small wind turbine design

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Design of a small wind turbine with CFD

Computational Simulation Techniques WS 2015

Final Exam Project

Submitted by: Iqbal Meskinzada

Matriculation #: 2267075

Lecturer: Prof. Dr.-Ing. Rainer Stank

Due Date: 31st of May 2016


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Hamburg University of Applied Sciences (HAW) |CWi Project 2

Table of ontents

1.

Introduction ………………………………………………………………………………3

1.1. Design conditions and Airfoils……………………………………………….............3

1.2. Calculation of Angles of Attack and Twist and the Lift Coefficients ……….............6

2. Building up Geometry ………………………...………………..………………...............6

2.1. Assembling the turbine……………………………………………………….............8

3.

Meshing…………………………………………………………………………...............9

4.

Pre-processing…………………………………………………………………….............11

5. Main processing…………………………………………………………………………..14

6.

Post-processing …………………………………………………………………………..15

7.

Theoretical power calculation ……………………………………………………. ……..18

8. Results and Discussions ………………………………………………………….............18

9.

References………………………………………………………………………………...19


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1. Introduction

Computational fluid dynamics, usually abbreviated as CFD, is a branch of fluid mechanics that uses

numerical analysis and algorithms to solve and analyze problems that involve fluid flows. In the

branch of Renewable Energies, a lot of procedures are based on transport processes like fluid or heattransport or on mixing processes. Therefore, fluid dynamics and especially computational fluid

dynamics are playing an important role.

The advantage of computational methods is that they provide

a practical solution of the exact governing equations for almost all engineering problems.

Computational methods can be characterized as an art of replacing the governing nonlinear partial

differential equations by numbers and advancing these numbers in space and or time to obtain a final

numerical description of the complete problem of interest. The end product of computational methods

is indeed a collection of numbers which is in contrast to a closed-form analytical solution. Generally,

the objective of most engineering analysis is a quantitative description of a problem [1].

1.1. Design conditions and Airfoils

This report is a detailed explaination of the designing of a small wind turbine using numerical methods

with ANSYS software package. The primary purpose of this task is to replace the blades on the

existing wind turbine,”TESPE”. Blades on this turbine were very simple in terms of design, there is no

twisting along the blades, constant angle of attack from the hub to the tip and a uniform cross

section. These properties reduce the performance of the turbine. In this report, the aim is to the

improve the performance of the turbine by changing the blades shape, determine the optimum angle of

attack and twist angle. The rotor blades of the turbine should consist of the two airfoils NREL S822

and S823.

F igure (1): Profile of the airfoil NREL S822, smooth and streamlined, selected for the tip

Since blade tickness decreases from root to the tip [2] and the airfoil NREL S822 is streamlined and

narrow, it is suitable for the tip position and NREL S823, shown in figure (3), for the hub.


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Fi gure (2): Polar diagram of airfoil NREL S822, with optimum angle of attack of 4.2°

and corresponding lift coefficient of 0.675.

Fi gure (3): Profile of the airfoil NREL S823. Thicker, selected for the hub


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Fi gure (4): Polar diagram of airfoil NREL S823. with optimum angle of attack

of 13.5° and corresponding lift coefficient of 12.8.

Chord length varies over the radius and has to be

determined at the given radii. The chord length between

the given airfoil section at the give radii varies linearly.For radii smaller than r=0.15R there is no airfoil.

The ratio of the chord length at the hub to the chord

length at the tip is 2.5. The twist of the airfoil sections is

done with respect to a normal axis at 0.25 chord length as

shown in figure (5). The blade radius is 3.8m and the

number of blades are 4.

F igure (5): Positions for twisting

Given tip speed ratio and rated wind velocity are and respectively.The angleof attack is defined as.

