Experimental Investigation of Runner Design Parameters on the Performance of Vortex Turbine ^†

Abstract

The vortex turbine (VT) is a micro-hydro turbine that extracts power from a water vortex that is artificially generated in a conical or cylindrical cross-section basin. The former cross-section gives rise to a stronger vortex than the latter, meaning it has more potential for power generation. For this reason, the present study experimentally analyzed the effect of sensitive geometric parameters on the VT performance, i.e., the rotor to basin diameter ratio (RB) and runner’s position in the conical basin (CB). The results show that the ideal runner in terms of RB for the best performance of VT is a runner with RB = 0.6, which has a maximum potential for the utilization of the forced vortex region of the Rankine vortex. Moreover, the best position for the installation of a VT runner is the location at the bottom near the orifice, as the strong vortex tangential velocity and maximum head drop at the mid-position is not a feasible option. Blades with a tilt in the vertical plane are suggested for use in the power extraction at the bottom of CB whereas crossflow blades suit the rotational flow region near the top of CB, i.e., the surface vortex.

1. Introductions

A huge amount of hydro energy has remained unutilized due to low flowrate and a low-head issue [1]. This problem can be addressed with the use of gravitational water vortex turbine (VT) technology, as all other conventional hydro turbines need one of the above-mentioned parameters in a sufficient amount. This type of turbine can easily operate from 0.7 m to 3 m under the water head [2]. It is used to extract energy from the water vortex, which is artificially generated when the water exits under the force of gravity from the orifice of the basin in either a cylindrical or conical configuration [3]. The VT performance reduces when the blades are varied from 6 to 12 on a runner [4]. The performance of the VT has also been analyzed with different profile blade runners, such as centrifugal, Francis, and impulse paddle type, while it is suggested that the runner should have a minimum hub diameter with straight blades at an angle for better VT performance [3,5]. It is reported that the CB provides a stronger vortex than a cylindrical basin under the same water head and flow rate [4]. Moreover, the vortex can re-originate itself in the CB once it is distorted through a runner that is fixed in the basin. Therefore, for the first time, Gheorghe-Marius et al. proposed the idea of using stepwise runners in a CB, and they reported a detailed analytical model [3]. Later, the same approach was used for the purpose of a detailed experimental investigation of multi-stage VT performance [6]; The authors’ recent study is particularly focused on the inter-stage and intra-stage performance evaluation of multi-stage VT [7].

This study is especially devoted to the sensitive runner’s design parameters, including the rotor diameter to corresponding basin diameter ratio (RB) and runner’s position in the CB for the purpose of maximum power extraction. The effect of the above parameters on the performance of the VT has been examined by plotting the brake Torque (T), power (P), and efficiency (η) vs. the rotational speed (rpm). To the best of our knowledge, the present manuscript is of worthy consideration for the further exploration of VT technology.

2. Experimental Setup and Methodology

The designed experimental rig used for the performance evaluation of the VT is shown in Figure 1a. The testing facility comprised a centrifugal pump, storage tank, overhead reservoir, conical basin, and assembly of a VT. The VT runner, which had four blades of the Savonius profile, was used to extract power from the water vortex. The bucket method was used for the measurement of flowrate [7]. An inlet flow rate of 4 L/s was kept constant for all the experiments. A tachometer and Prony brake mechanism were used for the measurement of rotational speed, N (rpm), and brake torque, T (N-m), respectively.

The following equations are used for the measurement of the experimental angular velocity ω (rad/s), brake power P (watt), and turbine efficiency η.

Ω = 2πN/60

(1)

P = Tω

(2)

η = P/HP

(3)

“HP” refers to the available hydraulic power. The developed mathematical model is used for the analytical measurements of the above performance parameters (PPs) [7].

3. Results and Discussion

In the operation of the turbine, it is important to know the value of torque and RPM for a specific power output under a constant water head and inlet flow rate. Therefore, the effect of the RB and position of a runner on the performance of the VT have been depicted by plotting the performance curves at various load conditions.

3.1. Analysis for Rotor Diameter to Corresponding Basin Diameter Ratio (RB)

The RB is among the major geometric parameters that greatly influence the PPs of the VT. The variation in the RB results in the change of the runner’s diameter. For this reason, the RB is varied by the fixing of the blades of Figure 1b, which were T1, T2, and T3 at stage 3 (S3), as shown in Figure 2. The first subscript terms R31, R32, and R33 stand for the stage of the VT, while the second subscript stands for the type of blade. For example, R31 is the runner fixed at stage 3 while having T1 blades. In Figure 2, the comparison between R33 and R31 showed that R33 performed better than R31 with a higher brake torque (38%↑) at the cost of low rpm (17%↓), thus providing more power. Under the same conditions, R32 competes with R33 in the same power production and efficiency with an advantage of less weight; thus, it may be termed as the optimum performing runner when RB = 0.6. Figure 2 also reflects that the VT runner with RB = 0.47, showing its installation within the rotational flow region of the Rankine vortex; it achieved the highest rpm. In the Rankine vortex, the rotational vortex has a greater ability for power generation due to the existence of a strong tangential velocity in it, unlike the irrotational flow region [8]. The maximum RB runner reflects its presence in the region of both free and forced vortex while providing the lowest rpm.

The optimum performer runner (R32) implies its installation on the interface of the rotational and irrotational vortex. R32, in terms of PPs, overshoots the analytical calculations due to having the highest capacity for the utilization of the rotational vortex region. Thus, based on the discussion above, a runner with RB = 0.6 is suggested as a runner for VTs.

3.2. Effect of Runner Position

The mounting position of an individual runner in a CB has a great impact on the performance of a VT. At the same input design parameters, the maximum energy can be harnessed from the vortex by fixing the turbine runner at its proper location along the height of the shaft. This is why the position of the runner is varied by fixing the T1 blades of Figure 1 at S1, S2, and S3, respectively (Figure 2). Thus, the insight runners in this section are R11, R21, and R31.The results achieved through the adopted technique are plotted in Figure 3.

The highest rotational speed of R11 at S1 is because of the maximum head drop at the same inlet flowrate. Among all the runners, the runner fixed at S2 (R21) underperformed the other two, R11 and R31, in all modes of PPs. Thus, it can be suggested that the mid-position of CB should not be utilized for power generation in the case of single-stage turbines. Between the runners R11 and R31, the more promising candidate is R11. It produced double the power compared with R31 with a lower water head utilization tendency, as indicated by its lower efficiency. On the other hand, R31 has more competency for water head utilization over R11, thus making it difficult to credit the S1 or S3 position for energy generation. An in-depth observation would shed an inkling on the selection of the blades profile. S1 and S3 are the location of the maximum and minimum head drops, respectively. Thus, we suggest that blades of the same profile are tilted in a vertical plane at S1 to utilize both head and vortex tangential velocity. The vortex tangential velocity resulting through the water head drop and artificial vortex generation in the CB is the major contributor to the higher power of R11, which is fixed near the bottom. In the position of S3, the available water head is minimum, but the blades of the selected profile suit S3 well. We conclude that in the case of a single-stage VT, the bottom position near the orifice while using the runner with blades tilted in a vertical plane is preferred for power generation. The top position is feasible for power generation if the runners have blades of the present profile.

4. Conclusions

The purpose of the present study is to trace the best diameter (RB) runner and fixing position of the vortex turbine in a conical cross-section basin for maximum power output. The analysis was carried out based on rotational speed (rpm), power (watt), and efficiency under different loads. For the best performance of the VT, it is recommended to use a runner with RB = 0.6 as it showed the maximum potential in utilizing the potential flow in the Rankine vortex. The bottom position near the orifice is preferred for the installation of the VT due to the presence of a strong vortex tangential velocity.