Experimental Study on the Effect of Foil Guide Vane on the Performance of a Straight-Blade Vertical Axis Ocean Current Turbine

4.1 Self-Starting Experiments / Eksperimenti sa spontanim pokretanjem

Self-starting is defined as the ability of the turbine to start rotating when subjected to a flow of water with a minimum current speed. The minimum current speed generated by the water tunnel in this study is 0 m/s (calm), while the maximum current speed is 0.18 m/s. The variations in current speed that a water tunnel can produce are from 0 m/s, 0.078 m/s, 0.096 m/s, 0.101 m/s, 0.106 m/s, 0.160 m/s, 0.162 m/s, and 0.180 m/s. Every variation of the current speed is investigated to determine the ability of the turbine rotation without and with FGV. The turbine is installed without loading so that no external factors other than the current speed interfere with its rotation. For 120 seconds, the turbine rotational speed data is collected without FGV and FGV, which is then averaged to obtain the turbine rotational speed. Figure 6 shows the results of an investigation for 120 seconds with the sample shown at the maximum current speed of 0.18 m/s. The data shows that the curve of the red diamond line (straight-blade with FGV) is above the blue triangle line (straight-blade without FGV), which is visible when the graph is enlarged at 0 to 40 seconds. FGV has been proven to increase the rotational speed of the straight-blade vertical axis ocean current turbine.

Figure 6. Time series comparison of rotational speed in investigating self-starting capability between straight-blade without FGV and with FGV (sample under conditions of maximum current speed, 0.18 m/s)

Slika 6. Usporedba vremenske serije rotacijske brzine u ispitivanju sposobnosti spontanog pokretanja između ravne lopatice bez FGV-a i s FGV-om (uzorak u uvjetima maksimalne brzine struje, 0,18 m/s)

Figure 7 shows a self-starting comparison chart between straight-blade without and with FGV. The x-axis indicates the current speed generated by the water tunnel from 0 - 0.18 m/s. The y-axis indicates the rotational speed of the turbine to investigate self-starting capability. At the minimum current speed of 0 m/s, it has been shown that the rotational speed of the straight blade without and with FGV is 0 rpm; this is due to the absence of kinetic power in the flow. When the water tunnel dimmer was increased to 0.078 m/s, the straight blade without FGV did not rotate because the kinetic energy produced was still very small, namely 0.003 watts. Previously it was also mentioned that the straight-blade vertical axis ocean current turbine has the disadvantage of low performance, one of which is self-starting. However, when the turbine is added FGV at a speed of 0.078 m/s, the turbine has been rotating with a rotational speed of 15.3 rpm. The addition of FGV to the straight-blade vertical axis ocean current turbine is proven to increase self-starting capability, which is more significant at minimum current speed compared to changing foil to flexible-foil which starts rotating at a current speed of 0.5 m/s with a rotational speed of 4 rpm [10]; compared to changing the blade to an inclined-blade which starts spinning at a current speed of 0.2 m/s with a rotational speed of 2.25 rpm [26]; and compared to changing the turbine rotor to a hybrid form (darrieus-savonius) which starts rotating at a current speed of 0.2 m/s with a turbine rotational speed of 1 rpm [27]. This means that adding FGV to the turbine can increase the excellent self-starting capability without having to modify the geometry of the turbine, either the foil, blade, or rotor.

Figure 7 shows that the curve of the red diamond line (straight-blade with FGV) is above the curve of the blue triangle line (straight-blade without FGV) at all variations of current speed 0 - 0.18 m/s. FGV is also proven to increase the turbine's rotational speed at all variations in current speed. When the water tunnel dimmer is raised again to 0.096 m/s, the straight-blade without FGV starts rotating at 21.6 rpm. In the same condition, the straight-blade with FGV showed a higher speed of 47.7 rpm. Likewise, when the water tunnel dimmer was increased to 0.101, 0.106, 0.160, 0.162, and 0.180 m/s, FGV was able to increase the rotational speed of the straight-blade vertical axis ocean current turbine of 35%, 36%, 41%, 38 %, and 40%, respectively. The success of FGV in increasing the straight-blade self-starting capability is a positive initial investigation result. The FGV process disturbs and accelerates the inlet velocity before attacking the turbine, influencing the self-staring capability [28,29,30]. It is believed to improve other performances, such as torque and power. That is because increasing the current speed around the turbine rotor can increase the kinetic energy absorbed by the turbine, based on Equation 6. This study shows that the current speed conditions are relatively low from 0 – 0.18 m/s, so the straight-blade with FGV is very suitable to be applied in Indonesia, which has a low current speed.

