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Development of a High-Performance Wind Turbine Equipped with a Brimmed Diffuser Shroud
Abstract
We have developed a new wind turbine system that consists of a diffuser shroud with a broad-ring brim at the exit periphery and a wind turbine inside it. The brimmed-diffuser shroud plays the role of a device for collecting and accelerating the approaching wind. Emphasis is placed on positioning the brim at the exit of the diffuser shroud. Namely, the brim generates a very low-pressure region in the exit neighborhood of the diffuser by strong vortex formation and draws more mass flow to the wind turbine inside the diffuser shroud. To obtain a higher power output of the shrouded wind turbine, we have examined the optimal form for the brimmed diffuser, such as the diffuser open angle, brim height, hub ratio, centerbody length, inlet shroud shape and so on. As a result, a shrouded wind turbine equipped with a brimmed diffuser has been developed, and demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about five compared to a standard (bare) wind turbine.
... A diffuser duct placed in a free flow has been proved to accelerate the airflow at its inlet 7,8) . Many researchers have proved that mating a turbine with a diffuser increases the power significantly to exceed the Betz limit, which is 59.3% 9,10) ; wind turbines shrouded with a diffuser are known as diffuser augmented wind turbine (DAWT) or Wind Lens 11) . ...
... The validation process was conducted in two steps; the first step is to use CFD to solve the flow around a hollow cylindrical flanged diffuser, which was tested in a wind tunnel by Ohya et al. 7) . In this experiment, the airspeed at the diffuser's centreline was measured using hot-wire; Figure 5a shows the comparison between CFD results and Experiment results. ...
... a) The velocity of the flow at the centerline of a hollow diffuser relative to the inlet speed calculated by CFD and wind tunnel testing by Ohya et al. 7) b) Power coefficient of a scaled NREL Phase VI wind turbine calculated by CFD and wind tunnel testing by Cho et al. 26) . ...
... In regards to the development of an efficient wind energy conversion system, this paper aims at finding out an effective method for collecting more wind flow and a suitable type of wind turbines which can generate reasonable amount of energy from the wind. One of the DAWT concepts used for wind acceleration with the name of 'wind-lens' has been developed by Ohya et al. [1][2][3][4][5][6][7][8][9][10]. Subsequently, a diffuser-shaped configuration able to collect and accelerate the oncoming wind has been developed. ...
... To the best of the authors' knowledge, no reliable computational study used to solve the full set of Navier-Stokes equations in transient mood was dedicated to assess the performance of this type of wind turbines. The novelty of this paper lies in the fact that the adopted approach here is depending on the full unsteady Reynoldsaveraged Navier-Stokes (URANS) or high-fidelity CFD simulations to demonstrate the aerodynamic characteristics of the wind-lens turbine developed by Ohya et al. [1][2][3][4][5][6][7][8][9][10]. Since most of the research in the existing literature about wind-lens concept have been conducted based on windtunnel measurements [2,[4][5][6][7], even the computational ones have adopted low-fidelity CFD simulation including the use of actuator disk (AD) model or RANS-BEM [10,31] to avoid higher computational cost and to reduce the computational time for their studies. ...
... The novelty of this paper lies in the fact that the adopted approach here is depending on the full unsteady Reynoldsaveraged Navier-Stokes (URANS) or high-fidelity CFD simulations to demonstrate the aerodynamic characteristics of the wind-lens turbine developed by Ohya et al. [1][2][3][4][5][6][7][8][9][10]. Since most of the research in the existing literature about wind-lens concept have been conducted based on windtunnel measurements [2,[4][5][6][7], even the computational ones have adopted low-fidelity CFD simulation including the use of actuator disk (AD) model or RANS-BEM [10,31] to avoid higher computational cost and to reduce the computational time for their studies. The AD model or RANS-BEM usually model the action of the rotor by introducing body 'circular disk' and the body forces are regarded to be originating from 2D airfoil data in combination with the BEM technique. ...
Wind-lens turbines (WLTs) exhibit the prospect of a higher output power and more suitability for urban areas in comparison to bare wind turbines. The wind-lens typically comprises a diffuser shroud coupled with a flange appended to the exit periphery of the shroud. Wind-lenses can boost the velocity of the incoming wind through the turbine rotor owing to the creation of a low-pressure zone downstream the flanged diffuser. In this paper, the aerodynamic performance of the wind-lens is computationally assessed using high-fidelity transient CFD simulations for shrouds with different profiles, aiming to assess the effect of change of some design parameters such as length, area ratio and flange height of the diffuser shroud on the power augmentation. The power coefficient (C p) is calculated by solving the URANS equations with the aid of the SST k-ω model. Furthermore, comparisons with experimental data for validation are accomplished to prove that the proposed methodology could be able to precisely predict the aerodynamic behavior of the wind-lens turbine. The results affirm that wind-lens with cycloidal profile yield an augmentation of about 58% increase in power coefficient compared to bare wind turbine of the same rotor swept-area. It is also emphasized that diffusers (cycloid type) of small length could achieve a twice increase in power coefficient while maintaining large flange heights.
... where; is the flow density, is the velocity of the flow, and is the cross-sectional area. A group of researchers in Japan has shown by wind tunnel testing that a diffuser placed in a free flow will accelerate the airspeed [7,8]. The diffuser configuration creates a pressure difference across its length such that the inlet will have a sub-atmospheric pressure, which encourages upstream air to flow through the diffuser. ...
... For validation purposes, the velocity at the diffuser center-line across its length resulted from this CFD model, and wind tunnel [7] are compared. It is found that the two results are agreeable with each other, thus validating this simulation work ...
