Kaplan turbines derive motive force from pure reaction. They work efficiently when there is a huge water flow available. Working and design principles of Kaplan turbine are discussed elaborately in this video lecture.
Detailed description of the video lecture is given below.
Kaplan Turbine - A Mammoth
Kaplan turbines are suitable for power extraction when water energy is available at low head and high flow rate. Following are the operating conditions Kaplan turbine is best suitable for
- Head = 2-25 m
- Flow rate = 70-800 m^3/s
|Fig.1 Kaplan turbines are efficient in power production when there is huge flow available|
Which means they are suitable for operation when water is stored in a big reservoir at a relatively shallow altitude. Owing to the fact that Kaplan turbines handles huge flow rate, Kaplan turbines come in really huge size. They are the biggest among hydro electric turbines.
Flow in Kaplan Turbine
In Kaplan turbine flow is entered through a spiral casing. Decreasing area of casing makes sure that flow is entered to the central portion almost at uniform velocity throughout the perimeter.Water after crossing the guide vanes passes over the runner.Finally it leaves through a draft tube.
|Fig.2 Water flow paths in Kaplan turbine in 2 different views|
Energy Extraction at Runner
Most important part of Kaplan turbine is its runner. Cross section of runner blade will have a curved shape, as shown in figure below. So 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.
|Fig.3 Force developed on runner blades due to airfoil effect is the driving force of Kaplan turbine|
So it is clear that in Kaplan force is derived due to pure reaction force of flowing water. Due to this reason water absolute velocity across the blade will remain same, but there will be a huge drop in pressure.
There will be efficient production of reaction force when flow rate is high. This is the reason why Kaplan turbines perform well under huge flow rate.
Kaplan Turbines - Axial Flow Machines
Kaplan turbines are axial flow machines, where absolute velocity of flow is parallel to axis of turbine.
Water is precisely made to pass through runner blades with help of a shroud as shown in figure below.
|Fig.4 In Kaplan flow over runner blade is parallel to axis of rotation|
Meeting for Varying Power Demand
Power demand may fluctuate over time. Controlling the water flow rate is the most efficient way to meet for power demand. A governing mechanism, which controls position of guide vanes is used to control water flow rate.When power demand is high guide vanes are opened and when power demand is low guide vanes are closed. Figure below shows 2 extreme cases of power generation.
|Fig.5 Governing mechanism to control flow: In first case flow is maximum, in second case flow is minimum|
Blade Design – Kaplan Turbine
Blades of Kaplan turbine are designed to operate under a wide range of operating conditions. A rotating blade experiences relative velocity of flow. Fundamental principle in blade design is that, relative velocity of fluid flow should be at optimum angle of attack at all cross sections. Such condition can generate maximum force production thum maximum energy extraction from fluid.
Even though absolute velocity is axial, relative velocity will be inclined depending upon blade velocity. Inclination of relative velocity increases as we move from root to tip due to increasing blade velocity. This variation is shown in figure below.
|Fig.6 Continuous twist is given to the blade from root to tip to make sure that angle of attack is optimum under all cross-sections|
So there should be continuous twist in blade from root to tip. Such twist will make sure at every cross section the angle of attack is optimum.
Adjustable Blades for Optimum Angle of Attack
With varying 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 blade should pitch vertically. If flow rate is low relative velocity of flow is more tangential. So blades are pitched in tangential direction. This pitching action is shown in figure below.
|Fig.7 Pitching action of blades: First figure is for high flow rate case Second figure is of low flow rate case|
In short pitching action of blade make sure that blade is still in optimum angle of attack even under varying flow.
Controlling Swirl of Flow – Guide Vanes
Apart from controlling the flow rate guide vanes have got one more function. They help in controlling swirl of flow. If guide vanes were not present the flow would be highly swirling in nature due to its tangential entry.
|Fig.8 Flow in runner region without guide-vanes|
Such a flow would reduce performance of turbine drastically due to its poor angle of attack. So guide vanes control swirl of flow by making the flow more radial and makes sure that flow is still under optimum angle of attack.
Overcoming Problem of Cavitation
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 because there is huge pressure drop assosiated with energy extraction from fluid. In most of the regions pressure goes very low.
But damage due to cavitation can be reduced by using suitable blade material. ASTM A487 Stainless steel is one of the commonly used blade material to prevent it from cavitation. Use of anti-cavitation fin is another technique. Draft tube which transforms dynamic pressure to static pressure due to its increasing area also helps in reducing cavitation effect.