Numerical Analysis of Flow Phenomena Related to the Unstable Energy-Discharge Characteristic of a Pump-Turbine in Pump Mode

Regions of positive slope in the pressure-discharge characteristics are one of the major concerns in design and operation of centrifugal pumps, as they are limiting the admissible operating range to values above the critical discharge. The industrial pump turbine of specific speed ν=0.42 (nq=66 min-1) proposed as QNET-CFD test case TA6-04 shows a marked saddle in the energy-discharge characteristic associated to a sudden drop of efficiency versus discharge at part load. The pump-turbine consists of a shrouded impeller with five blades, a diffuser with 22 guide and stay vanes and a spiral casing. CFD flow simulations on a reduced model were carried out with a finite volume Navier-Stokes code (CFX-5.7) using block- tructured hexahedral meshes and the Menter-SST Turbulence model. Control of numerical quality has been performed. Reduced models with relatively low computational effort (mixing plane interface) already permit to capture the drop in efficiency and energy coefficient to analyze the flow phenomena inducing the drop of the energy coefficient Ψ that occurs at partial discharge. Analysis of local flow patterns and energy and velocity distributions at the rotor-stator interface that are related to the onset of recirculation are presented

Flow Simulation in an Elbow Diffuser : Verification and Validation

Numerical simulation of the unsteady turbulent flow in a three-dimensional draft tube geometry is performed. The investigation is carried out with a commercial finite volume solver implementing the Reynolds averaged Navier-Stokes equations. The modeling of most practically relevant turbulent flows continues to be based on this equation system. For this reason it is important to evaluate the limitations of this approach. Verification and validation are presented; detailed measurements are compared with computations over a wide range of operating conditions

Numerical investigations of fluid structure coupling: oscillating hydrofoil

This paper presents an investigation of the hydro elastic behavior of vibrating blades in hydraulic machines, which is of strong interest for turbo machinery applications. As a representative case study for vibrating blade in hydraulic machines, a NACA 0009 oscillating hydrofoil is considered. The aim is to model the hydrodynamic moment acting on the oscillating hydrofoil. Two types of oscillation are investigated: forced and free motions. The fluid torque acting on the hydrofoil is modeled introducing an added moment of inertia, a fluid damping and a fluid stiffness coefficient. The model coefficients are identified through an investigation in the frequency domain of the forced motion. The influence of the frequency and the upstream velocity are investigated. The model is then validated in case of the free motion: numerical simulation and model prediction show good agreements in terms of frequency and dimensionless damping

Numerical Flow Analysis of the GAMM Turbine at nominal and off-design Operating Conditions

The flow in a Francis turbine runner (GAMM Turbine) is analysed numerically. Different operating points are calculated using two industrial software packages based respectively on a finite element method (N3S) and a finite volume method (TASCflow®) and compared to experimental results. The numerical results allow to observe physical phenomena in the runner that are important in the process of hydraulic turbomachinery design. Values of Cu and Cm velocity components, blade pressure distribution and recirculation in the flow are compared to experimental results at nominal and off-design flow conditions. The experimental and numerical results show a similar efficiency evolution in function of flow rate and head, however the absolute level of energetic losses are overestimated by the two numerical codes

Experimental and Numerical Analysis or Free Surface Flows in a Rotating Bucket

The aim of this study was to provide a complete experimental and numerical analysis of the flow in a rotating Pelton bucket. For the experimental analysis, several functioning points have been used in order to study differences which may occur in various situations. This analysis will be mainly provided by the pressure distribution in the bucket and will be supported by flow visualisations. Thanks to these results, five distinct zones within the bucket have been isolated each corresponding to different signal characteristics. The numerical simulation has been performed using the two-phase homogeneous method in an original way by considering the relative path of the jet and the relative velocity changing with the rotation of the wheel. These simulations were performed for the same experimental functioning points

Surface Parametrization of a Francis Runner Turbine for Optimum Design

The aim of this paper is to apply a surface parameterization to a blade. This geometric representation should be used as a practical tool in the process of design optimization. Most parameterizations are based on blade section approaches. The parameters are typically angles, lengths that have a clear meaning to the hydraulic designer. Spanwise functions are sometimes used to ensure coherence between the sections and the smoothness of the constructed blade surfaces. Here, the section-to-section approach is replaced by a purely surface method. The blade is modelled using a camber surface and thickness distributions, and the design parameters are kept as close as possible to their original physical meaning. Smooth blade surfaces are ensured, and a reduced number of variables are sufficient to describe realistic designs. The present line of research aims to introduce a surface parameterization approach, which provides a representation of the blade. One of the benefits of this methodology is the reduction of design parameters involved as this approach is no longer section dependent. Other advantages reside in the easiness to obtain smooth geometries. Finally, it is also important to point out that data exchange between programs (i.e. CAD, Mesh Generator) may now use surface representation. This entails that subjective reconstruction of this surface is no longer necessary. As a conclusion, with our approach and the reduction of design parameters, the design optimization process becomes shorter in terms of time and effort. In this sense, preliminary tests of geometry optimization will be presented

Experimental and Numerical Cavitation Flow Analysis of an Industrial Inducer

In the present study, a CFD model for cavitation simulation have been investigated and compared to experimental results in the case of 3-bladed industrial inducer. The model uses a multiphase approach, based on multiphase mixture assumption. A truncated form of Rayleigh-Plesset equation is used as a source term for the inter-phase mass transfer. The model allows a good prediction of the cavitation inception as well as the main cavity dimensions. The threshold corresponding to the head drop is also well predicted by the model. It was found that the cavitation induced head drop is mainly due to an increase of energy losses and a decrease of the supplied energy. The hydrodynamic mechanism of head drop is investigated through a global and local analysis of the flow field

Rapport I.9 Calcul de l'écoulement instationnaire dans une volute de pompe centrifuge

The non uniform velocity at the outlet of a centrifugal pump rotor produces an unsteady flow in the volute. This unstability generates hydraulic noise and fluctuating forces on the impeller itself. A numerical modelisation of this flow is proposed. The numerical flow must be able to transport the vorticity created by the rotation of the rotor. A first approximation is made considering that the viscous effects are not preponderant. Therefore the unsteady incompressible Euler equation with a U, V, P, formulation is selected. The flow is 3D in the volute and the code is developped in this objective, but of the purpose of testing the theorical and numerical technic, a 2D flow is first computed. The unsteady Euler equation are discretized with a finite volume technic on a orthogonal curvilinear mesh. The mesh is obtained by solving a steady potential flow in the volute with a constant velocity at the outlet of the impeller. In order to define the mesh lines, a singularity technic is used and the potential and streamlines are interpolated. First results are presented for a flow with imposed rotating boundary conditions at the outlet of the impeller. The rotating velocity profile is estimated with the available experimental data, 1,5 impeller rotation were performed.Un profil de vitesse non uniforme en sortie d'une roue de pompe centrifuge crée un écoulement instationnaire dans la volute. Du fait du couplage de l'écoulement roue-volute, ces instabilités génèrent des efforts fluctuants sur la roue elle-même et sont source de bruit hydraulique. Une modélisation instationnaire de l'écoulement dans la volute est proposé Elle doit être apte à transporter le rotationnal créé par la rotation de la roue. En première approximation ; les effets visqueux ne sont pas prépondérants La modélisation des équations d'Euler incompressibles instationnaires a donc été adoptée. Dans l'optique d'un calcul 3 D : une formulation en variables U . \l P avec des coordonnées curvilignes orthogonales a été développée. Le maillage est obtenu en interpolant les lignes de courant et équipotentielles de l'écoulement permanent calculé par une méthode de singularités. Un premier cas de calcul est présenté ici. Les conditions limites instationnaires en sortie de roue ont été estimées en se basant sur les résultats expérimentaux disponibles et le calcul pour un tour et demi de la roue a été réalisé.Kueny Jean-Louis, Papantonis Dimitris. Rapport I.9 Calcul de l'écoulement instationnaire dans une volute de pompe centrifuge. In: Machines hydrauliques. Conception et exploitation. Développements récents et Applications aux différents secteurs industriels. Vingtièmes journées de l'hydraulique. Lyon, 4-6 avril 1989. Tome 1, 1989