Use of draft tube in Francis turbine to increase output

AI Thread Summary
The draft tube in a Francis turbine enhances power output by lowering the static pressure at the runner outlet, which increases the effective head available for energy conversion. This increase in head allows for greater work output per kg of fluid, despite the constant blade RPM dictated by electricity frequency. The relationship between flow rate and tangential velocity is crucial, as the draft tube's design helps maintain optimal flow conditions. Additionally, the draft tube mitigates cavitation risks, which can otherwise diminish turbine efficiency. Overall, the draft tube plays a vital role in maximizing the performance of Francis turbines under steady flow conditions.
the_kool_guy
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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 ...
 
Engineering news on Phys.org
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
 

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