Blade angel of attack, …………. (1)

Chord length distribution, √ ………………. (2)


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1.2. Calculating the Angles of Attack and Twist and the Lift Coefficients

The first step is to determine the optimum value for aerodynamic parameters such as angles of attack,

chords length and twist angles at different rotor diameters. This can be done using the given polar

diagrams of the airfoils and design conditions. The aim is to determine the maximum left coefficients

over various angles of attack such that the ratio of the hub chord length to that of the tip remains less

than 2.5. The lift force increases with the increase in anagle of attack until the blades “stall” where the

the lift coefficient decreases. Therefore, there is a range for optimum angle of attack. Most often the

best angle of attack lies within a range of (4-16 degrees) for such small size wind turbine. This is a

trial-&-error method to find the best angles from the given polar diagrams. The determined angles are

presented in table (1). Since the blade cross-section will not be uniform, different chord lengths should

be calculated at the given radii usnig the given formula. Assumed lift coefficients and the

corresponding chord length and twist angle are calculated in table (1).

Table (1): Calculated values for chord lengths and twist angles

2. Building Up the Geometry

To create the geometry of the blades, first of all the data points pertaining to S823 should be importedin the ICEM and then connected all together to make the first airfoil S823, it consist of five curves.

Then this airfoil needs to be offset by a distance of 950cm from the origin and 20 degrees twisted. The

twist of the airfoil sections is done with respect to a normal axis at 0.25 chord based. Similarly, the

data points for the airfoil S823 are imported and merged with the exiting data points. Airfoil S822 will

be positioned at 2.85m from the center point and offset to the tip. Then it will be scaled and twisted

accordingly. All twist angles and offsets are based on the calculation of table (1).

Importing geometric points for both Scalling and offseting of the airfoils. Twisting

Fi gure (6): Building the blade: 6.1: imported data points. 6.2: scaled and offset airfoils, 6.3: twisted airfoils.

Airfoil R r(R) Lambda D Z AoA [°] chord length c(r) AoA Twist chord lenght ratio

S822 3.8 3.8 4.4 4 4.2 0.674 0.4 4.42 2.498

S822 3.8 2.85 4.4 4 4.2 0.674 0.53 7.22

S823 3.8 0.95 4.4 4 13.5 1.28 0.73 17.72

S823 3.8 0.57 4.4 4 13.5 1.28 1 31.79

21 3


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The next step is to create the surfaces and this can be done from the Create/Modify Surface option.

The following figure shows the generated sufaces over the blade.

d

F igure (7): Blade surface generation

To complete the blade geometry, the above created blade should be replicated around the center point

by 90 degrees to create a four-bladed turbine as indicated in figure (8).

F igure (8): Four-bladed rotor, surface generation


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2.1. Assembling The Turbine

Next task is to insert this rotor geometry to the old given tespe file “ICEM input file Tespe” which is a

complete geometry of small-sized wind turbine. However before doing this, the blade geometry in thisfile must be removed. In addition to that, there are some other adjustments as well that need to be

done. Gondel needs to be rotated (45 degrees) and inlined with blades’ direction. The inner and outer

interfaces should be resized such that they accommodate the large blades and leave some space

between the tip and outer interface. Initially there would be an intersection between the “lit-Bret” and

outer-interface of the blades, so some modifications are needed. After all the required corrections are

made, the geometry is ready for meshing. The completed geometry is presented in the following

figure.

Fi gure (9): Four-bladed turbine, adjusted into the tespe file


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3. Meshing

Meshing is one of the most important influences on CFD simulation accuracy since it depends on the

quality of the meshing. Upon completion of the geometry, the next step is meshing. Tetrahedron mesh

type is selected for this particular problem. To get a better control over the meshing process, it is

divided into two parts. The first part is meshing of the environment, the surrounding box including the

interfaces of the rotor. The second part is the blades and the interfaces corresponding to the hub and

rod. The student version of the ANSYS has limitations on the number of meshes (512,000), therefore

some trial and error work is involved until acceptable number of meshes achieved. In the first step,

the surrounding is meshed. In the “Global Mesh Setup”, a scale factor of 1 and maximum element of

1600 were selected. The maximum mesh sizes are entered in “ Part Mesh Setup” and a total of 120,000

meshes were obtained for the first part. The following figure shows the meshed surrounding.

Fi gure (10): Wind turbine surrounding area meshed.

