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Diffusion in the vane island passages of a centrifugal compressor diffuser is in part controlled by contouring the diffuser passage wall with low profile surface variations. The surface variations can be provided in the form of flow boundary disrupting protrusions disposed within the downstream port

Diffusion in the vane island passages of a centrifugal compressor diffuser is in part controlled by contouring the diffuser passage wall with low profile surface variations. The surface variations can be provided in the form of flow boundary disrupting protrusions disposed within the downstream portion of the vane island passages to prevent flow separation.

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1. A centrifugal compressor vane diffuser for receiving high velocity air from an impeller mounted for rotation about an axis of a gas turbine engine compressor, the diffuser comprising front and back walls defining an axial gap therebetween, a circumferential array of vane islands extending from th

1. A centrifugal compressor vane diffuser for receiving high velocity air from an impeller mounted for rotation about an axis of a gas turbine engine compressor, the diffuser comprising front and back walls defining an axial gap therebetween, a circumferential array of vane islands extending from the front wall to the back wall to define therewith a plurality of vane island passages, the vane islands having leading edges located on an inner circumference and trailing edges located on an outer circumference, the inner and outer circumferences being centered relative to the axis of rotation of the impeller, and a series of low profile flow boundary disrupting protrusions circumferentially staggered relative to said circumferential array of vane islands and disposed in said vane island passages, the low profile flow boundary disrupting protrusions projecting a short distance from one of said front and back walls to a flow boundary region of the vane island passages, each of the flow boundary disrupting protrusions having a chord length extending between a leading edge and a trailing edge, the chord length of the flow boundary disrupting protrusions being smaller than that of the vane islands, the flow boundary disrupting protrusions being contained between said inner and outer circumferences, and the leading edges of the flow boundary disrupting protrusions being located radially outward from said inner circumference. 2. The centrifugal compressor vane diffuser as defined in claim 1, wherein the trailing edges of the flow boundary disrupting protrusions are located radially inwardly of the outer circumference on which the trailing edges of the vane islands are disposed. 3. The centrifugal compressor vane diffuser as defined in claim 1, wherein the flow boundary disrupting protrusions are disposed in the downstream half portion of the vane island passages in the chordwise direction relative to a direction of flow of the air through the vane island passages. 4. The centrifugal compressor vane diffuser as defined in claim 3, wherein the chord length of the flow boundary disrupting protrusions is about 30% to about 50% of the chord length of the vane islands. 5. The centrifugal compressor vane diffuser as defined in claim 1, wherein the height of the flow boundary disrupting protrusions is about 1/10 to about ⅛ of the height of the vane islands. 6. The centrifugal compressor vane diffuser as defined in claim 1, wherein the flow boundary disrupting protrusions are provided in the form of wall surface deformations. 7. The centrifugal compressor vane diffuser as defined in claim 1, wherein each of the flow boundary disrupting protrusions is provided in the form of an elongated race track shape corrugation defined in said one of said front and back walls. 8. The centrifugal compressor vane diffuser as defined in claim 7, wherein each of the corrugations has a rounded cross-sectional shape. 9. The centrifugal compressor vane diffuser as defined in claim 1, wherein said protrusion have a chordwise curvature which generally corresponds to that of the vane islands. 10. A gas turbine engine centrifugal compressor comprising an impeller mounted for rotation about an axis and a vane diffuser disposed around an outer periphery of the impeller to decrease the velocity and increase the static pressure of the air from the impeller, the vane diffuser having a pair of axially spaced-apart flow boundary surfaces defining an axial gap therebetween, a circumferential array of vane islands spanning said axial gap between the axially spaced-apart flow boundary surfaces and defining therewith a plurality of vane island passages, and a circumferential array of low profile protrusions circumferentially staggered relative to said circumferential array of vanes islands, the circumferential array of low profile protrusions being contained in a downstream portion of said vane island passages relative to a flow direction of the air through the diffuser, the low profile protrusions forming geometrical surface variations at one of said flow boundary surfaces. 11. The centrifugal compressor defined in claim 10, wherein the low profile protrusions have a height corresponding to not more than about ⅛ of the height of the vane islands. 12. The centrifugal compressor defined in claim 10, wherein the low profile protrusions have a height which is comprised between about 1/10 to about ⅛ of the height of the vane islands. 13. The centrifugal compressor defined in claim 10, wherein said low profile protrusions are provided in the forms of localized wall deformations in said one of said flow boundary surfaces. 14. The centrifugal compressor defined in claim 10, wherein said low profile protrusions have a rounded cross-sectional shape. 15. The centrifugal compressor defined in claim 10, wherein said low profile protrusions have an elongated shape with a substantially constant thickness along a major portion of a length thereof. 16. The centrifugal compressor defined in claim 10, wherein said low profile protrusions have a chord length which ranges from about 30% to about 50% of that of the vane islands. 17. The centrifugal compressor wherein said low profile protrusions are localized indentations of said one of said flow boundary surfaces into the vane island passages. 18. A centrifugal compressor vane diffuser surrounding an impeller mounted for rotation about an axis of a gas turbine engine compressor, the diffuser comprising confronting front and back walls defining an axial gap therebetween, a circumferential array of vane islands extending from the front wall to the back wall to divide the axial gap into a plurality of vane island passages, the vane islands having leading edges located on an inner circumference and trailing edges located on an outer circumference, the inner and outer circumferences being centered relative to the axis of rotation of the impeller, wherein each of said vane island passages has a flow boundary surface area extending between adjacent vane islands on one of said front and back walls, said flow boundary surface area having an uneven surface profile configured to locally increase a velocity of a flow boundary layer in a downstream portion of each of the island vane passages. 19. A centrifugal compressor vane diffuser as defined in claim 18, wherein the uneven surface profile comprises a surface deformation in the form of a low profile protrusion extending between each pair of adjacent vane islands in the back wall of the diffuser. 20. A centrifugal compressor vane diffuser as defined in claim 18, wherein the uneven surface profile are provided in the form of elongated rounded indentations in one of said front and back walls.