Design of wind turbine hybrid airfoil for low Reynolds numbers (panel numerical method, CFD and experimental test in wind tunnel)

Dissertation for M.Sc master's degree

Mechanical engineering - energy conversion trend

Abstract

In the last century, with the increasing demand for energy and the reduction of fossil fuel resources, the role of renewable energy in the progress and development of countries is not hidden from anyone. In the meantime, wind energy has a special share and has the highest growth rate among other types of renewable energy. Horizontal axis wind turbines play an essential role in the production of energy and wind power, and many researches and financial resources have been carried out to advance their design and optimization. In recent research, an airfoil (wind turbine blade cross-section) has been designed by a combined method to work in low Reynolds conditions, and it has been studied and aerodynamically analyzed by three different methods. This work started with a brief introduction about wind turbines and continued in detail to the background and work records of researchers and institutions that have researched in the field of airfoils. Then, for the design, a combination of NACA 63-1015 upper surface and Wortmann FX63-168 lower surface, both of which are from the family of airfoils used in wind turbines, have been used. After that, it has been studied using numerical panel method based on the linear distribution of vorticity, Computational Fluid Dynamics (CFD) method and experimental test in the combined airfoil wind tunnel. For panel method, computer code in FORTRAN language was used and CFD method was performed using Spalart-Almaras turbulence model by FLUENT6.3.26 software. The experimental test was also carried out in the wind tunnel of the Islamic Azad University, Majlesi branch. The obtained results have been compared with previously designed airfoils for low Reynolds conditions in authoritative references and some conventional airfoils for use in wind turbines that have been simulated in CFD due to the unavailability of detailed information. Based on the obtained results, the recent combined airfoil has a very high ability to provide a suitable starting torque for low Reynolds turbines due to its high drag coefficient. It has also shown a very good performance in the predictions of the performance coefficient due to the appropriate ratio of forward to backward. In addition to the general analysis of wind turbine airfoils, the ability of different methods to predict the aerodynamic loads of wind turbine airfoils has been investigated, and the role of changing the Reynolds number in reducing or increasing the coefficients of aerodynamic loads has been investigated. Based on this, the panel method is only able to predict the drag coefficient with a percentage of error at low attack angles, and it is practically closed in calculating the drag coefficient. This method also does not have the ability to check the effect of Reynolds number. Based on CFD studies, the Spalart-Almaras model has a high ability to simulate the airfoil flow of wind turbines and it will encounter a slight error only at very high angles of attack. Based on experimental tests and computational fluid dynamics, with an increase in the Reynolds number, the drag coefficient will increase and the drag coefficient will decrease, as a result, the performance coefficient of the wind turbine will increase with an increase in the Reynolds number.

Key words: renewable energy, wind turbine airfoils, experimental aerodynamics, numerical aerodynamics, wind tunnel test, performance coefficient, computational fluid dynamics, effect of number Reynolds.

Introduction

The global energy crisis and the role of wind energy

With the increasing world population and decreasing fossil fuel reserves, energy has become a more important issue than before. The importance of the role of energy can be seen from the fact that it appears as a widely used word in all aspects of human life, from the family to the economy and politics.

At the beginning of the seventies, with the onset of the global oil price crisis, which was caused by the imbalance between discovery, extraction and market demand, the eyes turned towards alternative sources of energy. America, Denmark and Germany were among the first countries that invested in wind energy study [1]. Based on the statistical data of the World Wind Energy Association, this technology in 2009 has made a profit of about 50 billion euros and has employed more than 550,000 people worldwide [2].Figure 1-1 shows the total installed capacity of wind energy in the world in different years. As it is clear from this graph, in the previous decade, the growth of the use of wind energy is increasing at a very high rate.

Today, with the clarification of more efficient uses of oil, gas and coal, especially in developed countries, and considering the increase in the amount of greenhouse gases in the atmosphere and the issue of global warming, as well as due to the reduction of environmental pollution (chemical and thermal), the use of renewable energy has become a concern for countries. is It is interesting that in recent years, wind energy has taken the largest share of progress among other renewable energy sources [3]. In Iran, due to the increase in urban population and the resulting increase in energy demand, the use of renewable energy as a component of sustainable development plays a vital role [4]. Sustainable development means development in such a way that in addition to the progress of the current generation and meeting the current needs of society (social perspective) and man (individual perspective), it does not limit the ability and resources of the future generation [5]. Based on this, the advantages of using wind energy can be summarized as follows [6]:

It is available without spending any money.

It does not cause any chemical or thermal environmental pollution.

The cost of maintaining wind turbines in recent years has reached a very good balance compared to other power plant equipment.

It can be used in areas far from the main grid.

Iran is a suitable region for using wind energy due to its unique geographical location. It has been found that Iran is exposed to winds in both winter and summer that blow from the Atlantic Ocean in winter and from the Northeast, Central Asia, and in the summer from the Northwest, that is, around Iceland and Scandinavia, and from the South, that is, the Indian Ocean [7]. Figure 1-2 and Figure 1-3 respectively show the annual average energy and wind speed at a height of 20 meters and 40 meters above the ground in different regions of Iran [8]. Earth [8]

1-2- Wind turbines and their types

Wind turbines are equipment that absorb the kinetic energy of the wind flow and convert it into rotational energy of the rotor axis and then into electrical power [9]. These turbines include two main groups: vertical axis wind turbines[1] and horizontal axis wind turbines[2]. Vertical axis wind turbines are the first man-made wind turbines, but currently horizontal axis turbines have a much greater share in the production of wind power in terms of economy and production power. For this reason, most of the research in recent years has focused on horizontal axis wind turbines. Vertical axis wind turbines are usually used in places and conditions where horizontal axis wind turbines are not able to work in those areas or conditions. For example, in the conditions of very strong winds and with high turbulence [3], it is better to use vertical axis turbines [10]. In general, the advantages and disadvantages of vertical and horizontal axis wind turbines can be compared as follows [11]:

Vertical axis wind turbines do not need to face the local wind, while horizontal axis wind turbines must be aligned with the local wind flow.

Vertical axis turbines have less noise pollution than horizontal axis turbines because they are usually faster They work less frequently.

The cost of building vertical axis wind turbines is much lower than horizontal axis wind turbines with large, complex blades and with many changes in three dimensions due to the simplicity of their geometry.

Safety and working conditions for vertical axis turbines are more than horizontal axis turbines.

Performance factor for the turbine Horizontal axis turbines are much higher than the coefficient of performance for vertical axis turbines.