Compressor housing

Abstract

A compressor housing 14 comprises an inlet passage 26 defining a central axis 86, a first axially extending wall 48 defining at least part of the inlet passage, a wheel chamber 17 in communication wit

Description

Field of Invention The present invention relates to a compressor housing. The present invention also relates to a compressor, and to a turbomachine comprising the compressor housing. Background A compressor comprises an impeller wheel, carrying a plurality of blades (or vanes) mounted on a shaft for rotation within a compressor housing. Rotation of the impeller wheel causes gas (e.g. air) to be drawn into the impeller wheel and delivered to an outlet chamber or passage. In the case of a centrifugal compressor, which are also known as radial compressors, the outlet passage is in the form of a scroll volute defined by the compressor housing around the impeller wheel and in the case of an axial compressor the gas is discharged axially. Turbochargers are well-known device for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). The compressor of a turbocharger is driven by an exhaust gas turbine that is mounted on a common shaft. Exhaust gas from the internal combustion engine flows through the turbine and drives the turbine wheel in rotation, which, in turn, rotates the compressor impeller. Air is drawn through an axial inlet of the compressor housing and compressed air is delivered to the intake manifold of the internal combustion engine, thereby increasing engine power. One aspect of turbocharger control is to ensure stable operation by avoiding what is known as surge. If the turbocharger is operating at a relatively low compressor volumetric air flow rate and a high boost pressure the air flow into the compressor may stall and the operation of the compressor is interrupted. Following stall, the air flow tends to reverse through the compressor until a stable pressure ratio is reached at which the air can flow in the correct direction. This process repeats and results in pulsations in the air flow known as surging. Compressor recirculation passages are used to return a portion of flow from an outlet of the compressor to an inlet of the compressor. Recirculating the flow in his way helps to avoid the likelihood of the compressor undergoing surge in use. It is an object of the present invention to address one or more problems associated with known turbomachines, whether identified herein or otherwise. Summary In a first aspect of the invention there is provided a compressor housing. The compressor housing comprises an inlet passage configured to receive intake fluid. The inlet passage has an inlet opening. The inlet passage defines a central axis. The compressor housing further comprises a first axially extending wall. The first axially extending wall define at least part of the inlet passage. The compressor housing further comprises a wheel chamber. The wheel chamber is in fluid communication with the inlet passage. The wheel chamber is configured to receive a compressor wheel. The compressor housing further comprises an outlet passage. The outlet passage is in fluid communication with the wheel chamber. The compressor housing further comprises a first recirculation passage. The first recirculation passage fluidly links the wheel chamber to the inlet passage. The compressor housing further comprises a second recirculation passage. The second recirculation passage comprises a conduit. The conduit comprises a first end opening at the outlet passage and a second end opening that extends through an outer wall of the compressor housing in the region of the inlet passage. The second end opening is disposed in axial alignment with, and radially outboard of, the first axially extending wall. The conduit is provided with a valve. The second recirculation passage further comprises an annular channel that is defined between the outer wall and the first axially extending wall. The annular channel is configured to receive fluid flow from the second end opening and discharge fluid flow into the inlet passage. An outlet of the annular channel is disposed upstream of the of the first recirculation passage. The compressor housing may be a centrifugal compressor housing. Since the fluid from the conduit passes into the annular channel before entering the inlet passage, the fluid enters the inlet passage with an axial component to its direction. This results in less turbulence, as compared to radial flow, in the fluid in the inlet passage. Reducing the turbulence in the inlet passage is desirable because the pressure distribution along the leading edges of the compressor blades is more uniform as a result. This advantageously improves the efficiency of the compressor that the compressor housing forms a part of. The conduit may define a first conduit portion. The first conduit portion may adjoin the outer wall in the region of the inlet passage. The first conduit portion may define a length. The first conduit portion may extend non-radially. The first conduit portion extending non-radially may be understood to mean that a central axis of the first conduit portion is non-parallel to a radial direction. The radial direction may be a direction that extends radially from the central axis. A length of the first conduit portion may define a component that extends in the radial direction, while the first conduit portion extends non-radially. Where the first conduit portion extends non-radially, turbulence in the flow of fluid entering the inlet passage is further reduced in use. A component of the length of the first conduit portion may extend radially with respect to the central axis. A component of the length of the first conduit portion may extend in a first circumferential direction. The first conduit portion may extend in a non-radial direction. That is to say, a component of the length of the first conduit portion may extend in the radial direction, but may also have a component that extends in a non-radial