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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Kyriakos Vafiadis University of Western Macedonia, Greece
Co-authors:
Kyriakos Vafiadis (1) F P Antonios Tourlidakis (1) Theocharis Fintikakis (1) Ioannis Zaproudis (1)
(1) University of Western Macedonia, Kozani, Greece

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Mr. Vafiadis is a Mechanical Engineer and a PhD candidate of University of Western Macedonia, Greece. His research is focused on aerodynamics and blade optimisation of wind turbines while he is involved in research in the fields of turbomachinery, fluid mechanics, computational fluid dynamics applications and renewable energy.

Abstract

Computational Analysis of a Shrouded Small Scale Vertical Axis Wind Turbine

Introduction

Small wind turbines are generally intended to supply electric power to buildings (e.g. urban environment without favorable wind conditions). The installation of a wind turbine on a building has two major aspects, the efficiency of the wind turbine and its building integration. Research has been carried out from the early ‘80s in order to overcome the Betz limit and extract more wind energy using shroud augmented wind turbines (particularly horizontal axis machines). In this paper computational and experimental analysis of a shrouded small scale Vertical Axis Wind Turbine (VAWT) suitable for building integrated systems is carried out.

Approach

In this research work, the numerical and experimental study of a VAWT (bare and shrouded) is presented. Initially, a 2 m high, three bladed H-rotor vertical axis turbine with a radius of 1,045 m was designed. The three blades are constructed using the NACA 0015 airfoil sections. Two different configurations were considered. Firstly, a bare rotor was analysed and subsequently the rotor was placed inside a convergent-divergent casing in order to accelerate the incoming flow. The wind speed was fixed to be constant and equal to 10 m/s, while the rotational speed ranges from 70 to 143 rpm and from 286 to 621 rpm for the bare turbine and the shrouded turbine case, respectively. The shroud designed for this project has a total length of 7.012 m, a rectangular cross section all over its body, inlet and outlet areas each of 2.2x3.255 square meters, flat top and bottom walls and curved side walls. The area ratio for both converging and diverging sections of the shroud is 1.453. The experimental study was conducted in an in-house developed test rig.

Full three-dimensional transient CFD simulations were carried out for the slow simulation around the rotor, through the shroud and through the full rotor – shroud arrangement. The computational domain consisted of two regions with different frames of reference (a stationary and a rotating). In the stationary region, the domain around and inside the walls of the shroud was included. The wind turbine was included in the cylindrical rotating region. The rotating region rotational speed was that of the VAWT. The computational mesh for both bare and shrouded cases is tetrahedral unstructured and it counts around 12.6 milion elements for the bare rotor case and 11.8 million elements for the shrouded case. Inflation layers were inserted in order to accurately resolve the boundary layers.


Each simulation corresponded to a different rotational speed. All the simulations were performed using the commercial CFD software package ANSYS CFX. The standard k – epsilon turbulence model was utilised for the simulations.

Main body of abstract

The installation of a wind turbine on a building has two major aspects, the efficiency of the wind turbine and its building integration. The energy production using small scale wind turbines installed on urban areas depends on many factors. Such factors are the wind direction, the turbulence intensity and the effect of the neighboring buildings on the air flow characteristics. All these may have an adverse effect on the power production of a wind turbine. In order to extract a substantial amount of wind energy from such areas with unfavorable wind conditions, the wind turbines should be specifically designed for this environment. Studies on the energy yield of small wind turbines mounted on buildings for many areas have been presented by various researchers. The power extracted from the wind is related to the wind speed raised to the third power and thus any improvement to the wind speed will greatly affect the efficiency of the machine. Unfortunately, for conventional wind turbines in open flow the maximum power that can be extracted from the wind is limited by the Betz’s law (59.3% of the incident wind’s kinetic energy). Research has been carried out from the early 1980s in order to overcome this limit and extract more of wind’s energy from small wind turbines through acceleration of the wind flow with the addition of a converging inlet – diverging outlet shroud. These studies have shown that the Betz limit can be overcome by a factor relative to the air mass flow that is forced through the shroud. Simulations and experiments made to wind turbines with a shroud have shown that the power factor can be increased up to 4 times compared to a bare wind turbine. The outlet diffuser’s efficiency depends on the amount of flow separation inside the casing. Furthermore, the diffuser has a positive effect on the destruction of vortices that develop downstream of the rotor. In this direction, only a few researchers have studied the vertical axis wind turbine augmentation with ducts, shrouds or diffusers. The building integration problem has been studied mainly in order to achieve the development of a model that, i) suits to the building aesthetically, ii) saves useful place and iii) takes advantage of the building geometry (aerodynamically).
In the current research work, the numerical and experimental study of a VAWT (bare and shrouded) is presented. Initially, a 2 m high, three bladed H-rotor Darrieus turbine with a radius of 1,045 m was designed. The three blades are constructed using the NACA 0015 airfoil sections. The wind speed is constant and equal to 10 m/s, while the rotational speed ranges from 70 to 143 rpm and from 286 to 621 rpm for the bare turbine and the shrouded turbine case, respectively. The shroud geometry designed for this project has a total length of 7.012 m, a rectangular cross section all over its body, inlet and outlet areas each of 2.2x3.255 square meters, flat top and bottom walls and curved side walls. The area ratio for both converging and diverging sections of the shroud is 1.453. The experimental study was contacted in an in-house developed test rig. In addition, full three-dimensional transient CFD simulations were carried out for the modeling of the flow around the rotor, through the shroud and through the full rotor – shroud arrangement. The computational domain consisted of two regions with different frames of reference (a stationary and a rotating). Each simulation concerned a different rotational speed. All the simulations were performed using the commercial CFD software package ANSYS CFX. The standard k – epsilon turbulence model was utilised in all the simulations.
Detailed flow analysis results are presented, dealing with the various investigated test cases. By varying the rotational speed of the turbine the power coefficient versus tip speed ratio curves was obtained. The CFD resulting power coefficient curve is compared with the experimental one and good agreement was found. The results showed that there is a significant improvement in the performance of the wind turbine.

Conclusion

The computational investigation of the aerodynamics of a vertical axis wind turbine was carried out both numerically and experimentally.




The novelty and originality of the present work lies in the detailed analysis of the effect of the presence of a shroud on the performance of this wind turbine rotor and the attained results demonstrated that significant performance benefits can be accomplished overcoming the Betz limit.

The computational analysis of the unsteady flow field was proven to be necessary due to the nature of the complex flow field that develops due to the interaction of the rotating blades with the wakes of the upstream turbine blades.

The addition, of the converging section accelerates the wind flow approaching the rotor blade thus enhancing the power output. In order to maximise the performance of the turbine, a diffusing section needs to be inserted at the outlet which facilitates the reduction of static pressure immediately behind the rotor and enhances the wake mixing. The reference aerodynamic design of the wind turbine rotor used symetrical airfoil sections and in combinatiion with their setting angle in respect with the tangential direction, produced a considerably limited aerodynamic loading. However the inclusion of the rotor inside a converging – diverging casing was proven to be a useful modification for small wind turbines and especially at sites where the wind direction is comparatively steady, by setting the turbine in the wind direction. In general, as it was proven in previous studies by the authors for horizontal axis wind turbines, the placement of vertical turbines inside converging – diverging casings is worthy to be seriously considered when they are going to be integrated in buildings.


Learning objectives
This paper presents numerical and experimental work on a shroud augmented VAWT. The main objective of this work is to analyse the aerodynamics of this type of wind turbines and how they can be affected by the use of shroud. Due to its relative small size this arrangement could be easily integrated into a building, therefore contributing positive to the energy equilibrium of the building.


References
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