Specific Speed Turbines Research Articles

Optimization design of the elbow draft tube of the hydraulic turbine

The draft tube of the hydraulic turbine serves to recover the kinetic energy of the water flow that leaves the runner. The amount of this energy depends on the turbine type, its specific speed and flow capacity. For high specific speed turbines, influence of the draft tube on the turbine efficiency is more essential. The height of the elbow-type draft tube has a considerable influence on its performance. Statistics show that increasing the height of the draft tube leads to a considerable increase of the turbine efficiency, but it also increases the scope and cost of civil works. The present paper presents the solution of the multi-objective optimization problem directed to minimization of the height of the toroidal elbow-type draft tube and maximization of the overall efficiency of the turbine. During optimization, 11-19 parameters that determine the shape and dimensions of the draft tube were subject to variation. In optimization, efficiency was maximized in two operating points: best efficiency point and full-load point. Cavitation qualities were defined as constraints. In order to correctly assess the turbine efficiency and cavitation, the computational domain included one channel of the distributor, one channel of the runner and draft tube; whereas losses in the spiral case and stay ring were determined using empiric formulae. Within the optimization the flow analysis was performed using 3D steady-state Reynolds averaged Navier Stokes equations closed by the k-ε turbulence model. On solid walls the method of wall functions was used. The turbine head, being the difference of specific energies in the inlet and outlet cross sections, was pre-set as a constant value, while the discharge was determined in the course of solution of the problem. A multi-objective genetic algorithm was used to solve the optimization problem. For the draft tube shapes obtained in the course of optimization, their performance characteristics and flow character were compared by means of CFD. It was found that it is possible to significantly reduce the height of the classical toroidal draft tubes without considerably affecting their performance characteristics.

The relation between internal flow and loss mechanism of unshrouded runners

Abstract High efficiency, low-cost, and reliable hydraulic turbines are in high demand for hydroelectric power generation, especially in medium and small-scale power plants. The authors’ laboratory has been developing unshrouded, i.e., band-opened runners of turbines to meet the demand. Those runners feature highly efficient performance and less expensive manufacturing costs produced by cutting, not by casting. For the sake of developing further improved unshrouded runners, it is necessary to understand their loss mechanism in detail. Therefore, in this study, the internal flow and the loss mechanism are investigated experimentally and computationally, with two different specific speed hydro turbine runners. First, Euler’s theoretical head on each runner is discussed to identify if they are influenced by the distinct specific speed. It demonstrated that the low specific speed turbine consumes greater potential energy in centrifugal force. Secondary, the loss mechanism in the unshrouded runners is investigated. The result showed that the dominant loss is caused by a tip leakage vortex developed by the collision of a leakage flow at the tip clearance and a secondary flow.

A GEOMETRIC DESIGN METHOD OF RADIAL INFLOW TURBINE FROM 0D TO 3D FOR ORGANIC RANKINE CYCLE MICRO POWER GENERATION

Radial turbine is an essential component of Organic Rankine Cycle system and requires a medium to high specific speed turbine. Radial turbine has a compact structure that can easily be made with current additive manufacturing technology if the 3D geometry of turbine components is known. Current researches only conduct 2D geometry design then import it into third-party software to construct the 3D geometry. This paper will explain design methodology to design radial inflow turbines from 0D until 3D using simple tools. The methods used to determine the geometry were based on Aungier, with modification in determining value of a, b, and c in nozzle design and A1 in Volute design to simplify the design process. The tools used in design were MS Excel and Autodesk Inventor. Rotor design starts with determining the two-dimensional parameters. All parameters are calculated based on the angle and velocities occurring in the velocity triangle at the inlet and outlet of the rotor using equations proposed by Aungier. Then, the straight, radial and quasi-normal lines of the blades are drawn based on governing equations. The transformation from 2D to 3D blade coordinates is done by using vector equations. The nozzle is designed by drawing the camber line profile and calculating the nozzle thickness to get the profile based on the governing equations given by Aungier. The volute dimensions are obtained by calculating the area of volute inlet passage and mean radius from mass and momentum conservation equations. A case study is shown in this paper with R134a as working fluid with the following range inlet conditions: mass flow rate at 1-2 kg/s, inlet pressure at 1.5 to 5 bar, inlet temperature at 80 to 130 °C, and power output target between 20 to 25 kW. The CFD results show that the designed turbine performs well with slight wake flow at the pressure side on the rotor inlet. A further study needs to be done in order to check the validity of this method by conducting analysis through experimental.

