Effects of Hub-To-Tip Ratio and Tip Clearance on Hysteretic Characteristics of Wells Turbine for Wave Power Conversion

A Wells turbine for wave power conversion has hysteretic characteristics in a reciprocating flow. The hysteretic loop is opposite to the well-known dynamic stall of an airfoil. In this paper, the mechanism of the hysteretic behavior was elucidated by an unsteady 3-dimensional Navier-Stokes numerical simulation. It was found that the hysteretic behavior was associated with a streamwise vortical flow appearing near the blade suction surface. The effects of hub-to-tip ratio and tip clearance on the hysteretic characteristics of the Wells turbine have also been discussed in this paper.     

Comparison of Performances of Turbines for Wave Energy Conversion

The Wells turbine for a wave power generator is a self-rectifying air turbine that is available for an energy conversion in an oscillating water-air column without any rectifying valve. The objective of this paper is to compare the performances of the Wells turbines in which the profile of blade are NACA0020, NACA0015, CA9 and HSIM15-262123-1576 in the small-scale model testing. The running characteristics in the steady flow, the start and running characteristics in the sinusoidal flow and the hysteretic characteristics in the sinusoidal flow were investigated for four kinds of turbine. As a conclusion, the turbine in which the profile of blade is NACA0020 has the best performances among 4 turbines for the running and starting characteristics in the small-scale model testing.     

Effect of end plates on the performence of a wells turbine for wave energy conversion

In order to improve the performance of the Wells turbine for wave energy conversion,the effect of end plates onthe turbine characteristics has been investigated experimentally by model testing under steady flow conditions.The end plate attached to the tip of the original rotor blade is slightly larger than the original blade profile.Thecharacteristics of the Wells turbine with end plates have been compared with those of the original Wells turbine,i.e.,the turbine without end plate.As a result, it has been concluded that the characteristics of the Wells turbinewith end plates are superior to those of the original Wells turbine and the characteristics are dependent on the sizeand position of end plate. Furthermore, the effect of annular plate on the turbine performance,which encircles theturbine and is attached to the tip,was investigated as an additional experiment.However,its device was not effec-tive in improving the turbine characteristics.     

The performance of Wells turbine under bi-directional airflow

This paper presents the performance of a Wells turbine operating under unsteady bi-directional airflow conditions. In this study, four kinds of blade profile were selected, NACA0020, NACA0015, CA9 and HSIM 15-262123-1576. The experiments have been carried out for two solidities under sinusoidal and irregular unsteady flow conditions based on Irish waves (Site2). It was found that for a Wells turbine operating under bi-directional air flow, the rotor geometry preferred is the blade profile of CA9 with rotor solidity σ=0.64. In addition, the efficiency curve of the Wells turbine under unidirectional flow conditions fails to present the rapid rise in the instantaneous efficiency which occurs at low flow coefficient of bi-directional flow condition. A comparative analysis between the numerical simulation results and experimental results was carried out. As a result, an excellent agreement was found between the numerical and experimental results. In addition, the effect of blade profile and rotor solidity on hysteretic characteristics of the turbine has been clarified experimentally under bi-directional airflow.     

Effects of blade geometry on performance of wells turbine for wave power conversion

IntroductionThe fossil fuel energy with the problem of airpollution might run out by the middle of the 21stcentury. So many researchers have studied forscores of years on alternative, renewable energysources11] such as tidal, wave, salinity gradient,current, wind and solar energy.The energy density level of waves is higherthan the other energy sources stated above. Thereare various techniques for extraction of energyfrom waves[2]. Several of the wave energydevices using the principle of an os…     

Wells turbine blade profile optimization for better wave energy capture

Despite the fact that wave energy is available at no cost, it is always desired to harvest the maximum possible amount of this energy. The axial flow air turbines are commonly used with oscillating water column devices as a power take‐off system. The present work introduces a blade profile optimization technique that improves the air turbine performance while considering the complex 3D flow phenomena. This technique produces non‐standard blade profiles from the coordinates of the standard ones. It implements a multi‐objective optimization algorithm in order to define the optimum blade profile. The proposed optimization technique was successfully applied to a biplane Wells turbine in the present work. It produced an optimum blade profile that improves the turbine torque by up to 9.3%, reduces the turbine damping coefficient by 10%, and increases the turbine operating range by 5%. The optimized profile increases the annual average turbine power by up to 3.6% under typical sea conditions. Moreover, new blade profiles were produced from the wind turbine airfoil data and investigated for use with the biplane Wells turbine. The present work showed that two of these profiles could be used with low wave energy seas. Copyright © 2017 John Wiley & Sons, Ltd.     