Figure 7. Comparison of self-starting straight-blade without FGV and with FGV

Slika 7. Usporedba ravne lopatice sa spontanim pokretanjem bez FGV-a i s FGV-om

In Figure 7, it can also be observed that there are two other data with blockage correction applied. In order to calculate the blockage corrected result, the blockage ratio (BR) needs to be defined first. For the water channel, [31] proposed that the BR can be calculated from HD/H_wW₂, where H and D are the turbine’s height and diameter; H_w and W₂ are the water channel’s water depth and width. If the resulting BR is more than 7.5%, then blockage correction must be done [32]. Previous researchs [33,34] implement the improved Maskell’s correction method to obtain the corrected velocity value, given by [TeX:] U_{C} = \sqrt[2]{\left( \frac{1}{1 - m.BR} \right)U_{\text{UC}}^{2}} $U_C=\sqrt[2]{\left(\frac{1}{1-m.BR}\right)U_{\mathrm{\text{UC}}}^2}$ . U_UC denotes the uncorrected current speed, while m is the blockage correction factor. According to [35,36], m can be expressed as [TeX:] m = 8.14\left( \text{BR} \right)^{2} - 7.309\left( \text{BR} \right) + 3.23 $m=8.14{\left(\mathrm{\text{BR}}\right)}^2-7.309\left(\mathrm{\text{BR}}\right)+3.23$ . By using the calculation steps above, the corrected velocity can be determined and presented in Figure 7. Generally, the corrected current velocity becomes higher to accommodate the adverse wall effect.

4.2 Torque Experiments / Eksperimenti sa zakretnim momentom

Torque is the force the turbine releases to produce mechanical energy through rotational motion. The kinetic power received by each blade in a vertical turbine is not uniform because the locations of the blades are not all in front of the flow front, unlike in a horizontal turbine. This condition causes vertical turbines to have more significant torque fluctuations than horizontal turbines. Vertical turbine blades on the upstream side will receive a more significant hydrodynamic load than on the downstream so that the upstream torque is greater than the downstream, which causes torque fluctuations during the turbine rotation. Large torque fluctuations cause severe vibrations that fatigue the turbine [37,38], break the turbine shaft [22], and potentially damage turbine components [39]. Figure 8 shows the torque fluctuation curves on the straight-blade without and with FGV for 120 seconds. However, the graph shows that the curve of the red diamond line (straight-blade with FGV) is above the blue triangle line (straight-blade without FGV), which is then clarified with a zoomed view from 0 – 40 seconds. In addition, the graph also shows that torque fluctuations in straight-blade without FGV appear to have a more significant difference than with FGV. However, it needs to be considered to determine the torque fluctuation value of the straight-blade without FGV and with FGV.

Figure 8. Torque time series on straight-blade without FGV and with FGV

Slika 8. Vremenske serije zakretnog momenta na ravnoj lopatici bez FGV-a i s FGV-om

