Patent Publication Number: US-2017350410-A1

Title: Centrifugal compressor impeller

Description:
TECHNICAL FIELD 
     The present invention relates to a shape of a trailing edge of a blade in an impeller used in a centrifugal compressor. 
     BACKGROUND ART 
     Generally, a centrifugal compressor utilizes centrifugal force of an impeller rotating together with a rotating shaft to pump a fluid, taken in from the leading edge side of the impeller, outward in a rotating shaft radial direction from the trailing edge side of the impeller and discharge the fluid into a diffuser. Specifically, the fluid taken into the centrifugal compressor is increased in pressure while passing through a flow passage of the rotating impeller and then the speed of the fluid is decreased by flowing through a diffuser. For example, Patent Document 1 discloses such a conventional centrifugal compressor. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent No. 3383023 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     When the fluid passes through the flow passage of the impeller, friction is generated between the fluid and a flow passage wall surface. Accordingly, the total pressure of the fluid flowing on the flow passage wall surface side decreases at the trailing edge of the impeller which forms an exit of the flow passage, and this leads to pressure loss. Particularly, when the centrifugal compressor is operated at a low flow rate, separation of the fluid becomes significant on a flow passage wall surface of the diffuser, and this leads to further pressure loss. 
     Moreover, diffusers of centrifugal compressors include a vaned diffuser and a vaneless diffuser. Generally, the pressure loss can be suppressed by providing diffuser vanes in the diffuser. 
     The centrifugal compressor disclosed in Patent Document 1 listed above includes a vaned diffuser, and the shape of a trailing edge of an impeller is specified in order to reduce noise and pressure pulsation generated when a fluid flowing into the diffuser comes into contact with diffuser vanes. In other words, the shape of the trailing edge of the impeller disclosed in Patent Document 1 is not for dealing with pressure loss occurring in a vaneless diffuser. 
     The present invention solves the problem described above and an object thereof is to provide a centrifugal compressor impeller capable of improving machine operation efficiency by causing a fluid flowing into a vaneless diffuser to have even total pressure distribution. 
     Means for Solving the Problem 
     A centrifugal compressor impeller in a first aspect of the present invention for solving the aforementioned problem is a centrifugal compressor impeller which comprises a plurality of blades provided radially about a rotating shaft and which utilizes centrifugal force generated by rotating together with the rotating shaft to pump a fluid, taken in from a leading edge side of each of the blades, outward in a rotating shaft radial direction from a trailing edge side of the blade and then discharge the fluid into a vaneless diffuser, the impeller characterized in that 
     an edge front end and an edge rear end of the trailing edge are disposed outside an edge center portion of the trailing edge in the rotating shaft radial direction. 
     A centrifugal compressor impeller in a second aspect of the present invention for solving the aforementioned problem is characterized in that the edge front end and the edge rear end are disposed to be shifted from each other in the rotating shaft radial direction. 
     A centrifugal compressor impeller in a third aspect of the present invention for solving the aforementioned problem is characterized in that thicknesses of the edge front end and the edge rear end are smaller than a thickness of the edge center portion. 
     Effect of the Invention 
     In the centrifugal compressor impeller of the present invention, the edge front end and the edge rear end in the trailing edge of the blade are disposed outside the edge center portion in the trailing edge of the blade in the rotating shaft radial direction. The centrifugal force acting on the fluid passing the edge front end and the edge rear end can be thereby made greater than the centrifugal force acting on the fluid passing the edge center portion. Accordingly, the total pressure distribution of the fluid passing the trailing edge can be set such that the pressure gradually increases from the edge center portion toward the edge front end and the edge rear end. Hence, even when pressure loss occurs due to friction generated between the fluid discharged from the impeller into the vaneless diffuser and a wall surface of the diffuser, the fluid flowing through the diffuser can have an even (uniform) total pressure distribution. As a result, operation efficiency of the centrifugal compressor can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-sectional view illustrating a schematic configuration of a centrifugal compressor to which an impeller in one embodiment of the present invention is applied. 
         FIG. 2  is an enlarged view of a main portion of  FIG. 1  and is a view illustrating an example of a trailing edge shape in a blade. 
         FIGS. 3A to 3D  are views illustrating other examples of the trailing edge shape in the blade. 
         FIG. 4  is a graph illustrating a relationship between a trailing edge height of the blade and a trailing edge thickness of the blade. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     A centrifugal compressor in the present invention is described below in detail by using the drawings. 
     Embodiment 
     As illustrated in  FIG. 1 , the centrifugal compressor  1  includes a casing  10 , a rotating shaft  20 , an impeller  30 , and the like. Specifically, the casing  10  has a hollow shape and the rotating shaft  20  is rotatably supported in the hollow portion via a bearing. The impeller  30  is fitted on the rotating shaft  20 . 
     The impeller  30  includes a hub  31 , a front shroud  32 , and multiple blades  33 . The hub  31  is formed in an annular shape whose outer diameter gradually increases from front to rear in a rotating shaft direction, and the rotating shaft  20  is fitted in a center hole of the hub  31 . Moreover, the front shroud  32  is disposed outside the hub  31  in a radial direction, and is formed in an annular shape whose inner diameter gradually increases from front to rear in the rotating shaft direction. 
     Furthermore, the blades  33  are provided radially about the rotating shaft  20 , between an outer peripheral surface of the hub  31  and an inner peripheral surface of the front shroud  32 , and are formed to gradually curve toward the outside in the rotating shaft radial direction while extending from front to rear in the rotating shaft direction. 
     Specifically, multiple spaces which are each surrounded by the outer peripheral surface of the hub  31 , the inner peripheral surface of the front shroud  32 , and two blades  33  adjacent to each other in a rotating shaft circumferential direction are formed in the impeller  30  radially about the rotating shaft  20 , and each serve as a flow passage  34  through which a fluid G passes. A front wall surface  34   a  and a rear wall surface  34   b  of the flow passage  34  are formed by the inner peripheral surface of the front shroud  32  and the outer peripheral surface of the hub  31 , and the flow passage  34  as a whole is formed to gradually curve toward the outside in the rotating shaft radial direction while extending from front to rear in the rotating shaft direction. 
     The impeller  30  can thus utilize centrifugal force generated by rotating together with the rotating shaft  20  to take in the fluid G from the leading edge  33   a  side of the blade  33  forming an entrance of the flow passage  34 , and then discharge the fluid G outward in the rotating shaft radial direction from the trailing edge  33   b  side of the blade  33  forming an exit of the flow passage  34 . In this case, the fluid G taken into the impeller  30  is increased in pressure while passing through the flow passage  34 . 
     Meanwhile, an intake passage  11  and a diffuser  12  serving as a discharge passage are formed in the casing  10 . 
     The intake passage  11  is disposed in front of the impeller  30  in the rotating shaft direction (upstream of the impeller  30  in a fluid flow direction), and is an annular passage which guides the fluid G taken in from the outside of the casing  10  toward the leading edge  33   a  of the blade  33  in the impeller  30  in the rotating shaft direction. 
     Meanwhile, the diffuser  12  is disposed outside the impeller  30  in the rotating shaft radial direction (downstream of the impeller  30  in the fluid flow direction), and is an annular passage extending in the rotating shaft radial direction. In other words, a front wall surface  12   a  and a rear wall surface  12   b  which have an annular shape as a whole are formed in the diffuser  12 . An annular entrance  12   c  of the diffuser  12  is formed by an inner end of the front wall surface  12   a  in the radial direction (upstream end of the front wall surface  12   a  in the fluid flow direction) and an inner end of the rear wall surface  12   b  in the radial direction (upstream end of the rear wall surface  12   b  in the fluid flow direction), and is opposed to the exit (trailing edge  33   b  of the blade  33 ) of the flow passage  34  in the impeller  30  in the rotation shaft radial direction. 
     The diffuser  12  thus takes in the fluid G compressed in the flow passage  34  of the impeller  30 , between the front wall surface  12   a  and the rear wall surface  12   b  and then discharges the taken-in fluid G outward in the rotating shaft radial direction while reducing the speed of the fluid G. 
     Note that the diffuser  12  is a so-called vaneless diffuser and do not have diffuser vanes for suppressing pressure loss inside the diffuser. 
     Accordingly, when the centrifugal compressor  1  is operated, the rotating shaft  20  rotates and the impeller  30  also rotates together with the rotating shaft  20 . The fluid G sucked into the intake passage  11  of the casing  10  by this rotation is compressed by being taken into the flow passage  34  of the rotating impeller  30 , and is then discharged from the inside of the flow passage  34 . Next, the fluid G discharged from the impeller  30  is taken into the diffuser  12  such that the speed thereof is reduced and the flow thereof is regulated, and then discharged from the inside of the diffuser  12 . 
     Here, as illustrated in  FIG. 2 , in the impeller  30 , the trailing edge  33   b  of the blade  33  forming the exit of the flow passage  34  is recessed inward in the rotating shaft radial direction in an arc shape. 
     Specifically, the trailing edge  33   b  is formed to gradually curve inward in the rotating shaft radial direction from an edge front end  41  and an edge rear end  42  toward an edge center portion  43 . To be more specific, the edge front end  41  and the edge rear end  42  are disposed at the same position in the rotating shaft radial direction and are disposed outside the edge center portion  43  in the rotating shaft radial direction. 
     Note that the edge front end  41  is a portion of the trailing edge  33   b  located closest to the front shroud  32  and is joined to a downstream end of the inner peripheral surface of the front shroud  32  (downstream end of the front wall surface  34   a  in the flow passage  34 ). Moreover, the edge rear end  42  is a portion of the trailing edge  33   b  located closest to the hub  31  and is joined to a downstream end of the outer peripheral surface of the hub  31  (downstream end of the rear wall surface  34   b  in the flow passage  34 ). Furthermore, the edge center portion  43  is located in an intermediate portion between the edge front end  41  and the edge rear end  42  and is a portion where a main flow of the fluid G flowing through the flow passage  34  passes. 
