Reaction Turbines

REACTION TURBINES

Principle 

If, at the inlet of the turbine, the water possesses both kinetic energy as well as potential energy, the turbine is known as Reaction Turbine. It requires low / medium head with high rate of flow. All the pressure energy of water is not completely converted into kinetic energy, as in the case of the impulse turbine.

First, water enters the guide blades, which guide the water to enter the moving blades. In the moving blades, part of the pressure energy is converted into kinetic energy, which causes rotation of the runner. Water leaving the moving blades is at a low pressure.

Thus, there is a pressure difference between the entrance and the exit of the moving blades. This difference in pressure is called Reaction. Pressure acts on moving blades and causes the rotation of the wheel in the opposite direction.

1. FRANCIS TURBINE

Francis Turbine was developed by the American Engineer Francis in 1850. It is an inward flow radial type reaction turbine. It operates under medium head.

 Working Principle [Fig. 2]

 Francis turbine consists of a Spiral Casing, Fixed Guide Blades, Runner, Moving Blades and Draft Tube.

The spiral casing encloses a number of stationary guide blades. The guide blades are fixed around the circumference of an inner ring of moving blades. Moving blades are fixed to the runner.

Water at high pressure from the penstock pipe enters the inlet in the spiral casing. It flows radially inwards to the outer periphery of the runner through the guide blades. From the outer periphery of the runner, water flows inwards through the moving blades and discharges at the center of the runner at a low pressure. During its flow over the moving blades, water imparts kinetic energy to the runner, causing the rotation of the runner.

Draft tube is a diverging conical tube fitted at the center of the runner. It enables the discharge of water at low pressure. The other end of the draft tube is immersed in the discharging side of the water, called tail race.

2. KAPLAN TURBINE

Kaplan Turbine is a low head reaction turbine, in which water flows axially. It was developed by German Engineer Kaplan in 1916.

 All the parts of the Kaplan turbine (viz., spiral casing, guide wheel and guide blades) are similar to that of the Francis turbine, except the runner blades, runner and draft tube. The runner and runner blades of the Kaplan turbine resemble with the propeller of the ship. Hence, Kaplan turbine is also called as Propeller Turbine.

Working Principle [Fig. 3]

Water at high pressure enters the spiral casing through the inlet and flows over the guide blades. The water from the guide blades strikes the runner blades axially. Thus, the kinetic energy is imparted by water to the runner blades, causing the rotation of the runner. The runner has only 4 or 6 blades.

The water discharges at the center of the runner in the axial direction into the draft tube. The draft tube is of L-shape with its discharging end immersed into the tail race.