Similarly meshing is carried out for the second part which consisted of the rotor as well as the

interfaces covering it. A total of about 263,244 meshes were obtained for the second part. Making a

total of about 383,244 meshes. The following show shows the meshing of the blades and associated

interfaces.


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Fi gure (11): Resulted meshed blades, tetrahydrons

F igure (12): Resulted meshed for the “Nabel ”, “Holm ” and Blades

After the meshing is complete and accurate, the solver setup should be set for “ ANSY CFX”, for

writing the solver input for the pre-prosessing phase.


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4. Pre-possessing

After meshing process is completely done and checked. The next step is pre-processing. In general, in

pre-processing stage, the following major steps are involved.

.

Import or Create geometry

Define element type and real constants

Define material properties

Applying Loads

Applying constraints, boundary conditions (fixed edge, simply supported)

However, in our case we have the existing old tespe wind turbine file, in which all these steps have

already been performed and we just need to apply the same properties to our new file. Therefore, a

new case should be introduced into the CFX-pre and the meshed file should be imported. The general

idea in this phase is to create domains for the environment” the surrounding box” and the wind

turbine. Then inlet, outlet and circumference interfaces should be introduced. After these setting have

made, all the properties of the pre-file should be checked and compared against the original given

pre-file (tespe). Below figures are some of the snaphshots for the applied properties

F igure (13): Velocity inlet and constant pressure outlet on the surrounding area


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Fi gure (14): Velocity and pressure on the rotor interface


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Below figure indicates the properties that were incorporated into the new wind turbine pre-file which

is the same as the old tesp file.

F igure (15): Applied properties to the new pre-file, barrowed from the old tespe file

After these consideresations are given, we run the simulation process.


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5. Main Processing

After providing the input for all necessary parameters in Pre-Processing phase, simulation in Main

processing using CFX-Solver Manager shall be initiated. Graphs in the following step are attached

below.

F igure (16): Solution curves for wind turbine torque

F igure (17): Solution curves for momentum and mass


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6. Post-processing

Upon successful completion of the main-processing, the results are generated for the

post-processing. At this stage, the output power from the rotor needs to be calculated by using the following formula.

……….. (3) Where is the angular velocity, n rotational speed and M, the torque. The rotationalspeed can be calculated using the following formula.

…………….. (4)

= 121.63 rpm

The angular velocity can be calculated by using below formula:

……………. (5)

= 12.73 rad/sThe total output power of the wind can be calculated through formulla (3), where M is torque which

was determined from the post-prossesing phase ( M = 750 N*m). For an angular veclocity of 12.73

rad/s and a torque of this much, the power generated by the wind turbine would be:

= 9547.5 W

Figures 18 and 19 shows velocity and pressure contours respectively and figure 20 shows streamlines(100) coming from the blades.


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F igure (18): Velocity contour

Fi gure (19): Pressure contour


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F igure (20): Streamlines from the turbine blades indicating the nature of air flow lines leaving the blades


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9. References

[1]. Computational Simulation Techniques. Wind turbine design with CFD lecture notes, Chapter 1“ Introduction” Pages 2-6, Prof. Dr.-Ing. Rainer Stank, Hamburg University of Applied Sciences

HAW Hamburg.

[2] – Wind Turbine Lecture notes , Chapter 4 “Aerodynamics of Wind Turbines” Page 6, by Prof.

Dr. Timon Kampschulte,Hamburg University of Applied Sciences HAW Hamburg, Faculty of Life

Sciences.

[3] – Wind Turbine Power Calculations, RWE npower renewables Mechanical and Electrical

Engineering Power Industry, The Royal Academy of Engineering.

[4] – Aerodynamics of Wind Turbines 2nd

Edition, Martin O. L. Hansen, 2008.

[5] - AERODYNAMIC STUDY OF A SMALL-SCALE WIND TURBINE. E. Barbiera, P. Bucelloa, S.

D’hersa, P. Mosquera Michaelsenb, B. Pritzb, N. Bottinic and M. Micheloudc. aCentro de Mecánica

Computacional, Instituto Tecnológico de Buenos Aires, Av. E. Madero 399, Buenos Aires, Argentina,

[email protected].

small wind turbine design

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