Showing papers on "Wells turbine published in 2021"

Wells Turbine Stall Control Using Plasma Actuators

Abstract: Dielectric barrier discharge (DBD) plasma actuators were implemented on the leading edges of Wells turbine impeller blades for the purpose of controlling leading-edge separation, and the turbine pe...

Experimental investigation on performance improvement by mid-plane guide-vanes in a biplane-rotor Wells turbine for wave energy conversion

Abstract: The Wells turbine is the most frequently used or proposed self-rectifying air turbine for oscillating-water-column wave-energy converter application, largely because of its conceptual and mechanical simplicity. Biplane Wells turbines allow a higher total blade solidity to be attained than monoplane turbines do, but this results in larger aerodynamic losses associated with the swirl kinetic energy loss at turbine exit. This may be overcome by the presence of a row of guide vanes between the two rotor planes, a solution that had been proposed and investigated theoretically or by numerical modelling. Results of turbine overall performance and flow details are reported from laboratory tests of a biplane Wells turbine without guide vanes and with specially designed guide vanes. The presence of the guide vanes was found to increase the peak efficiency by seven percentual points, while reducing (for fixed rotational speed) the damping provided by the turbine. Measured losses in the guide vane row were much smaller than in the rotors. Experimental results are compared with previously published numerical results. A stochastic theoretical transform was applied to obtain averaged results for the turbine performance subject to the irregular bidirectional air flow induced by real sea waves.

Double Fed Induction Generator Control Design Based on a Fuzzy Logic Controller for an Oscillating Water Column System

Abstract: Oscillating water column (OWC) systems are water power generation plants that transform wave kinetic energy into electrical energy by a surrounded air column in a chamber that changes its pressure through the waves motion. The chamber pressure output spins a Wells turbine that is linked to a doubly fed induction generator (DFIG), flexible devices that adjust the turbine speed to increase the efficiency. However, there are different nonlinearities associated with these systems such as weather conditions, uncertainties, and turbine stalling phenomenon. In this research, a fuzzy logic controller (FLC) combined with an airflow reference generator (ARG) was designed and validated in a simulation environment to display the efficiency enhancement of an OWC system by the regulation of the turbine speed. Results show that the proposed framework not only increased the system output power, but the stalling is also avoided under different pressure profiles.

Combined Casing Groove and Blade Tip Treatment for Wave Energy Harvesting Turbine

Abstract: The Wells turbine is a self-rectifying air turbine, used in oscillating water column (OWC) to harvest wave energy. It produces unidirectional torque as the flow oscillates inside the OWC chamber. It has inherent disadvantage of narrow operating range due to stall at high airflow rate. Whereas, a wider operating range is essential to improve the turbine power output. A casing groove modifies the tip leakage flow pattern and improves the operating range. In addition, a radiused tip can alter the tip leakage flow and delay the stall. To enhance the performance further, this paper investigates the combined effect of tip groove and radiused tip (CG&RT) design modification. The flow was simulated by solving steady, incompressible Reynolds averaged Navier–Stokes equations in Ansys CFX 15.0. As expected, the CG&RT blade enhanced the relative operating range and the turbine power output by 44.4% and 23.8%, respectively.

A comparison of different approaches to estimate the efficiency of wells turbines

Abstract: Wells turbines are among the most interesting power takeoff devices used in oscillating water column (OWC) systems for the conversion of ocean-wave energy into electrical energy. Several configurations have been studied during the last decades, both experimentally and numerically. Different methodologies have been proposed to estimate the efficiency of this turbine, as well as different approaches to evaluate the intermediate quantities required. Recent works have evaluated the so-called second-law efficiency of a Wells turbine, and compared it to the more often used first-law efficiency. In this study, theoretical analyses and numerical simulations have been used to demonstrate how these two efficiency measures should lead to equivalent values, given the low pressure ratio of the machine. In numerical simulations, small discrepancies can exist, but they are due to the difficulty of ensuring entropy conservation on complex three-dimensional meshes. The efficiencies of different rotor geometries are analyzed based on the proposed measures, and the main sources of loss are identified.