     The radius of a circle centered on a rotation center of the rotating shaft  20  and passing the edge front end  41  and the edge rear end  42  is thus greater than the radius of a circle centered on the rotation center of the rotating shaft  20  and passing the edge center portion  43 . Centrifugal force greater than that acting on the fluid G flowing through a center portion of the flow passage  34  and passing the edge center portion  43  thereby acts on the fluid G flowing along the front wall surface  34   a  of the flow passage  34  and passing the edge front end  41  and the fluid G flowing along the rear wall surface  34   b  of the flow passage  34  and passing the edge rear end  42 . 
     Accordingly, a total pressure distribution P of the fluid G passing the trailing edge  33   b  can be set such that the pressure gradually increases from the edge center portion  43  toward the edge front end  41  and the edge rear end  42 . Hence, even when pressure loss occurs due to friction generated between the fluid G discharged from the impeller  30  into the diffuser  12  and each of the front wall surface  12   a  and the rear wall surface  12   b,  the fluid G flowing through the diffuser  12  can have an even (uniform) total pressure distribution. As a result, operation efficiency of the centrifugal compressor  1  can be improved. 
     Note that, although the recess shape of the trailing edge  33   b  is the arc shape in the aforementioned embodiment, the recess shape of the trailing edge  33   b  only needs to be such that the edge front end  41  and the edge rear end  42  are disposed outside the edge center portion  43  in the rotating shaft radial direction. For example, the recess shape of the trailing edge  33   b  may be the recess shapes illustrated in  FIGS. 3A to 3D . 
     Specifically, in the recess shape illustrated in  FIG. 3A , the trailing edge  33   b  of the blade  33  is recessed inward in the rotating shaft radial direction in an arc shape, and the edge front end  41  and the edge rear end  42  are shifted from each other in the rotation shaft radial direction. To be more specific, the edge front end  41  and the edge rear end  42  are disposed outside the edge center portion  43  in the rotating shaft radial direction and, in addition, the edge front end  41  is disposed outside the edge rear end  42  in the rotating shaft radial direction. Note that the configuration may be the opposite such that the edge rear end  42  is disposed outside the edge front end  41  in the radial direction. 
     Meanwhile, in the recess shape illustrated in  FIG. 3B , although the trailing edge  33   b  of the blade  33  is recessed inward in the rotating shaft radial direction in an arc shape, only the edge center portion  43  is recessed inward in the rotating shaft radial direction. Specifically, the edge front end  41  and the edge rear end  42  are disposed at the same position in the rotating shaft radial direction and are disposed outside the edge center portion  43  in the rotating shaft radial direction. 
     When only the edge center portion  43  is to be recessed inward in the rotating shaft radial direction as described above, the edge center portion  43  may be notched in a rectangular shape or a wedge shape as illustrated in  FIGS. 3C and 3D . 
     Furthermore, although the total pressure of the fluid G passing the edge front end  41  and the edge rear end  42  is made higher than the total pressure of the fluid G passing the edge center portion  43  by forming the trailing edge  33   b  of the blade  33  in the recess shape in the aforementioned embodiment, a further total pressure difference may be generated by additionally varying the thickness of the trailing edge  33   b  among the edge front end  41 , the edge rear end  42 , and the edge center portion  43 . 
     Specifically, as illustrated in  FIG. 4 , the thickness of the trailing edge  33   b  is made to gradually decrease from the edge center portion  43  toward the edge front end  41  and the edge rear end  42 . In the trailing edge  33   b  of the blade  33 , the smaller the thickness is, the more the pressure loss is suppressed. Accordingly, the total pressure of the fluid G passing the edge front end  41  and the edge rear end  42  can be made higher than the total pressure of the fluid G passing the edge center portion  43 . 
     Hence, even when large pressure loss occurs due to friction generated between the fluid G discharged into the diffuser  12  and each of the front wall surface  12   a  and the rear wall surface  12   b,  the fluid G flowing through the diffuser  12  can have an even (uniform) total pressure distribution. 
     INDUSTRIAL APPLICABILITY 
     The impeller of the centrifugal compressor in the present invention can improve the total pressure distribution of the fluid in the trailing edge. Accordingly, the impeller can be utilized to be highly beneficial in improving the operation efficiency of a machine. 
     EXPLANATION OF THE REFERENCE NUMERALS 
     
         
           1  centrifugal compressor 
           10  casing 
           11  intake passage 
           12  diffuser 
           20  rotating shaft 
           30  impeller 
           31  hub 
           32  front shroud 
           33  blade 
           33   a  leading edge 
           33   b  trailing edge 
           34  flow passage 
           41  edge front end 
           42  edge rear end 
           43  edge center portion 
         G fluid 
         P total pressure distribution