Flow Investigation in a Francis Draft Tube : the Flindt Project

The flow investigation in a elbow draft tube is presented. After a review of the existing works related to the flow analysis of such component, the need of an experimental validation of the CFD tools is discussed with respect the practical interest of improving the matching of a runner and the draft tube. The geometry and the main characteristics of the test model is described and the pressure recovery coefficient is provided in an extended operating range. The instrumentation of the draft tube and the LDA system for flow survey are described. After the discussion of the influence of the boundary conditions and the mesh size with respect to the CFD analysis, the results of flow analysis are compared to the available experimental data for 3 operating points

Cavitation test of hydraulic machines : procedure and instrumentation

Model testing of hydraulic turbines, storage pumps and pump-turbines is of great importance especially for large units involved in power generation. Among all the aspects of machine operation, cavitation plays a fundamental role with respect to the possible alteration of the efficiency and to the erosion risk. This paper describes the procedures of cavitation tests and the related instrumentation, depending both on the type of hydraulic machine and its operation range. Discussion is mainly directed toward the unavoidable distortions of the similitude requirements between model and industrial scales

Hydroacoustic Modeling of Rotor Stator Interaction in Francis Pump-Turbine

A one-dimensional hydroacoustic model is set up to perform the numerical simulation of the rotor-stator interaction of a Francis pump-turbine scaled model. The numerical results enable to identify both rotating diametrical modes in the vaneless gap between the 20 guide vanes interacting with the 9 rotating impeller blades and the standing waves in the spiral casing. Moreover, the simulations enlighten the interaction between these rotating diametrical modes and the standing waves

Introduction to Cavitation in Hydraulic Machinery

Design, operation and refurbishment of hydraulic turbines, pumps or pump-turbine are strongly related to cavitation flow phenomena, which may occur in either the rotating runner-impeller or the stationary parts of the machine. The paper presents the cavitation phenomena featured by fluid machinery including type of cavity development related to the specific speed of machines in both pump and turbine mode, the influence of the operating conditions, such as load, head and submergence. Therefore, for each type of cavitation illustrated by flow visualization made at the EPFL testing facilities, the influence of cavitation development on machine efficiency, operation and integrity are discussed

Cavitation erosion of the hydraulic machines : generation and dynamics of erosive cavities

The cavitation which causes severe erosion of hydraulic machine runners is produced by the development of fixed cavities attached at the leading edge of the blades. It is clearly shown, from a comparison between the observations of cavity patterns during model tests and the damage found in the prototype, that the damaged areas of the blades extend in the wak:e of the fixed cavities. In this paper we give experimental evidence that this damage is due to the collapse of transient cavities in the fixed cavity closure region. Visualizations, pressure measurements and LDA flow field measurements using the IMHEF High-Speed Cavitation Tunnel allow us to describe the generation and the dynamics of these transient cavities where the Reynolds number plays a leading role. Moreover the investigation carried out with a special facility, the Cavitation Vortex Generator, reveals that the spherical shock-waves emitted in the last stages of cavity collapse are strong enough (20 kbar) to damage any industrial alloy. These experimental results should be confirmed in order to provide a physical basis for numerical simulation of the fixed cavity development and the generation-collapse cycle of the related transient cavities

Toward design optimization of a Pelton turbine runner

International audienceThe objective of the present paper is to propose a strategy to optimize the performance of a Pelton runner based on a parametric model of the bucket geometry, massive particle based numerical simulations and advanced optimization strategies to reduce the dimension of the design problem. The parametric model of the Pelton bucket is based on four bicubic Bézier patches and the number of free parameters is reduced to 21. The numerical simulations are performed using the finite volume particle method, which benefits from a conservative, consistent, arbitrary Lagrangian Eulerian formulation. The resulting design problem is of High-dimension with Expensive Black-box (HEB) performance function. In order to tackle the HEB problem, a preliminary exploration is performed using 2’000 sampled runners geometry provided by a Halton sequence. A cubic multivariate adaptive regression spline surrogate model is built according to the simulated performance of these runners. Moreover, an original clustering approach is proposed to decompose the design problem into four sub-problems of smaller dimensions that can be addressed with more conventional optimization techniques

Pressure Wall Measurements in the whole Draft Tube : Steady and Unsteady Analysis

Flow in a elbow draft tube is a turbulent complex unsteady flow caused by its rotating nature and the draft tube's geometry. The understanding of the main phenomena that could appear near the BEP and at off-design conditions is important to master the instability of the machine. The results shown in this work are a summary of a very large experimental database. The aim of this investigation work is to analyze the steady and unsteady pressure fields at the wall of the draft tube through extensive pressure measurements. A particular evolution of the steady pressure field appeared according to the operating points around the BEP chosen to study the draft tube recovery break-off. Consequently, the flow is distributed between the 2 channels of the draft tube according to the operating point. The fluctuating pressure field at the cone can be divided mainly in 2 components: first a rotating component at the runner's frequency (f*) and second a syncronous component at 20f* resultant of the interaction rotor-stator frequency. The influence of the spiral casing on the fluctuating pressure is pointed out by representing all phase average signals in the same absolute angular position of the runner. The results at the low discharge operating point without vapour phase show the main pressure fluctuations at the cavitation free vortex frequency as well as the advection of these fluctuations in the elbow and the channel walls

Theoretical and experimental study of the inlet and outlet cavitation in a model of a Francis turbine

In order to study the inlet and outlet cavitation in a Francis turbine (nq = 48) a numerical model of the bubble dynamics is established. By using the pressure and velocity distribution obtained by a 3-D potential flow analysis of the turbine, the Rayleigh-Plesset equation is integrated for a wide range of head and bubble sizes and for two typical turbine operating points, where inlet and outlet cavitation respectively are observed. The numerical simulations agree very well with the main cavitation features observed in the turbine runner. The minimum explosive bubble radius is found to be an inverse function of the test head, as predicted by the equilibrium free nucleus stability theory, unless this theory overestimates the size of the minimum active nuclei, mainly in outlet cavitation. Hence, these results, in addition to a measured water nuclei radii histogram, can explain fairly well both the influence of water nuclei content in outlet cavitation and the inlet cavitation independence of this content for usual test heads. The test head is then found to influence the maximum explosive bubble sizes by an H^(-1/2) law

Dynamics of vortex cavitation involved in the erosion of hydraulic machines

In order to investigate the dynamics of the vortex cavitation, which is found to be responsible of the cavitation erosion in hydraulic machinery, a cavitation vortex generator (CVG) is used. By producing the cyclic growth and collapse of cavitation vortex, this device provides a good simulation of the realistic hydrodynamics situation corresponding to the travelling cavities downstream a fixed leading-edge cavity. The simultaneous use of the shock and the pressure time-history recordings and the high speed Cranz-Schardin visualizations allows us to describe the instantaneous cavity motion in the CVG test section . The most important result is the evidence of a strong spherica1 shock waves emission at the time of the cavitation vortex collapse, leading to overpressure of more than 10 kbar