On the potential of the ideal diffuser augmented wind turbine: an investigation by means of a momentum theory approach and of a free-wake ring-vortex actuator disk model (original) (raw)
Related papers
Effects of the duct thrust on the performance of ducted wind turbines
This work investigates the performance of ducted wind turbines (DWTs) through the axial momentum theory (AMT) as well as through a semi-analytical approach. Although the AMT points out that the duct thrust plays a key role in the enhancement of the power extraction, it does not allow for the evaluation of the flow field around the duct. For this reason, a semi-analytical model is also used to investigate the local and global features of the flow through a DWT. In comparison to the AMT, the proposed semi-analytical method can properly evaluate the performance of the device for each prescribed rotor load distribution and duct geometry. Moreover, in comparison to other linearised methods, this approach fully takes into account the wake rotation and divergence, and the mutual interaction between the turbine and the shroud. The analysis shows the opportunity to significantly increase the power output by enclosing the turbine in a duct and that the growth in the duct thrust has a beneficial effect onto the device performance. Finally, some insights on the changes occurring to the performance coefficients with the rotor thrust and the duct camber are obtained through a close inspection of the local features of the flow field.
International Journal of Wind Engineering and Industrial Aerodynamics, 2023
The study investigates the soundness of a popular uncoupled design strategy for diffuser-augmented wind turbines (DAWTs), namely the use of an annular wing to enclose an existing open-rotor. To this aim, the paper presents a numerical analysis of the NREL-Phase-VI rotor enclosed into a shroud whose cross-section consists of the Selig-S1223 airfoil. Particular attention is devoted to the analysis of the blade pressure fields, velocity triangles, blade forces, tip-vortex and wake development. The data show that the duct induces a gain in the rotor inlet axial velocity and, therefore, in the local flow-angle. Consequently, the blade forces, the extracted work, and the risk of flow separation considerably rise. Thanks to the simultaneous increase in the ingested mass flow rate and extracted work, the DAWT experiences a higher power coefficient (C_{P,exit}) which, however, would be further improved if a coupled design-procedure was used. Indeed, in the present case, the maximum C_{P,exit} is obtained for the wind-speed value corresponding to the duct optimal flow behaviour. However, in this condition, the rotor operates at off-design with an extensive flow-separation on the blade suction-side. Finally, while the inefficiencies magnitude is specific of the analysed case, the conceptual relevance of the achievements remains valid in general.
The Joukowsky rotor for diffuser augmented wind turbines: design and analysis
Energy Conversion and Management, 2022
Diffuser-augmented wind turbines are known for the potential improvement in power extraction in comparison with open wind turbines. Despite the large number of research works dealing with this subject and unlike the open rotor case, an optimum ducted rotor model is still missing. Since the Joukowsky (free-vortex) optimum rotor exhibits the best power coefficient in the open configuration, this paper presents a newly developed ring-vortex free-wake approach for the performance evaluation of an optimum Joukowsky rotor enclosed in a duct of general shape. The method, which is extensively verified, relies on the exact solution of the steady, incompressible, inviscid and axisymmetric flow, and it naturally takes into account the wake divergence and rotation. The procedure is used to obtain, for the first time, the maximum-power-coefficient/tip-speed-ratio characteristic curve for a diffuser augmented wind turbine. The proposed ducted rotor beats the Betz limit by 14.5% when the power coefficient is referred to the device frontal (exit) area. Additionally, the device experiences a slower decrease in performance with the reduction of the tip-speed ratio, thus extending the design range of ducted rotors in comparison with the open ones. Finally, taking into account the mutual influence of the disk and duct, a new rotor design strategy, capable to evaluate the optimum distribution of the chord and pitch-angle along the blade span, is also proposed. A complete design exercise is carried out and the rotor geometry is obtained for three different values of the nominal tip speed ratio. The paper also proves that a two-dimensional design procedure, which strongly couples the duct and the rotor induced flow, is mandatory to properly evaluate the optimum rotor geometry.
Performance analysis of open and ducted wind turbines
In this paper the analysis of the aerodynamic performance of ducted wind turbines is carried out by means of a nonlinear and semi-analytical actuator disk model. It returns the exact solution in an implicit formulation as superposition of ring vortices properly arranged along the duct surface and the wake region. In comparison with similar and previously developed models, the method can deal with ducts of general shape, wake rotation and rotors characterised by radially varying load distributions. Moreover, the nonlinear mutual interaction between the duct and the turbine, and the divergence of the slipstream, which is particularly relevant for heavily loaded rotors, are naturally accounted for. Present results clearly show that a properly ducted wind turbine can swallow a higher mass flow rate than an open turbine with the same rotor load. Consequently, the ducted turbine achieves a higher value of the extracted power. The paper also presents a detailed comparison between the aforementioned nonlinear and semi-analytical actuator disk method and the widely diffused CFD actuator disk method. The latter is based on the introduction of an actuator disk model in a CFD package describing the effects of the rotor through radial profiles of blade forces distributed over a disk surface. A set of reference numerical data, providing the inviscid axisymmetric velocity and pressure field distributions, are generated with controlled accuracy. Owing to an in-depth analysis of the error generated by the semi-analytical method and to the exactness of the solution in its implicit form, the collected data are well-suited for code-to-code validation of existing or newly developed computational methods.
