In an axial-flow compressor, what primarily determines the compressor ratio?

• A. Decrease in area of the succeeding stages
• B. Increase in area of the stages
• C. Number of rotors
• D. Ambient temperature

Answer: (A)

The main idea here is how the flow area through an axial-flow compressor controls the pressure rise. As air moves through each stage, the rotor adds energy and the stage is designed with a progressively smaller annulus area. This decrease in area forces the flow to accelerate and, with the energy input from the rotor, produces a larger increase in total pressure for each stage. Taken across all stages, the overall compressor ratio rises as the area is reduced from stage to stage.

If the area were increased instead of decreased, the same amount of work input would produce a smaller velocity change and a smaller pressure rise per stage, lowering the overall compressor ratio. The number of rotors (stages) does affect the total energy added, but the fundamental control of how much pressure is generated per stage comes from the geometry that reduces flow area. Ambient temperature influences density and performance but does not directly determine the compressor’s pressure ratio for a given design.