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
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|
|Fig.2 Water flow through Francis turbine runner|
|Fig.3 Airfoil cross section shape of Francis blades & production of reaction force|
|Fig.4 Francis turbine derive energy from combined action of reaction and impulse force|
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|
|Fig.6 Stay vanes and guide vanes used in Francis turbine|
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|
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|