Numerical Investigation of flow instabilities in Speed No-Load operation of a Bulb turbine

When operating in Speed-No-Load (SNL), turbines function at their synchronous rotating speed with a fraction of discharge at best efficiency point, ready for electrical grid connection. The flow in SNL condition is characterized by high swirling component leading to complex flow structures inside the runner, which generate wide-band pressure fluctuations. When operating in SNL condition, high specific speed turbines, such as axial turbines, can be damaged by significant pressure oscillations attributed to rotating vortex phenomena, alike rotating stall. However, understanding the fundamental flow dynamics behind the phenomena in SNL condition remains challenging due to the stochastic nature of flow. The present research highlights unsteady flow characteristics and flow instabilities in SNL condition of a bulb turbine by URANS simulations based on scale-resolving techniques. The simulation results provide evidence that the development of a rotating vortex array in precession with the runner is a major source of pressure oscillations and torque fluctuations in SNL operation of this bulb turbine.

Flow Inside the Sidewall Gaps of Hydraulic Machines: A Review

The paper critically reviews the current state of the art in flow inside sidewall gaps of hydraulic pumps and turbines. It describes the consequences of the presence of this type of flow in turbomachinery and then relates it to other physical phenomena that determine the behavior, operating characteristics, and overall performance of the machine. Despite the small dimensions of the rotor-stator spaces, the flow in these regions can significantly affect the overall flow field and, consequently, efficiency. The circulation of the fluid inside the gaps and secondary flow that is caused by rotating elements influences the disk friction losses, which is of great importance, especially in the case of low specific speed pumps and turbines. The flow pattern affects the pressure distribution inside a machine and, thus, generates axial thrust. The presence of secondary flow also significantly changes the rotordynamics and can bring about undesirable vibrations and acoustics issues. This article aims to review and summarize the studies that were conducted on the mentioned phenomena. Experimental and numerical studies are both taken into consideration. It proposes some requirements for prospective research in order to fill current gaps in the literature and reveals the upcoming challenges in the design of hydraulic machines.

Differences of Flow Patterns and Pressure Pulsations in Four Prototype Pump-Turbines during Runaway Transient Processes

Frequent working condition conversions in pumped-storage power stations often induce stability problems, especially when the operating point enters the S-shaped region, during which flow transitions and pressure fluctuations are serious. The pump-turbines with different specific speed values show different characteristics, but their differences in stability features are still not clear. In this study, four different pump-turbines were selected to simulate the runaway processes from turbine modes. The similarities and differences of flow patterns and pressure fluctuations were analyzed. For the similarities, pressure pulsations increase gradually and fluctuate suddenly once the backflows occur at the runner inlets. For the differences, the evolutions of backflows and pressure pulsations are related to specific speeds and runner shapes. Firstly, it is easier for the lower specific speed turbines to enter the reverse pump mode. Secondly, the blade lean angle influences the position where backflows occur, because it determines the pressure gradient at the runner inlets. Thirdly, the runner inlet height influences pressure pulsations in the vaneless space, because the relative range of backflow transitions will be enlarged with the decrease of specific speed. Overall, investigating the mechanisms of flow pattern transitions and pressure variations is important for runner design and transient process control.

