its direction can be controlled by these adjustable gates. After leaving the
runner, the water enters a draft tube which delivers the flow to the tail race.
There are two main types of reaction turbine, the Francis and the propeller
(also known as Kaplan or bulb) turbine (Fig. 12.9).
Turbines may also be classified according to the main direction of
flow of water in the runner as: (a) tangential flow turbine (Pelton wheel);
(b) radial flow turbine (Francis, Thompson, Girard); (c) mixed-flow
turbine (modern Francis type); (d) axial-flow turbine of fixed-blade (pro-
peller) type or movable blade (Kaplan or bulb) type.
For history of the development of hydraulic turbines see Viollet (2005).
12.8.2 Specific speed, Ns, speed factor, , and turbine classification
The specific speed, Ns, of a turbine is its most important characteristic, and
is of paramount importance in design. It is defined as the speed at which a
geometrically similar runner would rotate if it were so proportioned that it
would develop 1 kW when operating under a head of 1 m, and is given by
NsNP1/2/H5/4 (12.2)
506 HYDROELECTRIC POWER DEVELOPMENT
Fig. 12.9 Reaction turbines
