Optimisation of Pelton turbine jet deflector using CFD analysis

Numerical simulation of three-dimensional turbulent flow and heat transfer was performed in a multipass rectangular (AR = 2:1) rotating cooling channel with and without turning vane in the hub region under various flow conditions, with two different Reynolds numbers of 10,000 and 25,000, two different channel orientations of 45-deg and 90-deg, and the rotation number varies from 0 to 0.2. This study shows that the addition of the turning vane in the hub turn region does not cause much impact to the flow before the hub. However, it significantly alters the flow reattachment and vortex distribution in the hub turn region and after the hub turn portion. The local heat transfer is deeply influenced by this complex flow field and this turning vane effect lasts from the hub turn region to the portion after it.

The numerical simulation of a flow through a duct requires an externally specified forcing that makes the fluid flow against viscous friction. To this end, it is customary to enforce a constant value for either the flow rate (CFR) or the pressure gradient (CPG). When comparing a laminar duct flow before and after a geometrical modification that induces a change of the viscous drag, both approaches lead to a change of the power input across the comparison. Similarly, when carrying out direct numerical simulation or large-eddy simulation of unsteady turbulent flows, the power input is not constant over time. Carrying out a simulation at constant power input (CPI) is thus a further physically sound option, that becomes particularly appealing in the context of flow control, where a comparison between control-on and control-off conditions has to be made. We describe how to carry out a CPI simulation, and start with defining a new power-related Reynolds number, whose velocity scale is the bulk flow that can be attained with a given pumping power in the laminar regime. Under the CPI condition, we derive a relation that is equivalent to the Fukagata–Iwamoto–Kasagi relation valid for CFR (and to its extension valid for CPG), that presents the additional advantage of naturally including the required control power. The implementation of the CPI approach is then exemplified in the standard case of a plane turbulent channel flow, and then further applied to a flow control case, where a spanwise-oscillating wall is used for skin-friction drag reduction. For this low-Reynolds-number flow, using 90 % of the available power for the pumping system and the remaining 10 % for the control system is found to be the optimum share that yields the largest increase of the flow rate above the reference case where 100 % of the power goes to the pump.

Hydro energy still occupies an important place among renewable energy sources. In special operating conditions, Pelton turbines are irreplaceable and can be used for extremely small hydropower plants and also large hydro power plants. Pelton turbines can operate with high head and relatively small flow rates. In many cases, the height differences of the water are very large. Sometimes it is necessary to stop the operation of the turbine very quickly and the consequences of water hammer can be very severe. The responsible part to minimize the consequences of this phenomena is jet deflector, which can be in two different technical designs. The steps for 3-D geometry definition, pre-processing and post-processing, flow modelling and FEM analysis are presented. In the paper is presented the new optimized design of push-out jet deflector shape. Optimization consider 3-D CFD analysis of free surface flow and stress analysis. The main goal of the research was to minimize the influence of all force components on the torque of deflector servomotor. The final results present the geometry of the deflector, with a significant reduction in the stresses and deformations. These have been achieved with a crucial reduction in the hydrodynamic force and torque.

Many hydroelectric power plants use a Francis turbine, known for its versatility and efficiency in converting hydraulic energy into mechanical energy.Two integral components of this hydroturbine are the spiral case and the guide vane, which play a key role in controlling the flow of water through the hydroturbine system, ensuring optimal performance and energy production. The process of developing new flow space of hydroturbines consists ofseveral consecutive stages, in particular: selection of design parameters of the hydroturbine for given conditions of the hydroelectric power plant anddesign of flow space elements (as a rule, several options are considered, differing in calculation methods, geometric, kinematic and other parameters); computational study of flow characteristics, power and torques, energy losses in flow space elements and determination of the best options; experimental study of model hydroturbine units on energy cavitation stands, and the dimensions of the model, test conditions and stand parameters must meet the requirements of the international IEC standard. Based on the results of experimental and computational studies of the model, the characteristics are calculated and guarantees for the operation of a full-scale hydroturbine are issued. The article presents some results of a computational study of the spatial turbulent flow of a viscous fluid in the flow space of a high-head turbine Fr500, performed using the CFXTASCflow application software package. To improve the energy characteristics at the stage of preliminary turbine design, numerical simulation of the flow should be carried out. Such a CFD approach reduces costs and time compared to the experimental approach and makes it possible to improve and analyze the characteristics of the hydroturbine and its design before manufacturing the model. The computational complex of programs provides an opportunity to see the pressure distribution pattern, the velocity vector field and the motion of fluid particles for substantiation and analysis of the results. Numerical simulation of the spatial flow in the flow section of the hydroturbine was carried out to determine changes in energy characteristics, therefore, the k-ε turbulence model was chosen. As a result of the calculation, the distribution of speeds and pressures in various elements of the hydroturbine was determined for different profiles of the guide vane. An analysis of energy losses in the flow part of the high-head hydroturbine was carried out: a spiral case, a stator with flat rings, a guide vane, an runner and a suction pipe for the optimal operation mode of the hydroturbine, and an analysis of the influence of opening the guide vane on the change in energy losses in various elements of the flow space was also carried out. The issue of increasing the energy efficiency of the flow space of the high-head hydroturbine was also considered.