... The increase in the torque occurred when the flanged-diffuser shrouds the turbine is clearly shown in Figure 7; there is an increase in torque that range between 4 % and 107% compared to the bare turbine; this result agrees with results from previous researches conducted by Ohya et al., [16]. Figure 7 also emphasizes the role of the flange because the torque's increase is negligible when the flange is removed from the shroud part, which agrees with the experimental work results from Ohya et al., [7,16] that also emphasize the importance of the flange. ...
In this research work, a new design concept for shrouded turbines is introduced using a circular flange ring instead of a diffuser. The performance of a wind turbine shrouded with the conventional configuration of flanged diffusers is firstly investigated. This allows comparison to be made with the performance of the new design proposed in this project. The bare turbine used in this research work is a horizontal axis wind turbine with a diameter of 0.6 m, and the flange height is 10% of the diameter. The performance of the shrouded turbine is investigated using computational fluid dynamics; by solving the flow field using three-dimensional Reynolds Averaged Navier-Stokes for incompressible flow. Findings indicate that the performance of the turbine shrouded with the flange ring is superior to the bare turbine by 33%. It also has benefit over the conventional design as this new design concept is simpler and uses lesser material since it is not using the diffuser, which favorably in turn will reduce the cost. Subsequently, the findings of this research may have the potential to expedite the developments of green-source energy that undoubtedly could benefit the entire growing wind turbine industry.
... Ducts with an outlet flange typically have been viewed as having a somewhat distinct mechanism of augmentation. Separated flow, which will be discussed further in Section 1.2.3, exists on a flange's downstream side; some authors described augmentation directly in terms of this region having low pressure and drawing more flow through [59,[83][84][85], while others specified that the low pressure results from vortex formation [33,37,[86][87][88][89][90]. One study attributed the augmentation to obstruction by the flange causing the flow to have an 'easier path' through the duct [91], while explanations by analogy to trailing edge flaps on aerofoils were not found. ...
... Generally such investigations were part of the design process for a duct [e.g. 67,83,89,112] or rotor [e.g. 109,[113][114][115], and involved measurements of the power extracted or velocity at the rotor plane. ...
... A range of rotor representations were used in these studies, including rotors designed for use inside a duct [39,116] or without a duct [67,117], mesh screens designed to induce a pressure drop [39,56,62,104,118], and physical rotor models of unclear or unspecified design [35,59,89,115,119]. Some compared the performance of the same rotor in and out of a duct [e.g. ...
Ducted turbines are designed to augment the flow through a rotor and consequently increase power extraction, with the aim of reducing the cost of wind energy. Despite many years of research, however, much uncertainty remains on a fundamental level: uncertainty that is not conducive to maximising performance or to commercial success.
This work reduces the problem’s complexity and improves understanding of the fundamentals by examining the underlying inviscid behaviour of ducted turbines, which are also known as diffuser augmented turbines. Numerical results show that the Betz limit does not apply, even using duct exit area, confirm the applicability of inviscid simulations to attached viscous flow, and clarify the influence of duct geometry.
A comparison demonstrates that the diffuser conceptual model, which has dominated research thus far, is outperformed by an aerofoil conceptual model. The latter gives a closer match between intuition and actual performance, is easier to work with, and allows the influence of the rotor to be thought of as a change in the flow seen by the duct. It is therefore recommended as the standard for future studies.
Theoretical examinations establish that invalid simplifying assumptions in existing theories leave the requirement for empirical parameters intact, and that velocity at the rotor may better fill the empirical parameter role than exit pressure or duct drag. A detailed derivation for the relationship between inviscid duct drag and augmentation is also described for the first time.
An analysis suggests that ducts inherently reduce the optimum rotor loading in inviscid flow, with increases in rotor loading decreasing duct performance by reducing the effective duct wall angle and effective free stream velocity magnitude. Viscous effects may then increase the optimum, play a larger role than otherwise appears, and have greater potential for performance improvements than previously thought.
... Consequently, accelerate the air flow and increase mass flow rate going through the lens. Lens Concept was developed by Ohya et al. (2006). As a result, the overall power production increased about 2-3 times compared to SRWT without duct. ...
... As a result, from this optimization model, the mass flow rate obtained is 26.28 kg/s which represent an increase of 33.6 % and gives a drag coefficient (Cd) of 3.357. Figure 7 demonstrates the new optimized lens profile that has been used in the rest of this study against the one adopted by previous study of Ohya et al. (2006). ...
Due to growing needs for energy in our life, research in the wind energy field has increased significantly. There has been global concern towards the development of smart techniques and devices that could optimize the energy conversion and maximize the output power from the wind. Investigating such alternative solutions are required in order to meet the continuous increase in the power demand.
The Dual Rotor Wind Turbine system (DRWT) offers higher energy extraction rates from the wind. In the present study, it is proposed to utilize the dual rotor configuration in a ducted system using wind lens in order to enable its application in regions of low wind speeds. The aerodynamic performance of ducted dual rotor wind turbine is investigated using CFD to solve three dimensional, turbulent-steady incompressible flow equations, using the k-ε Realizable and k-ω shear stress transport (SST) turbulence models. Several difficulties due to complexity of geometry and meshing requirements have been encountered. Mesh independence study was conducted to ensure the accuracy and validate the results. Power curves were obtained, detailed investigation of the wind turbine performance in different configurations are highlighted in order to explore the benefit and effect of each configuration to the output power. The final results of combined configuration for dual rotor wind turbine (DRWT) with lens show a considerable improvement to the performance of wind turbine over wide range of wind speeds.
