University of Duisburg-Essen

Transcription

1 University of Duisburg-Essen Institute for Energy and Environmental Engineering Chair of Fluid Mechanics Bachelor Thesis Numerical and Experimental Investigation of The Rotor Blades of An HAWT With A Profile HKAS Inspired by a Maple Seed Supervisors: Prof. Dr. Ing. Ernst von Lavante Prof. Dr.-Ing. Wojciech Kowalczyk Dipl. Ing. Harun Kaya Author: Kinaci, Mustafa Efe Matriculation Number: October 3, 2011

2 Eidesstattliche Erklärung Ich versichere, dass ich die eingereichte Bachelorarbeit ohne fremde Hilfe verfasst und andere als die in ihr angegebene Literatur nicht benutzt habe und dass alle ganz oder annahernd übernommenen Textstellen sowie verwendeten Grafiken und Tabellen kenntlich gemacht sind; ausserdem versichere ich, dass die Ausarbeitung in dieser oder ähnlicher Form noch nicht anderweitig als Prüfungsleistung vorgelegt und bewertet würde. Datum Unterschrift

3 Acknowledgments Firstly, I would like to thank the head of Fluid Mechanics department Prof. Dr.-Ing. Ernst von Lavante for introducing me to fluid mechanics and fluid dynamics. His vast knowledge on these and many other subjects has captured my total attention and made me advance in these subjects. I would also like to thank Prof. Dr.-Ing. Wojciech Kowalczyk for his support. This task could not have been completed without the help of Dipl. Ing. Harun Kaya, who has given me this topic, and I thank him especially for showing me the support to accomplish this task and giving me advices for my future endeavors. I would also thank Dipl. Ing Ali Gedikli for giving me the start to learn the needed software s and helping me advance in them. I thank them for sharing their knowledge. Last but not least I would like to thank all of my friends for morally helping me during this hard period. I thank my family for always pushing me forward into a more successful career and helping me in every aspect of my life, without their help and support none of this could have been done. I thank them for picking me up whenever I stumbled.

4 Table of Contents 1. Introduction Motivation Goal of Thesis Overview 6 2. Theoretical Background Wind Turbines Origins Aerodynamics of a Wind Turbine Laminar and Turbulent Flows Numerical and Experimental Setup Gridgen Star-CCM Experimental Setup Results and Comparison Star-CCM Experiment Comparison of Results Conclusion Bibliography 47

5 List of Tables 2 List of Tables Table 1: Continuum Physic Models and Definitions. 25 Table 2: The Different Regions and Their Explanations. 29 Table 3: Simulation Properties. 31 Table 4: The measurements gathered from experimental work. 43 Table 5: A table of the angle of attack for different installation angles. 44

6 List of Figures 3 List of Figures Figure 1: Heron's Windmill Design. 8 Figure 2: Types of Vertical Axis Rotors. 10 Figure 3: Horizontal Axis Wind Turbine. 11 Figure 4: Detailed drawing of the parts of a HAWT. 11 Figure 5: Flow Model of Betz's Momentum Theory. 12 Figure 6: Figure of airfoil and the affecting forces. 15 Figure 7: A sketch of the boundary layer on an airfoil. 16 Figure 8: Angle of incidence. 17 Figure 9: Flow physics for a two-dimensional airfoil undergoing dynamic stall. 18 Figure 10: Laminar and Turbulent Flows. 19 Figure 11: The Database imported from Solid Works. 20 Figure 12: End top side view 21 Figure 13: The rotor domains' skewness. 22 Figure 14: The Volume Conditions 23 Figure 15: The length of the airfoil 27 Figure 16: Periodic Parts of the Model. 29 Figure 17: Outer Wall with Slip. 29 Figure 18: The turbine profile made out of maple seed. 32 Figure 19: The design of the experimental setup. 32 Figure 20: The experimental Set-up. 33 Figure 21: The location of the rotor profile inside the test tunnel. 33 Figure 22: Power Output for 30 Degree Installation Angle. 34 Figure 23: Power Output for 40 Degree Installation Angle. 35 Figure 24: Power Output for 60 Degree Installation Angle. 35 Figure 25: The velocity magnitude scene of 300rpm. 36 Figure 26: The velocity magnitude from negative y to plus z [the pressure outlet section]. 37 Figure 27: Velocity magnitude display right after the turbine. 38 Figure 28: Schematic diagram showing the air loads. 38 Figure 29: Pressure coefficient distributions of the HKAS airfoil for 0.03m, 0.05m, 0.07m, 0.08m and shows the streamline plots from CFD simulations for the corresponding distances [0.03, 0.05, 0.07, 0.08 meters]. 40 Figure 30: Streamline results for 60 in the CFD simulation. 41 Figure 31: Streamline results for 30 in the CFD simulation. 41 Figure 32: Streamline results for 45 in the CFD simulation. 41 Figure 33: Streamline results for 70 in CFD simulation. 42