GPU-accelerated numerical analysis of jet interference in a six-jet Pelton turbine using Finite Volume Particle Method

The Pelton turbine is an impulse turbine typically installed for high head hydroelectric power plants. For a site with a rated head H and discharge Q, a higher specific speed turbine results in a more compact generating unit with reduced manufacturing costs but requires a larger number of jets. However, by increasing the number of jets and specific speed, the water jets tend to interfere, creating a significant energy loss. In the present research, the interaction between two adjacent jets in a six-jet Pelton runner is simulated using a GPU-accelerated particle-based in-house solver based on the 3-D Finite Volume Particle Method (FVPM). The numerical simulations are performed at eight operating points ranging fromN/NBEP=0.89 to N/NBEP=1.31,where N is the runner rotational speed, and BEP is the Best Efficiency Point. The torque and efficiency trends, as well as the speed range in which the jets interfere, are well-captured, which provides confidence in the use of the numerical simulations for the design optimization of Pelton turbines. The simulations, in particular, evidence a significant torque and efficiency drop at high rotational speeds, due to jet interference. Furthermore, jet disturbance yields load fluctuations at rotational speeds both lower and higher than the NBEP, which is likely to amplify fatigue damage. Both phenomena are worth considering that in the design process of a Pelton machine.

Three-Dimensional Inverse Design of Low Specific Speed Turbine for Energy Recovery in Cooling Tower System

A three-dimensional inverse design of a low specific speed turbine is studied, and a set of design criteria for low specific speed turbine runner is proposed, including blade loading distributions and blade lean angles. The characteristics of the loading parameters for low specific speed turbine runner are summarized by analyzing the suction performance of different loading positions, loading slopes and blade lean angles based on the orthogonal experiment design and range analysis. It is found that the blade loading distribution at the band plays a more important role than it does at the crown and it should be fore loaded for both band and crown. The blade lean angle at the blade leading edge should be negative. Then, the blade is optimized through the inverse method by fixing blade lean angle, based on the response surface method. After seeking the optimal value of the response surface function, the optimal result of the design parameters is obtained, which is in conformity with the design criteria and verifies the rationality of the established design criteria for low specific speed turbine.

Study on the impact of blades wrap angle on the performance of pumps as turbines used in water supply system of high-rise buildings

The use of pumps as turbines (PATs) in different applications has attracted increasing attention in recent years. Based on our previous research, the PAT could be used in water supply system of high-rise buildings for water head reduction and hydropower generation, but the efficiency is relatively low. Due to the high water head and low flow rate, PATs used in water pipelines are usually low specific speed turbines. According to the impeller inlet and outlet velocity triangles, the blades wrap angle of a centrifugal pump is not optimal for low specific speed turbines, so the impact of blades wrap angle on the PAT performance should be studied. In this paper, the impact is investigated by numerical and experimental methods. Firstly, a 3D model of an original PAT was built in CAD software, then the model was imported into the ICEM to generate grids and then imported into Ansys CFX for simulation. After simulation, a PAT prototype was tested in the lab test rig to validate the simulation results. The comparison between simulation and experimental results indicates that although some deviations exist, the simulation could provide an acceptable prediction of the PAT’s performance. Finally, four PAT models with different blades wrap angles were developed to compare their performance in terms of both power output and water head reduction. The results show that the blades wrap angle does have effects on the PAT’s efficiency and hydraulic loss, to achieve a good balance between water head reduction and power output. The model with −60° wrap angle has the best performance.

New JSME standard S008 “Performance Conversion Method for Hydraulic Turbines and Pump-Turbines”

JSME Standard S008 “Performance Conversion Method for Hydraulic Turbines and Pump-Turbines” is now being revised and will be published in 2016. This new revision follows the main theory of previous version S008-1999. It enables us to convert the performance of each flow passage component of spiral case, stay vane, guide vane, runner and draft tube of model turbines and pump-turbines to that of prototypes with one-step calculation. The relevant values needed for the performance conversion, e.g. dimension factor, flow velocity factor, relative scalable loss of components δECO, etc. are newly organized as functions of specific speeds of turbines and pump-turbines using polynomial expressions. Additional data for high specific speed turbines are included. The resultant factors for conversion of the specific energy efficiency scale factor FE, the discharge efficiency scale factor FQ and the power efficiency scale factor FT are determined by considering friction coefficient ratio for prototype to the model.

Optimization of hydraulic turbine diffuser

Hydraulic turbine diffuser recovers pressure energy from residual kinetic energy on turbine runner outlet. Efficiency of this process is especially important for high specific speed turbines, where almost 50% of available head is utilized within diffuser. Magnitude of the coefficient of pressure recovery can be significantly influenced by designing its proper shape. Present paper focuses on mathematical shape optimization method coupled with CFD. First method is based on direct search Nelder-Mead algorithm, while the second method employs adjoint solver and morphing. Results obtained with both methods are discussed and their advantages/disadvantages summarized.

