The Francis turbine converts energy at high heads which are often not available; hence, a turbine was required to convert energy at low heads, given a sufficiently large quantity of water. It was easy to convert high heads to power easily but difficult to do so for low-pressure heads. Therefore, an evolution took place that converted the Francis turbine to the Kaplan turbine, which generates power at even low heads efficiently.
Turbines are sometimes differentiated according to the type of inlet flow, whether the inlet velocity is in an axial direction, radial direction, or a combination of both. The Francis turbine is a mixed turbine (the inlet velocity has radial and tangential components) while the Kaplan turbine is an axial turbine (the inlet velocity is purely axial). The evolution mainly consisted of the change in the inlet flow.
Nomenclature of a velocity triangle:
A general velocity triangle consists of the following vectors:[1][2]
Generally, the Kaplan turbine works on low 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 and high heads.
In the figure, it can be seen that the increase in specific speed (or decrease in head) has the following consequences:
Hence, these are the parameter changes that have to be incorporated in converting a Francis turbine to a Kaplan turbine.
