Experimental Study on the Performance of Diffuser-Augmented Wind Turbines

The provided content discusses the research and experimental studies conducted on wind turbines, particularly focusing on the use of diffusers to augment their performance. The studies employed various techniques such as particle image velocimetry (PIV) to visualize flow structures behind diffuser and rotor cross-sections. Researchers also analyzed the performance of diffuser-augmented vertical axis wind turbines using computational models and experimental tests in water tunnels. The findings suggest that the presence of a diffuser can lower the power output of the turbine at low speeds, but improve its overall efficiency.

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A COMPARATIVE STUDY AND DESIGN OF CUSTOM CONSTRUCTED WIND
AUGMENTATION SHROUD TO IMPROVE EFFICIENCY OF POWER OUTPUT

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Introduction
Alternative sources of energy are vital for application of efficient power sources
in the future. The other sources such as fossil fuel are reducing at a very high rate and
causing issues such as environmental degradation and global warming. Creation of
better ways of generating energy as well as utilising renewable and clean energy should
be highly invested on. Among others, innovations in wind energy have been developing
rapidly and are expected to play a fundamental role in the field of energy (Isensee &
Abdul, 2012). However, a comparative study between the rate of using wind power and
the overall demand for energy is still subtle especially the level of development is
negligible in the remote area.
Remote areas suitable for the development of wind power plants, complex
terrains and the turbulent nature of the local winds are affecting the installation of wind
turbines. Therefore, the introduction of new and innovative wind energy systems that
produce high output regardless of the wind speed and patterns are desired.
It is imperative to understand better various ways of improving the efficiency of
wind turbines, as well as bringing down the cost to an economical level. The cost of oil
and other non-renewable resources are on the rise every day (Carroll, 2014). Improving
the efficiency of wind turbines helps in reducing the energy limits being experienced
today. In the future, turbines will be able to harness more energy from the wind with
limited technological improvements.
A considerable number of high-efficiency turbines lowers the cost of energy as well
as generate power at a lower cost. Wind shroud system is one of the major techniques
that can be utilized to improve the efficiency of the turbines. The wind shroud systems
have a duct that surrounds the blades that increase the cross-sectional area of flowing

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wind. The resultant sub-atmospheric pressure within the duct draws more air through
the turbine blades and hence providing more power to be generated compared to the
traditional turbines (Bet & Grassman, 2013).
Generation of wind power is proportional to the speed of wind cubed(Phillips,
2003). Therefore, an increase in output is caused by the possibility to increase the speed
of the wind and using the wind fluid dynamic around the structure, the output of power
of the wind turbines can be increased considerably. Even though there have been some
studies on shrouded wind systems reported, it has not provided attractive subjects
conventionally. Specific research conducted extensively is the examination of shrouded
wind turbines by Çetin, Yurdusev, Ata, & Ozdamar (2006).
In most of the recent studies, the focus concentrated on the techniques used to
concentrate the wind in the duct with a wide open angle, and a boundary segment
controlled with a number of flow slots is used in order to concentrate the flow that goes
inside the surface of the duct. Therefore, the technique of using boundary segments
reduces the loss of pressure through flow separation and increasing the mass flow
within the conduit.
Based on this concept, a company in New Zealand developed the Vortex 7 diffuser
augmentation turbine (Abe, at al., 2006). They utilized a slotted duct that would help
prevent separation within the duct. Additionally, they built a ring profiled structure in
the wide opening of the turbine. It was reported that the new modified structure
increased Power output due to the wing system by a factor of 1.8 compared to the bare
turbines (Abe, at al., 2006). Even though there have been, other ideas being reported,
most of them are yet to reach commercialization.

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The primary objective of the present study concerning developing a wind system
with higher output is determining the best way to harness wind energy in a more
efficient way as well as the right shroud that can be used to generate energy more
efficiently. As an alternative, the shrouded wind system technology might provide
energy to areas with turbulent wind patterns as well as other remote areas, where
electricity is yet to be installed. In fact, shrouded wind systems are an attractive
alternative for the developing market especially for areas that lack electricity or areas
with energy deficiency.
Shrouded wind systems have many advantages over the bare turbines. The newly
innovated turbines are easy to install since they have a shorter construction period that
is extending the utility grid line. This turbine technology has been reported to be lesser
complex to construct (Hirahara, Hossain, Kawahashi & Nonomura, 2005). As for the
reasons, local manufacturing is usually a preferred option in remote areas or developing
areas that could, in turn, boost economic development as well as reducing the cost of
production.
This model of turbine technology would also increase the wind velocity through the
blades which include small inlet shroud and a diffuser. A low-pressure area is usually
generated behind the diffuser as vortices are created. Not only there is a huge
development in the wind industry in regard to power production through the wind
turbines, but also on the effectiveness of the turbines used. Recent research shows that
researcher is looking for much better ways of improving and increasing the
effectiveness and applicability of the present wind turbines (Hirahara, et al., 2006).

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A growth and the improvisation in the uses of wind energy in order to generate the
electricity is being adhered in the past two decades due to the concerns which are
related to the global warming and perpetual maximisation in the cost of fossil fuels. The
generated wind power maximises due to a factor of eight at the time when the speed of
the wind the double that demonstrates that the energy which is present in the wind
replicates to the wind power directly. Further, the amount of power that the wind is
competent of producing is shown by the following equation i.e. p=0.5 ρAV3. The
equation demonstrates that the wind power is majorly impacted by the speed of the
wind as it is proportional to the wind speed cube directly.
As it is being observed that the wind is the most natural and occurring source in
order to produce energy. There is an excessive importunity for the energy because of
the industrialisation and the rapid growth of the economy. Further, there is a great
maximisation in the fossil fuel consumption in order to generate energy (Đurišić, Ž., &
Mikulović, J. 2012). In the year 2013, the content of global carbon dioxide increased
from 280 ppm to 400 ppm in the environment. The recent report which is given by the
NOAA (National Atmospheric and Oceanic Administration) in the year 2016, the
content of the global carbon dioxide was maximised to 402.59 ppm from the 399.29
ppm in the environment.
The negative impact of the fossil fuels discharging carbon dioxide and the various
other harmful gases, perpetual maximization in the cost of fossil fuels and the huge
consumption which further leads to abatement of resources for the upcoming
generations and various parts of the world has demonstrated great interest in the
technologies of wind energy in order to produce power. Further, the wind turbine’s

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power output relies highly on the velocity of the wind. It is responsible for the blades
rotation to transform it into electrical energy from the mechanical energy. Moreover,
the velocity of the wind is amused towards the turbines of the wind in order to produce
the power at maximum level.
Further, the usage of wind energy was started 3000 years ago, and it was initially
accustomed in order to grind grain through the windmills drive to pump water and
shove ships. There are two types of energy sources which are available in order to
produce the power i.e. conventional and non-conventional. Wind energy is the
component which is non-polluting and eco-friendly and further it is a technique which
is being used in order to generate power, and it doesn't emanate carbon dioxide and
various other harmful gases in comparison to fossil fuels and the generation of nuclear
power.
The wind turbines are the most effective sources which are producing power by
transforming kinetic energy to the mechanical energy, and further, it informs to the
electrical energy through the generator. Furthermore, adjudicating the placement of the
turbines of the wind in the wind far, accustomed to contain simplified models and the
guess work but the numerical techniques are empowering optimization and the
understanding of the turbine's wake interactions (Kubik et al., 2013). Without the usage
of these techniques, the wind energy would not be exploited to its potential. ABL
(Atmospheric Boundary Layer) was also the concern of various speculations but the
dimensions coupled with the new techniques of numeric which have helped in making
the accurate measurements of wind energy of the site according to its potential. In these
areas, the advances have been made, but it still requires more improvisation and
improvisation can be done through optimization and combination of the present
contrived models.

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The numeric methods and the dynamics of the computational fluids can be utilized
in order to anatomize the flow over the complicated terrain to better micro-site the farm
of wind. The coastal areas are the most fascinated locations due to moderately high
speeds of the wind but these regions mostly have an unsteady flow of the wind and the
complicated terrain. The dynamics of the computational fluids can be coupled along
with the algorithms that use spacing, cost, inter-connections of the turbine and
geography in the form of variables to find the great layout of the farm. Further, the
single kind of the turbine at height can be anatomized, and the various other restrictions
and the price are not being considered (McWilliam et al., 2012).
The impact of support structures on the flow of wind might have an effect on the
performance of the turbine. The impacts can be caused by using the characteristics of
the flow, interference of the blade tower, lift forces by using the dynamics of the
computational fluids on the upwind turbine. Further, this model predicted successfully
the flow deterrent from the tower when the blade will pass through it. Despite this, it
doesn't demonstrate that the outcomes were substantiated by experimental data.
Moreover, the interference of tower gives rise to vibrations. The simulation of numeric
and the anatomy of the wind turbine structures of the support and the generation of
noise are the main areas to study because they are accustomed to augur the fatigue life
and minimise a load of fatigue towards the support structure of the turbines of wind.
The wind turbines can be characterised in two kinds relied on the axis in which the
rotation of the turbines takes place i.e. wind turbine of the horizontal axis and the wind
turbines of the vertical axis. Further, in the horizontal axis, the turbine blades rotate
around the axis of horizontal, and they are mainly pointed to the direction of the wind.
Whereas, the wind turbines of the vertical axis are the kind of turbine of the wind in
which it rotates vertically. The wind turbines of the vertical axis are the independent of

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the direction of the wind and further, there is no need of creating a control system for
the dictate adjustment of the blades, and the turbine can be accustomed in various
places where the direction of the wind is largely variable. The gearboxes and the
generators can be put near to the ground, and as a result, there will be a minimum load
on the tower and the maintenance will be easy.
The main objective behind the development of the wind turbine is maximising the
power outcome of the turbine. Moreover, there are two parameters which impact the
power's value i.e. the blade's swept area and the speed of the wind. The power outcome
can be maximised by maximising one out of the two given parameters. As a result, the
conventional turbines of the wind the coefficient of the power is a restricted parameter.
Moreover, the method to maximise the velocity of the wind is to utilise the duct across
the rotor (Song et al., 2012). The design is being pertained as the wind turbine of
diffuser augmented. It is being observed that the duct across the rotor maximises the
air's flow rate through the swept area with the help of the rotor and maximises the
velocity of the wind at the rotor. The effective pasture of the speed of the wind for
producing the urged power which is broader than that connate to the conventional
system. The majority of the development of the diffuser were don on the basis of the
wind turbine of a horizontal axis, and it is being observed that the design of the
diffusers was mainly focused on the applications of the wind turbines of the vertical
axis.

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Literature Review
Shrouded Wind turbines are newly developed wind turbine system that not only
utilizes a diffusing structure but also a large flanged structure that was developed by
Ohio, Karasudani, and Inoue (2008), from Kyushu University, Japan. The research
conducted by Ohio karasudani in 2011 reported that the Shrouded wind system was
increasing the power augmentation by a factor of about 5, in a given wind speed and the
diameter of the wind turbine (Ohya & Karasudani, 2012).
A simulation result conducted by Bet and Grossman (2003) has reported a
substantial increase in coverage of low-pressure areas behind the turbine. Along with
the studies above, this section will present the main theories for the assessment of
shrouded wind turbine as compared to the bare turbine for turbine aerodynamic
evaluation. A preliminary review of the literature is made to understand the
fundamental concepts of the Shrouded wind turbines. Some of the key contributions
made by researchers in the field in the past few years will be discussed.
The invention of accurate models of aerodynamic aspects of wind turbines is one
significant way of analysing different wind energy systems. The operation of wind
turbines induces phenomena like wind flow components where the direction and the

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magnitude of the wind are the rotor changes and the turbine blades rotate (Cetin,
Yurudusev, Ata & Ozdamar, 2005). Furthermore, wind flow separation becomes more
complicated in the new model. The instabilities of the wind interacting with the blades,
hub and the tip of the vortices impact the character of the entire flow. The aerodynamics
of the wind turbines becomes more complex with all the wind instabilities as well as
flow interaction. (Burton, Sharpe, Jenkins & Bossanyi, 2001).
First, in order to understand the complex physics and aerodynamic used in
developing the shrouded wind turbines one must analyse and understand a simple one-
dimensional model (Archer & Caldeira, 2009). To understand the concept of the
shrouded rotor, it is also important to examine the theoretical concept which governs
the performance of shrouded wind turbines.
According to the Augmentation ratio era, it is easy to describe the enhanced energy
produced by the turbines combined by the diffuser. Thus, the augmentation ratio is
expressed as (Aranake, Lakshminarayan & Duraisamy, 2013): ra= (Cp, d)/0.593, these
Cp,d represents the power coefficient for a turbine with the diffuser.
Shrouded Wind turbines can combine various systems to assist in concentrating and
accelerating the wind. A hollow structure is used for surrounding the turbine as a way
of enhancing the flow of wind. Applying a nozzle in a converging shape at the inlet of
the shrouded wind system is beneficial especially in turbulent wind flow condition,
which is common in urban areas (Hirahara, et al., 2005).
The shrouded turbines exploit the venturi effect, where a reduction in pressure of the
wind and the velocity of wind are produced by the passage of the fluid in a contraction.
For a shrouded wind system, contraction is performed by the shroud surrounding the
turbines (Isensee, et al., 2012). At the nozzle section, the diameter of the inlet should be

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larger than the outlet in order to lower the wind velocity. Also, in regards to the outlet,
the velocity is increased due to a decrease in the area (Vermillion, Grunnagle, Lim &
Kolmanovsky, 2014).
Albert Betz, a German physicist, calculated that no wind turbine could convert more
than 59.3% of the kinetic energy of the wind into mechanical energy turning a rotor.
This is known as the Betz Limit and is the theoretical maximum coefficient of power
for any wind turbine. In bare wind turbines operating at a maximum Betz limit, the flow
of air decreases to approximately 2/3 of the velocity of the free stream. This decrease in
airflow causes an increase in pressure in front of the rotor inducing a small section of
the mass flow being pushed around the rotor (Khunthongjan & Janyalertadun, 2012).
A way of increasing the flow of air can be incorporated by using an annular lifting
device around the turbine. This device is what is known as a shroud. The increase in the
exit plane of the shroud together with a reduction in the exit pressure enhances the mass
flow leading to higher extraction of energy.
The philosophy behind a flange at the exit of the shroud diffuser is to create a low-
pressure system at the downstream to increase the velocity of incoming air. Past
research conducted on the flange diffuser only focused on the impact of the width of the
vertical Fringe (Carroll, 2014).
Grady & Hader (2012) developed a diffuser to a bare wind turbine so as to increase
the efficiency of the wind turbine and also conducted an analysis of the fluid flow. The
outcomes of the simulation indicate that there was an increase in velocity of the wind by
approximately 60% (Isensee, et al., 2012).

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Toya, Kusakabe & Yuji (2007) developed a convergent and a divergent shroud wind
system using an any. Later analysis of the wind flow was conducted on the turbine with
a shroud and a bare turbine. The outcome showed that the output of the turbine with a
shroud increases three times compared with the one with no turbine (Toya, et al., 2007).
According to the researchers (Toshimitsu et al., 2008), the system of the wind
turbine has been developed along with the shroud over the efficacy power can be
improvised and a cone to enable the upcoming flow of the air from the turbine of the
wind for the higher outcome. The researches have demonstrated that the shroud along
with the angle of 30 degrees has been connected to the turbine of the wind in order to
deliver the improvised performance and the efficacy of the power when the comparison
is made with the other shrouded devices of the wind augmentation through varied
angles. The large device of the wind augmentation was constructed in which the inlet's
angle remained 30 degrees only. The performance of the shrouded turbine of the wind is
being investigated numerically and experimentally at the various speeds of the wind and
outcome of the power as well. Further, the parameter of the power outcome was
compared among the shrouded turbine of the wind with and without the cone. As a
result, the shrouded turbine of the wind along with a cone has shown the augmentation
of the power by a factor and it was 65 percent similar in comparison with the turbine of
the wind apart from the cone.

