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A centrifugal compressor for compressing a fluid comprises a compressor wheel having a plurality of circumferentially spaced blades, and a compressor housing in which the compressor wheel is mounted. The compressor housing includes an inlet duct through which the fluid enters in an axial direction a

A centrifugal compressor for compressing a fluid comprises a compressor wheel having a plurality of circumferentially spaced blades, and a compressor housing in which the compressor wheel is mounted. The compressor housing includes an inlet duct through which the fluid enters in an axial direction and is led by the inlet duct into the compressor wheel, and a wheel shroud located radially adjacent the tips of the blades. A bleed port is defined in the wheel shroud at a location intermediate the leading and trailing edges of the blades for bleeding off a bleed portion of the fluid being compressed by the compressor wheel. A converging injection nozzle is defined in the duct wall upstream of the leading edges of the blades for injecting the bleed portion of the fluid back into the main fluid flow stream approaching the compressor wheel.

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What is claimed is: 1. A centrifugal compressor for compressing a fluid, comprising: a centrifugal compressor wheel having a hub defining a rotational axis and having a plurality of circumferentially spaced blades each having a root joined to the hub and extending generally radially outwardly to a

What is claimed is: 1. A centrifugal compressor for compressing a fluid, comprising: a centrifugal compressor wheel having a hub defining a rotational axis and having a plurality of circumferentially spaced blades each having a root joined to the hub and extending generally radially outwardly to a tip of the blade, each of the blades having a leading edge and a trailing edge spaced downstream from the leading edge along a flow direction; a compressor housing in which the compressor wheel is mounted so as to be rotatable about the rotational axis of the compressor wheel, the compressor housing including an inlet duct through which the fluid enters in a direction generally parallel to the rotational axis of the compressor wheel and is led by the inlet duct into the compressor wheel, the fluid flowing along a main flowpath defined by the compressor wheel and compressor housing and exiting the compressor wheel in a generally radially outward direction, the inlet duct comprising a duct wall that encircles the rotational axis, the compressor housing further including a wheel shroud located radially adjacent the tips of the blades; a bleed port defined in the wheel shroud at a location intermediate the leading and trailing edges of the blades for bleeding off a bleed portion of the fluid being compressed by the compressor wheel, the bleed port being sized in flow area, in relation to a flow area of the main flowpath, such that at a predetermined operating condition the bleed portion constitutes from about 15% to about 25% of the total mass flow entering the compressor; and a converging injection nozzle defined in the duct wall upstream of the leading edges of the blades for injecting the bleed portion of the fluid back into the main fluid flow stream approaching the compressor wheel, the injection nozzle being configured to inject the bleed portion in a direction that makes an angle of from 0° to 90° with respect to the rotational axis, and wherein a flow area of the injection nozzle decreases in the flow direction such that the bleed portion is accelerated before being injected into the main fluid flow stream. 2. The centrifugal compressor of claim 1, wherein the bleed port and injection nozzle each comprises a substantially uninterrupted 360° annular passage. 3. The centrifugal compressor of claim 1, wherein the injection nozzle is defined between a first wall and a second wall, the first wall axially overlapping the second wall such that the bleed portion is injected with a non-zero axial component of velocity. 4. The centrifugal compressor of claim 1, wherein the bleed port is connected to the injection nozzle by a connecting passage defined in the compressor housing. 5. The centrifugal compressor of claim 4, wherein the flowpath defined by the connecting passage and the injection nozzle is free of abrupt steps. 6. The centrifugal compressor of claim 4, further comprising a plurality of guide vanes disposed in the connecting passage and configured to alter a degree of swirl in the bleed portion prior to the bleed portion being discharged through the injection nozzle. 7. The centrifugal compressor of claim 6, wherein the guide vanes are configured to give the bleed portion substantially zero swirl as the bleed portion is injected into the main fluid flow stream. 8. The centrifugal compressor of claim 6, wherein the guide vanes are configured to give the bleed portion a swirl in an opposite direction to the direction of rotation of the compressor wheel as the bleed portion is injected into the main fluid flow stream. 9. The centrifugal compressor of claim 1, wherein the blades at the leading edges have a span S, and the exit plane of the injection nozzle is spaced upstream of the leading edges by a distance of from 0% to about 100% of S. 10. A method for controlling surge of a centrifugal compressor, the compressor comprising a centrifugal compressor wheel having a hub defining a rotational axis and having a plurality of circumferentially spaced blades each having a root joined to the hub and extending generally radially outwardly to a tip of the blade, each of the blades having a leading edge and a trailing edge spaced downstream from the leading edge along a flow direction, and a compressor housing in which the compressor wheel is mounted so as to be rotatable about the rotational axis of the compressor wheel, the compressor housing including an inlet duct through which the fluid enters in a direction generally parallel to the rotational axis of the compressor wheel and is led by the inlet duct into the compressor wheel, the fluid flowing along a main flowpath defined by the compressor wheel and compressor housing and exiting the compressor wheel in a generally radially outward direction, the inlet duct comprising a duct wall that encircles the rotational axis, the compressor housing further including a wheel shroud located radially adjacent the tips of the blades, the method comprising the steps of: bleeding off a bleed portion of the fluid being compressed by the compressor wheel via a bleed port defined in the wheel shroud at a location intermediate the leading and trailing edges of the blades, the bleed portion constituting from about 15% to about 25% of the total mass flow entering the compressor; and injecting the bleed portion of the fluid back into the main fluid flow stream approaching the compressor wheel via a converging injection nozzle defined in the duct wall upstream of the leading edges of the blades, the injection nozzle being configured to inject the bleed portion in a direction that makes an angle of from 0° to 90° with respect to the rotational axis, and wherein a flow area of the injection nozzle decreases in the flow direction such that the bleed portion is accelerated before being injected into the main fluid flow stream. 11. The method of claim 10, further comprising altering a degree of swirl in the bleed portion prior to the bleed portion being discharged through the injection nozzle. 12. The method of claim 11, wherein the bleed portion exits the injection nozzle with substantially no swirl. 13. The method of claim 11, wherein the bleed portion exits the injection nozzle with swirl opposite to the rotational direction of the compressor wheel. 14. The method of claim 10, wherein the bleed portion is bled at a location proximate the leading edges of the blades. 15. The method of claim 10, wherein the bleed portion is injected back into the main fluid flow stream at a location that is at a distance of from 0% to about 100% of a span of the blades at the leading edges thereof.