NUMERICAL APPROACH OF THE BLADE SHAPE AND NUMBER ON THE PERFORMANCE OF MULTIPLE BLADE CLOSED TYPE IMPULSE WIND TURBINE

An impulse turbine uses drag force on its blades to produce torque on its rotor. As fluid flows over the blades, pressure changes occur at the nozzle, which increases the fluid's velocity and reduces the static pressure at the nozzle outlet. The high-momentum fluid then impinges on the rotor blades, generating frictional force and resulting in torque production. To study the impact of blade shape and number on the turbine's performance, simulations were conducted. The results indicate that blades with an angle of 0° and 180° are optimal for creating high-pressure vortices on the concave surface of the blade. Addition-ally, more blades always result in higher torque and power out-put by increasing the active area of the blades. However, in the case of blades with an angle of 0° and 180°, 8 blades produced more torque than 12 blades with an angle of 0° and 90°. There-fore, blades with an angle of 0° and 180° are highly effective at generating drag force and producing torque.


INTRODUCTION
Indonesia has a vast potential for renewable energy, which amounts to 6,811.3 GW, consisting of 6,749.3 GW of solar power, 7,308.8 GW of pumped hydro energy storage, 6.3 GW of micro hydro power, 106 GW of wind power, and 30.73 GW of biomass power. This potential is much higher than the 443 GW stated in the National Energy General Plan document. It is more than enough to achieve deep decarbonization and reach the target of zero emissions by 2050. However, with the anticipated rise in electricity demand in the future, it is crucial to develop technologies to fulfill the growing needs [1] [2]. In order to meet the rising demand for electricity in the future, technological advances are required. This is particularly true given the growing popularity of electric vehicles, which has led to a need for improved battery and motor technology [3]. One innovative solution to this problem is the implementation of an "Energy Harvesting System," which harnesses the energy generated by the movement of the vehicle to produce electricity. Specially designed wind turbines can be installed on various parts of the vehicle to collect wind energy. However, it's important to select and design these turbines appropriately for maximum effectiveness.
Previous studies have explored the drag force present in impulse turbines by examining the effects of blade shape, size, and number under specific inflow momentum and turbine rotation conditions. Experimental research has found that the installation of a directional cylinder in front of the returning blade is an effective method for improving the performance of vertical-axis Savonius wind turbines, increasing the power coefficient [4]. The guide cylinder enhances the coefficient of moment by exploiting the synergy between drag forces on the returning and advancing blades [5]. The power coefficient can also be increased by installing the guide cylinder at a certain distance near the advancing blade, as evidenced by numerical simulations conducted by Setiawan et al. The flow acceleration that occurs in the gap between the cylinder and the advancing blade results in a stronger drag force on both blades.
Mahmoud et al. [6] conducted a study on Savonius turbines, where the parameters varied included the number of turbine blades (two, three, or four), the number of turbine stages (one or two), the aspect ratio ( ) of 0.5, 1, 2, 4, and 5, the overlap ratio ( ) of 0, 0.2, 0.25, 0.3, and 0.35, and the presence or absence of an end plate on the Savonius wind turbine [7]. The experimental results indicated that a two-blade turbine had higher efficiency compared to a three-blade and four-blade turbine. A two-stage turbine performed better than a single-stage turbine. A turbine without an overlap ratio performed better than a turbine with an overlap ratio. A turbine with an end plate had higher efficiency compared to a turbine without an end plate. An increase in the aspect ratio also led to an improvement in turbine performance [7].
The aim of the experimental study on Pelton turbines was to assess the power and efficiency produced by varying the number of blades and nozzle diameter. The nozzle diameter ranged from 0,5 to 0,75 inches, with the highest power obtained from 18 blades and a 0.5-inch diameter. Moreover, the research revealed that increasing the number of blades resulted in higher power and efficiency [8]. Additionally, the study explored different shapes of Pelton turbine blades, such as bowl and half-cylinder, and valve openings of 60°, 75°, and 90°. The findings indicated that bowl blades had greater efficiency and power than halfcylinder blades [9].
The purpose of this paper is to numerically model and simulate the performance of an impulse turbine and investigate the impact of the number of blades and blade shape on its efficiency. The study will evaluate the turbine's performance by analyzing the flow field pattern occurring in the rotor. The study uses 8 and 12 blades, and blades with semicircular curvature ( 1 = 2= 0° relative to the tangent to the rotor circle) and blades with curvature ( 1 = 0° and 2= 90° relative to the tangent to the rotor circle). Numerical and simulation approaches will be utilized to determine the optimal turbine performance.

METHODS
Numerical approach using computational methods applied in this research. There are three stages in this numerical computation method: the pre-processing, processing, and postprocessing stages. Pre-processing is the stage to prepare the simulation domain. Processing is the core stage in the simulation which is the iteration stage to get the final simulation result. The final results of the simulation will then be processed in the post-processing stage to collect qualitative and quantitative data which are then interpreted. The software used in this numerical computation method includes ANSYS 2021 R2 and ANSYS FLUENT.
In the pre-processing stage, the turbine geometry is made using Ansys Space Claim Geometry which is used for the simulation. Then after the turbine geometry is obtained, the center of the turbine is taken to be used as a simulation domain. The simulation domain will be meshed, and the boundary conditions are defined with ANSYS 2021 R2. In the processing stage, several conditions from the ANSYS FLUENT 21 simulation are determined, such as physical phenomena, fluid properties, solution method and re-sidual monitoring before running the simulation. The simulation results are then processed at the post-processing stage to collect quantitative and qualitative data such as velocity distribution, pressure con-tours [10].
The geometry of turbine and blade shape is shown in Figure 1 and 2. The variations used in this study are the number of blades in the turbine and the shape of the blades shown in Table 1, including the definition in Figure 3. The control variable used in this study is the similarity of the inlet velocity and rotor rotation for each variation.    After the turbine geometry is formed, meshing is carried out. The shape of the mesh used is a quad-rilateral mesh which shows in Figure 4 with the concentration of elements on the turbine blade. The boundary conditions for the simulation are determined where the flow will enter the turbine through the inlet and exit through the circular outlet in the middle of the turbine housing as shown be-low. The rest of the simulation domain geometry is considered as a flow barrier (wall) which is shown in Figure 5. The turbulence model is the standard k-∈ turbulence model, this model is suitable for free shear layer flow with a small gradient wall function. This turbulence model also uses less memory and produces good convergence values. Numerical simulation uses a working fluid in the form of air with a constant density of 1.225 kg/ 3 and dynamic viscosity constant of 1.7894 x105 kg/m.s . A stationary condition is given relative to the coordinate center, while for the rotating and blade domains, the mesh motion feature is selected with an angular velocity of 300 rpm. Boundary conditions are made as follows in Table 2.

Table 2. Simulation Boundary Conditions
Solution method on the pressure-velocity coupling is set using the coupled scheme. The spatial discretization used for pressure, momentum, turbulent kinetic energy, and turbulent dissipation rate is a second order upwind. The residual value is targeted to reach convergence at 10-6 in this numerical sim-ulation as a sign of the success of the simulation. Initialization is intended to determine the initial con-ditions of a new simulation and then discretization will begin to achieve a convergent residual value. In this simulation, the initialization used is hybrid initialization. The calculation is done with a timestep of 500 and a timestep size of 0.001s.
A grid independence test is carried out so that the results of this simulation do not depend on the quality and density of the existing mesh. Therefore, a grid independence test was carried out on the turbine blades in this simulation. This area was chosen because this area is the review area that is used to determine the convergence of the simulation and the area to be compared for each variation shown in Table 3.  Figure 6. Grid independence test between moment and nodes Figure 6 shows that the moment is stable at the number of nodes above 52073. After the numerical simulation results are obtained, the data will be processed through post processing to be visualized. The results of this visualization will later be in the form of research data obtained. The data obtained is in the form of qualitative and quantitative data consisting of qualitative data of velocity and pressure contours, and quantitative data of the value of the rotor moment and the power generated in the turbine. The rotor moment that will be obtained is the average moment that is obtained every 1.8° rotation, within a span of 0.5 seconds for each rotation variation. The moment obtained will be multiplied by the rotation of the rotor to obtain the power [4]. A blade pressure coefficient will be made to determine the separation point that occurs in the blade. Turbine blade coordinate system to present blade surface static pressure coefficient is shown in Figure 7.