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In the recent study, it has been observed that the researchers are making the use of
conical shroud in the tunnel of the wind because the member of the conical is having a
frequently maximising the diameter in the direction of the wind flow. Also, the member
of the conical expands smoothly in the direction of the wind flow. The efficacy of the
conventional turbine of the wind is restricted. The system of the wind turbine which
consists shroud along with a cone was created in order to face these challenges.
According to the researchers, the global warming is described as one of the greatest
threat that everyone must be aware and should act responsibly in controlling and
managing the causes of global warming. It has been researching that approximately 80
percent of the global warming is caused by the emission of the carbon dioxide through
the fossil fuels. Further, the wind power has the capacity the control the disaster of the
environment because of it consumers around 0 percent of the water and it is
environmentally friendly, and it largely reduces the emissions of the carbon dioxide
content (Wan et al., 2012).
The researchers have stated that the capacity of the power of the wind turbine
mainly relies upon the design and the speed of the wind. Moreover, it has been observed
that the only disadvantage of the technology of wind turbine is that the low speed of the
wind possesses minimum energy of the density per volume of the air which hits the
blades of the turbine and maximises the cost of production in comparison to the fossil
fuels. The researchers have conducted various researches in order to improvise the
density of the energy in the wind as it will further improve the capacity of the power
and would make the resources cost effective.
According to the researchers, the predicting and modelling the attributes of the wind
are significant to the power sitting and the prediction. Further, the accurate models of

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wind prediction exist but the methods of numeric improvises the outcome. The example
is the technique of predicting the potential of the wind power through a program which
is called WAsP (wind analysis of the Atlas and the application program). Further, the
program uses the velocity of the wind and the direction at the individual elevation.
Moreover, the method so the minimum squares are being used in order to convert the
wind data which is being taken at three or even more elevations to the data of
equivalence which is being taken at one so that the accurate data can be used as the
WAsP’s input.
Some of the researchers (Saha, U. K., & Rajkumar, M. J. 2006) have also used the
simulation models of ABL in order to appraise the wind farms locations. Further, the
simulation model of MM5 ABL was used to impose the potential area of the China’s
coastal area. The model estimated the distribution of the velocity of the wind and the
energy accurately in the ABL. The connate study was being used the model of WRF in
order to stimulate the wind with the wind farm of the country through the complicated
terrain. The model has the distinct boundary layer and the schemes are proceeding for
the distinct levels of the terrain and the types of the vegetation.
The researchers had planned to run the model of the CFD coupling and the wind
prediction through two distinct resolutions and further validate it through the various
observations. Therefore, the methodology was proved to be accurate apart from such
cases where there was the existence of the sharp and abrupt transformations in the
terrain like cliffs and hills.
Some of the researchers (Akhgari, A. 2011) have also focused on the analysis of the
fatigue by using the techniques of numeric. Further, the model of FEA is being used in
order to identify the blade's critical zone and accelerated the fatigue defect modelling to

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predict the fatigue life of the blades of the wind turbine. Furthermore, the wind turbines
of the horizontal axis are mainly used to produce the power, and there are various
experimental and numeric studies which are related to the applications of the wind
turbines of the horizontal axis. Further, the researchers have studied the wake
architecture of the turbines in a tunnel of water by using the turbine of small scale. The
researchers have made the comparisons on the basis of the experimental outcomes
through the calculation of the numeric depending on the method of rotor vortex lattice
and the code of the custom.
The researchers (Fujisawa, N., & Shibuya, S. 2001) also authenticated the
experiment through the experiment of the full scale in the tunnel of the wind and
moreover the flow architecture of the turbines with the diffuser has been studied. The
technique of PIV is being used in order to illustrate the structure behind the diffuser and
also in the rotor’s cross section. Further, the researchers used the design of the diffuser
and also established the blockage vertices formation delinquent the diffuser because of
the pressure drop.
According to the researchers (Islam et al., 2008), the experimental study was
conducted in order to analyse the performance of the turbine of the diffuser augmented
in the visualization flow of the water tunnel. It is the experiment which is being done in
order to analyse the hydrodynamic behaviour of the sunken bodies in the flow of the
water. The tunnel of the water is comparable to the re-circulating tunnel of the wind
apart from the working fluid which is water, not the air. Further, in accordance with the
adequate scaling, the experiments can be done through the turbine model of the small
scale. Another difference which is being observed between the air and water tunnel is
the passage that the driving force is being produced in each system. Therefore, the
power outcome of the turbine when there is the presence of the diffuser across the rotor

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which is lower as compared to the power outcome of the bare rotor for the ratios of the
low speed.

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