16 research outputs found

    Pelton turbine:identifying the optimum number of buckets using CFD

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    A numerical case study on identifying the optimum number of buckets for a Pelton turbine is presented. Three parameters: number of buckets, bucket radial position and bucket angular position are grouped since they are found to be interrelated. By identifying the best combination of the radial and angular position for each number of buckets it is shown that reduction in the number of buckets beyond the limit suggested by the available literature can improve the efficiency and be beneficial from the manufacturing complexity and cost perspective. The effect of this reduction in the amount of buckets was confirmed experimentally

    Optimisation and efficiency improvement of pelton hydro turbine using computational fluid dynamics and experimental testing

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    The aim of this PhD research was to develop a generic optimisation method for Pelton turbine runners and assess the key design parameters using Computational Fluid Dynamics (CFD). This optimisation was applied on a modern commercial Pelton turbine runner taken as a base design. The design together with the field knowledge and experience was provided by a turbine manufacturing company Gilbert Gilkes and Gordon Ltd. to establish the state of the art starting point. The work described in this thesis can be divided into three main parts: 1) developing of numerical modelling technique by combining current commercial CFD models with engineering assumptions to produce results of acceptable accuracy within reasonable timescales and verifying this technique, 2) optimising the Pelton runner provided by Gilkes to produce better efficiency and simplify its design, 3) manufacturing of original and optimised design model runners and experimentally testing them. The numerical techniques created during part 1) included many numerical and physical assumptions to simplify the problem. This was necessary because accurate modelling of impulse turbines (Pelton in this case) that include complex phenomena like free surface flow, multi fluid interaction, rotating frame of reference and unsteady time dependent flow is a challenge from a computational cost point of view. These simplifications included the usage of symmetry plane and modelling of only two consecutive buckets to reduce the size of the computational domain. Casing and any backsplash effects were not modelled at all expecting that a runner with higher hydraulic efficiency would reduce these effects since the remaining energy in the water that leaves the bucket would be reduced. For domain discretisation it was decided to use two types of mesh sizing. Fine mesh simulation was mesh independent but the required time to solve was still unfeasible for parametric optimisation. Therefore, this fine mesh sizing was used only at the key points to verify the design changes. Coarse mesh simulation was not mesh independent but reduced the timescale by the factor of 5; therefore, making it possible to acquire the results within a reasonable timescale. It was observed that the coarse meshes slightly underpredict the efficiency as compared to the fine mesh simulations. However, it was assumed that this underprediction is going to be constant when comparing small changes in geometry. Based on this assumption the coarse mesh simulations were chosen for design optimisation. In part 2) some of the design parameters were expected to be interrelated and therefore were grouped together and analysed using Design of Experiments technique, some of the parameters were assumed to have low relation to other parameters and were analysed individually. In the end, CFD was predicting a 2.5 % increase of the original efficiency. Moreover, a reduction in the amount of buckets to 15 (originally the runner contained 18 buckets) was investigated and provided some promising results. This reduction can be very beneficial from the manufacturing complexity and cost point of view. In part 3) which was the final stage, three model runners were manufactured and experimentally tested in the Laboratory of Hydraulic Turbomachines at the National Technical University of Athens. It was decided to manufacture the original runner, the runner that contains 18 optimised buckets and the runner that contains 15 optimised buckets. The experimental results confirmed the increase in the efficiency and proved this optimisation technique to be valid

    Development of hydro impulse turbines and new opportunities

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    Hydro impulse turbines are often referred to as a mature technology having been invented around 100 years ago with many of the old design guidelines producing machines of a high efficiency. However with recent advances in Computational Fluid Dynamics (CFD) it is now possible to simulate these highly turbulent multiphase flows with good accuracy and in reasonable timescales. This has opened up an avenue for further development and understanding of these machines which has not been possible through traditional analyses and experimental testing. This paper explores some of the more recent developments of Pelton and Turgo Impulse turbines and highlights the opportunities for future development

    Flow modeling in Pelton turbines by an accurate Eulerian and a fast Lagrangian evaluation method

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    The recent development of Computational Fluids Dynamics (CFD) has allowed the flow modeling in impulse hydro turbines that includes complex phenomena like free surface flow, multi fluid interaction, and unsteady, time dependent flow. Some commercial and open-source CFD codes, which implement Eulerian solving methods, have been validated against experimental results showing satisfactory accuracy. Nevertheless, further improvement of the flow analysis accuracy is still a challenge, while the computational cost is very high and unaffordable for multi-parametric design optimization of the turbine’s runner. In the present work, a CFD Eulerian approach is applied at first, in order to simulate the flow in the runner of a Pelton turbine model installed at the laboratory. Then, a particulate method, the Fast Lagrangian Simulation (FLS), is used for the same case, which is much faster than the Eulerian approach, and hence potentially suitable for numerical design optimization, providing that it can achieve adequate accuracy. The results of both methods for various operation conditions of the turbine, as also for modified runner and bucket designs, are presented and discussed in the paper. In all examined cases the FLS method shows very good accuracy in predicting the hydraulic efficiency of the runner, although the computed flow evolution and torque curve during the jet-runner interaction exhibit some systematic differences from the Eulerian results

    Experimental investigation and analysis of the spear valve design on the performance of Pelton turbines:3 case studies

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    The impact of the nozzle and spear valve configuration on the performance of a Pelton turbine is investigated both experimentally and computationally. A previously published computational fluid dynamics (CFD) study has shown that injectors with noticeably steeper nozzle and spear angles, 110° and 70° respectively, attain a higher efficiency than the industry standard 80° and 55°. As a result, three injector design cases were manufactured for experimental testing. Two of those cases were the standard (80/55) design, with nozzle and spear tip angles of 80° and 55° and the Novel 1 design (110/70) with nozzle and spear tip angles of 110° and 70° based on previously published CFD optimisation studies. These studies showed that increasing the nozzle and spear angles to the upper limit of the investigated test plan gave higher efficiencies. The response surfaces suggested that the optimum nozzle and spear angles could be even steeper. That is why, an additional case, a third design (Novel 2) with even steeper angles (150/90) was also manufactured and tested. The experimental tests were carried out in a single jet operation using the upper injector on the Gilkes Pelton runner with series Z120 buckets. The results show that two novel injector design cases produce higher efficiencies than the standard design, when tested with a Pelton runner. An important gain of about 1% in efficiency is achieved at the Best Efficiency Point of the turbine. Furthermore, the improvement is even more pronounced at lower flow rates, where the spear valve opening is smaller and the geometry of the injector has even larger effect. To discuss and analyse these experimental observations, a further 2D axisymmetric CFD analysis is performed. This analysis shows a similar trend to the experimental results. The CFD results show that the largest amount of energy is lost at the region upstream of the nozzle exit, where the static pressure is converted into the dynamic pressure. This conversion starts earlier in case 1, the Standard injector design, at about twice the distance compared to the Novel designs, cases 2 and 3. Consequently, the flow must travel in this region at an increased velocity and it is shown that this region is longer in the Standard injector. Hence, its friction losses are higher. However, the differences between the designs calculated in CFD are about a factor of 2 lower than the experimental results, indicating that the 3D secondary flow mechanisms arising from the geometry upstream of the nozzle and spear tip also affect the performance of the spear valve and the Pelton runner. The mismatch between the efficiency increase magnitude observed experimentally and modelled using the axisymmetric case suggests that the steeper angle injectors cope better with secondary velocities in the flow

    Investigating the influence of the jet from three nozzle and spear design configurations on Pelton runner performance by numerical simulation

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    This paper reports the initial results of three dimensional CFD simulations of the jet – runner interactions in a twin jet horizontal axis Pelton turbine. More specifically, the analysis examines the impact of the nozzle and spear valve configuration on the performance of the runner. Previous research has identified that injectors with notably steeper nozzle and spear angles attain a higher efficiency than the industry standard. However, experimental testing of the entire Pelton system suggests that there appears to be an upper limit beyond which steeper angled designs are no longer optimal. In order to investigate the apparent difference between the numerical prediction of efficiency for the injector system and the obtained experimental results, four different jet configurations are analysed and compared. In the first configuration, the interaction between the runner and an ideal axisymmetric jet profile is investigated. In the final three configurations the runner has been coupled with the jet profile from the aforementioned injectors, namely the Standard design with nozzle and spear angles of 80° & 55° and two Novel designs with angles 110° & 70° and 150° & 90° respectively. The results are compared by examining the impact the jet shape has on the runner torque profile during the bucket cycle and the influence this has on turbine efficiency. All results provided incorporate the Reynolds averaged Navier Stokes (RANS) Shear Stress Transport (SST) turbulence model and a two-phase Volume of Fluid (VOF) model, using the ANSYS® FLUENT® code. Therefore, this paper offers new insights into the optimal jet – runner interaction

    Experimental investigation and analysis of three spear valve designs on the performance of Turgo impulse turbines

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    Several numerical investigations into the impact of the spear and nozzle configuration of impulse turbine injectors can be found in the literature, however there is little or no experimental data available for the effect on Turgo impulse turbine performance. A recent 2D numerical Design of Experiments (DoE) study found that much steeper nozzle and spear angles than the industry standard produced higher efficiencies. This work was extended to compare the performance of an industry standard injector (with nozzle and spear angles of 80° and 55°) and an improved injector with much steeper angles of 110° and 70° using a full 3D simulation of the injector, guide vanes and first branch pipe. The impact of the jets produced by these injectors on the performance of a Turgo runner was also simulated. The results for both CFD tools used suggest that steeper injector nozzle and spear angles reduce the injector losses, showing an increase in efficiency of 0.76% for the Turgo 3D injector. In order to investigate the numerical results from the previous studies further, three Turgo impulse turbine injectors were manufactured by Gilbert Gilkes & Gordon Ltd for testing on the 9” Gilkes HCTI Turgo rig at the Laboratory of Hydraulic Machines, National Technical University of Athens (NTUA). The injector designs tested were the standard (80/55) design, with nozzle and spear tip angles of 80° and 55° and the Novel 1 design (110/70) with nozzle and spear tip angles of 110° and 70° based on previously published CFD optimisation studies. The optimisations in the previous studies showed that the nozzle and spear angles in the upper limit of the investigated test plan, which was much higher than current industry guidelines, gave higher efficiencies. The DoE response surfaces in that study suggested that the optimum nozzle and spear angles may be even steeper and therefore an additional, third design (Novel 2) with even steeper angles (150/90) was also manufactured and tested. This paper presents the experimental data obtained for the three injector designs which were tested in a Turgo model turbine at various rotating speeds and flow rates. The 70 kW Turgo was coupled to a 75kW DC generator which allowed continuous speed regulation. The inlet conditions into the Turgo model turbine were controlled by a high head adjustable speed multistage pump of nominal operation point Q=290 m3/h, H=130 m (coupled via a hydraulic coupler to a 200 kW induction motor) which pumped water from the 320 m3 main reservoir of the Lab. The tests were carried out in single jet and twin jet operation. Testing and calibration of all the sensors was carried out according to testing standard IEC 60193 Hydraulic turbines, storage pumps and pump-turbines – Model acceptance tests (IEC 60193:1999). The results show that the Novel 2 injector performs best overall, which is consistent with the results obtained in previous 2D injector simulations. The achieved turbine efficiency with this injector is of the order of 0.5-1% higher than the Standard design, for both single and twin jet operation. The Novel 1 injector’s performance is between the Standard and Novel 2 injectors overall. Some images of the jets were also taken at various openings and are presented to qualitatively analyse the impact of each injector design on the disturbances on the outside of the jet. A further 2D axisymmetric CFD analysis is carried out to validate the measurements and to analyse the mechanisms which lead to injector losses. The results found that the majority of the losses occur in the region just upstream of the nozzle exit, where the static pressure is converted into dynamic pressure and the flow accelerates. In the Standard design, this conversion begins sooner and the flow travels over a longer distance at higher velocities leading to an increase in the losses. The CFD results found the differences between the designs to be smaller than the experiments however the trend of the results was similar, suggesting that the steeper angle injectors achieve higher efficiencies and better jet quality. The next stage of this research is to carry out a CFD analysis of the three injector designs in 3D, including the guide vanes and branch pipes, to investigate the impact of the steeper angles on the secondary velocities within the jet and the impact this has on the runner performance

    Investigating the influence of the jet from three nozzle and spear design configurations on Pelton runner performance by numerical simulation

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    This paper reports the initial results of three dimensional CFD simulations of the jet – runner interactions in a twin jet horizontal axis Pelton turbine. More specifically, the analysis examines the impact of the nozzle and spear valve configuration on the performance of the runner. Previous research has identified that injectors with notably steeper nozzle and spear angles attain a higher efficiency than the industry standard. However, experimental testing of the entire Pelton system suggests that there appears to be an upper limit beyond which steeper angled designs are no longer optimal. In order to investigate the apparent difference between the numerical prediction of efficiency for the injector system and the obtained experimental results, four different jet configurations are analysed and compared. In the first configuration, the interaction between the runner and an ideal axisymmetric jet profile is investigated. In the final three configurations the runner has been coupled with the jet profile from the aforementioned injectors, namely the Standard design with nozzle and spear angles of 80° & 55° and two Novel designs with angles 110° & 70° and 150° & 90° respectively. The results are compared by examining the impact the jet shape has on the runner torque profile during the bucket cycle and the influence this has on turbine efficiency. All results provided incorporate the Reynolds-averaged Navier Stokes (RANS) Shear Stress Transport (SST) turbulence model and a two-phase Volume of Fluid (VOF) model, using the ANSYS® FLUENT® code. Therefore, this paper offers new insights into the optimal jet – runner interaction

    State of the art in numerical modeling of Pelton turbines

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    Pelton turbine (or Pelton wheel) is among the most efficient impulse turbines and has retained its existence in hydropower for well over a century. However unlike in the development of the reaction turbines, where Computational Fluid Dynamics (CFD) have been successfully applied for more than 20 years now, up until recently it was not feasible to perform CFD analysis of Pelton turbines due to the nature of the flow which is much more complex than in the reaction turbines. The recent developments in CFD models and tools together with the continuous increase in computational resource are bringing the CFD modelling up to a level suitable for industrial applications in development of Pelton turbines. Current published research in the field worldwide can be divided into two distinct branches of CFD models: the Eulerian specification of flow field, which tends to be more accurate, but also more computationally expensive, and the Lagrangian specification which is known to be less computationally demanding, however to date it cannot compete with Eulerian specification in terms of accuracy. This review paper is aiming at establishing the state of the art in numerical modeling of Pelton Turbines and would serve as guidance when choosing the optimum CFD modeling methodology and software available
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