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

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.

Published in:
Proceedings of the 22nd IAHR Symposium on Hydraulic Machinery and Systems, Stockholm, Sweden, 1, 1-17
Presented at:
22nd IAHR Symposium on Hydraulic Machinery and Systems, Stockholm, Sweden, June 29 – July 2, 2004
International Association For Hydraulic Research

Note: The status of this file is: EPFL only

 Record created 2005-11-04, last modified 2018-05-08

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