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Impulse turbines are widely used in hydropower plants around the world due to their high efficiency in a larger operating range compared to reaction turbines. State of the art Pelton turbines can give over 90% efficiency at the designed operation load. However, there lies a possibility of improving the efficiency at off design condition for these turbines by optimizing the design of the nozzles, runner and/or housing. Prediction of the performance of the turbines by numerical techniques allows a quicker design optimization and at a much lower cost. In the case of impulse turbines, the use of CFD is limited by the complex nature of the flow, interaction of the jets, water/air mixture and interference of water after the impact on the successive buckets. Unlike reaction turbines, where the performances of the turbines could be studied through time-independent analysis to some extent, transient simulations are inevitable for the impulse turbines. Moreover, need for the multiphase models add to the overall complexities of CFD. Some recent development includes the use of Lagrangian scheme, which has reduced the computational efforts significantly. In the conventional Eulerian schemes, it is found that the numerical domains are usually simplified, either by using symmetry (half bucket) or rotational periodicity options. With the development of the computational capacity, some recent studies have also performed full turbine's simulation, using nearly 50 million mesh elements. The design optimizations are performed on the shapes of the bucket, jet, deflectors as well as the turbine's casings. However, validation of the numerical solutions remains a challenging topic with the quality of the mesh being used currently. Nevertheless, progresses in the numerical techniques and computing power within the last 15 years have strengthened the prospects of utilizing CFD and FEM as primary tools for validating and optimizing the design of the impulse turbines. • There is a remarkable prospect for design and optimization of impulse turbines using numerical technique. • Lagrangian approach was found to be convenient for impulse turbines to study the free surface flows. • Eulerian approach was found to be more popular with the commercially available codes. • Validation of the numerical solutions in the case of these turbines remains a challenging topic. [ABSTRACT FROM AUTHOR]

Abstract Expander is one of the core components in organic Rankine cycle (ORC). High rotating speed and limited working range are the key limitations on using turbine expander in small-scale ORC system. In this work, an axial turbine, consisting of 6 nozzles and 36 blades with mean diameter of 160 mm, is designed with partially admitted method to evaluate the feasibility of reducing rotating speed for small mass flow conditions and extending working range with R245fa used as working fluid. The results from 3D simulation show reasonable performance in design and off-design conditions. In design condition, the mass flow rate, turbine power output and turbine efficiency using Laval nozzle are 1.035 kg/s, 17.0 kW and 54.1%, higher than using convergent nozzle. However, the performance using convergent nozzle is better than using Laval nozzle when total mass flow is lower than 0.6 kg/s. Furthermore, reducing the number of nozzles can effectively increase power output of turbine and thermal efficiency of ORC for small mass flow conditions. With the designed turbine using 6 Laval nozzles and rotating at 10,000 RPM, the maximum net power output and thermal efficiency of bottoming ORC are 18.3 kW and 7.8%, respectively. Highlights • Turbine with low rotating speed is specially designed for small-scale ORC. • Two type nozzles with small partially admitted rate are compared. • The real power output and efficiency at design condition are 17.0 kW and 54.1%. • The maximum net work output and thermal efficiency of ORC are 18.3 kW and 7.8%. • Adjusting nozzle number can effectively extend working range of turbine. [ABSTRACT FROM AUTHOR]

Abstract In Pelton turbine, two-phase homogeneous model is the fundamental base to investigate the flow inside the turbine. Meanwhile, flow pattern in section of the distributor and bifurcation pipe produces the secondary flow which influences the shape of the jet downstream 6 nozzles. This paper numerically investigated the dynamic process using CFX commercial code under different operating conditions. Volume of Fluid(VOF) model was used to capture the air/water interface. This work focuses on the jet bifurcation problem which can seriously influence the Pelton turbine performance and furthermore it is not yet investigated in hydraulic research. Results indicate that the shape of the distributor induces the secondary flow and Dean vortex structure which are symmetric with the central horizontal plane of the pipe and nozzles. On the other hand, this sequence of vortex develops and transits along the flow and finally affects the shape of jet. Effect of secondary flow in the distributor stretches the jet flow in vertical direction and compress it in horizontal direction. Meanwhile, bifurcating phenomenon is observed outside the nozzle which is also obtained in classical experimentations. This study reveals that the secondary flow is important to be taken into consideration to optimize the performance of Pelton turbine. Highlights • Six nozzles of a Pelton turbine are numerically simulated with unsteady two-phase model. • Secondary flow is detected in the flowing parts and Dean vortex are observed clearly. • Jet bifurcation is visualized and quantitatively confirmed by the former experimentation in the literature. [ABSTRACT FROM AUTHOR]