7 List of Figures 4 Figure 34: Simulation Results for HKAS Figure 35: Experimental Results for HKAS

8 Introduction 5 1. Introduction 1.1 Motivation The significant rise in the cost of petroleum oil and as the environment is becoming worse the search for viable alternative technologies have increased. As one type of renewable energy generation, wind power is not only environmental friendly but also more mature in technology and able to be explored in larger scale compared with other types of green energy. The EU has committed itself to the promotion of renewable energy sources to tackle climate change [1]. National research programs were initiated around the world to investigate the possibilities of using wind energy. Wind turbines in Denmark generated 0.7 billion kwh in 1991, or 2.4% of Denmark s total electrical consumption. The California Energy Commission (CEC) reports more than 15,000 turbines generated 2.5 billion kwh in California in 1990 [2]. Since August 2009, the first German offshore wind farm Alpha Ventus is also operational [3]. Though wind turbines and windmills have been used for centuries, the application of rotor aerodynamics technology to improve reliability and reduce costs of wind generated energy has only been pursued in earnest for the past 25 years. Since wind energy is a low-density source of power, it is important to maximize the efficiency of wind turbines. Among other problems to be solved about wind turbines the aerodynamic performance prediction is the basic one. With the development of modern computer technology and simulation method of turbulence, numerical simulation method has the potential to provide physically simulation of the wind turbine flow field. Another critical aspect of the wind turbine that has not been evaluated until recently is the blade itself. The blades angle of attack as well as its shape has an effect on the performance of the turbine. A recurring problem is that increasing the angle eventually forces the blade to stall. If a technology could be developed to boost the operation of wind turbines, it would be less stress on the countries with high dependence on fossil fuels and also an environment friendlier solution for everyone.

9 Introduction Goal of Thesis Generating energy without harming the environment is one of the top priorities, and using the earth s wind power to do so is just another advantage. The accurate prediction of the performance of wind turbines is of great importance when designing economical and reliable wind turbines [4]. The aim of the present work is to obtain aerodynamic predictions of HKAS turbine profile which is inspired by a maple seed. The power output was to be determined off four different installation angles [ and a constant velocity of 10 m/s. The simulation results are compared with the experimental data from University Duisburg-Essen s laboratories, a small wind tunnel test, for validation. The testing tunnel size was 12 cm in diameter and the located turbine profile was approximately near that size. Plexiglas was used as the material to craft this profile shape out of, with an approximated thickness of 1.2 cm. The simulations were carried out in Star-CCM+ v and the design was created with Solid Works and meshing was done in Gridgen V Overview The structure of this study is as follows. Information and detailed history of wind turbine origins are stated at the first section of chapter 2. The following section explains the different types of flows. At the end of chapter 2 the experimental set-up as well as theory of the programs used and detailed information on how the simulations were made is explained. The closing chapter summarizes and draws a conclusion based also on these results. The results of the simulation and experimental testing, and a comparison is also stated at the closing chapter.

10 Theoretical Background 7 2. Theoretical Background 2.1 Wind Turbines The use of wind energy is not newly discovered. Simply, it was only easier to find other more powerful means of energy rather than looking for alternate ways. The procedures used to create energy from fossil fuels are slowly coming to a stopping point. Most countries have realized the hazardous effects of these fuels. According to the UK HM Treasuries Stern Review posted in 2006 the effective mitigation of climate change will require deep reductions,up to 60-80%, in greenhouse gas emissions by Also the near depletion of fossil fuels is another reason for the search of an effective and reliable energy sources. Wind power all starts with the sun. When the sun heats up a certain area of land, the air around that land absorbs the heat. After a certain temperature and due to the fact that hot air is lighter than cold air, the heated air rises. When the hot air rises, immediately the cooler air flows in to replace the free space and hence it creates wind. Wind turbines use the kinetic energy of the wind to generate electricity. In order to capture this energy and convert it to electrical energy, one needs to have a device that is capable of touching the wind. This device, or turbine, is usually composed of three major parts: the rotor blades, the drivetrain and the generator. The blades are the part of the turbine that touches the wind and rotates about an axis. Extracting energy from the wind is typically accomplished by first mechanically converting the velocity of the wind into a rotational motion of the wind turbine by the means of the rotor blades, and then converting the rotational energy of the blades into electrical energy by using a generator [5].

11 Theoretical Background Origins The earliest example of windmills was first designed by the Greek engineer Heron of Alexandria in the 1 st century. It is also claimed that the Babylonian emperor Hammurabi had planned the use of wind powered machines due to his interest in irrigation projects around the 17 th century B.C. Figure 1: Heron's Windmill Design. It is not entirely clear where the European s have seen the windmills, but it has been said that the Chinese were using windmills for draining rice fields. Some people speculate that the European s have learned about windmills from the Persians. The first verifiable information about windmills in Europe is that they were used in the Duchy of Normandy around the 1180 s [6]. Later on, Netherlands used these windmills and made improvements on them. There are several kinds of windmills; The first one is the Post mill. As the name suggests the mill s main structure was balanced on a large upright post so that it could turn to face the wind. The milling machines were then stationed inside the hub. These windmills had four sails and they were usually covered with fabric.

