Compressor

Abstract

A compressor housing 202 has a recirculation passage 218 defined between a primary baffle 214 and an outer wall 212 with a first channel for fluid flow from a wheel chamber 206 to the recirculation pa

Description

The present invention relates to a compressor housing. In particular, the present invention relates to a compressor housing having a so-called “map width enhancement structure” comprising a primary baffle and a secondary baffle, including a first channel extending between a wheel chamber and a recirculation passage configured to permit fluid flow from the wheel chamber to the recirculation passage, and a second channel extending between the recirculation passage and the inlet passage configured to permit fluid flow from the recirculation passage to the inlet passage. Compressors are used to increase the pressure of a working fluid. In the context of an internal combustion engine system, compressors are used to increase the pressure of the intake air to a pressure above atmospheric so as to enable a greater quantity of air to enter the engine. This increases the mass of oxygen available to support combustion, enabling a greater quantity of fuel to be used and therefore increasing the engine’s power output. In internal combustion engine systems, such compressors may be driven by an electric motor, a turbine propelled by exhaust gas (i.e. forming a turbocharger), or a combination of the two. Compressors typically comprise an inlet, a compressor (impeller) wheel contained within a compressor wheel (impeller) chamber, and an outlet. During use, flow conditions within the compressor wheel may cause the mass flow rate of working fluid through the compressor wheel to slow down, for example due to flow separation from the compressor wheel blades or due to the formation of vortices between adjacent blade pairs and the compressor wheel chamber of the compressor housing. If such flow conditions are allowed develop to a sufficient extent, the compressor may experience a so-called “surge” event. This may manifest in transient fluctuations of mass flow rate through the compressor wheel and, in the worst case, may cause backwards flow through the compressor wheel. If a compressor is operated 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 may be 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 airflow known as surging. Maximum operating efficiency of the engine is achieved by operating close to the surge limit and a surge margin is built into the control process to ensure that the turbocharger operates at a safe distance from the surge condition. It is an object of the present invention to address one or more problems associated with known turbomachines, whether identified herein or otherwise. A first aspect of the present invention provides a compressor housing for a compressor, the compressor housing comprising: an inlet, the inlet comprising: an inlet passage configured to receive intake fluid, the inlet passage being defined at least in part by a primary baffle; and a recirculation passage defined between the primary baffle and an outer wall of the compressor housing; a wheel chamber in fluid communication with the inlet passage, and configured to receive a compressor wheel supported for rotation about a compressor wheel axis; an outlet in fluid communication with the wheel chamber; a first channel extending between the wheel chamber and the recirculation passage configured to permit fluid flow from the wheel chamber to the recirculation passage; a second channel extending between the recirculation passage and the inlet passage configured to permit fluid flow from the recirculation passage to the inlet passage; the primary baffle comprising a first end proximal the wheel chamber, a second end distal from the wheel chamber, and a wall extending between the first end and the second end; a secondary baffle defining at least part of the recirculation passage, the secondary baffle having an end proximal and axially spaced from the second end of the primary baffle, where a distance between the compressor wheel axis and the radially outermost point of the end of the secondary baffle defines a first radius (RB); wherein a distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle defines a second radius (RW); wherein a distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle at a maximum axial distance from the compressor wheel chamber, defines a third radius (RD); wherein the first radius (RB) is greater than the second radius (RW), and the first radius (RB) is less than or equal to the third radius (RD); wherein a minimum distance between the primary baffle and the end of the secondary baffle defines a gap (G); and wherein a ratio (G I Rw) of the gap (G) to the second radius (RW) is around 0.014 to around 0.133. It has been found that by the first radius being greater than the second radius, the first radius being less than or equal to the third radius and the ratio of the gap to the second radius being around 0.014 to around 0.133 an improvement in efficiency close to the surge region of a compressor stage map is obtained. That is to say, by controlling the gap G between the distal end of the primary baffle and the end of the secondary baffle, and the relative radial position relationship between the first, second, and third radii, improvements in efficiencies when operating close to surge can be obtained. The term “compressor housing” encompasses a single, integrally formed component or an assembly of two or more separately formed components. The outlet may further comprise a volute portion, where the term “volute portion” encompasses a portion of the compressor housing having a generally scroll-like shape configured to receive intake air that has been compressed by the compressor wheel in a radial direction and to discharge the compressed intake air in a tangential direction relative to the compressor wheel axis. The volute portion may be connected to an exit portion. The term “exit portion” encompasses a portion of the compressor housing having a generally pipe-like or conduit-like shape that is configured to receive intake air from the volute portion in a tangential direction relative to the compressor wheel axis and to discharge the intake air to one or more downstream components of an engine or fuel cell system, for example a charge air cooler (i.e. intercooler), an internal combustion engine, or a fuel cell. In some embodiments, the exit portion may be a straight pipe. In other embodiments, the exit portion may be an extension of the volute portion, such that the exit portion is considered to define the most downstream portion of the volute. In further embodiment, the exit portion may comprise additional fluid-interacting features such as a diffuser, a nozzle or the like. The distance between the compressor wheel axis and the radially outermost point of the end of the secondary baffle which defines the first radius (RB) is the radial distance, i.e. the shortest distance between the compressor wheel axis and the radially outermost point of the end of the secondary baffle. Likewise, the distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle which defines the second radius (RW) is the radial distance, i.e. the shortest distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle. The distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle at a maximum axial distance from the compressor wheel chamber, which defines the third radius (RD) is the radial distance, i.e. the shortest distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle at a maximum axial distance from the compressor wheel chamber. The minimum distance between the primary baffle and the end of the secondary baffle encompasses the minimum distance between the primary baffle and the radially outermost point of the end of the secondary baffle. In particular, the radially outermost point of the end of the secondary baffle may be the point of the end of the secondary baffle that is closest to the primary baffle. The gap (G) may comprise an axial component relative to the compressor wheel axis. The gap (G) may comprise a radial component relative to the compressor wheel axis. The gap (G) may comprise both a radial and an axial component relative to the compressor wheel axis. An axial distance between the distal end of the primary baffle and the end of the secondary baffle may define the gap (G). The wall of the primary baffle may comprise a variable profile portion, where along a length of the variable profile portion the radial distance between the compressor wheel axis and the wall varies. That is to say, a profile of the wall extending between the first end of the primary baffle and the second end of the primary baffle may vary relative to the compressor wheel axis. In other words, the distance between the compressor wheel axis and the wall of the primary baffle may change along the axial length of the variable profile portion of the primary baffle, i.e. the variable profile portion comprises a transition in geometry. In particular, the variable profile portion of the primary baffle may include a tapered portion. The profile of the primary baffle may include an inclined portion. The profile of the primary baffle may include a curved portion. The profile portion of the primary baffle may comprise any two or all three of tapered, inclined and curved portions. The second end of the primary baffle may comprise the variable profile portion. The variable profile portion may terminate at the second end of the primary baffle. The variable profile portion may comprise the second end of the primary baffle. The termination of the variable profile portion may define the radially innermost point of the wall of the primary baffle at a maximum axial distance from the compressor wheel chamber, The variable profile portion may comprise a tapered portion relative to the compressor wheel axis. The tapered portion may include the wall of the primary baffle, tapering in a direction towards or away from the compressor wheel axis. The tapered portion may be at the second end of the primary baffle. The tapered portion may end at a point. The end of the tapered portion may be the radially innermost point of the wall of the primary baffle at a maximum axial distance from the compressor wheel chamber. The variable profile portion may comprise a curved surface. That is the say, a portion of the profile of the wall of the primary baffle may curve in a direction towards the compressor wheel axis, or may curve in a direction away from the compressor wheel axis. One or more other portions of the wall of the primary baffle may curve in the same or different directions, i.e. if one portion is curved towards the compressor wheel axis another portion may also be curved towards the compressor wheel axis, or may be curved away from the compressor wheel axis. The rate of curvature may not be constant along the length of the compressor wheel axis, i.e. the rate of curvature may vary along the length of the compressor wheel axis. The variable profile portion may comprise an inclined portion relative to the compressor wheel axis. The term “inclined portion” encompasses a region, wherein a length of the variable profile portion of the wall extends towards or away from the compressor wheel axis. The second end of the primary baffle may comprise a radially extending portion, such that the third radius (RD) is the radial distance between the compressor wheel axis and the radially innermost point of the radially extending portion of the second end of the primary baffle. The second end of the primary baffle is distal from the compressor wheel. The term “radially extending portion” encompasses a portion of a wall that extends in a generally radial direction relative to the compressor wheel axis. The radially extending portion may extend in entirely a radial direction relative to the compressor wheel axis. The radially extending portion of the second end of the primary baffle may extend from where the variable profile portion terminates. As such, a point at which the variable portion terminates and the radially extending portion of the second end of the primary baffle may be at the same axial point relative to the compressor wheel axis. The ratio of the gap (G) and the second radius (RW) may be between around 0.029 to around 0.066. The ratio of the gap (G) and the second radius (RW) may be between around 0.016 to around 0.116. The ratio of the gap (G) and the second radius (RW) may be between around 0.021 to around 0.1. The ratio of the gap (G) and the second radius (RW) may be between around 0.025 to around 0.076. The gap (G) may be between around 1 mm and around 2 mm. The gap (G) may be between around 1.15 mm and around 1.75 mm. The gap (G) may be between around 1.15 mm and around 1.5 mm. The gap (G) may be between around 1.5 mm and around 1.75 mm. The gap (G) may be around 1 mm. The gap (G) may be around 1.15 mm. The gap (G) may be around 1.5 mm. The gap (G) may be around 1.75 mm. The gap (G) may be around 2 mm. The end of the secondary baffle may comprise a radially extending portion. The term “radially extending portion” encompasses a portion of a wall of the secondary baffle that extends in a generally radial direction relative to the compressor wheel axis. The radially extending portion may extend in an entirely radial direction relative to the compressor wheel axis, i.e. it may not include an axial component, or it may include an axial component, which will typically be smaller in magnitude than the radial component such that the end of the secondary baffle may still be considered as extending in a generally radial direction. The secondary baffle may extend from the outer wall of the compressor housing towards the primary baffle. That is to say, the secondary baffle may extend from the outer wall in an axial direction, relative to the compressor wheel axis, towards the primary baffle. The secondary baffle may also extend in a radial direction, relative to the compressor wheel axis. A second aspect of the present invention provides a compressor comprising the compressor housing according to the first aspect of the present invention, which may incorporate any of the above-defined optional features of the housing of the first aspect of the present invention. The compressor according to the second aspect may be a compressor for a turbocharger. A third aspect of the present invention provides a turbomachine comprising: a turbine housing; a compressor housing according to the first aspect of the present invention, optionally including any of the above-defined optional features of said compressor housing; and a bearing housing disposed between the turbine housing and the compressor housing. The turbomachine according to the third aspect of the present invention may be a turbocharger. A fourth aspect of the present invention provides a compressor housing for a compressor, the compressor housing comprising: an inlet, the inlet comprising: an inlet passage configured to receive intake fluid, the inlet passage being defined at least in part by a primary baffle; and a recirculation passage defined between the primary baffle and an outer wall of the compressor housing; a wheel chamber in fluid communication with the inlet passage, and configured to receive a compressor wheel supported for rotation about a compressor wheel axis; an outlet in fluid communication with the wheel chamber; a first channel extending between the wheel chamber and the recirculation passage configured to permit fluid flow from the wheel chamber to the recirculation passage; a second channel extending between the recirculation passage and the inlet passage configured to permit fluid flow from the recirculation passage to the inlet passage; the primary baffle comprising a first end proximal the wheel chamber, a second end distal from the wheel chamber, and a wall extending between the first end and the second end; a secondary baffle defining at least part of the recirculation passage, the secondary baffle having an end proximal and axially spaced from the second end of the primary baffle, where a distance between the compressor wheel axis and the radially outermost point of the end of the secondary baffle defines a first radius (RB); wherein a distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle defines a second radius (RW); wherein a distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle at a maximum axial distance from the compressor wheel chamber, defines a third radius (RD); wherein the first radius (RB) is greater than the second radius (RW), and the first radius (RB) is less than or equal to the third radius (RD); wherein a minimum distance between the primary baffle and the end of the secondary baffle defines a gap (G); and wherein the gap (G) is between around 1 mm and around 2 mm. It has been found that by the first radius being greater than the second radius, the first radius being less than or equal to the third radius and the gap being between around 1 mm and 2 mm an improvement in efficiency close to the surge region of a compressor stage map is obtained. That is to say, by controlling the gap, G, between the distal end of the primary baffle and the end of the secondary baffle, and the relative radial position relationship between the first, second, and third radii, improvements in efficiencies when operating close to surge can be obtained. Further, unexpectedly, and without wishing to be bound by any particular theory the gap being between 1 mm and 2 mm results in an improvement in efficiencies when close to the surge region of a compressor stage map, for different sized compressor housings (i.e. different sized compressor wheels), when the first radius is greater than the second radius and the first radius is less than or equal to the third radius. The gap (G) may be between around 1.15 mm and around 1.75 mm. The gap (G) may be between around 1.5 mm and around 1.75 mm. The gap (G) may be around 1 mm. The gap (G) may be around 1.15 mm. The gap (G) may be around 1.75 mm. The gap (G) may comprise an axial component relative to the compressor wheel axis. The gap (G) may comprise a radial component relative to the compressor wheel axis. The gap (G) may comprise both a radial and an axial component relative to the compressor wheel axis. An axial distance between the distal end of the primary baffle and the end of the secondary baffle may define the gap (G). The axial distance between the distal end of the primary baffle and the end of the secondary baffle defines a gap (G) is relative to the compressor wheel axis. Where appropriate, any of the optional features discussed above in relation to one of the aspects of the invention, may be applied to another aspect of the invention. A detailed description of the invention will now be provided with reference to the accompanying drawings, in which: Figure 1 shows a cross-sectional view of a turbocharger according to an embodiment of the present invention; Figure 2 is a cross-sectional side view of a prior art compressor housing; Figure 3 is a schematic cross-section side view of a compressor housing according to the present invention; Figure 4 is an enlarged portion of the schematic cross-section side view of the compressor housing of Figure 3; Figure 5 is a chart showing how a change in a map width enhancement outlet channel affects efficiency; and Figure 6 shows an enlarged portion of a schematic cross-section side view of another compressor housing. 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 wheel 12. The turbine wheel 12 is disposed in a turbine wheel chamber 13. Similarly, the compressor 6 comprises a compressor housing 14 and a compressor wheel (impeller) 16. The compressor wheel 16 is disposed in a compressor wheel chamber 17. The turbine wheel 12 and compressor wheel 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 wheel 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 wheel 12. Rotation of the turbine wheel 12 rotates the compressor wheel 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 wheel 12, the shaft 18 and the compressor wheel 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 prior art compressor housing 102 for a compressor. The compressor housing comprises an inlet 104, a wheel chamber 106 and an outlet 108. The inlet 104 comprises a baffle 110, an outer wall 112, an end cap 114 and a retaining ring 116. The baffle 110 and outer wall 112 are integrally formed and may be said to define a compressor body, from which the end cap 114 and retaining ring 116 are separable. The baffle 110 is generally tubular and extends axially along a compressor wheel axis 118. The centre of the baffle 110 defines an inlet passage 120 configured to provide fluid to the compressor wheel chamber 106, and, in particular, to an inducer of a centrifugal impeller (omitted for clarity) contained within the wheel chamber 106. The outer wall 112 concentrically and circumferentially surrounds the baffle 110. The outer wall 112 is spaced apart from the baffle 110 to define a generally annular recirculation passage 122 therebetween. The end cap 114 is formed as a generally frusto-conical piece of thinwalled material. The end cap 114 comprises a radially outer edge that is received within a circumferentially extending stepped groove 124 of the outer wall 112. The retaining ring 116 is received within the stepped groove 124 to hold the end cap 114 in position within the inlet 104. The compressor wheel chamber 106 is defined in part by a wheel chamber surface 125. The wheel chamber surface 125 closely conforms to the geometry of the compressor wheel (impeller) so as to contain the working fluid so that it is compressed by the action of the compressor wheel blades. Accordingly, the wheel chamber surface 125 is generally trumpet-shaped. The baffle 110 comprises a first fluid communication passage 126 that is defined by a circumferential slot extending between the recirculation passage 122 and the impeller chamber surface 125. The first fluid communication passage 126 is positioned so that, during use, it is downstream of the leading edges of the blades of the compressor wheel (not shown). A second fluid communication passage 128 extends between the recirculation passage 122 and the inlet passage 120. The second fluid communication passage 128 is defined by an annular gap between the end cap 114 and a distal end of the baffle 110 which are spaced apart along the compressor axis 118 by a small amount. The second recirculation passage 128 is positioned upstream of the leading edges of the blades of the impeller during use. The baffle 110 is supported by three identical struts 130 that extend axially along the compressor axis 118. The struts 130 extend radially across the recirculation passage 122 to the baffle 110 and across the first fluid communication passage 126 so as to mechanically support the baffle 110 within the compressor inlet 104. The struts 130 are equi-spaced about the compressor axis 118 so that the centroid of a strut 130 is approximately 120 0 apart from the centroid of the next strut 130. During use, working fluid may enter the recirculation passage 122 from the compressor wheel chamber 106 via the first fluid communication passage 126. Accordingly, the first fluid communication passage 126 may be considered to be an inlet of the recirculation passage 122. Since the working fluid entering the recirculation passage 122 has been to some degree compressed by the action of the compressor wheel, the working fluid in the recirculation passage 122 has a higher pressure than the working fluid in the inlet passage 120. As a result, the working fluid leaves the recirculation passage 122 and reenters the inlet passage 120 via the second fluid communication passage 128. The second fluid communication passage 128 may therefore be considered to be an outlet of the recirculation passage 122. The above-described structure is an example of a so-called “map width enhancement” structure. By allowing a small amount of compressed fluid to escape from the compressor wheel (impeller), for a given pressure ratio, the minimum mass flow required to avoid surge events can be reduced, and hence the “width” of the operating map of the compressor 102 is said to be enhanced. Figure 3 shows a compressor housing 202 for a compressor. The compressor housing 202 comprises an inlet 204, a wheel chamber 206, and an outlet 208. The inlet 204 comprises an inlet passage 210. The inlet passage 210 is configured to receive a working fluid (e.g. air), that is drawn in by a compressor wheel (not shown) in the wheel chamber 206. The inlet passage 210 is defined in part by a generally annular outer wall 212 of the compressor housing 202, and by a generally annular primary baffle 214. The primary baffle 214 is spaced apart from the outer wall 212, with the outer wall 212 concentrically surrounding the primary baffle 214. The primary baffle 214 is integrally formed with the compressor housing 202 and is supported by one or more struts, which are out of plane in this view, but are shown in Figure 4. The primary baffle 214, may be supported by any suitable means, by way of example, in some embodiments, the primary baffle 214 may be held in a retaining means; and in other embodiments, the primary baffle may not be integrally formed with the compressor housing, it may instead be an insert. The wheel chamber 206, is defined in part by a wheel chamber surface 216. The wheel chamber surface 216 closely conforms in shape to the geometry of the compressor wheel, so as to contain the working fluid (usually air) that is compressed by the action of the compressor wheel blades. Accordingly, the wheel chamber surface 216 is generally trumpet shaped. The inlet passage 210 is in fluid communication with the wheel chamber 206 and is configured to provide the working fluid to the compressor wheel that is contained within the wheel chamber 206. The wheel chamber 206 is also in fluid communication with the outlet 208, where working fluid that is compressed by the compressor wheel is delivered to the outlet 208. The primary baffle 214 comprises a first end 226 that is proximal the wheel chamber 206, and a second end 228 that is axially spaced from the wheel chamber 206. A wall 230 of the primary baffle extends between the first end 226 and second end 228. A recirculation passage 218 is defined between the primary baffle 214 and the outer wall 212. A first channel 220, is defined by an opening between the first end 226 of the primary baffle 214, and an end 232 of the wheel chamber surface 216. In other embodiments, the first channel 220 may be a slot in the primary baffle 214. The first channel extends between the inlet passage 210 and the recirculation passage 218. The first channel 220 is positioned so that so that, during use, it is downstream of the leading edges of the compressor wheel blades (not shown). The recirculation passage 218, is further defined, in part by a secondary baffle 222. The secondary baffle 222, is generally annular, and extends in a radial direction from the outer wall 212 towards a compressor wheel axis 224, the secondary baffle 222 also extends in an axial direction relative to the compressor wheel axis 224. The secondary baffle 222 has an end 236 that is proximal and axially spaced from the second end 228 of the primary baffle 214. The secondary baffle 222 extends from the outer wall 212 of the compressor housing towards the primary baffle 214. The secondary baffle 222 is integrally formed with the outer wall 212 of the compressor housing 202, however, in other embodiments the secondary baffle 222 may be formed as a separate component to the outer wall 212 and then connected thereto. A second channel 234 is defined between the end 236 of the secondary baffle 222 and the second end 228 of the primary baffle 214 The second channel 234 is a generally annular channel, but it may be any suitable shape. The second channel 234 is positioned upstream of the leading edges of the blades of the compressor wheel during use. The second channel 228 permits fluid flow from the recirculation passage 218 to the inlet passage 210. During use, working fluid (e.g. air) may enter the recirculation passage 218 from the compressor wheel chamber 206 via the first channel 220. Accordingly, the first channel 220 may be considered to be an inlet of the recirculation passage 218. Since the working fluid in the recirculation passage 218 has been, to some degree, compressed by the action of the compressor wheel, the working fluid in the recirculation passage 218 has a higher pressure than the working fluid in the inlet passage 210. As a result, the working fluid leaves the recirculation passage 218 and re-enters the inlet passage 210 via the second channel 228. The second channel 228 may therefore be considered to be an outlet of the recirculation passage 218. The geometry of the primary baffle, secondary baffle and compressor housing will be further described, with reference to Figure 4. Figure 4 shows an enlarged view of the portion 240 indicated in Figure 3, taken through a different cross-sectional plane so that a strut 242 which supports the primary baffle 214 can be seen. A distance between the compressor wheel axis 224 and the radially outermost point 244 of the end of the secondary baffle defines a first radius (RB). A distance between the compressor wheel axis 224 and a radially innermost point 246 of the wall 230 of the primary baffle 214 defines a second radius (Rw). A distance between the compressor wheel axis 224 and a radially innermost point 248 of the wall 230 of the primary baffle at a maximum axial distance from the compressor wheel chamber 206 (not labelled in Figure 4), defines a third radius (Rd). The first radius (RB) is greater than the second radius (Rw), and the first radius (RB) is less than or equal to the third radius (Rd). It would not be desirable to have the first radius Rb equal to the second radius Rw, because this would reduce the inlet diameter of the compressor, and reduce the working fluid intake. Further, in the embodiment shown in Figure 4, a distance between the distal end of the primary baffle 214 and the end of the secondary baffle 222 defines a gap (G). The gap (G) is the minimum distance between the primary baffle 214 and the radially outermost point 244 of the end of the secondary baffle 222. In the embodiment shown in Figure 4, the minimum distance