Wave energy exploitation is of great interests nowadays due to its sustainability, reliability and large potential. This paper aims to firstly represent the turbine designs used in oscillating water column (OWC) system for wave energy conversion, and secondly to improve the rotor blade performances of a bidirectional radial impulse turbine which is in use in this system. Symmetrical blade profile based on use of circular arcs and straight lines has been adopted. A new procedure for optimizing the geometrical blade parameters has been introduced in this work. The design of experiments (DOE) method has been adopted with four blade geometrical parameters and two levels for reducing the number of numerical tests to determine the optimal profile in order to maximize and equilibrate the efficiency between exhalation and inhalation modes. Numerical tests in the whole turbine geometry have been carried out with ANSYS FLUENT 15.0. Significant differences have been noticed by varying the rotor blade profile. A flow analysis for the duct and the turbine elements; IGV, rotor and OGV has been performed. Performances of the turbine equipped with rotor designed with circular profiles in the present study have been compared to those of a previous design based on use of elliptical profiles. An increase of about 18% in terms of the peak efficiency in the inhalation mode has been noted for the improved design, and the performances have been equilibrated in both mode; inhalation and exhalation. [ABSTRACT FROM AUTHOR]

Wave energy harvesting systems mostly have low power production capability because of unoptimized design of the system components. A bidirectional flow impulse-turbine used in such a system has efficiency less than 40%, and it is required to design the turbine for a higher efficiency. Present work finds an optimal design and shows design-variable sensitivity to the turbine efficiency. The problem is solved using numerical analysis technique. The flow through the turbine was analyzed by solving the Reynolds-averaged Navier-Stokes equations (RANSE). The design variables; namely number of rotor blades and number of guide vane, guide vane angle and guide vane profile were modified to maximize the turbine efficiency. Using the Latin hypercube sampling technique, sample points were selected from a design space defined by lower and upper limits of the variables. Then, several surrogates were constructed using the RANSE calculated results, and the turbine performance was optimized. The results show that guide vane angle is the most sensitive parameter, while the guide vane profile has negligible effect on efficiency. A hybrid genetic algorithm searched the optimal design point. The relative mean efficiency enhancement over a wide range of flow coefficient was approximately 24%, while it was 28% at maximum efficiency point. [ABSTRACT FROM AUTHOR]

The axial-flow impulse turbine is recently utilized for oscillating water column wave energy conversion because of its wider operating range and having no stalling point. Most previous experimental studies conducted in the oscillating air flow testing rigs can give little information of the steady-state performance of impulse turbines. A constant flow test rig was set up in this study. The steady state performance of an axial-flow impulse turbine was studied by carrying out over 140 cases. The total pressures at the chamber and atmosphere sides and torque output under various incident flow velocities and prescribed rotation speeds are presented. Effects of Reynolds number on the dimensionless coefficients are reported, which are also compared by a previous study. The scatter diagram of efficiencies provides detailed information of turbine performance under different testing conditions. The peak value of turbine efficiency is found to be 0.48 at the flow coefficient of 1.0. [ABSTRACT FROM AUTHOR]

In this paper the problem of the response evaluation of nonlinear systems under multiplicative impulsive input is treated. Such systems exhibit a jump at each impulse occurrence, whose value cannot be predicted through the classical differential calculus. In this context here the correct jump evaluation of nonlinear systems is obtained in closed form for two general classes of nonlinear multiplicative functions. Analysis has been performed to show the different typical behaviors of the response, which in some cases could diverge or converge to zero instantaneously, depending on the amplitude of the Dirac's delta. [ABSTRACT FROM AUTHOR]

