francis turbines are the most preferred
hydraulic turbines
it contributes about 60 percent of the
global hydro power capacity
mainly because it can work efficiently
under a wide range of operating
conditions
most important part of france's turbine
is its runner
it is fitted with a collection of
complex shaped blades
in runner water is entered radially and
leaves axially
during the course of flow water glides
over runner blades
francis turbine blades are specially
shaped
you can note that blades are having thin
airfoil cross section
so when water flows over it a low
pressure will be produced on one side
and high pressure on the other side
this will result in a lift force
you can also note one more peculiar
thing about the blade
it is having a bucket kind of shape
towards the outlet
so water will hit and produce an impulse
force before leaving the runner
both impulse force and lift force will
make the runner rotate
so francis turbine is not a pure
reaction turbine
a portion of force comes from impulse
action also
thus as water flows over runner blades
both its kinetic and pressure energy
will come down
since flow is entering radially and
leaves axially they are also called
mixed flow turbine
runner is connected to generator via a
shaft for electricity production
this arrangement is fitted inside a
spiral casing
flow is entered via an inlet nozzle
flow rate of water will get reduced
along length of casing
since water is drawn into the runner
but decreasing area of spiral casing
will make sure that flow is entered to
run a region almost at uniform velocity
stay veins and guide veins are fitted at
entrance of runner
the basic purpose of them is to convert
one part of pressure energy to kinetic
energy
flow which is coming from the casing
meets stay veins
they are fixed
stay veins steers the flow towards the
runner section
thus it reduces swirl of inlet flow
demand for power may vary over time
the guide vane mechanism is used to
control water flow rate and make sure
that power production is synchronized
with power demand
apart from controlling flow rate guide
vanes also control flow angle to inlet
portion of runner blade
thus guide vanes make sure that inlet
flow angle is at optimum angle of attack
for maximum power extraction from fluid
low pressure at exit side of turbine may
cause serious problems of cavitation
so a carefully designed draft tube is
fitted at exit side to discharge the
fluid out
draft tube will transform velocity head
to static head due to its increasing
area and will reduce effective
cavitation
hope you had a nice session on working
of francis turbines thank you