At single-regulated Francis pump turbines a vortex rope is created outside the optimal operating range due to a swirling flow at the runner outlet. The vortex causes low-frequency pressure pulsations, more over vibrations and noise. Pressure pulsations and vortex rope have the same rotating frequency, which is usually between 20 % and 40 % of the rotational speed of the runner. In case of insufficient suction height the vortex rope begins to cavity and thus becomes visible and it is called cavitation vortex rope. For the selected model of the Francis pump turbine we have experimentally identified the operating points where the cavitation vortex rope is created. We defined how the vortex looks and the values of pressure pulsations that are generated by the vortex rope. In the paper are also presented the first results measuring the absolute static pressure inside the cavitation vortex rope. For the pressure measurements inside the cavitation vortex rope a special measurement system was developed. Depending on the different selected operating points, the measured absolute static pressure readings ranged from 90 mbar to 260 mbar. Measurements were carried out according to IEC 60193 standard at a laboratory for turbine machines at a test rig for measuring Francis type turbine models.
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