small wind turbine design
TRANSCRIPT
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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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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,