Numerical and Experimental study of the effect of increasing aspect ratio of self-starting force to vertical axis wind turbine

Three airfoils NACA0015, NACA0018 and NACA0021 were selected and in the Q-Blade software, the coefficients of shear, post and the maximum ratio of the exponential coefficients were determined and, finally, the Airfoil Nakata 0015 at a speed of 5 and 10 meters per second, was selected and best. Fluent software was used to solve it, based on the finite volume method. For numerical analysis, the turbulence method K-ω SST was validated with experimental results. The wind turbine schematics was designed in Catia software and the height of the blades was 35 and 75 centimeters, the radius of the blade was 18.5 cm and the length of the airfoil 6.4 Cm is. The results show that in order to operate the turbine of porous blade windings at 35 cm height at speeds of 1, 2, 3, 4, 5, 7, 7.45, 8.25, 8.5 m / s 50% 50%, 33%, 50%, 50%, 60%, 57%, 55%, 50%, and for setting up a porous blade wind turbine at a height of 75 cm at a speed of one, two, three, four, Five, seven, 7.45, 8.25, 8.5, 9, 9.5 m / s 66.6%, 75%, 80%, 71.4%, 66.6%, 76.9%, 80%, 82%, 89%, 100% of the launching force Smooth wind turbine is required at the same height.


Introduction
The choice of energy for electricity generation depends on the policies of each country. France generates 75% of its energy needs from nuclear power plants, while China and the United States depend on coal and oil for 65.2% and 37%, respectively, to produce most of their energy needs. [1] and make the United States and China the largest producers of greenhouse gases in the world, and with the enactment of strict environmental laws and the energy crisis, industrialized countries take renewable energy seriously [2].
The wind turbine uses kinetic energy and converts it into mechanical power and then electrical power. Rotor design and aerodynamic performance of turbine blades are very important and it is impossible to expect maximum efficiency without optimization operations [3]. One of the reasons for the low progress of vertical turbines compared to horizontal axis turbines is their inability to start up [4].
In a 2010 study, Howell et al. Concluded that increasing the number of blades reduces turbine efficiency, and that three blades have a higher coefficient of performance than other blades [5]. Leicher et al. (2010) showed that the combination of Darius and Savinius wind turbines could help to better launch the Darius vertical axis wind turbine [6]. In 2017, Duak et al. Concluded that the three-blade Darius wind turbine was capable of high initial start-up at lower wind speeds [7].
The wind turbine blade plays a very important role in the efficiency and output power of the wind turbine and several researches have been done in the field of wind turbine blade optimization that Fogelsang and Thomson, Benini, Tufalo, Hu and Rao optimize the wind turbine feature. Such as chord, ball, number of blades and rotor speed [8,9,10].
In this study, the effect of increasing the visibility ratio in porous and direct blades on the driving force of Darius vertical axis wind turbine is investigated. To porous the blade space, rhombus-shaped embossed sheet was used and for straight blade, a simple aluminum sheet with a thickness of 0.3 mm with a thickness equal to embossed sheet was used.

Choose airfoil
To select Darius vertical axis wind turbine blade airfoil, three airfoils NACA0015, NACA0018 and NACA0021 were selected and in Q-Blade software, the coefficients of lift, drag and maximum ratio of lift to drag coefficient were obtained, and finally, the airfoil at speeds of 5 and 10 M/s is selected to have the maximum ratio of the coefficient of elevation to the coefficient of high drag.

Solution theory and governing equations
The equations governing airflow in a Darius vertical axis wind turbine are the same as the equations of mass survival or continuity and the magnitude of motion, or the equations of momentum.

Numerical simulation
Due to the complexity of the airfoil geometry, in order to improve the quality of the networking, the computational range was divided into four areas and each area was networked separately. This has increased the quality of the grid, especially near the airfoil surface, which has improved the orthogonal nature of the grid lines on the airfoil surface at the attack edge. The geometry of the blade cross section and the airfoil meshing are done in Ansys and the created network is structured and a C-shaped computational amplitude is used around the airfoil.

Conclusion
The aim of this study was to investigate the effect of aspect ratio in a porous and straight blade on the driving force of a vertical axis Darius wind turbine. To implement this design, Darius vertical axis wind turbine with straight and porous blades has been designed, built and tested. Experimental experiments have been performed on a four-fan blower at speeds of one to 10 meters per second. Numerical results are also obtained in Fluent and Q-Blade software for analysis of airfoil and turbine blades and the results show the improvement of aerodynamic properties of porous blades compared to straight blades. [1]