Nonlinear eddy viscosity models have received significant interest because of the shortcomings of linear eddy viscosity models used in the numerical simulation of flow around building structures. There are many kinds of nonlinear eddy viscosity models. This paper adopts one of them to simulate backward facing step flow, which aims to evaluate the properties of this turbulence model. Compared with linear eddy viscosity model and experimental measurements, the simulation of nonlinear eddy viscosity model gives better results, especially for the reattachment length and pressure coefficient values. For this reason, this nonlinear eddy viscosity model is more suitable for numerical simulation of complex flow around building structures than linear eddy viscosity model.

The Stagnation Point Flow Experiment is a popular means for studying blood-surface interactions in a surface strain rate gradient. The strain rate distribution on the test surface of the flow chamber used in this experiment was originally assumed to be simply proportional to the radial distance from the stagnation point. A numerical simulation of the flows through the five most frequently used chambers shows that this is true only for one of the chambers, approximately so for another, and not true for the other three. An additional numerical simulation indicates the blood should purge the saline priming liquid within a few seconds at the beginning of the experiment.

The main objectives of this work are the formulation, implementation and applications of a numerical algorithm to simulate turbulent, incompressible, isothermal flows. The transient three-dimensional flow is analyzed using an explicit Taylor-Galerkin scheme and the finite element method with hexahedrical eight-node element. Turbulence is simulated using Large Eddy Simulation. For sub-grid scales two models were implemented, the classical Smagorinsky's model and the dynamic eddy viscosity model. For the second filtration, which is necessary in the dynamic model, a new method was developed based on independent finite elements that involve each node in the original mesh. The implemented scheme is efficient and good results with low additional computational cost were obtained. Results for two classical problems, the driven cavity and the backward facing step are presented. Comments about the model applicability for flows with high Reynolds numbers are also presented.

We investigate both laminar and turbulent flow regimes in a rough-walled rock fracture via numerical CFD simulations. While previous studies were limited to either fully viscous Darcy or inertial Forchheimer laminar flow regimes, we chose to cover the widest possible Reynolds number range of 0.1–10^6. We introduce CFD simulation of a turbulent flow for rough-walled fractures, implementing RANS approach to turbulence modeling. We focus on 2D fracture geometries and implement changes in both shear displacement and wall roughness, systematically examining their effect on fracture permeability and friction factor in a manner similar to the fundamental studies of the flow in rough-walled pipes. For a curvilinear fracture, laminar flow becomes non-stationary between Re , {sim } , 10^2–10^3, earlier for larger shear displacement and wall roughness. Laminar–turbulent transition starting at Re_{textrm{cr}} , {sim } , 2300 may lead to a sharp drop in permeability depending on the fracture geometry; this gap vanishes for larger shear displacement and wall roughness. Depending on the fracture geometry, bottlenecks closing with shear become a major negative factor for the overall permeability.

We propose a relationship between the water surface elevation and the mean water depth for partially wetted rectangular grid cells in the numerical simulation of shallow water flow with complex topography, and apply it to the numerical simulation of shallow water flow to verify the correctness of the relationship. Many natural terrains have complicated surface topography. It is very important to accurately predict the steep-fronted flows that occur after heavy rainfall flash floods or as inundation from dyke breaches. When modeling any terrain, rectangular grid cells are used to facilitate grid generation. In order to achieve a numerical balance of flux gradient and the source terms and to avoid numerical instability, a relationship between the water surface elevation and the mean water depth was derived if wetted and dry parts coexist within a rectangular grid cell. To estimate the momentum flux at the grid cell boundaries, we applied the second-order spatial accuracy Godunov finite volume algorithm with Roe approximation solver using the MUSCL method. Next, these relationships are applied to numerical simulations of three-dimensional shallow water flows with three humps and the validity of the proposed relationship is discussed. The relationship presented in this paper accurately reflects time-varying water regimes and boundaries in flow problems with moving wetting and drying zone interfaces and can be used for numerical simulations of three dimensional shallow water flows with arbitrary topography.

