Abstract
Hydraulic machines operating in a wider range are subjected to cavitation developments inducing undesirable pressure pulsations which could lead to potential instability of the power plant. The occurrence of pulsating cavitation volumes in the runner and the draft tube is considered as a mass source of the system and is depending on the cavitation compliance. This dynamic parameter represents the cavitation volume variation with the respect to a variation of pressure and defines implicitly the local wave speed in the draft tube. This parameter is also decisive for an accurate prediction of system eigen frequencies. Therefore, the local wave speed in the draft tube is intrinsically linked to the eigen frequencies of the hydraulic system. Thus, if the natural frequency of a hydraulic system can be determined experimentally, it also becomes possible to estimate a local wave speed in the draft tube with a numerical model.In the present study, the reduced scale model of a Francis turbine (v=0.29) was investigated at off-design conditions. In order to measure the first eigenmode of the hydraulic test rig, an additional discharge was injected at the inlet of the hydraulic turbine at a variable frequency and amplitude to excite the system. Thus, with different pressure sensors installed on the test rig, the first eigenmode was determined. Then, a hydro-acoustic test rig model was developed with the In-house EPFL SIMSEN software and the local wave speed in the draft tube was adjusted to obtain the same first eigen frequency as that measured experimentally. Finally, this method was applied for different Thoma and Froude numbers at part load conditions.
Highlights
Nowadays, the pump-storage power plants are a proven solution for storing electricity at large scale and offering flexibility to the power management
An alternative method based on experimental data and a numerical model is presented in this paper to identify the local wave speed as a function of the Thoma number for a cavitation vortex rope
As was pointed out to the natural frequency, there is always a strong dependence of the wave speed with the pressure in the draft tube, and with the Thoma number linked to the implementation of the turbine
Summary
The pump-storage power plants are a proven solution for storing electricity at large scale and offering flexibility to the power management. At off-design operating points, a swirling flow at the outlet runner remains and gives rise to a cavitation vortex rope. This cavitation vortex rope is described as an excitation source for the hydraulic system with a frequency of about 0.2 to 0.4 times the rotational frequency [1]. Interaction between this excitation source and system eigen frequency may result in resonance effect and induce a draft tube surge and electrical power swings [2][3]. There is an intrinsic link between the wave speed in the draft tube and all eigen frequencies
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