analysis Of Blade Shape On 12 Blades Turbines
The shape of the blade 1 = 0° and 2= 180°, seems to have a curved fore side of the stern which tends to induce strong adverse pressure resulting in earlier flow separation. The flow acceleration that occurs on this side immediately stops due to the occurrence of flow separation. This blade is more widely covered by wake at a low, almost constant pressure that is not as low as it would be if the accel-eration continued until it approached the trailing edge. As it moves across the inlet of the separation rotor the flow on the front side of the blade shifts faster towards the leading edge. However, streams that experience separation on the convex part tend to hold back the flow on the concave surface of the blade in front of it so that it results in a vortex or high-pressure flow on the concave surface. The pres-sure difference between the concave and convex surfaces of this blade shape is very large.
As the blade moves, the position of the apex of the curvature relative to the inflow velocity vector changes. Move down closer to the entrance. This causes the position of the minimum static pressure on the front surface of the blade to tend to approach the leading edge. This is what causes the adverse pressure zone to shift towards the leading edge and ultimately shifts the occurrence of flow separation towards the leading edge. The graph of the pressure coefficient and position of the separation point for blade A is shown in Figure 8. The graph shows that the Cp value decreases at the front of the blade starting from the leading edge to the trailing edge due to flow acceleration. Then getting closer to the trailing edge the pressure increases slightly before finally reaching a constant point Figure 9. Turbine blade pressure coefficient N = 12, blade shape 1 = 0° and 2= 180°, n = 300 rpm Flow contour of the turbine at a rotational speed of 300 rpm analyzed by the color counter. The re-sult is turbine managed to maintain the yellow color contour or high relative velocity which shows in Figure 10. The fore side of the stern in Figure 11 which is perpendicular to the casing tends to induce an ad-verse pressure gradient which is not strong enough, occurs in turbines with blade curvature 1 = 0° and 2= 90°, so that separation delayed. The attached flow on the front side of the blade last longer and the pressure ratio on the back and the front is lower than the shape of the blade 1 = 0° and 2= 180°. This is what causes this blade shape to produce less effective drag force compared to the blade shape 1 = 0° and 2= 180°. The velocity vector that the flow separation occurs at the front of blade A very close to the trailing edge later. In contrast to blades with curved shapes 1 = 0° and 2= 180° where separa-tion occurs earlier, almost in the middle of the blade. Then the blade pressure coefficient graph for blades A and B is shown in Figure 12 where the Cp value decreases for the front of the blade where the flow accelerates so that the pressure decreases. Then getting closer to the trailing edge the pressure. rises and reaches a constant point which indicates that the flow has undergone separation at that constant point. The chart shows that a new separation occurred when X'/C passed 0.9. It can be seen on the Cp graph that the pressure difference between the concave and convex parts is low. The velocity and pressure contours are analyzed in Figure 13, the flow contour produced by the tur-bine has managed to maintain the red color contour or a high relative velocity.

Analysis Of Blade Shape On 8 Blades Turbine
A turbine with 8 blades is less effective because it has less active blade area, which is affected by the flow, so it is less effective in generating drag force. In Figure 14 shows vector image of the relative velocity of the 8 blades turbine with blade shape 1 = 0° and 2= 180°. wind flow entering the rotor gap strikes blade A and separates before reaching the center of the blade The graph of the pressure coefficient for blade A is shown in Figure 15. The graph of blade A for the front immediately experiences very early separation at X'/C 0.4 while for the concave or rear part it is filled with vortexes with high pressure. From the following graph we can see that the pressure difference between the concave and convex surfaces is high. Thus, in a turbine with 8 blades with blade shapes 1 = 0° and 2 = 180° the separation occurs faster when compared to 12 blades with the same blade shape, because the flow on the convex surface is not restrained by the flow that wants to hit the concave surface blade in front of him. This happens because of the number of blades. 8, the distance between blades is farther apart when compared to the number of blades 12. Figure 17 shows the velocity vector on turbine blade 8 with a blade shape of 1 = 0° and 2 = 90°. The flow entering the rotor only strikes the 2 blades on the rotor side. In blade A, fluid flows first and it is seen that it is experiencing separation near the trailing edge. This separated flow hits the left blade so that the wind flow does not come straight out to the outlet. Figure 17. Relative velocity vector N = 8 with blade shape 1 = 0° and 2= 90°, n = 300 rpm Then a graph of the blade pressure coefficient is made to find out the separation point on blade A. The graph is shown in Figure 18 where it can be seen that the front for blade A experiences separation after passing through the middle of the blade and reaching a constant pressure point at X'/C 0.7 and the concave part is filled with flow at low pressure. The graph shows that the pressure difference between the front and rear is not too different. Then the pressure and velocity contours are analyzed. The pressure and relative velocity contours at 300 rpm rotation are shown in Figure 19. Figure 19. Relative velocity and pressure contour N = 8, blade shape 1 = and 2= 90°, n = 300 rpm Figure 20 shows the turbine blade velocity vector 1 = 0° and 2= 90° with the number of blades 12 and 8 at the same blade position. The turbine with blade 8 undergoes separation earlier than the turbine blade 12 so that the pressure difference in blade A is higher in the turbine blade12 compared to blade 8. Turbine blade 12 also appears to have 3 blades that are actively traversed by wind flow through the rotor inlet namely blades A, B, and C while in blade 8 turbine only has 2 blades that are actively exposed to wind flow, namely A and B. On the front blade A, for turbine blades 12 the flow is separated after passing through the middle of the blade and on the concave side of the blade there is no vortex separation. There is only a very slow flow blockage by a concave surface so that the static pressure is high. Towards the exit of the blade this pressure slowly increases because it is more restrained by the concave surface, but at the trailing edge it drops sharply because it can escape from the basin at high velocity. 180° on 300 rpm

