Reaction Turbine

REACTION TURBINE

Principle

Reaction is the force obtained on a body when a fluid leaves the body with a higher relative velocity.

Examples: Lawn (Water) sprinkler, Recoil of a gun, Jet plane, etc.

In Fig. 6(i), in the lawn sprinkler, water under pressure flows into the sprinkler. The sprinkler rotates in the direction shown, due to the reaction of the high velocity water jet.

See Fig. 6(ii). Assume that high pressure high temperature steam from the boiler is sent to a hollow cylindrical rotor.

The rotor has a few openings arranged radially through tubes. The ends of the tubes are shaped as Nozzles. Steam expands as it passes through the nozzleş. .

This expansion of steam causes a backward thrust on the nozzles. This backward thrust is known as Reaction. Due to this reaction, the rotor will rotate in a direction opposite to the direction of steam flow.

1. PARSON'S REACTION TURBINE

Pure reaction turbine is never used in actual practice. In the actual reaction turbine, power is obtained by the impulsive force of incoming steam and reactive force of outgoing steam.

Descriptions

See Fig. 7(i). C.A. Parson has built a Reaction Turbine with three stages. It consists of a wheel or rotor and casing, both are of varying diameters. Equal number of fixed and moving blades are attached alternately to the casing and the wheel.

The moving and fixed blades are not symmetrical in shape and are also curved in the opposite direction.

The shape of the moving blades is so designed to have the reactive force, when the jet of steam is leaving the blades. Also, there will be some pressure drop in the moving blades.

Fixed blades are designed in such a way that the passages between them act as nozzles. Hence, the velocity increases with the decrease of pressure.

Working

Steam passes over the first fixed blade F₁. The fixed blade changes the direction of steam and at the same time, allows it to expand to a higher velocity, with decrease of pressure.

Then, the steam passes over the first moving blade M₁. The moving blade converts the kinetic energy into mechanical work with decrease of velocity. But, at the same time, the steam expands as it flows over the moving blade and there is a fall of pressure. This produces a reaction on the blade by the expanding steam.

Thus, in the reaction turbine, the steam expands both in fixed and moving blades continuously as steam passes over them. So, the pressure drop occurs gradually and continuously over both fixed and moving blades.

Fig. 7(ii) shows the Pressure Velocity Diagram of Parson's Reaction Turbine.

2.  COMPARISON OF IMPULSE AND REACTION TURBINES