Towards improving the aerodynamic performance of a ducted wind turbine: A numerical study
Journal of Physics: Conference Series, 2018
This paper aims to study the aerodynamic performance of ducted wind turbines (DWT) using inviscid and viscous flow calculations by accounting for the mutual interaction between the duct and the rotor. Two generalized duct cross section geometries are considered while the rotor is modelled as an actuator disc with constant thrust coefficient. The analysis shows the opportunity to significantly increase the overall aerodynamic performance of the DWT by a correct choice of the optimal rotor loading for a given duct geometry. Present results clearly indicate that the increased duct cross section camber leads to an improved performance for a DWT. Finally, some insights on the changes occurring to the performance coefficients are obtained through a detailed flow analysis.
The effects of shape and size on duct-augmented horizontal axis turbine performance
Wind Engineering, 2020
This article seeks to contribute to knowledge on duct-augmented turbines by investigating the influence of the key geometric parameters of the duct on the turbine performance: (i) duct expansion angle and length, (ii) position of the duct relative to the rotor and (iii) added geometric features to the duct. A new analytic model is proposed for the duct-augmented turbine and used for the investigation. The proposed analytic model used in this study was developed with existing momentum and blade element analysis methodologies serving as its basis. Using the proposed analytic model, the duct length is found to be more influential on the duct turbine system performance than the duct expansion angle. In addition, the performance can be enhanced by addition of a flange to the duct trailing edge. The study also highlights that the optimum rotor location within a duct is slightly behind the minimum duct area.
Axisymmetric Analysis and Design of Ducted Rotors
This report describes the analytical formulation used for the design and analysis of a ducted propulsor with single or multiple blade rows. It is based on a lifting-line representation of the rotor blade together with an axisymmetric panel representation of the duct and centerbody. The induced velocities associateed with blade-row loading are represented by vortex sheets shed into the flowfield. Blade element models are used for blade row sections using two-dimensional lift, moment and profile drag characteristics to account for loading and viscous losses. The modeling fidelity of the present approach lies between classical vortex/blade-element methods of Betz [1] and Glauert [2], and a general 3-D vortex-lattice or panel method. Even the general 3-D formulations would have to make the same time-averaging assumptions for the unsteady counterrotating flow as the present method, and hence would not be more sophisticated or more accurate in this regard. The chief advantage of the present method is that it is extremely fast computationally and is has simple inputs, making it ideal for interactive design work.
Power Enhancement of a Vertical Axis Wind Turbine Equipped with an Improved Duct
Energies
Efforts to increase the power output of wind turbines include Diffuser Augmented Wind Turbines (DAWT) or a shroud for the rotor of a wind turbine. The selected duct has three main components: a nozzle, a diffuser, and a flange. The combined effect of these components results in enriched upstream velocity for the rotor installed in the throat of the duct. To obtain the maximum velocity in the throat of the duct, the optimum angles of the three parts have been analyzed. A code was developed to allow all the numerical steps including changing the geometries, generating the meshes, and setting up the numerical solver simultaneously. Finally, the optimum geometry of the duct has been established that allows a doubling of the flow velocity. The flow characteristics inside the duct have also been analyzed in detail. An H-Darrieus Vertical Axis Wind Turbine (VAWT) has been simulated inside the optimized duct. The results show that the power coefficient of the DAWT can be enhanced up to 2.9 ...
Review on Ducts for Diffuser Augmented Wind Turbine
2019
The paper describes the development of ducted wind turbines which harness wind energy more efficiently than a conventional wind turbine. It consists a brief discussion on different ducted turbine designs, their advantages and shortcomings. These designs include Invelox which operates in varying wind conditions, Wind tamer specifically designed for low cut in speeds, Ogin (FloDesign) turbine which utilizes a mixer-ejector shroud for accelerated flow of air particles, Buoyant turbine which is an airborne wind turbine, Skywolf turbine which is under the class of hybrid turbine. The current trend in wind power industry is presented along with a section of likely future development and application of ducted wind turbines. Keywords— Ducted wind turbine, Betz Limit, Wind-lens turbine, Diffuser augmented wind turbine
Computational Fluid Dynamics Investigation of a Novel Multiblade Wind Turbine in a Duct
A novel manufacturing approach similar to filament winding is able to produce high-performance and lightweight composite wheels. The production can be rapid, inexpensive, and utilize commercially available winding machines. One potential application of the wheel is as a wind turbine. It is widely accepted that placing a duct around a wind turbine can enhance its performance, especially when a new designed turbine with unique advantages has a relatively low power coefficient, it is necessary to examine the benefits and economics of a turbine in a duct. In this study, a numerical analysis of a ducted multiblade composite wind turbine using computational fluid dynamics (CFD) is evaluated and compared with a bare wind turbine of the same turbine area. This investigation was performed using FLUENT in conjunction with the GAMBIT meshing tool. The extracted power is calculated and compared for these two modeling designs. Through the comparison of power coefficient variation with thrust coefficient, it was found that a ducted turbine can be 2–3 times that of the power extracted by a bare turbine. The results of the analysis provide an insight into the aerodynamic design and operation of a ducted wind turbine in order to shorten the design period and improve its technical performance.