Flow direction in a centrifugal pump — entry vs exit Complete the statement: In a centrifugal pump, the water enters the impeller ________ and leaves the vanes axially (at design point).

Introduction:
Centrifugal pumps are rotodynamic machines that add energy to the fluid by imparting tangential velocity through rotating blades, converting this to pressure in the volute or diffuser. Understanding inlet and outlet flow directions is essential for interpreting velocity triangles and avoiding inlet pre-swirl problems.

Given Data / Assumptions:

• Proper inlet design with negligible pre-swirl (guide vanes may or may not be present depending on pump type).
• Design operation near the best-efficiency point.

Concept / Approach:
At the impeller eye, the fluid is guided to enter radially (relative velocity directed into blade passages with minimal whirl). The impeller then imparts tangential momentum, and through diffuser/volute action, flow is guided such that the absolute discharge direction can be arranged axially (for mixed/axial diffuser arrangements) or with controlled whirl depending on design. The provided statement uses the common teaching idealization: radial entry, axial leaving from vanes (in the absolute sense) at the design condition of zero whirl at outlet in certain pump designs with appropriate diffuser/guide geometry.

Step-by-Step Solution:

Verification / Alternative check:
Velocity triangles at design point often target zero whirl (Vw2 ≈ 0) for certain designs, leading to axial absolute exit in diffuser passages.

Why Other Options Are Wrong:

• Axially (at inlet): contradicts the standard radial-entry assumption for typical centrifugal impellers.
• Tangentially opposite to rotation: describes whirl components, not the ideal entry direction.
• Spiral path: qualitative and ambiguous.
• Pre-swirl same as rotation by default: undesirable unless specifically introduced; increases NPSH requirement.

Common Pitfalls:
Confusing relative, absolute, and blade angles; assuming the same direction at all operating points; ignoring inlet pre-swirl control.

Final Answer: