Reaction turbine rotor behavior – pressure and velocity trends As steam flows through the moving (rotor) blades of a reaction turbine stage, how do pressure and velocity change?

Introduction / Context:Unlike a pure impulse stage, a reaction stage experiences expansion (pressure drop) in both stator and rotor passages. The rotor thus acts as a nozzle, accelerating the fluid while extracting work through momentum change.

Given Data / Assumptions:

• Finite degree of reaction R > 0.
• Well-designed blade passages with smooth area variation.
• Steady, adiabatic flow for conceptual analysis.

Concept / Approach:In the moving blades of a reaction stage, part of the enthalpy drop occurs, resulting in a fall in static pressure and a corresponding increase in relative (and typically absolute) velocity. This converts pressure energy into kinetic energy within the rotor, contributing to torque via change in whirl velocity.

Step-by-Step Solution:Define reaction: portion of total stage enthalpy drop in rotor.Pressure drop in rotor → fluid accelerates through blade passages.Hence, pressure decreases while velocity increases through the moving blades.

Verification / Alternative check:Velocity triangles for 50% reaction (Parsons) show acceleration in both stator and rotor; measured static taps confirm pressure reduction across both rows.

Why Other Options Are Wrong:

• (a) Reverse of nozzle-like behavior in rotor.
• (c) and (d) contradict energy conversion trends.
• (e) Constant pressure would imply impulse behavior, not reaction.

Common Pitfalls:Assuming the rotor only turns the jet; that is valid for ideal impulse stages, not for reaction stages.

Final Answer:pressure decreases while velocity increases