A type of turbine that develops torque
by reacting to the pressure or weight of a fluid; the operation of reaction
turbines is described by Newton's third law of motion (action and reaction
are equal and opposite).
| Reaction and impulse turbines compared.
In a reaction turbine, unlike in an impulse
turbine, the nozzles that discharge the working fluid are attached to
the rotor. The acceleration of the fluid leaving the nozzles produces a
reaction force on the pipes, causing the rotor to move in the opposite direction
to that of the fluid. The pressure of the fluid changes as it passes through
the rotor blades. In most cases, a pressure casement is needed to contain
the working fluid as it acts on the turbine; in the case of water
turbines, the casing also maintains the suction imparted by the draft
tube. Alternatively, where a casing is absent, the turbine must be fully
immersed in the fluid flow as in the case of wind
turbines. Francis turbines
and most steam turbines use the
reaction turbine concept.
Main types of reaction turbine
A propeller turbine generally has a runner with three to six blades
in which the water contacts all of the blades constantly. Picture a
boat propeller running in a pipe. Through the pipe, the pressure is
constant; if it isn't, the runner would be out of balance. The pitch
of the blades may be fixed or adjustable. The major components besides
the runner are a scroll case, wicket gates, and a draft tube. There
are several different types of propeller turbines:
- Bulb turbine
The turbine and generator are a sealed unit placed directly in the
The generator is attached directly to the perimeter of the turbine.
- Tube turbine
The penstock bends just before or after the runner, allowing a straight
line connection to the generator.
Both the blades and the wicket gates are adjustable, allowing for
a wider range of operation.
A Francis turbine has a runner with fixed buckets (vanes), usually nine
or more. Water is introduced just above the runner and all around it
and then falls through, causing it to spin. Besides the runner, the
other major components are the scroll case, wicket gates, and draft
Kinetic energy turbines, also called free-flow turbines, generate electricity
from the kinetic energy present in flowing water rather than the potential
energy from the head. The systems may operate in rivers, man-made channels,
tidal waters, or ocean currents. Kinetic systems utilize the water stream's
natural pathway. They do not require the diversion of water through
manmade channels, riverbeds, or pipes, although they might have applications
in such conduits. Kinetic systems do not require large civil works;
however, they can use existing structures such as bridges, tailraces