Nature of expansion in reaction turbines As steam flows over the blades of a reaction turbine stage, which ideal thermodynamic process best represents the expansion within the blade passages?

Introduction / Context:
When analyzing steam turbine stages, we often idealize the flow through fixed and moving blades as adiabatic and reversible to establish limits and to construct velocity triangles. This makes it crucial to know which thermodynamic process serves as the ideal reference for the expansion through blades.

Given Data / Assumptions:

• Reaction turbine stage with pressure drop across both stator and rotor.
• Idealized analysis (no heat transfer, no friction), i.e., adiabatic and reversible flow.
• Steam properties referenced on an absolute temperature scale.

Concept / Approach:

The ideal flow through a nozzle or an expanding blade passage is taken as isentropic. Isentropic means adiabatic and reversible, resulting in constant entropy while static enthalpy drops and kinetic energy rises. Real blades incur losses (non-isentropic), but the isentropic model provides the benchmark for efficiency calculations (nozzle efficiency, stage efficiency, and turbine internal efficiency).

Step-by-Step Solution:

Verification / Alternative check:

Nozzle and blade efficiency definitions compare actual enthalpy drop with the isentropic enthalpy drop between the same inlet and outlet pressures. This confirms the isentropic process is the intended idealization.

Why Other Options Are Wrong:

Isothermal: would require heat transfer to maintain constant temperature, not typical in fast turbine passages.Throttling: involves pressure drop at essentially constant enthalpy (not expansion work producing kinetic energy).Free expansion: irreversible, with no useful work and undefined path for turbine modeling.

Common Pitfalls:

Assuming “adiabatic” automatically means “isentropic.” Adiabatic plus reversible is required; in practice, friction makes real expansion non-isentropic, but the isentropic model remains the standard reference.

Final Answer:

isentropic process