We present two control strategies for an oscillating water column-wave energy converter (OWC-WEC) in the time domain. We consider a fixed OWC-WEC on the open sea with an impulse turbine module. This system mainly consists of a chamber, turbine and electric generator. For the time domain analysis, all of the conversion stages considering mutualities among them should be analyzed based on the Newtonian mechanics. According to the analysis of Newtonian mechanics, the hydrodynamics of wave energy absorption in the chamber and the turbine aerodynamic performance are directly coupled and share the internal air pressure term via the incompressible air assumption. The turbine aerodynamics and the dynamics of the electric generator are connected by torque load through the rotor shaft, which depends on an electric terminal load that acts as a control input. The proposed control strategies are an instant maximum turbine efficiency tracking control and a constant angular velocity of the turbine rotor control methods. Both are derived by Lyapunov stability analysis. Numerical simulations are carried out under irregular waves with various heights and periods in the time domain, and the results with the controllers are analyzed. We then compare these results with simulations carried out in the absence of the control strategy in order to prove the performance of the controllers. [ABSTRACT FROM AUTHOR]

The Turgo Impulse turbine provides a unique and novel solution to increasing the capacity of a hydraulic impulse turbine while maintaining the nozzle and spear injector system (as used in Pelton turbines) for flow regulation. This has produced a turbine which operates in the higher flow ranges usually reserved for Francis machines while maintaining a relatively flat efficiency curve, characteristic of impulse machines. Since its invention nearly 100 years ago, the Turgo turbine has been installed in thousands of locations across the globe. The majority of the development of the Turgo turbine design has been through the use of paper based and experimental studies however recent advances in computational fluid dynamics (CFD) tools have allowed the simulation of the complex, highly turbulent, multiphase flows associated with impulse turbines and some work has been done in applying this to the Turgo design. This review looks at the development of the of the Turgo turbine since its invention in 1919 and includes the paper-based analyses, experimental studies and the more recent CFD analyses carried out on the design. [ABSTRACT FROM AUTHOR]

Impulse turbine is used in recent times as a self-rectifying turbine in OWC wave energy convertor. It has been found from field test data in Indian OWC plant that the profiles of air flow velocity is not symmetric in exhalation and reverse directions. The velocity is higher during the exhalation (chamber to atmosphere) than in the reverse direction. The modification proposed in this paper is to set the rotor blade pitch asymmetrically with a non-zero value of setting angle. It is expected to give a better performance in a wave cycle under the above air flow conditions. A 3D CFD model based on Fluent is validated by corresponding experimental data and is used for the numerical simulation of the turbine performance for different setting angles under steady conditions. All the calculations were carried out under the steady conditions. Pseudo-sinusoidal velocity pattern was used to represent the realistic air flows in the pilot plant. Quasi-steady analysis was then employed for calculating the mean efficiency for a certain variation of air flow velocity with time during a wave cycle. The clear flow separations were found at the suction side near the trailing edge and the high velocity domain occurs at the suction side near the midstream flow path. The pressure distribution show similar characteristics on both pressure and suction sides. Input coefficient, torque coefficient and mean efficiency are calculated for evaluating the turbine performance under various rotor blade setting angles. For the typical value of 0.6 for the ratio of velocity amplitude at inhalation to that at exhalation, the rotor blade setting angle of 5° was found to be optimum for the modified impulse turbine to achieve the best mean efficiency in a wave cycle. [ABSTRACT FROM AUTHOR]

Impulse turbines with tailwater depression are an efficient choice for energy recovery in drinking water systems. To reduce negative effects of detrained air downstream of the turbine casing, such as corrosion and reduced transport capacity, main processes and possible reduction of air detrainment have been investigated in detailed experimental studies on a micro-hydro Pelton machine with tailwater depression. The experimental setup has been designed to distinguish between dissolved and undissolved air detrainment. In case of dissolved air detrainment there is a clear relation between the amount of dissolved air and the counter pressure as well as the flow rate parameter. Experimental results for the amount of undissolved air detrainment show dependencies on geometrical dimensions, velocity coefficient, casing water level and the flow rate of the turbine. To avoid undissolved air detrainment, design equations are deducted for dimensioning the casing height and the casing diameter. Therefore the presented investigations show how air detrainment can be controlled or rather minimized by optimised casing-design of impulse turbines. [ABSTRACT FROM AUTHOR]

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 in the hydraulic design of Pelton and Turgo Impulse turbines and highlights the opportunities for future development. [ABSTRACT FROM AUTHOR]

