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

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

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.     

基于气液两相的导叶对液力透平内部流动规律的影响

为了研究气液两相条件下导叶对液力透平内部流动特性的影响,现选取比转速为55.7的离心泵反转作为液力透平,并在液力透平叶轮进口添加一组负曲率导叶,设计出含导叶的水力模型,研究含气工况下导叶对液力透平性能的影响.研究发现:添加导叶前蜗壳和叶轮流道内压力分布和气相分布不均匀,且含气率越高均匀性越差,过流部件内流动较为紊乱,蜗...     

Performance of Wells Turbine with Guide Vanes for Wave Energy Conversion

ＰｅｒｆｏｒｍａｎｃｅｏｆＷｅｌｌｓＴｕｒｂｉｎｅｗｉｔｈＧｕｉｄｅＶａｎｅｓｆｏｒＷａｖｅＥｎｅｒｇｙＣｏｎｖｅｒｓｉｏｎＭａｎａｂｕＴａｋａｏ；ＴｏｓｈｉａｋｉＳｅｔｏｇｕｃｈｉ；ＫｅｎｊｉＫａｎｅｋｏ（ＤｅｐａｒｔｍｅｎｔｏｆＭｅｃｈａｎｉｃａ...     

Counter-rotating turbines for solar chimney power plants

An efficiency model at design performance for counter-rotating turbines is developed and validated. Based on the efficiency equations, an off-design performance model for counter-rotating turbines is developed. Combined with a thermodynamic model for a solar chimney system and a solar radiation model, annual energy output of solar chimney systems is determined. Two counter-rotating turbines, one with inlet guide vanes, the other without, are compared to a single-runner system. The design and off-design performances are weighed against in three different solar chimney plant sizes. It is shown that the counter-rotating turbines without guide vanes have lower design efficiency and a higher off-design performance than a single-runner turbine. Based on the output torque versus power for various turbine layouts, advantageous operational conditions of counter-rotating turbines are demonstrated.     

Wave energy harvesting using a novel turbine rotor geometry

Wells turbines provide a practical solution for wave energy harvesting. The low aerodynamic efficiency of Wells turbines tangibly reduces their output power. Both the turbine efficiency and output power depend on the turbine solidity. The turbine solidity decreases from rotor hub to rotor tip for the commonly used rotors with constant chord‐length blades. The present work introduces a novel Wells turbine rotor geometry. This geometry was obtained by numerically optimizing the rotor's radial solidity distribution. The turbine performance with different rotor geometries was numerically simulated by solving the three‐dimensional Reynolds‐averaged Navier–Stocks equation under incompressible and steady state flow conditions. Simple and multi‐objective optimization were implemented in order to obtain the optimum rotor geometry. The present work showed that an improved turbine performance can be achieved by optimizing the turbine radial solidity distribution. Two different optimized rotor geometries were obtained and presented. The first rotor geometry improved the turbine efficiency by up to 4.7% by reducing its pressure drop. The second rotor geometries enhanced the turbine output power by up to 10.8%. Copyright © 2016 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.     

Multi-objective optimization of the airfoil shape of Wells turbine used for wave energy conversion

Wells turbine is one of the technical systems allowing an efficient use of the power contained in oceans’ and seas’ waves with a relatively low investment level. It converts the pneumatic power of the air stream induced by an Oscillating Water Column into mechanical energy. The standard Wells turbines show several well-known disadvantages: low tangential force, leading to low power output from the turbine; high undesired axial force; usually a low aerodynamic efficiency and a limited range of operation due to stall. In the present work an optimization process is employed in order to increase the tangential force induced by a monoplane Wells turbine using symmetric airfoil blades. The automatic optimization procedure is carried out by coupling an in-house optimization library (OPAL (OPtimization ALgorithms)) with an industrial CFD (Computational Fluid Dynamics) code (ANSYS-Fluent). This multi-objective optimization relying on Evolutionary Algorithms takes into account both tangential force coefficient and turbine efficiency. Detailed comparisons are finally presented between the optimal design and the classical Wells turbine using symmetric airfoils, demonstrating the superiority of the proposed solution. The optimization of the airfoil shape leads to a considerably increased power output (average relative gain of +11.3%) and simultaneously to an increase of efficiency (+1%) throughout the full operating range.     

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.     

Computed effect of guide vane shape on performance of impulse turbine for wave energy conversion

This paper deals with the computational fluid dynamics (CFD) analysis on effect of guide vane shape on performance of impulse turbine for wave energy conversion. Initially, experiments have been conducted on the impulse turbine to validate the present CFD method and to analyse the aerodynamics in rotor and guide vanes, which demonstrates the necessity to improve the guide vanes shape. The results showed that the downstream guide vanes make considerable total pressure drop leads low performance of the turbine and hence three‐dimensional (3‐D) inlet and downstream guide vanes have been designed based on well‐known vortex theory to improve the efficiency of the turbine. In order to prove the improvement in efficiency due to 3‐D guide vanes, CFD analysis has been made on impulse turbine with 2‐D and 3‐D guide vanes for various flow coefficients. As a result, it is seen that the present CFD model can predict the experimental values with reasonable accuracy. Also, it is showed from the numerical results that the efficiency of the turbine can be improved by average of 4.5 percentage points by incorporating 3‐D guide vanes instead of 2‐D guide vanes. The physical reason for improvement in efficiency of the turbine due to 3‐D guide vanes has been explained with the CFD flow insight pictures. As the turbine operates in fluctuating flow conditions, the performance of the turbine with 2‐D and 3‐D guide vanes have been calculated numerically using quasi‐steady analysis. Furthermore, the performance of the turbine has been predicted for one year based on Irish wave climate to show the feasibility of using 3‐D guide vanes in actual sea wave conditions. Copyright © 2005 John Wiley & Sons, Ltd.     