direction. Where a component of the length of the first conduit portion extends radially with respect to the central axis and a component of the length extends in a first circumferential direction, turbulence in the flow of fluid entering the inlet passage is further reduced in use. A component of the length of the first conduit portion may extend in an axial direction. The axial direction may be understood to refer to a direction that is parallel to the central axis. Where a component of the length of the first conduit portion extends in the axial direction, turbulence in the flow of fluid entering the inlet passage is further reduced in use. The first conduit portion may extend tangentially with respect to a radially outer surface of the first axially extending wall. The first conduit portion may extend tangentially with respect to the radially outer surface of the first axially extending wall in a circumferential direction. Where the first conduit portion extends tangentially with respect to a radially outer surface of the first axially extending wall, turbulence in the flow of fluid entering the inlet passage is further reduced in use. The compressor housing may further comprise a partition. The partition may be disposed in the annular channel. The partition may be configured to block fluid flow. The compressor housing may comprise only a single partition in the annular channel. Where a partition is provided, merging of separate portions of flow in the annular channel is reduced, which advantageously reduces the amount of turbulence in the fluid both within the annular channel, and within the inlet passage. The partition may be disposed circumferentially adjacent the second end opening of the conduit. The partition may extend in a direction that is generally parallel to the first conduit portion. Where the partition extends in a direction that is generally parallel to the first conduit portion, turbulence in the fluid within the annular channel is advantageously reduced. The partition may be disposed at a position that is diametrically opposed to the second end opening of the conduit. The partition may define an arc angle. The arc angle of the partition may be at least 5° and / or up to 180°. The arc angle of the partition may be at least 5° and / or up to 90°. The arc angle of the partition may be at least 8° and / or up to 90°. The arc angle of the partition may be chosen based on, for example, the geometry of the compressor housing and of the compressor rotor that is to be received by the compressor housing. Flow may enter the annular channel and flow through the annular channel in opposed circumferential directions in two portions. Where the partition is disposed at a position that is diametrically opposed to the second end opening of the conduit, merging of the two portions of flow is inhibited. This advantageously reduces the amount of turbulence within the annular channel and within the inlet passage. The partition may comprise a tapered portion. The tapered portion may be configured to encourage fluid to flow from the annular channel into the inlet passage. The partition may define one or more helical portions. The annular channel may be defined between the partition and an axial end wall of the compressor housing. An axial length of the annular channel may reduce in a direction towards the axial end wall. The axial length of the annular channel may reduce in a direction towards the axial end wall about the central axis. The axial length of the annual channel may be understood to a distance in a direction that is parallel to the central axis from the partition to the axial end wall of the compressor housing. A radial width of the annular channel may be non-uniform. The partition may extend from a point that is circumferentially adjacent the second end opening of the conduit. The partition may define an arc angle of at least 90 degrees and / or up to 180 degrees. The point that is circumferentially adjacent the second end opening of the conduit may circumferentially adjoin, or be circumferentially offset from, the second end opening. At least part of the annular channel may be disposed radially outboard of, and in axial alignment with, the first recirculation passage. Where at least part of the annular channel is disposed radially outboard of, and in axial alignment with, the first recirculation passage, the compressor housing occupies less space in the axial direction. The first axially extending wall may be defined by a baffle that is disposed in the inlet passage. The first axially extending wall may extend from the outer wall of the compressor housing. The baffle may be disposed upstream of the first recirculation passage. The baffle may be entirely disposed upstream of the first recirculation passage. In a second aspect of the invention there is provided a compressor. The compressor comprises the compressor housing of the first aspect of the invention. In a third aspect of the invention there is provided a turbomachine. The turbomachine comprises a compressor housing according to the first aspect of the invention. The turbomachine comprises a bearing housing coupled to the compressor housing. The turbomachine may comprise a compressor according to the second aspect of the invention. In a fourth aspect of the invention there is provided a turbomachine. The turbomachine comprises a turbine. The turbomachine further comprises a compressor housing according to the first aspect of the invention. The turbomachine further comprises a bearing housing disposed between the turbine and the compressor housing. The turbomachine may comprise a compressor according to the second aspect of the invention. In a fifth aspect of the invention there is provided a compressor housing. The compressor housing comprises an inlet passage. The inlet passage is configured to receive intake fluid. The inlet passage has an inlet opening and defines a central axis. The compressor housing further comprises a first axially extending wall. The first axially