Experimental Investigation on Effect of Hoop Thickness on Performance of Hooped Pelton Wheel

A recent improvement in Pelton runner design, the hooped runner, is based on a redistribution of behaviour among the buckets and hoops and which allows stresses to be minimized and efficiency is nearly equivalent to conventional Pelton Runner apart from low nozzle opening of 40% and 60%. This advanced design having a couple of superiority from the mechanical orientation as well as from the assembly point of view without any different difficulty from the hydraulic orientation that establishes the significance of this unique concept. In the present work, an actual laboratory-scale hooped Pelton runner is developed. The thickness of its hoop is varied as 2 mm, 3 mm and 4 mm and to prove the competence of this advanced design and the influence of hoop thickness on the hydraulic performance of the Pelton wheel, detailed experimental studies have been performed. From the results it has been concluded that the power output and efficiency of hooped Pelton runner are nearly the same as the conventional Pelton runner at all nozzle opening.


Introduction
Hydropower is a renewable energy source that does not require the use of fossil fuels.Pelton turbines are among the most basic and extensively used turbines in hydropower plants [1].The Pelton turbine is characterised by high heads and small volume flows, and it captures energy from a high-speed water jet.Manufacturers are very interested in predicting the performance efficiency and dynamic performance of any Hydraulic turbine/machine under various operating conditions.In today's highly competitive market, determining the performance that ensures turbine upgrades and overhauls in a short period of time is quite tough.
Lester Allen Pelton designed the Pelton turbine in 1880 and the currently available design has incorporated a very limited number of changes.The technology is to identify many innovations like forged, welded, cast, and frequently with a separated bucket.In recent times, the one-piece casting used to be the most common design of Pelton runners [2].To work around difficulties inherent to cast steel (quality of the materials, foundry lead times, price variations) hydro manufactures turned to propose fully machined runners from a forged disk or mixed forged and welded runner.The basic design of this part has changed little during the past decades.Performance improvements have led to an increase in the ratio of jet diameter to bucket width, resulting in an increment in weight and, therefore, an increase in stresses [2].
In connection with the operation, little modification or no change has taken place, since the maintenance of runners worn by silt-laden water.It is repaired by welding and heat treatment, and these operations leave stresses in the base material.This eventually leads to the replacement of the entire runner.The current priority among operators is to reduce maintenance and repair expenses while increasing component reliability.To meet the needs of its customers and to mitigate industrial and operational risks, a new Pelton runner design is developed known as the hooped runner [2,13].Classically, in Pelton runners, the buckets are encased onto a central rim, either in the case of a onepiece runner or of mechanically fixed separated buckets.The attachment zone is then subjected to cycled high bending stresses as the bucket repeatedly passes into the jets.Furthermore, once the pressure on the bucket has been released, its cantilever structure gets vibrating according to its natural modes and, if not properly designed and/or manufactured, a resonance may occur and a severe increase in the dynamic stress amplitude is observed.
In the new design, the separated buckets keep their main hydraulic function which is the conversion of the jet's driving force into a tangential force, but their structures are not solicited to transform this force into torque by involving shear and bending at their connections with the rim.This latter function is accomplished by two hoops on which the buckets are mounted, allowing stresses to be more efficiently distributed all around the runner [3].A valuable contribution to the Pelton turbine is available by some of the researchers, Reiner Mack et.al.[4], Helene Garcin et.al.[5], Perrig and et.al [6], J C Marongiu et.al.[7], C. Pellone [8], Filip Sadloet.al [9], Liu Jie et.al.[10], H. P. Hauser and Staubli [11], Muller and Zhang [11], Lowys et.al [12].
In the literature, Maryse Francois, Pierre [2] and Bernard Michel, Georges Rossi [3] have found that the efficiency of hooped Pelton runners is marginally less than the Conventional Pelton runner and hence they suggested further modification and parametric study of hoop runner for improving its performance.However, the studies related to the influence of flow passage variations due to change on hoop thickness are not readily available in the literature.Therefore, the focus is set to study the influence of these thickness variations on the hydraulic performance of hooped Pelton runner.In the current research, a detailed experimental investigation is carried out for both the conventional and hooped Pelton runner of the same size on a 390 mm diameter with varying hoop thicknesses of 2 mm, 3 mm, and 4 mm to determine the impact of hoop thickness on Pelton wheel hydraulic performance.Figures 1 and 2 show the dimensions of a conventional runner and a hooped runner with the same number of buckets and tip diameters as those already in use.This dimension of conventional runner corresponds to the laboratory setup available at the institute.

Experimental Test Rig
All the measurements are carried out on the horizontal type of test rig at C. K. Pithawalla College of Engineering and Technology.The test rig is suited for single injector machines and is equipped with 1 pump that can be operated separately which is powered by a 15 HP synchronous motor.The rotational speed can thus be adjusted over a large band.Table 2 summarizes the main specifications of the test rig.The facility is equipped with manual-controlled injector needles.The casing is fitted with a medium-size acrylic window allowing observation of the runner and injector.The casing of the Pelton turbine with the Pelton runner is shown in Figure 7. Pressure gauges monitor pressure within the tank and at the entrance of the nozzle.To measure the flow rate, a calibrated flow meter is employed which is connected between the pump and the nozzle.A continuous flow of water is maintained by connecting the turbine discharge into the water tank which is further connected to the pump.The flow rate is controlled by the valve below the pipe, which controls the needle valve in the inflow line.
The conventional and hooped Pelton runner fitted in the casing is shown in Figure 8.The water drops out from the bottom of the glass to the tank

Experimental Analysis
Experimental studies are carried out in two phases.However, before actual investigations, at first, the verification of the working of various instruments is done.After adequate verification, the first phase of studies are carried out with the conventional Pelton runner.In the second phase, the conventional runner was replaced with the hooped runner and similar investigations have been carried out.The results of these experimental investigations are discussed in a subsequent section.

Variation of Power developed with Specific Speed at different Nozzle Opening
The measurement of power developed and efficiency is accomplished for conventional runners and hooped runners.By increasing the thickness of the hoop the influence of these thickness variations on the hydraulic performance of hooped Pelton runners are studied along with different nozzle openings (40 % to 100 %).The load is increased from 5 kg to 20 kg with a 5 kg increment on the rope brake dynamometer, which is used to measure torque.
The change in power developed with particular speed at different nozzle openings is shown in Figures 9 to 12.Both the conventional Pelton Runner and the Hooped Pelton Runner create more power as their particular speed increases.In general, the power and efficiency of a hooped Pelton runner are slightly lower than that of a conventional runner.This is most likely due to the water's interaction with the hoop.To eliminate interference, this result suggests that the buckets and hoop should be optimised.According to the literature, Maryse Francois, Pierre [2] and Bernard Michel, Georges Rossi [3] discovered that the efficiency of the hooped Pelton runner is marginally lower than that of the Conventional Pelton runner, and thus recommended further modification and parametric study of the hoop runner to improve its performance.Figures 13 to 16 show the difference in efficiency with Specific Speed for Conventional and Hooped Pelton Runners at various nozzle openings.By increasing the Specific Speed the Efficiency is increasing in both the cases, which is quite obvious.In general, the power developed and efficiency of the hooped Pelton runner are reduced marginally.This is practically due to jet interaction with hoops which result in loss of kinetic energy due to frictional effects of hoop.But, at 40% and 60 % nozzle opening of (Figure.9, 10 and 13), the power developed and efficiency of Hooped Pelton runner is higher than that of Conventional Pelton Runner.This is most likely due to jet diffusion at lower nozzle openings, which reduces efficiency in Conventional runners, whereas diffusion is rigorously limited in hooped runners due to hoops, and hence hooped runners have higher power and efficiency than Conventional Pelton runners at part loads.
The water distribution between the buckets and the hoop reaches a peak, resulting in improved efficiency for the hooped runner compared to the Conventional runner at a given nozzle opening.The initial shape of the hoops cause an interference with bucket outflow, as shown by these results.To regain the performance level of a hooped Pelton runner, some changes to the hoops are required.
The hooped Pelton runner's power and efficiency are nearly identical to that of a Conventional runner.However, with 2 mm thickness of hoop, higher efficiency levels are achieved compared to higher hoop thickness.This is because, at 2 mm thickness of hoop, the flow passage is more compared to higher thickness of hoop.It is also evident that at lower nozzle opening, both the power and efficiency of hooped runner is superior to Conventional runner for 2 mm hoop.However with 4 mm hoop thickness at certain conditions (nozzle opening of 40% and 60%) an increase in efficiency is witnessed.Which needs further investigations to deduce the cause and understand the phenomena.

Conclusion
The present work investigated through experiments, the effect of hoop thickness on power output and efficiency of conventional and hooped Pelton runners.From the experimental result, it is shown that the power output, as well as the efficiency of the conventional and hooped Pelton runner, increases as the specific speed of the turbine increases, which is quite obvious.The influence of the hoop's thickness on power output and efficiency of the hooped Pelton wheel is not much affected.The power output and efficiency of hooped Pelton runner are nearly the same as the conventional Pelton runner at all nozzle opening.
The experimental results suggest that at the lower nozzle opening the performance of hooped Pelton runner is marginally superior to Conventional runner due to controlled diffusion.At higher nozzle opening the marginal performance deterioration of hooped runner may be due to the interaction of water with the hoop and also from the experimental results, with 2 mm, 3 mm, and 4 mm hoop thickness, the efficiency of 2 mm hooped runner is more compared to the higher thickness of hoop which is due to improved flow passage.However, 4 mm thickness hoop has shown slight high efficiency and power output compared to 2 mm thickness which needs further investigations.

Figure 3
gives the photographic views of Individual cast Bucket & Figure 4 (a), (b) and (c) gives the various photographic views of Hoop thickness of 2 mm, 3 mm & 4 mm. Figure 5 show different views of the laboratory-scale hooped runner.

Speed 12 Figure 7 .
Figure 7. Casing of the Pelton Turbine Test Rig.

Figure 9 .
Figure 9. Power Variations for Conventional and Hooped Pelton Runners Developed with Specific Speed at 40% Nozzle Opening.

Figure 10 .
Figure 10.Power Variations for Conventional and Hooped Pelton Runners Developed with Specific Speed at 60 % Nozzle Opening.

Figure 11 .
Figure 11.Power Variations for Conventional and Hooped Pelton Runners Developed with Specific Speed at 80 % Nozzle Opening.

Figure 12 .Figure 13 .
Figure 12.Power Variations for Conventional and Hooped Pelton Runners Developed with Specific Speed at 100 % Nozzle Opening.

Figure 14 .
Figure 14.Variations in Efficiency for Conventional and Hooped Pelton Runners at 60 % NozzleOpening with Specific Speed.

Figure 15 .
Figure 15.Variations in Efficiency for Conventional and Hooped Pelton Runners at 80 % NozzleOpening with Specific Speed.

Figure 16 .
Figure 16.Variations in Efficiency for Conventional and Hooped Pelton Runners at 100 % NozzleOpening with Specific Speed.

Table 1 .
Table 1 gives the data on the conventional runner Conventional Pelton wheel data.
2. Development of Hooped RunnerThe data in Table1are used to develop the hooped runner.The thickness of the hoop is varying as 2 mm, 3 mm, and 4 mm & the disc diameter is kept as 390 mm which is fabricated from Mild Steel.

Table 2 .
Specifications of Test Rig.