Francis turbine classification — inward or outward flow? State whether a Francis turbine is an outward flow reaction turbine or not. Choose the correct classification for standard Francis machines used in hydropower.

Introduction:
Turbines are categorized by how water flows through the runner (radial, axial, mixed) and the type of energy conversion (impulse vs reaction). The Francis turbine is a widely used reaction machine for medium heads and is frequently misdescribed regarding the direction of flow.

Given Data / Assumptions:

• Conventional Francis design with spiral casing and guide vanes.
• Runner passages shaped for combined pressure and velocity changes (reaction action).

Concept / Approach:
Francis turbines are predominantly inward radial flow at entry, with the flow turning and leaving in an axial direction—thus often described as a mixed-flow reaction turbine. They are not outward radial flow machines. Inward flow reduces runner diameter toward the exit and suits the conversion of pressure head to mechanical work across the blades.

Step-by-Step Solution:

Verification / Alternative check:
Velocity triangles at inlet show significant whirl and radial components pointing inward; at exit, the design often targets reduced whirl and axial discharge into the draft tube.

Why Other Options Are Wrong:

• “True” (outward flow): contradicts the standard Francis configuration.
• “Axial flow”: describes Kaplan/propeller class, not Francis.
• “Tangential impulse”: describes Pelton-type impulse turbines.

Common Pitfalls:
Assuming “radial flow” means outward; confusing the inlet/outlet directions; overlooking that many Francis runners are explicitly described as mixed-flow.

Final Answer: