Air refrigeration cycles: An air refrigerator (as used in aircraft) operates on which idealized cycle?

Introduction / Context:Aircraft air-cycle machines and simple air refrigerators use the working fluid (air) directly. Their thermodynamic model is the Bell–Coleman or reversed Brayton cycle, not the vapour-compression cycle used with refrigerants like ammonia or R-134a.

Given Data / Assumptions:

• Working fluid is air (treated as an ideal gas).
• Processes are ideally isentropic compression/expansion with heat exchange at constant pressure.
• Engine bleed air or ram air often serves as the heat source/sink.

Concept / Approach:The Bell–Coleman cycle consists of two isentropic processes (compression and expansion in a turbine) and two constant-pressure heat transfer processes (cooling after compression and heating after expansion), producing refrigeration by expansion work.

Step-by-Step Solution:

Verification / Alternative check:Temperature–entropy diagrams and p–h surrogates for air show the characteristic Brayton rectangles/curves, not the dome typical of vapour systems.

Why Other Options Are Wrong:

• Reversed Carnot is the theoretical upper limit, but not the practical air-cycle model.
• “Both” is incorrect because the standard air refrigerator is specifically the reversed Brayton.
• Reversed Rankine applies to vapour-compression, not air-cycle machines.

Common Pitfalls:Confusing “reversed” with “Carnot” generically; always map processes to the actual hardware (compressor, cooler, turbine, heat exchanger).

Final Answer:Bell–Coleman (reversed Brayton) cycle