Reaction Turbines — Role of Guide Vanes What is the principal function of the stationary guide vanes (wicket gates) in a reaction turbine such as a Francis machine?

Introduction:
Guide vanes in reaction turbines condition the inflow to the runner. They set both the flow rate and the flow angle to maximize efficiency and avoid hydraulic losses from incidence or separation.

Given Data / Assumptions:

• Steady operation; incompressible flow.
• Adjustable wicket gates used to regulate discharge and swirl.

Concept / Approach:
The guide apparatus provides two key controls: quantity control (opening changes flow area) and quality control (exit angle sets whirl component), thereby minimizing shock on entry and suppressing eddies.

Step-by-Step Solution:
Set proper incidence: vane exit angle matches runner inlet velocity triangle to avoid shock.Regulate discharge: gate opening sets the required flow rate.Shape flow: smooth passages reduce eddy formation and pressure losses.

Verification / Alternative check:
Runner performance maps show sharp efficiency penalties when incidence is off-design, highlighting the importance of guide vane angle control.

Why Other Options Are Wrong:

• Each single function is true, but the complete and correct choice is “all of the above”.

Common Pitfalls:
Assuming guide vanes only throttle flow; their primary role also includes direction control to match velocity triangles.

Final Answer:
all of the above