This paper mainly summarizes the recent progresses for the cavitation study in the hydraulic machinery including turbo-pumps, hydro turbines, etc.. Especially, the newly developed numerical methods for simulating cavitating turbulent flows and the achievements with regard to the complicated flow features revealed by using advanced optical techniques as well as cavitation simulation are introduced so as to make a better understanding of the cavitating flow mechanism for hydraulic machinery. Since cavitation instabilities are also vital issue and rather harmful for the operation safety of hydro machines, we present the 1-D analysis method, which is identified to be very useful for engineering applications regarding the cavitating flows in inducers, turbine draft tubes, etc. Though both cavitation and hydraulic machinery are extensively discussed in literatures, one should be aware that a few problems still remains and are open for solution, such as the comprehensive understanding of cavitating turbulent flows especially inside hydro turbines, the unneglectable discrepancies between the numerical and experimental data, etc.. To further promote the study of cavitation in hydraulic machinery, some advanced topics such as a Density-Based solver suitable for highly compressible cavitating turbulent flows, a virtual cavitation tunnel, etc. are addressed for the future works.

A review of cavitation in hydraulic machinery

Reaction turbines basically Francis turbines and propeller/Kaplan turbines are suitable for medium and low head hydropower sites. The management of the small hydropower plants is an important factor, for achieving higher efficiency of hydro turbines with time. Turbines show declined performance after few years of operation, as they get severely damaged due to various reasons. One of the important reasons is erosive wear of the turbines due to cavitation. Reaction turbines, however are more prone to cavitation especially Francis turbines where a zone in the operating range is seriously affected by cavitation and considered as forbidden zone. Cavitation is a phenomenon which manifests itself in the pitting of the metallic surfaces of turbine parts because of the formation of cavities. In the present paper, studies undertaken in this field by several investigators have been discussed extensively. Based on literature survey various aspects related to cavitation in hydro turbines, different causes for the declined performance and efficiency of the hydro turbines and suitable remedial measures suggested by various investigators have been discussed.

Study of cavitation in hydro turbines—A review

The dynamics of the rotating vortex taking place in the discharge ring of a Francis turbine for partial flow rate operating conditions and cavitation free conditions is studied by carrying out both experimental flow survey and numerical simulations. 2D laser Doppler velocimetry, 3D particle image velocimetry, and unsteady wall pressure measurements are performs to investigate thoroughly the velocity and pressure fields in the discharge ring and to give access to the vortex dynamics. Unsteady RANS simulation are performed and compared to the experimental results. The computing flow domain includes the rotating runner and the elbow draft tube. The mesh size of 500,000 nodes for the 17 flow passages of the runner and 420,000 nodes for the draft tube is optimized to achieve reasonable CPU time for a good representation of the studied phenomena. The comparisons between the detailed experimental flow field and the CFD solution yield to a very good validation of the modeling of the draft tube rotating vortex and, then, validate the presented approach for industrial purpose applications.

Experimental Study and Numerical Simulation of the Flindt Draft Tube Rotating Vortex

