Wells turbine for wave energy conversion : a review

In the past twenty years, the use of wave energy systems has significantly increased, generally depending on the oscillating water column (OWC) concept. Wells turbine is one of the most efficient OWC 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 computational fluid dynamics (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 five years, indicates that there is a trend that considerably depends on the CFD method

Computational and Experimental Investigations of an Omni-Flow Wind Turbine

This document is the Accepted Manuscript version, made available under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License CC BY NC-ND 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/ ). The final, definitive version of this paper is available online at doi: https://doi.org/10.1016/j.apenergy.2015.01.067.Both numerical and experimental studies were conducted to evaluate the performance of an omni-flow wind turbine designed to provide renewable electricity on the top of urban buildings like skyscrapers. The numerical approach was based on Finite Volume Method (FVM) and the turbulence flow was studied with several commonly used Reynolds-averaged Navier–Stokes turbulence models. The results of the study were evaluated with the wind tunnel test results over a range of tip speed ratios. The numerical results showed the effect of blade number on both power output and starting capability. Although both the power and torque coefficient were improved significantly by the optimisation of the blade number, there was only a slight change when the blade number was greater than twenty. The results from wind tunnel testing also showed excellent starting capability with a starting wind velocity as low as 1.6 m/s. A numerical simulation was also conducted for the wind turbine working under non-uniform flow conditions. The numerical results have shown that the peak power coefficient of such a wind turbine under non-uniform flow, was lower than that under the uniform flow. Additionally, the applied thrust on a blade was subject to frequent and periodical changes. However, the effect of the change of thrust in magnitude and frequency was not significant. Therefore the omni-flow wind turbine has the potential to meet the challenge of unpredictable wind velocity and direction as a consequence of the urban environment.Peer reviewedFinal Accepted Versio

A COMPARISON BETWEEN EXPERIMENTAL AND NUMERICAL ANALYSIS OF A WELLS TURBINE

Wave energy is one of the renewable energy sources with the highest potential. Several pilot plants have been built based on the principle of the Oscillating Water Column (OWC). Among the different solutions that have been suggested, the Wells turbine has gained particular attention due to its simplicity and reliability. The majority of available studies concentrate on the steady operation of the Wells turbine, while only few analyze its performance under an unsteady and bi-directional air flow, as determined by the presence of the OWC system. In this work, experimental and numerical performance of a high-solidity Wells turbine with NACA0015 profiles under the bi-directional flow generated by a hydraulic piston is compared. The numerical simulations have been conducted using commercial CFD software and focus on unsteady predictions, with particular attention to the behavior of the flow upstream and down-stream of the rotor, flow hysteresis between acceleration and deceleration phases and differences between intake and exhaust strokes due to the non-symmetrical configuration of the machine

Cfd Analysis Of A Uni-directional Impulse Turbine For Wave Energy Conversion

Ocean energy research has grown in popularity in the past decade and has produced various designs for wave energy extraction. This thesis focuses on the performance analysis of a uni-directional impulse turbine for wave energy conversion. Uni-directional impulse turbines can produce uni-directional rotation in bi-directional flow, which makes it ideal for wave energy extraction as the motion of ocean waves are inherently bi-directional. This impulse turbine is currently in use in four of the world’s Oscillating Wave Columns (OWC). Current research to date has documented the performance of the turbine but little research has been completed to understand the flow physics in the turbine channel. An analytical model and computational fluid dynamic simulations are used with reference to experimental results found in the literature to develop accurate models of the turbine performance. To carry out the numerical computations various turbulence models are employed and compared. The comparisons indicate that a low Reynolds number Yang-shih K-Epsilon turbulence model is the most computationally efficient while providing accurate results. Additionally, analyses of the losses in the turbine are isolated and documented. Results indicate that large separation regions occur on the turbine blades which drastically affect the torque created by the turbine, the location of flow separation is documented and compared among various flow regimes. The model and simulations show good agreement with the experimental results and the two proposed solutions enhance the performance of the turbine showing an approximate 10% increase in efficiency based on simulation results

