How pressure increases in centrifugal compressor?

[Science20]

TL;DR Summary

Confusion about operation principle of centrifugal compressor

I have read this in my textbook about principles of operation of centrifugal compressor:

"The impeller is rotated at high speed by the turbine and air is continuously induced into the center of the impeller. Centrifugal action causes it to flow radially outwards along the vanes to the impeller tip, thus accelerating the air and also causing a rise in pressure to occur. The engine intake duct may contain vanes that provide an initial swirl to the air entering the compressor. The air, on leaving the impeller, passes into the diffuser section where the passages form divergent nozzles that convert most of the kinetic energy into pressure energy. In practice, it is usual to design the compressor so that about half of the pressure rise occurs in the impeller and half in the diffuser."

I thought that the pressure increase is only obtained in the diffuser part of the compressor as kinetic energy is converted to pressure, how compressor impeller also contributes to pressure increase?

 

[jrmichler]

The answer to your question is in the following figure from Chapter 3: Theory of the Centrifugal Pump Impeller in the book Centrifugal and Axial Flow Pumps by A.J. Stepanoff.

Please don't ask me to interpret it. I read the rest of the book and only skimmed the theory chapter. This figure is from a book about pumps, so is based on incompressible flow. Your question adds the additional factor of compressible flow. I expect that a better discussion would be in the following book by the same author, which I have not read: https://www.amazon.com/dp/B0007E6V9I/?tag=pfamazon01-20. Out of print, but should be in some engineering libraries.

Or an oversimplified answer would be that the impeller spins, and centrifugal force pushes the gas outward hard enough to compress it.

 

[jack action]

The impeller can also be a diffuser. Looking at the following velocity triangles, when you consider the relative velocities (i.e. ignoring the fact that the impeller rotates, just the flow through it) you can see that $v_{r2}$ can be set smaller than $v_{r1}$, meaning that while traveling through the impeller, the air pressure rises.

 

[Science20]

Thank you so much. The concept is now clear to me.