Hydropower represented in 1999 19% of the world electricity production and the absolute production is expected to grow considerably during the next 30 years. Francis turbines play a major role in the hydroelectric production due to their extended range of application. Due to the deregulated energy market, hydroelectric power plants are increasingly subjecting to off design operation, start-up and shutdown and new control strategies. Consequently, the operation of Francis turbine power plants leads to transients phenomena, risk of resonance or instabilities. The understanding of these propagation phenomena is therefore paramount. This work is a contribution to the hydroacoustic modelling of Francis turbine power plants for the investigation of the aforementioned problematic. The first part of the document presents the modelling of the dynamic behavior and the transient analysis of hydroelectric power plants. Therefore, the one-dimensional model of an elementary pipe is derived from the governing equations, i.e. momentum and continuity equations. The use of appropriate numerical schemes leads to a discrete model of the pipe consisting of a T-shaped equivalent electrical circuit. The accuracy in the frequency domain of the discrete model of the pipe is determined by comparison with the analytical solution of the governing equations. The modelling approach is extended to hydraulic components such as valve, surge tanks, surge shaft, air vessels, cavitation development, etc. Then, the modelling of the Francis, Pelton and Kaplan turbines for transient analysis purposes is presented. This modelling is based on the use of the static characteristic of the turbines. The hydraulic components models are implemented in the EPFL software SIMSEN developed for the simulation of electrical installations. After validation of the hydraulic models, transient phenomena in hydroelectric power plants are investigated. It appears that standard separate studies of either the hydraulic or of the electrical part are valid only for design purposes, while full hydroelectric models are necessary for the optimization of turbine speed governors. The second part of the document deals with the modelling and analysis of possible resonance or operating instabilities in Francis turbine power plants. The review of the excitation sources inherent to Francis turbine operations indicates that the draft tube and the rotor-stator interaction pressure fluctuations are of the major concern. As the modelling of part load pressure fluctuations induced by the cavitating vortex rope that develops in the draft tube at low frequencies is well established, the focus is put on higher frequency phenomena such as higher part load pressure fluctuations and rotorstator interactions or full load instabilities. Three hydroacoustic investigations are performed. (i) Pressure fluctuations identified experimentally at higher part load on a reduced scale model Francis turbine are investigated by means of hydroacoustic simulations and high speed flow visualizations. The resonance of the test rig due to the vortex rope excitation is pointed out by the simulation while the special motion and shape of the cavitating vortex rope at the resonance frequency is highlighted by the visualization. A description of the possible excitation mechanisms is proposed. (ii) A pressure and power surge measured on a 4 × 400 MW pumped-storage plant operating at full load is investigated. The modelling of the entire system, including the hydraulic circuit, the rotating inertias and the electrical installation provides an explanation of the phenomenon and the related conditions of apparition. A non-linear model of the full load vortex rope is established and qualitatively validated. (iii) The rotor-stator interactions (RSI) are studied in the case of a reduced scale pump-turbine model. An original modelling approach of this phenomenon based on the flow distribution between the stationnary and the rotating part is presented. The model provides the RSI pressure fluctuation patterns in the vaneless gap and enables to predict standing waves in the spiral case and adduction pipe. The proposed one-dimensional modelling approach enables the simulation, analysis and optimization of the dynamic behavior of hydroelectric power plants. The approach has proven its capability of simulating properly both transient and periodic phenomena. Such investigations can be undertaken at early stages of a project to assess the possible dynamic problems and to select appropriate solutions ensuring the safest and optimal operation of the facility.

Hydroacoustic modelling and numerical simulation of unsteady operation of hydroelectric systems

An LDA technique and phase-averaging analysis were used to study unsteady precessing flow in a model vortex burner. Detailed measurements were made for Re=15,000 and S=1.01. On the basis of the analysis of phase-averaged data and vortex detection by the λ2-technique of Joeng and Hussain (1995), three precessing spiral vortex structures were identified: primary vortex (PV), inner secondary vortex (ISV), and outer secondary vortex (OSV). The PV is the primary and most powerful structure as it includes primary vorticity generated by the swirler; the ISV and OSV are considered here as secondary vortical structures. The jet breakdown zone is the conjunction of a pair of co-rotating co-winding spiral vortices, PV and ISV. The interesting new feature described is that the secondary vortices form a three-dimensional vortex dipole with a helical geometry. The effect of coupling of secondary vortices was suggested as a mechanism of enhanced stability reflected in their increased axial extent.

Coherent structures in unsteady swirling jet flow

The complex three-dimensional unsteady flow developing in the draft tube of a Francis turbine is responsible for pressure fluctuations, which could prevent the whole hydropower plant from operating safely. Indeed, the Francis draft tube is subjected to inlet swirling flow, divergent cross section, and the change of flow direction. As a result, in low discharge off-design operating conditions, a cavitation helical vortex, so-called the vortex rope develops in the draft tube and induces pressure fluctuations in the range of 0.2–0.4 times the runner frequency. This paper presents the extensive unsteady wall pressure measurements performed in the elbow draft tube of a high specific speed Francis turbine scale model at low discharge and at usual plant value of the Thoma cavitation number. The investigation is undertaken for operating conditions corresponding to low discharge, i.e., 0.65–0.85 times the design discharge, which exhibits pressure fluctuations at surprisingly high frequency value, between 2 and 4 times the runner rotation frequency. The pressure fluctuation measurements performed with 104 pressure transducers distributed on the draft tube wall, make apparent in the whole draft tube a fundamental frequency value at 2.5 times the runner frequency. Moreover, the modulations between this frequency with the vortex rope precession frequency are pointed out. The phase shift analysis performed for 2.5 times the runner frequency enables the identification of a pressure wave propagation phenomenon and indicates the location of the corresponding pressure fluctuation excitation source in the elbow; hydroacoustic waves propagate from this source both upstream and downstream the draft tube.

Experimental Evidence of Hydroacoustic Pressure Waves in a Francis Turbine Elbow Draft Tube for Low Discharge Conditions

Scale model of high specific speed Francis turbines may present at upper part load operation under low cavitation number pressure fluctuation in the range 2 to 4 times fn, the rotating frequency. In the framework of the FLINDT project, pressure measurements on the draft tube wall of a Francis turbine scale model at 104 locations revealed such phenomenon at a frequency of almost 2.5 fn. The phase shift analysis of the measured pressure fluctuations in the draft tube at this frequency points out a pressure source located in the inner part of the draft tube elbow. The spectral analysis of the pressure signal at this location shows that there is energy uniformly distributed in the range 0 to 7 fn. This results from impacts of the vortex rope during its precession on the draft tube wall. The wave speed along the draft tube is calculated using the experimental results of the phase shifts and allows modeling the entire test rig with SIMSEN. The simulation of the hydro acoustic behavior of the entire test rig, including the scale model and the piping system, and considering white noise excitation at the pressure source location provides eigen frequencies of the full hydraulic system. An eigen frequency at 2.46 fn is identified and the corresponding mode shape agrees well with the experimental results. A forced excitation composed of the synchronous pressure fluctuation measured in the draft tube cone added to pressure impulsion peaks at the vortex rope precession frequency is also simulated. This excitation represents the synchronous part of the vortex rope excitation and the energy provided by the impacts on the draft tube wall. The analysis of the resulting pressure fluctuation in the entire test rig shows significant pressure amplitude mainly at 2.46fn, which evidences the excitation mechanism.

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Identification and Modeling of Pressure Fluctuations of a Francis Turbine Scale Model at Part Load Operation

Note: Using Smart Source Parsing 9-12 September pp 3 Reference LMH-CONF-2002-002 Record created on 2005-11-04, modified on 2017-05-10

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

At early stage of a hydroelectric project, 1D transient simulations are performed to determine the basic layout of power plant. In this phase, the design of surge tanks is decisive to achieve good dynamic performances of the power plant, with respect to water hammer and mass oscillations induced by the hydraulic machines for normal, exceptional, and accidental operation. As the head losses between the gallery and the surge tank have strong influence on the transient behavior of the hydraulic system, they are usually optimized by means of 1D transient simulation to avoid low pressure in gallery or surge tank overflow. An asymmetric diaphragm is often placed at the surge tank inlet to achieve the optimum inlet and outlet head losses. Thus, the design of such diaphragm is a challenging task usually performed through an iterative process on reduced-scale model. In this context, 3D CFD simulations can significantly improve the design process to select the appropriate geometry of the diaphragm. In this chapter, head losses coefficients of a surge tank scale model are derived from CFD simulations performed with ANSYS CFX. Results are compared with measurements on reduced-scale physical model and analytical approach. The good agreement of CFD computations with measurements demonstrates that a design optimization with 3D flow simulations can be performed preliminary to scale model tests in order to reduce the number of geometries to be tested to achieve the expected head losses.

Determination of Surge Tank Diaphragm Head Losses by CFD Simulations

Note: Using Smart Source Parsing et novembre 3 Reference LMH-CONF-2000-008 Record created on 2005-11-04, modified on 2017-05-10

Jorge Arpe

Papers

Experimental Evidence of Hydroacoustic Pressure Waves in a Francis Turbine Elbow Draft Tube for Low Discharge Conditions

Identification and Modeling of Pressure Fluctuations of a Francis Turbine Scale Model at Part Load Operation

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

Determination of Surge Tank Diaphragm Head Losses by CFD Simulations

Etude du champ instationnaire de vitesse en sortie de roue de turbine : étude expérimentale et numérique