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.

Avellan, François
Lausanne, EPFL

Note: The status of this file is: EPFL only

 Record created 2005-03-16, last modified 2018-03-17

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