Axial-flow compressor design practice: typical degree of reaction In axial flow compressors used in gas turbines and process plants, designers generally aim for a balanced degree of reaction so that the static enthalpy rise is shared between rotor and stator. What value is the degree of reaction usually kept close to for most stages?

Introduction / Context:
The degree of reaction is fundamental in axial-flow compressor design. It indicates how the static enthalpy (or static pressure) rise is split between the rotating blades (rotor) and the stationary blades (stator). Choosing an appropriate value affects efficiency, blade loading, and stall margin.

Given Data / Assumptions:

• Axial-flow compressor with repeating stages.
• Conventional subsonic design intent for good efficiency and operability.
• Standard definition: degree of reaction R = static enthalpy rise in rotor / stage static enthalpy rise.

Concept / Approach:
A stage with R ≈ 0.5 shares the work nearly equally between rotor and stator. This symmetry yields well-proportioned velocity triangles, moderate diffusion factors, and a good compromise between efficiency and stall resistance. Many classical stage designs target R in the range 0.4–0.6, with 0.5 being the canonical textbook value.

Step-by-Step Solution:
Define R = (static rise in rotor) / (total stage static rise).For balanced velocity triangles and manageable blade loading, select R near 0.5.Such staging supports consistent flow turning and diffusion in both rotor and stator rows.Hence, the usual target is approximately 0.5.

Verification / Alternative check:
Stage loading and diffusion-factor limits commonly plotted against R show favorable regions around 0.5. Empirical compressor maps also reflect robust operation near this value.

Why Other Options Are Wrong:

• 0.2, 0.3: too rotor-light; places excessive rise in stator, often leading to high stator diffusion.
• 0.4: feasible but slightly below the standard balanced point.
• 0.65: workable for some high-loaded stages but not the usual general target.

Common Pitfalls:
Confusing degree of reaction with isentropic efficiency or with stage loading coefficient; they are related but distinct non-dimensional measures.

Final Answer:
0.5