Patent Publication Number: US-2010129209-A1

Title: Centrifugal Compressor

Description:
TECHNICAL FIELD 
     The present invention relates to a centrifugal compressor of a turbo charger or the like. 
     BACKGROUND ART 
     Conventionally, a centrifugal compressor of a turbo charger or the like to be used for an internal combustion engine of a motor vehicle for example, is commonly known. 
       FIG. 5A  is a sectional view showing a relevant section of a conventional centrifugal compressor. A centrifugal compressor  10  shown in the drawing compresses fluid such as gas and air introduced from outside a housing  11 , by rotating an impeller  13  having a large number of blades  12  within the housing  11 . The fluid flow (airflow) formed in this way travels through an impeller exit (Hereunder, also referred to as “diffuser entry”)  14 , being an outer circumferential end of the impeller  13 , then through a diffuser passage  15  and a scroll  16 , and is then delivered to the outside. Reference symbol  17  in the drawing denotes a shaft axis about which the impeller  13  rotates. 
     The diffuser passage  15  mentioned above is provided in between the impeller exit  14  and the scroll  16 , being a passage for recovering static pressure by slowing down the airflow discharged from the impeller  14 . The diffuser passage  15  is normally formed from a pair of opposing wall surfaces. In the description below, one of the pair of the opposing wall surfaces is called a shroud side wall surface  15   a  and the other is called a hub side wall surface  15   b.    
     In a turbo charger of a motor vehicle to be used in combination with an internal combustion engine, since a wide compressor operation range is required, a type of diffuser that does not have vanes (vaneless diffuser) is normally employed. 
     Incidentally, in recent years, in response to an increase in flow amount and pressure ratio of the centrifugal compressor  10 , distortion of the airflow flowing into the diffuser passage  15  is becoming greater. It is thought that a flow with a large distortion flowing into the diffuser passage  15  results in the occurrence of a phenomenon known as surging, which determines the operation limit on the small flow amount side in the diffuser passage  15 . 
     In the mechanism that gives rise to surging it is thought that surging occurs when a backflow area of the airflow reaches an exit side end section of the diffuser passage  15 . Moreover, since the occurrence of the backflow area mentioned above is thought to be caused by a flow on the compressor shroud side within the diffuser passage  15 , that is, distortion of the flow along the shroud side wall surface  15   a , a diffuser structure for reducing such distortion in the flow is proposed. 
     The distortion in the flow mentioned above refers to the flow velocity distribution or pressure distribution being in a non-uniform state. Conventional techniques for making this uniform employ a structure or method that changes a flow passage sectional area of the diffuser passage  15 , or that utilizes a circulation passage or the like. Each of such conventional techniques has a focus on reducing distortion that occurs on the entry side (impeller exit  14  side) of the diffuser passage  15 . 
     As a conventional technique for reducing the distortion mentioned above, for example, provision of a convex section or a concave section for changing the flow passage sectional area on the wall surface of the diffuser passage  15  has been proposed. Such a convex section or concave section regularize the airflow along the circumferential direction by changing the passage shape in the circumferential direction, thereby enabling an improvement in compression efficiency. (for example, refer to Patent Document 1). 
     Patent Document 1: Japanese Unexamined Patent Application, Publication No. Hei 10-176699 
     DISCLOSURE OF INVENTION 
     However, in a small centrifugal compressor such as a turbo charger for a motor vehicle in particular, it is difficult to measure the internal flow of the airflow flowing within the diffuser passage. Therefore, since distortion in actual internal flow has yet to be sufficiently understood and furthermore the phenomena up to the point of surging have not been explained, it is necessary to understand these and develop an effective surging prevention method to widen the operation range of the centrifugal compressor. 
     As shown in  FIG. 5B , the conventional diffuser passage  15  generally has a shape in which a pair of opposing wall surfaces, namely a shroud side wall surface  15   a  and a hub side wall surface  15   b , are in a parallel form, so that an axial direction flow passage width W of the diffuser passage  15  is constant in the circumferential direction. Having examined a flow pattern of up to surging, by carrying out internal flow measurement using a model for the conventional diffuser passage  15  configured in this way, it was revealed that in the downstream area of the backflow area (shown with an arrow A in the drawing) becomes a flow area that reaches the diffuser exit  18  side end section of the diffuser passage  15 , the airflow separates from the hub side wall surface  15   b  in the vicinity of the diffuser exit  18 , forming a backflow area (shown with an arrow B in the drawing). That is to say, it is thought that it is not a shroud side backflow area A, but a backflow area B which causes surging to occur. 
