Design and Analysis of 3D Blades for Wells Turbine

Transcription

1 IJIRST International Journal for Innovative Research in Science & Technology Volume 1 Issue 11 April 2015 ISSN (online): Design and Analysis of 3D Blades for Wells Turbine Shyjo Johnson Saintgits College of Engineering, Pathamuttom, Kottayam, Tom B Thachuparambil Saintgits College of Engineering, Pathamuttom, Kottayam, Sriram S Kumar Saintgits College of Engineering, Pathamuttom, Kottayam Vivek Joseph John Saintgits College of Engineering, Pathamuttom, Kottayam, G. Anil Kumar Saintgits College of Engineering, Pathamuttom, Kottayam, Abstract Wells turbine is a self-rectifying air turbine which is widely used in oscillating water column energy converter. The Wells turbine will always rotate in the same direction irrespective of the direction of the oscillating airflow. Furthermore the Wells turbine has a simple configuration and structure. This is why the Wells turbine is very commonly used for conversion of wave energy. At present 2-dimensional blades are being used for the conversion of wave energy. We are proposing 3-dimensional blades that can improve the steady characteristics of the current wells turbine. The effect of 3-dimensional blade on the turbine characteristics has been analyzed based on different inlet velocities and steady characteristics were found out. Further, the aim of the use of 3-dimentional blade for Wells turbine is to prevent flow separation on the suction surface near tip, which is a major drawback in the case of existing one. The chord length is constant with radius and the blade thickness increases gradually from hub to tip. The blade profiles are NACA0015 at hub, NACA0020 at mean radius and NACA0025 at tip. The performance of wells turbine with 3-dimensional blades has been compared with those of original Wells turbine, i.e., the turbine with 2- dimensional blades. As a result, it has been concluded that both the efficiency and turbine characteristics can be improved by the use of 3-dimensional blade. Keywords: Wells Turbine, Oscillating Water Column, Steady Characteristics, Stall I. INTRODUCTION Several of the wave energy devices being studied under any wave energy program make use of the principle of an oscillating water column (OWC). In such wave energy devices an oscillating water column due to wave motion is used to drive an oscillating air column, which is converted into mechanical energy. The energy conversion from an oscillating air column can be achieved by using a system of non-return valves for rectifying the airflow, together with a conventional turbine. However, such a system is complicated and difficult to maintain, and the average cycle efficiency in an oscillating airflow is likely to be considerably smaller. The non-return valves can be eliminated by the use of a self-rectifying air turbine, which inherently provides a unidirectional rotation for an alternating airflow. The Wells turbine is of this type and is one of the simplest and probably the most economical turbines for wave energy conversion. However, according to the previous studies, the current Wells turbine has inherent disadvantages such as lower efficiency, poorer starting characteristics and higher noise level in comparison with conventional turbines. On the other hand, in order to overcome these weak points, a number of self-rectifying air turbines with different configurations have been proposed.rather, different we are concentrating on the design of a three dimensional blade, in place of the existing 2 dimensional one. Figure 1 shows the conversion of energy taking place inside a wells turbine. The potential hydro energy of ocean waves is converted to pneumatic energy. This pulsated air passes through a turbine blade and is converted to mechanical energy. The turbine is coupled to a generator which converts the mechanical energy into electrical energy All rights reserved by 202

2 Fig. 1: Outline diagram of wells turbine Figure 2 shows the outline of wells turbine. In this study, in order to enhance the characteristics of Wells turbine for wave energy conversion, the effect of 3-dimensional (3D) blade on the turbine characteristics has been investigated analytically under steady flow conditions. The chord length is constant with radius and the blade profile changes gradually from hub to tip in the study. The aim of 3D blade is to prevent flow separation on the suction surface near tip and to gain much energy at tip. Fig. 2: Wells turbine The blade profiles are NACA0015 at hub, NACA0020 at mean radius and NACA0025 at tip. And then, the characteristics of Wells turbine with 3-dimensional have been compared with those of the original Wells turbine, i.e., the turbine with 2- dimensional (2D) blade. II. OBJECTIVES The first objective of our project is to design three dimensional blades using CATIA V5 for wells turbine in place of existing two dimensional blades. A blade is called 2D because the blade thickness is constant with radius and has uniform cross section. Blade profiles such as NACA0015 or NACA0020 or NACA0025 can be used. The existing two dimensional blades have certain drawbacks such as poor starting characteristics, lower efficiency and problem of flow separation at suction surface. The above drawbacks could be overcome by three dimensional blades. Secondly, to compare the steady characteristics of the current 2Dimensional blade with 3Dimensional blades. Turbine performance under steady flow conditions is evaluated by, Turbine efficiency, Torque coefficient, Pressure drop coefficient against axial velocity III. PROBLEM DEFINITION AND BACKGROUND Analyses of the flow through the Wells turbine have been carried out by means of analytical, and numerical methods. The flow domain is divided into annular elements where the two-dimensional assumption is used; the lift and drag coefficients are obtained either from experimental data for isolated airfoils. In the last years, thanks to the development achieved in CFD, the numerical simulation of the three-dimensional turbulent flows, such as that through a Wells turbine, became practicable and All rights reserved by 203

