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International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:19 No:06 97
E N S I J IJENS © December 2019 IJENS -IJMME-2727-406191
An Experimental and Numerical Investigation on
Darrieus Vertical Axis Wind Turbine Types at
Low Wind Speed Nawfal M. Ali1, Dr. Sattar Aljabair2, Dr. Abdul Hassan A.K.3
1 Department of Mechanical Engineering , University of Technology, Baghdad, Iraq:
E-mail: me.21326@uotechnology.edu.iq 2Department of Mechanical Engineering , University of Technology, Baghdad, Iraq:
E-mail:20010@uotechnology.edu.iq 3 Department of Mechanical Engineering , University of Technology, Baghdad, Iraq:
E-mail:dr.abdulhassn@uotechnology.edu.iq
Abstract-- This paper presents a model for the evaluation of
the optimal design of Darrieus vertical axis wind turbine by
CFD analysis and experimental tests, through analyzing six
models of Darrieus wind turbines, number of blades and tip
speed ratio. For this purpose, a full investigation campaign
has been carried out through a systematic comparison of
numerical simulations with wind tunnel experiments data.
The airfoil profile used in the turbine blades was DU06W200
and constant geometry dimensions to turbines. The
experiments were done for all Darrieus wind turbine models
by using a subsonic wind tunnel under open type test section
with airflow speed range (3-7.65) m/s and different tip speed
ratio TSR. The results show that Darrieus WT straight type
can be self-starting at the wind velocity 3 m/s, where other
types cannot be starting at less than wind speed 5 m/s. The
rotational speed (N) increases for all models with the wind
velocity increase. The power coefficient (CP) increases when
the TSR increases at experimental results for all models. The
performance of Darrieus WT with 2 blades rotor is better
than other models. At low wind velocity (3 m/s) the value of
CP (0.2495), the CT (0.174), the rotational speed (198 rpm) and can be self-starting at this wind velocity.
Index Term-- Darrieus wind turbine; Straight-type;
Twisted-type; Helical-type; Airfoil profile DU06W200; low
wind velocity.
Nomenclature
As swept area of turbine (m2) T dynamic torque (N.m) Cp power coefficient Ta ambient temperature (K)
CT torque coefficient Tor torque (N.m)
c blade chord length (mm) TSR tip speed ratio
R rotor radius (mm) u relative velocity of fluid
e overlap distance (mm) RANS Reynolds Averaged Navier-Stokes
F force (N) SST Shear Stress Transport
H blade height (mm) VAWT Vertical axis wind turbine
Rg gas constant (287 J/kg.K) WT Wind Turbine
𝑟 position vector rotational speed (rpm)
rp radius of pulley (mm) viscosity (Pa.s)
Patm atmospheric pressure (Pa) ρ air density (kg/m3)
PAV available power in the wind (W) angular velocity (rad/s)
PT power produced from turbine (W) Sui Centrifugal and Coriolis force
P static pressure τij Average shear stress
1. INTRODUCTION
In the latest years, wind energy has become one of the
most important technology in economic renewable
energy. Today, wind turbines use proven and tested
technology for generating electrical power and provide a
secure and sustainable energy supply. To compete with others it had to cope with the least affirmative conditions
to maximum positive conditions. The usual challenge for
the turbines is performing at low wind speed. Wind power
has many advantages, that makes it's the fastest-growing
energy source in the world. Wind energy doesn't pollute
the air like a power plant [1,2]. Darrieus is a lift-type
VAWT, it can be a rotation at tip speed ratio greater than
one, the torque generated by Darrieus wind turbine is less
than Savonius wind turbine but it rotates fastest. Darrieus
wind turbine is much better to use in generating
electricity. Darrieus turbine generates very large
centrifugal forces act on the turbine, there are many types of Darrieus turbines such as H-rotor, Eggbeater, Helical
blades, twisted blades ... etc. [3].
The effect of the blade geometrical section on the energy
performance and aerodynamic forces working on a small
straight-Darrieus type vertical axis wind turbine studied
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by [4], CFD software was used for the calculation of rotor
performance. The results are suggested on the two kinds
of airfoil sections NACA0021 (classical symmetrical)
blade profile and other type is DU06W200 (non-
symmetric) and laminar blade profile. Results show that
overall aerodynamic performance for DU06W200 is better than NACA0021 (up to nearly 2%), because of an
increased blade performance during downwind operation.
[5] studied the effect of some design parameters such as
airfoil type, the number of blades and turbine solidity on
the performance. Numerical and experimental study on
small scale Darrieus straight-bladed VAWT into the
aerodynamics and performance. The airfoils in this study
two types (NACA0018 and DU06W200) and the CFD
software was used to the transient simulations. Results
show the comparison of a straight-bladed VAWT between
the airfoils NACA0018, DU06W200 and S1210 at wind
velocity 10 m/s, that the VAWT with DU06W200 airfoil increases self-starting ability and maximum efficiency
more than the other types. [6] studied the designed
aerodynamic modeling of a VAWT by using software
tools for Darrieus VAWT type straight blades (H-type)
with airfoil NACA0012, VAWT has three blades. The
results show that the rotational speed increases when wind
speed increase with maintains a linear relationship. The
TSR of VAWT increases with increase the wind speed
until reaching a certain value after that the TSR decreases
with wind speed increase. Analysis of different blade
architectures on small VAWT performance studied by [7]. Five models were chosen to make a comparison between
them, four cases the airfoil type NACA0018 ((2-3) blades
H-Darrieus and 3 blades helical) and other DU06W200 (3
blades H-Darrieus). the helical turbine is less efficient at
starting TSR. The results suggested using (DU06W200)
airfoil to increased rotor performance at starting TSR (at
low rotor speed, relieve start-up problems in VAWT), the
helical turbine is less efficient at starting TSR, the blade
manufacturing is higher cost more than the straight blade
type. The power was obtained at H-Darrieus with the
airfoil DU06W200 higher than the helical rotor. A
reviewing of the various literature by [8] on the VAWT
has a symmetrical airfoil, and what the airfoil profile
gives good performance. The results of the study show
that aerodynamic blade profile NACA 0012, NACA 0015,
NACA 0018, NACA 0021, NACA 0025, NACA 0063 are
suitable for Darrieus rotor. Also the blade profile NACA
0021 has better starting performance due to its thickness. [9] studied improving the output power of a Vertical Axis
Wind Turbine by utilizing different methods. The blade
profile at VAWT was NACA 0018, the turbine contained
three blades. The results show, no significant effect for
changing off the angle of Attack form 0 degrees to 10 and
-10 degrees, also no effect of the mechanical turbulator
that added to the turbine blade on the output power of
VAWT. The modification of the original strut of the
VAWT was improvement the output power of VAWT
because the resistance to flow was decreased. [10] studied
the effect of the central shaft of a straight-bladed Darrieus
VAWT on the aerodynamic performance by using numerical simulations on a three-bladed rotor
configuration, four different shaft diameters were studied.
The results show that when the shaft diameter increases
the overall rotor performance was a reduction, the
decrease in rotor aerodynamic efficiency is caused by a
reduction of the wind velocity inside the rotor disk,
especially in the downwind portion due to the wake of the
shaft.
In this study, execute a comparison between six
models from Darrieus wind turbine has airfoil profile
DU06W200 and consist of different blades number to
study the behavior and performance of these models at
wind speed range (3-7.65) m/s, especially at (3 m/s) with
different tip speed ratio. Experiments and numerical
analyses were carried out to choose the best model that can be work in the low wind velocity conditions.
2. DARRIEUS WIND TURBINE (DWT) PERFORMANCE:
The performance of Darrieus VAWTs is explained by
(CT), (CP), () given by [11]. Fig. (1) shows the geometry and dimensions of the Darrieus wind turbine:
As = 2RH (1.1)
Fig. 1. The geometry and dimensions of the Darrieus wind turbine
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:19 No:06 99
E N S I J IJENS © December 2019 IJENS -IJMME-2727-406191
= ωR
u (1.2)
CT = T
0.5 ρAsu2R
(1.3)
CP = PTurbine
PAvailable =
T ω
0.5 ρA𝑠u3 =
T
0.5 ρA𝑠u2R
Rω
u = CT × (1.4)
In this study, six models were taken to studies and make a comparison between them to find out the best model of DWT,
the study condition at low wind velocity (3 m/s).
3. DARRIEUS WT MODELS: The blades of Darrieus VAWT were used airfoil type
DU06W200. This airfoil designed to use for small
VAWTs, it's asymmetrical profile. DU06W200 airfoil was
improved in 2006 by Delft University of Technology also known as TU Delft, located in Delft, Netherlands [12].
One advantages of this airfoil increase rotor performance
at starting TSRs (at low rotor speed, relieve start-up
problems in VAWT) Fig. (2), the details (Max. thickness
19.8% at 31.1% chord and Max. camber 0.5% at 84.6%
chord). The types of six models of DWT were a straight,
twisted 70o and helical 120o, all types with two and three
blades. Figs. (3,4,5) shows DWT type Straight, Twisted and Helical respectively. The dimensions of DWT were
listed in the table (1).
DWT blades were fabricated by utilized 3D Printer and the materials PLA (polylactic acid) for all models. After finished
from fabricating the blades, then coated by Aluminum sheets had thickness 0.5 mm.
Fig. 3. Darrieus WT type Straight blades with two and three blades
Fig. 2. Airfoil type DU06W200 asymmetrical profiles
Fig. 4. Darrieus WT type Twisted 70o blades with two and three blades
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Table I
The dimensions of Darrieus wind turbine models
Turbine Type Number of
blades Chord length (c)
(mm)
Rotor radius
(R) (mm)
Blade
Height (H) (mm)
Shaft
Diameter (e) (mm)
Straight
2 , 3 100
250
540
25
Twisted 70o
Helical 120o
4. THE EXPERIMENTAL WORK
4.1 Experimental setup:
The experiments were conducted using a subsonic
wind tunnel under open type test section, shown in fig. (6 - a,b,c) with the test section contains the turbine model
and the measurement devices assembly with a cross-
section of frontal view of 1m x 1.25m. Double butterfly
valve used to control and regulate airflow rate, the
airspeed range is (0 - 9) m/s. The turbine models are fixed
at 250 mm between the rotor shaft and exit of the wind
tunnel.
Digital tachometer used to measure the rotational speed of
the wind turbine. Digital Force gauge is used to measure
the force (F) produced from the rotor shaft, the torque can
be calculated by:
Tor = F x rp (1.5)
the radius of pulley (rp) = 50 mm. The static pitot tube is used to measure airflow speed in
the wind tunnel, it's connected with macroscopic
manometer. The digital thermometer used to measure
ambient temperature. The pressure gauge is used to
measure atmospheric pressure. The air density is given
by:
𝛒 =𝐏𝐚𝐭𝐦
𝐓𝐚×𝐑𝐠 (1.6)
Fig. 5. Darrieus WT type Helical 120o blades with two and three blades
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Fig. 6-a. A subsonic wind tunnel
Fig. 6-b. Schematic diagram for wind tunnel with dimensions details
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5. NUMERICAL MODEL
To obtain a successful design and suitable geometry
for the Darrieus wind rotor, a modeling style should be
applied. To achieve this aim and proof the obtained
experimental results, six models of Darrieus wind rotor
geometry have been modeled.
In this study used ANSYS-CFX software for simulation. The results of the simulation were compared with
experimental results and the error was assessed. Transient
conditions are considered for this modeling. Fig. (7-a)
shows the 3D computational domain for the Darrieus rotor
model. Two separated parts by a sliding interface in the
domain. Wind tunnel testing zone represents the
stationary, that is the first part of the domain.
The dimensions of the wind tunnel (stationary domain)
are assumed 2 m x 2 m x 5 m. The second part is the
rotational domain (rotor), that rotating around a
perpendicular axis. The rotor is fixed at 1 m from the inlet face of the stationary domain. Location of wind rotor in
the middle of the rotational domain, it is rotating with the
angular velocity of the domain. Wind velocity at the
entrance stationary domain is 3 m/s (as in real condition),
this value is considered as the inlet boundary condition
and distance from the axis of the rotational domain is 1 m.
The pressure in the outer face is equal to atmospheric
pressure. The blade of the rotor is set as non-slip smooth
wall and is considered as a boundary condition on the
blade. To obtain good results and more strictly study the
flow in the boundary layers of rotor blade this study, prismatic mesh applied on sides of rotor blades to obtain
correctly boundary layer. Accordingly, near the wall of
rotor blades, the density of meshes was higher and
interface. Fig. (7-b,c,d) shows the mesh on the rotating
domain, the mesh on the rotor and mesh on the endplate in
the rotor, also appears the mesh near the blade show
boundary layers. The flow around the rotor is turbulent,
thus the simulation of CFD around the rotor is very
complex. Simulations of CFD were applied to solve the
cases based on 3D steady finite volume incompressible
Reynolds Averaged Navier-Stokes (RANS) equations.
Controlling equations of turbulence flow are continuity and Navier - Stokes equations, these equations in
conservative forms are [13,14]: ∂ui
∂xj = 0
(1.7) ∂
∂xj (ρuiuj ) =
−∂P
∂xi+
∂
∂xj (τij − ρuiuj ) + Sui (1.8)
where:
Sui = - ρ [2 Ω × u + Ω × (Ω × r )] (1.9) τij = - μ ( ∂ui
∂xj + ∂uj
∂xi ) (1.10)
To solve the case of the flow field around the rotor
numerically, the turbulence models were added in RANS
CFD solvers. The best model that should be obtains
acceptance results and agreement with experimental
results.
5.1 Turbulence models:
The precedent studies used a 3D SST k- turbulence model which agrees with the experimental work of [15,16,17]. This model employs to analyze the transient
forces that affect DWTs. The advanced turbulence models
used in this work need a very fine mesh near the wall so
that y+ < 5 [18,19], to get a good result. So in this study,
the SST k- turbulence model was applied in numerical simulation.
Fig. 6-c. The test section rigs with turbine model and dimensions details
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6. RESULTS AND DISCUSSION
The effect of turbine types and blades number shown
in Fig. (8-12). Fig. (8-a and b) illustrates the relationship
between the rotational speed (N) of the DWT models have 2 blades and 3 blades with various values of wind speed
(3 - 8 m/s) for experimental data results. The DWT types
twisted and helical were starting rotation at a wind
velocity of over 5 m/s. Fig. (8-a) represented the
experimental results for DWT models have 2 blades, the
rotational speed (N) for DWT type straight was higher
than other models for all wind speed values, thus the
rotational speed (N) for straight DWT at 3 m/s was 198
rpm. Fig. (8-b) shows the relationship between the
rotational speed (N) and wind speed (u) for the DWT
models have 3 blades, also the DWT types twisted and
helical were starting rotation at a wind velocity of 5 m/s. The rotational speed (N) for DWT type straight was
higher than other models for all wind speed. Table (2)
shows the experimental results of the rotational speed at
various wind velocity for DWT models, from the table
shows that the DWT type straight with 2 blades has higher
values more than other types. The rotational speed (N)
increases for all models when the wind velocity increase.
Table II
The rotational speed (N) at various wind velocity (u) for DWT models
DWT type Number of
Blades
Wind velocity (m/s)
3 4 4.5 4.85 5.15 6.45 7.65
Straight 2 198 279 370 435 500 720 ---
3 165 215 279 305 458 590 ---
Twisted 70o 2 0 0 0 0 75 150 245
3 0 0 0 45 90 170 275
Helical 120o 2 0 0 0 0 75 130 190
3 0 0 0 60 110 200 295
The rotational speed (N) in (rpm).
The DWT types twisted and helical could not be starting rotation at a wind velocity of less than 5 m/s, thus there
are no values for power or torque coefficients for this WT
types at wind velocity below 5 m/s in experimental works, but in the numerical simulations it has a values shown in
Fig. (9-a and b). Fig. (9-a) represented the relationship
Fig. (7-a). Computational mesh (view of both domains) Fig. (7-b). Computational mesh
for rotating domain
Fig. (7-d). Top view of mesh on the
endplate in the rotor Fig. (7-c). Mesh near the blade show boundary layers
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between power coefficient (CP) and tip speed ratio TSR
for the numerical results to DWT models that have 2
blades at constant wind speed 3 m/s, the Cp values for
straight DWT was higher than other models. Fig. (9-b)
shows the relationship between (CP) and (TSR) for the
numerical results to DWT models that have 3 blades at constant wind speed 3 m/s, also the CP values for straight
DWT was higher than other models. Table (3) shows the
experimental results of the (CP) at various wind velocity
for DWT models, from the table shows that the DWT type
straight with 2 blades has higher values more than other
types. The power coefficient (CP) increases for all models
when the wind velocity increase. Fig. (10-a and b) shows
a comparison of the relation Cp-TSR between 2 and 3
blades straight DWT at constant wind velocity (3 m/s), the
experimental and numerical results shown that the values
of CP for 2 blades were higher than values of 3 blades.
Fig. (11 - a and b) represented the relation of torque
coefficient (CT) and TSR for the DWTs models that have 2 and 3 blades at constant wind velocity (3 m/s). Fig. (11 -
a) shows the relation of CT and TSR for DWTs with 2
blades, the CT value for straight DWT was higher than
other models, Fig. (11-b) for the DWTs with 3 blades and
also the result similar to the 2 blades model. Table (4)
shows the experimental results of the (CT) at various
wind velocity for DWTs models.
