In the Brayton cycle, the heat addition process occurs at

• A. Constant pressure.
• B. Constant volume.
• C. Constant temperature.
• D. Variable pressure.

Answer: (A)

Heat addition in the Brayton cycle happens at essentially constant pressure. In the ideal Brayton model, air is first compressed, then fuel is burned in the combustor while the flow remains at the same pressure as it enters and leaves the combustor. This isobaric (constant-pressure) heating raises the gas temperature and enthalpy without a large change in pressure. The flame adds energy to the flowing gas, but the overall pressure is kept fixed by the upstream compressor and downstream turbine, so the state moves along a constant-pressure line on a P–H or T–s diagram. If the process were at constant volume, the added heat would cause a significant pressure rise, which isn’t observed in the actual combustor. If it were at constant temperature, the fuel would need to be removed or the heat transfer controlled in a way that isn’t how a typical combustor operates. Real systems do have small pressure losses, but they’re usually neglected in the ideal cycle analysis, so the heat addition is treated as constant-pressure.