     It is thought that such a flow separation from the hub side wall surface  15   b  occurring in the vicinity of the diffuser exit  18  cannot be suppressed by a conventional technique, as disclosed in Patent Document 1 for example, in which airflow is regularized along the circumferential direction by changing the passage shape in the circumferential direction. 
     In consideration of the above circumstances, an object of the present invention is to provide a centrifugal compressor having a diffuser structure in which airflow is unlikely to separate from a hub side wall surface on the hub side wall surface downstream side within the diffuser passage (in the vicinity of the diffuser exit). 
     The present invention employs the following means to solve the above problems. 
     A centrifugal compressor according to the present invention is a centrifugal compressor provided with a diffuser passage for recovering static pressure by slowing down discharged air from an outer circumferential end of an impeller that rotates within a housing, wherein a hub side wall surface of the diffuser passage is provided with an inclined area that approaches toward a shroud side, in a position on a downstream side of a portion parallel with a normal line direction of a section surface of an impeller exit. 
     According to such a centrifugal compressor, since the hub side wall surface of the diffuser passage is provided with the inclined area that approaches the shroud side, in the position on the downstream side of the portion parallel with the normal line direction of the section surface of the impeller exit, the radial direction velocity of the low velocity area that occurs on the hub side wall surface increases in the inclined area, and the radial direction velocity distribution within the diffuser passage is made uniform. 
     The inclined area in this case is a portion of an inclined plane, a curved surface, or a stepped plane, formed on the hub side wall surface for example, and must be such that, in this inclined area, the axial direction flow passage width of the diffuser passage narrows down from the upstream side toward the downstream side by inclining the hub side wall surface so as to approach toward the opposed shroud side wall surface as it approaches the downstream side. 
     In the above aspect of the invention, a preferable position in the diffuser passage length direction for providing the inclined area, in the diffuser passage in a range between the diffuser entry taken as a base point (0) and the diffuser exit (1), is within a range on the downstream side (exit side) having a ratio of 0.3 to 0.7. 
     Moreover, in the above aspect of the invention, it is preferable that the inclined area provided in the diffuser passage be such that the maximum value of an amount of projection from the hub side wall surface toward the shroud side wall surface is set to approximately ⅓ to ⅕ of the passage width, being equivalent to the size of the measured backflow area. 
     Furthermore, in the above aspect of the invention, the preferable inclination angle is less than or equal to 20 degrees based on the normal line of the impeller exit section when the inclined area is taken as a plane. However, a more preferable inclination angle is between 2 degrees and 10 degrees, inclusive, based on the normal line of the impeller exit section. An excessively large inclination angle is not preferable, as the airflow is re-accelerated due to the reduction in the passage area in this case. 
     According to the present invention described above, since on the hub side wall surface of the diffuser passage there is provided the inclined area that approaches the shroud side in the position on the downstream side of the portion parallel with the normal line of the impeller exit section, the radial direction velocity of a low velocity area that occurs on the hub side wall surface increases in this inclined area. Therefore, the radial direction velocity distribution within the diffuser passage is made uniform, and separation of the airflow is unlikely to occur locally, so that the surge flow rate can be reduced and the compressor operation range can be widened. 
     Moreover, the present invention described above is suited for widening the operation range of a small centrifugal compressor provided with a vaneless diffuser, such as a turbo charger for a motor vehicle, which requires a particularly wide compressor operation range. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       [ FIG. 1 ] is a sectional view of a diffuser passage showing a first embodiment of a centrifugal compressor according to the present invention. 
       [ FIG. 2 ] is a sectional view showing a first modified example of  FIG. 1 . 
       [ FIG. 3 ] is a sectional view showing a second modified example of  FIG. 1 . 
       [ FIG. 4 ] is a sectional view of a diffuser passage showing a second embodiment of a centrifugal compressor according to the present invention. 
       [ FIG. 5A ] is a sectional view showing a relevant section of the centrifugal compressor. 