12 Theoretical Background 9 The other type of windmills used in Europe was Hollow-post mill or Wipmolen. For this type of windmill, the body mounted is hollowed out to accommodate a drive shaft. This type of windmill was used in the Netherlands first to drain the wetlands, later for graining and wood cutting. Tower windmill was widely spread in the Mediterranean regions. This structure had no yaw, so it could only be turned in the direction of the wind manually. The most advanced windmills were a type called the Dutch windmill. The entire housing was a fixed structure, and only the top cap could rotate where the sails were located. This allowed for the building of larger and more powerful windmills. The model could support various machines like grain millstones and heavy pan grinders. Naturally after the invention of steam engines the number of windmills started to decrease all over Europe. Some new wind wheel ideas were thought of during the Renaissance period but it wasn t deeply considered until Wilhelm Leibniz, who proposed numerous impulses for the construction of windmills. Bernoulli later on applied his recently formulated basic laws of fluid mechanics to the design of windmill sails. Also Euler did some research on the windmill sails. He was the first to calculate the twist of the sails [7]. The turning point from wind wheels into modern wind turbines is marked by the Danish scientist Poul la Cour. He was a pioneer of electricity generation by means of wind power generation in the 19 th century. He did many researches on how to make the wind wheel more aerodynamic, and tested some of his ideas in a wind tunnel which he had built himself. La Cour later on started building wind turbines, which were widely used until the end of World War I, but afterwards the construction decreased due to the fact that the oil prices became cheaper. However, the dynamic properties of his wind turbines were not so high and needed to be worked on. This job was taken gladly by many countries including Germany and U.S.A. Since the basic parts of a wind energy converter were known, the different types of wind turbines were obtained mostly by the changes made in the wind rotor. However, the wind rotor is not the only component. Gearbox, generator and control systems are just as well necessary to change the rotational energy into electricity. There are two main types of wind rotors, Vertical Axis Rotors and Horizontal Axis Rotors. Vertical Axis Rotors were the earliest designs in wind turbines. These wind turbines were not able to successfully use aerodynamic lift, until recently. French engineer Darrieus modeled a new type of Vertical axis rotor known as the Darrieus Rotor. This model was hard to build due to its shape and had no real advantages

13 Theoretical Background 10 with respect to horizontal axis rotors. The production costs were higher and the power output could not be controlled by pitching the rotor blades. Another type for vertical axis rotors is the Savonius design. This model is especially used for driving small water pumps, but it has no real electricity generating value. Figure 2: Types of Vertical Axis Rotors. Horizontal axis wind turbine (HAWT) is the common equipment in wind turbine generator systems in recent years. This is due to the fact that in propeller designs, rotor speed and power output can be controlled by pitching the rotor blades about their longitudinal axis. Rotor blade pitching is the most effective protection against over speeding and extreme wind speeds. Another reason for the widely use of HAWT is the rotor blade shape can be aerodynamically optimized. This is an advantage because it can be optimized to achieve the highest efficiency, which is known to be when the aerodynamic lift is exploited to a maximum degree.

14 Theoretical Background 11 Figure 3: Horizontal Axis Wind Turbine. Every part of a wind turbine, even the oil can, be changed to have a better power output but the most significant one is the rotor blades because of the aerodynamic structure. Some description of the parts of a Horizontal Axis Wind Turbine is shown in Figure 4. This figure also shows that the gearbox and the generator are stationed in the nacelle of the turbine. The gearbox and the generator of a Vertical Axis Wind Turbine are stationed at the base of the turbine. Figure 4: Detailed drawing of the parts of a HAWT.

15 Theoretical Background Aerodynamics of a Wind Turbine The most effective change that can be done to the wind turbine is finding different shapes for the rotor. The capability of the rotor to convert a maximum proportion of the wind energy flowing through its swept area into mechanical energy is obviously the direct result of its aerodynamic properties. With different shapes come different properties. The German aerodynamicist Albert Betz has formulated the physical laws of energy conversion and the theory of wind rotor. Betz published writings in which he was able to show that by applying elementary physical laws, the mechanical energy extractable from air passing through a given cross sectional area is restricted to a certain fixed proportion of the energy or power contained in the air stream. Betz s simple momentum theory, which assumes an energy converter working without losses in a frictionless airflow, is based on the modeling of a two-dimensional flow through the actuator disc. The airflow is slowed down and the flow lines are deflected only in one plane. Although it contains simplifications its results are quite usable for performing rough calculations. Figure 5: Flow Model of Betz's Momentum Theory.

16 Theoretical Background 13 To understand the Betz s momentum theory thoroughly, here are some background calculations. The kinetic energy of an air mass m moving at velocity v can be expressed as: Considering a cross sectional area A, which the velocity v passes, the volume flowing through a certain time unit, the so called volume flow is: And the mass flow with the air density : The equation expressing the kinetic energy of the moving air and the mass flow yield the amount of energy passing through the cross section A per unit time. However, as shown in Figure 5 the velocity before the converter ( ) and the velocity after the converter ( ) are different. That is why we must consider the conditions before and after the converter. Thereby changing the power P to: Maintaining the mass flow (continuity equation) requires that the mass flow before and after the converter is equal. Thus; From this equation it is shown that the maximum power is reached when is equal to zero. However, this does not make sense physically. That would mean that the velocity before the converter also has to be zero.

17 Theoretical Background 14 Using the law of conversation the force that air exerts on the converter can also be calculated. From the force we can calculate the power With respect to this equation we can see that the flow velocity through the converter is equal to the arithmetic mean of and : Mechanical power output of the controller can be expressed as The reference power for this output is the free-air stream power across the same cross-sectional area but without the extraction of the mechanical power of the converter. The ratio between the mechanical power output and that of the undisturbed free-air stream is the power coefficient. According to Betz, the maximum ideal power coefficient should be Since Betz was the first to derive this value it is called the Betz factor. This value is the maximum fraction of the power that can be extracted from the wind flow. To calculate this value Betz had certain assumptions, the first one was that the rotor does not have a hub, so an ideal rotor with infinite number of blades that does not have any drag. Second, the flow would be incompressible so the density would be constant, and heat transfer between the rotor and flow would not occur. Third, the rotor has to be massless and the flow in and out of the rotor has to be axial. This