Simulation on Balde Duct Vortex in a Francis Turbine Runner Based on S-A Model

If deviating from the optimal operation conditions, flow separation will occur on the blade of the runner in a low specific speed turbine. At this time, the turbulent flow of flow field in the blade duct will be in a strong non-equilibrium state, and thus the blade duct vortexes will be generated. To further study the mechanism of blade duct vortexes and to control the generation of these vortexes, Spalart-Allmaras (S-A) model was used to numerically simulate and calculate the internal flow in the low specific speed turbine runner under low load conditions. The blade duct vortexes in the turbine runner were accurately predicted. The effect of short blade in eliminating and reducing the vortexes in the low specific speed turbine runner was analyzed and compared.

Simulation on Balde Duct Vortex in a Francis Turbine Runner Based on S-A Model

If deviating from the optimal operation conditions, flow separation will occur on the blade of the runner in a low specific speed turbine. At this time, the turbulent flow of flow field in the blade duct will be in a strong non-equilibrium state, and thus the blade duct vortexes will be generated. To further study the mechanism of blade duct vortexes and to control the generation of these vortexes, Spalart-Allmaras (S-A) model was used to numerically simulate and calculate the internal flow in the low specific speed turbine runner under low load conditions. The blade duct vortexes in the turbine runner were accurately predicted. The effect of short blade in eliminating and reducing the vortexes in the low specific speed turbine runner was analyzed and compared.

The Operating Point Determination of Special Turbine in Cooling Tower

This article introduces the theory analysis and experimental study on the performance of special turbine in cooling tower. The author analyses the speed-flow characteristics of turbine-fan unit and the speed-flow characteristics of ultra-low specific speed turbine itself. At the same time the author points out the principles and methods to determine the best operating point of turbine-fan unit. So it is very important to determine whether the specific turbine is in the best condition.

Exit-flow velocity survey of two single-tangential-inlet vaneless turbine volutes

The most cited analytical technique for designing turbine volutes is to assume the throughflow is free from torque, although for this assumption to hold, the volute walls must lie near what would be streamlines in an unbounded free vortex-plus-sink flow. The single tangential inlet design, with inlet offset decreased and diameter increased to attain the weak exit swirl required by high specific speed turbines, deviates from such a shape, and the volute’s internal geometry is no longer torque-free. It is desired to know the actual time-averaged flow leaving such a volute, so that a rotor can be designed to compliment it. For two existing single tangential inlet volutes, time-averaged radial and tangential velocity and static pressure measurements of exit flow have been obtained on a cylindrical cut plane through the radial-inflow section using a three-port yawmeter in air. The Reynolds numbers based on inlet pipe mean conditions, around 10 5, are well into the fully-turbulent regime and on the order of comparable water turbines. A comprehensive map of time-averaged exit flow of both volutes is presented. The integrated values of gross angular momentum flux change and total pressure loss coefficient are tabulated. Circumferential variation of flowrate and swirl strength highlight unexpected differences in outlet flow between the two volute designs. Results are presented alongside corresponding numerical results from the commercial package Fluent (Fluent, Inc., Lebanon, NH, USA) using Reynolds stress, k– ω, and inviscid flow models. In both volutes, measured gross exit angular momentum flux was more than 1.7 times what the zero-torque assumption would predict when blindly applied to the volute as a whole. This discrepancy is attributed to significant turning near the volute’s inlet region leading to an updated view of what an appropriate control volume is when applying the zero-torque assumption. Additionally, variation of both radial and tangential velocity in both the circumferential and axial directions on the order of 15% of the mean value reveal that volute swirl characterization by a single measurement would have a significant associated uncertainty.

Virtual model test for a Francis turbine

High accuracy performance prediction method based on entire flow passage for a Francis turbine is presented. The performance is predicted by solving unsteady Reynolds-Averaged Navier-Stokes equations combined with Reynolds- Stress model and Bubble two-phase flow model. The prediction accuracy was evaluated to compare with the model test results for three different specific speed turbines. By comparing the simulation results with model test results for efficiency characteristic, pressure fluctuation characteristic and cavitation characteristic, it was proved that numerical model test presented in this paper could predict important characteristics of Francis turbine with high accuracy not only quantitatively but also qualitatively. As a result, it was concluded that numerical model test is going to be a more realistic estimation tool for hydraulic performance of Francis turbine and hence contributes to cost reduction in the development of Francis turbine.

Development of a Positive Displacement Micro-Hydro Turbine

The objective of this study is to develop an efficient turbine that can be used to extract micro hydropower potential of a water supply system. For the case of high head and critical low flow rate range of micro hydropower resources, it requires very low specific speed turbines which are lower than conventional impulse turbines’ specific speed. For this purpose, we develop a new Positive Displacement Turbine (PDT). In order to reveal the performance characteristics of the new turbine, one conventional impulse turbine, which is used for automatic water faucet system, was tested for comparison. The test results show that the PDT was much more efficient than a conventional turbine and it can sustain high efficiency under the wide range of operating conditions. In addition, the efficiency of the PDT is much improved when reducing its side clearance. The pressure pulsations at the inlet and outlet of the PDT can be considerably minimized by using simple dampers.

Dynamic orifice model on waterhammer analysis of high or medium heads of small hydropower schemes

The most severe hydropower transients are induced in long hydraulic circuits due to extreme operating conditions. A computational model was developed in order to on one hand ensure waterhammer system control and, on the other hand, provide a more reliable and easier analysis for different specific speed turbines and alternative solutions of the system as a whole, through interaction between different hydraulic components. In reaction turbines, runaway conditions and guide vane closure cause significant discharge variations and pressure fluctuations that can affect the design of conveyance systems. A new approach for groups modelling as dynamic orifices concept was developed enabling the characterisation of the integrated system. The simulation results were compared with laboratory tests. This model can be used in the initial stages of civil works design as an efficient way to better characterise the hydrodynamic behaviour of the system when equipped with reaction turbines.

Simplified Tidal Barrage for Small-Scale Applications

The performance of a tidal barrage with no gates and continuous flow through its turbines has been investigated by simulation. Double-effect generation could be achieved with an axial-flow turbine that reverses direction after each half of the tidal cycle. Such a system might be attractive for unmanned operation in remote areas, perhaps on a smaller scale than for conventional barrage systems. Predictions have been made about the energy obtainable from such a system, and the conditions necessary to maximize energy capture have been determined. A mathematical model of a high specific speed turbine, incorporated into the prediction procedures, allowed the behavior of the system under varying tidal range (and hence varying head) to be examined. Differential heads across the turbine are somewhat less than those achievable with a gated barrage, and performance is correspondingly affected in very small tidal ranges. Variable-speed turbine operation was found to be beneficial in these circumstances. Overall levels of predicted output were very encouraging and compare surprisingly well with conventional barrage schemes. The concept would seem to merit further investigation.

Achievements in High Specific-Speed Water Turbines

The steady increase in the capacity of generating sets created a demand for the high specific-speed turbine which was met by several experimenters. “Specific speed” is the speed at which a turbine will run under unit head when developing unit power, and nowadays a “high specific-speed” water turbine denotes one having a runner of the propeller type and a specific speed between 100 and 230 r.p.m. Difficulties were encountered in the development of propeller turbines, especially in connexion with cavitation. Laboratory tests and the use of visual study methods have played an important part in the solution of these difficulties. The method of fixing suction head in conjunction with laboratory test results is explained, and a comparison is made between the various forms of suction tube in use. Much work has been done to overcome the effects of localized cavitation, and stainless steel has been found to be very effective, especially when runners are cast entirely of that material. A method of operation has been developed to dispense with the use of inlet sluice gates in large machines. Several methods in use for operating the movable runner blades are described. The introduction of the high specific-speed turbine has led to a large increase in the number of automatic stations. The great size attained by these turbines has entailed the construction of equally large generators, the development of which has had its own problems.