Use of draft tube in Francis turbine to increase output

[the_kool_guy]

Work output(deal) for a hydraulic machine per kg of incoming fluid is V(TANGENTIAL)*SPEED OF BLADE (assuming zero out swirl velocity.

Draft tube causes static pressure ont runner outlet to fall below zero. Thus available head is increased by an amount equal to this drop. But power output from francis turbine is dependent on blade rpm( which is constant as per electricity frequency) and inlet velocity.
How does draft tube increases power output of a francis turbine if flow of water is constant . ( this flow rate is directly proportional to tangential velocity component )...

My alternate question is on the values of power generated in the absence of draft tube ...

 

[Tom.G]

See the website below. It states that the Draft Tube is used to reduce or eliminate cavitation within the turbine.

http://www.learnengineering.org/2014/01/how-does-francis-turbine-work.html