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.

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Wells turbine for wave energy conversion: a review

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

Comparison of LIMPET contra-rotating wells turbine with theoretical and model test predictions

Computed effects of tip clearance on performance of impulse turbine for wave energy conversion

Detailed entropy generation analysis of a Wells turbine using the variation of the blade thickness

The Wells turbine is one element in a chain of devices by which the oceans' wave power resource can be tapped This paper reports a study of the performance and aerodynamics of a Wells turbine using a three-dimensional, flow solver for the Reynolds-averaged Navier-Stokes equations Calculations have been performed for a monoplane device comprised of NACA 0015 blades under the conditions: Reynolds number 8×10 5 , tip Mach number 04, hub-to-tip ratio 06 and tip clearance 2% To study the effect of solidity calculations were performed for different numbers of blades It is shown that the predicted effect of solidity on the turbine pressure drop, torque and efficiency agrees qualitatively and quantitatively with experimental data The discrepancies can be partly explained by geometric differences between the experimental and numerical turbines It is also shown that increasing turbine solidity increases the resistance to stall by strengthening the blade-to-blade interactions in the hub region

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Computed Effects of Solidity on Wells Turbine Performance.

The aerodynamics of the Wells air turbine has been studied using a three-dimensional, unstructured mesh flow solver for the Reynolds-averaged Navier-Stokes equations. Calculations have been performed for a monoplane device comprised of six blades at a stagger angle of 90 degrees under the conditions: Reynolds number equal to 8/spl times/10/sup 5/, tip Mach number 0.4, solidity 0.5 and hub to tip ratio 0.6. Four flow coefficients representative of low, medium and high loading conditions have been considered. It is shown that the method is able to predict with reasonable accuracy the variation of pressure drop across and efficiency of the turbine with flow coefficient. However, stalling of the device is predicted too early. It is suggested that, counter to the received wisdom, the effect of the tip clearance flow is more significant in the process of turbine stall than the flow at the hub. The incidence at the hub is actually lower than at the tip, and it is hypothesized that this is due to the higher local solidity at the hub.

Investigation of Wells turbine performance using 3-D CFD

Ocean wave power can be harnessed by means of a Wells turbine driven by a bi-directional airflow above an oscillating water column. While the Wells turbine research budget remains small, full-scale experiments are prohibitively expensive. Computational fluid dynamics (CFD) is a possible way of generating full-scale data. This paper summarizes a numerical study of Wells turbine performance and aerodynamics using a CFD method, and makes recommendations about its use for Wells turbine studies. Calculations have been performed for a monoplane turbine comprised of straight NACA 0015 blades at a stagger angle of 90 deg, Reynolds number 8 x 105, tip Mach number 0.4, and hub-to-tip ratio 0.6. Flow coefficient, tip clearance, and blade number were varied. The predictions agree favorably with experimental data. Differences can be explained partly by geometric differences between the experimental and numerical turbines. Because of the 90-deg stagger angle of the turbine, accurate predictions require fine resolution of the blade leading-edge region. This makes performance calculations expensive, and it is probably more prudent to use simpler predictive methods. CFD, however, is most useful for the study of turbine aerodynamics. Nomenclature CT = torque coefficient c - blade chord h = ratio of hub diameter to tip diameters N = number of blades in annulus p^ p 2 = static pressure up- and downstream of turbine Q = volume flow rate QR - volume flow rate ratio Rt - tip radius of turbine T = torque Ut = blade tip velocity Vx = axial velocity Ap* = nondimensional ized pressure drop j] = efficiency 6 = tangential coordinate cr = turbine solidity, Ncl[7rRt(\ + h)] TC = blade clearance at tip as percentage of chord 4> = flow coefficient fl = angular velocity of turbine

Numerical analysis of wells turbine aerodynamics

The results of numerical studies of Wells turbine aerodynamics are presented. In the first study, the ability of a hybrid active/passive device to alleviate post-stall buffet of an isolated NACA0012 aerofoil at 18/spl deg/ is evaluated. The passive boundary layer control device is a porous surface/plenum chamber which covers the 1% to 10% chord portion of the aerofoil, and active control is provided by applying suction to the plenum chamber. It is shown that the unsteady load experienced by the solid aerofoil at post-stall incidence is indeed alleviated by the active device; while the passive device has minimal impact on performance at pre-stall angles. The second study uses a three-dimensional numerical method to predict the performance of typical Wells turbines with various numbers of blades and tip clearances. The effect of solidity on turbine performance is predicted well both qualitatively and quantitatively, and the effect of clearance is qualitatively correct.

J.K. Watterson

Papers

Computed Effects of Solidity on Wells Turbine Performance.

Investigation of Wells turbine performance using 3-D CFD

Numerical analysis of wells turbine aerodynamics

Applications of computational fluid dynamics to a wave energy conversion device

Numerical analysis of Wells turbine aerodynamics