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.     

Performance estimation of bi-directional turbines in wave energy plants

Oscillating water column(OWC)based wave energy plants have been designed with several types of bidirec-tional turbines for converting pneumatic power to shaft power.Impulse turbines with linked guide vanes andfixed guide vanes have been tested at the Indian Wave Energy plant.This was after initial experimentation withWell's turbines.In contrast to the Well's turbine which has a linear damping characteristic,impulse turbines havenon-linear damping.This has an important effect in the overall energy conversion from wave to wire.Optimizingthe wave energy plant requires a turbine with linear damping and good efficiency over a broad range of flow co-efficient.This work describes how such a design can be made using fixed guide vane impulse turbines.The In-dian Wave Energy plant is used as a case study.     

A novel radial self-rectifying air turbine for use in wave energy converters. Part 2. Results from model testing

The paper presents results from model testing of a self-rectifying radial-flow air turbine, that is being developed as an alternative to the axial-flow self-rectifying turbines for applications in wave energy conversion. In the new machine, named biradial turbine, the flow into, and out of, the rotor is radial. The rotor is surrounded by a pair of radial-flow guide-vane rows. The downstream guide vanes are prevented from obstructing the flow coming out of the rotor by axially displacing the whole guide vane set. The turbine model, with a 0.488 m diameter rotor, was tested in unidirectional flow. Experimental results are shown, in dimensionless form, for efficiency, power and pressure head versus flow rate. They are compared with predictions from CFD computations. The results from model testing were used to estimate the time-averaged efficiency of the turbine subject to the irregular bidirectional air flow induced by random waves.     

Design Method of Guide Vane for Wells Turbine

Guide vanes are installed in the Wells turbine in order to improve its efficiency, self-rotating characteristics and off design performance with stall. This work attempts to explain the role of these guide vanes on the basis of momentum theory. It is shown that the upstream vanes are more effective in enhancing efficiency than the downstream ones. A design method for guide vanes is suggested based on experimental data and potential theory. Experimental studies carried out by the author confirm the theory proposed.     

Comparison of modelling approaches and layouts for solar chimney turbines

The turbogenerator is a core component of any solar chimney power plant. Various layouts for the turbogenerator have been proposed in the literature. In this paper the performance of these layouts is compared using analytical models and optimization techniques, and the important design parameters are discussed. The turbine layouts under consideration are single rotor and counter rotating turbines, both with or without inlet guide vanes.In contrast to similar investigations found in the literature, various radial sections along the blades are analysed in the turbine model. This approach is more appropriate than using a simple mean line analysis when dealing with turbines with high blade aspect ratio and low hub to tip ratio. Furthermore, a limit to the degree of reaction of the turbine has been introduced to avoid diffusion at the hub.It is shown in this paper that these slight changes in modelling approach have a significant impact on the performance prediction. Further it can be concluded that the single rotor layout without guide vanes performs very poorly; the efficiency of the other three layouts is much better and lies in a narrow band. The counter rotating layouts provide the highest peak efficiencies, but at relatively low speeds, which leads to an undesirable higher torque for the same power output.     

A review of impulse turbines for wave energy conversion

Oscillating Water Column based wave energy plants convert wave energy into low pressure pnuematic power in the form of bi-directional air flows. Air turbines which are capable of rotating uni-directionally in bi-directional air flow, otherwise also known as self-rectifying turbines, are used to extract mechanical shaft power which is further converted into electrical power by a generator. This paper reviews the state of the art in self-rectifying impulse air turbines. New results on optimum parameters for the fixed-guide-vane impulse turbine are also presented. Starting characteristics and conversion efficiencies of two types of impulse turbines are compared with the well known Wells turbine.     

Experimental studies on effect of guide vane shape on performance of impulse turbine for wave energy conversion

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.     

Study on performance and flow field of an undershot cross-flow water turbine comprising different number of blades

Recently,small hydroelectric generators have gained attention as a further development in water turbine technology for ultra low head drops in open channels.The authors have evaluated the application of cross-flow water turbines in open channels as an undershot type after removing the casings and guide vanes to substantially simplify these water turbines.However,because undershot cross-flow water turbines are designed on the basis of cross-flow water turbine runners used in typical pipelines,it remains unclear whether the number of blades has an effect on the performance or flow fields.Thus,in this research,experiments and numerical analyses are employed to study the performance and flow fields of undershot cross-flow water turbines with varying number of blades.The findings show that the turbine output and torque are lower,the fluctuation is significantly higher,and the turbine efficiency is higher for runners with 8 blades as opposed to those with 24 blades.     

A twin unidirectional impulse turbine topology for OWC based wave energy plants – Experimental validation and scaling

The twin unidirectional turbine topology was recently proposed with the promise of very significant improvements in the energy capture in Oscillating Water Column (OWC) based wave energy plants. Here, we present the initial results of the experimental validation of the twin unidirectional impulse turbine topology. A scale model of the concept was built and tested using simulated bidirectional flow. The model consists of two 165 mm impulse turbines each individually coupled to 375 W grid connected induction machines. An oscillatory flow test rig was used to simulate bidirectional flow to test the model. The results of the experiments validate the concept of the twin turbine configuration. The proposed topology utilizes no moving parts and achieves more than 50% efficiency over a broad range of flow coefficients. A comparison with other competing turbines (viz, a twin Wells’ turbine, a linked guide vane impulse turbine and a fixed guide vane impulse turbine) is done, based on actual measurements in the Indian wave energy plant. The results from the experiments are scaled to evaluate the design features of a 50 GWh wave energy plant.