extending wall defines at least part of the inlet passage. The compressor housing further comprises a wheel chamber in fluid communication with the inlet passage and configured to receive a compressor wheel. The compressor housing further comprises an outlet passage in fluid communication with the wheel chamber. The compressor housing further comprises a first recirculation passage. The first recirculation passage comprises a first conduit that extends from an outer wall of the compressor housing in the region of the inlet passage and comprises a valve mount. The first recirculation passage further comprises a second conduit that extends from the outlet passage. The first recirculation passage further comprises a recirculation valve that is coupled to the valve mount such that a valve axis of the valve extends in a generally radial direction with respect to the central axis. The recirculation valve is operable to regulate the flow of fluid between the wheel chamber and the inlet passage via the first conduit and the second conduit. At least part of an outlet of the first conduit is disposed in axial alignment with, and radially outboard of, the first axially extending wall. Since the valve is mounted to the first conduit portion, which extends from the inlet passage, the valve is subject to lower operating temperatures as compared to if the valve were mounted to the second conduit portion, which extends from the outlet passage. This is because the temperature of the fluid in the inlet passage is lower than the temperature of the fluid in the outlet passage. The valve may be mounted to the valve mount such that the valve is urged towards the closed position by the pressure of fluid in the inlet passage in use. Conventionally, the valve would be mounted such that it is urged to the open position by the pressure of the fluid in the outlet passage. The pressure and temperature of the fluid in the outlet passage are greater than that in the inlet passage. To overcome the pressure of the outlet passage, a biasing member is typically required to urge the valve towards the closed position, and an actuator is required to overcome the biasing force provided by the biasing member to move the valve between the opening and closed positions. Alternatively or additionally, the valve may comprise a pressure balancing passage to balance the pressure immediately upstream of the valve with the pressure immediately downstream of the valve. High temperature, fluid flows from the outlet passage to the inlet passage via the pressure balancing passage. Where the valve is mounted such that it is urged to the closed position by virtue of the pressure of the fluid in the inlet passage, the biasing member and actuator of the valve can be smaller. Furthermore, a pressure balancing passage need not be provided and so the valve is protected from high temperature conditions in use. This advantageously reduces the likelihood of failure of the valve. The valve may comprise a pressure balancing passage. The pressure balancing passage may be configured to allow flow to pass from within the valve to the inlet passageway. This advantageously reduces the likelihood of failure of the valve because the high temperature fluid that may leak into the valve is able to exit the valve, therefore avoiding overheating of the valve. Features disclosed in relation to one aspect of the invention may be combined with other aspects of the invention. Brief Description of the Drawings Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a cross-sectional view of a turbocharger; Figure 2 shows a cross-sectional view of a compressor housing of the turbocharger of Figure 1; Figure 3 shows a perspective view of the compressor housing of Figure 2; Figure 4 shows a cross-sectional view of a compressor housing according to a further embodiment of the invention; Figure 5 shows a cross-sectional view of a compressor housing according to a further embodiment of the invention; Figure 6 shows a perspective view of an inlet passage of the compressor housing of Figure 5; Figure 7 shows a cross-sectional view of a compressor housing according to a further embodiment of the invention; Figure 8 shows an inlet passage of a compressor housing according to a further embodiment of the invention; Figure 9 shows a cross-sectional view of a compressor housing according to a further embodiment of the invention; and Figure 10 shows a cross-sectional view of a compressor housing according to a further embodiment of the invention. Detailed Description Figure 1 shows a turbocharger 2. The turbocharger 2 comprises a turbine 4 joined to a compressor 6 via a bearing housing 8. The compressor 6 is a centrifugal compressor. The turbine 4 comprises a turbine housing 10 and a turbine rotor 12. The turbine rotor 12 is disposed in a turbine wheel chamber 13. Similarly, the compressor 6 comprises a compressor housing 14 and a compressor impeller 16. The compressor impeller 16 is disposed in a compressor wheel chamber 17. The turbine rotor 12 and compressor impeller 16 are mounted on opposite ends of a shaft 18 which is supported on a bearing assembly 20 that is disposed within the bearing housing 8. The turbine housing 10 is provided with an exhaust gas inlet (not visible in Figure 1 - it is out of the plane of the cross-section) and an exhaust gas outlet 22. The exhaust gas inlet directs incoming exhaust gas to an annular inlet volute 24 surrounding the turbine rotor 12. The exhaust gas flows through the turbine 4 and out of the exhaust gas outlet 24 via a circular outlet opening which is co-axial with the turbine rotor 12. Rotation of the turbine rotor 12 rotates the compressor impeller 16 which draws in air through an inlet opening 25 and an inlet passage 26 and delivers compressed air to the engine intake via an outlet passage 28 in the form of an annular outlet volute. The turbine rotor 12, the shaft 18 and the compressor impeller 16 are co-axial and rotate about a turbocharger axis 30. The bearing housing 8 provides a