18 Theoretical Background 15 proves that the Betz factor is just an ideal coefficient. Reaching this value even with today s technology is hard because of the other constraints. Today, the designing of an airplane wing or a rotor is simulated and experimented upon by inspecting the airfoil cross-section of a blade element. If a horizontal blade element is cut by a vertical plane parallel to the centerline the resulting section is called airfoil section. The generated characteristics (generated lift and stall characteristics) of the blade depend strongly on the geometry of the airfoil sections that make up the blade [8]. Figure 6: Figure of airfoil and the affecting forces. The motion of the air around the blade section produces pressure and velocity variations which produce the aerodynamic forces and moments. It is possible to use the equations of the motion for an inviscid flow to determine the pressure distribution around the vehicle and hence the velocity of the air particles at the edge of the boundary layers. For many high-reynolds number flows, the flow field may be divided into two regions: (1) a viscous boundary layer adjacent to the surface of the rotor and (2) the essentially inviscid flow outside the boundary layer. The

19 Theoretical Background 16 Laminar boundary layer is a relatively thin layer with limited mass transfer and a low velocity gradient near the wall can be seen. There is low skin friction in this boundary layer. In Turbulent boundary layer the layer is thicker with considerable amount of mass transfer, and with higher skin friction and higher velocity near the surface [9]. Figure 7: A sketch of the boundary layer on an airfoil. The shear and pressure forces can be resolved as the lift and drag forces along the axis as shown in fig. 6. The lift force is the force component acting upward, perpendicular to the direction of the undisturbed free-stream velocity. The aerodynamic lift is produced primarily by the pressure forces acting on the vehicle surface. The drag force is the net aerodynamic force acting in the same direction as the undisturbed free-stream velocity. The aerodynamic drag is produced by the pressure forces and by skin friction forces that act on the surface. The lift and drag coefficients of an airfoil are functions of the angle of incidence and fig. 8 shows the curves for a typical airfoil, the drag being drawn to five times the scale of the lift. The lift coefficient varies linearly with the angle of incidence for a certain range and then attains a maximum value at the critical angle of incidence. The important working range of an airfoil is represented by the linear part of the lift curve and in this range the drag is small compared with the lift, but on approaching the critical angle the drag increases rapidly.

20 Theoretical Background 17 Figure 8: Angle of incidence. After the critical angle of incidence of the airfoil is exceeded, stall starts to occur. The stall occurs when the airflow separates from the upper side of the airfoil. The Reynolds number dependency becomes smaller. The Reynolds number dependency is related to the point on the airfoil where the boundary layer transition from laminar to turbulent flow occurs [Fig. 7]. The way an airfoil stalls is very dependent on the geometry. Thin airfoils with sharp nose tend to stall more abruptly than thick airfoils. There are different stall behaviors and the explanation lies in the way the boundary layer separates from the upper side of the airfoil. The behavior of the viscous boundary layer is very complex and depends on the curvature of the airfoil, the Re number, surface roughness and for high speed also on the Mach number. During the past decades, dynamic stall phenomenon has received a great deal of attention of aerodynamicists and researchers from different research institutions all over the world. Methodologies, through which the dynamic stall is represented, mainly include three categories: experimental approaches, various semi-empirical models, and computational fluid dynamics solutions.

21 Theoretical Background 18 Farren [10] first carried out experiments on various British airfoils in the 1930s to measure the dynamic stall phenomenon. In the 1970s and the early 1980s, Mc.Alister and McCroskey did notable experimental dynamic stall studies on different airfoils. Schreck and Helin were the first to provide a visual representation of DSV (dynamic stall vortex) using dye in a water tunnel and a detailed set of surface pressure measurements on a finite wing [11]. 2.2 Laminar and Turbulent Flows Experiments in flow pipes demonstrated that two different flow regimes exist laminar and turbulent. When laminar flow exists in a system, the fluid flows in smooth layers called laminae. A fluid particle in one layer stays in that layer. The layers of fluid slide by one another without apparent eddies or swirls. Turbulent flow, on the other hand, exists at much higher flow rates in the system. In this case eddies and vortices mix the fluid by moving particles tortuously about the cross section. Figure 9: Flow physics for a two-dimensional airfoil undergoing dynamic stall. The existence of two types of flow is easily visualized by examining results of experiments performed by Osborne Reynolds. In his experiments a transparent tube is attached to a constant-head tank with water as the liquid medium. The opposite end of the tube has a valve to control the flow rate. Dyed water is injected into the

22 Theoretical Background 19 water at the tube inlet and the resulting flow pattern is observed. For low rates of flow the pattern is regular and forms a single line like thread. There is no lateral mixing in any part of the tube, and the flow follows parallel streamlines. This type of flow is called laminar or viscous flow. As the flow rate of water is increased beyond a certain point, the dye is observed not to follow a straight threadlike line but to disperse. The dyed water mixes thoroughly with the pipe water as a result of erratic fluid behavior in the pipe. This type of flow is called turbulent flow [12]. So the transition from the laminar to the turbulent state occurs if the momentum exchange by molecular transport cannot compete sufficiently effectively with the transport due to macroscopic fluctuations in flow velocity. Reynolds argued that the transition from the laminar to the turbulent state occurs when a dimensionless parameter exceeds a certain critical value [13]. The Reynolds number: Where and v are the fluid density and viscosity, respectively and V and D are the typical velocity and size scale of the flow, respectively. For straight circular pipes the flow is always laminar for a Reynolds number less than about The flow is usually turbulent for Reynolds numbers over However, the situation is also more complex than what was originally presumed by Reynolds. For instance, the numerical value of the critical Reynolds number depends on the flow and a number of other factors such as the initial disturbance level. Figure 10: Laminar and Turbulent Flows.