This paper uses two modern commercial CFD software packages to compare the performance of a standard and improved impulse turbine injector developed in a previous study. The two injector designs are compared by simulating the two-dimensional (2D) axissymmetric cases as well as full three-dimensional (3D) cases including the bend in the branch pipe and the guide vanes. The resulting jet profiles generated by these simulations are used to initialize the inlet conditions for a full Pelton and Turgo runner simulation at different operating conditions in order to assess the impact of the injector design on the performance and efficiency of a real impulse turbine. The results showed that the optimized injector design, with steeper nozzle and spear angles, not only attains higher efficiencies in the 2D and 3D injector simulations but also produces a jet which performs better than the standard design in both the Pelton and the Turgo runner simulations. The results show that the greatest improvement in the hydraulic efficiency occurs within the injector with the improved design, showing an increase in efficiency of 0.76% for the Turgo 3D injector and 0.44% for the Pelton 3D injector. The results also show that in the case of the 3D injector, the improved injector geometry produces a jet profile which induces better overall runner performance, giving a 0.5% increase in total hydraulic efficiency for the Pelton case and 0.7% for the Turgo case. [ABSTRACT FROM AUTHOR]

The work presented here focuses on evaluating the interaction between ocean waves and a device designed to harness their energy. The device, called Blow-Jet, consists of a narrowing structure that concentrates waves and transforms their energy from potential and kinetic to kinetic, turning the oscillatory flow into an intermittent jet that can be easily guided to an impulse turbine. The new device is designed to feed low energy demands such as secondary fuel generators (e.g. hydrogen production). Several experimental tests were carried out to investigate the hydraulic behavior of the Blow-Jet. Results indicate that even for low energetic wave trains, the device is able to capture energy and amplify the water velocity, which is indeed energy conversion capacity. In order to evaluate the flow field inside the device, a high resolution 3D numerical model OpenFOAM is utilized. Good agreement was found between the pressures exerted to the walls of the device with that computed with the numerical tool. The combination of numerical and experimental results confirmed the optimal geometry, location and position of the Blow-Jet, while the need for some geometric changes was detected after the analysis of numerical model results. The Blow-Jet has shown itself to be a potential WEC with acceptable efficiency. [ABSTRACT FROM AUTHOR]

Real-life engineering problems have multiple objectives, which mostly are conflicting in nature, and these problems can be solved through multi-objective optimization (MOO) procedure. In the present problem, a high-fidelity computational fluid dynamics model coupled with multiple-surrogate assisted genetic algorithm based MOO has been solved for performance enhancement of a wave energy extracting axial impulse turbine. Response surface approximation, Kriging, neural network and a weighted-average surrogate (WAS) were used to generate population for the MOO procedure and Pareto optimal fronts (PoF) of the objectives were produced. The design variables were number of rotor blade and guide vanes and the objectives were minimization of pressure drop and maximization of shaft power of the turbine. It was found that a cross-validation error analysis is inevitable to find the degree of fitness of a surrogate. The WAS-produced PoF shows better performance as compared to that of the other surrogates. The surrogates based on minimum cross-validation errors produce slightly lesser performance than the WAS. The efficiency, which is a function of both the objectives, was relatively increased by ∼11% through the current investigation. [ABSTRACT FROM AUTHOR]

OWC (Oscillating water column) wave power plants are widely used in ocean wave energy conversion. It is universally accepted that for a good efficiency of the plant, the turbine and the chamber should match each other according to the wave climate site. In this work an alternative methodology is presented for coupling the chamber and the turbine efficiently, carrying out a whole dimensional analysis of the OWC plant. Basically, the wave data and the chamber geometry are the input data. The behaviour of the chamber is numerically simulated. From the numerical results, the optimum damping could be identified for every wave condition. Once the optimum chamber damping is found, the objective is to determine which turbine matches this damping. Numerical results, based on the quasi-steady assumption, were used to simulate turbine performance under periodic conditions. The calculation of the diameter is based on the damping caused by the turbine on the OWC, whereas the rotational speed is fixed to maximize the turbine mean efficiency. Furthermore, this methodology is worked out to compare the size and the rotational speed of different kinds of turbines. Finally, the influence of sea state changes on the parameters of the optimum turbine is also studied. [ABSTRACT FROM AUTHOR]