lubricating system for the turbocharger 2. The bearing housing 8 includes a series of channels 32 (only one of which is labelled in Figure 1) through which oil is supplied to the bearing assembly 20. The channels 36 receive oil from an engine oil circuit (not shown). The bearing housing 8 also includes a turbine seal assembly 38 and a compressor seal assembly 40. Figure 2 shows a cross-sectional view of the compressor housing 14. A first baffle 58 is provided in the inlet passage 26. The first baffle 58 defines at least part of the inlet passage. The first baffle 58 is provided as a separate component to the compressor housing 14. However, in some embodiments, the first baffle 58 may be integrally formed with the remainder of the compressor housing 14. In some, non-depicted, embodiments, the first baffle 58 need not be provided. The compressor housing 14 comprises a first recirculation passage 42. The first recirculation passage 42 fluidly links the wheel chamber 17 to the inlet passage 26. The first recirculation passage 42 maybe referred to as a map width enhancement feature. The first recirculation passage comprises an annular passage 44. The annular passage 44 maybe referred to as a map width enhancement slot. The first recirculation passage 42 comprises a first annular channel 46. The annular passage 44 extends from the wheel chamber 17 to the first annular channel 46. The first annular channel 46 is defined between a first axially extending wall 48 and a second axially extending wall 50. The second axially extending wall 50 is disposed radially inboard of the first axially extending wall 48. The first axially extending wall 48 and the second axially extending wall 50 are disposed radially inboard of an outer wall 60 of the compressor housing 14. An outlet 52 of the first recirculation passage 42 is defined between a first axial end 54 of the second axially extending wall 50 and an axial end 56 of the first baffle 58. The first baffle 58 is secured to the first axially extending wall 48. Referring to Figure 3. The compressor housing 14 comprises a second recirculation passage 62. The second recirculation passage 62 may be referred to as the first recirculation passage, and the first recirculation passage 42 may be referred to as the second recirculation passage. The second recirculation passage 62 is provided to allow, and control, the flow of fluid from the outlet passage 28 to the inlet passage 26. The second recirculation passage 62 comprises a conduit 64. The conduit 64 comprises a first conduit portion 68 and a second conduit portion 70. The conduit 64 comprises a first end opening (not visible in Figure 3 - it is disposed within the outlet passage). The first end opening is disposed at the outlet passage. The conduit comprises a second end opening (not visible in Figure 3 - it is disposed within the inlet passage). The second end opening extends through the outer wall 60 of the compressor housing 14 in the region of the inlet passage 26. The conduit 64 is provided with a valve 66. The valve 66 may be referred to as a recirculation valve. The valve 66 is disposed between the first conduit portion 68 and the second conduit portion 70. The first conduit portion 68 defines the second end opening of the conduit, and the second conduit portion 70 defines the first end opening of the conduit 64. The first conduit portion 68 comprises a valve mount 84. The first conduit portion 68 extends in a generally radial direction with respect to a central axis 86 of the compressor housing 14. The valve 66 is mounted to the valve mount 84. The valve 66 is orientated such that a valve axis 88 extends in a generally radial direction. Since the valve 66 is mounted to the first conduit portion 68, which extends from the inlet passage 26, the valve is subject to lower operating temperatures as compared to if the valve 66 were mounted to the second conduit portion (not visible in Figure 3), which extends from the outlet passage 28. This is because the temperature of the fluid in the inlet passage 26 is lower than the temperature of the fluid in the outlet passage 28. The valve 66 is mounted such that the valve 66 is urged to the closed position by virtue of the pressure of the fluid in the inlet passage 26. The valve 66 is mounted such that the pressure of the fluid in the outlet passage 28 acts radially with respect to a valve axis 93. The valve 66 is mounted such that the pressure of the fluid in the outlet passage 28 has an insignificant influence, or no influence, on the position of the valve 66. Conventionally, the valve would be mounted such that it is urged to the open position by the pressure of the fluid in the outlet passage. The pressure and temperature of the fluid in the outlet passage are greater than that in the inlet passage. To overcome the pressure of the outlet passage, a biasing member is typically required to urge the valve towards the closed position, and an actuator is required to overcome the biasing force provided by the biasing member to move the valve between the opening and closed positions. Alternatively or additionally, the valve may comprise a pressure balancing passage. The pressure balancing passage may balance the pressure of the fluid within the valve with the pressure of the fluid in the outlet passage. By mounting the valve such that it is urged to the closed position by virtue of the pressure of the fluid in the inlet passage 26, the biasing member and actuator of the valve can be smaller. This advantageously reduces the likelihood of failure of the valve 66. Furthermore, a pressure balancing passage need not be provided and so the valve 66 is protected from high temperature conditions in use, which further reduces the likelihood of failure of the valve 66. However, in some embodiments, a pressure balancing passage may be provided to allow flow that leaks into the valve 66 from the outlet passageway to pass to the inlet passageway. This advantageously reduces the likelihood of failure of the valve because the