Potential Flow Calculations of Axisymmetric Ducted Wind (original) (raw)
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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.
Energy Conversion and Management, 2020
Diffuser-augmented wind-turbines are drawing increasing attention since they can beat the Betz-limit referred to the rotor-area. However, their diffusion is still prevented by some issues including: 1) the attainable power has not yet been shown to be larger than that of an open-turbine with the same frontal-area, 2) the classical analysis methods rely on the one-dimensional-flow and no-tip-gap assumptions whose impact has never been quantified. The paper addresses these two items investigating the potential of ideal diffuser-augmented wind-turbines using a newly-developed Axial-Momentum-Theory approach, and an extended version of a free-wake ring-vortex actuator-disk model. In comparison with similar methods, the novelty of the first approach is that it accounts for the two-dimensional effects and the tip-gap presence. Since this approach cannot evaluate the performance of a turbine for a given duct-geometry, a ring-vortex method is also developed. This is the first low-computational-cost method relying on the exact solution of the inviscid-flow through a uniformly-loaded ducted-turbine with a finite-size tip-gap. It strongly couples the flow induced by the duct and the wake which are modelled as the superposition of ring-vortices. The combined use of axial-momentum and ring-vortex methods leads to the following results. Firstly, it is clearly shown that an ideal diffuser-augmented turbine can extract more power than a Betz disk with the same frontal-area. To strengthen this statement, a new duct geometry with a remarkable value of the exit-area power-coefficient equal to 0.6098 is presented. This value is significantly higher than that of a base-line NACA5415 duct profile, i.e. 0.4800. Secondly, the impact of the one-dimensional-flow and no-tip-gap assumptions is evaluated. It is also shown that the tip-gap has negligible effects. Moreover, the one-dimensional-flow hypothesis has a low impact for high values of the rotor load, while the errors grow up decreasing the rotor thrust.
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.
Design of a ducted wind turbine for offshore floating platforms
Wind Engineering, 2016
Within the Marine Energy Laboratory project, funded by the Italian Ministry for Education, University and Research, one of the first offshore wind plants on floating hulls, hosting ducted wind turbines, has been considered. The confinement of horizontal-axis wind turbines inside divergent ducts is reconsidered, in light of material innovation and direct drive coupling. Ducted wind turbines can take advantage of the flow rate increase due to the effect of the divergent shrouds. The conventional blade element momentum theory has been reformulated in order to deal with ducted turbines. Furthermore, computational fluid dynamics simulations have been carried out based on the solution of the steady two-dimensional Reynolds-averaged Navier–Stokes equations for axisymmetric swirling flows. In order to avoid any expensive mesh refinement near the actual rotor blades, the turbine effect on the flow field is taken into account by means of source terms for the momentum equations solved inside t...
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.
Numerical Studies Of the Upstream Flow Field Around A Horizontal Axis Wind Turbine
33rd Wind Energy Symposium, 2015
The aerodynamics of a wind turbine is governed by the flow around the rotor. Prediction of the velocity field, both upstream and downstream, is one of the challenges for wind turbine performance in terms of the aerodynamic loads and the generated power at different operational conditions. For simplicity, the wind velocity at the rotor plane is assumed to be equal to far upstream flow where the interaction of the rotor blades with upstream flow, close to the rotor plane, is not taken into account. This paper aims to study the effect of the rotor blade azimuthal position and the trailing wake, on upstream and downstream flow near to the rotor plane. For this purpose, an in-house Vortex Lattice Free Wake (VLFW) code, based on the potential, inviscid and irrotational flow, is developed. The results are compared with the MEXICO wind tunnel measurements. They show that the wind speed decreases in the axial direction upstream the rotor plane because of the induced velocity field by the rotor blades and the trailing wake vortices. This leads to a power reduction of the wind turbine. Furthermore, contrary to the traditional actuator disk model, the VLFW simulations predicts a tangential velocity component upstream the rotor due to the blade rotation which is in agreement with the measurement data. Finally, it is found that the flow field downstream and upstream the rotor blades depends on the blade azimuthal direction.
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 ...
Vortex theory of the ideal wind turbine
INCAS BULLETIN, 2009
Based on the concepts outlined by Joukowsky nearly a century ago, an analytical aerodynamic optimization model is developed for rotors with a finite number of blades and constant circulation distribution. In the paper we show the basics of the new model and compare its efficiency with results for rotors designed using the optimization model of Betz.
Numerical Evaluation of the Flow around a New Vertical Axis Wind Turbine Concept
Sustainability, 2021
In order to develop a sustainable economy based on the efficient use of green energy resources, it is necessary to research and innovate systems such as wind turbines. In this paper, a new configuration for vertical axis wind turbines was proposed and numerically analyzed using CFD methods. The concept is based on solving the starting problem of lift-based vertical axis wind turbines. The new concept consists of three blades with different chords, arranged at different radii so that the interaction between the blades is reduced and the operation in the vortex wake is minimal, thus reducing the losses. Through comparing a classic case of an H-Darrieus wind turbine with the new concept, not only were satisfying results regarding the blade-to-blade interaction presented, but an increased efficiency of up to 10% was also observed. Among the presented results is the variation of the vorticity magnitude at different positions of the blades, thus, the concept’s blade-to-blade interaction i...