... The pressure variation along the structure can be seen in Figure 11. It can be observed that minimum pressure occurs in the region between convergent nozzle and flanged diffuser where wind turbine is to be placed [9]. The velocity variation along the structure is shown in Figure 12 which is computed using CFD. ...
... The velocity variation along the structure is shown in Figure 12 which is computed using CFD. By observing all the CFD analysis results of pressure and velocity variations, it can be proved that maximum wind velocity occurs in between convergent nozzle and divergent diffuser where wind turbine is to be placed [9]. ...
Now-a-days, wind energy is a major contributor in power generation and the modern day horizontal axis wind turbine (HAWT) is the most suitable for power generation where the wind velocities are greater than 4-6 m/s. This restricts the suitable location for a potential wind farm to limited numbers. Thus introduction of new wind turbines which can generate usable power at lower speeds will be a contributing factor for the ever increasing demands of power. This paper focuses on increasing the conversion efficiency of a wind turbine by installing a conical duct at the inlet of wind turbine in combination with flanged diffuser at outlet of wind turbine.
... The pressure variation along the structure can be seen in Figure 11. It can be observed that minimum pressure occurs in the region between convergent nozzle and flanged diffuser where wind turbine is to be placed [9]. The velocity variation along the structure is shown in Figure 12 which is computed using CFD. ...
... The velocity variation along the structure is shown in Figure 12 which is computed using CFD. By observing all the CFD analysis results of pressure and velocity variations, it can be proved that maximum wind velocity occurs in between convergent nozzle and divergent diffuser where wind turbine is to be placed [9]. ...
Now-a-days, wind energy is a major contributor in power generation and the modern day horizontal axis wind turbine (HAWT) is the most suitable for power generation where the wind velocities are greater than 4-6 m/s. This restricts the suitable location for a potential wind farm to limited numbers. Thus introduction of new wind turbines which can generate usable power at lower speeds will be a contributing factor for the ever increasing demands of power. This paper focuses on increasing the conversion efficiency of a wind turbine by installing a conical duct at the inlet of wind turbine in combination with flanged diffuser at outlet of wind turbine.
... In addition, it was confirmed that the diffuser works better at places where the wind direction is constant. Optimal shape for a brimmed diffuser, like the height of the brim, the angle of the brim, inlet shape of the shroud, etc. was studied experimentally by Ohya et al. (Ohya et al. 2006). The results show that wind speed and power are increased by a factor of about 5 times more than a bare wind turbine. ...
Wind energy is rapid and mature energy, which is vastly growing in many countries. Therefore, it is needed to introduce new technologies of wind energy to improve efficiency and reduce costs. Shrouded wind turbines are believed to be a promising technology in wind energy conversion that can improve its overall efficiency and output. The special structure of the shroud causes a low-pressure area behind the turbine. Therefore, more mass flow is drawn into the turbine for higher power outputs. This new technology enables the use of wind turbines within urban areas with low wind speeds. The objective of this paper is to review all the papers in the shrouded wind turbine field and to state the research gaps in the subject. It is conceived that the main focus of shrouded wind turbine studies is on experimental studies, which have proved the beneficial idea of using a shroud. The main purposes of these experiments were to maximize the extracted and to improve efficiency while keeping the size of the diffuser small and improving the economics of the system. Therefore, this method improves the power output higher than 2–5 times.
... DAWT, Diffuser Augmented Wind Turbine, can be a promising solution to overcome those problems by adding a shroud to the turbine, then, might increase the power output of wind turbines [4][5][6]. DAWT technology has developed in the early 1950s where the results obtained flow velocity 1.3 times higher than the freestream velocity [7]. These theoretical results were then carried out by experiments on small-scale wind turbines with a shroud and obtained an increase in power by 4% compared to without a shroud in wind turbines [8]. ...
... Thus, an effective effort is needed to maximize the use of wind energy in Indonesia by manipulating the wind speed so that it has a higher local wind speed. DAWT, Diffuser Augmented Wind Turbine, can be a promising solution to overcome those problems by adding a shroud to the turbine, then, might increase the power output of wind turbines [3][4][5]. This technology is a development of the horizontal axis wind turbine. ...
Wind energy is one of the promising alternative energy resources after solar and hydropower. Most of wind turbine technologies are designed at high speed, whereas, not effectively operated in low wind speed areas. An effective technology is required to enhance the possible use of wind energy at low wind speeds. Diffuser Augmented Wind Turbine (DAWT) has been used recently to improve the use of wind turbine in a low wind speed area by manipulating the wind speed. The main concept of this technology is the pressure difference between inside and outside of DAWT which is occurred, hence, it might enhance the wind velocity and the power is increased as well. In this paper, simulation using ANSYS was conducted to investigate the performance of Horizontal Axis Wind Turbine (HAWT) in low wind speed area applying DAWT by modifying the angle and the length of diffuser. The variation of the diffuser angle was in the range 4-16 o at L=1.25D. The simulation results showed a good agreement with the reference literature which obtained the increased power around 1.4-2.9 times higher than the non-diffuser wind turbine. The parameter of diffuser length was also investigated at L=0.25D-2.5D, with the significant impacts are obtained until L=1.25D.
... The augmentation ratio in power output mainly depends on the brim height, diffuser shape and length. The results of the parametric studies were summarized in the references [18,19]. In parallel with the development of the shroud structure, the turbine blade and its airfoil section were designed to achieve the best performance by configuring the blade and brimmed diffuser to obtain maximum aerodynamics. ...
We developed a new wind turbine system that consists of a diffuser shroud with a broad-ring brim at the exit periphery and a wind turbine inside it. The shrouded wind turbine with a brimmed diffuser, which we called a “wind lens turbine” (WLT), has demonstrated power augmentation by a factor of about 2–5 compared with a bare wind turbine for a given turbine diameter and wind speed. The increase in power output depends on the diffuser shape and length and the brim height. However, a simple theory presented in this paper argues that only two performance coefficients are needed to predict the performance of WLT. The coefficients are the back pressure coefficient of the brim and the pressure recovery coefficient of the diffuser. We theoretically showed that the back pressure coefficient was particularly important for the performance of WLT. Finally, the simple theory was evaluated with experimental results. The results showed good agreement with each other.
... The maximum speed is observed as expected at the diffuser inlet and hence it is recommended to place in this position the runner which will extract energy from the wind. a) The experimental setup b) streamlines through the diffuser Further work by the same research team and other researchers showed a significant power coefficient improvement that reaches 4-5 times the power of the bare rotor using a shroud (Ohya et al., 2006;Matsushima et al., 2006;Ohya et al., 2008). The diffuser outlet cross section in this case was about twice that of the inlet cross section. ...
... While maintaining the same expansion angle and inlet diameter, they observed how the diffuser's length influenced the flow's maximum axial velocity, u max , in a standalone diffuser. Matsushima et al. [92] and Ohya et al. [103], both in 2006, observed a 30% increase on u max when increasing the L e /D i ratio from 0.5 to 2; increasing the length further produced only marginal results. ...
A systematic review and analysis of the literature of diffuser-augmented horizontal-axis turbines are presented. A collection of 155 articles in the area is analyzed and classified. The work sample is divided into 16 main research branches for discussion. Performance assessment metrics are proposed based on power coefficient and tip-speed-ratio, to quantify and compare all diffuser-augmented turbines in a unified, meaningful manner. Design suggestions for the development of diffuser-augmented turbines are pointed out based on the analysis of 73 cases. A power coefficient assessment on the work sample presented that, in 58% of the cases, the diffuser-augmented turbines surpassed the power coefficient of scaled bare turbines of the same diameter. A tip-speed-ratio assessment presented that almost 90% of the diffuser-augmented turbines developed a narrower operational interval. Five high-performing diffuser-augmented turbines are discussed, highlighting their methodologies and contributions. Caution is advised when coupling a diffuser to a bare turbine with an already high power coefficient; the diffuser-augmented turbine, especially in those cases, should be designed employing a simultaneous diffuser-rotor optimization.
... However, it generates vortex behind the diffuser, where a low-pressure region appears and raises the pressure in front of the flange to draw more wind inside the diffuser. Ohya [46] discussed how the flange highly increases the wind velocity compared to a non-flanged diffuser. The flange hight should be optimized to increase the mass flow rate, as it affects downstream separations and the upstream pressure. ...
The present paper investigates the aerodynamic and aeroacoustic characteristics of H-rotor Darrieus vertical axis wind turbine combined with a promising energy conversion technology, namely wind-lens, employing computational approaches. The Darrieus turbine adequate for low wind speed and urban area conditions. However, its aerodynamic and aeroacoustic characteristics are very complicated. Thus, the main scope of the current work is to enhance the aerodynamic performance of the introduced turbine and then assess the noise production accomplished with such enhancement. The studies are taken on two phases, the first phase is parametric 2D CFD simulations, employing the unsteady Reynolds-averaged Navier-Stokes (URANS) approach, to optimize the design parameters of the wind-lens. The second phase is a 3D CFD simulation of the full turbine using a higher order numerical scheme employing a hybrid RANS/LES method. An estimate of the aeroacoustic noise of the turbine is quantified using the Ffowcs Williams-Hawkings (FW-H) method. As a result, the optimal design parameters of the wind-lens are achieved and the optimized configuration shows a superior augmentation in power generated of about 82% increase in the power coefficient at a speed ratio of 2.75. However, the turbine equipped wind-lens produces more noisy operation compared to the non-shrouded turbine.
... While there have existed few researches on the flow past a pipe (Hirata et al., 2013), we can find several researches on the flow past a ring, a torus or a washer (Takamoto, 1987;Leweke et al., 1993;Hirata et al, 2001;Sheard et al., 2005;Hirata et al., 2006;Hirata et al., 2007). For renewable-energy applications, the flow has attracted our attention in the context of a new-concept windmill design (Ohya et al, 2006). However, in all the previous researches, a pipe is not in rotation but stationary. ...
The present purpose is to reveal the mechanism of a flying pipe from an aerodynamic point of view. At first, we conduct field observations of a flying pipe using a pair of high-speed video cameras, together with three-dimensional motion analyses. In addition, we conduct numerical analyses by a finite difference method based on the MAC scheme. As a result, the observed orbit is approximated to be not an obvious parabolic curve but rather a straight line, after an initial instable and complicated curve. The stable flight with this approximately-straight orbit suggests the importance of aerodynamics in flying mechanism. More specifically, the model is in an unstable and complicated flight during an initial flight, afterwards becomes in a stable and approximately-straight flight. In the initial instable and complicated flight, the model flies fluctuating its posture upward, downward, left-ward and right-ward. As flight distance increases, the absolute value and the amplitude of moment becomes small to zero. During such a decaying and stabilising process, the gyroscopic effect plays a primary role balancing not angular acceleration of the model but aerodynamic fluid moment. In the stable and approximately-straight flight, the flow in the stable and approximately-straight flight is nearly the velocity-potential one, and accompanies very-small drag force. And, we could ignore the influence of model's rotation upon the flow and the orbit. In this context, the model's rotation is only to stabilise its posture, and gives negligible contribution upon its aerodynamics.
