Experimental investigation and mitigation of pressure pulsations in Francis turbines

Hydraulic turbomachines, such as Francis turbines, have been utilized for more than a century to generate renewable energy. Research and development has led to an outstanding level of efficiency and reliability, with the Francis turbine at the top end with almost 96% efficiency. To achieve such levels of efficiency, the turbine has to be design for a specific operating point. The development of a continental transmission grid and introduction of intermittent renewables such as wind and solar has led to a fundamental change in the energy market. The hydraulic turbomachines in hydro power has now become one of the stabilizing force of an energy market demanding flexibility. Amongst other reasons, this has led to operation of hydraulic turbines outside the design point. Operating turbines outside the design point may lead to heavy vibrations and potentially mechanical failure. In addition, modern design is focused on increasing efficiency, possibly at the expense of the runner characteristics outside the design load. The vibrations are a consequence of the turbine, which generates pressure pulsations. The pressure pulsations propagates into the water conduit. In case of resonance, the pressure pulsations will increase in amplitude and induce further vibrations. However, they can be reduced. The main objective of this thesis has been to investigate and possibly reduced the pressure pulsations occurring at part load operation. An experimental investigation of different methods to reduce pressure pulsation in Francis turbines has been carried out. Air injection was investigated at La Higuera Hydro Power Plant (HPP) in Chile. A free rotating runner cone extension (FRUCE) was developed and investigated at the Waterpower Laboratory and in Leirfossene HPP. The air injection through the runner cone and draft tube wall was tested in La Higuera HPP. Both options gave a significant dampening of pressure pulsations at part load by reducing the maximum value of the peak-to-peak values by 60%. However, an increase in the vortex rope frequency was observed when air was injected. At the Waterpower Laboratory, the free rotating runner cone extension was tested with different lengths. The developed FRUCE is an extension of the runner cone where the outer shell is mounted on bearings allowing it to rotate independently from the turbine. It was concluded that the FRUCE had some dampening effect, but that the diameter was too small to achieve a significant dampening effect of the pressure pulsations. Three FRUCEs with different length and diameter were designed and tested at Leirfossene HPP. A dampening in pressure pulsations was achieved at part load operation. However, different FRUCEs worked best at different loads. The FRUCE also reduced pressure pulsations at full load, but the drawback is reduced efficiency at full load. An adjustment of the FRUCE length and diameter is necessary to achieve the maximum dampening. The FRUCE had development potential and may be possible solution to reduce pressure pulsations in Francis turbines. However, it will require numeric simulations, further prototype testing and possibly implementation of technology to actively control the FRUCE length and possibly the rotational speed