Where expansion provides useful output in a turbojet After heat addition in the combustor of a turbojet (Brayton cycle), in which component does the working fluid expand essentially isentropically to deliver shaft work to drive the compressor?

Introduction / Context:
The ideal Brayton cycle features isentropic compression, constant-pressure heat addition, and isentropic expansion. In a turbojet, the turbine extracts enough work to drive the compressor, while the nozzle converts remaining enthalpy into jet kinetic energy for thrust.

Given Data / Assumptions:

• Idealized processes with negligible losses (isentropic compressor and turbine in the ideal model).
• Combustion approximated as constant-pressure heat addition.
• Nozzle treated separately for thrust generation.

Concept / Approach:
Work transfer to the compressor comes from the turbine. The turbine expansion is modeled as isentropic in the ideal cycle. The exit nozzle is not constant-volume; it is an accelerating, near-isentropic flow passage that primarily produces thrust rather than shaft power.

Step-by-Step Solution:
Heat addition raises total temperature at essentially constant pressure.Turbine expands the hot gas isentropically, extracting shaft work for the compressor via the shaft.Remaining enthalpy is then converted in the nozzle into jet kinetic energy for propulsion.Thus, the component providing shaft work is the turbine, with essentially isentropic expansion.

Verification / Alternative check:
T-s diagrams of the Brayton cycle show the compressor-turbine pair as vertical (isentropic) lines in the ideal case, confirming the answer.

Why Other Options Are Wrong:
Exit nozzle produces thrust, not shaft work; there is no constant-volume process in turbine or nozzle in Brayton engines.

Common Pitfalls:
Confusing the roles of turbine (shaft work) and nozzle (thrust) in turbojets.

Final Answer:
Turbine blades, which is essentially an isentropic process