between the primary baffle 214 and the radially outermost point 244 of the end of the secondary baffle 222 is the distance between radially innermost point 248 of the wall 230 of the primary baffle at a maximum axial distance from the compressor wheel chamber 206 and the radially outermost point 244 of the end of the secondary baffle 222. In the embodiment shown in Figure 4, the gap (G) is thus the axial distance between the distal most end of the primary baffle 214 and the end of the secondary baffle 222 that is closest to the distal most end of the primary baffle 228. That is to say, in the embodiment shown in Figure 4 the gap (G) is entirely axial, as it comprises an axial component but no radial component. In other embodiments, as shown and described below in relation to Figure 6, the gap (G) may not be an entirely axial gap and may comprise both axial and radial components. A ratio (G / Rw) of the gap (G) to the second radius (Rw) is around 0.014 to around 0.133. That is to say, that for compressors with an inner diameter of around 140 mm (e.g. Rw is around 70 mm) to around 30 mm (e.g. Rwis around 15 mm), the gap (G) is between around 1 mm and 2mm. Figure 5 is a chart showing how as the width of the gap (G) changes for two compressors (each having a different diameter) the difference from the target best efficiency point varies. As would be appreciated by the skilled person, the target best efficiency is the efficiency in the compressor map region, between the surge line and peak efficiency area of the map, where compressor performance is critical to engine performance. As can be seen in Figure 5, the compressor with a first inner diameter (illustrated by a dashed line and diamond markers, on the chart in Figure 5 and labelled “Compressor-First Diameter), the theoretical peak efficiency is achieved when the gap (G) is between around 1.5 mm to around 2mm. When the gap (G) increases beyond around 2 mm, the difference from the theoretical peak efficiency reduces. Similarly, when the gap (G) decreases between around 1.5 mm and around 1 mm, the difference from the theoretical peak efficiency also decreases. Further, when considering another compressor, with a second inner diameter that is different in size to the first inner diameter (illustrated by a solid line and ‘X’ markers, on the chart in Figure 5 and labelled “Compressor- Second Diameter). In particular, the second inner diameter is around 1.4 times greater than the first inner diameter it can be seen that the theoretical peak efficiency is also achieved when the gap (G) is between around 1.5 mm to around 2 mm. When the gap (G) increases beyond 2mm to around 4 mm, the difference from the theoretical peak efficiency reduces by 1 point. Likewise, as seen with the compressor with the first inner diameter, as the gap (G) decreases below 1.5 mm, the difference from the theoretical peak efficiency also decreases. Providing a compressor, irrespective of size, with a gap (G) in the range of around 1 mm to around 2.5 mm, when the first radius (Rb) is greater than the second radius (Rw), and the first radius (Rb) is less than or equal to the third radius (Rd), provides an unexpected benefit of improved efficiency that is not available outside of this range. This improved efficiency is particularly notable when the gap (G) is in the range of around 1 mm to around 2 mm. It is further surprising, that having a gap (G) of between around 1mm to around 2 mm provides a benefit in improved efficiency across compressors with different sized inner diameters. The gap (G) may be between 1.15 mm and around 1.75 mm. The gap (G) may be between around 1.15 mm and around 1.5 mm. The gap (G) may be between around 1.5 mm and around 1.75 mm. The gap (G) may be around 1 mm. The gap (G) may be around 1.15 mm. The gap (G) may be around 1.5 mm. The gap (G) may be around 1.75 mm. The gap (G) may be around 2 mm. Further, where the compressor has an inner diameter of between around 140 mm to around 30 mm (i.e. where the second radius Rw is between around 70 mm to around 15 mm), the ratio of the gap (G) to the second radius (Rw) is around 0.014 to around 0.133. The ratio of the gap (G) to the second radius (Rw) may be around 0.029 to around 0.066. The ratio of the gap (G) to the second radius (Rw) may be around 0.016 to around 0.116. The ratio of the gap (G) to the second radius (Rw) may be around 0.021 to around 0.1. The ratio of the gap (G) to the second radius (Rw) may be around 0.025 to around 0.076. The ratio of the gap (G) to the second radius (Rw) may be around 0.014, 0.016, 0.034, 0.046, 0.054, 0.021, 0.025, 0.029, 0.066, 0.068, 0.076, 0.091 0.1,0.109, 0.116, or 0.133. Returning to Figure 4, along a portion of the primary baffle 214, the distance of the wall 230 of the primary baffle 214 from the compressor wheel axis 224 varies along the axial length of the primary baffle 214. That is to say, that the wall 230 of the primary baffle 214 comprises a region 249 towards the compressor wheel chamber 206, where the distance between the wall 230 and the compressor wheel axis 224 remains constant. The primary baffle 214, further comprises a variable profile portion 250, whereby the radial distance between the compressor wheel axis 224 and the wall 230 varies. In the embodiment shown in Figure 4, in the variable profile portion 250 the radial distance between the compressor wheel axis 224 and the wall 230 increases along the length of the wall 230 in a direction away from the compressor wheel chamber 206. The variable profile portion 250 starts at the point marked 246 on Figure 4 and ends at the point marked 248. The variable profile portion 250 terminates at the second end 228 of the primary baffle 214. The variable profile portion 250, in particular, tapers in a direction away from the compressor wheel axis 224, in an axial direction away from the compressor wheel chamber 206. It will, however, be appreciated that in other embodiments, the variable profile portion 250 may taper towards the compressor wheel axis 224, and in some embodiments, may comprise one or more regions that taper away