Impulse Turbine – Exit-Whirl Condition for Maximum Efficiency State whether, for maximum blade efficiency in a pure impulse turbine stage, the steam should leave the moving blades with a velocity component at right angles to blade motion (i.e., zero whirl at exit).

Introduction / Context:
Blade or diagram efficiency in an impulse stage depends on how effectively the rotor converts jet kinetic energy to shaft work. The tangential (whirl) components of velocity at rotor inlet and exit determine the torque and thus the work per unit mass. The question tests the condition for maximizing this efficiency in a pure impulse stage.

Given Data / Assumptions:

• Pure impulse stage: nearly all pressure drop in stator nozzles; negligible pressure change across rotor.
• Friction is present but small for conceptual analysis; optimal inlet angle and speed ratio are considered.
• Blade speed and nozzle angle are adjusted for best efficiency.

Concept / Approach:

Shaft work per unit mass in a turbine stage is proportional to the change in whirl velocity: w = U*(Vw1 − Vw2), where U is blade speed, Vw1 is inlet whirl, and Vw2 is exit whirl. For a given inlet condition and blade speed ratio, maximizing w (and diagram efficiency) requires minimizing the magnitude of exit whirl. The ideal target is Vw2 = 0, meaning the absolute exit velocity is orthogonal to the wheel motion (purely axial for an axial-flow stage). This condition eliminates kinetic energy associated with tangential swirl in the exhaust, thus minimizing losses.

Step-by-Step Solution:

Verification / Alternative check:

Classical velocity-triangle analysis and diagram-efficiency derivations show the maximum occurs near zero exit whirl when blade-friction losses are modest and incidence is optimized.

Why Other Options Are Wrong:

Reaction stages intentionally retain pressure drop in the rotor; the stated condition is specific to impulse optimization and is not restricted to partial admission or reheat considerations.

Common Pitfalls:

Confusing axial exit (Vw2 = 0) with zero absolute velocity; the magnitude of exit velocity is not zero—it is oriented axially to avoid tangential kinetic-energy carryover.

Final Answer:

Correct