Numerical Study of The Airfoil Pitch Angle on The Darrieus-Savonius Combination Turbine

Turbin Darrieus banyak digunakan sebagai konverter arus laut untuk menghasilkan energi listrik terbarukan. Salah satu cara untuk meningkatkan performa turbin adalah dengan mencari sudut pitch optimal. Oleh karena itu tujuan penelitian ini adalah menentukan sudut optimal sudut pitch dari airfoil NACA 0018 dengan variasi sudut -6°, -2°, 0° dan 2°. Metode Numerik dengan bantuan software Ansys Fluent dilakuan untuk mengevaluasi turbin tersebut. Hasil penelitian adalah sudut 2° menghasilkan koefisin daya dan torsi terbesar dibandingakan sudut yang lain. Hal ini terjadi karena arah gaya angkat pada airfoil kedepan sesuai dengan perputaran turbin.


Introduction
Renewable energy is starting to be widely used in everyday life. This happens because of the awareness of the dangers of using fossil energy that can cause global warming so that there will be a future climate crisis. One of the renewable energy is ocean currents. This energy is available and will never run out. In some places there are ocean currents that can be used to generate electricity. The tool to convert energy from ocean currents into electrical energy is a turbine. Ocean currents turn a turbine, where the turbine drives a dynamo to produce electrical energy.
In this study, the turbine used is a vertical axis turbine and a combination of the Darrieus turbine and the Savonius turbine which has been studied previously [1]. Turbine optimization is carried out to produce greater torque and power than the previous design. One of the methods used is the optimization of the pitch angle of the Darrieus turbine blades. Similar research has been carried out, namely discussing the effect of the Darrieus turbine angle of attack on the lift and drag forces [2]. In this study, the effect of changing the pitch angle on the torque and power coefficient is studied. So that the angle that produces good torque or power is obtained. The pitch angle variations selected are -6°, -2°, 0°, and 2°.
The same research has been carried out on vertical axis turbine, combination of Darrieus and Savonius: M. Abid et. al [3] perfected the Darrieus turbine which was not effective in self starting by combining it with Savonius. The design of this combination turbine can be seen in Figure 1.

Figure 2. D-S Combination Turbine [4]
Research on the Darrieus turbine was carried out by Iwan Kurniawan [5] where the previously straight turbine blades were converted into a helix shape with variations in angles of 60° and 120°. The design of the turbine can be seen in Figure 3.

Figure 3. Darrieus Turbine Helix Blade [6]
Meanwhile, in this study, variations in pitch angle on the D-S combination turbine were carried out. Turbine model can be seen in Figure 4.

Method
The flow of research completion can be seen in Figure 4: The method used for this research is a numerical method with the help of Ansys Fluent software with 2D analysis.

Turbine Model
The turbine model can be seen in Figure 3 and the turbine size is in Table 1 below:

Table 1. Turbine Sizes
The paddle diameter is the diameter of the Savonius type turbine which is semi-circular. The paddle in Figure 4 is of the overlap type, with an overlap value of 0.4 m. The type of airfoil used is NACA 0018 which can be seen in Figure 5 below:

Figure 5. NACA Profile 0018
The model is made in four variations of angles, namely -6°, -2°, 0° and 2°. Next, we will do a mesh and determine the limits of numerical simulation.

Numerical Simulation
The Meshing used is triangles. The number of mesh in this simulation is 447,100 and 262,336 nodes. Numerical model and solver details can be seen in Table 2 below: Numerical simulation results will produce some necessary data such as power and torque produced by the turbine at a predetermined speed. These results will be used in the following Formulas 1, 2 and 3 to determine the performance of the turbine [6]:

Analysis of Numerical Simulation Results
The analysis was carried out on the simulation results from the variation of angles in the form of tables and graphs. And conclusions are drawn from these results.

Results and Discussion
The simulation results for turbines with variations in pitch angle can be seen in Table 2, Table  3, Figures 6 and 7.  Table 3.

Power Coefficient At Variation Of Pitch Angle
Gambar 7. TSP versus CT Chart With Pitch Angle Variation Based on the results of this study, it can be concluded that the airfoil that has the best performance is the one that has an angle of 2°. Figure 8 shows the static pressure contour on the airfoil with an angle of 2°.

Figure 8. Static Pressure Contour at 2° Angle
An angle of 2° produces lift towards the front of the turbine. This direction corresponds to the movement of the turbine which rotates counterclockwise. In contrast to the angle below 2° where the direction of the turbine is opposite to the turbine rotation. Based on the results of the study, it can be concluded that the angle of 2° has the best performance compared to the angle of -6°, -2°, 0°. This is because angle 2 provides a lifting force towards the front of the turbine in accordance with the direction of rotation of the turbine. As further research, it is recommended to increase the pitch angle variations so that a wider picture is obtained.