       [ FIG. 5B ] is a sectional view showing a conventional structure of a diffuser passage. 
     
    
    
     EXPLANATION OF REFERENCE SIGNS 
     
         
           10 : Centrifugal compressor 
           11 : Housing 
           12 : Impeller 
           14 : Impeller exit (diffuser entry) 
           15 ,  30 : Diffuser passage 
           15   a ,  30   a : Shroud side wall surface 
           15   b ,  30   b : Hub side wall surface 
           16 : Scroll 
           18 : Diffuser exit 
           20 : Inclined plane (inclined area) 
           21 : Inclined curved surface (inclined area) 
           22 : Inclined inflected line (inclined area) 
           33 : Shroud side inclined section (inclined area) 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereunder an embodiment of a centrifugal compressor according to the present invention is described, based on the drawings. 
     As shown in  FIG. 5A , a centrifugal compressor  10  is provided with a diffuser passage  15  that recovers static pressure by reducing the velocity of a discharged air from an outer circumferential end of an impeller  13  that rotates within a housing  11 . The diffuser passage  15  is provided so as to connect between an impeller exit (diffuser entry)  14  and a scroll  16 , and is formed in between a pair of opposing wall surfaces that comprise a shroud side wall surface  15   a  and a hub side wall surface  15   b.    
       FIG. 1  is a sectional view of the diffuser passage  15  showing a first embodiment. This diffuser passage  15  introduces from the diffuser entry  14 , the discharged air (indicated with white arrows in the drawing) from the outer circumferential end of the impeller  13 , and allows the airflow guided to the passage between the shroud side wall surface  15   a  and the hub side wall surface  15   b  to flow out from a diffuser exit  18  into the scroll  16 . 
     In the embodiment shown in the drawing, the hub side wall surface  15   b  of the diffuser passage  15  is provided with an inclined plane  20  that approaches the direction of the shroud side wall surface  15   a  in a position on the downstream side in a portion parallel with a normal line of the section of the impeller exit. This inclined plane  20  is an inclined area, formed on the hub side wall surface  15   b  of the diffuser passage  15 , that becomes closer to the shroud side wall surface  15   a  as the distance from the diffuser entry  14  increases, approaching the diffuser exit  18  and narrowing an axial direction flow passage width W, which is a distance between the opposing wall surfaces, down to Wa. 
     That is to say, in the diffuser passage  15  of normal line direction flow passage length L, the inclined area formed by the inclined wall surface  20  inclines toward the direction of the shroud side wall surface  15   a  from the upstream side of a passage of a flow passage length La parallel with the normal line direction, and is provided in a downstream portion of flow passage length Lb. Here, taking 1 as the flow passage length L, the preferred length of the downstream portion Lb, which becomes the inclined area, is a portion remaining on the downstream side where the length La of the upstream portion parallel with normal line direction is 0.3 to 0.7. In other words, the preferred position for providing the inclined area in the lengthwise direction of the diffuser passage  15  may be set so that, where the flow passage length from the diffuser entry  14  which is taken as a base point (0), to the diffuser exit  18 , which is taken as an end point (1), is taken as L (L=1), the length La of the upstream portion is set at a ratio of 0.3 to 0.7, and the length Lb of the downstream portion is set at a ratio of 0.7 to 0.3. Therefore, since the flow passage length L is the total length of the length La of the upstream portion and the length Lb of the downstream portion (L=La+Lb), the total length L of the upstream portion length La and the downstream portion length Lb is always “1”. 
     Moreover, it is preferable that the inclined area provided in the diffuser passage  15  be such that a projection amount of the inclined wall surface from the hub side wall surface  15   b  to the shroud side wall surface  15   a  is set to be approximately ⅓ to ⅕ of the passage width at the diffuser exit  18 , where the projection amount of the inclined wall surface  20  reaches its maximum value Wb. That is to say, since the maximum value Wb of the projection amount is ⅓ to ⅕W (Wb≈⅓ to ⅕W), the axial direction flow passage width Wa narrowed by the inclined wall surface  20  is set to be approximately ⅔ to ⅘ of the axial direction flow passage width W (Wa≈⅔ to ⅘W). 
     Moreover, as is the case with the inclined wall surface  20  mentioned above, the angle of inclination θ where the inclined area is a plane surface, is preferably set less than or equal to 20 degrees based on the normal line of the impeller exit section. However, a further preferable angle of inclination θ is greater than or equal to 2 degrees and less than or equal to 10 degrees based on the normal line of the impeller exit section. 
     The normal line and normal line direction of the impeller exit section mentioned above refers to a straight line or direction that extends radially outward from an axial center line  17  of rotation of the impeller  13  and passes the impeller exit section, and it practically approximates the airflow direction. 
     As described above, the diffuser passage  15  provided with the inclined area formed by the inclined wall surface  20  is provided with: an upstream side area where the shroud side wall surface  15   a  and the hub side wall surface  15   b  are both parallel with the normal line direction and the axial direction flow passage width W is constant; and an inclined area on the downstream side where the axial direction flow passage width W to the diffuser exit  18  side is narrowed by the inclined wall surface  20 , in which the hub side wall surface  15   b  inclines toward the shroud side wall surface  15   a.    
     Therefore the airflow introduced from the diffuser entry  14  is slowed down by flowing through the diffuser passage  15 , and recovers its static pressure. However, at this time, on the downstream side closer to the diffuser exit  18 , the airflow in a low velocity area, which occurs in the vicinity of the wall surface and is thought to cause the airflow to separate from the hub side wall surface  15   b , is guided to the inclined wall surface  20  so that it gradually flows toward the shroud side wall surface  15   a.    
     The low velocity area in this case is an area where a velocity component in the radial direction from the diffuser entry  14  toward the diffuser exit  18  is low. In the example shown in the drawing, the radial direction corresponds to the normal line direction. 
     As a result, the airflow in the low velocity area that has occurred in the vicinity of the wall surface of the hub side wall surface  15   b  increases its velocity component in the radial direction. Therefore, in the diffuser passage  15 , the velocity distribution in the radial direction becomes uniform and flow separation is unlikely to occur locally. 
     As described above, if flow separation is unlikely to occur locally in the diffuser passage  15 , the surge flow rate can be reduced, enabling the operation range of the centrifugal compressor to be widened. In particular, if the invention is applied in the case where a wide range of compressor operation is required from a small sized centrifugal compressor such as a turbo charger provided with a vaneless diffuser, widening of the range can be easily achieved. 
     Incidentally, considering the case of providing a similar inclined area on the shroud side wall surface  15   a , the axial direction flow passage width W becomes narrower in the opposite direction as it approaches the diffuser exit  18 . However, since there is not a low velocity area which is thought to be a cause of flow separation, present in close proximity to the wall surface of the shroud side wall surface  15   a  in the vicinity of the diffuser exit  18 , the airflow guided to the inclined wall surface  20  is accelerated by gradually flowing toward the hub side wall surface  15   b . Therefore, a difference between the accelerated velocity on the shroud side wall surface  15  and the velocity on the hub side wall surface  15   b , in which the low velocity area is present, increases, and hence non-uniformity of the velocity distribution in the radial direction increases. 
     Next, a first modified example of the inclined area mentioned above is described, based on  FIG. 2 . The same reference symbols are given to parts that are the same as in the above embodiment, and detailed descriptions thereof are omitted. 
     In this first modified example, in place of the inclined plane  20  in  FIG. 1 , an inclined curved surface  21  forms an inclined area. This inclined curved surface  21  is the same as the inclined plane  20  with respect to the preferred position along the diffuser passage length direction in which the inclined area is to be provided (ratio of length Lb), and with respect to the maximum value Wb of the projection amount from the hub side wall surface  15   b  toward the shroud side wall surface  15   a , and the curvature may be appropriately set to satisfy these conditions. The curved surface in this case may be either a concave curved surface or convex curved surface when seen from inside the diffuser passage  15 . 
     Since the radial direction velocity component of the low velocity area airflow that has occurred in the vicinity of the wall surface of the hub side wall surface  15   b  also increases even when the inclined curved surface  21  provided is formed by such an inclined area, the velocity distribution in the radial direction is made uniform so that flow separation becomes unlikely to occur locally. 
     Therefore, the surge flow rate can be reduced, and hence the operation range of the centrifugal compressor can be widened and, in particular, if the invention is applied in a case where a wide range of compressor operation is required of a small size centrifugal compressor provided with a vaneless diffuser, the compressor operation range can be easily widened. 
     Next, a second modified example of the inclined area mentioned above is described, based on  FIG. 3 . The same reference symbols are given to parts that are the same as in the above embodiment, and detailed descriptions thereof are omitted. 
     In this second modified example, in place of the inclined plane  20  in  FIG. 1 , an inclined inflected line  22  forms an inclined area. This inclined inflected line  22  is formed from a plane inclined section  22   a  and a parallel section  22   b  on the diffuser exit  18  side, the parallel section  22   b  in this case being parallel to the shroud side wall surface  15   a  and the hub side wall surface  15   b.    
     Moreover, this inclined inflected line  22  is the same as the inclined plane  20  with respect to the preferred position along the diffuser passage length direction in which the inclined area is to be provided (ratio of length Lb), and with respect to the maximum value Wb of the projection amount from the hub side wall surface  15   b  toward the shroud side wall surface  15   a.    
     Since the radial direction velocity component of the low velocity area airflow that has occurred in the vicinity of the wall surface of the hub side wall surface  15   b  also increases even when the inclined inflected line  22  provided is formed by such an inclined area, the velocity distribution in the radial direction is made uniform so that flow separation becomes unlikely to occur locally. 
     Therefore, the surge flow rate can be reduced, and hence the operation range of the centrifugal compressor can be widened and, in particular, if the invention is applied in a case where a wide range of compressor operation is required of a small size centrifugal compressor provided with a vaneless diffuser, the compressor operation range can be easily widened. 
     The inclined inflected line  22  shown in the drawing is a combination of the inclined section  22   a  and the parallel section  22   b . However, an inclined section  22   a  of two or more steps may be combined, and furthermore, a curved surface may also be combined. 
       FIG. 4  is a sectional view showing a second embodiment of the centrifugal compressor according to the present invention. The same reference symbols are given to parts that are the same as in the first embodiment, and detailed descriptions thereof are omitted. 
     In the present embodiment, a diffuser passage  30  is divided into three areas. Specifically, from the upstream side, a hub side inclined section  31 , a parallel section  32  parallel with the normal line direction of the impeller exit section, and a shroud side inclined section  33  are integrally connected. Therefore, compared to the first embodiment shown in  FIG. 1 , the hub side inclined section  31  is added on the most upstream side, and, in the shroud side inclined section  33 , the shroud side wall surface  30   a  and the hub side wall surface  30   b  are arranged parallel with each other so as to incline toward the shroud side at the same inclination angle. 
     When employing this kind of configuration also, in the shroud side inclined section  33  of the diffuser passage  30 , the hub side wall surface  30   b  is provided with an inclined area that approaches the shroud side in a position on the downstream side of the parallel section  32 , which is a portion parallel with the normal line of the impeller exit section. That is to say, by having the hub side wall surface  30   b  of the shroud side inclined section  33  approach the shroud side, an inclined area having the same effect as that of the inclined plane  20  mentioned above is formed. 
     As a result, since radial direction velocity component of the low velocity area airflow that has occurred in the vicinity of the wall surface of the hub side wall surface  30   b  is increased by the hub side wall surface  30   b  that serves as an inclined plane similar to the inclined plane  20 , the velocity distribution in the radial direction is made uniform so that flow separation becomes unlikely to occur locally in the diffuser passage  30 . 
     Therefore, the surge flow rate can be reduced, and hence the operation range of the centrifugal compressor can be widened and, in particular, if the invention is applied in a case where a wide range of compressor operation is required of a small size centrifugal compressor provided with a vaneless diffuser, the compressor operation range can be easily widened. 
     As described above, on the hub side wall surface  15   a  of the diffuser passage  15 , since the inclined area that approaches the shroud side is provided in a position on the downstream side of the portion parallel with the normal line direction of the impeller exit section, the radial direction velocity of the low velocity area that occurs on the hub side wall surface  15   a  increases, and the radial direction velocity distribution within the diffuser passage  15  is made uniform. Therefore, airflow is unlikely to separate locally from the wall surface in the vicinity of the diffuser exit  18  of the diffuser passage  15 , and hence the surge flow rate can be reduced and the compressor operation range can be widened. 
     The present invention is not limited to the above embodiments and can be appropriately modified without departing from the scope of the present invention.