Definition check – diagram (blading) efficiency in steam turbines Which ratio correctly defines the diagram efficiency associated with turbine blades for a given stage?

Introduction / Context:
Diagram efficiency (also called blading efficiency) quantifies how effectively the kinetic energy presented to a set of blades is converted into useful work on the rotor. It focuses on the velocity triangles at blade entry and exit, separating blade performance from other losses (nozzle, mechanical, and leakage losses).

Given Data / Assumptions:

• Consider one turbine stage with known inlet and outlet velocity triangles.
• Energy supplied to the blades refers to the available jet kinetic energy at blade entry.
• Workdone on the blades is the specific work extracted from the jet by the rotor (per kg steam).

Concept / Approach:

Diagram efficiency η_d is defined as the ratio of specific work obtained from the blades to the specific kinetic energy supplied to the blades. In symbols: η_d = (work on blades per kg) / (energy supplied to blades per kg). This is distinct from stage efficiency (which uses isentropic heat drop for the whole stage) and from overall plant efficiency (shaft power over boiler heat input).

Step-by-Step Solution:

Verification / Alternative check:

From velocity triangles, maximizing the exit whirl reduction (Vw2) for a given inlet whirl (Vw1) increases W_blade and thus η_d, consistent with the definition.

Why Other Options Are Wrong:

Option (b) mixes blade and stage quantities; stage energy includes nozzle and other drops.Option (c) inverts the ratio; η must be less than or equal to 1.Option (e) refers to overall thermal efficiency, not diagram efficiency.

Common Pitfalls:

Confusing diagram efficiency with stage or nozzle efficiencies; always check the numerator and denominator definitions.

Final Answer:

workdone on the blades to the energy supplied to the blades