from the compressor wheel axis and one or more regions that taper towards the compressor wheel axis 224. The variable profile portion 250 is a generally annular curved surface in the embodiment shown in Figure 4. In other embodiments, the variable profile portion 250, and the remainder of the primary baffle 214 may take any suitable shape. By way of example, in some embodiments, the primary baffle 214 may comprise one or more stepped portions. Put another way, the primary baffle 214 may comprise an inclined portion relative to the compressor wheel axis 224. The primary baffle 214 having a non-uniform profile relative to the compressor wheel axis 224 may aid in returning working fluid to the inlet passage 210 through the second channel 234 whilst reducing flow disturbance to a primary flow in the inlet passage 210. The second end 228 of the primary baffle 214 further comprises a radially extending portion 252. Accordingly, in this embodiment, the third radius Rd, is the radial distance between the compressor wheel axis 224 and the radially innermost point 248 of the radially extending portion 252. The radially extending portion 252 partly defines the second channel 234. The radially extending portion thus aids in directing working fluid that has been compressed from the recirculation passage 218 through the second channel 234 to the inlet passage 210. It will be appreciated, that in other embodiments the primary baffle 214, may not comprise a radially extending portion 252. In a similar manner, the end 236 of the secondary baffle 222 also comprises a radially extending portion 254, that extends parallel to the radially extending portion 252 of the primary baffle 214. The radially extending portion 252 of the primary baffle 214 partly defines the second channel 234, and thus also aids in directing working fluid from the recirculation passage 218 through the second channel 234 to the inlet passage 210. It will be appreciated that, in other embodiments, the end 236 of the secondary baffle 222, may not comprise a radially extending portion 254, by way of example, the end 236 of the secondary baffle 222 may extend to a point. The radially extending portion 254 of the secondary baffle 222 and the radially extending portion 252 of the primary baffle 214 may not have the same radial extent. For example, the radial extent of the radially extending portion 252 of the primary baffle 214 may be less than or greater than the radial extend of the radially extending portion 254 of the secondary baffle 222. The radially extending portion 254 of the secondary baffle 222 and the radially extending portion 252 of the primary baffle 214 may, in some embodiments, start at the same radial distance from the compressor wheel axis 224. In other embodiments, the radially extending portion 254 of the secondary baffle 222 may start at a distance closer to the compressor wheel axis 224 that the radially extending portion 252 of the primary baffle (as is the case in the embodiment shown in Figure 4), or vice versa. Figure 6 shows an enlarged portion 340 of a schematic cross-section side view of another compressor housing 302. The portion 340 of the compressor housing 302 differs only from the compressor housing 202 in Figures 3 and 4 in that it comprises a secondary baffle 322 with a different profile. For ease of understanding, only the differences between the compressor housings 202 and 302 will be described. Features of the compressor housing 302 which are the same as the compressor housing 202 are provided with the same reference signs, and for differing but corresponding features the reference signs are increased by 100. As with the compressor housing 202 in Figures 3 and 4, a distance between the compressor wheel axis 224 and the radially outermost point 344 of the end 336 of the secondary baffle 322 defines a first radius (Rb). A distance between the compressor wheel axis 224 and a radially innermost point 246 of the wall 230 of the primary baffle 214 defines a second radius (Rw). A distance between the compressor wheel axis 224 and a radially innermost point 248 of the wall 230 of the primary baffle at a maximum axial distance from the compressor wheel chamber 206 (not labelled in Figure 4), defines a third radius (Rd). The first radius (Rb) is greater than the second radius (Rw), and the first radius (Rb) is less than or equal to the third radius (Rd). It would not be desirable to have the first radius Rb equal to the second radius Rw, because this would reduce the inlet diameter of the compressor, and reduce the working fluid intake. In the embodiment shown in Figure 6, and as with the embodiment in Figures 3 and 4, the gap (G) is defined by the minimum distance between the primary baffle 214 and the radially outermost point 344 of the end 336 of the secondary baffle 322. As can be seen in Figure 6, because the secondary baffle 322 has a greater radial and axial extent compared to the baffle 222 in Figure 4, the gap (G) differs in that it is not entirely axial, i.e. the gap (G) has both an axial and a radial component relative to the compressor wheel axis 224. As with the embodiment shown in Figures 3 and 4, a ratio (GI Rw) of the gap (G) to the second radius (Rw) is around 0.014 to around 0.133. That is to say, for compressors with an inner diameter of around 140 mm (e.g. Rw is around 70 mm) to around 30 mm (e.g. Rwis around 15 mm), the gap (G) is between around 1 mm and 2 mm. The various ratios of (GI Rw) described above in relation to earlier embodiments therefore also apply to the embodiment of Figure 6. It will be appreciated that features of the compressor housing 202 shown in Figures 3 and 4 can be combined with features of the compressor housing 302 shown in Figure 6 and vice versa. With respect to any of the embodiments above, it will be appreciated that in alternative embodiments a compressor housing according to the present invention may comprise substantially any number of first and second fluid channels. For example, instead of single channels, a series of radially extending holes or the like may alternatively be used.