3 several numerical studies on the flow-field through Wells turbine have been presented. In consideration of the low sea wave frequencies, previous works have carried out the fluid dynamic analysis of such a turbine by means of a quasi-steady approach. Recently, hysteretic phenomena have been detected under oscillating flow conditions, especially when the oscillating flow amplitude grows. Sea-wave energy power plants experienced a renewed interest after the introduction of the Wells turbine. In fact, the Wells turbine is commonly adopted in OWC wave energy converters, where, due to the wave motion, the pressure at the inlet of the vertical duct generates pressure fluctuations in the plenum thus producing an oscillating air-flow able to drive a turbine. At present two dimensional blades are being utilized for the conversion of mechanical energy, which has its own disadvantages. It includes poor starting characteristics of the turbine, lower efficiency and the problem of flow separation at the suction surface. In order to overcome these disadvantages 3 dimensional blades are designed which helps to overcome the disadvantages of current one. The three dimensional blades are of two types namely 3D-A and 3D-B,which are classified according to the blade profiles arrangement at the hub, mean radius and tip. IV. DESIGN OF TURBINE BLADE The effective energy conversion is possible due to the use of the self-rectifying axial Wells turbine. The turbine blades have symmetrical profiles (commonly four digit double zero NACA profiles).in order to investigate the characteristics of wells turbine NACA0015 with constant chord length for 2D blades and for 3d blades with NACA0015,NACA0020 and NACA0025.The details of 2D blades and 3D blades are shown on table 1 Table -1: Specifications of blade Figure 3 shows the dimensions of wells turbine experimental setup. The airfoil used in wells turbine are symmetrical airfoils. Fig. 3: Dimensions of wells turbine The design of wells turbine has done in CATIA v5.the design of turbine rotor are shown below: All rights reserved by 204

4 (a) 3D-B blade (b) 3D-A blade Fig. 4: 3D blade profile V. RESULT AND DISCUSSION The analysis of wells turbine has done using ANSYS14. We used Moving Reference Frame (MRF) method to analyze the turbine. MRF method is a method used to analyze rotating bodies. The boundary conditions are inlet velocity was varied in the order of 3 to 9 m/s and outlet pressure as gauge pressure and the speed of rotation is limited to 2000 rpm using speed governor. For the analysis of 3D-A blade results are calculated at 2000rpm using MRF method. The pressure contour obtained shown in the below Fig.The maximum pressure was found to be 473.5pa Fig. 5: Pressure contour at 5m/s for 3D blade We are analyzing 2D, 3D-A and 3D-B type of blades by giving different inlet velocities such as 3, 5, 7 and 9m/s. As a result force acting on the turbine, area and the net torque acting on the turbine can be obtained from the fluent software. Based on the above values steady characteristics such as flow coefficient, pressure drop coefficient and efficiency can be calculated. The turbine performance under steady flow conditions is evaluated by turbine efficiency, torque coefficient and pressure drop coefficient against flow velocity and is being tabulated as shown in table 2: All rights reserved by 205

5 Table -2: Steady characteristics of turbine From the graph shown below it can be infer that as axial velocity increases torque coefficient increases. Maximum torque coefficient is found out to be for 3D-A which is 0.16 at 9m/s when compared to other two blades. Fig. 6: Variation of torque coefficient with axial velocity From the graph shown below it can be infer that as axial velocity increases pressure drop coefficient increases. For 3D-A blade it is found out that there is slight increase in pressure drop coefficient with increase in axial velocity Fig. 7: Variation of pressure drop coefficient with axial velocity All rights reserved by 206

6 From the graph, efficiency increases with axial velocity and efficiency for 3D-A is found to be more than other two blades. Max efficiency for 3D-A is found to be 66.32% Fig. 8: Variation of efficiency with axial velocity VI. CONCLUSIONS In this study, the effect of 3-dimensional blade on turbine characteristics was investigated analytically under steady flow conditions, in order to enhance the performance of Wells turbine for wave energy conversion. As the results, it seems that the turbine characteristic in the case of 3D-A which the blade thickness increases with radius is better than the case of 2-dimensional blade. The turbine characteristics such as pressure drop coefficient, torque coefficient and efficiency is found to be higher for 3D-A blade. The maximum efficiency was found to be 66.32% for 3D-a blade. Further, it can be concluded that the stall characteristic in the case of 3-dimensional blade depends on the profile at tip than that at hub. REFERENCES [1] David G. Dorrell1 and Min-Fu Hsieh, Performance of Wells Turbines for use in Small-Scale Oscillating Water Columns, ISOPE Conference, 1-6 July, 2007 [2] Masami Suzuki, Design Method of Wave Power Generating System with Wells Turbine, Proceedings of the Twelfth (2002) International Offshore and Polar Engineering Conference Kitakyushu, Japan, May 26 31, 2002 [3] M. Torresi, S. M. Camporeale, P. D. Strippoli and G. Pascazio Accurate numerical simulation of a high solidity Wells turbine, Renewable Energy, vol. 33, issue 4, 2008, pp All rights reserved by 207