Axial discharge condition For a turbine designed for axial discharge at the stage exit, what is the value of the whirl (tangential) component of absolute velocity at outlet?

Introduction / Context:
Velocity triangles in turbines decompose absolute velocity into axial (flow), whirl (tangential), and sometimes radial components. Power transfer depends on the change in whirl component across the rotor. Some stages are designed for axial discharge to simplify downstream flow and reduce exit swirl losses.

Given Data / Assumptions:

• Axial discharge means the absolute exit velocity is aligned with the turbine axis.
• Steady flow and well-designed outlet guide geometry.
• No significant swirl-producing leakage or secondary flows at design point.

Concept / Approach:

If the absolute exit velocity vector is purely axial, its tangential (whirl) component is zero. This choice reduces kinetic energy loss in the exhaust (swirl loss) and simplifies matching to the next stage or diffuser. In the Euler turbine equation, the work per unit mass is proportional to the difference in whirl components (Vw1 − Vw2); setting Vw2 = 0 makes the work equal to Vw1 * u, a common design target for impulse stages.

Step-by-Step Solution:

Verification / Alternative check:

Measured exit swirl angles near zero in well-tuned stages confirm negligible whirl; residual small swirl may exist off-design but the design intent is Vw2 ≈ 0.

Why Other Options Are Wrong:

Maximum/minimum (non-zero) contradict the definition of axial discharge.Equal to blade speed is unrelated; blade speed is a rotor property, not a flow component at exit.

Common Pitfalls:

Confusing relative velocity (in the blade frame) with absolute exit velocity; axial discharge refers to the absolute velocity direction.

Final Answer:

zero