... In this way, a gain in the mass flow swallowed by the rotor and in the power output seems obtainable. In the last years, several studies dealing with this topic have been proposed [13][14][15][16][17][18][19][20][21][22]. ...
Over the recent years, augmentation of turbine proves the significant performance growth of wind, hydrokinetic, and marine energy turbines. The design and number of blades and their hydrodynamics in case of the water turbine and aerodynamic in case of wind turbines are most important parameter to optimize the efficiency of turbine and power extraction. Recent studies show some potential advantages of ducted or " diffuser-augmented " current turbines are explored. These include improved safety, protection from weed growth, increased power output and reduced turbine and gearbox size for a given power output. Ducted turbines are not subject to the so-called Betz limit, which defines an upper limit of 59.3% of the incident kinetic energy that can be converted to shaft power by a single actuator disk turbine in open flow. The present study allows to over through the various achievement in augmented wind and water turbine rotor design, and effect of critical parameters on the performances.
... [8][9][10][11][12] More recently, Abe et al. presented an extensive analysis of DAWT equipped with large flanges at the diffuser exit. [13][14][15] It was reported that the flange geometry induces flow separation in the region behind the diffuser exit thus creating an unsteady and low pressure region. It results in a larger air mass flow swallowed by the rotor, that increases the power extracted by a DAWT. ...
In this paper, a numerical investigation on the effect of gurney flap (GF) on the performance of a diffuser augmented wind turbine (DAWT) is presented. The flow-field around the DAWT is obtained by solving the Reynolds-averaged-Navier-Stokes (RANS) equations. The turbine is modelled as an uniformly loaded actuator disc (AD) that imposes a resistance to the passage of the flow. Comparison of the numerical results with experimental measurements in similar conditions shows that the numerical approach used satisfactorily reproduces the mean flow field. GF heights equal to 2% and 4% of the diffuser chord length were investigated. Results show that separation induced by GF creates a low pressure region at the diffuser exit, that increases the mass flow through the diffuser and the power coefficient of the DAWT.
... In this way a gain in the mass flow swallowed by the rotor and in the power output seems obtainable. In the last years several studies dealing with this topic have been proposed, see for example [27][28][29][30][31][32][33][34][35], but this list should be not considered as exhaustive. ...
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.
... Recently, Ohya et al. 1) have developed an effective wind-acceleration system. Although it adopts a diffuser-shaped structure surrounding a wind turbine like the others previously proposed, the feature that distinguishes it from the others is a large flange attached at the exit of diffuser shroud. ...
Unsteady 3-D direct numerical simulations based on FDM are carried out for flow fields around a wind turbine equipped with a flanged diffuser. Generally, it is difficult to simulate numerically the flow around rotational bodies like rotors of wind turbines, because of unsteadiness due to a moving body and complex geometry. Therefore, we have devised an actuator-disc model for a wind turbine for simulating the resistance and rotational forces on fluid. Introduced those volume forces derived from the actuator-disc model into the external terms in N-S equations, the unsteady flow around a wind turbine can be simulated. The results of numerical simulations are compared with the wind tunnel tests and show a good agreement for the velocity and pressure fields. 1 INTRODUCTION The power in wind is well known to be proportional to cubic power of the wind velocity approaching the wind turbine. This means that even a small amount of acceleration gives a large increase in the energy output. Therefore, many research groups have tried to find a way to accelerate the approaching wind effectively. Recently, Ohya et al. 1) have developed an effective wind-acceleration system. Although it adopts a diffuser-shaped structure surrounding a wind turbine like the others previously proposed, the feature that distinguishes it from the others is a large flange attached at the exit of diffuser shroud. Figure 1 illustrates an overview of the present wind-acceleration system. A flange generates a large separation behind it, where a very low-pressure region appears to draw more wind compared to a diffuser with no flange. Owing to this effect, the flow coming into the diffuser can be effectively concentrated and accelerated. In this system, the maximum velocity is obtained near the inlet of diffuser and thus a wind turbine is located there as shown in Figure 1.
... Such studies mainly added a diffuser to the wind turbine from the early 1980s [2] to the beginning of this century [3,4]. In [5,6] an effective wind-acceleration system was produced, which contains a large diffuser with a flange creating large separation in the flow. This configuration generates low-pressure region behind it, which assists the turbine to capture more wind energy compared to a system with the diffuser without a flange. ...
In this paper, the effect of the diverging part (diffuser) configuration on the flow field pattern inside wind concentrator is investigated by computational fluid dynamics (CFD) simulations. Diffusers with different diameter ratio, lengths and profiles are examined to estimate the optimum location of the turbine rotor inside wind concentrator and to attain minimum wake losses. The obtained results show that the optimum diverging diameter ratio (d3/d2) is 2, and diverging length to cylinder diameter ratio (l3/d2) is 1.33, respectively. The optimal diffuser shape is the convex that has a radius of curvature (R2/d2) of 2.293. The corresponding wind speed-up ratio (acceleration factor) K (= Ux/Uo) equals 2.33 at a corresponding axial distance measured from wind concentrator inlet, (x/d2) equals 1.34 which is the optimum location of the wind turbine rotor.
... Although there have existed few researches on the flow past a pipe, we can find several researches on the flow past a ring, a torus or a washer, which is not in rotation but stationary [2] [8]. Recently, the flow has attracted our attention in the context of a new-concept wind- mill design [9]. ...
The present aim is to reveal the flow past a rotating pipe which is immersed parallel to the mainstream. At first, we conduct field observations of a flying pipe using a pair of high-speed video cameras, together with motion analyses based on their recorded images, which quantitatively reveal both paths and angular velocities of the flying pipe. In addition, we conduct numerical simulations by a finite difference method, whose results suggest that the pipe-rotation effect becomes remarkable for a rotation parameter Ω*> 0.4.
Nowadays renewable energy sources play an important role in partially meeting the global energy demand and protecting the environment. Wind energy technology among the renewable sources is developing rapidly around the World as it becomes a challenging renewable energy technology. One of the latest developed technologies that enable small size wind turbines to operate in the most efficient way is the diffuser augmented wind turbine (DAWT) system utilization. In this study, concentrators involving nozzle-diffuser-flange systems were aimed to be reviewed thoroughly by studying the variety of geometries that are mostly available in the literature. Also, the diffuser augmented wind turbine technologies, from the viewpoint of designs, their aerodynamic characteristics, performances, and efficiencies were reviewed extensively. The benefits obtained from cased wind turbines in terms of wind speed and power coefficient enhancements were analyzed considering a variety of different studies found in the literature.
Sustainable utilization of earth resources have been highly demanded, which is under improvement and development for a long-time. A rapid increase in demand and higher cost for energy and declining fossil fuels in the near future induced further stress in research and development on renewable energies. But these are to be feasible in view of the utilization of renewable energy sources. Side by side, the overall efficiency and cost is also counted as a matter of debate. Sustainability has become a major highlight in modern civilization. Progressive depletion of conventional fossil fuels with increasing energy consumption and greenhouse gas (GHG) emissions has led to a move toward renewable and sustainable energy sources. The production of sustainable energy based on renewable sources is a challenging task for replacing fossil-based fuels to get a cleaner environment and also to reduce the dependency on other countries and uncertainty of fuel price. The life cycle assessment (LCA) of renewable energy sources is one of the key terms which influences the sustainability of the related resource under study. Study and Dynamic model Simulation of such hybrid ORC based Solar, Geothermal and Wind power generation energy systems explore a ray of light in this renewable energy prospect. A thorough study has been made for hybrid Solar, Geothermal and Wind power generation energy system simulation with Journal of Renewable Energy and Resources Volume 6 Issue 1 2 Multi-Criteria Decision Analysis (MCDA) for sustainable energy future utilization in the present paper. Several attempts have been made to utilize low-temperature Geothermal fields and low potential Winds incorporating ORC Solar based technological concept. The presently proposed hybrid ORC-based Solar, Geothermal and wind energy system will be beneficial. Discussions promised that the suggested idea is competitive with other renewable energy sources. Further studies are necessary to achieve more realistic results to start actual study in reality.
The present purpose is to reveal the mechanism of a flying pipe from an aerodynamic point of view. At first, we conduct field observations of a flying pipe using a pair of high-speed video cameras, together with three-dimensional motion analyses. In addition, we conduct numerical analyses by a finite difference method based on the MAC scheme. As a result, the observed orbit is approximated to be not an obvious parabolic curve but rather a straight line, after an initial instable and complicated curve. The stable flight with this approximately-straight orbit suggests the importance of aerodynamics in flying mechanism. More specifically, the model is in an unstable and complicated flight during an initial flight, afterwards becomes in a stable and approximately-straight flight. In the initial instable and complicated flight, the model flies fluctuating its posture upward, downward, left-ward and right-ward. As flight distance increases, the absolute value and the amplitude of moment becomes small to zero. During such a decaying and stabilising process, the gyroscopic effect plays a primary role balancing not angular acceleration of the model but aerodynamic fluid moment. In the stable and approximately-straight flight, the flow in the stable and approximately-straight flight is nearly the velocity-potential one, and accompanies very-small drag force. And, we could ignore the influence of model’s rotation upon the flow and the orbit. In this context, the model’s rotation is only to stabilise its posture, and gives negligible contribution upon its aerodynamics.
Measurement of service strains of the blade of Wind-lens turbine was conducted by using wireless strain measurement system in a wind tunnel apparatus. Wind-lens turbine has demonsrated power augmentation by a factor about 2-5 compared with a bare wind turbine, for a given turbine diameter and wind speed. Mean strain and vibrational strain variation range at the root of the blade was increased with the increase of uniform flow velocity of the wind tunnel. The phenomenon of vibration of the blades of Wind-lens turbine was due to a flutter or flow-induced vibration, self-excited oscillation induced by vortices around the blades, in a certain range of flow velocity. The increase of strain variation range at the root of the blades in the case of intense variation of wind direction was caused by a gyro-moment due to a yawing movement of the Wind-lens turbine. The intense variation of wind velocity, in addition to that of wind direction, caused extreme large vibration amplitude, or strain variation.
Measurement of service strains of the blade of Wind-lens turbine was conducted by using wireless strain measurement system in a wind tunnel apparatus. Wind-lens turbine has demonsrated power augmentation by a factor about 2-5 compared with a bare wind turbine, for a given turbine diameter and wind speed. Mean strain and vibrational strain variation range at the root of the blade was increased with the increase of uniform flow velocity of the wind tunnel. The phenomenon of vibration of the blades of Wind-lens turbine was due to a flutter or flow-induced vibration, selfexcited oscillation induced by vortices around the blades, in a certain range of flow velocity. The increase of strain variation range at the root of the blades in the case of intense variation of wind direction was caused by a gyro-moment due to a yawing movement of the Wind-lens turbine. The intense variation of wind velocity, in addition to that of wind direction, caused extreme large vibration amplitude, or strain variation.