Comparison Between Two Different Blades Number
The following graphs 22-25 show the relative velocity and pressure contours for the turbine with the number of blades 12 and 8 for each variation of the blade shape. As explained above, the 12-blade turbine managed to pound 3 blades through the rotor inlet, while the 8blade turbine was only able to pound 2 blades.

Rotor Moment and Generated Power
The rotor in an impulse turbine rotates due to the resistance (drag force) created by the rotor blades. In a closed type impulse turbine with a single inlet flow section, the most significant rotor motion is caused by the combined drag forces on multiple blades during a specific period, which are part of the effective relative flow field. The turbine's generated power is calculated by multiplying the time averaged rotor moment with the rotor's angular speed. The study focuses on four rotor configurations, and the effective flow fields surrounding the blades as they pass through the inlet flow section are examined above. This section provides a comparison of the rotor moment and power generated for all the configurations analyzed, as demonstrated in figures 26 (for rotor motion) and 27 (for power generated).

Figure 26. Rotor moment for 4 examined configurations
According to the analysis of the relative flow field in sections 3.1 and 3.2, a rotor with an exit blade angle of 2 = 1800 is found to generate a more powerful drag force compared to a rotor with an exit blade angle of 1 = 900. This is backed up by the data on the relative velocity field and static pressure distribution around the blade in the inlet flow section.
Furthermore, the data in section 3.3 shows that a rotor with a blade number of N = 12 produces a stronger drag force than a rotor with a blade number of N = 8 at a rotational speed of n = 300 rpm. This corresponds with the information illustrated in Figure 26, which suggests that a rotor with a stronger drag force also produces a stronger torque. Given the same rotational speed of n = 300 rpm for both rotors, the rotor with a stronger torque will undoubtedly produce more power, as depicted in Figure 27 below. The following table 4 presents a summary of the quantitative output data from the Multiple blade closed type impulse wind turbine research, including the moment and power generated. The highest moment data was obtained on turbines with blade shapes 1 = 0° and 2 = 180° with 12 blades at 300 rpm rotation. The highest power is obtained in turbines with blade shapes 1 = 0° and 2 = 180° with 12 blades at 300 rpm rotation. Table 4. Quantitative data of moment and power of each variation.

CONCLUSION
The conclusion of this research is: 1. Blades with the shape 1 = 0° and 2= 180° are very sensitive in causing flow separation on the front surface. In directing the flow, which is experiencing separation at the front, this blade shape tends to allow the flow towards the outlet rather than the concave part of the blade at the front. However, the flow that leads to this outlet restrains the flow that is on the concave sur-face of the blade in front of it to form a vortex with high pressure. The high-pressure differ-ence between the concave and convex surfaces of the blade makes this blade shape very effec-tive in generating drag force.
2. Blades with the form 1 = 0° and 2 = 90° are very good at delaying the occurrence of flow separation on the front surface. In directing the flow, which is separated on the front surface, the blade tends to direct the flow to the concave part of the blade in front of it. However, the concave surface is always filled with vortexes or also streams that have a velocity that is not too low so that high pressure conditions are not obtained on the concave surface. The absence of conditions of a high-pressure difference between the concave and convex surfaces makes this blade shape ineffective in generating drag force.

3.
A turbine with a blade number of 12 has a blade that is more active in receiving wind flow that enters the rotor gap so that the concave part of the blade is also more effective in receiving flows that experience separation on the front surface of the previous blade.
4. Turbines with 8 blades have fewer active blades in receiving wind flow entering the rotor gap so that the reduced number of blades can reduce the utilization of wind flow entering through the rotor gap.