Kaplan turbine is a famous type of the reaction turbine. Kaplan turbine is a countercurrent axial turbine with regulating buckets. A turbine is called an axial turbine if fluid flows horizontally on the shaft axis of rotation. Kaplan turbines are axial reaction turbines because the water flows in an axial path.
In axial reaction turbines, the turbine shaft is perpendicular. The lower end of the shaft is larger and is called the boss or hub. Since the buckets attaches to the hub, the hub works as an impeller for the axial reaction turbines.
Professor Viktor Kaplan from Austria it in 1913. The Kaplan turbine is the development of the Francis turbine. The development of this turbine enables efficient power generation in low head applications that was impossible with Francis turbines. These turbines are widely using worldwide for low head, high flow power generation.