Claims

1. A compressor housing for a compressor, the compressor housing comprising:an inlet, the inlet comprising:an inlet passage configured to receive intake fluid, the inlet passage being defined at least in part by a primary baffle; anda recirculation passage defined between the primary baffle and an outer wall of the compressor housing;a wheel chamber in fluid communication with the inlet passage, and configured to receive a compressor wheel supported for rotation about a compressor wheel axis;an outlet in fluid communication with the wheel chamber;a first channel extending between the wheel chamber and the recirculation passage configured to permit fluid flow from the wheel chamber to the recirculation passage;a second channel extending between the recirculation passage and the inlet passage configured to permit fluid flow from the recirculation passage to the inlet passage;the primary baffle comprising a first end proximal the wheel chamber, a second end distal from the wheel chamber, and a wall extending between the first end and the second end;a secondary baffle defining at least part of the recirculation passage, the secondary baffle having an end proximal and axially spaced from the second end of the primary baffle, where a distance between the compressor wheel axis and the radially outermost point of the end of the secondary baffle defines a first radius (Rb);wherein a distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle defines a second radius (Rw);wherein a distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle at a maximum axial distance from the compressor wheel chamber, defines a third radius (Rd);wherein the first radius (Rb) is greater than the second radius (Rw), and the first radius (Rb) is less than or equal to the third radius (Rd);wherein a minimum distance between the primary baffle and the end of the secondary baffle defines a gap (G); andwherein a ratio (GI Rw) of the gap (G) to the second radius (Rw) is around 0.014 to around 0.133.

2. A compressor housing according to claim 1, wherein the gap (G) comprises an axial component relative to the compressor wheel axis.

3. A compressor housing according to claim 1 or 2, wherein the gap (G) comprises a radial component relative to the compressor wheel axis.

4. A compressor housing according to claim 1 or claim 2, wherein an axial distance between the distal end of the primary baffle and the end of the secondary baffle defines the gap (G).

5. A compressor housing according to any preceding claim, wherein the wall of the primary baffle comprises a variable profile portion, where along a length of the variable profile portion the radial distance between the compressor wheel axis and the wall varies.

6. A compressor housing according to claim 5, where the variable profile portion terminates at the second end of the primary baffle.

7. A compressor housing according to claim 5 or claim 6, wherein the variable profile portion comprises a tapered portion relative to the compressor wheel axis.

8. A compressor housing according to any of claims 5 to 7, wherein the variable profile portion comprises a curved surface.

9. A compressor housing according to any of claims 5 to 8, wherein the variable profile portion comprises an inclined portion relative to the compressor wheel axis.

10. A compressor housing according to any preceding claim, wherein the second end of the primary baffle comprises a radially extending portion, such that the third radius (Rd) is the radial distance between the compressor wheel axis and the radially innermost point of the radially extending portion of the second end of the primary baffle.

11. A compressor housing according to claim 10, when dependent on claim 6, wherein the radially extending portion of the second end of the primary baffle extends from where the variable profile portion terminates.

12. A compressor housing according to any preceding claim, wherein the ratio of the gap (G) and the second radius (Rw) is between around 0.029 to around 0.066.

13. A compressor housing according to any preceding claim, wherein the gap (G) is between around 1 mm and around 2 mm.

14. A compressor housing according to claim 13, wherein the gap (G) is between around 1.15 mm and around 1.75 mm.

15. A compressor housing according to claim 14, wherein the gap (G) is around 1.5 mm.

16. A compressor housing according to any preceding claim, wherein the end of the secondary baffle comprises a radially extending portion.

17. A compressor housing according to any preceding claim, wherein the secondary baffle extends from the outer wall of the compressor housing towards the primary baffle.

18. A compressor comprising the compressor housing according to any of claims 1 to 17.

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

20. A compressor housing for a compressor, the compressor housing comprising:an inlet, the inlet comprising:an inlet passage configured to receive intake fluid, the inlet passage being defined at least in part by a primary baffle; anda recirculation passage defined between the primary baffle and an outer wall of the compressor housing;a wheel chamber in fluid communication with the inlet passage, and configured to receive a compressor wheel supported for rotation about a compressor wheel axis;an outlet in fluid communication with the wheel chamber;a first channel extending between the wheel chamber and the recirculation passage configured to permit fluid flow from the wheel chamber to the recirculation passage;a second channel extending between the recirculation passage and the inlet passage configured to permit fluid flow from the recirculation passage to the inletnQCCQflQ'pdoodyc,the primary baffle comprising a first end proximal the wheel chamber, a second end distal from the wheel chamber, and a wall extending between the first end and the second end;a secondary baffle defining at least part of the recirculation passage, the secondary baffle having an end proximal and axially spaced from the second end of the primary baffle, where a distance between the compressor wheel axis and the radially outermost point of the end of the secondary baffle defines a first radius (RB);wherein a distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle defines a second radius (Rw);wherein a distance between the compressor wheel axis and a radially innermost point of the wall of the primary baffle at a maximum axial distance from the compressor wheel chamber, defines a third radius (Rd);wherein the first radius (Rb) is greater than the second radius (Rw), and the first radius (Rb) is less than or equal to the third radius (Rd);wherein a minimum distance between the primary baffle and the end of the secondary baffle defines a gap (G); andwherein the gap (G) is between around 1 mm and around 2 mm.

21. A compressor housing according to claim 20, wherein the gap (G) comprises an axial component relative to the compressor wheel axis.

22. A compressor housing according to claim 20 or 21, wherein the gap (G) comprises a radial component relative to the compressor wheel axis.

23. A compressor housing according to claim 20 or 21, wherein an axial distance between the distal end of the primary baffle and the end of the secondary baffle defines the gap (G).

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