Effect of Microcylinder and D-Cylinder at the Leading Edge of a Wells Turbine Harvesting Wave Energy

Abstract: Wells turbine is a self-rectifying axial flow reaction turbine used to harvest energy from the ocean waves. It suffers from a premature stall at higher flow rates. The present study discusses a comparative performance analysis with a turbine-blade leading-edge (LE) microcylinder (LEM) and D-cylinder (LED). The space between the LE and the cylinder was fixed as 1.5% of chord length (c). The sizes of the cylinder were varied from 0.5% to 0.75% of the chord. The unstructured tetrahedral mesh elements were used to discretize the computational flow domain that consists of a single blade passage with periodic boundary conditions. The Reynolds-Averaged Navier-Stokes equations with the k-ω shear stress transport (SST) turbulence equations were solved in a commercial CFD code Ansys CFX 18.1. The flow was considered incompressible. The present numerical study was compared with available open literature. The modified rotor blades showed a significant performance enhancement compared to the reference turbine. The peak efficiency was improved by 11.29% at a particular flow coefficient in 0.5%c radius LED-turbine. The presence of the cylinders delayed the flow separation and enhanced the operating range up to 11.11%.

Radiused Edge Blade Tip for a Wider Operating Range in Wells Turbine

Abstract: The narrow operating range of a Wells turbine restricts its energy extraction capability from the ocean waves. In this work, the concept of a radiused edge tip blade (RETB) was introduced to overcome such an issue. The RET modifies the tip and changes the tip leakage flow behaviour. The flow through the turbine annulus was simulated numerically and compared with the existing experimental results of the reference turbine. Three-dimensional Reynolds-averaged Navier–Stokes equations with a two-equation turbulence closure model available in ANSYS CFX 14.5 was used for the simulations. The computational domain was discretized with unstructured tetrahedral elements, and the grid independence study gave an optimal grid. The RETB altered the tip leakage flow characteristics and delayed the stall inception. The RETB enhanced the relative operating range by 25% and peak torque by 37%.

Experimental Study of an Axial Turbine for Wave Energy Conversion

Abstract: With the growing menace of increase in population and pollution, renewable sources of energy are being preferred over the fast depleting fossil fuels. Ocean wave energy is one such form of renewable source with a huge potential all around the globe. Wave energy conversion (WEC) devices are used to harness the energy from ocean waves. An oscillating water column (OWC) is a WEC device, which uses an axial turbine to convert wave energy into useful electrical energy. The experimental setup designed and assembled at Wave Energy and Fluids Engineering Laboratory, IIT Madras tested the turbine subjected to bidirectional airflow. The objective of the preliminary experiment is to measure the rotational speed and pressure drop across the turbine. The variable parameters are stroke length and time period of the oscillating piston, which simulates different ocean wave conditions. A comparative study of the parameters is carried out and reported in this article.

Optimization of Gap-to-Chord Ratio of Wells Turbine with Guide Vanes for Wave Energy Conversion

Abstract: Wells turbine (WT) used for harnessing wave energy has narrow operating range and poor starting characteristics. To enhance its starting and running characteristics, guide vanes (GV) are installed upstream and downstream of the rotor. This paper studies the effect of the gap-to-chord (\(G/l_{r}\)) ratio of the WT GV on the turbine performance numerically. The work incorporates validation of the numerical simulations with experimental results and application of optimization techniques to find the optimal \(G/l_{r}\) ratio. A range for \(G/l_{r}\) ratio is suitably selected for the analysis and metamodel based on Kriging algorithm is developed. Following this, the optimization algorithm of screening is utilized to find the optimal conditions to achieve the objective function.