Effect of Guide Vanes on the Performance of a Variable Chord Self—Rectifying Air Turbine

INTRODUCTIONWiththeconventionalenergyresourceslikelytogetexhaustedinafewdecades,theinexhaustiblesourcesofenergyhavetotaketheirplace.Alternateenergyfromtheoceanisattractingtheattelltionoftheresearchersinrecentyearsduetoitsperennialavailabilityandminimumhealthhazards.Ofthemanypossibleformsofoceanenergy,waveenergyispromising.Waveenergyisanalternateformenergy,whichispollutantfreeandinnearfutureitislikelytobeeconomicallyviable.Countrieswhicharesurroundedbyseaandpossessremotelysituatedislandcom…     

Numerical investigation on the effect of blade sweep on the performance of Wells turbine

A Wells turbine is one of the simplest and promising self-rectifying air turbines which is basic to the needs of the near future and likely to be economically viable. With the recent development in computer hardware and software, it has now become practicable to conduct a reasonable computation of three-dimensional turbulent flows through complex geometry. To investigate the effect of blade sweep on the performance of the Wells turbine, the numerical investigation was carried out under steady flow condition with a fully 3-D Navier–Stokes code for two kinds of blades, NACA0020 and CA9. As a result, it was found that the performance of the Wells turbine is considerably influenced by the blade sweep. The optimum blade sweep ratio (f=0.35) for the NACA0020 was found. This value is just the same as one obtained experimentally by the authors in the past. It was also found that the overall turbine performance for the NACA0020 is better than that for the CA9. It was shown that the numerical method is able quite well to predict the effect of blade sweep of the Wells turbine. The detailed flow patterns for several blade sweeps were also shown and discussed in this paper.     

Effect of guide vanes on the performance of a self-rectifying air turbine with constant and variable chord rotors

A Wells turbine is a self-rectifying air flow turbine capable of converting pneumatic power of the periodically reversing air stream in Oscillating Water Column into mechanical energy. The Wells turbine has inherent disadvantages; lower efficiency, poorer starting characteristics, higher axial force and low tangential force in comparison with conventional turbines. Guide vanes before and after the rotor suggest a means to improve the tangential force, hence its efficiency. Experimental investigations are carried out on a Wells turbine with the constant chord and variable chord blade rotors fitted with inlet and outlet guide vanes to understand the aerodynamics. Experiments were also conducted for the above said rotors with various stagger angles to validate the design stagger angle. In addition, the starting and running characteristics of the rotors have been studied and compared with the case without guide vanes. Studies were done at various flow coefficients covering the entire range of flow coefficients over which the turbine is operable. The efficiency, starting characteristics of the turbines with guide vanes have improved when compared with the respective turbines without guide vanes.     

Experimental investigation in a Wells turbine under bi-directional flow

In this work an experimental study of flow through a Wells turbine with NACA0015 profiles submitted to an unsteady and bi-directional flow is presented.The experimental set-up of the Department of Mechanical, Chemical and Materials Engineering of the University of Cagliari (DIMCM), can simulate the real operation of a wave energy conversion device based on the principle of an oscillating water column (OWC) equipped with a Wells turbine. The set-up consists of a piston, controlled by a hydraulic system, that moves inside a cylindrical chamber open at the top where the Wells turbine is placed. The piston movement generates the airflow driving the turbine.Experimental investigations were carried out in proximity of the rotor blade using three-dimensional aerodynamic probes to perform a careful characterization of the flow field upstream and downstream of the turbine. The dynamic characteristic of the turbine in terms of dimensionless flow parameters was also determined. The real entity of the hysteresis phenomenon was highlighted for the phases of acceleration and deceleration of the unsteady flow through the turbine. Moreover, the existence of an appropriate correlation between the conventional dimensionless coefficients and a measurable and reliable physical variable was investigated.     

振荡水柱波浪发电空气透平工作特性数值模拟

为更深入地探讨振荡水柱式波能发电装置的原理,对用于波力发电装置二次能量转换的威尔斯式透平进行了三维全流道模拟计算。针对对称翼型叶片和非对称翼型叶片的威尔斯式透平,分别考察了其在不同叶片安装角下的效率和出力随流量系数的变化趋势,得出了不同叶片安装角时威尔斯式透平运行的最佳参数。在相同工况下,对比分析了不同翼型叶片威尔斯式透平的效率、出力、输入系数及扭矩系数随流量系数的变化,并比较了叶片表面的压力分布情况。结果表明,随着叶片安装角增大,装置最高效率值增大,且在安装角大于9°后变化缓慢,而装置最大出力却随着安装角的增大而减小;采用非对称翼型叶片时,装置的出力、输入系数、扭矩系数均大于相同工况下对称翼型叶片装置。     

A modified Wells turbine for wave energy conversion

The method of wave energy conversion utilises an oscillating water column (OWC). The OWC converts wave energy into low-pressure pneumatic energy in the form of bi-directional airflow. Wells turbine with its zero blade pitch setting has been used to convert this pneumatic power into uni-directional mechanical shaft power. Measurements in OWC based wave energy plants in India and Japan show that the airflow velocity is not equal in both directions. The velocity is more when the airflows out to the atmosphere (exhalation) than in the reverse direction. It may be advantageous to set the rotor blade pitch asymmetrically at a positive pitch so as to achieve a higher mean efficiency in a wave cycle. Towards this objective, performance characteristics of a turbine with different blade setting angles in steady flow were found by experimentation. Quasi-steady analysis was then used to predict the mean efficiency for a certain variation of air velocity with time. This variation with time was taken as pseudo-sinusoidal wherein the positive part of the cycle was taken as a half sine-wave whose amplitude is greater than that of the negative half sine-wave. Such a variation is representative of what happens in reality. For exhalation velocity amplitude to inhalation velocity ratios 0.8 and 0.6, a rotor blade setting angle of 2° was found to be optimum.     

Numerical optimization of axial turbine with self‐pitch‐controlled blades used for wave energy conversion

Wells turbines are among the most practical wave energy converters despite their low aerodynamic efficiency and power produced. It is proposed to improve the performance of Wells turbines by optimizing the blade pitch angle. Optimization is implemented using a fully automated optimization algorithm. Two different airfoil geometries are numerically investigated: the standard NACA 0021 and an airfoil with an optimized profile. Numerical results show that each airfoil has its own optimum blade pitch angle. The present computational fluid dynamics optimization results show that the optimum blade pitch angle for NACA 0021 is +0.3° while that of the airfoil with an optimized profile equals +0.6°.The performance of the investigated airfoils is substantially improved by setting the blades at the optimum blade pitch angle. Both the turbine efficiency and tangential force coefficient are improved, especially at low flow rate and during turbine startup. Up to 4.3% average increase in turbine efficiency is achieved by optimizing the blade pitch angle. A slight improvement of the tangential force coefficient and decrease of the axial force coefficient are also obtained. A tangible increase of the stall‐free operating range is also achieved by optimizing the blade pitch angle. Copyright © 2013 John Wiley & Sons, Ltd.     

Computational analysis of performance and flow investigation on wells turbine for wave energy conversion

Wells turbine is a self-rectifying airflow turbine capable of converting pneumatic power of the periodically reversing air stream in oscillating water column into mechanical energy. This paper reports the computational analysis on performance and aerodynamics of Wells turbine with the NACA 0021 constant chord blades. Studies have been made at various flow coefficients covering the entire range of flow coefficients over which the turbine is operable. The present computational model can predict the performance and aerodynamics of the turbine quantitatively and qualitatively. The model also predicted the flow coefficient at which the turbine stalls, with reasonable accuracy.     

非定常流下Wells涡轮机准静态分析方法研究

以Wells涡轮机为研究对象,通过求解RANS方程和Spalart-Allmaras模型实现数值仿真模拟,研究非定常流下Wells涡轮机的准静态分析方法。通过网格独立性分析和已有文献对比,验证计算模型的准确性。计算并对比定常流、振荡流和往复流工况中Wells涡轮的性能,结果显示非定常流下Wells涡轮机会发生迟滞现象。通过振荡流和往复流的流动频率影响分析,说明非定常流下Wells涡轮机准静态分析方法的可行性和局限性:准静态分析对于较低流动频率的非定常流工况准确性较高,但对于较高流动频率误差较大。     

Comparative study of turbines for wave energy conversion

IlltnductionSeveral of the wave energy devices cuntiy stUdiedin the United kingdom, Japan, POhogal, India and othercountries make use of the principle of the oscillatingwater-air coltUnn for convening wave energy to lowPneqmatic energy Which in tUrn can be converted intomechAncal energy. In this case, the developmellt of a bidirechonal air theme has come lip as an importantProblem. So far, a number of self-rechfying air onnesWith different configurations have been ProPOsed, and a; Wells…     

Effect of guide vane shape on the performance of a Wells turbine

A Wells turbine has inherent disadvantages: lower efficiency and poorer starting characteristics. Providing guide vanes on either side of the rotor could be one of the most effective ways of improving its performance. Several papers have demonstrated the usefulness of 2D guide vanes so far. In order to achieve further improvement in the performance of the Wells turbine, the effect of 3D guide vanes has been investigated experimentally by testing a model under steady flow conditions. Then, the running and starting characteristics under irregular flow conditions have been obtained by a computer simulation using quasi-steady analysis. It is found that the running and starting characteristics of the Wells turbine with 3D guide vanes are superior to those with 2D guide vanes.     

Wells turbine for wave energy conversion: a review

In the past 20 years, the use of wave energy systems has significantly increased, generally depending on the oscillating water column concept. Wells turbine is one of the most efficient oscillating water column technologies. This article provides an updated and a comprehensive account of the state‐of‐the‐art research on Wells turbine. Hence, it draws a roadmap for the contemporary challenges, which may hinder future reliance on such systems in the renewable energy sector. In particular, the article is concerned with the research directions and methodologies, which aim at enhancing the performance and efficiency of Wells turbine. The article also provides a thorough discussion of the use of CFD for performance modeling and design optimization of Wells turbine. It is found that a numerical model using the CFD code can be employed successfully to calculate the performance characteristics of W‐T as well as other experimental and analytical methods. The increase of research papers about CFD, especially in the last 5 years, indicates that there is a trend that considerably depends on the CFD method. Copyright © 2016 John Wiley & Sons, Ltd.     

基于滑移网格的垂直轴潮流水轮机的三维数值模拟

为了探究翼型对垂直轴水轮机水力效率的影响,基于叶素理论分析建立了垂直轴潮流水轮机在水槽中的物理模型,采用滑移网格技术在Fluent软件中对模型的流场进行了三维数值模拟。在保持转速一定、更改来流速度即改变叶尖速比的条件下,分析了两种不同NACA翼型直叶片的潮流水轮机内部流场以及水力性能。结果表明,翼型以及叶片安装角对垂直轴潮流水轮机的利用效率影响很大,其流动特性与来流速度、叶片布置形式有重要联系,为新型潮流水轮机的设计和翼型的选择应用提供了依据。     

水泵水轮机转轮加装短叶片前后的流动特性对比

为对比高水头水泵水轮机的转轮加装短叶片前后的能量特性及流动特性,基于SST湍流模型,选取4个具有代表性的水泵及水轮机工况,对有/无短叶片的水泵水轮机进行全流道三维定常计算。数值模拟结果表明,以水泵运行时加装短叶片可抑制脱流与漩涡等二次流现象,降低单个叶片承受的水力载荷,提高转轮进出口、导叶区及蜗壳静压,使泵获得更高的扬程。水轮机运行时添加短叶片可减小转轮出口环量,改善在尾水管内形成的复杂漩涡流,提高其水力效率。相同边界条件下,长短叶片转轮改善了转轮区的流动条件,从而提升了机组的能量特性及水力稳定性。