Terminology – what is “stage efficiency” also called? In turbine performance terminology, the stage efficiency is also commonly referred to as:

Introduction / Context:
Turbine efficiencies come in several flavors: nozzle efficiency, diagram (blading or stage) efficiency, mechanical efficiency, and overall/isentropic efficiencies. Consistent terminology avoids confusion when comparing designs or solving examination problems.

Given Data / Assumptions:

• Single stage under consideration.
• Energy quantities are per unit mass of working fluid.
• Loss accounting distinguishes fluid-dynamic from mechanical losses.

Concept / Approach:
Stage efficiency—especially in many textbooks and exam syllabi—is often synonymous with diagram efficiency. It measures how effectively the stage converts the available kinetic/enthalpy change into rotor work (before mechanical losses). Nozzle efficiency is specific to conversion inside stationary nozzles. Gross or overall efficiency includes additional plant-level factors.

Step-by-Step Solution:
Identify what “stage efficiency” evaluates → blade-row energy conversion performance.Match the common synonym → “diagram efficiency.”Hence, option (a) is correct.

Verification / Alternative check:
Reference definitions: η_diagram = (work on blades per kg) / (kinetic energy at rotor inlet per kg) for impulse; equivalent stage forms exist for reaction stages, collectively termed diagram/stage efficiency.

Why Other Options Are Wrong:

• Nozzle efficiency refers only to stator performance.
• Gross efficiency is ill-defined at the stage level and may imply plant-wide or multi-stage performance.
• Mechanical efficiency accounts for bearings, seals, windage, and gearing, not fluid-dynamic stage conversion.

Common Pitfalls:
Interchanging mechanical and diagram efficiencies; they quantify different loss categories.

Final Answer:
diagram efficiency