Calculation of the torque fluctuation value can be calculated using the torque ripple factor (TRF) formulation, namely, calculating the difference between the maximum torque coefficient ( [TeX:] C_{t}\max $C_{t}max$ ) and the minimum torque coefficient ( [TeX:] C_{t}\min $C_{t}min$ ) during the time the turbine rotates [40]. Figure 9 compares the TRF values of all [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ ranges on straight-blade without FGV and with FGV for both the corrected and uncorrected velocity. The graph shows that the red diamond line curve (straight-blade with FGV) is below the blue triangle line curve (straight-blade without FGV). This curve shows excellent results because FGV can reduce the difference in torque fluctuations in the straight-blade vertical axis ocean current turbine. Thus, it will reduce the potential damage to turbine components, such as vibration on the shaft and even break. The TRF_max values for straight-blade without FGV and with FGV, respectively, occur at [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 0.03 and 0.04, with a difference of up to 44%. TRF_min in straight-blade without FGV and with FGV, respectively, occurs at [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 1.28 and 1.52, with a difference of up to 58%. A nearly identical trend can also be found for the corrected data. The TRF value is the same, while the TSR decreased slightly.

Figure 9. Comparison of torque ripple factor on straight-blade without FGV and with FGV

Slika 9. Usporedba faktora valovitosti zakretnog momenta na ravnoj lopatici bez FGV-a i s FGV-om

The torque fluctuated during the sampling time, starting from the turbine starting to rotate for 120 seconds, and then the average value was taken from each [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ range. This average torque value is investigated in this study to determine the effect of FGV on the torque generated by the turbine. The greater the torque value, the turbine has better performance because it can produce more mechanical power by adjusting the optimal turbine rotational speed value, as in Equation 4. The torque coefficient ( [TeX:] C_{t} $C_t$ ) can also represent the turbine torque value, which is calculated using Equation 3. Figure 10 compares [TeX:] C_{t} $C_t$ to [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ in turbines without FGV and with FGV. The graph shows that the red diamond line curve (straight-blade with FGV) is above the blue triangle line curve (straight-blade without FGV) in all [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ ranges. The [TeX:] C_{t}\max $C_{t}max$ value in a straight-blade without FGV is 0.56 at a [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ of 0, meaning that in this condition, the turbine does not rotate but produces ample torque. The straight-blade with FGV can make a [TeX:] C_{t}\max $C_{t}max$ of 0.83 at a [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ of 0.04, meaning that even though the turbine rotation is very slow, it produces ample torque. In this case, the [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ represents the value of the turbine's rotational speed, where a small [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ value indicates the turbine rotates slowly. The corrected data is also in Figure 10, with relatively lower values for both the x and y-axis components. This is because the corrected current velocity affects both the x and y-axis parameters.

Figure 10. Comparison of torque coefficient between straight-blade without FGV and with FGV

Slika 10. Usporedba koeficijenta zakretnog momenta između ravne lopatice bez FGV-a i s FGV-om

Conversely, an enormous [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ value indicates the turbine rotates quickly. If the [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ value is 0, there are two possibilities; first, the turbine does not rotate because of the more significant torque force released compared to the flow force hitting the turbine; second, there is no kinetic power absorbed by the turbine. Figure 10 shows that the lower the TSR value, the greater the torque coefficient. This is due to the increased torque braking load so that the torque value becomes prominent and the rotating speed of the turbine decreases until the [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ point is zero, and the turbine stalls; the same result has also been done by Talukdar et al. [19]. However, FGV can make the straight-blade vertical axis ocean current turbine generate ample torque but not stall the turbine to produce better mechanical power production. This study has proven that FGV can increase the [TeX:] C_{t} $C_t$ value of straight-blade vertical axis ocean current turbines in all [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ ranges. The value of the increase in [TeX:] C_{t}\max $C_{t}max$ and [TeX:] C_{t}\min $C_{t}min$ due to FGV in a straight-blade is 32% and 6%, respectively.

4.3 Power Coefficient Experiments / Eksperimenti s koeficijentom snage

Power coefficient ( [TeX:] C_{p} $C_p$ ) is a non-dimensional form of turbine mechanical power obtained from the formulation in Equation 5. In simple terms, [TeX:] C_{p} $C_p$ can be interpreted as the turbine's ability to absorb kinetic energy from current flow or commonly known as efficiency. The amount of turbine mechanical power is influenced by the torque ( [TeX:] T $T$ ) and the rotational speed of the turbine (ω), as in Equation 4. So if certain [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ conditions have shown ample torque, it does not necessarily produce sizeable mechanical power or [TeX:] C_{p} $C_p$ because a factor ω also affects it. As previously explained, a very low [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ of 0.04 can make the [TeX:] C_{t}\max $C_{t}max$ in a straight-blade with FGV. This condition has greater power compared to turbines without FGV. However, compared to other [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ s in the same straight-blade conditions (e.g. straight-blade with FGV), it does not necessarily produce greater power because a ω factor also affects it. This section discusses the results of the investigation of the effect of FGV on [TeX:] C_{p} $C_p$ produced by the straight-blade vertical axis ocean current turbine.

Figure 11 compares [TeX:] C_{p} $C_p$ to [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ in straight-blade without FGV and with FGV. The graph shows that the red diamond line curve (straight-blade with FGV) is above the blue triangle line curve (straight-blade without FGV) in all [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ ranges. The benefit of using FGV is that it can accelerate the current velocity with the same function as a flow deflector [41,42, 43]. The graph also shows that the [TeX:] C_{p} $C_p$ value increases with increasing [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ . This differs from the torque case in that the value of [TeX:] C_{t} $C_t$ increases as the [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ decreases. This means that the turbine rotational speed factor (ω) also plays an essential role in influencing the magnitude of the turbine's mechanical power performance. The [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ value that could be produced in this experiment was low because it was below [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 2. For the corrected data, the trend is found to be similar to the [TeX:] C_{t} $C_t$ curves. The turbine generates a notably lower [TeX:] C_{p} $C_p$ while operating at a lower TSR than the uncorrected data.

In contrast, [TeX:] C_{p} $C_p$ generally began to stall after passing a high [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ of 2-3 range, while a very high [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ of 4-6 [TeX:] C_{p} $C_p$ had experienced a significant decrease. The low [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ in this study is due to the shallow current speed conditions generated by the water tunnel, 0 - 0.18 m/s. In this case, it shows that the straight-blade without FGV has three blue triangle line points at a very low [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ (closer to [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 0), while the straight-blade with FGV only has one red diamond line point at a very low [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ (closer to [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 0). That is, FGV can make the straight-blade move faster towards a greater than zero TSR. It can be seen on the chart that the second and third red diamond line points are at [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 0.4 and 1, respectively, while the second and third blue triangle line points are at [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 0.03 and 0.09, respectively, very far behind. The lowest [TeX:] C_{p} $C_p$ produced by a straight-blade with FGV is 0.033 at a [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ of 0.04, while for a straight-blade without FGV, the lowest [TeX:] C_{p} $C_p$ is 0 and at a [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ of 0. A straight-blade with FGV can absorb better kinetic energy than without FGV. The [TeX:] C_{p}\max $C_{p}max$ in the straight-blade with FGV occurs at [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 1 with a value of 0.519, while the [TeX:] C_{p}\max $C_{p}max$ in the turbine without FGV occurs at [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 1.28 with a value of 0.415. That is, FGV can make straight-blade produce [TeX:] C_{p}\max $C_{p}max$ faster than without FGV and increase the [TeX:] C_{p}\max $C_{p}max$ value by up to 20%. The use of FGV (50˚ pitch angle) in this study can produce a better [TeX:] C_{p}\max $C_{p}max$ (0.519) than guide-vane-augmented at pitch angles of 0˚, -30˚, and 30˚ with [TeX:] C_{p}\max $C_{p}max$ values of 0.37, 0.31, and 0.28 respectively, by Liu et al. [17].

