Abstract:
A two-stage radial compressor having a ported second-stage shroud, forming a passive surge control system. Thus, some proportion of the fluid is allowed to flow between the second-stage of the compressor in the vicinity of the impeller blade tips, through a port in the shroud into an upstream portion of the compressor flow path that leads into the second-stage impeller inlet. The fluid is allowed to flow in either direction, depending on the pressure difference existing between the two locations. Surge margin at higher pressure ratios is significantly increased by the ported second-stage shroud.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is related to U.S. patent application Ser. No. 11/004,467, filed on Dec. 3, 2004, currently pending. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to two-stage radial compressors and to turbochargers that include such compressors.  
         [0003]     Radial compressors are used in various types of turbomachinery, including turbochargers for internal combustion engine systems. A radial compressor generally includes at least one compressor stage formed by a rotating impeller mounted on a shaft within a compressor housing. The housing defines an inlet flow path that typically leads into the impeller in a generally axial direction. The impeller includes a hub and a plurality of blades spaced about its circumference and extending out from the hub. The impeller is configured to receive fluid in the axial direction and to compress the fluid and discharge the fluid in a generally radially outward direction into a volute defined by the compressor housing. The housing includes a wall or shroud that extends proximate the tips of the impeller blades and, together with the hub of the impeller, defines the main flow path through the impeller.  
         [0004]     In some applications requiring pressure ratios above that achievable by a single-stage radial compressor, two-stage radial compressors are employed. A second stage is formed by a second-stage impeller, which receives the fluid from the first-stage impeller and further compresses it to a higher pressure. Examples of two-stage radial compressors are described in U.S. Pat. Nos. 6,062,028 and 6,834,501, the disclosures of which are incorporated herein by reference.  
         [0005]     In any compressor, there is a limit to the pressure ratio that can be attained at a given flow rate before surge occurs; the locus of points at which surge occurs, as a function of flow, is referred to as the surge line on the compressor map. There is also a limit to how much flow can be passed through the compressor before choking occurs somewhere in the compressor. The useful operation range of the compressor is defined between the surge line and the flow rate at choke. It is desirable to have a wide range of operation. In particular, on a compressor map of pressure ratio versus flow rate, it is desirable to push the surge line as far toward the upper left-hand corner of the map as possible. Many different approaches for controlling surge in compressors have been proposed over the years. Some approaches involve relatively complicated active control systems using feedback control techniques and/or variable-geometry mechanisms in the compressor. For many applications, such as turbochargers, such complex approaches are not practical.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The present invention addresses the above needs and achieves other advantages by providing a two-stage radial compressor having a simple passive surge control system. It has been discovered that surge in a two-stage radial compressor can be delayed to lower flows and higher pressure ratios by allowing some proportion of the fluid to flow between the second-stage of the compressor in the vicinity of the impeller blade tips and an upstream portion of the compressor flow path that leads into the second-stage impeller inlet. The fluid is allowed to flow in either direction, depending on the pressure difference existing between the two locations. Thus, at some operating conditions at which the pressure at the impeller blade tips is higher than that at the upstream flow path location, the fluid will flow in a recirculating fashion from the blade tip region to the upstream location. At other operating conditions, the pressure gradient may be opposite and the fluid may flow in the opposite direction.  
         [0007]     The second-stage of the compressor includes a second-stage shroud that forms the radially outer wall of the compressor flow path through the second stage and extends proximate the tips of the blades. The shroud includes at least one port through which fluid can pass. A flow passage is defined by the compressor housing, leading from the port back into the flow path upstream of the second-stage impeller inlet. The compressor housing can have various geometries for conducting the fluid from the first-stage impeller into the second-stage impeller, and the particular configuration for providing fluid communication between the upstream flow path and the flow passage connected to the shroud port(s) depends on the compressor housing geometry.  
