Head Delivered by Turbine and Draft Tube
To enable the turbine to be set above the water level, Figure a kind of diffuser is provided and connected between the water exit and the tail race level. This is called a draft tube.
A partial vacuum is produced at the upper end of the draft tube, which compensates for the height at which the turbine runner is set within certain limits. The turbine rotor could be set at different elevations without altering the available head. Another function to draft tube is to reduce the exit velocity from the turbine in such a way that the kinetic energy rejected in tail race could be reduced.
Schematic diagram of reaction turbine installation
H = the geometric difference between the upstream and the downstream.
1 H = the losses in the tunnel between the turbine and the U.S.
V g e 2 / 2 = kinetic energy rejected in the tail race where e V is the water exit
V g o 2 / 2 = kinetic energy at inlet.
Applying energy equation (Bernoulli's) between points 1 and 2 (the turbine exit and the draft tube exit);
The term represents the tail losses; L h represents thehydraulic losses due to friction and enlargement and represents the losses due to the kinetic energy rejected in the tail race.
The losses could be presented as follows:
The absolute pressure head at point 1 (turbine exit) could be presented as follows:
When K equals zero, there will be no energy losses and hence the efficiency of the draft tube could be written as;
The theoretical regain in head in draft tube due to kinetic energy g h ,
Introducing the draft tube efficiency D.T. η , the above expression will be;