François Avellan

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

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.

Analysis of the swirling flow downstream a Francis turbine runner

An experimental and theoretical investigation of the flow at the outlet of a Francis turbine runner is carried out in order to elucidate the causes of a sudden drop in the draft tube pressure recovery coefficient at a discharge near the best efficiency operating point. Laser Doppler anemometry velocity measurements were performed for both axial and circumferential velocity components at the runner outlet. A suitable analytical representation of the swirling flow has been developed taking the discharge coefficient as independent variable. It is found that the investigated mean swirling flow can be accurately represented as a superposition of three distinct vortices. An eigenvalue analysis of the linearized equation for steady, axisymmetric, and inviscid swirling flow reveals that the swirl reaches a critical state precisely (within 1.3%) at the discharge where the sudden variation in draft tube pressure recovery is observed. This is very useful for turbine design and optimization, where a suitable runner geometry should avoid such critical swirl configuration within the normal operating range.

Experimental Evidence of Rotating Stall in a Pump-Turbine at Off-Design Conditions in Generating Mode

An experimental investigation of the rotating stall in reduced scale model of a low specific speed radial pump-turbine at runaway and turbine brake conditions in generating mode is achieved. Measurements of wall pressure in the stator are performed along with high-speed flow visualizations in the vaneless gap with the help of air bubbles injection. When starting from the best efficiency point (BEP) and increasing the impeller speed, a significant increase of the pressure fluctuations is observed mainly in the wicket gates channels. The spectral analysis shows a rise of a low frequency component (about 70% of the impeller rotational frequency) at runaway, which further increases as the zero discharge condition is approached. Analysis of the instantaneous pressure peripheral distribution in the vaneless gap reveals one stall cell rotating with the impeller at sub-synchronous speed. High-speed movies reveal a quite uniform flow pattern in the guide vanes channels at the normal operating range, whereas at runaway the flow is highly disturbed by the rotating stall passage. The situation is even more critical at very low positive discharge, where backflow and vortices in the guide vanes channels develop during the stall cell passage. A specific image processing technique is applied to reconstruct the rotating stall evolution in the entire guide vanes circumference for a low positive discharge operating point. The findings of this study suggest that one stall cell rotates with the impeller at sub-synchronous velocity in the vaneless gap between the impeller and the guide vanes. It is the result of rotating flow separations developed in several consecutive impeller channels which lead to their blockage.

Flow in a Pelton Turbine Bucket: Numerical and Experimental Investigations

The flow model used for the numerical simulations is based on the generalized homogeneous multiphase flow model developed by Ishii 15, with the additional sources of momentum for the effects of the Coriolis and centrifugal accelerations in a steady rotating frame of reference. The governing equations are described below

Cavitation Influence on von Kármán Vortex Shedding and Induced Hydrofoil Vibrations

The present study deals with the shedding process of the von Karman vortices at the trailing edge of a 2D hydrofoil at high Reynolds number. This research focuses mainly on the effects of cavitation and fluid-structure interaction on the mechanism of the vortex generation. The vortex shedding frequency, derived from the flow-induced vibration measurement, is found to follow the Strouhal law provided that no hydrofoil resonance frequencies are excited, i.e., lock-off. For such a regime, the von Karman vortices exhibit strong spanwise 3D instabilities and the cavitation inception index is linearly dependent on the square root of the Reynolds number. In the case of resonance, the vortex shedding frequency is locked onto the hydrofoil eigenfrequency and the spatial coherence is enhanced with a quasi-2D shape. The measurements of the hydrofoil wall velocity amplitude and phase reveal the first torsion eigenmotion. In this case, the cavitation inception index is found to be significantly increased compared to lock-off conditions. It makes clear that the vortex roll-up is amplified by the phase locked vibrations of the trailing edge. For the cavitation inception index, a new correlation relationship that encompasses the entire range of Reynolds numbers, including both the lock-off and the lock-in cases, is proposed and validated. In contrast to the earlier models, the new correlation takes into account the trailing edge displacement velocity. In addition, it is found that the transverse velocity of the trailing edge increases the vortex strength linearly. This effect is important in the context of the fluid-structure interaction, since it implies that the velocity of the hydrofoil trailing edge increases the fluctuating forces on the body. It is also demonstrated that cavitation developing in the vortex street cannot be considered as a passive agent for the turbulent wake flow. In fact, for fully developed cavitation, the vortex shedding frequency increases up to 15%, which is accompanied by the increase of the vortex advection velocity and reduction of the streamwise vortex spacing. In addition, a significant increase of the vortex-induced vibration level is found at cavitation onset. These effects are addressed and thought to be a result of the increase of the vorticity by cavitation.

