Quantifying blade friction effect in impulse turbines By approximately what percentage does blade friction reduce the steam velocity as it passes over the moving blades of an impulse turbine?

Introduction / Context:
In impulse turbine rotors, ideally there is no pressure drop; the rotor simply turns the jet and extracts work. In practice, surface roughness and viscous effects cause friction, reducing the relative velocity across the blade passage and thus decreasing the work recovered and the diagram efficiency.

Given Data / Assumptions:

• Impulse-type moving blades with negligible pressure drop across the rotor in the ideal model.
• Friction present due to surface finish, incidence, and secondary flows.
• Typical well-designed stage, not severely eroded or fouled.

Concept / Approach:

Engineering practice often assumes a relative-velocity reduction factor across the rotor (sometimes denoted by k or a blade friction coefficient). For clean, well-designed blades, the relative velocity at exit is commonly taken as about 0.85 to 0.90 of the inlet relative velocity, implying a reduction on the order of 10 to 15 percent. This rule-of-thumb is used in preliminary design and performance calculations.

Step-by-Step Solution:

Verification / Alternative check:

Velocity-triangle based design examples and textbook problems frequently adopt 10–15% reduction, matching test data for smooth, modern impulse blades at design incidence.

Why Other Options Are Wrong:

Higher ranges (20–40%) imply unusually high losses inconsistent with good design and would severely depress diagram efficiency.Below 5% is overly optimistic and rarely achieved in practice.

Common Pitfalls:

Confusing relative velocity reduction (rotor passage) with absolute velocity changes across the stage; the former is the correct metric for blade friction in impulse rotors.

Final Answer:

10 to 15%