Kaplan turbines are suitable for power extraction when water energy is available
at low head and high flow rate
which means they are suitable for operation when water is stored in big reservoir
at relatively shallow altitude
In kaplan turbine flow is entered through a spiral casing
decreasing area of casing make sure that flow is entered through center portion almost at uniform velocity throughut the perimeter
water after crossing the guide vanes passes over the runner
finally it leaves through the draft tube
most important part of kaplan turbine is it's runner
cross section of runner blade will have a curve shape
when water flows over it
it will induce a lift force due to airfoil effect
tangential component of lift force will make the runner rotate
this rotation is transferred to a generator for electricity production
kaplan turbines are axial flow machines where absolute velocity of flow is parallel to axis of turbine
water is precisely meet to pass through the runner blades with the help of shroud
power demand may fluctuate over time
this can meet for varing power demand
blades of kaplan turbine are designed to operate under wide range of operating conditions
A rotating blade experiences relative velocity of flow
the fundamental thing in blade design is
relative velocity of fluid flow should be at optimum angle of attack at all cross sections
even though absolute velocity is axial
relative velocity will be inclined depending upon blade velocity
Inclination of relative velocity increases as removed from rotor tip
there should be continuous twist in blade from rotor tip
so that at every cross section the angle of attack is optimum
with varing flow condition relative velocity will change drastically
kaplan turbine blades are adjustable
when flow rate is high relative velocity of flow will be more axial
so blades should pitch vertically
If flow rate is low relative velocity of flow is more tangential
So blades are pitched in tangential direction
in short the pitching action of blade make sure that blade is still in optimum angle of attack
even under varying flow
apart from controlling the flow rate guide vanes have got one more function
they help in controlling swirl of flow
if guide vanes will not present the flow would be highly swirling in nature
such a flow will reduce performance of turbine drastically due to its poor angle of attack
so guide vanes control swirling flow for optimum angle of attack
biggest challenge in design of kaplan turbine is how to overcome problem of cavitation which causes material erosion
and vibration
cavitation is unavoidable in kaplan turbine since in most of the regions pressure goes very low
but damage due to cavitation can be reduced by using suitable
draft tube which transforms dynamic pressure to static pressure