Reaction turbines – interpreting degree of reaction When the degree of reaction (R) in a reaction steam turbine stage is zero, what does this imply about where the enthalpy (heat) drop occurs?

Introduction / Context:
The degree of reaction is a key metric that distinguishes impulse from reaction behavior in turbine stages. It partitions the total stage enthalpy drop between fixed (stator) and moving (rotor) blades. Understanding extreme values of this ratio helps identify limiting cases such as pure impulse staging.

Given Data / Assumptions:

• Degree of reaction R is defined on a single stage.
• Total stage heat (enthalpy) drop = drop in fixed blades + drop in moving blades.
• Steady, adiabatic flow is assumed for the ideal discussion.

Concept / Approach:
The degree of reaction is R = (enthalpy drop in moving blades) / (total enthalpy drop in the stage). When R = 0, the numerator is zero, meaning the rotor contributes no enthalpy drop. All the pressure/enthalpy drop then occurs in the fixed nozzles; the moving blades merely deflect the jet without further expansion. This is the textbook definition of a pure impulse stage (De Laval type).

Step-by-Step Solution:
Write R = Δh_rotor / (Δh_stator + Δh_rotor).Set R = 0 → Δh_rotor = 0.Therefore, the entire stage enthalpy drop occurs in the fixed (stator) blades.Conclusion: there is no heat (enthalpy) drop in the moving blades.

Verification / Alternative check:
Velocity triangles for a pure impulse stage show large nozzle exit velocity and predominantly constant static pressure through the rotor. Any kinetic-to-work conversion happens via momentum change, not further pressure drop in the moving row.

Why Other Options Are Wrong:

• No heat drop in fixed blades: opposite of impulse behavior.
• Maximum heat drop in moving blades: corresponds to high reaction, not zero.
• Maximum heat drop in fixed blades: while true qualitatively, the precise implication of R = 0 is “no drop in moving blades,” which is the most accurate statement among the options.

Common Pitfalls:
Confusing momentum exchange (which still produces rotor work) with enthalpy drop. Zero reaction does not mean zero power; it means zero rotor enthalpy drop.

Final Answer:
no heat drop in moving blades