A flow modeling for the meridional flow calculation in the aerodynamic design of the half-ducted turbomachinery blade rows has been presented. It is assumed that the meridional flow is axisymmetric and viscous one. To take into account the blade loading, a blade force is introduced as a body force to the axisymmeric Navier-Stokes equations. The blade force contains the inviscid blade effect only, namely, the pressure difference across a blade. Numerical examples have been presented for a wind-lens turbine and a half-ducted propeller fan in order to demonstrate the validity of the present flow modeling. For the wind-lens turbine, the present flow modeling can predict non-uniform flow distributions at the inlet and outlet of the turbine rotor. For the half-ducted propeller fan, however, the formation of the several separated and vortical flow structures results in the well prediction of the inlet flow angle only. It is found that the present flow modeling is useful to the blade row design tool.
The present aim is to reveal the flow past a pipe which is immersed parallel to the mainstream at high Reynolds numbers. In a wind tunnel, we carry out (1) base-pressure measurements, (2) velocity-fluctuation measurements using a hot-wire anemometer and (3) flow visualisations by a smoke-wire method with PIV analyses, where we take consecutive picutures using a high-speed camcorder to obtain quantitative flow-field information such as velocity vector and vorticity. The tested parameter ranges are Re = 2.0-10 3 - 1.3-10 4, d/t = 4.0 - 10.0 and l/t = 1.0 - 10.0, where Re, d, t and / are the Reynolds number, mean diameter, thickness and length of the pipe, respectively. As a result, the Re effects are negligible. The base-suction coefficient -C pb monotonously decreases with decreasing d/t, or with increasing l/t. We propose a unified formula to predict -C pb, which are consistent with both a two-dimensional prism and a rod for l/t < 4.0 in addition to a ring. In contrast, the Strouhal number St almost coincides with that for a two-dimensional prism at any l/t, if we can detected any dominant frequencies. In addition, we conduct flow visualisations, and reveal the effects upon axisymmetry of wake. Finally, we classify the flow into three modes based on both periodicity and axisymmetry. Such a modal classification reveals that the enhancement of flow irregularity corresponds to the decrease of -C pb.
This paper explains a novel concept for wind power density enhancement through the use of specially shaped static wind deflector structures. The design and fabrication of the deflectors and the experimental results are explained. The general wind enhancement concept is further divided into two separate but related versions. The first deflector version discussed herein is a closed or constrained-flow device proposed for energy recapture from industrial-scale ventilation exhaust air. The second deflector version discussed herein is an open or unconstrained-flow device proposed for low wind velocity (< 4 m/s) applications and thus offers potential for wind power generation in built-up urban environments. Experiments associated with the two design adaptations were performed on a test-duct and in a wind tunnel, respectively. The test results showed velocity increases (factors of 3 and 1.7 respectively). The test-duct results for the constrained-flow device also showed turbine power output enhancement by a factor of approximately 20. Neglecting other losses, such a ratio would provide a theoretical power improvement of about 5 times over the base flow velocity. The term Wind Powered Deflector (WPD) was coined to describe the devices discussed herein.
A novel idea of hybrid power generation system has been proposed that can be characterized as a simple and renewable method to achieve sustainable utilization of renewable resources. The new hybrid system utilizes a solar chimney to produce electricity with a windmill. Geothermal heat will be used as a heat source to achieve desirable air speed inside the chimney. Calculations suggest the idea may be competitive with other renewable energy sources. Further studies are necessary to achieve more realistic results to start an actual project.
The diffuser-augmented wind turbine (DAWT) is one of the advanced concepts being investigated to improve the economics of wind energy conversion systems (WECS). Application of modern boundary-layer control techniques has reduced the surface area requirements of an efficient diffuser by an order of magnitude. Many parameters that affect the performance of the diffuser system have been examined in small-scale wind tunnel tests with a family of compact diffusers, using screens and centerbodies to simulate the presence of a turbine. Flowfield surveys, overall performance, the effect of ground proximity, and the prospects for further improvement are described. The baseline configuration is a conical, 60 deg included angle diffuser with an area ratio of 2.78 controlled by two tangential injection slots. This first-generation DAWT can provide about twice the power of a conventional WECS with the same turbine diameter and wind. Economic estimates show that this DAWT can be as much as 50% cheaper than coventional WECS for the same rated power.
Numerical investigations are carried out for flow fields around flanged diffusers to develop small-type wind turbines under . In the calculations, an advanced closure approximation is adopted, within the framework of non-linear eddy-viscosity modeling, which aims specifically at an improved representation of turbulence anisotropy. Comparison of the computed results with the corresponding experimental data shows that the present calculation has the capability of providing reasonable predictions for the present complex turbulent flows. Furthermore, by processing the computational results, the input-power coefficient is estimated under various conditions of diffuser opening angle and loading coefficient. It is shown that the performance of a flanged diffuser strongly depends on the loading coefficient as well as the opening angle because it greatly affects the nature of the separation appearing inside the diffuser. The present investigation suggests that the loading coefficient for the best performance of a flanged diffuser is considerably smaller than that for a bare wind turbine.