23 Theoretical Background Numerical and Experimental Setup In this chapter a brief introduction to the simulation programs that were used to do this thesis is going to be given, as well as how it was done. The general programs that are used to do are; Gridgen a meshing toolkit and for the simulation part Star- CCM+ is used. The experimental part which took place at one of University of Duisburg Essen s laboratories is also shown in this chapter Gridgen Gridgen is an interactive, graphical software package used to create 2D and 3D quadrilateral, hexahedral, triangular, and tetrahedral grid meshes and finite element models. Gridgen is employed in the construction of hybrid meshes as well, through the use of prisms and pyramids. In this purpose, it serves as a preprocessor to Computational Fluid Dynamics and Finite Element Analysis [14]. A database model from Solid Works was imported to Gridgen V15.10 R1. This allowed the drawing of the outlining connectors for the project easier, thus making the meshing easier as well. The most important aspect that had to be looked out for was the meshing that had to be done around the turbine rotor. This area had to be done with fine-structured meshing in order for the simulation to give the desired results. The meshing can be thought of as the process of replacing a continuous medium like air, water or metal with finite number of pieces. These pieces, called cells, can be any shape. It s a lot easier to solve CFD problems by dividing the domain into these small cells. Figure 11: The Database imported from Solid Works.

24 Theoretical Background 21 In our case there are in total of 605 connectors, 477 domains, in which 44 are unstructured, and 118 blocks, in which 25 are unstructured. The amount of structured meshes is greater than those of unstructured meshes since it is mostly accepted that structured meshes are higher quality than unstructured meshes but it is really hard to construct in hard topologies. From the above given database the connectors were drawn using the create connector on DB (database) entities tool. This gave the basic shape of the turbine rotor and the 120 arc that it is in. The arc is only 120 because it is unnecessary for all of the rotor blades to be drawn, when it could be done in Star-CCM+ digitally. In the process of drawing the rest of the connectors dimensioning had to be taken seriously. Dimensioning the connector s shows at which point a node has to be situated. The distance between these nodes had to be arranged so that the meshing would flow continuously. The distance between the nodes had to be equal. This dimensioning process is needed for the meshes to be structured. In general, when there shouldn t be a denser area switching into a less dense area. This would mean an error in the simulation part. Figure 12: End top side view

25 Theoretical Background 22 The designing of the domains, which are made by combining four dimensioned connectors if structured or just combining the connectors to form a closed area for unstructured, had to take the best skweness( ) into account. The skewness around the rotor blades also had to be especially at the best ratio. This area is the most important part since the fluid flow is going to be thoroughly examined in this part for that reason even the structured meshes had to undergo some methods to better the mesh. There also ways to improve the quality of structured grids by applying Gridgen's elliptic PDE methods. These methods iteratively solve Poisson's equation. While the defaults have been set to provide the nominal grid, the control functions can be fine-tuned at any time using the following techniques [15]: LaPlace (smoothness) Thomas-Middlecoff (clustering) Fixed Grid (smoothness) von Lavante-Hilgenstock-White, and Steger-Sorenson (orthogonality). Figure 13: The rotor domains' skewness.

26 Theoretical Background 23 The blocks are created after the domains. Domains can be considered as surface areas, and blocks as the volume. Blocks are created by combining six domains, if structured, together. When a block is made Gridgen automatically fills the blocks with grids according to the grid structure of the domains. If an unstructured domain exists inside a block, the block has to be unstructured as well. In this case the volume just has to be closed and for the blocks to work another action has to be taken, where we define to the program that these blocks also need to be filled with meshes. Blocks are needed to define the volume conditions, and the domains to define the boundary conditions. These conditions are defined for the comfort of the user, so they are highly needed. Boundary conditions define the velocity inlet, pressure outlet, periodic areas, etc. and volume conditions define the blocks in one area in general. This helps the user to see which area is the inlet and outlet. It also helps to do some specific changes in Star-CCM+ such as the transformation or rotation of one volume condition. If these conditions are not met problems would occur in the simulation part. The simple task of just rotating on area would be more complicated. Velocity inlet, pressure outlet, rotational part and wing section are the four volume conditions for this work. The rotational part is the area where the turbine will rotate when the simulation starts. The wing section is the circular area which covers the rotor blade and this part is inside the rotational part. Wing section had to be circular in order for us to change the angle of the turbine so that the meshes wouldn t overlap. Figure 14: The Volume Conditions

27 Theoretical Background 24 The four volume conditions as shown in the figure above also had different boundary layers. The boundary layers also needed to be separate even if they defined the same thing, each area had to have their own pressure outlet wall, which is the outer arc. Also there are in total of 6 periodic areas. These are the areas which can be considered left and right parts of the triangular section of the design. The left side is considered periodic left and had to be separated by for the three different areas used, and the right side is considered periodic right also as for the areas used. This periodicity allows the design to be completed to a 360 in Star- CCM+. After the VC and BC (volume and boundary conditions) were specified and all of the meshes generated, the designing process was done. The output of the file was made as a fluent model, which was specified at the startup of Gridgen so that Star- CCM+ would be able to open it. Analysis S/W command of the program contains the type of the model, whether it is a 2-D or 3-D model, and the output format of the mesh design is also arranged here Star-CCM+ Star-CCM+ is an intuitive and modern analysis environment that guides users effortlessly through setting up, running and analyzing complex engineering simulations. The program has an easy to use and practical interface. The main usage of this program is mainly computational fluid dynamics. The program s main advantages are: CAD & PLM integration, fully integrated with most common design programs such as Pro-Engineer, SolidWorks and Gridgen; Built-in meshing technology, polyhedral meshing, which also will be used later to see the results; Intuitive simulation user environment; Multi-disciplinary solutions; and Engineering analysis, especially for fluid mechanics department [16].