Analysis of the Cavitating Draft Tube Vortex in a Francis Turbine Using Particle Image Velocimetry Measurements in Two-Phase Flow

Partial flow rate operation of hydroturbines with constant pitch blades causes complex unstable cavitating flow in the diffuser cone. A particle image velocimetry (PIV) system allows investigating the flow velocity field in the case of a developing cavitation vortex, the so-called vortex rope, at the outlet of a Francis turbine runner. The synchronization of the PIV flow survey with the rope precession allows applying the ensemble averaging by phase technique to extract both the periodic velocity components and the rope shape. The influence of the turbine setting evel on the volume of the cavity rope and its centerline is investigated, providing a physical knowledge about the hydrodynamic complex phenomena involved in the development of the cavitation rope in Francis turbine operating regimes.

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

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.

Analysis and Prevention of Vortex Breakdown in the Simplified Discharge Cone of a Francis Turbine

We perform a numerical analysis of the decelerated swirling flow into the discharge cone of a model Francis turbine operated at variable discharge and constant head, using an axisymmetric turbulent swirling flow model and a corresponding simplified computational domain. Inlet boundary conditions correspond to velocity and turbulent kinetic energy profiles measured downstream the Francis runner. Our numerical results are validated against experimental data on a survey section further downstream in the cone, showing that the Reynolds stress turbulence model with a quadratic pressure-strain term correctly captures the flow field. It is shown that the diffuser performance quickly deteriorates as the turbine discharge decreases, due to the occurrence and development of vortex breakdown, with a central quasistagnant region. We investigate a novel flow control technique, which uses a water jet injected from the runner crown tip along the axis. It is shown that the jet discharge can be optimized for minimum overall losses, while the vortex breakdown is eliminated. This flow control method is useful for mitigating the Francis turbine flow instabilities when operating at partial discharge.

3D PIV AND LDV MEASUREMENTS AT THE OUTLET OF A FRANCIS TURBINE DRAFT TUBE

For certain geometries of elbow draft tubes of a hydraulic turbine, a drop in the pressure recovery coefficient is observed for a small variation of the flow rate. In order to determine the possible causes of this characteristics shape, the flow field analysis for 4 nearby operating points have been investigated. For velocity and turbulence fields investigation in the outlet section of the studied draft tube, LDV measurements were performed in a transversal section and the 3D-PIV system was qualified for global velocity measurements in longitudinal sections, with an accuracy of less than 3%. By correlating the LDV and PIV results, the quantification of the flow rate through each channel, related to the operating points, and the description of the secondary flow in the outlet zone are possible.Copyright

Dynamics and Intensity of Erosive Partial Cavitation

An experimental work has been carried out to investigate the dynamic behavior and the intensity of erosive partial cavitation on a 2-D hydrofoil. Both sheet (stable) and cloud (unstable) cavitation have been tested in a cavitation tunnel for various free stream velocities. Special attention has been given to validate the use of acceleration transducers for studying the physical process. In particular, the modulation in amplitude of the cavitation induced vibrations in a high frequency band has allowed us to determine the shedding frequency and the relative intensity of the collapse process for each testing condition. Regarding the cavity dynamics, a typical Strouhal value based on its length of about 0.28 has been found for cloud cavitation; meanwhile, for sheet cavitation, it presents a value of about 0.16. Furthermore, the level of the vibration modulation in the band from 45 kHz to 50 kHz for cloud cavitation shows a power law dependency on the free stream velocity as well as a good correlation with the pitting rate measured on stainless steel samples mounted on the hydrofoil.