of the seventh European Wave and Tidal (EWTEC), Oporto; 2007. [11] Setoguchi T, Kaneko K, Taniyama H, Maeda M, Inoue M. Impulse turbine with self-pitch-controlled guide vanes for wave power conversion: guide vanes connected by links. International Journal of Offshore and Polar Engineering 1996;6:76e80. [12] Santhakumar S, Jayashankar V, Atmanand M, Pathak A, Ravindram R, Setoguchi T, Takao M, Kaneko K. Performance of an impulse turbine based wave

ZT Energy per cicle 0

T0 u dt

The better performance in the M16 is caused by the well balanced behavior between inhalation and exhalation. This causes that the efciency along a periodical and bidirectional ow was higher in the M16.

7. Conclusions In this paper, we have analyzed the optimization of the turbines design by means of a numerical model. This model, which was previously validated, allows us to study in depth the ow through the machine and its sources of energy loss. The new proposed geometry means a remarkable advance in turbine performance. The guide vanes are an important source of energy loss as much in inhalation as in exhalation (mainly the inner ones). When the guide vanes are upstream of the rotor and they guide the ow towards the rotor, the loss is mainly associated with the curvature of the vane and the variation of the channels passage section. The vane curvature is related to the intensity of the tip ow, whereas the variation of the passage section is important in order to avoid ow disattachment in the inner part of the guide vanes. These two aspects have more relevance in the inner guide vanes because ow velocity is higher there.

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B. Pereiras et al. / Renewable Energy 36 (2011) 1477e1484 energy plant. In: Proceedings of eighth international offshore ad polar engineering, 1, Montreal; 1998. Maeda H, Santhakumar S, Setoguchi T, Takao M, Kinoue Y, Kaneko K. Performance of an impulse turbine with xed guide vanes for wave energy conversion. Renewable Energy; 1999:533e47. Setoguchi T, Shanthakumar S, Maeda H, Takao M, Kaneko K. A review of impulse turbines for wave energy conversion. Renewable Energy; 2000:261e92. Kim T, Takao M, Setoguchi T, Kaneko K, Inoue M. Performance comparison of turbines for wave energy conversion. International Journal of Thermal Science 2001;40:681e9. Setoguchi T, Takao M. Current status of self rectifying air turbines for wave energy conversion. Energy Conversion and Management; 2006:2382e96. Setoguchi T, Santhakumar S, Takao M, Kaneko K. A performance study of a radial impulse turbine for wave energy conversion. Journal of Power and Energy 2002;216(A1):15e22. McCormick M, Rehak J, Williams B. An experimental study of a bidirectional radial turbine for pneumatic wave energy conversion. Mastering the Oceans through Technology 1992;Vol. 2:866e70. [19] Dixon SL. In: BUTTERWORTH-HEINEMANN, editor. Fluid mechanics and thermodynamics of turbomachinery. 5th ed. Elsevier, ISBN 978-0-7506-78704; 2005. [20] Takao M, Fujioka Y, Setoguchi T. Effect of pitch-controlled guide vanes on the performance of a radial turbine for wave energy conversion. Ocean Engineering; 2005:2079e87. [21] Marjani AE, Castro F, Rodrguez M, Parra M. Numerical modelling in wave energy conversion systems. Energy 2008;33:1246e53. [22] Pereiras B, Castro F, Rodrguez M. Tip clearance effect on the ow pattern of a radial impulse turbine for wave energy conversion. ISOPE09. Osaka, Japan; 2009. ISSN 1098-6189. [23] Pereiras B, Castro C, Marjani AE, Rodrguez M. Radial impulse turbine for wave energy conversion, A new geometry. OMAE08. Portugal: Estoril, ISBN 978-07918-4823-4; June 15e20, 2008. [24] Inoue M, Kaneko K, Setoguchi T, Saruwatari T. Studies on the Wells turbine for wave energy generation (turbine characteristics and design parameter for irregular wave). JSME International Journal 1988;31: 676e82.

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an improved radial impulse turbine for owc

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