28 Theoretical Background 25 The program has the capability of solving the most complex engineering problems, with a good accuracy. For this the program has a wide range of usable physical models. These physical models have to be determined by the user, and chosen at the beginning of the simulation. The physical models cover time, flow, motion, regime and even heat transfer and combustion. The physics values used for this simulation were discussed with supervisors and the conclusion was that; Simulation Properties Explanations Continua Physics 1 Models Constant Density Gas: Air with density kg/m^3 and dynamic viscosity of E-5 Pa-s Implicit Unsteady Laminar Segregated Flow Three Dimensional Reference Values Initial Conditions Reference Pressure: Pa 1 Pressure: 0.0 Pa 1 Velocity: [0.0, -10.0, 0.0] m/s 2 Table 1: Continuum Physic Models and Definitions. 1. Default Values in Star-CCM+ 2. Velocity is not negative; the minus sign defines the direction of the flow.

29 Theoretical Background 26 The model constant density was chosen because there is no change of density happening during the experiment. Air cannot be treated as a steady flow; it has to be regarded as implicit unsteady. In some cases of an airfoil simulation the model turbulent is chosen instead of laminar. The reason for this is because of the critical Reynolds number is 5*10 5. If we look at the Reynolds number equation The critical length l can be calculated with the equation: Where is fluid viscosity, the fluid density and the velocity. If the critical length is greater than the length of the airfoil ( ) the flow is considered to be laminar. If the critical length is smaller than the length of the airfoil ( the flow is considered to be turbulent. So we can calculate as The length L of the airfoil can be withdrawn from the drawing itself which is. This makes the case of true, so the flow can be considered to be laminar. The airfoil s length is smaller than the laminar boundary layer, and the flow cannot convert into a turbulent flow.

30 Theoretical Background 27 Figure 15: The length of the airfoil The velocity, pressure and other properties of fluid flow can be functions of time (apart from being functions of space). To determine if a flow is steady or unsteady, these properties are taken into consideration. If a flow is such that the properties at every point in the flow do not depend upon time, than the flow is considered to be steady. Mathematically speaking for steady flows, Where P is any property like pressure, velocity or density. Unsteady flow is one where the properties do depend on time [17]. Turbulent flows are unsteady by definition. The implicit unsteady approach is appropriate if the time scales of the phenomena of interest are of the same order as the convection and/or diffusion processes (for example, vortex shedding) or are due to some relatively low frequency external excitation (for example, time-varying boundary conditions or boundary motion). With implicit unsteady approach you are required to set the physical time-step size, the Courant number, and the number of inner iterations to be performed at each physical time-step [18]. The Segregated Flow model solves the flow equations (one for each component of velocity, and one for pressure) in a segregated, or uncoupled, manner. The linkage between the momentum and continuity equations is achieved with a predictorcorrector approach. This model has its roots in constant-density flows. Although it is capable of handling mildly compressible flows and low Raleigh number natural

31 Theoretical Background 28 convection, it is not suitable for shock-capturing, high Mach number and high Raleigh-number applications [19]. After the correct continuum models have been chosen, the next step is to configure the rest of the simulation properties. Some specifications were already given as boundary and volume layers in Gridgen. Due to that after the imporing was done to Star-CCM+ the regions were already separated as desired. The properties of these regions are as shown in Table 2. Simulation Properties Explanations Regions Wing Section Overview Covers the Ahorn-Samen (Turbine Rotor) section of the design and the general interface section. The interface between the wing section and rotational part. Pressure Outlet Rotation Part Physics Values Section Overview Boundaries Periodic Left & Right Pressure Outlet Wall Section Overview Where the motion of the wing part can be chosen as stationary or rotation. If for a rotational part a stationary motion is chosen, the reference frame can be arranged to be for rotation. The section which is considered the end part of the shape. The part which shows the pressure outlet. The boundaries which were specified as periodic are chosen and made clear to the program that it will be periodic. Physics Condition: Slip in order for the program not to recognize the part as a wall. The middle part which covers also the wing section. It has interface with all other sections. The periodicity and wall physics are the same as pressure outlet region.

32 Theoretical Background 29 Velocity Inlet Section Overview It is the inlet of the shape. The direction of the velocity is determined by the velocity physics values. The periodicity and wall physics are the same as pressure outlet region. Table 2: The Different Regions and Their Explanations. The figures 16 and 17 show the periodic regions and outer wall parts. As explained on the table above the outer wall has to have slip condition for the program to not to recognize it as a normal wall. Otherwise the data would change and the results would not be symmetric. Figure 16: Periodic Parts of the Model. Figure 17: Outer Wall with Slip.