Experimental analysis of the unsteady flow inside a Wells turbine

Abstract: One of the most promising technologies for sea-wave energy conversion is the one based on the Oscillating Water Column (OWC) principle. The system is composed of two units, an open chamber that converts the sea water motion into an alternating air-flow, and a turbine driven by the latter. The alternating flow of air requires a turbine capable of maintaining the same direction of rotation. The Wells turbine represents the simplest and most reliable device for this purpose. It is a self-rectifying axial turbine characterized by a rotor with symmetric blades staggered at 90 degrees relative to the axis of rotation. The vast majority of experimental works on Wells turbines and OWC devices analyzed their performance from a global point of view, often under steady conditions, in order to evaluate the pressure drop through the rotor, the torque produced and thus the turbine efficiency. This paper presents an experimental analysis of the three-dimensional flow inside a Wells turbine which operates in a facility capable of reproducing the alternating air-flow typical of an OWC system. The investigation is based on local flow measurements using several probes in order to describe the non-stationary air-flow, both up- and down-stream of the rotor at different heights, along the span of the blade. The investigation, conducted on a high-solidity turbine, details the behavior of the flow field inside the machine, aiming to provide a detailed description that can guide the aerodynamic optimization of the entire system (chamber and turbine) for a better energy conversion.

Modular Pod Wells Turbine Array: a Preliminary Assessment

Abstract: Renewable energy sources are becoming increasingly important in the development of technologies for the generation of electricity. This is where the air turbines are located which, depending on their location, are declined as air turbines or, if they use the mass of air displaced by the OWCs, they are installed as Wells turbines. Since the mass of air oscillates, the Wells have the ability to always rotate in the same direction regardless of the direction of the air. In the Mediterranean, where the excursion of the waves level is more modest than the oceanic one, it is necessary to resort to other methodologies such as the implementation of an array of small autonomous turbines that come into operation in a modular way according to the energy of the sea in order to exploit even modest energy.

Overview of OWC Mathematical Model

Abstract: The OWC system is usually constructed on the shoreline of the ocean, as shown in Fig. 2.1 (Mishra et al. 2016d). The OWC consists of four walls. It is open at the bottom where the ocean waves hit. The OWC chamber is partly watered, while the top part of the column is filled with air. The Wells turbine is installed in a circular cabinet at the top of the chamber and is driven by a DFIG connected to it. Based on rising and falling levels of seawater, the air in the chamber is compressed and decompressed. As a consequence, the oscillatory movement of the water causes the bidirectional airflow. Notwithstanding the alternating path of the airflow, the Wells turbine is constructed in such a manner that its rotation is always one-way. Next, mathematical models of ocean waves, OWC chamber, Wells turbine and DFIG are described.

DBD Plasma Actuation on the Blades of Axial-Flow Turbomachinery

Abstract: Flow separation, or stall, in axial flow turbomachinery results in a loss of pressure or compression in the case of fans and compressors, or the loss of power or thrust generation in the case of turbines. Wave-power-based Wells turbines, in particular, suffer so acutely from blade stall during normal operation, that it compromises their viability as a major renewable energy resource. In this research, pulsed dielectric barrier discharge (DBD) plasma actuators were implemented on the blades of a mono-plane Wells turbine impeller and its full-bandwidth performance was evaluated. An initial parametric study indicated that blade-tip reduced frequencies ≥2.5 produced the greatest impeller acceleration from rest. The corresponding physical pulsation frequency was then used as a basis for conducting nominally steady-state experiments as well as experiments involving acceleration and deceleration of the impeller. Data so acquired, corresponding to a reduced frequency range of 0.9 to 2.5, was compiled to construct an impeller performance map. Plasma pulsations dramatically increased the effective impeller bandwidth by producing useful net power well beyond flow ratios where mono-plane impellers spin down to a standstill. In fact, the shaft power at a 17° blade-tip angle of attack exceeded the plasma input power by a factor of 33. These findings are potentially game-changing for wave energy generation and axial flow turbomachinery in general.