Steam Turbine Classification – What Defines a Reaction Turbine? In a reaction steam turbine stage, how and where does the steam expand, and what distinguishes it from a pure impulse stage in terms of pressure drop and blade function?

Introduction / Context:
Steam turbines are broadly categorized as impulse or reaction. Understanding where the pressure drop occurs is fundamental to velocity triangles, stage efficiency, and blade design. This question asks you to identify the hallmark of a reaction stage.

Given Data / Assumptions:

• Single turbine stage with fixed and moving blade rows.
• Quasi-steady flow and negligible leakage in the ideal description.
• Steam properties change according to expansion through blade passages.

Concept / Approach:

In a pure impulse stage (e.g., De Laval), nearly all pressure drop occurs in stationary nozzles, converting pressure energy to kinetic energy; the moving blades ideally only turn the jet with negligible pressure change. In a reaction stage (e.g., Parsons), pressure drops occur in both the fixed and moving passages; both act as nozzles to some degree, and static enthalpy decreases across the rotor as well as the stator. The degree of reaction quantifies the fraction of enthalpy drop in the rotor.

Step-by-Step Solution:

Verification / Alternative check:

Velocity triangles for Parsons stages show changing absolute and relative velocities consistent with pressure reduction across the rotor. Measured pressure taps confirm a drop across both blade rows in reaction stages.

Why Other Options Are Wrong:

Options (a) and (c) describe impulse operation; (d) is false because pressure and temperature do not remain constant; (e) incorrectly asserts no rotor pressure change, which again matches impulse, not reaction.

Common Pitfalls:

Assuming all turbines are either 100% impulse or 100% reaction—most practical stages have a degree of reaction between 0 and 1.

Final Answer:

The expansion of steam occurs partly in fixed (stator) blades and partly in moving (rotor) blades