Abstract
Today's energy market has a high demand of flexibility due to introduction of other intermittent renewables as wind and solar. To ensure a steady power supply, hydro turbines are often forced to operate more at part load conditions. Originally, turbines were built for steady operation around the best efficiency point. The demand of flexibility, combined with old designs has showed an increase in turbines having problems with hydrodynamic instabilities such as pressure pulsations. Different methods have been investigated to mitigate pressure pulsations. Air injection shows a significant reduction of pressure pulsation amplitudes. However, installation of air injection requires extra piping and a compressor. Investigation of other methods such as shaft extension shows promising results for some operational points, but may significantly reduce the efficiency of the turbine at other operational points. The installation of an extension of the runner cone has been investigated at NTNU by Vekve in 2004. This has resulted in a cylindrical extension at Litjfossen Power Plant in Norway, where the bolt suffered mechanical failure. This indicates high amplitude pressure pulsations in the draft tube centre. The high pressure pulsation amplitudes are believed to be related to high tangential velocity in the draft tube. The mentioned runner cone extension has further been developed to a freely rotating extension. The objective is to reduce the tangential velocity in the draft tube and thereby the pressure pulsation amplitudes.
Highlights
Hydropower is a renewable energy source with high efficiency
The results found a stationary runner cone to be beneficial for the draft tube performance
The largest rotational speed is reached at full load, where the freely rotating runner cone extension (FRUCE) is rotating the opposite direction of the turbine
Summary
Hydropower is a renewable energy source with high efficiency. its storage capacity and flexibility in power generation makes hydropower an excellent form of energy generation to ensure steady power supply. Increased amount of intermittent energy generation introduced the last years has increased the flexibility demand of the energy market. This has led to increased part load operation of hydro turbines. Direct consequences of pressure pulsations may be cavitation erosion and fatigue damages. Rotating machinery such as turbines tend to create periodic pressure fluctuations known as pressure pulsations. Cassidy and Falvey [4] discovered that a helical vortex could form downstream of the vortex breakdown This phenomenon is often referred to as the rotating vortex rope (RVR) in the hydropower literature [5]. For fixed blade runners, such as Francis, a vortex rope tends to occur in the draft tube at part load operation
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