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In previous works, the authors presented computational fluid dynamics (CFD) results, which showed that injectors with noticeably steeper nozzle and needle tip angles 110 deg & 70 deg and 150 deg & 90 deg, respectively, attain higher efficiency than the industry standard, which, according to available literature on the public domain, ranges from 80 deg to 90 deg for nozzle and 50-60 deg for needle tip angles. Moreover, experimental testing of the entire Pelton system showed that gains of about 1% in efficiency can be achieved; however there appears to be an upper limit beyond which steeper designs are no longer optimal. This study aims at providing further insight by presenting additional CFD analysis of the runner, which has been coupled with the jet profile from the aforementioned injectors. 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. It can be concluded that the secondary velocities, which contribute to the development of more significant free-surface degradations as the nozzle and needle tip angles are increased, result in a nonoptimal jet runner interaction. [ABSTRACT FROM AUTHOR]

The recent development of CFD has allowed the flow modeling in impulse hydro turbines that includes complex phenomena like free surface flow, multifluid interaction, and unsteady, time dependent flow. Some commercial and open-source CFD codes, which implement Eulerian methods, have been validated against experimental results showing satisfactory accuracy. Nevertheless, further improvement of accuracy is still a challenge, while the computational cost is very high and unaffordable for multiparametric 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 and hence potentially suitable for numerical design optimization, providing that it can achieve adequate accuracy. The results of both methods for various turbine operation conditions, 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 the torque curve exhibit some systematic differences from the Eulerian results. [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]

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]

Pelton hydraulic turbines are impulse-type turbomachines commonly used in hydroelectric plants with medium-to-high water head and in various energy recovery applications. The aim of the present work is to provide detailed performance measurements on a Pelton turbine model, along with the design and geometrical dimensions of its runner/buckets and injectors. Such a complete set of data would be useful for further development and evaluation of numerical modelling tools of the complex unsteady free-surface flow developed in the turbine, and is missing from the literature. The two-nozzle Pelton model was designed using standard guidelines and was fully constructed in the Laboratory of Hydraulic Turbomachines, NTUA. The measurements include the net water head and flowrate, the injector characteristic curves, and the torque and rotation speed of the runner, from which the corresponding overall efficiency and shaft power are computed. The model was tested with one injector (upper or lower) and with both injectors in operation, using either constant or variable rotation speed mode. The comparative results were satisfactory and in line with the theory, verifying similarity and repeatability, and allowing for an estimation of mechanical losses. The measurements covered the entire operation range of the turbine, in order to draw complete hillcharts for various operation modes. [ABSTRACT FROM PUBLISHER]

This work constitutes an extension of two previous experimental studies, examining the flow field in a square tube T-junction with a time-dependent periodic inlet flow rate (zero to a maximum value) and equal branch flow rates. Based on numerical predictions, more details of the flow field are revealed, not being easily detectable by the experiment. Emphasis is given on the recirculation regions examining the velocity, pressure, and wall shear stress distributions as well as the limiting streamlines as a function of time. During acceleration an adverse pressure gradient builds up at the junction due to the streamlines curvature, causing flow separation before flow peak, which initiates at the corners of the tube cross section. Flow separation and reattachment lines, determined by the limiting streamline topology, move apart during deceleration. Streamwise cross-flow vortices appear in both branches, being much stronger in the 90° branch. The characteristics of the latter vortices, namely their circulation, swirl ratio, velocity distribution, as well as the process of their birth at the beginning of each cycle are examined in detail. Wall shear stress distributions exhibit maximum values at the entrance of the 90° branch, taking values proportional to the inlet time-dependent flow rate. Finally, comparisons with steady inlet conditions showed that in the unsteady case the flow can sustain much higher adverse pressure gradients before separating. [ABSTRACT FROM AUTHOR]