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 modelling of Pelton Turbines and would serve as guidance when choosing the optimum CFD modelling methodology and software available. [ABSTRACT FROM AUTHOR]

This study utilizes two modem computational fluid dynamics (CFD) software packages (ANSYS:® CFX® and ANSYS® FLUENT®) to analyze the basic geometric factors affecting the efficiency of a typical impulse turbine injector. A design of experiments (DOEs) study is used to look at the impact of four primary nozzle and spear design parameters on the injector losses over a range of inlet pressures. Improved injector designs for both solvers are suggested based on the results and comparisons are made. The results for both CFD tools suggest that steeper injector nozzle and spear angles than current literature describes will reduce the losses by up to 0.6%. [ABSTRACT FROM AUTHOR]

Oscillating water column based wave energy extracting system has a low efficiency due to the poor performance of its principal power extracting component, the bidirectional turbine. In the present work, flow over a bidirectional impulse turbine was simulated using CFD technique and optimized using multiple surrogates approach. The surrogates being problem dependent may produce unreliable results, if a wrong surrogate is selected. Hence, multiple surrogates such as response surface approximation, radial basis function, Kriging and weighted average surrogates were incorporated in this problem. Same design points were used to generate multiple optima via multiple surrogates to enhance the robustness of the optimization process. Numbers of guide vanes and rotor blades were chosen as the design variables, and the objective was to maximize the blade efficiency. Reynolds-averaged Navier–Stokes equations were solved for analyzing the flow physics. The computed results were used to train the surrogates and find the optimal points via hybrid genetic algorithm. The surrogates were further applied to find the optimal flow parameters by changing flow velocity and turbine speed. The relative efficiency enhancement through our present approach was about 16%. Detailed methodologies, analysis of the results and surrogate applicability have been presented in this paper. [ABSTRACT FROM AUTHOR]

This paper presents a technology that can accelerate the development of hydro turbines by fully automating the initial testing process of prototype turbine models and automatically converting the acquired data into efficiency hill charts that allow straight forward comparison of prototypes' performance. The testing procedure of both reaction and impulse turbines is illustrated using models of Francis and Pelton turbines respectively. For the development of an appropriate hill chart containing no less than 780 points the average duration of the fully automated test is 4 h while the acquired data files can be processed into descriptive standard efficiency hill charts within less than a minute. These hill charts can then be used in research and development to quickly evaluate and compare the performance of initial turbine prototype designs before proceeding to much lengthier and more expensive development stage of the chosen design. [ABSTRACT FROM AUTHOR]

Purpose – This paper is concerned with improving the flow pattern in the nozzle-rotor axial gap in impulse turbines using a genetic algorithm (GA) and 3D numerical analysis. The paper aims to discuss these issues. Design/methodology/approach – The appropriate model was used to estimate the turbine performance introduced in the beginning of the work. Then, the nozzle design parameters that are effective in the axial gap flow pattern are optimized using a non-linear optimization code. This code works based on the GA theory. Since the GA results are not conclusive, the selected cases were evaluated using 3D numerical analysis. For a detailed comparison of the flow pattern in initial and improved cases, a transient analysis was done. Experimental tests were performed in order to validate the work. For this purpose, the characteristic curves of the turbines were studied and compared with each other. Findings – Improving the nozzle-rotor axial gap flow pattern leading to increase in the total-to-total efficiency of the turbine by more than two points. Research limitations/implications – Partially injected flow forced to use the full model computational analysis. Practical implications – Weight reduction in a feeding system. Originality/value – New loss modeling method presented for partial admission condition. [ABSTRACT FROM AUTHOR]

This paper presents the numerical modelling techniques for the simulation of the energy conversion chain from wave to electricity in an Oscillating Water Column (OWC) equipped with a full-scaled self rectifying turbine. The performance of the OWC device has been assessed for the stand-alone power system of a typical Irish climatic wave condition. The results showed that the overall performance of the complete device depends on the level of turbine damping, which in turn depends on the wave climate, especially the significant wave height. Furthermore, turbine efficiency predicted using quasi-steady showed qualitatively favourable agreement with the experimental results. [ABSTRACT FROM AUTHOR]