high temperature fluid that may leak into the valve is able to exit the valve, therefore avoiding overheating of the valve 66. Referring back to Figure 2, the second recirculation passage 62 comprises a second annular channel 72. The second annular channel 72 is defined between the first axially extending wall 48 and the outer wall 60 of the compressor housing 14. In particular, the secondary circulation passage 62 is defined between a radially outer surface of the first axially extending wall 48 and a radially inner surface of the outer wall 60. At least part of the second annular channel 72 is axially aligned with the at least part of the first recirculation passage 42. This allows the compressor housing 14 to occupy less axial space. The second recirculation passage 62 receives fluid from the second end opening 74 of the conduit 64. The second end opening 74 is in axial alignment with and, and is disposed radially outpoured of, the first axially extending wall 48. Since the second end opening 74 of the conduit 64 is disposed radially outboard of, and in axial alignment with, the first axially extending wall 48, turbulence of the flow through the inlet passage 26 is reduced. This is as compared to, for example, if the flow from the conduit 64 entered the inlet passage 26 such that the flow from the conduit 64 enters the inlet passage flow in a radial direction. This advantageously improves the efficiency of the compressor because the pressure distribution at the leading edge of the compressor impeller is more uniform. An outlet 76 of the annular channel 72 is defined between a first axial end 78 of the first axially extending wall 48 and an axial end wall 80 of the compressor housing 14. A second baffle 82 is disposed at the inlet opening 25. The second baffle 82 is integrally formed with the axial end wall 80. However, in other embodiments, the second baffle 82 may be provided as a separate component to the axial end wall 80. The second baffle 82 is disposed radially inboard of the outlet 76 of the second annular channel 72. A radially outer surface of the second baffle 82 tapers in a radially inwards direction (along an upstream axial direction). The second baffle 82 acts to guide flow from the second recirculation passage 62 into the inlet passage 26. This advantageously reduces the likelihood of turbulence in the flow in the inlet passage way occurring in use. In some, non-depicted embodiments, the second baffle 82 need not be provided. In some embodiments, the axial end wall 80, the first baffle 58, the second baffle 82, the first axially extending wall 48, and the second axially extending wall 50 may be provided to the compressor housing 14 as an insert. That is to say, these components may be separately formed from the remainder of the compressor housing. In some embodiments, the axial end wall 80, the first baffle 58, the second baffle 82, and the first axially extending wall 48 may be provided as an insert. Figure 4 shows a modified embodiment of the compressor housing 14. In this embodiment, a partition 90 is disposed in the second annular channel 72. The partition 90 is configured to block fluid flow. In particular, the partition 90 is configured to block the flow of fluid in the region of the second annular channel 72 to which it is provided. The partition 90 defines a first circumferential end (not shown in Figure 4 - it is out of the plane of the cross section of Figure 4) and a second circumferential end (not visible in Figure 4 - it is out of the plane of cross section of Figure 4). The first circumferential end of the partition 90 is disposed circumferentially adjacent to the second end opening 74 of the conduit 64. The first circumferential end of the partition is disposed circumferentially adjacent to the second end opening 74 of the conduit 64. The partition 90 defines an arc angle. The arc angle of the partition 90 is the angle that is defined between a first radial axis that extends through the first circumferential end of the partition and a second radial axis that extends through the second circumferential end of the partition. The arc angle of the partition 90 is at least 90° and / or up to 180°. Advantageously, the arc angle of the partition 90 being in this range concentrates fluid flow within the second annular channel 72, while providing sufficient space for fluid to pass through the second annular channel 72. Concentrating fluid flow reduces the likelihood of stagnation of the fluid in the second annular channel 72 occurring in use. Figure 5 shows a further embodiment of a compressor housing 14. In this embodiment, a partition 90 is again provided to the second annular channel 72. The partition 90 is disposed at a position that is diametrically opposed to the second end opening 74 of the conduit 64. However, in some, non-depicted, embodiments, the partition 90 may be disposed at another position within the second annular channel 72. The partition 90 is disposed in the second annular channel 72. The partition 90 comprises a first circumferential end 92 and a second circumferential end 94. An arc angle of the partition 90 is at least 5° and / or up to 180°. An arc angle of the partition 90 may be at least 5° and / or up to 90°. An arc angle of the partition 90 is at least 8° and / or up to 90°. The arc angle of the partition 90 may be chosen based on, for example, the geometry of the compressor housing 14 and of the compressor rotor (not shown in Figure 5). A minimum circumferential width of the partition 90 may be 5mm. The partition 90 is also configured to block a flow of fluid. In use, fluid enters the second annular channel 72 from the second end opening 74 of the conduit 64. The fluid then splits such that a portion of the flow flows clockwise about the central axis (which extends into the page of Figure 5) of the compressor housing 14, and a portion of the flow flows anti-clockwise about the central axis of the compressor housing. With the absence of the partition, these two portions of flow would