33 Theoretical Background 30 The wing section is the part that contains the turbine rotor. This part has the option to be rotated using Star-CCM+. To compare results the rotors were rotated for four different angles ( ). However these angles are not the angle of attack but they are the angle of installation. The angle which if the turbine was built the rotors would have directly from construction. The results from four different simulations were than taken into account for the experimental setup. This simple action of rotating the wing part was also already concluded in the designing of the mesh grids. The rest of the simulation properties were all done so that the rotor blade would have rotational motion. These properties can be seen in Table 3 below. Simulation Properties Interfaces Explanations There are in total of six interfaces created. Three are in-place interfaces, which are in between the four different areas (velocity inlet, pressure outlet, rotational part and wing section). The other three interfaces are periodic interfaces. Derived Parts Solvers The parts which are made by the user. They show the simulation area. There are five derived parts created to determine the results. One part is a cylindrical section that covers from velocity inlet to pressure outlet. The other four parts are on the wing section with different distances (0.03m-0.05m-0.07m-0.08m). These parts are necessary to get out the lift and drag coefficients at different lengths of the rotor. The solvers are the implicit unsteady solver, in which the time-step is determined to be s, and segregated flow solver. Stopping Criteria Reports Stopping criteria as can be understood from the name, is the property where the maximum inner and maximum iteration number is decided. Reports are created in order to plot the desired force values in a certain direction and part. The reports were created for drag coefficient, lift coefficient, moment and moment coefficient. The

34 Theoretical Background 31 results of these reports are plotted. Monitors Reports are defined here to be simulated through the simulation iterations. Plots Residuals and charts depending on the reports/monitors were generated. An additional pressure coefficient plot was generated for ever derived part. Scenes Tools Scalar and vector scenes were generated to view the results. Rotation with 2500rpm and 300rpm is added to motions bar. The periodicity is also arranged here with the transforms bar. It is decided on how many times the periodic should continue, in this case 2 times. Table 3: Simulation Properties. By changing the simulation properties, the plots for desired coefficients were created. With the data the lift and drag coefficients were calculated. After the desired number of iterations was reached, by the help of derived cylindrical section symmetry was controlled, this helped to check if the periodic parts were working as desired. A vector scene was created to check the initial directions. This scene creates an arrow like structure on the desired parts such as velocity inlet, the ahorn samen or pressure outlet. The rotation of the blade is also controlled with this scene. The rotation speed was set as 2500rpm for the beginning iterations, than changed to 300rpm. For normal 12 meter diameters, this speed is not reachable other. The experimental setup would only allow for a 12 cm diameter blade. Due to that the rotational speed was calculated with 2500 and 300(rpm).

35 Theoretical Background Experimental Setup Figure 18: The turbine profile made out of maple seed. The experimental work took place after the simulations were finished. The experiment took place in one of the University of Duisburg-Essen s Essen laboratories. The rotor blades were cut from a Plexiglas material, each blade with the length of 6 cm. A cylindrical pipe not much wider than the blades combined was connected to an air motor which gave out the desired the air at the desired velocity. The desired value was determined by a turbine meter. The rotor blades were connected to a nacelle-like structure (see Fig. 18), which also was connected to a generator. The ammeter, voltmeter and resistor were connected to generate as shown in Fig. 19 below. Figure 19: The design of the experimental setup. The resistance was changed manually, starting from 50Ω until 500Ω. With each resistance a different voltage was calculated. The rotor blades were first run without any resistance. This was done to find out the unloaded revolution speed of the blades. The blade revolution was determined by using a stroboscope.

36 Theoretical Background 33 Figure 20: The experimental Set-up. The Figure 20 shows the testing tunnel for experiment. The air motor is at the end and blows air with a magnitude of 10 m/s. From a hole near the end the center of the tunnel is measured and the speed is checked. The rotor blades are inserted at this location afterwards and screwed down for stabilization. Figure 21 shows the location and the ending of the pipe. The Turbine blades can be seen connected to the tunnel and to the voltmeter and ampermeter. The turbine rotates inside the tunnel with the incoming wind and shows how much voltage and current it creates. With this information known a calculation of the power output for different angles is made. General testing is done with normal edged profiles but as a comparison a round edged profile type is also tested. The round edged profile has shown to have better results than square-edged one, but since the simulation is done with only square-edged profile type in the comparison between the experiment and simulation the square-edge profile values are taken. Figure 21: The location of the rotor profile inside the test tunnel.

37 P [W] Results and Comparison Results and Comparison In this chapter the results from the simulation part and the results of the experimental work are explained. As it was previously stated the simulation and experiment took place in four different angles with same velocity and revolution speed. However, only the simulation results for 60 are explained since it was the best power output that was gathered. The other simulation results are added to the appendix section at the end. The power output of each angle starting from 30 and ending at 60 are shown in the Figures The power output of 70 could not be calculated with the results from the simulation. Power-rotational speed w [U/min] Figure 22: Power Output for 30 Degree Installation Angle.

38 P [W] P [W] Results and Comparison 35 Power-rotational speed w [U/min] Figure 23: Power Output for 40 Degree Installation Angle. Power-rotational speed w [U/min] Figure 24: Power Output for 60 Degree Installation Angle.

39 Results and Comparison Star-CCM+ The simulations are done with the same simulation properties which were described in chapter Maximum number of iterations for 60 is 5000 iterations for 300 rpm and 8000 iterations for 2500 rpm. These numbers of iterations are approximately the same for all the other simulations. The first thing to check in the simulations is the symmetry of the scalar scene of the velocity magnitude. The accuracy of the simulation can also be checked with this, if the simulation properties are wrong than the scene would not be symmetrical. Since the velocity is flowing from positive to negative y, the perspective to best look at these is from the positive x looking down onto negative z. As another control method the scene looking up from negative y at 90 degrees is also acceptable, since with this scene the pressure outlet can be seen from a straight point of view (see Figure 26). The scalar scene for a velocity magnitude is plotted upon the derived parts (see chapter 2.3.2). From the figure below it can be seen that the flow continues from the upper part to the lower part. This means that the periodicity has worked and that the program is calculating for three rotor blades. Figure 25: The velocity magnitude scene of 300rpm.

