an experimental and numerical investigation on darrieus vertical axis wind...

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: [email protected] 2Department of Mechanical Engineering , University of Technology, Baghdad, Iraq:

E-mail:[email protected] 3 Department of Mechanical Engineering , University of Technology, Baghdad, Iraq:

E-mail:[email protected]

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



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



= 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



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



Fig. 6-a. A subsonic wind tunnel

Fig. 6-b. Schematic diagram for wind tunnel with dimensions details



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



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



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



(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



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



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



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



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

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