combine at a position that is diametrically opposed to the second end opening 74 of the conduit 64, which can cause turbulence. Turbulent flow can reduce the efficiency of the compressor, create noise, and the stability of the fluid flow through the compressor rotor (not shown in Figure 5). The partition 90 reduces the likelihood of turbulence occurring in use because the two portions of flow are separated by the partition. Figure 6 shows a perspective view of the inlet passage 26. The remaining portions of the compressor housing have been removed for clarity. As can be seen, the circumferential width of the partition 90 is non-uniform. The partition 90 comprises a tapered portion 89 and a linear portion 91. The tapered portion 89 adjoins the linear portion 91. A first axial end 85 of the partition 90 adjoins the axial end wall 80. The first axial end 85 forms a part of the linear portion 91. The linear portion 91 extends in a direction that is generally parallel to the central axis 86 of the compressor housing. The second axial end 87 forms a part of the tapered portion 89. The circumferential width of the partition 90 decreases from the second axial end 87 to the linear portion 89. The circumferential width of the partition 90 decreases from the second axial end 87 to the linear portion 89 in a linear manner. In some embodiments, the circumferential width of the partition 90 may decrease from the second axial end 87 to the linear portion 89 in a non-linear manner. The tapered portion 91 advantageously encourages fluid to flow from the second annular channel 72 and into the inlet passage 26. In some embodiments, the circumferential width of the partition 90 may be uniform. In some embodiments, the entirety of the partition 90 may be tapered. Where the entirety of the partition 90 is tapered, a circumferential width of the partition 90 may increase from the axial end wall 80. Figure 7 shows a further embodiment of the compressor housing 14. In this embodiment, when viewed in a plane that is perpendicular to the central axis (which extends in to the page of Figure 6) of the compressor housing 14, the first conduit portion 68 of the conduit 64 extends in a non-radial direction. That is to say, a component of a length of the first conduit portion 68 extends circumferentially and a component of the length of the first conduit portion 68 extends radially with respect to the central axis (which extends into the page of Figure 6). This advantageously reduces the amount of space occupied by the compressor housing 14, as compared to a radially extending first conduit portion 68. In some embodiments, a component of the length of the first conduit portion 68 may extend in an axial direction. Where a component of the length of the first conduit portion 68 extends in an axial direction, the first conduit portion 68 may be orientated such that a component of the flow of fluid in the first conduit portion 68 extends in an upstream direction of the flow in the inlet passage 26. A central axis 93 of the first conduit portion 68 extends tangentially with respect to a radially outer surface 95 of the first axially extending wall 48. However, in other embodiments, this need not be the case. The first conduit portion 68 is orientated such that flow entering the second annular channel 72 flows through the second annular channel 72 in a first circumferential direction. This advantageously allows a circumferential component to be introduced to the flow in the inlet passageway. This circumferential component may be referred to as pre-swirl. Introducing pre-swirl advantageously improves compressor efficiency and reduces noise because the incidence angle of flow onto the compressor rotor is reduced. As viewed in Figure 7, the first circumferential direction is the anti-clockwise direction. However, the flow may enter the channel in the clockwise direction. Since the first conduit portion 68 of this embodiment extends in a non-radial direction, turbulence in the flow exiting the first conduit portion 68 is reduced. This is because the angle of impingement of the flow is reduced. Reducing the amount of turbulence in the flow advantageously improves compressor efficiency. The presence of the partition 90 further reduces turbulence because flow already in the second annular channel 72 is prevented from merging with the flow entering the second annular channel 72. The first conduit portion 68 of any of the embodiments disclosed in this document may be non-radial in any manner described above. The compressor housing 14 of this embodiment also comprises a partition 90. The partition 90 is disposed adjacent the send end opening 74 of the first conduit portion 68. The partition 90 is disposed circumferentially adjacent the second end opening of the first conduit portion in the first circumferential direction. The partition 90 extends in a direction that is generally parallel to the central axis 92 of the first conduit portion 68. Therefore, the partition 90 extends in a non-radial direction. However, in some embodiments, this need not be the case. The partition 90 may take any suitable form. The partition 90 may be trapezoidal in shape. Furthermore, in some embodiments in which the first conduit portion 68 extends in a non-radial direction, the partition 90 need not be provided. Figure 8 shows a perspective view of an inlet passage 26 of a compressor housing (only the inlet passage 26 is shown in Figure 8 for clarity) in accordance with a further embodiment of the invention. The inlet passage 26 of Figure 8 may be used with any of the compressor housings disclosed in this document. In this embodiment, the partition 90 is helical. The partition 90 extends about the central axis 86 of the compressor housing. The second annular channel 72 extends axially between the partition 90 and the axial end wall 80. An axial length of the second annular channel 72 reduces in a direction towards the axial end wall 80. This advantageously encourages flow to exit the second annular channel 72 and enter the inlet flow of the compressor housing such that the flow from entering the inlet flow from the second annular channel 72 is more uniform. More uniform flow is desirable because the amount of turbulence in the flow is reduced. Reducing the turbulence in the flow reduces noise and improves the efficiency of the compressor. The partition 90 of this embodiment may be particularly effective at encouraging flow to exit the second annular channel 72 where the first conduit portion (not shown in Figure 8) extends in a non-radial direction, as for the embodiment of Figure 7. However, a helical partition 90 may be provided where the first conduit portion extends in a radial direction. In some embodiments, the partition 90 may comprise two helical portions. The helical portions may extend from the axial end wall 80 in opposite directions about the central axis 86. That is to say, a first helical portion may extend about the central axis 86 in a clockwise direction and a second helical portion may extend about the central axis 86 in an anticlockwise direction. An arc angle of each helical portion may be less than 180 degrees. Figure 9 shows a compressor housing 14 in accordance with a further embodiment of the invention. In this embodiment, the radial width of the second annular channel 72 is non-uniform. This is achieved by the outer wall 80 of the compressor housing 14 and the first axially extending wall 48 being non-concentric. The radial width of the second annular channel 72 is greatest in the region of the second end opening 74 of the conduit 64. The radial width of the second annular channel 72 tapers about the central axis 86 of the compressor housing 14 from the region of the second end opening 74 of the conduit 64. The radial width of the second annular channel 72 may taper to zero, or may taper to a non-zero value. This advantageously encourages fluid to flow from the second annular channel 72 and into the inlet passage 26 such that the flow from entering the inlet flow from the second annular channel is more uniform. More uniform flow is desirable because the amount of turbulence in the flow is reduced. Reducing the turbulence in the flow reduces noise and improves the efficiency of the compressor. In some embodiments, a radial width of the first axially extending wall 48 and / or a radial width of the outer wall 80 of the compressor housing 14 may increase from the region of the second end opening 74 of the conduit 64 about the central axis 86. As a result, the radial width of the second annular channel 72 may be non-uniform. Figure 10 shows a compressor housing 14 in accordance with a further embodiment of the invention. In this embodiment, the first axially extending wall 48 is in the form of a baffle. The first axially extending wall 48 of this embodiment extends from the outer wall 60 of the compressor housing 14. As with previous embodiments, the second annular channel 72 is defined between the first axially extending wall 48 and the outer wall 60 of the compressor housing 14. In this embodiment, the first annular channel 46 is defined between a second axially extending wall 50 and a third axially extended wall 96. The third axially extending wall 96 may be said to correspond to the first axially extending wall 48 of previous embodiments. In this embodiment, the entirety of the second annular channel 72 is disposed axially upstream of the first recirculation passage 42. In the depicted embodiment, at least part of the second annular channel 72 is radially aligned with at least part of the first recirculation passage 42. As with the previous embodiments, the second end opening 74 of the conduit 64 is in axial alignment with and radially outboard of the first axially extending wall 48. The conduit 64 of this embodiment follows the same arrangement of the previous embodiment. That is to say, the valve 66 is mounted to a valve mount 84, and the valve mount 84 forms a part of the first conduit portion 68, which extends from the outer wall 60 in a region of the inlet passage 26. The second annular channel 72 of this embodiment may be provided with any of the partitions discussed in relation to Figures 4-8. Furthermore, the first conduit portion 68 may be non-radial, as with the embodiment shown in Figure 7. Furthermore, the radial width of the second annular channel 72 may be non-uniform, as with the embodiment of Figure 9. 5 Although the above description has been with reference to a turbocharger, the present invention is applicable to any type of turbomachine. For example, the present invention may be applied to an electric turbocharger, or a supercharger. While specific embodiments of the invention have been described above, it will be 10 appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. A compressor housing comprising:an inlet passage configured to receive intake fluid, the inlet passage having an inlet opening and defining a central axis;a first axially extending wall that defines at least part of the inlet passage;a wheel chamber in fluid communication with the inlet passage and configured to receive a compressor wheel;an outlet passage in fluid communication with the wheel chamber;a first recirculation passage that fluidly links the wheel chamber to the inlet passage;a second recirculation passage that comprises:a conduit that comprises a first end opening at the outlet passage and a second end opening that extends through an outer wall of the compressor housing in the region of the inlet passage, the second end opening being disposed in axial alignment with, and radially outboard of, the first axially extending wall, the conduit being provided with a valve; andan annular channel that is defined between the outer wall and the first axially extending wall, the annular channel being configured to receive fluid flow from the second end opening and discharge fluid flow into the inlet passage, and wherein an outlet of the annular channel is disposed upstream of the of the first recirculation passage.

