The influence of the free runners on the decelerated swirling flow from the draft tube cone of hydraulic turbines

Nowadays the renewable energy plays an key role in the development of safe and clean energy. Worldwide and especially in Europe the development of renewable energy is focused on development of new wind and solar power plants. Accordingly, these energy technologies introduce high power fluctuations in the electrical system, which are necessary to compensate by other energy sources. To compensate these power fluctuations, the electrical systems use new technologies as high-capacity electric batteries or classical technologies as hydropower. The new technologies as the batteries are used rarely, while the hydropower remain the only technology to response quickly at the electrical systems requirements. The hydropower with the hydraulic turbines has the capability to adapt faster at the electrical system requirements, but for hydraulic turbines this requirement comes with hydrodynamic consequences. Accordingly, at the outlet of the hydraulic turbines (in the conical diffuser), the hydraulic instabilities are developed (vortex rope occurs) accompanied by high pressure fluctuations. The present paper proposes adding free runner at the inlet of the conical diffuser, which rotates on an axle, with null momentum. For this purpose, have been designed and manufactured a series of four free (additional) runners (with different numbers of blades) to assess the performances of this method. Experimental investigations of unsteady pressure and velocity profiles using LDV system were performed downstream of the free runner. The velocity measurements have been performed on three survey axes, by measuring the meridian and circumferential velocity profiles. The unsteady pressure measurements were achieved at the conical diffuser wall on two levels. To better analyse the relevance of the free runners, the Fourier transform is applied on pressure signals. The results will clarify the functionality and limitations of this method for swirling flow control.