The radial vanes are the main compression channels, where air gets squeezed into 5 - 10 bar and gets slung out of the tip traveling at 500 m/s.
The rotating guide vanes scoop the incoming air into the impeller vanes, kind of like a pre-whirl tower. They’re at the inside diameter and travel at much lower linear speed, subject to lower centrifugal force.
This picture was used in an earlier answer, and illustrates the different roles of the channels and vanes. As the pressure ratio on the horizontal axis increases, we can observe the following:
- Efficiency decreases: losses are higher for higher single stage pressure ratios, which is the main reason that commercial aircraft turbine engines use axial compressors.
- Impeller height increases. The curved shape allows for a more gradual change of direction of the airflow: inflow is in axial direction, outflow in radial direction. A large potential source for efficiency loss.
- Inside vane shape points more and more in radial direction: it scoops the airflow into the impeller, and pre-whirls into the radial direction that will be imposed by the impeller.
The right-most impeller combines an axial stage with a centrifugal stage. In an axial compressor the rotor is followed by a stator, in our axial-centrifugal combination there must be no stator because we want whirling flow now.
With advances in production technology, this has resulted in impeller shapes with double curved inlet vanes, such as above. The inner bits of the impeller are subject to much lower stresses and may be shaped much more funkily than the impeller outer diameter.