Research and technology

The NASA Lewis Research Center's research and technology accomplishments for fiscal year 1987 are summarized. It comprises approximately 100 short articles submitted by staff members of the technical directorates and is organized into four sections: aeronautics, aerospace technology (which includes space communications), space station systems, and computational support. A table of contents by subject was developed to assist the reader in finding articles of special interest

Aerodynamic Analysis of a Novel Wind Turbine for an Omni-flow Wind Energy System

The purpose of this research is to propose a novel wind turbine for an omni-flow wind energy system and investigate its aerodynamic performance. The geometry of the novel wind turbine is based upon the impulse turbine technology which has been successful in wave energy. In this study, both numerical and experimental studies were conducted to evaluate the aerodynamic features of this wind turbine. The numerical method was validated by a comparison between numerical and experimental results over a range of tip speed ratios. Results obtained from experiments and simulations indicate that the proposed wind turbine can be employed. Additionally, on the basis of the analyses performed, this new wind turbine has the potential for having a good startup feature, which means that this wind turbine can be suitable for applications in an urban environment. As an important component, the stator of this wind turbine can increase the passing flow velocity by 20%. Meanwhile, the passing flow direction also can be optimised by the stator. Aerodynamics of the wind turbine was analysed under the non-uniform flow condition, because the flow is non-uniform inside the omni-flow wind energy system. It was found that the maximum power coefficient of such a turbine under the non-uniform flow condition is lower than that under the uniform flow condition. Due to the non-uniform flow, the blades experience different flow velocities, and as a consequence, undergo different aerodynamic loads during one operation cycle. Thus the generated torque and thrust on a blade are subjected to frequent and periodical changes. Influences of the geometrical parameters on the aerodynamic characteristics of this wind turbine were investigated. From the initial study, it was found that changes of hub-to-tip ratios, numbers of blades, aerofoils and numbers of guide vanes, can significantly affect the II power performance. Additionally, the wind turbine obtained high values of maximum torque coefficients with changing geometrical parameters

Turbine design and application volumes 1, 2, and 3

NASA has an interest in turbines related primarily to aeronautics and space applications. Airbreathing turbine engines provide jet and turboshaft propulsion, as well as auxiliary power for aircraft. Propellant-driven turbines provide rocket propulsion and auxiliary power for spacecraft. Closed-cycle turbine engines using inert gases, organic fluids, and metal fluids have been studied for providing long-duration electric power for spacecraft. Other applications of interest for turbine engines include land-vehicle (cars, trucks, buses, trains, etc.) propulsion power and ground-based electrical power. In view of the turbine-system interest and efforts at Lewis Research Center, a course entitled 'Turbine Design and Application' was presented during 1968-69 as part of the In-house Graduate Study Program. The course was somewhat revised and again presented in 1972-73. Various aspects of turbine technology were covered including thermodynamic and fluid-dynamic concepts, fundamental turbine concepts, velocity diagrams, losses, blade aerodynamic design, blade cooling, mechanical design, operation, and performance. The notes written and used for the course have been revised and edited for publication. Such a publication can serve as a foundation for an introductory turbine course, a means for self-study, or a reference for selected topics. Any consistent set of units will satisfy the equations presented. Two commonly used consistent sets of units and constant values are given after the symbol definitions. These are the SI units and the U.S. customary units. A single set of equations covers both sets of units by including all constants required for the U.S. customary units and defining as unity those not required for the SI units. Three volumes are compiled into one

Turbine blade-tip clearance excitation forces

The results of an effort to assess the existing knowledge and plan the required experimentation in the area of turbine blade tip excitation forces is summarized. The work was carried out in three phases. The first was a literature search and evaluation, which served to highlight the state of the art and to expose the need for an articulated theoretical experimental effort to provide not only design data, but also a rational framework for their extrapolation to new configurations and regimes. The second phase was a start in this direction, in which several of the explicit or implicit assumptions contained in the usual formulations of the Alford force effect were removed and a rigorous linearized flow analysis of the behavior of a nonsymmetric actuator disc was carried out. In the third phase a preliminary design of a turbine test facility that would be used to measure both the excitation forces themselves and the flow patterns responsible for them were conducted and do so over a realistic range of dimensionless parameters