Why is a centrifugal compressor in aircraft engine less susceptible to stall and surge?

Question

I have came across a similar question here. Although the answer doesn't explain much about why and is somewhat vague. I have read that centrifugal compressor can achieve a higher pressure rise per stage than axial compressor. So technically they are less susceptible to stall and surge.

How is a stall and surge occurring in a centrifugal compressor different from that occurring in axial compressor. What are the scenarios under which a centrifugal compressor stall and/or surge (Is it same as those scenarios in axial compressor. If so I do have a rough idea)? And where does stall/surge first occur in a centrifugal compressor (impeller or diffuser)?

Is it because the rotor blades of centrifugal compressors doesn't act/function like airfoils?

Answer 1

The reason comes down to the principle of operation of the compressors - a centrifugal compressor, as it's name implies, just spins around really fast, and the centrifugal force compresses the air. As you can see in the image below, the vanes of the impeller of a centrifugal compressor just help push the air around in a circle (unlike an airfoil) and centrifugal force does the compressing: The centrifugal compressor is circled in blue. As you can see, when the impeller rotates, it pushes air to the side.

Image source - https://upload.wikimedia.org/wikipedia/commons/6/6f/Saturn_MD-120_cutaway.jpg

An axial compressor has blades that work like little rotors (which are really just rotating wings) - the rotors accelerate the air, and the stators slow the air down again, using that kinetic energy to compress the gas. Just like wings, these blades can stall and lose function whenever the input flow is disturbed enough (such that the local angle of attack of a portion of the blade is too high for the given local flow conditions). This stall can then potentially spread to other blades, or to the entire engine.

As you said, the vanes of an impeller in a centrifugal compressor don't work like airfoils (instead, they kinda just rotate and push air around, kinda like a turnstile), so if they encounter airflow disturbances, they won't stall like an airfoil would. Of course, if said airflow disturbance is something like severely restricting airflow, the compressor will surge, but this is considered rare in practice.

Therefore, it's much easier to get an axial compressor to surge or stall (just by disturbing the airflow a lot) in comparison to a centrifugal compressor, which requires airflow to be restricted.

Answer 2

For a compression system operating in normal condition, a mass-flow reduction leads to an increase of pressure rise. By reducing continuously the mass-flow, you reach the point of maximum pressure ratio after which the compressor operates differently. This could bring aerodynamic stall in flow and degrade performance.

• Rotating stall (often called stall) is a stable operating mechanism which allows compressor to adapt to a very small mass-flow and affects overall performance. Some stalled zones called 'cells' appear which have a very little flow (low velocity). It's possible to compare this with stall on airfoils but the blockage caused by cells will affect incidence of adjacent blades (or downstream rows). Note that stall can (quite often actually) precede surge but not always.

• Surge is an instable process where the annulus average mass-flow varies with time and cyclically. The compressor pass from an un-stalled phase to a stalled one and back again. That flow instability is not only related to the compressor alone but on the whole system where it operates. It is common to modelling this with a throttle valve (see picture bellow).

In nominal operating zone, compressor is unconditionally stable since compressor and throttle lines have « opposite » behaviour. A small reduction in mass flow leads to increase the pressure rise through compressor and decrease pressure drop in throttle. Therefore, the flow will accelerate until an equilibrium.

In unstable region, the throttle do not compensate changes in pressure rise due to mass flow unsteady.

To answer your question, mechanisms presented before are valid for any compression system.

Indeed, centrifugal compressors sometimes operate with stall: near inducer tips or over the impeller due to downstream asymmetry. But they could remain stable even with stall present, and surge is less likely to occur.

The reason for this is that a large part of the compression is done by centrifugal effects with or without rotating stall. The compressor line in the stalled region (on previous diagram) is probably flatter (and so more stable) for a centrifugal.

Work done by a compressor can be estimated by Euler theorem (considering inlet/outlet moment of momentum):

This implies two ways to make work through a compressor: deviate flow (V theta) and/or enlarge section (radius r). The last one is obviously related to centrifugal forces.

If stalls occur, the deviation (V theta) will be affected, at least locally (since rotating stall is a 3D non-axisymmetric phenomena). For axial compressor, most of the pressure rise is done by deviation. This is not as significant for centrifugal compressor.