Impeller shroud frequency tuning rib

A centrifugal compressor comprising:

• an impeller rotatable about a central axis, the impeller having blades extending from a hub to blade tips between an inlet and an outlet;
• and
• a shroud annularly extending around the blade tips of the impeller and extending in a streamwise direction between an inducer end at the inlet of the impeller and an exducer end at the outlet of the impeller, the shroud having a gaspath surface facing the impeller and a back surface opposed to the gaspath surface, the back surface having a tuning rib extending therefrom at either or both the inducer end and the exducer end of the shroud, the tuning rib configured to alter a natural frequency of the shroud so as to avoid coincidence with aerodynamic excitation frequencies to which the shroud is configured to be exposed to during use;
• wherein the tuning rib is provided at the exducer end of the shroud, wherein the exducer end has a wall thickness A, wherein the tuning rib has a length B in an axial direction and a height C in a radial direction relative to the central axis, and wherein:
• 0.1·A≤B≤3·A
• 0.1·B≤C≤3·B.