Jet Propulsion/Compressors

Axial Compressors
In axial compressors the fluid flows mainly parallel to the rotation axis. They are widely used in gas turbines. Axial flow compressors have large mass flow capacity and high efficiencies, but have a smaller pressure rise per stage than centrifugal compressors. Most commercial jet engines used in transport employ axial compressors. Smaller engines such as those used in helicopters use centrifugal compressors which provide larger pressure ratios.

Axial compressors can achieve pressure ratios of about 2 per stage, limited by the blade velocity and diffusion capability. With aspirated blades providing better boundary layer control much higher pressure ratios can be achieved.

Axial compressors have higher mass flow per frontal area, and this is a major advantage for jet aircraft.

Centrifugal Compressors
Although the inlet flow to a centrifugal compressor is more or less axial, the rotor exit flow is in a radial direction. Downstream of the rotor, a diffuser slows the flow down to recover static pressure. The diffuser also normally restores the flow to a more axial direction , ready for the next process (e.g. another centrifugal compressor or reverse flow combustor).

Although high stage pressure ratios (more than 10:1) are achievable, rotor stress considerations significantly limit the stage pressure ratio that can be used when the overall pressure ratio is high (e.g 30:1).

Nowadays, centrifugal compressors are less commonly used in jet aircraft. Their prime applications are in small turbojets/turbofans and turboprop/turboshaft engines, where the simplicity, compactness, lower weight and lower cost are needed.

In small size axial compressor, the aerofoils on the rear stages are not very rugged, because they are too thin (a bit like razor blades). Consequently a centrifugal compressor is often used as the rear stage on a small compressor. Common configurations are axial-centrifugal or, at really small sizes, double centrifugal (i.e. two centrifugal compressors in series). At small sizes, centrifugal compressors are more efficient than the equivalent axial units.

Their very design limits their use. They are only used in small engines because of the fact that they're made of a relatively heavy piece of metal that spins at very high speed (30'000 RPM and more). That creates an extreme centrifugal force. Today's materials cannot endure the force that would be generated by a compressor of more than three to five feet in diameter.

One disadvantage of an centrifugal compressor is the relatively large overall diameter across the diffuser unit, which can pose installation problems.

Diagonal or Mixed Flow Compressors
As the name might suggest, a diagonal (or mixed) flow compressor is a cross between an axial and centrifugal compressor, where the flow takes a diagonal route through the compressor, exiting diagonally (rather than radially) at a significantly higher higher mean radius than at inlet. The compression process is part axial and part centrifugal.

So far, only a few examples of these units have been tested worldwide. They do, however, offer the advantage of a relatively high pressure ratio per stage, without the need for a relatively large diameter exit diffuser, which can cause installation problems.