Etude de l'influence des instationnarités des écoulements sur le développement de la cavitation

Flows in hydraulic machinery show interactions between guiding and rotating components. The wakes of the fixed components will be part responsible for the pressure fluctuations sensed on the blades. Those fluctuations become more important when the distance between the fixed and rotating components decreases. Global instabilities induced by the rope in the draft tube will also be sensed all over the installation. Cavitation on the blades will react in some ways to those fluctuations. This work is devoted to the influence of flow unsteadiness on the cavitation phenomenon. Two different flow configurations are tested, both subject to macroscopic fluctuations of the flow variables. The first part of this work involves the use of a fixed and oscillating NACA hydrofoil in the test section of the High Speed Cavitation Tunnel. Different leading edge configurations corresponding to hydrodynamically smooth and rough surfaces are tested. Observations of cavitation figures are done in parallel with pressure measurements on the hydrofoil suction side. It is observed that the pressure field leads the hydrofoil motion. This positive phase lag increases with the oscillation reduced frequency. For a particular reduced frequency, the phase lag between the movement and the pressure signals increases from the leading to the trailing edge of the hydrofoil. The transition location of the boundary layer on the suction side is modified by the movement. This transition follows the hydrofoil movement if compared with the fixed incidence case. Leading edge cavitation on the hydrofoil suction side interacts with the fluctuating pressure field. Cavitation appearance becomes intermittent when the leading edge is smooth. It disappears over all oscillation cycles if the oscillation frequency is further increased. This phenomenon is not observed when the leading edge is rough. Cavitation is also observed on the smooth parts of the leading edge when a rough patch is put at mid span. Intermittence is therefore not related to the modification of the pressure field, but to the state of the laminar boundary layer in the low pressure region. The positive phase lag of the pressure field also act to lower cavitation aggressiveness when the leading edge is rough on the oscillating hydrofoil. Those observations are validated by the measured acceleration. The second part of this work is devoted to the study of cavitating flow field in Francis turbines. A model turbine is used for pressure measurements on the blades without cavitation. The operating conditions correspond to the best efficiency point, highest power output at highest head, part load at the lowest head and part load at the highest head. It is observed that pressure fluctuations measured on the blades have the highest amplitude at part load. Those pressure fluctuations are related to the swirl of the flow in the draft tube. Other pressure fluctuations sources are the distribution of velocity at runner inlet and the wakes of the guide vanes. Experiments with cavitation on the blades show that the distribution of the flow at the runner inlet has a strong influence on the pressure pulses measured. Those pulses are detected in the pressure signals before cavitation becomes visible on the blades. An increase in the Reynolds number of the flow acts to higher the incipient cavitation parameter. This observation suggests that cavitation tests should be done at the highest Reynolds number reachable with the model turbine.

The influence of flow unsteadiness on erosive cavity dynamics

François Avellan, Jean-François Caron, Mohamed Farhat

This paper presents investigations of unsteady flow conditions influence over cavitation dynamics. Measurements of wall pressure fluctuations in a model turbine are first introduced. It is shown that the wake of the spiral casing tongue modulates cavity dynamics near the trailing edge of runner blades. A new device, which permits a 2D hydrofoil to oscillate around its mean incidence angle, is used in order to recreate flow conditions similar to those of blade row wakes experienced in industrial flows. This device is mounted in the LMH High-Speed Cavitation Tunnel test section. Flow visualizations and surface pressure measurements are performed concurrently. Perturbations strongly affect leading edge cavitation at some flow conditions. Complete disappearance of leading edge cavitation can be imposed at a particular operating point. It is proposed that this behavior is associated with turbulent boundary layer transition. Pressure time signals obtained with the oscillating hydrofoil and the model turbine share common features that are different from those obtained with the hydrofoil set at fixed incidences.

