Axial-flow reaction turbine — flow direction relative to shaft Complete the statement: In an axial flow reaction turbine, the water flows ________ to the axis of the wheel (runner).

Introduction:
Turbine classification by flow direction guides selection for head and discharge. Axial-flow turbines, such as Kaplan and propeller types, are preferred for low heads and high discharges, where the fluid ideally moves along the shaft axis through the runner passages.

Given Data / Assumptions:

• Reaction turbine with guide vanes and adjustable or fixed-pitch runner blades.
• Design near best-efficiency point with minimal swirl at exit.

Concept / Approach:
In axial-flow turbines, the absolute velocity of water is substantially along the axis at both inlet and outlet (with small swirl components adjusted by blade pitch). This reduces secondary losses and allows a compact draft tube to recover kinetic energy efficiently at low heads.

Step-by-Step Solution:

Verification / Alternative check:
Velocity triangles for Kaplan turbines show small whirl components targeted so that discharge is nearly axial into the draft tube, maximizing pressure recovery.

Why Other Options Are Wrong:

• Perpendicular: describes radial flow, not axial.
• Radially inward then axial: describes Francis (mixed flow).
• Tangential: impulse action along periphery (Pelton).
• Spiral with large pre-swirl: increases losses; not a defining feature.

Common Pitfalls:
Confusing axial with radial-mixed configurations; ignoring that small swirl may still exist but the dominant direction is axial.

Final Answer: