The Working Mechanism of a Francis Turbine
The turbine consists of an outer spiral casing, followed by a set of fixed blades called stay vanes. Next comes a collection of moving blades called guide vanes, then a bunch of centrally placed blades called runner and lastly, an outgoing duct called a duct draft tube.
The flow enters the Francis turbine through the spiral casing. Decreasing the cross-sectional area of the casing ensures that the flow enters the central part of the turbine with uniform velocity throughout the perimeter.
The flow next passes through two sets of blades before entering the runner, namely – the outer stay vanes and the inner guide vanes. The stay vanes are fixed and help steer the water towards the runner section. They help in reducing the swirl at the inlet flow as well.
The guide vanes sitting in between the stay vanes and the runner have a more critical role to play. They manage the flow rate based on the power demand. But the power demand fluctuates over time. The guide vanes govern the water flow rate and ensure that power production is in sync with demand. In addition, guide vanes control the flow angle directed towards the runner blades. They try to ensure that the inlet flow angle is at the optimal angle of attack to harness maximum water power.