Four hydrofoils were investigated for designing a 50 W class tidal current turbine rotor blade. Figure 3 shows the four selected hydrofoils (MNU26, NACA63421, DU91_W2_250, and DU93_W_210LM). Each hydrofoil has a different thickness ratio and hydrodynamic characteristics. Tidal current turbines must be capable of operating under sea water environment. It must be able to withstand the fatigue loading induced by the tidal current, water turbulence, temperature effects, and waves. A thick blade has a relatively high structural strength as compared to a thin blade. Therefore, in this study, hydrofoils with three different thicknesses were selected.

Figure 3: 
Selected hydrofoils for rotor blade

The MNU26 hydrofoil is a combined hydrofoil type that was used in a previous study [7]. The suction side of the DU91_W2_250 hydrofoil and the pressure side of the S814 hydrofoil were combined to produce the MNU26 hydrofoil. Some points at the leading and trailing edges were also changed to provide a smooth curve. The MNU26 hydrofoil has a 26% thickness ratio that can be applied throughout the blade length, giving good structural strength. The NACA63421 [8] and DU93_W_210LM hydrofoils have a 21% thickness ratio, and the DU91_W2_250 hydrofoil has a 25% thickness ratio.

The hydrodynamic characteristics are very important in the selection of a high-performance hydrofoil. The performance of a turbine depends on three important parameters: the lift coefficient (C_L), lift to drag ratio (C_L/C_D), and Reynolds number. These three parameters vary with respect to the angle of attack (AOA) [5]. In this study, the design tidal current velocity is 1 m/s, and the Reynolds number is about 1 × 10⁵.

Figure 4 shows the C_L and C_L/C_D curves at Reynolds number 1 × 10⁵, which is calculated using X-Foil open source code [9]. The Reynolds number that is used in this study (1 × 10⁵) for the small tidal current turbine is relatively low compared to conventionally used Reynolds number (over 1 × 10⁶) for tidal current turbines. Therefore, C_L and C_L/C_D curves have many variations for a low Reynolds number, and it is relatively difficult to obtain smooth hydrodynamic characteristics for AOA. Comparing the four hydrofoils, it was observed that MNU26 hydrofoil exhibited a higher C_L and a relatively lower C_L/C_D in contrast to the other three hydrofoils. The MNU26 hydrofoil showed different hydrodynamic characteristic compared to previous study [7] because of the different design parameters resulting from using a low Reynolds number.

Figure 4: 
Hydrodynamic characteristics of four hydrofoils investigated at Reynolds number 1 × 10⁵

Table 2 presents the fundamental design parameters for the blades designed in this study. Using these design parameters and the four hydrofoils, four rotor blades were designed.

For the blade design, output power P can be calculated with respect to current speed as in equation (1). Since P is directly proportional to the cube of the inflow current velocity, with the consideration of the rotation number of a generator in order to obtain the maximum output at tip speed ratio (TSR) λ = 5 as defined in equation (2).

Here, ρ is the sea water density, A is the swept area, V_∞ is the inflow current velocity, and the blade radius R is set as 0.265 m.

The power coefficient is given by equation (3), where P_T is the power from the turbine, which is the product of torque and angular velocity of the turbine rotor blade, and the denominator represents the power available, which is given by equation (1).

Figure 5 presents the twist angle, radius length, and chord length of the 50 W class optimized tidal current turbine rotor blade model designed in this study. When the blade was designed, each hydrofoil was used for the total length of the rotor blade.

The NACA63421 hydrofoil was used for the total length of the rotor blade. Circular profile sections are designed near the root section of the blade in order to strengthen the base of the blade as shown in Figure 6. The other three blades were also designed by applying this method with each hydrofoil.

Figure 5: 
Twist angle, radial length and chord length of 50W class rotor blade model designed

Figure 6: 
50W class tidal current turbine rotor blade model designed using NACA63421 hydrofoil

The computational grid of the 50 W class tidal current turbine is presented in Figure 7. Considering the computer capacity and calculation time, the model was meshed using ICEM CFD with a grid having a total of 6.5 million nodes. Hexahedral structural grids were used to increase the convergence of the calculation and its reliability.

The grid density of the blade surface is higher than that of the flow field surface because an O-Grid was used for the blade surface. A high grid density is required for the blade surface to achieve reliable CFD analysis results. Thus, the blade was refined to ensure that the y⁺ value was less than 3. However, for efficient calculation time, the flow field is formed with a lower density with a gradual change in node distance. The steady state simulation was applied with a high resolution advection scheme until the simulation reached the required RMS residual stress value between 1 × 10^-5 and 1 × 10^-6.

Figure 7: 
Numerical grid of total flow field for 50W class tidal current turbine rotor blade model

The size of the total flow field is presented in Figure 8. The inlet and outlet surfaces were set at 8 and 12 times the blade radius, respectively, and the radial distance was set at 5 times the blade radius, as a large distance is required for a smooth transition of the flow.

For numerical analysis of the 50 W class tidal current turbine rotor blade performance and fluid flow, ANSYS CFX [10], a commercial CFD code, was used as a solver in this study. Table 3 presents the calculation conditions used for the numerical analysis of the rotor blade. For the boundary conditions of the computational flow fields, the design current speed was set at the inlet, and the static pressure was set at the outlet. Sea water at 25 oC was used as the working fluid, and the steady state calculation was conducted. In order to confirm the performance and fluid flow for TSR values ranging between 3 and 8, the angular velocity was analyzed in the range of 11.32–30.19 rad/s. The shear stress transport (SST) turbulence model was utilized, which has been known to estimate both separation and vortex occurring on the wall of the blade.

Figure 8: 
Size of total flow field of 50W class tidal current turbine rotor blade model

Equation (3) is utilized in plotting the power coefficient curves for the four rotor blades investigated. The power coefficient curves are shown in Figure 9.

From the power coefficient curves, it is observed that blades having a hydrofoil with a relatively low thickness ratio showed a higher efficiency than those with a relatively higher thickness ratio. The maximum power coefficient of two turbines is about the same at around 0.45. NACA63421 and DU93_W_210LM blades had the maximum efficiency at TSR 6. NACA63421 blade had the maximum efficiency at a lower TSR compared to the other three turbines. In addition, DU93_W_210LM blade had the maximum efficiency at a higher TSR compared to the other three turbines. These results indicate that at a high current speed, the NACA63421 turbine performs better than the other three turbines. Moreover, the DU93_W_210LM turbine performs better than the other three turbines at a low current speed.

In this study, MNU26 blade shows a low C_P value compared to the results of previous study [5]. This is because of the difference in the Reynolds number. The Reynolds number used in this study is 20 times different. The MNU26 hydrofoil shows different hydrodynamic characteristics because of a low Reynolds number whereas a higher Reynolds number was used in the previous study.

Figure 9: 
Power Coefficient Curve.

The value of the pressure coefficient (C_P) surrounding the blade pressure surface and suction surface is compared quantitatively as shown in Figure 10, at 50%, 75%, 95% of the local radius for TSR 5. The area of the pressure coefficient curves denotes the pressure difference between the pressure surface and suction surface. A larger area indicates that the blade performance is good. As shown in Figure 10, X on the abscissa in the figures represents the chord length, which is the length from the leading edge to the trailing edge of the hydrofoil.

The maximum area of the pressure difference between the pressure and suction surfaces is observed at 75% of the local radius. By examining the pressure coefficient curves, it is observed that the maximum power is generated from approximately 75% of the blade radius to the tip. The area of the pressure difference between the pressure and suction surfaces at 75% of the local radius is observed at a relatively low thickness ratio of the blade, NACA63421 and DU93_W_210LM. However, the tidal current blade needs enough structural strength to withstand the extreme thrusts of the tidal current. Therefore, a small amount of power can be sacrificed for an optimum design. Further thinning of the blade would perhaps lead to fracture and a low lifespan.

Figure 10: 
Pressure coefficient curves at 50%, 75%, 95% of the local radius

Figure 11 presents a clear streamline distribution on the blade surface for TSR 5, which is the design point of the turbine. From this figure, it can be observed that all the blades show secondary flow at the suction side surface. Moreover, the suction side surface shows the formation of radial flow. This radial flow creates an irregular flow on the blade surface and affects the hydrodynamic characteristics of the turbine by decreasing the output power and power coefficient.

However, pressure side surfaces show relatively uniform flow compared to the suction side surface. MNU26 and DU91_W2_250 hydrofoils showed the formation of secondary flow near the tip of the pressure side. This small difference in flow separation may be the reason for a lower power output.

Figure 11: 
Streamlines on blade surface at TSR5