American Society of Mechanical Engineers1999

Détection vibratoire de l'érosion de cavitation des turbines Francis

Cavitation erosion of Francis turbines continues to impose costly repairs and loss of revenues. This erosion is primarily caused by leading edge cavitation. A revue of the literature reveals the non-existence of an absolutely validated detection technique of this cavitation in operating Francis turbines. Considering the impulsive character of the implosion of a cavitation structure, the assimilation of cavitation erosion to a process of mechanical fatigue of materials and the vibratory response at high frequencies of a linear structure to multiple random excitations whose responses add on a power basis, we have proposed that the vibratory response of a Francis turbine at high frequencies is tightly related to its cavitation erosion rate. The absolute measurement of cavitation aggressiveness on the blades can be made by the normalization of lower guide bearing vibration measurements by an average Transmissibility function measured by reciprocity with the runner underwater. The proposed method has been validated and it is now possible to state that the MSV of inferred forces at the cavitation impact rate of hydrodynamic origin is proportional to the erosion rate. The approach has been validated in excellent manner in laboratory set-ups with little or no flows by comparison to absolute cavitation aggressiveness assessment methods such as the DECER electrochemical technique and the pit counting technique on polished material samples. The validation was then pursued with laboratory homologous flows on a NACA 009 profile in the LMH high-speed cavitation tunnel with blade mounted DECER sensors as reference. Once again, impact frequencies of hydrodynamic origin, following here a Strouhal law, were observed and the MSV of the profile acceleration at high frequencies was proportional to the average erosion rate on the profile. Notwithstanding the difficulty of the measurements, the results were excellent and fared well with the erosive power of the flow in V3L2 (V= flow velocity, L= length of the attached vapour cavity). Preliminary measurements on a Francis model with blade mounted DECER sensors agreed well with the MSV of lower guide bearing acceleration at high frequencies. Later measurements on the Rapide Blanc model confirmed the damage model observed on the NACA profile. Hydrodynamic impact frequencies were however not governed by the tunnel Strouhal law but were found to be determined by the blade passage frequency. Stationary measurements at the lower guide bearing were equivalent to runner on board measurements. Reciprocity, typical to linear structures, was verfied. As on the profile, maximum aggressiveness on the runner blade occurs in the leading edge cavity closure area. This was confirmed by the application of "Prescale" pressure sensitive film on the blades for operating conditions generating maximum vibratory response at the cavitation hydrodynamic frequencies. High-speed video recordings shot at 2000 frames/s allowed detailed observation of the dynamics of the cavitation structures at the blade passage frequency. Visual observations of cavitation developments along with implosion frequencies allowed observing that the MSV of vibratory response at high frequencies at the cavitation impact occurrence rate was coherent with the calculated value of the flow erosive power inasmuch as the implosions occurred on the blades and not beyond the blade trailing edges. These results on the Francis model therefore supported the proposed vibratory detection approach. The application of the developed techniques, including the measurements of Transmissibility with the runners underwater, allowed assessing the difference in absolute cavitation aggressiveness on an existing and proposed replacement model runner for the Beauharnois power plant. The applicability of the proposed measurement and calibration methods was confirmed on prototype N°6 of the Rapide Blanc power plant. Measurements on fixed and rotating elements of the machine are equivalent and reciprocity also applies. The recurrence frequency of the cavitation impacts was however different from that of the model. The difference appears to be due to the semi-spiral case utilized on the model. It was not fully homologous to the full spiral case of the prototype. It would thus appear that on the model, the wake of the semi-spiral case tongue plays a determinant role in the generation and implosion process of cavitating structures while on the prototype the wakes of the wicket gates fill that function. The ultimate validation of the vibratory approach to the detection of erosive cavitation on Francis prototypes occurred in 1995 on unit 44 of the Manic 5 PA power plant. This was done using on board sensors (pressure, DECER and accelerometer) on severely eroded blade 4 and polished 316L stainless steel disks mounted in this same blade after removal of the pressure sensor housings. Paint removal and pit counting tests had previously correlated well in 1993 with the MSV of global modulation of high frequency acceleration at the lower guide bearing at the cavitation hydrodynamic impact occurrence rate which was for this prototype the wicket gate passing frequency for a rotating observer. The 1995 correlation of the MSV of the blade 4 high frequency acceleration modulation at the wicket gate passage hydrodynamic frequency with the volume pitting rate of the 316L stainless steel disks and the DECER erosion current was excellent. The existence in the flow of the 60Hz (rotation speed, 180 r.p.m., 20 wicket gates) hydrodynamic frequency was confirmed by dynamic pressure measurements on blade 4 and in the draft tube just below the runner. The per blade inferred forces were calculated for each set of test conditions with the average Transmissibility measured by reciprocity on five blades with the runner underwater. The MSV of high frequency inferred forces agrees better with the real aggressiveness than does the wideband acceleration but not as well as the MSV of the acceleration modulation at the hydrodynamic frequencies of the cavitation impacts. Improvements on this subject have been incorporated in the cavitation erosion monitoring system now in operation at Hydro-Québec. Tu summarize, the MSV of the vibratory response at the lower guide bearing modulated at the cavitation hydrodynamic frequencies describes well its relative aggressiveness with varying operating conditions and the use of an averaged Transmissibility measured with the runner underwater allows to estimate it on an absolute basis.

