Centrifugal compressor with inlet duct having swirl generators

1. A centrifugal compressor comprising:

an inlet; and

an impeller comprising a hub and blades;

the inlet for conveying flow to the impeller comprising guide means arranged to guide the flow through the inlet to induce swirl in the flow adjacent tips of the blades, the direction of swirl being the direction of rotation of the blades, without substantially disturbing flow adjacent the hub;

wherein the flow is incident on the guide means and the guide means is arranged to guide the flow by providing at least one surface that is substantially oblique to the flow incident on the guide means, wherein each of the at least one surfaces is embodied by a passageway, wherein each passageway is a hole.

2. The centrifugal compressor of claim 1 , wherein the guide means comprises structure that deflects the flow.

3. The centrifugal compressor of claim 1 , wherein the at least one surface is substantially oblique to the axis of the inlet.

4. The centrifugal compressor of claim 1 , wherein each passageway is formed in the structure of the inlet.

5. The centrifugal compressor of claim 1 , wherein each passageway is substantially in a plane tangential to the structure of the inlet and parallel to the axis of the inlet.

6. The centrifugal compressor of claim 5 , wherein each passageway is substantially at the same angle in its respective plane to the axis of the inlet as each other passageway.

7. The centrifugal compressor of claim 1 , wherein the inlet comprises a restriction of smaller inner diameter than the inner diameter of the remainder of the inlet.

8. The centrifugal compressor of claim 7 , wherein the restriction comprises a face substantially perpendicular to the axis of the inlet.

9. The centrifugal compressor of claim 8 , wherein the restriction is shaped to comprise a surface which slopes from the perpendicular face to the wall of the inlet.

10. The centrifugal compressor of claim 7 , wherein each passageway is located in the restriction.

11. The centrifugal compressor of claim 10 , wherein the passageways are holes and pass through the restriction.

12. The centrifugal compressor of claim 10 , wherein there is a plurality of passageways, each angularly distributed in the restriction around, and with respect to, the axis.

13. A turbocharger comprising a centrifugal compressor according to claim 1 .

14. An inlet for conveying flow to an impeller of a centrifugal compressor, the inlet comprising guide means arranged to guide the flow through the inlet to induce swirl in the flow adjacent tips of blades of the impeller, the direction of swirl being the direction of rotation of the blades, without substantially disturbing the flow adjacent the hub of the impeller;

wherein the flow is incident on the guide means and the guide means is arranged to guide the flow by providing at least one surface that is substantially oblique to the flow incident on the guide means, wherein each of the at least one surfaces is embodied by a passageway, wherein each passageway is a hole.

15. Guide means for fitting to an inlet of a centrifugal compressor, the guide means arranged to guide the flow through the inlet to induce swirl in the flow adjacent tips of blades of an impeller of the centrifugal compressor, the direction of swirl being the direction of rotation of the blades, without substantially disturbing the flow adjacent the hub of the impeller;

wherein the flow is incident on the guide means and the guide means is arranged to guide the flow by providing at least one surface that is substantially oblique to the flow incident on the guide means, wherein each of the at least one surfaces is embodied by a passageway, wherein each passageway is a hole.