TURBINE

1. FRANCIS TURBINE
THE FRANCIS TURBINE IS A REACTION TURBINE AND IT WAS
DEVELOPED BY JAMES B FRANCIS. IT IS AN INWARD FLOW
REACTION TURBINE THAT COMBINES RADIAL AND AXIAL FLOW
CONCEPTS. THE FRANCIS TURBINE IS THE MOST COMMON WATER
TURBINE USED TODAY.
THE FRANCIS TURBINE OPERATES A HEAD RANGE OF 10 METERS
TO SEVERAL HUNDRED METERS AND IS PRIMARILY USED
FOR ELECTRICAL POWER PRODUCTION.

3. The Main Parts of a Francis Turbine:
A Francis Turbine consists of the 5 main parts those are:
•Spiral Casing
•Stay Vanes
•Guide Vanes
•Runner Blades
•Draft Tube

4. Spiral Casing:
It provides an encased water path to contain the water pressure.
The water flowing from the reservoir or dam is made to pass through
this pipe with high pressure. The blades of the turbines are circularly
placed, which means the water striking the blades of the turbine should
flow in the circular axis for efficient striking. So, the spiral casing is
used, but due to the circular movement of the water, it loses its
pressure.
To maintain the same pressure, the diameter of the casing is gradually
reduced, to maintain the pressure uniformly, thus uniform momentum or
velocity striking the runner blades.
Stay Vanes:
This guides the water to the runner blades.
Stay vanes remain stationary at their position and reduces the swirling
of water due to radial flow and as it enters the runner blades. Hence,
makes the turbine more efficient.

5. Guide Vanes:
Guide vanes are also known as wicket gates. The main function or usages of
the guide vanes are to guide the water towards the runner. The water flow
must be an angle and that is appropriate for the design.
Runner Blades:
Absorbs the energy from the water and converts it to rotational motion of the
main shaft. The runner blades design decides how effectively a turbine is
going to perform. The runner blades are divided into two parts. The lower half
is made in the shape of a small bucket so that it uses the impulse action of
water to rotate the turbine.The upper part of the blades uses the reaction
force of water flowing through it. These two forces together make the runner
rotate.

6. Its cross-section area increases along its length, as the water coming out of
runner blades, is at considerably low pressure, so its expanding cross-
section area helps it to recover the pressure as it flows towards the tailrace.
Let's See How a Francis Turbine Work?
First, the water is allowed to enter into the spiral casing of the turbine, which
guides the water through the stay vanes and guide vanes. The spiral case is
kept here in decreasing diameter so that to maintain the flow pressure.
The stay vanes being stationary at their place removes the swirls from the
water, which are generated due to flow through the spiral casing and tries it
to make the flow of water more linear to be deflected by adjustable guide
vanes.

7. The angle of guide vanes determines the angle of strikes of water at the
runner blades thus make sure the output of the turbine. The runner blades
are stationary and can-not pitch or change their angle. In short, the guide
vane controls the power output of a turbine.
The performance and efficiency of the Francis turbine are dependent on the
design of the runner blades.
In a Francis turbine, the runner blades are divided into two parts. The lower
half is made in the shape of a small bucket so that it uses the impulse action
of water to rotate the turbine.
The other or you can say the upper part of the blades uses the reaction
force of water flowing through it. Thus, runner blades make use of both
pressure energy and kinetic energy of water and rotate the runner most
efficiently.

8. The water which is coming out of runner blades would lack both the kinetic
energy and pressure energy, so we use the draft tube to recover the
pressure as it advances towards tailrace, but still, we cannot recover the
pressure to that extent that we can stop air to enter into the runner
housing thus causing cavitation.
Cavitation:
The difference in the pressure of water when the entrance to the turbine and
exists to the turbine after striking the runner blades is too high, due to more
amount of pressure difference, the air molecules which are relatively at high
pressure then the water comes out, enters the turbine casing in the form of
bubbles.
These bubble keeps on exploding near the surface of the runner blades
continuously causing a shock wave, which produces a kind of defect at
runners’ surface called cavitation.
Thus, causing a serious problem for turbines efficiency.