Table III
The (CP) at various wind velocity for DWT models
DWT type Number
of Blades
Wind velocity (m/s)
3 4 4.5 4.85 5.15 6.45 7.65
Straight 2 0.2495 0.2506 0.2635 0.275 0.2895 0.3076 ----
3 0.2407 0.2494 0.2606 0.2678 0.2846 0.3065 ----
Twisted 70o 2 0 0 0 0 0.0372 0.0757 0.1216
3 0 0 0 0.0195 0.0597 0.1008 0.1323
Helical 120o 2 0 0 0 0 0.0449 0.0690 0.0889
3 0 0 0 0.0427 0.0789 0.1332 0.1465
Table IV
The (CT) at various wind velocity for DWT models
DWT type Number
of Blades
Wind velocity (m/s)
3 4 4.5 4.85 5.15 6.45 7.65
Straight 2 0.1740 0.1396 0.1212 0.1163 0.1119 0.1050 ----
3 0.2015 0.1803 0.1615 0.1590 0.1243 0.1276 ----
Twisted 70o 2 0 0 0 0 0.098 0.1242 0.1444
3 0 0 0 0.0665 0.1069 0.1240 0.1306
Helical 120o 2 0 0 0 0 0.1181 0.1308 0.1365
3 0 0 0 0.146 0.1414 0.1641 0.15
The Darrieus WTs models can be self-starting at the wind velocity as in table (5). Performance
parameters in terms of (CP, CT) for Darrieus WT with 2 blades at constant wind velocity (3 m/s) were
shown in Fig. (12-a and b) related to TSR for the experimental and numerical results respectively.
Table V
The wind velocity that the Darrieus WTs models can be self-starting
DWT type Number of
Blades
Wind velocity self-
starting (m/s)
Straight 2 3
3 3
Twisted 70o 2 5.75
3 5
Helical 120o 2 6.5
3 6
From the results in the tables (2, 3, 4 and 5) shown the
performance of Darrieus WT with 2 blades rotor is better
than other models. When the wind velocity 3 m/s the
values of the parameters for DWT with 2 blades are the
rotational speed (198 rpm), the CP (0.2495), the CT
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(0.174) and self-starting rotation in this value of wind
velocity (3 m/s).
7. CONCLUSIONS
Subsonic wind tunnel open type was used in the
experiments for Darrieus WT with different types and blades number at various airflow velocity and focus on
low wind velocity (3 m/s) to study the effect of Darrieus
turbine types and blades number on the performance of
Darrieus WTs. In this study, the types of six models of
DWT were a straight, twisted 70o and helical 120o, all
types with two and three blades. Results show the
followings:
1. The blades of Darrieus VAWT were used airfoil type
DU06W200. This airfoil designed to use for small
VAWTs, it's asymmetrical profile.
2. The Darrieus WTs straight types can be self-starting at
the wind velocity 3 m/s, but the twisted types starting over 5 m/s and the helical types starting over 6 m/s.
Thus the straight WTs models are better than other
types in the low wind velocity.
3. The rotational speed (N) increases for all models with
the wind velocity increase, the rotational speed (198
rpm) at wind velocity 3 m/s for DWTs straight type
with 2 blades and (165 rpm) for DWTs straight type
with 3 blades at same conditions.
4. The power coefficient (CP) increases when the TSR increases at experimental results, but at numerical
results increases when the tip speed ratio increase to a
certain value then the value decreases with increases
TSR. However, the torque coefficient (CT) decreases
when the TSR increases.
5. The power coefficient values for DWTs straight model
with 2 blades are higher than other models in this
study.
6. The performance of Darrieus WT with 2 blades rotor
is better than other models. At low wind velocity (3
m/s) the value of CP (0.2495), the CT (0.174), the
rotational speed (198 rpm) and can be self-starting at this wind velocity.
Fig. (8-a). The relationship between the rotational speed (N) and wind speed (u) for the DWT models have 2 blades
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Fig. (8-b). The relationship between the rotational speed (N) and wind speed (u) for the DWT models have 3 blades
Fig. (9-a). The numerical relationship between CP and TSR for the DWT models have 2 blades
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Fig. (9-b). The numerical relationship between CP and TSR for the DWT models have 3 blades
Fig. (10-a). The experimental relationship between CP and TSR for the DWT models have 2 and 3 blades at wind velocity 3
m/s
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Fig. (10-b). The numerical relationship between CP and TSR for the DWT models have 2 and 3 blades at wind velocity 3
m/s
Fig. (11-a). The numerical relationship between CT and TSR for the DWT models have 2 blades at wind velocity 3 m/s
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Fig. (11-b). The numerical relationship between CT and TSR for the DWT models have 3 blades at wind velocity 3 m/s
Fig. (12-a). Performance parameters in terms of (Cp , CT) for DWT with 2 blades Experimental results
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Fig. (12-b). Performance parameters in terms of (Cp , CT) for DWT with 2 blades Numerical results