40 Results and Comparison 37 Figure 26: The velocity magnitude from negative y to plus z [the pressure outlet section]. From the figure above the velocity is marked with the same colors that show the same speed from left to right. Since the velocity is the same of the two sides it can be called that this is symmetrical if the scene would not be symmetrical than the simulation had to be started again with different properties. The above figure is from the pressure outlet view, which is the end of the geometrical shape. The vortexes that were created by the rotor have disappeared slowly. However if a closer plane section is made, Figure 27, the exact shape of the wind after it has made contact with the rotating turbine blade can be seen. The flow is distinctly shown in these figures to be continuing form left to right in motion, which also proves that the simulation is running periodically. After the simulation was evaluated and the accuracy checked, the rest of the information needed could be read from the other results that are calculated by the simulation. The air loads, lift, drag and moment coefficients, that are acting on the rotor are plotted from the reports created in the program. This is done for every angle of installation. The details for these plots were given at the beginning of the simulation, and because of how the rotor moved and the incoming way of velocity showed that the rotor was moving on towards minus z axis. For the reports of these plots to be correct the movement axis has to be defined before starting the simulation unlike other data.

41 Results and Comparison 38 Figure 27: Velocity magnitude display right after the turbine. The results show that it is not possible to get the data for the coefficients from a 45 degree installation angle. For 45 degrees the values for the coefficients do not stabilize at one point but instead they are changing with every iteration step. Figure 28 shows the average values which are the results of the reports for the other calculated installation angles, degrees. The lift and drag is important for the stability of the rotor (see chapter 2.2), because of stall. As it can be seen from the graphs on the Figure 28 the drag coefficient is rising as the installation angle rises. It reaches a maximum of 0.16 at 70 degrees, while at 60 degrees it is 0.14 and at 30 degrees it is The drag is minimum at 30 degrees. However, the lift is also minimum at 30 degrees, The lift coefficient for 60 degrees is the highest with approximately At 70 degrees the lift coefficient drops to This means that at 70 degrees the drag is more than the lift coefficient which would create a stall. The moment coefficient, which is not the moment Figure 28: Schematic diagram showing the air loads.

42 Results and Comparison 39 generated, also rises as the installation angle closes to 90 degrees. The moment coefficient for all the angles is a negative value. These are not the only coefficients that were calculated with the simulation. The pressure coefficient at different lengths was also calculated for all of the different installation angles. The derived parts that were created at 0.03, 0.05, 0.07, 0.08 meters were the base of calculating the pressure coefficients.

43 Results and Comparison 40 Figure 29: Pressure coefficient distributions of the HKAS airfoil for 0.03m, 0.05m, 0.07m, 0.08m and shows the streamline plots from CFD simulations for the corresponding distances [0.03, 0.05, 0.07, 0.08 meters]. The above velocity magnitude scenes are only for the wing section. They show the velocity change and distribution in different lengths of the rotor. The pressure coefficient distribution plots are from the Ahorn-Samen section, which means the pressure coefficients are from only around the rotor. These did not need to be made for the whole system since the changes that are most important are in the areas near the rotor. From the velocity magnitude diagrams we can see how the velocity is changed by the shape of the rotor. The rotor is not a straight figure but it is a bended figure which makes the location of the rotor from different cross-sections also different. The red are in the velocity diagrams, which is the part where velocity is highest, is changing according to the taken distance. The velocity around the tip can be seen to be higher than the velocity at the hub. Also the separation lines of velocities are distinctly seen (as the red color in the plots). A general streamline function starting from velocity inlet and ending at pressure outlet, which also goes over the rotor, is shown on Figure 30. The continuing figures (Fig. 31, Fig. 32 and Fig. 33) show the streamline of the other installation angles, which are 30, 45 and 70 degrees. The streamline function is different for every installation angle since it is different for every installation angle how the incoming velocity is affected. All of the following streamline functions are simulated for the rotational speed of the turbine blade as 300 rotations per minute.

44 Results and Comparison 41 Figure 30: Streamline results for 60 in the CFD simulation. In this figure it can be seen that a vortex is created after the rotor blade. This happens due to the pressure drop from the upper part of the rotor to the lower part. The vortex however dissolves as it continues on. Figure 31: Streamline results for 30 in the CFD simulation. Figure 32: Streamline results for 45 in the CFD simulation.

45 Results and Comparison 42 Figure 33: Streamline results for 70 in CFD simulation. The figures above show that the vortex is created in every installation angle, which is a sign that a good meshing was done to be able to show these vortexes. The streamline function change for every angle, and at 70 it is clearly depicted that the vortex is too great and does not stabilize as the other angles. 3.2 Experiment For the experimental work there was in total of eight different results for every angle twice, and also four measurements were made with a round edged rotor blade rather than a rectangular shaped one. The resistance was given by hand, but the values that were read were the current, voltage and with the use of a stroboscope the revolution speed was measured.first the revolution speed was read from the stroboscope, for every angle with no resistance at all. Afterwards the resistance was gradually increased starting from 50Ω and going up until 500Ω. All of the measurements were taken with the constant velocity of 10m/s. The results of the rectangular edged measurements are given at the table below. 30 with 10m/s. Resistance Free Revolution Speed: 668rpm [rpm] R[Ω] I[A] U[V] P[W]