CT: A Kaplan turbine is basically a propeller with adjustable blades inside a tube. It is an axial-flow turbine, which means that the flow direction does not change as it crosses the rotor. Figure 1 shows a simplified Kaplan turbine.
The inlet guide-vanes can be opened and closed to regulate the amount of flow that can pass through the turbine. When fully closed they will stop the water completely and bring the turbine to rest. Depending on the position of the inlet guide-vanes they introduce differing amounts of ‘swirl’ to the flow, and ensure that the water hits the rotor at the most efficient angle for the highest efficiency. The rotor blade pitch is also adjustable, from a flat profile for very low flows to a heavily-pitched profile for high flows (see Figure 2). This adjustability of both inlet guide-vanes and rotor blades means that the flow operating range is very wide (a characteristic from the inlet guide-vanes) and the turbine efficiency is high and the efficiency curve very flat (a characteristic from the adjustable rotor blades allowing optimum alignment of the blade to the oncoming flow).
S: http://www.renewablesfirst.co.uk/hydro-learning-centre/kaplan-turbines/ (last access: 27 December 2014)
N: 1. The Kaplan turbine is an Austrian invention by Viktor Kaplan (1876-1934) and is chiefly used for large masses of water and low drop heights. The rotor is similar to a ship’s propeller. Fitting it takes place mostly vertically, so that the water can flow over the turbine from top to bottom. Directly above the turbine is the generator. The generated kinetic energy is transferred from the turbine to the generator with the help of a vertical shaft.
2. Kaplan turbines are primarily used in the low head range with large volumes of water. The adjustability of the guide vanes and runner blades allows optimal use of varying water flow.
3. The nose cone on a Kaplan turbine is important hydro-dynamically to reduce losses and prevent the formation of a core ‘rope vortex’, and also provides the space for the complex blade pitching mechanism inside. The draft tube is also a critically important part. Although a static fabricated part, the geometry of the draft tube is carefully designed to extract any remaining kinetic energy from the flow by reducing the water pressure at the exit of the rotor.
4. There are variants of Kaplan turbines that only have adjustable inlet guide-vanes or adjustable rotor blades, which are known as semi-Kaplan’s.
S: 1. http://www.verbund.com/pp/en/turbinen-typen/turbinen-kaplan (last access: 27 December 2014). 2. http://www.koessler.com/en/kaplan-turbines (last access: 27 December 2014). 3 & 4. http://www.renewablesfirst.co.uk/hydro-learning-centre/kaplan-turbines/ (last access: 27 December 2014); http://www.hydroquebec.com/learning/hydroelectricite/types-turbines.html (last access: 27 December 2014).
CR: [hydroelectric energy], hydroelectric power plant.