EPFL2000

Etude du comportement dynamique des turbines Francis

A common phenomenon in Francis turbines is draft tube surge. In these fixed-bladed machines, a strong runner outlet swirl can develop under off-design conditions. The action of the diffuser, and in particular the draft tube elbow, on the rotating flow field induces synchronous pressure and discharge fluctuations. These low frequency disturbances are of special interest, because they can easily propagate throughout the whole hydraulic system. The dynamic response of the system to the natural excitations can inhibit the normal usage of the powerplant. If the natural excitation occurs at frequencies close to an eigenfrequency of the hydraulic system, an important amplification of the hydraulic fluctuations can be expected. Passive measures, such as air admission or minor design modifications, are commonly taken as a last resort to try to reduce these phenomena. This work explores an active control approach to alleviate the problem. It is shown that the hydroacoustic fluctuations in a hydraulic installation can strongly be reduced by "injecting" the inverse signal of the turbine's natural excitation. While the main theme is the application of the active control approach to improve the operation stability of Francis turbines, the scope of this study is larger and addresses the following scientific topics: Modeling of an hydroelectric installation The dynamic behavior of an installation is studied based on an one-dimensional model. A compilation of the basic modeling tools was done. Using these tools, the dynamic model of a hydraulic installation and an external hydroacoustical source is presented. It is used to explain how the overall hydroacoustic fluctuations can be reduced using a hydraulic exciter mounted in the wall of a draft tube's cone. Improving the operation stability by active control The hydraulic fluctuations associated with off-design operating conditions of Francis turbines are often very periodic and characterized by the presence of a dominant frequency component. The aim is to cancel out this component for which the amplitudes are often excessive. An active control system, composed of a hydraulic exciter and its controller, has been developed for a Francis turbine. The exciter employs a pulsed flow injection in the draft tube to generate an antiexcitation at a given frequency, which is then synchronized with the turbine's natural excitation. A self-tuning extremum control algorithm optimizes the operating parameters of the exciter to minimize the overall fluctuations in the hydraulic installation. Laboratory tests show the efficiency of the active control system. The amplitude of the pressure and discharge fluctuations at the dominant frequency component are reduced to background noise levels. Multiple configurations of the exciter have been tested. A spectral analysis of the pressure fluctuations is presented, the system's energy balance has been performed and the control algorithm of the system is analysed. The energy balance is very encouraging: the exciter system needed only about 1 percent of the hydraulic power of the Francis turbine. Prediction of the operation stability of turbines based on model tests Laboratory tests on a scale model, homologous to the turbine, are an indispensable step in the development of a hydroelectric powerplant. Unlike the static characteristics, the dynamic behavior of a turbine installation is difficult to predict. An original identification method of the dynamic characteristics of a model turbine is proposed. The method is verified on a theoretical case study and experimentally tested. It constitutes the basis of the prediction of the operation stability of the powerplant based on model tests.

EPFL2000

Etude du comportement dynamique des turbines Francis

EPFL2000

Détection vibratoire de l'érosion de cavitation des turbines Francis

EPFL2000