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Francis turbine converts energy at high pressure heads which are not easily available and hence a turbine was required to convert the energy at low pressure heads, given that the quantity of water was large enough. It was easy to convert high pressure heads to power easily but difficult to do so for low pressure heads. Therefore, an evolution took place that converted the Francis turbine to Kaplan turbine, which generated power at even low pressure heads efficiently.
Turbines are sometimes differentiated on the basis of the type of inlet flow, whether the inlet velocity is in axial direction, radial direction or a combination of both. The Francis turbine is a mixed hydraulic turbine (the inlet velocity has Radial and tangential components) while the Kaplan turbine is an axial hydraulic turbine (the inlet velocity has only axial velocity component). The evolution consisted of the change in the inlet flow mainly.
Nomenclature of a velocity triangle:
A general velocity triangle consists of the following vectors:[1][2]
- V : Absolute velocity of the fluid.
- U : Tangential velocity of the fluid.
- Vr: Relative velocity of the fluid after contact with rotor.
- Vw: Tangential component of V (absolute velocity), called whirl velocity.
- Vf: Flow velocity (the axial component in the case of axial machines, the radial in the case of radial).
- α: Angle made by V with the plane of the machine (usually the nozzle angle or the guide blade angle).
- β: Angle of the rotor blade or angle made by relative velocity with the tangential direction.
Generally, the Kaplan turbine works on low pressure heads (H) and high flow rates (Q). This implies that the specific speed (Ns) on which a Kaplan turbine functions is high, as specific speed (Nsp) is directly proportional to flow (Q) and inversely proportional to head (H). On the other hand, the Francis turbine works on low specific speeds i.e., high pressure heads.
In the figure, it can be seen that the increase in specific speed (or decrease in head) have following consequences:
- A reduction in inlet velocity V1 .
- The flow velocity Vf1 at inlet increases, and hence allows a large amount of fluid to enter the turbine.
- Vw component decreases as moving to the Kaplan turbine, and here in the figure, Vf represents the axial (Va) component.
- The flow at the inlet, in the figure, to all the runners, except the Kaplan impeller, is in the radial (Vf) and tangential (Vw) directions.
- β1 decreases as the evolution proceeds.
- However, the exit velocity is axial in Kaplan runner, while it is the radial one in all other runners.
Hence, these are the parameter changes that have to be incorporated in converting a Francis turbine to a Kaplan turbine.
- The efficiency of a Kaplan turbine is higher than that of a Francis turbine.
- A Kaplan turbine has a smaller cross-section and has lower rotational speed than a Francis turbine.
- In a Kaplan turbine, the water flows in axially and out axially, while in a Francis turbine it flows in radially and out axially.
- A Kaplan turbine has fewer runner blades than a Francis turbine because a Kaplan turbine's blades are twisted and cover a larger circumference.
- Friction losses in a Kaplan turbine are less.
- The shaft of a Francis turbine is usually vertical (in many of the early machines it was horizontal), whereas in a Kaplan turbine it is always vertical.
- A Francis turbine's specific speed is medium (60–300 RPM); a Kaplan turbine's specific speed is high (300–1000 RPM).
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