For a Francis turbine, the guide vanes are used primarily to
- ASupport the runner
- BRegulate the flow rate and impart tangential velocityCorrect
- CIncrease the pressure head
- DConvert kinetic energy into pressure energy
Solution
Concept
Operating functions of guide vanes in Francis turbines
Formula
NONE
Step-by-step solution
1. In a Francis turbine, water passes from the casing through a ring of guide vanes (wicket gates) before reaching the runner. 2. The guide vanes are pivoted and regulated by a governor to adjust the flow rate to match electric generator load. 3. They are shaped to impart a tangential whirl velocity so water strikes runner blades smoothly without hydraulic shock. 4. Thus, guide vanes regulate flow rate and impart tangential velocity. Answer: B
Explanation
In a Francis reaction turbine, water enters radially through a stationary ring of adjustable guide vanes (wicket gates). The guide vanes perform two primary functions: regulating the volumetric flow rate into the runner according to power output demand, and directing water at the optimum tangential angle to enter the runner vanes smoothly without shock.
Why the other options are wrong
A) Guide vanes do not support the runner; the runner is supported on the main turbine shaft and bearings. C) Guide vanes convert pressure head into kinetic energy, not vice versa. D) The draft tube converts kinetic energy into pressure energy, not the guide vanes.
Shortcut
Guide vanes in Francis turbine = Regulate flow rate + Impart whirl / tangential velocity.
Common mistake
Thinking guide vanes support the runner shaft or convert kinetic energy into pressure.
Correct answer: B) Regulate the flow rate and impart tangential velocity