Which stage of the compressor does the centrifugal compressor represent in an APU?

The correct choice represents the second stage of the compressor in an Auxiliary Power Unit (APU) where a centrifugal compressor is typically employed.

In APUs, centrifugal compressors are often utilized after a primary stage, such as an axial or another centrifugal stage. The function of the centrifugal compressor is to increase the pressure of the air taken from the first stage and subsequently propel it into the combustion chamber. It achieves this through its design, which involves a rotating impeller that imparts kinetic energy to the air, converting that energy into increased pressure as the air moves through a diffuser.

Selecting the second stage denotes the sequential process in which the centrifugal compressor works to maximize efficiency. Thus, recognizing the placement of centrifugal compressors within the compressor stages helps in understanding their role in optimizing APU performance and contributing significantly to the engine's air delivery system.