Hydraulic turbines play an important role in stabilizing electrical grids due to their fast response times. Comparative to other hydraulic turbines, a jet deflector of a Pelton turbine enables rapid response for load reduction. However, jet deflectors are subjected to high forces, torques and the splashing water impinges on the runner and elements within the casing. Thus, the design of the jet deflector is important. The analytical calculations are complex due to the splashing and backscattering of the deviated jet. Though the prediction of forces and deflector torque based on the impulse–momentum equation is feasible, but results are not sufficiently accurate, and thus measurements are required. Subject of this study is a test rig equipped with the physical model of a Pelton turbine injector with deflector control and a high-quality torque transducer for precise measurement. Calibration of instruments used, and uncertainty analysis of the test rig has been carried out. The overall uncertainty of the torque resulted in ±1.58%. The experimental results are compared with the analytical results to verify the primary data. It is observed that as the deflector initiates its action, the torque undergoes an increasing trend. At a deflection angle of 12°, the torque reaches its maximum value, and subsequently, upon achieving complete deflection, the torque is observed to attain its minimum magnitude. The empirical relationship of the deflector torque uncertainty has been developed using the regression analysis of the evaluated uncertainty data.

Centrifugal fans are widely used in several applications, and in some cases, the noise generated by these machines has become a serious problem. The centrifugal fan noise is frequently dominated by tones at the blade passing frequency as a consequence of the strong interaction between the flow discharged from the impeller and the volute tongue. In this study, a previously published aeroacoustic prediction methodology (Cho, Y., and Moon, Y.J., 2003, “Discrete Noise Prediction of Variable Pitch Cross-Flow Fans by Unsteady Navier-Stokes Computations,” ASME J. Fluids Eng., 125, pp. 543–550) has been extended to three-dimensional turbulent flow in order to predict the noise generated by a centrifugal fan. A three-dimensional numerical simulation of the complete unsteady flow on the whole impeller-volute configuration has been carried out using the computational fluid dynamics code FLUENT®. The unsteady forces applied by the fan blades to the fluid are obtained from the data provided by the simulation. The Ffowcs Williams and Hawkings model extension of Lighthill’s analogy has been used to predict the aerodynamic noise generated by the centrifugal fan from these unsteady forces. Also, the noise generated by the fan has been measured experimentally, and the experimental results have been compared to the numerical results in order to validate the aerodynamic noise prediction methodology. Reasonable agreement has been found between the numerical and the experimental results.

Direct numerical simulations of turbulent free shear flows have been shown to be in generally good agreement with self.-similarity theory and laboratory data. The temporal development of a number of important statistical quantities has been shown to have the proper behavior, and the spatial evolution of the flow field is consistent with predictions. The main potential difficulties with the application of these methods are the inadequate initialization procedure and statistical scatter in the data. However, we do not see these problems as being insurmountable. The evolution of turbulence in a free shear flow is governed by a complex balance among the following mechanisms: generation of energy from the mean flow, turbulent diffusion by the turbulence, intercomponent transfer of energy, and dissipation. Our statistical results indicate that the direct numerical simulations are indeed faithfully representing these physical processes. We are quite optimistic about the future usefulness of direct numerical simulations as a scientific research tool.

Direct numerical simulation of compressible turbulent channel flows using the discontinuous Galerkin method

Zonal Embedded Grids for Numerical Simulations of Wall-Bounded Turbulent Flows

This paper introduces the 3D numerical simulation of unsteady turbulent flow in the entire flow passage of a Francis turbine with computational fluid dynamics (CFD) technology. The boundary conditions have been implemented based on the 3D averaged Navier-Stokes equations. The governing equations are discreted on space by the finite volume method and on time step by the finite difference method. The 3D unsteady turbulent flow in an entire Francis turbine model is calculated successfully using the CFX-TASCflow software and RNG k-ɛ turbulence model. Transient flow fields are simulated in the spiral case, the distributor, the runner, and the draft tube for the optimum operating condition. Meanwhile, the velocity and pressure at any points in the flow fields can be obtained so as to provide the great value on the performance prediction. By means of the numerical simulation in a flow field, it is verified that every component in a Francis turbine model is designed reasonably. The basis for the further researches on hydraulic turbines is also built.