Figure 11. Comparison of power coefficient between straight-blade without FGV and with FGV

Slika 11. Usporedba koeficijenta snage između ravne lopatice bez FGV-a i s FGV-om

After the [TeX:] C_{p}\max $C_{p}max$ , the turbine loses its lift force which causes [TeX:] C_{p} $C_p$ to stall on the straight-blade without FGV or with FGV. However, the stall in straight-blade without FGV is earlier than with FGV. This means that FGV can slow down stall events in straight-blade which means it can maintain a more extended lift force. [TeX:] C_{p} $C_p$ stall on the straight-blade without FGV and with FGV, respectively 0.312 ( [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 1.02) and 0.451 ( [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ 1.31). Even in stall conditions, FGV can increase the [TeX:] C_{p} $C_p$ of the straight-blade by 31%. Post-stall [TeX:] C_{p} $C_p$ on straight-blade without FGV and with FGV starts to increase again, which means the lift force is maintained. In general, post-stall turbine [TeX:] C_{p} $C_p$ will continue to decrease significantly, but in this study, there was an increase in post-stall [TeX:] C_{p} $C_p$ . This event is possible because the [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ range is still very low (0 – 1.52), so there is a possibility that [TeX:] C_{p} $C_p$ will continue to rise as the [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ increases so that there will be a significant stall at a high [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ . This research can be further investigated in a water tunnel or open channel facility capable of producing higher current speeds to produce a high [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ to determine the distribution of [TeX:] C_{p} $C_p$ over a more comprehensive [TeX:] \text{TSR} $\mathrm{\text{TSR}}$ range.

4.4 Flow Visualization Experiments / Eksperimenti vizualizacije protoka

Overall, the foil guide vane (FGV) is proven to improve the straight-blade vertical axis ocean current turbine performance regarding self-starting ability, torque coefficient, and power coefficient. Improved performance in straight-blade due to adding FGV can also be proven by looking at the flow visualization through the blades and FGV. This study carried out a flow visualization experiment using ink injection placed in front of the turbine. The camera is placed next to the mini water tunnel to make it easier to take videos of the movement of the injected ink. In contrast, the video is not taken from the top because the upper structure covers the turbine, making its movement invisible. The mini water tunnel is set at a current speed of 0.101 m/s because, from the test results, it is easiest to detect ink movement compared to more significant current speeds making ink movement too fast and difficult to visualize. The results of the flow visualization video recording are played slowly to determine the movement of the ink every second.

Figure 12 compares the flow visualization experiments' results on straight-blade without FGV and with FGV. The ink distribution from injection to behind the turbine shows a very significant difference in Figures 12 (a) and 12 (b). When the video is taken 1 second after the ink is injected, the straight-blade without FGV shows that the ink hits the blade marked with a red box line in Figure 12(a). That is, this condition indicates the straight-blade's readiness to extract the flow's kinetic power. In straight-blade with FGV, at 1 second, the ink enters the FGV slot, then slightly comes out of the FGV slot on the other side marked with a yellow box line. That is, this condition indicates a slight force exerted by the straight-blade to eject ink so that by that time, the turbine has already extracted the kinetic power of the flow. So that at that 1 second, the straight-blade with FGV can extract kinetic energy earlier than without FGV. This event can prove that straight-blade with FGV has better self-starting capabilities than without FGV.

Furthermore, 2 seconds after the ink is injected, the straight-blade without FGV shows that the ink spreads widely behind the turbine rotor, more or less along 2D and looks very thick. Meanwhile, on the straight-blade with FGV, it can be seen that the ink is visible very little behind the turbine rotor, approximately 0.5D long and looks very thin. Furthermore, within 3 seconds after the ink is injected, the straight-blade without FGV shows that the ink behind the turbine rotor continues to expand until it reaches approximately 3.5D, but it looks thinner. Meanwhile, in the straight-blade with FGV, ink is not visible behind the turbine rotor, but it is visible on the side of the FGV, and it hits the leg structure, making it darker. The events of the 2nd and 3rd seconds illustrate that the straight-blade with FGV slows down the current speed behind the turbine rotor, meaning that the kinetic energy behind the turbine rotor becomes smaller because some of it has been extracted by the straight-blade with FGV into mechanical power, and vice versa what happens to the straight-blade without FGV. Therefore, FGV is proven to increase the power coefficient of the straight-blade vertical axis ocean current turbine.

Figure 12. Comparison of flow visualization on straight-blade without FGV and with FGV

Slika 12. Usporedba vizualizacije protoka na ravnoj lopatici bez FGV-a i s FGV-om