Evolution from Francis turbine to Kaplan turbine

Francis turbines convert energy under high-pressure water heads that are not easily obtained, so considering that the water volume is large enough, turbines are required to convert energy under low-pressure water heads.It is easy to convert a high head to power, but it is difficult with a low head.Hence the evolution that took place to convert Francis turbines into Kaplan turbines, which generate electricity efficiently even at low head.

Changes 

• Turbines are sometimes differentiated by the type of inlet flow, whether the inlet velocity is axially oriented,radially oriented, or a combination of both.Francis turbines are hybrid turbines (inlet velocity has radial and tangential components), while axial turbines are axial turbines (inlet velocity has only axial velocity components).Evolution mainly includes changes in ingress traffic.

• Nomenclature of Velocity Triangle:

• A general velocity triangle consists of the following vectors:

• V: Absolute velocity of the fluid.

• U: Tangential velocity of the fluid.

• Vr: The relative velocity of the fluid after contact with the rotor.

• Vw: Tangential component of V (absolute velocity), called whirling velocity.

• Vf: Flow velocity (axial component in case of axial machines, radial component in case of radial machines).

• α: The angle V makes with the plane of the machine (usually the nozzle angle or vane angle).

• β: Angle or relative velocity of the rotor blades to the tangential direction.

Typically, Kaplan turbines operate at low head (H) and high flow (Q).This means that Kaplan turbines operate at high specific speed (Ns) because specific speed (Nsp) is directly proportional to flow (Q) and inversely proportional to head (H).On the other hand, Francis turbines work at low specific speed, i.e. high head.As can be seen from the figure, an increase in Specific speed (or a decrease in Head) has the following consequences:

• The inlet velocity V1 is reduced.

• The flow velocity Vf1 at the inlet is increased, thus allowing a large volume of fluid to enter the turbine.

• The Vw component decreases moving to a Kaplan turbine, and in the figure, Vf represents the axial (Va) component.

• In the figure, the flow at the inlet is in radial (Vf) and tangential (Vw) directions for all runners except the Kaplan impeller.

• β1 decreases as evolution progresses.

• However, the exit velocity of the Kaplan runner is axial, while the exit velocity of all other runners is radial.So these are the parameter changes that have to be incorporated when converting a Francis turbine to a Kaplan turbine.

General differences between Francis and Kaplan turbines

Efficiency of Kaplan turbine is higher than Francis turbine.Kaplan turbine is more compact in cross-section and has lower rotational speed to that of Francis turbine.In Kaplan turbine, the water flows axially in and axially out while in Francis turbine it is radially in and axially out.The runner blades in the Kaplan turbine are less in number as the blades are twisted and covers a larger circumference.Friction losses in Kaplan turbine are less.The Shaft of a Francis Turbine is usually vertical, but in many of the early machines it was horizontal, whereas in Kaplan Turbines it is always vertical.The Francis Turbines specific speed is medium (60-300 rpm) and Kaplan Turbines specific speed is high (300-1000 rpm).