         [0008]     In one embodiment of the invention, the compressor includes a vane assembly in the inlet flow path of the second-stage impeller, the vane assembly comprising a wall and a plurality of circumferentially spaced vanes affixed to the wall. The wall of the vane assembly defines at least one opening connected with the flow passage that leads from the second-stage shroud port(s) into the inlet flow path such that the inlet flow path is in fluid communication with the flow passage via such opening(s). Advantageously, the wall of the vane assembly defines a plurality of such openings spaced circumferentially apart.  
         [0009]     In another embodiment of the invention, the wall of the vane assembly is connected to the compressor housing such that a gap exists between an edge of the wall and the compressor housing, the gap being connected with the flow passage that leads from the second-stage shroud port(s) into the inlet flow path.  
         [0010]     In still another embodiment of the invention, the compressor housing defines a first-stage volute that receives fluid from the first-stage impeller, the first-stage and second-stage volutes each extending circumferentially at least partially about the first-stage and second-stage impellers, respectively. The compressor includes an interstage duct for conducting fluid from the first-stage volute to the second-stage inlet flow path. The interstage duct comprises first and second conduits connected at circumferentially spaced positions to the first-stage volute, the first and second conduits passing radially outward of the second-stage volute and then extending radially inwardly and connecting at circumferentially spaced positions to the inlet flow path of the second-stage impeller. The opening(s) into the inlet flow path of the second stage are formed in the walls of the first and second conduits, or as gaps between such walls and other portions of the compressor housing.  
         [0011]     In addition to the ported second-stage shroud, a compressor in accordance with some embodiments of the invention can also include a ported first-stage shroud.  
         [0012]     The invention also encompasses turbochargers having a two-stage radial compressor as described herein, as well as a method for enhancing performance of a two-stage radial compressor. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0013]     Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:  
         [0014]      FIG. 1  is a cross-sectional view of a turbocharger in accordance with one embodiment of the present invention;  
         [0015]      FIG. 2  is a perspective view of the vane assembly of the turbocharger of  FIG. 1 ;  
         [0016]      FIG. 3  is a compressor map obtained from testing of a two-stage radial compressor having non-ported shrouds in accordance with the prior art;  
         [0017]      FIG. 4  is a compressor map obtained from testing of a two-stage radial compressor differing from the compressor of  FIG. 3  only in that the second-stage shroud is ported;  
         [0018]      FIG. 5  is a schematic cross-sectional view of a two-stage compressor in accordance with a first embodiment of the invention;  
         [0019]      FIG. 6  is a schematic cross-sectional view of a two-stage compressor in accordance with a second embodiment of the invention;  
         [0020]      FIG. 7  is a schematic cross-sectional view of a two-stage compressor in accordance with a third embodiment of the invention;  
         [0021]      FIG. 8  is a schematic cross-sectional view of a two-stage compressor in accordance with a fourth embodiment of the invention;  
         [0022]      FIG. 9  is a view into the compressor housing of  FIG. 8 , as viewed toward the left in  FIG. 8 ;  
         [0023]      FIG. 10  is a schematic cross-sectional view of a two-stage compressor in accordance with a fifth embodiment of the invention;  
         [0024]      FIG. 11  is a view into the compressor housing of  FIG. 10 , as viewed toward the left in  FIG. 10 ;  
         [0025]      FIG. 12  is a schematic cross-sectional view of a two-stage compressor in accordance with a sixth embodiment of the invention; and  
         [0026]      FIG. 13  is a view into the compressor housing of  FIG. 12 , as viewed toward the left in  FIG. 12 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.  
         [0028]      FIG. 1  shows a turbocharger  10  having a two-stage compressor in accordance with one embodiment of the invention. The turbocharger  10  has a configuration generally as described in U.S. Pat. No. 6,834,501, the disclosure of which is incorporated herein by reference. The turbocharger  10  includes a rotary shaft  12  on one end of which a turbine wheel  13  is mounted. The turbine section of the turbocharger  10  includes a turbine housing  14  that defines a turbine volute  15  arranged to direct fluid to the turbine wheel. The turbine housing also defines an outlet  16 . Exhaust gases from an engine (not shown) are fed into the turbine volute  15 . The gases then pass through the turbine and are expanded so that the turbine wheel  13  is rotatably driven, thus rotatably driving the shaft  12 . The expanded gases are discharged through the outlet  16 . The turbine can be a radial turbine in which the flow enters the turbine in a generally radially inward direction; however, the invention is not limited to any particular turbine arrangement. Furthermore, the turbocharger could include means other than a turbine for driving the shaft  12 , such as an electric motor.  
         [0029]     The shaft  12  passes through a center housing  17  of the turbocharger. The center housing connects the turbine housing  14  with a compressor housing assembly  28  of the turbocharger as further described below. The center housing contains bearings  18  for the shaft  12 . A rear end of the compressor housing assembly  28  is affixed to the center housing  17  in suitable fashion, such as with threaded fasteners or the like.  
         [0030]     Mounted on an opposite end of the shaft  12  from the turbine is a two-stage compressor wheel comprising a first-stage impeller  24  and a second-stage impeller  26 . Surrounding the compressor wheel is the compressor housing assembly  28 . A forward portion of the compressor housing assembly defines a compressor inlet  30  leading into the first-stage impeller  24 . As further described below, a rear portion of the compressor housing assembly defines a series of flow paths for leading the pressurized fluid that exits the first-stage impeller into the second-stage impeller and for receiving and discharging the pressurized fluid that exits the second-stage impeller.  
         [0031]     More particularly, the rear portion of the compressor housing assembly defines: a first-stage diffuser  32  that receives the fluid discharged from the first-stage impeller and diffuses (i.e., reduces the velocity and hence increases the static pressure of) the fluid; an interstage duct  34  that receives the fluid from the first-stage diffuser  32 ; an arrangement  36  of deswirl vanes that receive the fluid from the interstage duct and reduce the tangential or “swirl” component of velocity of the fluid, as well as lead the fluid into the second-stage impeller  26 ; a second-stage diffuser  33  that receives the fluid discharged from the second-stage impeller and diffuses the fluid; and a second-stage volute  38  that receives the fluid from the second-stage diffuser and surrounds the second-stage impeller. Although not visible in  FIG. 1 , and as further described below, the compressor housing assembly also defines a discharge duct that connects with the second-stage volute  38  and routes the fluid from the volute out of the compressor for feeding to the engine intake manifold or to a charge air cooler before being fed to the engine intake manifold.  
         [0032]     The first-stage impeller  24  and second-stage impeller  26  are mounted back-to-back; that is, the downstream side of the first-stage impeller  24  is nearer the turbine than is the upstream side of the impeller, while the downstream side of the second-stage impeller  26  is farther from the turbine than is the upstream side of the impeller. The second-stage volute  38  is located generally concentrically within the interstage duct  34 . More specifically, the interstage duct  34  is a generally annular structure formed by an outer wall  40  that extends substantially 360 degrees about a central axis of the interstage duct (which axis generally coincides with the axis of the shaft  12 , although it does not have to so coincide), and an inner wall  42  that extends substantially 360 degrees about the duct axis and is spaced radially inwardly from the outer wall  40 . The interstage duct  34  defined between the inner and outer walls is generally U-shaped in cross-section such that fluid entering the duct is flowing generally radially outwardly (i.e., with little or no axial component, although it does have a substantial swirl component); the duct then turns the fluid so that it is flowing generally axially (again, with substantial swirl component, but with little or no radial component), and finally turns the fluid to a generally radially inward direction (with little or no axial component, but with substantial swirl component) as the fluid enters the deswirl vane arrangement  36 . The second-stage volute  38  is located generally concentric with and radially inward of the inner wall  42  of the interstage duct. The volute  38  is delimited at its radially outward side by the inner wall  42 , and at its radially inward side by an extension  44  of the wall  42 .  
         [0033]     The first-stage diffuser  32  is defined between the forward portion of the compressor housing assembly  28  and a stationary seal plate  46 . The seal plate separates the diffuser  32  from the second-stage volute  38  and also forms the forward wall of the second-stage diffuser  33 . The seal plate engages the compressor wheel with a suitable rotating sealing surface to prevent higher-pressure air discharged from the second-stage impeller from leaking into the lower-pressure first-stage diffuser  32 . Other types of seal arrangements can be used instead of the arrangement illustrated in  FIG. 1 .  
         [0034]     The deswirl vane arrangement  36  includes a ring  54  of generally annular form. With reference to  FIG. 2 , the vane ring  54  comprises a plurality of deswirl vanes  56  that are spaced apart about a circumference of the ring. The vanes  56  are oriented generally radially with respect to the axis of the compressor. The vanes are cambered and arranged in such a way that the leading edges of the vanes (at the outer diameter of the ring) are directed generally in the same direction as the swirling flow entering the vanes from the interstage duct, while the trailing edges (at the inner diameter of the ring) are directed substantially in the direction in which it is desired for the flow to exit the vanes, i.e., with little or no swirl component of velocity. The vanes thus reduce the swirl component of velocity before the flow enters the second-stage impeller.  
         [0035]     The vanes  56  are affixed to (and can be integrally formed with) a wall  58  of generally annular form that extends generally radially with respect to the compressor axis. The axial extent of each vane  56  is oriented generally perpendicular to the wall  58 . As shown in  FIG. 1 , a radially inner end of the wall  58  engages the inward extension  44  of the wall of the second-stage volute  38  and an O-ring or the like (not shown) is arranged therebetween for sealing this connection.  
         [0036]     The compressor housing includes a first-stage shroud  60  that extends circumferentially about the first-stage impeller  24  closely adjacent to the tips of the blades of the impeller; the main flow path through the first-stage impeller is defined between the first-stage shroud and the hub of the impeller. The housing also includes a second-stage shroud  62 , formed by the aforementioned inward extension  44  of the housing wall  42 , that extends circumferentially about the second-stage impeller  26  closely adjacent to the tips of the blades of the impeller; the main flow path through the second-stage impeller is defined between the second-stage shroud and the impeller hub.  
         [0037]     The upstream portion of each impeller that the fluid first encounters is often referred to as the inducer of the impeller. When the flow rate through the compressor is reduced while maintaining pressure ratio at a relatively high level, at some point the surge line of the compressor map is encountered. Surging at relatively high pressure ratios typically occurs because of stalling of the inducer of one or both impellers, wherein the flow at the blade tips of the inducer begins locally to recirculate, thereby reducing the effective flow area of the inducer. In contrast, below a certain pressure ratio, surging typically is the result of stalling of one or both diffusers. The surge line of many compressors has a kink or “knee” above which surging is caused by inducer stall, and below which surging is caused by diffuser stall.  
         [0038]     The present invention particularly addresses surging above the knee caused by inducer stall. In accordance with the invention, a ported second-stage shroud  62  is employed in order to delay the onset of inducer stall of the second-stage impeller to higher pressure ratios at flow (or, stated differently, to lower flows at pressure ratio). Thus, with particular reference to  FIG. 5 , the second-stage shroud  62  includes at least one port  64  that extends through the shroud into a flow passage  66  defined in the compressor housing. Advantageously, the shroud includes a plurality of ports  64  spaced circumferentially about the shroud, and the flow passage  66  is a generally annular cavity defined in the compressor housing surrounding the second-stage impeller  26 . In the embodiment of  FIG. 5 , the flow passage  66  is defined by a recess or cavity in the wall of the housing that forms the second-stage volute  38 ; that recess is closed by the wall  58  of the vane arrangement  36 . The wall  58  at its radially inner end engages the second-stage shroud  62  and an O-ring or other seal (not shown) is disposed therebetween for sealing the interface.  
         [0039]     The wall  58  includes at least one opening  68 , and preferably includes a plurality of circumferentially spaced openings  68  as shown in  FIG. 2 , that connect the flow passage  66  with the inlet flow path through the vane arrangement  36 . The openings  68  can be located between adjacent vanes  56  as shown. As a result, fluid can pass from the inducer region of the second-stage impeller through the ports  64  in the second-stage shroud  62  into the flow passage  66 , and through the openings  68  back into the inlet flow path of the second-stage impeller. Fluid can also pass in the reverse direction from the inlet flow path through the openings  68  into the flow passage  66  and through the ports  64  into the inducer of the second-stage impeller. The direction of flow depends on the sense of the pressure gradient between the inducer location and the upstream inlet flow path location.  
         [0040]     At higher pressure ratios, where inducer stall of the second-stage impeller  26  would ordinarily begin to occur, it has been found that the ported second-stage shroud (in comparison with an otherwise identical non-ported shroud) delays the onset of surge. It is believed that at near-surge conditions the ported shroud allows fluid to pass into the flow passage  66  and through the openings  68  back into the inlet flow path, and thereby prevents or reduces the local flow recirculation in the inducer tip region that normally attends inducer stall and surge.  
         [0041]     As shown in  FIG. 1 , a two-stage compressor in accordance with the invention can also include a ported first-stage shroud  60 . Thus, the first-stage shroud can include one or more ports  61  that connect with a flow passage  63  that leads back to the compressor inlet  30 . Alternatively, the compressor can have a non-ported first-stage shroud.  
         [0042]     To determine the effectiveness of the second-stage ported shroud  62 , a series of tests were conducted.  FIG. 3  is a compressor map obtained from testing of a two-stage radial compressor configured generally as depicted in  FIG. 1 , but having non-ported first-stage and second-stage shrouds. For comparison,  FIG. 4  shows a compressor map obtained from testing of a compressor configured generally as in  FIG. 1 , having the ported second-stage shroud  62  and non-ported first-stage shroud. With reference to  FIG. 3 , it can be seen that the prior-art compressor with non-ported shrouds has a kink or knee in the surge line at a pressure ratio of about 2.3 and a corrected flow of about 28 lb/min. Below the knee, it is believed the surge line is limited by diffuser stall. Above the knee, it is believed that surge is limited by inducer stall, and particularly inducer stall of the second-stage impeller. This appears to be borne out by the test results. Below the knee, the surge line of the compressor with ported second-stage shroud ( FIG. 4 ) is substantially the same as that of the compressor with non-ported shroud ( FIG. 3 ). Above the knee, however, the surge line of the compressor with ported second-stage shroud is substantially higher than that of the compressor with non-ported shroud. The compressor map with ported second-stage shroud has a substantially wider range of operation as measured by the ratio of the flow at choking to the flow at surge. For instance, at a pressure ratio of 3.5, for the compressor with non-ported shroud, the flow at choking is approximately 124 lb/min and the flow at surge is about 68 lb/min, yielding a choke/surge flow ratio of about 1.8. In contrast, at the same pressure ratio of 3.5, the compressor with ported second-stage shroud has a flow at choking of about 124 lb/min and a flow at surge of about 47 lb/min, yielding a choke/surge flow ratio of about 2.6, which is about 45 percent greater than that with the non-ported shroud.  
         [0043]     At a flow rate of 80 lb/min, the compressor with non-ported shroud has a maximum pressure ratio at surge of about 4.45, while the compressor with ported second-stage shroud has a maximum pressure ratio of about 5.06, which is an increase of about 13.7 percent. The benefits of the ported second-stage shroud thus are quite significant.  
         [0044]      FIG. 6  shows an alternative embodiment generally similar to that of  FIG. 5 . In this embodiment, the flow passage  66  is in fluid communication with the inlet flow path into the second-stage impeller  26  via a gap  70  defined between the radially outer edge of the vane ring wall  58  and the inner wall  44  of the interstage duct  34 . The gap  70  can comprise a plurality of gaps spaced about the circumference of the vane ring wall  58 ; at locations between the gaps, the radially outer edge of the vane ring wall  58  engages the wall  44 .  
         [0045]      FIG. 7  illustrates yet another embodiment that essentially combines the openings  68  of the  FIG. 5  embodiment with the gaps  70  of the  FIG. 6  embodiment.  
         [0046]      FIGS. 8 and 9  illustrate still another embodiment of the invention in which the compressor housing has a different type of configuration from that of the previously disclosed embodiments. The compressor housing of  FIGS. 8 and 9  is generally similar to that described in co-pending U.S. patent application Ser. No. 11/004,467, filed on Dec. 3, 2004. The compressor housing  128  of this embodiment includes a first-stage volute  130  that comprises first and second segments or portions  130   a ,  130   b . Each portion  130   a ,  130   b  extends about 180 degrees around the compressor wheel and is fluidly connected to the second-stage inlet  132  by a respective one of first and second passages or conduits  134   a ,  134   b . Each conduit  134   a ,  134   b  extends axially between the respective first-stage volute  130   a ,  130   b  and the second-stage inlet  132  and passes radially outward of the second-stage volute  138 . Each conduit  134   a ,  134   b  includes a wall  136   a ,  136   b  that extends generally radially inwardly and faces generally axially away from the second-stage volute  138 , and is roughly analogous to the vane ring wall  58  of the prior embodiments, except that each wall  136   a ,  136   b  extends for only a part of the circumference and is bounded on its opposite edges by walls  137  ( FIG. 9 ) that extend generally radially inwardly and face each other generally in the circumferential direction. If desired, deswirl vanes (not shown) can be employed upstream of the second-stage impeller.  
         [0047]     In this embodiment, the second stage of the compressor includes a ported second-stage shroud  162  having one or more ports  164  leading into a flow passage  166  generally as in the prior embodiments. Each of the walls  136   a ,  136   b  of the conduits  134   a ,  134   b  joins with the second-stage shroud  162 . The flow passage  166  is in fluid communication with the second-stage inlet flow path  132  via a number of apertures  168  in the walls  136   a ,  136   b . For example, as shown, each wall  136   a ,  136   b  can include a plurality of circumferentially spaced apertures  168 .  
         [0048]      FIGS. 10 and 11  illustrate yet another embodiment of the invention. This embodiment is generally similar to that of  FIGS. 8 and 9 , except that the walls  136   a ,  136   b  of the conduits  134   a ,  134   b  do not include apertures. Instead, the walls  137  that bound the opposite circumferential edges of the walls  136   a ,  136   b  include cutouts  170  at the edges of the walls  137  that abut an opposite wall  172  of the compressor housing. The wall  172 , the walls  137 , and the wall  136   a ,  136   b  of each passage collectively form a pair of substantially closed flow paths that lead the fluid into the second-stage inlet from diametrically opposite radially inward directions. The cutouts  170  allow fluid to move from the flow passage  166  into these inlet flow paths and hence back to the second-stage impeller inlet. Fluid can also move in the opposite direction from the inlet flow paths into the flow passage  166  and then through the ports  164  to the inducer of the second-stage impeller.  
         [0049]     Another embodiment of the invention is depicted in  FIGS. 12 and 13 . This embodiment is generally similar to the previously described one, except that the walls  137 ′ define “cutouts”  170 ′ that extend the full lengths of the walls  137 ′—i.e., there is a gap  170 ′ all along the lengths of the walls  137 ′ between those walls and the adjacent wall  172  of the compressor housing.  
         [0050]     In the various embodiments described above, during some operating conditions at relatively high pressure-ratios where surge is typically related to inducer stall, a portion of the fluid entering the inducer region of the second-stage impeller flows through the shroud ports  64 ,  164  into the flow passage  66 ,  166  and then through the openings and/or gaps and/or cutouts  68 ,  70 ,  168 ,  170 ,  170 ′ back into the second-stage inlet flow path. It is believed that at near-surge conditions the ported shroud prevents or reduces the local flow recirculation in the inducer tip region that normally attends inducer stall and surge.  
         [0051]     It is also possible to design the ported shroud in such a manner that the flow at a choke condition is increased relative to that obtained with a non-ported shroud. More particularly, additional flow passes through the openings and/or gaps and/or cutouts  68 ,  70 ,  168 ,  170 ,  170 ′ into the flow passage  66 ,  166  and through the shroud ports  64 ,  164  into the second-stage impeller.  
         [0052]     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.