2. The compressor housing of claim 1, wherein the conduit defines a first conduit portion that adjoins the outer wall in the region of the inlet passage and defines a length.

3. The compressor housing of claim 2, wherein the first conduit portion extends non-radially.

4. The compressor housing of claim 2 or claim 3, wherein a component of the length of the first conduit portion extends radially with respect to the central axis and a component of the length extends in a first circumferential direction.

5. The compressor housing of any of claim 2 to 4, wherein a component of the length of the first conduit portion extends in an axial direction.

6. The compressor housing of any of claims 2 to 5, wherein the first conduit portion extends tangentially with respect to a radially outer surface of the first axially extending wall.

7. The compressor housing of any preceding claim, further comprising a partition that is disposed in the annular channel, the partition being configured to block fluid flow.

8. The compressor housing of claim 7, wherein the partition is disposed circumferentially adjacent the second end opening of the conduit.

9. The compressor housing of any of claims 2 to 6 and claim 7 or claim 8, wherein the partition extends in a direction that is generally parallel to the first conduit portion.

10. The compressor housing of claim 7, wherein the partition is disposed at a position that is diametrically opposed to the second end opening of the conduit.

11. The compressor housing of claim 10, wherein the partition comprises a tapered portion that is configured to encourage fluid to flow from the annular channel into the inlet passage.

12. The compressor housing of claim 7, wherein the partition defines one or more helical portions.

13. The compressor housing of claim 12, wherein the annular channel is defined between the partition and an axial end wall of the compressor housing, and wherein an axial length of the annular channel reduces in a direction towards the axial end wall.

14. The compressor housing of any preceding claim, wherein a radial width of the annular channel is non-uniform.

15. The compressor housing of claim 7, wherein the partition extends from a point that is circumferentially adjacent the second end opening of the conduit and defines an arc angle of at least 90 degrees and / or up to 180 degrees.

16. The compressor housing of any preceding claim, wherein at least part of the annular channel is disposed radially outboard of, and in axial alignment with, the first recirculation passage.

17. The compressor housing of any of claims 1 to 15, wherein the first axially extending wall is defined by a baffle that is disposed in the inlet passage, and wherein the first axially extending wall extends from the outer wall of the compressor housing.

18. A compressor comprising the compressor housing of any preceding claim.

19. A turbomachine comprising:a compressor housing according to any of claims 1 to 17; and a bearing housing coupled to the compressor housing.

20. A turbomachine comprising:a turbine;a compressor housing according to any of claims 1 to 17; anda bearing housing disposed between the turbine and the compressor housing.

21. A compressor housing comprising:an inlet passage configured to receive intake fluid, the inlet passage having an inlet opening and defining a central axis;a first axially extending wall that defines at least part of the inlet passage;a wheel chamber in fluid communication with the inlet passage and configured to receive a compressor wheel;an outlet passage in fluid communication with the wheel chamber; anda first recirculation passage that comprises:a first conduit that extends from an outer wall of the compressor housing in the region of the inlet passage and comprises a valve mount;a second conduit that extends from the outlet passage; anda recirculation valve that is coupled to the valve mount such that a valve axis of the valve extends in a generally radial direction with respect to the central axis, wherein the recirculation valve is operable to regulate the flow of fluid between the wheel chamber and the inlet5 passage via the first conduit and the second conduit;wherein at least part of an outlet of the first conduit is disposed in axial alignment with, and radially outboard of, the first axially extending wall.

22. The compressor housing of claim 21, wherein the valve is mounted to the valve10 mount such that the valve is urged towards the closed position by the pressure of fluid in the inlet passage in use.

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