We have developed a shrouded wind turbine with a brimmed-diffuser. To realize higher power augmentation of it, we have investigated the optimum parameters of the diffuser angle and hub ratio, and the optimum shapes of the inlet shroud and center body. As a result, the new wind turbine has demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about 5 compared with a bare wind turbine.
To improve the performance of wind rotors by increasing an energy density of the wind, various power augmentation systems have been studied. The concentrator (nozzle) augmentation system is one of them. However, until now, the optimum design of concentrator is not revealed. The purpose of this study is to clarity the effect and to decide the optimum design of the concentrator. In conclusion, the effect of power augmentation was increased as following condition. 1. The position of the rotor is backward the outlet of concentrator. 2. The outlet diameter of the concentrator is smaller than the rotor diameter. 3. The inlet diameter is much larger than the rotor diameter.
The experiments conducted by the authors have showed that the power output of a diffuser-shrouded wind turbine is enhanced significantly by coupling a brim to the diffuser exit plane. This paper presents a simple theory to estimate the power augmentation effect of the brimmed diffuser. The augmentation ratio of inlet velocity and the power coefficient are the functions of the base-pressure coefficient of brim, the pressure recovery coefficient of diffuser and the loading coefficient of turbine. The loading coefficient to provide the maximum power coefficient is linked to the aerodynamic performance of the turbine blade based on the present theory.
Momentum theory of diffuser augmented wind turbine (DAWT) or all-directional wind acceleration tower (ADWAT) was treated, basically with Bernoulli's equation, the actuator disk theory and the equation of continuity. Then, output coefficients of the turbine were deduced as functions of load factor of the turbine, area ratio and efficiency of the diffuser. Through the numerical computation and the investigation of the resulting equations, the following has been ascertained. If there is no momentum loss in the diffuser flow, the maximum output coefficient of the turbine, based on the diffuser exit area, is found to be 0.385 with suitable combinations of the load factor and the area ratio. The effect of the diffuser efficiency becomes severer in the case of a larger area ratio of the diffuser and a lighter load factor of the turbine. Hence, it has been suggested that the reasonable area ratio of the diffuser and the load factor of the turbine are around 1:3 and 0.5, respectively. The output coefficient of the turbine based on the turbine area may be expected to be about 0.7.
In all previous numerical investigations of spherical Couette flow only axisymmetric regimes were considered. At the same time, in experiments [1–4] it was found that when both spheres rotate and the layer is thin centrifugal instability of the main flow leads to the appearance of nonaxisymmetric secondary flows of the azimuthal traveling wave type. The results of an initial numerical investigation of these flows are presented below. Solving the linear problem of the stability of the main flow and simulating the secondary flows on the basis of the complete nonlinear Navier-Stokes equations has made it possible to supplement and explain many of the results obtained experimentally. The type of bifurcation and the structure of the disturbances whose growth leads to the appearance of three-dimensional nonstationary flows are determined, and the transitions between different secondary regimes in the region of weak supercriticality are described.
We have developed a new wind turbine system that consists of a diffuser with a broad-ring brim at the exit periphery and a conventional wind turbine inside it. The new wind turbine has demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about 2 3 compared with a bare wind turbine. This is because a very low-pressure region due to strong vortex formation behind the broad brim draws more mass flow to a turbine inside the diffuser.
The diffuser-augmented wind turbine (DAWT) is one of the advanced concepts being investigated to improve the economics of wind energy conversion systems (WECS). Application of modern boundary-layer control techniques has reduced the surface area requirements of an efficient diffuser by an order of magnitude. Many parameters that affect the performance of the diffuser system have been examined in small-scale wind tunnel tests with a family of compact diffusers, using screens and centerbodies to simulate the presence of a turbine. Flowfield surveys, overall performance, the effect of ground proximity, and the prospects for further improvement are described. The baseline configuration is a conical, 60 deg included angle diffuser with an area ratio of 2.78 controlled by two tangential injection slots. This first-generation DAWT can provide about twice the power of a conventional WECS with the same turbine diameter and wind. Economic estimates show that this DAWT can be as much as 50% cheaper than coventional WECS for the same rated power.
The initial stages of the experimental development of the diffuser-augmented wind turbine (DAWT) employed various screen meshes to simulate the energy extraction mechanisms of a wind turbine. In this investigation in a 2 multiplied by 3 m wind tunnel, a three-bladed constant chord, untwisted turbine model was incorporated into a DAWT model. The objectives were to add real turbine characteristics such as swirl, and centerbodies effects, to the flow. Although this turbine model was not well matched to the diffuser, the model DAWT system increased the power output by more than four times that of the model turbine operating as a conventional wind energy conversion system; more than 3. 4 times the power potential of an ideal wind turbine was measured.
Introduction of Wind Turbine
- I Ushiyama
Ushiyama, I.: Introduction of Wind Turbine, Sanseido Press, Tokyo,
1997, pp. 77-84 (in Japanese).
Wind-Velocity Acceleration by Hollow Bodies
- T Karasudani
- Y Ohya
- N Fukamachi
- K Watanabe
Karasudani, T., Ohya, Y., Fukamachi, N. and Watanabe, K.: Wind-Velocity Acceleration by Hollow Bodies, J. Jpn. Soc. Fluid Mech.,
Nagare, 22 (2003), pp. 337-343 (in Japanese).
- K Abe
- Y Ohya
Abe, K. and Ohya, Y.: An Investigation of Flow Fields around
Flanged Diffusers Using CFD, J. Wind Eng. Ind. Aerodyn., 92
(2004), pp. 315-330.