How does a Francis turbine work ?
Francis turbines are the most preferred hydraulic turbines. They are the most reliable workhorse of hydroelectric power stations. It contributes about 60 percentage of the global hydropower capacity, mainly because it can work efficiently under a wide range of operating conditions. This video is aimed at giving a conceptual overview of working of Francis turbine.
Webpage version of the video gives more elaborated information on its working.
Water head and flow rate are the most vital input parameters that govern performance of a hydraulic turbine. But these parameters are subjected to seasonal variation in a hydroelectric power station. Francis turbine is capable of delivering high efficiency even if there is a huge variation in these flow parameters. Following are the head and flow rate under which Francis turbine is preferred to operate.
- Head = 45-400 m
- Flow rate = 10-700 m^3/s
In this article we will understand working of Francis turbine and will also realize why it is capable to work under varying flow conditions.
Runner – At the heart of the system
Most important part of Francis turbine is its runner. It is fitted with a collection of complex shaped blades as shown in Fig.1
|Fig.1 Runner - The most vital part of Francis turbine|
In runner water enters radially, and leaves axially. During the course of flow, water glides over runner blades as shown in figure below.
|Fig.2 Water flow through Francis turbine runner|
Blades of Francis turbine are specially shaped. One such blade is shown in Fig.2. It is clear from the figure that shape of blade cross-section is of thin airfoils. So when water flows over it, a low pressure will be induced on one side, and high pressure on the other side. This will result in a lift force.
|Fig.3 Airfoil cross section shape of Francis blades & production of reaction force|
You can also note one more peculiar thing about the blade. It is having a bucket kind of shape towards the outlet. So water will hit, and produce an impulse force before leaving the runner. Both impulse force and lift force will make the runner rotate.
|Fig.4 Francis turbine derive energy from combined action of reaction and impulse force|
So Francis turbine is not a pure reaction turbine, a portion of force comes from impulse action also. Thus as water flows over runner blades both its kinetic and pressure energy will come down.
Since flow is entering radially and leaves axially, they are also called ‘mixed flow turbine’.
Runner is connected to generator, via a shaft, for electricity production.
Use of Spiral Casing
Runner is fitted, inside a spiral casing. Flow is entered via an inlet nozzle. Flow rate of water will get reduced along length of casing, since water is drawn into the runner. But decreasing area of spiral casing will make sure that, flow is entered to runner region almost at uniform velocity.
|Fig.5 Spiral casing makes sure that flow is entered uniformly along the periphery of runnner|
Stay vanes and guide vanes are fitted at entrance of runner. The basic purpose of them is to convert one part of pressure energy into kinetic energy.
|Fig.6 Stay vanes and guide vanes used in Francis turbine|
Flow which is coming from the casing, meets stay vanes, they are fixed. Stay vanes steers the flow towards the runner section. Thus it reduces swirl of inlet flow.
Governing of Francis Turbine
Demand for power may vary over time. The guide vane mechanism is used to control water flow rate and makes sure that power production is synchronized with power demand.
|Fig.7 First figure shows guide vanes in closed position; In 2nd figure guide vanes in open position|
Apart from controlling flow rate guide vanes also control flow angle to inlet portion of runner blade. Thus guide vanes make sure that inlet flow angle is at optimum angle of attack for maximum power extraction from fluid.
Living with Cavitation
Most often local pressure at exit side of runner goes below vapor pressure of water. This will result in formation water bubbles and eventually damage to turbine blade material.This phenomenon is known as caviation. It is impossible to prevent cavitation completely. So a carefully designed draft tube is fitted at exit side to discharge the fluid out. Draft tube will transform velocity head to static head due to its increasing area and will reduce effect of cavitation.
|Fig.8 Conversion of velocity head to static head with help of drafttube|