Air-cycle (reversed Brayton) refrigeration In a reversed Brayton (air) refrigeration cycle, heat is absorbed from the cold space during the:

Introduction / Context:
The reversed Brayton (also called reversed Joule) cycle is the basis of air-cycle refrigeration used in aircraft environmental control systems. Understanding which process provides the cooling effect is crucial.

Given Data / Assumptions:

• Cycle steps: isentropic compression → constant-pressure heat rejection → isentropic expansion → constant-pressure heat absorption.
• Working fluid: air, treated as an ideal gas.

Concept / Approach:
After the turbine, the air is at low temperature and low pressure. It then absorbs heat from the refrigerated space at essentially constant low pressure, producing the refrigeration effect before returning to the compressor inlet.

Step-by-Step Solution:
1–2: Isentropic compression raises T and p; no external heat absorbed.2–3: Constant-pressure cooling rejects heat to ambient.3–4: Isentropic expansion through the turbine drops T greatly.4–1: Constant-pressure heat absorption in the low-pressure heat exchanger → refrigeration effect.

Verification / Alternative check:
On a T–s diagram, the horizontal (constant-pressure) segment at low pressure corresponds to heat addition to the cycle, i.e., cooling the space.

Why Other Options Are Wrong:
(A) and (B) are on the high-pressure (hot) side; (C) is an internal work-producing expansion with negligible external heat transfer; (E) does not exist in the ideal air cycle.

Common Pitfalls:
Confusing the isentropic turbine expansion (which lowers temperature) with the actual heat absorption step that follows at low pressure.

Final Answer:
constant-pressure expansion (low-pressure heat absorption) process