Patent Publication Number: US-2023135302-A1

Title: Compressor

Description:
TECHNICAL FIELD 
     The present disclosure relates to a compressor. 
     Priority is claimed on Japanese Patent Application No. 2020-045777 filed on Mar. 16, 2020, Japanese Patent Application No. 2020-045783 filed on Mar. 16, 2020, and Japanese Patent Application No. 2020-045650 filed on Mar. 16, 2020 and the contents thereof are incorporated herein. 
     BACKGROUND ART 
     In a turbo machine including a compressor, noise occur while rotation elements of the machine are operated. If such noise transmits to a stationary component, there is a risk that a structural failure of the stationary component may occur. Here, for the purpose of noise prevention, an acoustic liner being provided in an exit flow path of the compressor has been proposed (see Patent Document 1 below). The acoustic liner includes an introduction hole which is opened toward the flow path and an acoustic space which is connected to a downstream side of the introduction hole. 
     CITATION LIST 
     Patent Document(s) 
     
         
         Patent Document 1: US Patent No. 2002/0079158 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, since the flow velocity in the flow path is high in the above-described compressor, the acoustic resistance becomes large when guiding sound waves from the introduction hole of the acoustic liner to the acoustic space. Specifically, vortexes occur in fluid to be compressed at an exit of the introduction hole (that is, an entrance of the acoustic space) and the vortexes prevent sound waves from being introduced to the acoustic space smoothly. As a result, there is a possibility that a sufficient sound reduction effect may not be obtained. 
     The present disclosure has been made to solve the above-described problems and an object thereof is to provide a compressor having an excellent noise-reduction property. 
     Solution to Problem 
     In order to solve the above-described problems, a compressor according to the present disclosure includes: a rotation shaft which is allowed to rotate around an axis; an impeller which is configured to pressure-feed a fluid from one side in an axial direction toward an outside in a radial direction by rotating together with the rotation shaft; a casing surrounding the rotation shaft and the impeller and in which an exit flow path guiding the fluid pressure-fed from the impeller is formed; and an acoustic liner which is installed to face an inside of the exit flow path in the casing, wherein the acoustic liner includes an acoustic space which is formed inside the acoustic liner, an introduction hole communicating the acoustic space with the exit flow path, and a vortex suppressor which is placed in a connection area between the introduction hole and the acoustic space and is configured to suppress vortexes which occur in the connection area. 
     A compressor according to the present disclosure includes: a rotation shaft which is allowed to rotate around an axis; an impeller which is configured to pressure-feed a fluid from one side in an axial direction toward an outside in a radial direction by rotating together with the rotation shaft; a casing surrounding the rotation shaft and the impeller and in which an exit flow path guiding the fluid pressure-fed from the impeller is formed; and an acoustic liner which is installed to face an inside of the exit flow path in the casing, wherein the acoustic liner includes an acoustic space which is formed inside the acoustic liner, and a plurality of introduction holes communicating the acoustic space with the exit flow path, and wherein the plurality of introduction holes are formed to communicate with the acoustic space while coming closer to each other as the introduction holes are extended from the exit flow path toward the acoustic space and performs as a vortex suppressor suppressing vortexes which occur in a connection area between the plurality of introduction holes and the acoustic space. 
     A compressor according to the present disclosure includes: a rotation shaft which is allowed to rotate around an axis; an impeller which is configured to pressure-feed a fluid from one side in an axial direction toward an outside in a radial direction by rotating together with the rotation shaft; a casing surrounding the rotation shaft and the impeller and in which a diffuser flow path guiding the fluid pressure-fed from the impeller toward an outside in a radial direction is formed; and a plurality of diffuser vanes which are provided in the diffuser flow path at intervals in a circumferential direction of the axis, wherein each of the diffuser vanes includes a vane body extending toward a rotation direction of the rotation shaft as the diffuser vane is extended toward an outside in a radial direction and a sound reducer which is formed on a surface of the vane body. 
     A compressor according to the present disclosure includes: a rotation shaft which is allowed to rotate around an axis; an impeller which is configured to pressure-feed a fluid from one side in an axial direction toward an outside in a radial direction by rotating together with the rotation shaft; a casing surrounding the rotation shaft and the impeller and in which an exit flow path guiding the fluid pressure-fed from the impeller is formed; a speaker wall which is provided to face an inside of the exit flow path in the casing; a pressure sensor which is configured to detect a pressure inside the exit flow path; and a computing device which is configured to send a signal to the speaker wall to emit a sound having a frequency for canceling a target sound on the basis of a detection value of the pressure sensor. 
     Advantageous Effects of Invention 
     According to the present disclosure, it is possible to provide a compressor having exceptional noise-reduction properties. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view showing a configuration of a compressor according to a first embodiment of the present disclosure. 
         FIG.  2    is a perspective view showing a configuration of an acoustic liner according to the first embodiment of the present disclosure. 
         FIG.  3    is a cross-sectional view showing the configuration of the acoustic liner according to the first embodiment of the present disclosure. 
         FIG.  4    is a cross-sectional view showing a configuration of an acoustic liner according to a second embodiment of the present disclosure. 
         FIG.  5    is a cross-sectional view showing a modified example of the acoustic liner according to the second embodiment of the present disclosure. 
         FIG.  6    is a cross-sectional view showing a configuration of an acoustic liner according to a third embodiment of the present disclosure. 
         FIG.  7    is a cross-sectional view showing a configuration of a compressor according to a fourth embodiment of the present disclosure. 
         FIG.  8    is a cross-sectional view showing a configuration of a sound reducer according to the fourth embodiment of the present disclosure. 
         FIG.  9    is a cross-sectional view showing a configuration of a sound reducer according to a fifth embodiment of the present disclosure. 
         FIG.  10    is an explanatory diagram showing a behavior of the sound reducer according to the fifth embodiment of the present disclosure. 
         FIG.  11    is a cross-sectional view showing a configuration of a compressor according to a sixth embodiment of the present disclosure. 
         FIG.  12    is a cross-sectional view showing a configuration of a speaker wall according to the sixth embodiment of the present disclosure. 
         FIG.  13    is a hardware configuration diagram of a computing device according to the sixth embodiment of the present disclosure. 
         FIG.  14    is a functional block diagram of the computing device according to the sixth embodiment of the present disclosure. 
         FIG.  15    is a plan view of a speaker wall according to a seventh embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     (Configuration of Compressor) 
     Hereinafter, a compressor  100  according to a first embodiment of the present disclosure will be described with reference to  FIGS.  1  to  3   . As shown in  FIG.  1   , the compressor  100  includes a rotation shaft  1 , an impeller  2 , a casing  3 , a return vane  4 , and an acoustic liner  5 . 
     The rotation shaft  1  extends along an axis Ac 1  and is rotatable around the axis Ac 1 . The impeller  2  is fixed to the outer peripheral surface of the rotation shaft  1 . The impeller  2  includes a disk  21  and a plurality of blades  22 . The disk  21  is formed in a disk shape centered on the axis Ac 1 . An outer peripheral surface (main surface  21 A) of the disk  21  is formed in a curved surface shape which is curved from the inside toward the outside in the radial direction as the outer peripheral surface is expanded from one side toward the other side in the direction of the axis Ac 1 . 
     A plurality of blades  22  are provided on the main surface  21 A at intervals in the circumferential direction. Although not shown in detail, each blade  22  is curved from the front side toward the rear side in the rotation direction of the rotation shaft  1  as the blade is extended from the inside toward the outside in the radial direction. The impeller  2  rotates together with the rotation shaft  1  to pressure-feed a fluid, introduced from one side in the direction of the axis Ac 1 , toward the outside in the radial direction. 
     The casing  3  surrounds the rotation shaft  1  and the impeller  2  from the outer peripheral side. A compression flow path Pa, for compressing a fluid introduced from the outside of the casing  3 , in which the impeller  2  is accommodated and an exit flow path Fa which is connected to the radial outside of the compression flow path Pa are formed inside the casing  3 . The compression flow path Pa gradually increases in its diameter, corresponding to the outer shape of the impeller  2 , as the compression flow path is expanded from one side toward the other side in the direction of the axis Ac 1 . The exit flow path Fa is connected to the radially outer exit of the compression flow path Pa. 
     The exit flow path Fa includes a diffuser flow path Fa 1  and an exit scroll Fa 2 . The diffuser flow path Fa 1  is provided to recover the static pressure of the fluid introduced from the compression flow path Pa. The diffuser flow path Fa 1  is formed in an annular shape which extends from the exit of the compression flow path Pa toward the outside in the radial direction. In the cross-sectional view including the axis Ac 1 , the flow path width of the diffuser flow path Fa 1  is constant over the entire area in the extending direction. The plurality of return vanes  4  are provided in the diffuser flow path Fa 1 . The plurality of these return vanes  4  are arranged at intervals in the circumferential direction. 
     The exit scroll Fa 2  is connected to the radially outer exit of the diffuser flow path Fa 1 . The exit scroll Fa 2  is formed in a swirl shape extending in the circumferential direction of the axis Ac 1 . The exit scroll Fa 2  has a circular flow path cross-section. Part of the exit scroll Fa 2  is provided with an exhaust hole for guiding a high-pressure fluid to the outside (not shown). 
     (Configuration of Acoustic Liner) 
     The acoustic liner  5  is provided on the wall surface on the other side of the direction of the axis Ac 1  in the diffuser flow path Fa 1 . The acoustic liner  5  is embedded in this wall surface to face the diffuser flow path Fa 1 . The acoustic liner  5  is formed in an annular shape centered on the axis Ac 1 . The acoustic liner  5  is provided to absorb and reduce noise caused by the fluid flowing through the diffuser flow path Fa 1 . 
     As shown in  FIG.  2   , the acoustic liner  5  is formed in a plate shape and one side surface thereof is provided with a plurality of introduction holes h which are opened in the diffuser flow path Fa 1 . More specifically, as shown in  FIG.  3   , the acoustic liner  5  includes an acoustic space V, the introduction hole h, and a vortex suppressor  6  which are formed therein. 
     The acoustic space V is a space formed inside the acoustic liner  5 . The introduction hole h communicates the diffuser flow path Fa 1  with the acoustic space V. A plurality of pairs of such acoustic spaces V and introduction holes h are formed inside the acoustic liner  5  over the entire extension area. 
     The vortex suppressor  6  is provided at a connection area (throat portion S) between the introduction hole h and the acoustic space V. The vortex suppressor  6  is provided to reduce and suppress vortexes which occur when a sound wave introduces from the introduction hole h into the acoustic space V. In this embodiment, the vortex suppressor  6  which is formed of a foam metal m is provided to cover the introduction hole h from the side of the acoustic space V. 
     (Operation and Effect) 
     Next, the operation of the compressor  100  will be described. When operating the compressor  100 , the rotation shaft  1  is first rotated around the axis Ac 1  by an external drive source. As the rotation shaft  1  rotates, the impeller  2  also rotates such that an external fluid is introduced to the compression flow path Pa. The fluid guided to the blades  22  of the impeller  2  in the compression flow path Pa is compressed by a centrifugal force to a high pressure state. This high-pressure flow path is taken out to the outside through the diffuser flow path Fa 1  and the exit scroll Fa 2 . 
     Here, noise occurs while the impeller  2  rotates in the compressor  100 . Among such noise, especially the noise called NZ sound is likely to cause resonance with each part of the compressor  100 . As a result, it is important to reduce and suppress the noise. The NZ sound is noise (discrete frequency sound) of a frequency based on the sum of the number of blades (that is, the number of blades  22 ) N of the impeller  2  and the number of revolutions Z of the rotation shaft  1 . 
     For the purpose of reducing and suppressing such NZ sound, the acoustic liner  5  is provided in the diffuser flow path Fa 1  in this embodiment. The sound wave which is introduced into the acoustic space V through the introduction hole h is attenuated inside the acoustic space V. Accordingly, it is possible to suppress the leakage of noise to the outside. 
     Incidentally, since the flow velocity of the fluid is high in the diffuser flow path Fa 1 , acoustic resistance is increased when the sound wave is introduced from the introduction hole h of the acoustic liner  5  to the acoustic space V. Specifically, vortexes occur in the fluid at the exit of the introduction hole h (that is, the throat portion S) and the vortexes prevent the sound wave from introducing to the acoustic space smoothly. As a result, there is a possibility that a sufficient sound reduction effect may not be obtained. 
     However, according to the above-described configuration, since the throat portion S is provided with the vortex suppressor  6 , it prevents vortexes from occurring inside the acoustic space V. Accordingly, the resistance (acoustic resistance) generated when the sound wave is introduced to the introduction hole h is reduced. As a result, since the sound wave is likely to be introduced into the acoustic liner  5 , it is possible to more efficiently absorb and reduce noise. 
     Particularly, in the above-described configuration, the vortex suppressor  6  formed of the foam metal m is provided to cover the introduction hole h. Since the vortexes are dispersed by the foam metal m, it is possible to significantly reduce the resistance (acoustic resistance) generated when the sound wave introduces to the introduction hole h. Accordingly, it is possible to significantly reduce the noise of the compressor  100 . 
     Second Embodiment 
     Next, a second embodiment of the present disclosure will be described with reference to  FIG.  4   . Additionally, the same reference numerals will be given to the same configurations as those of the above-described first embodiment and detailed descriptions thereof will be omitted. As shown in the same drawing, in the acoustic liner  5   b  according to this embodiment, a plurality of plate members  7  are provided in the throat portion S as the vortex suppressor  6 . These plate members  7  extend from the introduction hole h toward the acoustic space V and are arranged at intervals in a direction including a plane orthogonal to the extending direction of the introduction hole h. Accordingly, a plurality of slits are formed between the plate members  7 . 
     According to the above-described configuration, since the vortexes are dispersed when passing through the slits as the vortex suppressor, it is possible to reduce the resistance (acoustic resistance) when the sound wave introduces to the introduction hole. As a result, since the sound wave is likely to be introduced into the acoustic liner  5   b , it is possible to more efficiently absorb and reduce noise. 
     Additionally, a configuration shown in  FIG.  5    can be also adopted as a modified example of the second embodiment. In an acoustic liner  5   c  of the same drawing, a cavity C having an opening area larger than that of the introduction hole h is formed at a portion of the introduction hole h on the side of the acoustic space V. The plurality of plate members  7   b  are arranged inside the cavity C similarly as described above. A plurality of slits are formed between these plate members  7   b.    
     According to the above-described configuration, since the vortexes are dispersed when passing through the slits as the vortex suppressor  6 , it is possible to reduce the resistance (acoustic resistance) when the sound wave introduces to the introduction hole h. Also, since the plate members  7   b  forming the slits are placed inside the cavity C, it is possible to secure a large effective volume as the acoustic space V. Accordingly, it is possible to significantly reduce noise. 
     Third Embodiment 
     Next, a third embodiment of the present disclosure will be described with reference to  FIG.  6   . Additionally, the same reference numerals will be given to the same configurations as those of the above-described embodiments and detailed description thereof will be omitted. As shown in the same drawing, in an acoustic liner  5   d  according to this embodiment, a plurality of (two as an example) introduction holes h 2  are formed in each acoustic space V. These plurality of introduction holes h 2  are extend to come closer to each other as the introduction holes are extended from the side of the diffuser flow path Fa 1  toward the side of the acoustic space V. These introduction holes h 2  come into contact with each other inside the acoustic space V to form a junction M which is the vortex suppressor  6  for suppressing occurrence of the vortexes of the fluid in the connection area between the plurality of introduction holes h 2  and the acoustic space V. The vortex suppressor  6  is formed to reduce and suppress the vortexes which occur when the sound wave introduces from the plurality of introduction holes h 2  into the acoustic space V. 
     According to the above-described configuration, the sound waves from the plurality of introduction holes h 2  interfere with each other when introducing to the acoustic space V. Accordingly, the occurrence of the vortexes of the fluid is suppressed and thereby the resistance (acoustic resistance) generated when the sound wave introduces to the introduction hole h 2  is reduced. As a result, since the sound wave is likely to be introduced into the acoustic liner  5   d , it is possible to more efficiently absorb and reduce noise. 
     Fourth Embodiment 
     (Configuration of Compressor) 
     Next, a compressor  200  according to a fourth embodiment of the present disclosure will be described with reference to  FIGS.  7  and  8   . As shown in  FIG.  7   , the compressor  200  includes a rotation shaft  201 , an impeller  202 , a casing  203 , and a diffuser vane  204 . 
     The rotation shaft  201  extends along an axis Ac 2  and is rotatable around the axis Ac 2 . The impeller  202  is fixed to the outer peripheral surface of the rotation shaft  201 . The impeller  202  includes a disk  221  and a plurality of blades  222 . The disk  221  is formed in a disk shape centered on the axis Ac 2 . The outer peripheral surface (main surface  221 A) of the disk  221  is formed in a curved surface shape which is curved from the inside toward the outside in the radial direction as the outer peripheral surface is expanded from one side toward the other side in the direction of the axis Ac 2 . 
     The plurality of blades  222  are provided on the main surface  221 A at intervals in the circumferential direction. Although not shown in detail, each blade  222  is curved from the front side toward the rear side in the rotation direction of the rotation shaft  201  as the blade is extended from the inside toward the outside in the radial direction. The impeller  202  rotates together with the rotation shaft  201  to pressure-feed a fluid, introduced from one side in the direction of the axis Ac 2 , toward the outside in the radial direction. 
     The casing  203  surrounds the rotation shaft  201  and the impeller  202  from the outer peripheral side. A compression flow path Pb, for compressing a fluid introduced from the outside of the casing  203 , in which the impeller  202  is accommodated and an exit flow path Fb which is connected to the radial outside of the compression flow path Pb are formed inside the casing  203 . The compression flow path Pb gradually increases in its diameter, corresponding to the outer shape of the impeller  202 , as the compression flow path is expanded from one side toward the other side in the direction of the axis Ac 2 . The exit flow path Fb is connected to the radial outside of the exit of the compression flow path Pb. 
     The exit flow path Fb includes a diffuser flow path Fb 1  and an exit scroll Fb 2 . The diffuser flow path Fb 1  is provided to recover the static pressure of the fluid introduced from the compression flow path Pb. The diffuser flow path Fb 1  is formed in an annular shape which extends from the exit of the compression flow path Pb toward the outside in the radial direction. In the cross-sectional view including the axis Ac 2 , the flow path width of the diffuser flow path Fb 1  is constant over the entire area in the extending direction. The plurality of diffuser vanes  204  are provided in the diffuser flow path Fb 1 . The configuration of the diffuser vane  204  will be described later. 
     The exit scroll Fb 2  is connected to the radial outside of the exit of the diffuser flow path Fb 1 . The exit scroll Fb 2  is formed in a swirl shape extending in the circumferential direction of the axis Ac 2 . The exit scroll Fb 2  has a circular flow path cross-section. Part of the exit scroll Fb 2  is provided with an exhaust hole for guiding a high-pressure fluid to the outside (not shown). 
     (Configuration of Diffuser Vane) 
     The plurality of diffuser vanes  204  are arranged in the diffuser flow path Fb 1  at intervals in the circumferential direction. Each diffuser vane  204  includes a vane body  241  extending toward the rotation direction of the rotation shaft  201  as the vane body is extended outward in the radial direction and a sound reducer  205  which is formed on a surface  241 S of the vane body  241 . 
     As shown in  FIG.  8   , the sound reducer  205  according to this embodiment is a plurality of recessed portions  205 R which are arranged at intervals on the surface  241 S of the vane body  241 . Each recessed portion  205 R is recessed from the surface  241 S toward the inside of the vane body  241 . The cross-sectional area of the portion (entrance portion Ra) on the side of the surface  241 S of the recessed portion  205 R is larger than that of the other portion (bottom portion Rb). Also, a step is formed between the entrance portion Ra and the bottom portion Rb. The cross-sectional area of the entrance portion Ra is constant over the entire area in the extending direction and the cross-sectional area of the bottom portion Rb is also constant over the entire area in the extending direction. 
     The depth L of the recessed portion  205 R from the surface  241 S (that is, the sum of the length of the entrance portion Ra and the bottom portion Rb) is a quarter length of the wavelength λ of the sound to be reduced. That is, L=(¼)×λ. 
     (Operation and Effect) 
     Next, the operation of the compressor  200  will be described. When operating the compressor  200 , the rotation shaft  201  is first rotated around the axis Ac 2  by an external drive source. As the rotation shaft  201  rotates, the impeller  202  also rotates, so that an external fluid is introduced to the compression flow path Pb. The fluid guided to the blades  222  of the impeller  202  in the compression flow path Pb is compressed by a centrifugal force to a high pressure state. This high-pressure flow path is taken out to the outside through the diffuser flow path Fb 1  and the exit scroll Fb 2 . 
     Here, noise occurs while the impeller  202  rotates in the compressor  200 . Among such noise, especially the noise called NZ sound is likely to cause resonance with each part of the compressor  200 . As a result, it is important to reduce and suppress the noise. The NZ sound is noise (discrete frequency sound) of a frequency based on the sum of the number of blades (that is, the number of blades  222 ) N of the impeller  202  and the number of revolutions Z of the rotation shaft  201 . 
     For the purpose of reducing and suppressing such NZ sound, the sound reducer  205  is provided in the vane body  241  disposed in the exit flow path Fb in this embodiment. Accordingly, it is possible to absorb and reduce noise when the fluid passes through the surface  241 S of the vane body  241 . 
     According to the above-described configuration, the sound wave is trapped in the recessed portion  205 R which is the sound reducer  205  and is attenuated inside the recessed portion  205 R. Accordingly, it is possible to reduce the leakage of noise to the outside. 
     According to the above-described configuration, a non-reflective boundary condition in which Zi is equal to pc is realized at the entrance of the recessed portion  205 R by setting the depth L of the recessed portion  205 R from the surface  241 S to a quarter of the wavelength λ of the sound at the frequency to be reduced. Additionally, Zi is the acoustic impedance, ρ is the density, and c is the speed of sound. Accordingly, the sound wave of the reduction target frequency trapped by the recessed portion  205 R can be attenuated without being reflected to the outside. 
     According to the above-described configuration, the cross-sectional area of the portion (entrance portion Ra) on the side of the surface  241 S of the recessed portion  205 R is larger than that of the other portion (bottom portion Rb). Accordingly, it is possible to trap the sound wave in a wider area of the surface  241 S of the vane body  241 . 
     Fifth Embodiment 
     Next, a fifth embodiment of the present disclosure will be described with reference to  FIGS.  9  and  10   . Additionally, the same reference numerals will be given to the same configurations as those of the fourth embodiment and detailed description thereof will be omitted. As shown in  FIG.  9   , in this embodiment, a plurality of passages  205 P are formed on the surface  241 S of the vane body  241  which is the sound reducer  205 . Both ends of the passage  205 P are opened to the surface  241 S. That is, the passage  205 P has a U shape in a cross-sectional view. The length Lp (that is, the length from one end t 1  to the other end t 2 ) of the passage  205 P is set to twice the wavelength λ of the sound to be reduced (Lp=2×λ). 
     According to the above-described configuration, the phase of the sound wave is changed in the passage  205 P and radiated from the other end t 2  by introducing the sound wave from one end t 1  of the passage  205 P. Since the sound wave radiated from the other end t 2  interferes with the sound wave incident on the other end t 2 , the sound wave can be attenuated. 
     Particularly, according to the above-described configuration, the length of the passage  205 P is twice the wavelength of the sound to be reduced. Accordingly, a sound (for example, a sound with a positive phase: the solid line arrow in  FIG.  10   ) entering the passage  205 P from one end t 1  is radiated from the other end t 2  as a sound with a negative phase. Accordingly, it is possible to cancel the positive phase sound (broken line arrow in  FIG.  10   ) entering the other end t 2 . As a result, it is possible to significantly reduce noise of the compressor  200 . 
     Sixth Embodiment 
     (Configuration of Compressor) 
     Next, a compressor  300  according to a sixth embodiment of the present disclosure will be described with reference to  FIGS.  11  to  13   . As shown in  FIG.  11  or  12   , the compressor  300  includes a rotation shaft  301 , an impeller  302 , a casing  303 , a diffuser vane  304 , a speaker wall  305 , a pressure sensor Sp, and a computing device  90 . 
     As shown in  FIG.  11   , the rotation shaft  301  extends along an axis Ac 3  and is rotatable around the axis Ac 3 . The impeller  302  is fixed to the outer peripheral surface of the rotation shaft  301 . The impeller  302  includes a disk  321  and a plurality of blades  322 . The disk  321  is formed in a disk shape centered on the axis Ac 3 . The outer peripheral surface (main surface  321 A) of the disk  321  is formed in a curved surface shape which is curved from the inside toward the outside in the radial direction as the outer peripheral surface is expanded from one side toward the other side in the direction of the axis Ac 3 . 
     A plurality of blades  322  are provided on the main surface  321 A at intervals in the circumferential direction. Although not shown in detail, each blade  322  is curved from the front side toward the rear side in the rotation direction of the rotation shaft  301  as the blade is extended from the inside toward the outside in the radial direction. The impeller  302  rotates together with the rotation shaft  301  to pressure-feed a fluid, introduced from one side in the direction of the axis Ac 3 , toward the outside in the radial direction. 
     The casing  303  surrounds the rotation shaft  301  and the impeller  302  from the outer peripheral side. A compression flow path Pc, for compressing a fluid introduced from the outside of the casing  303 , in which the impeller  302  is accommodated and an exit flow path Fc which is connected to the radial outside of the compression flow path Pc are formed inside the casing  303 . The compression flow path Pc gradually increases in its diameter, corresponding to the outer shape of the impeller  302 , as the compression flow path is expanded from one side toward the other side in the direction of the axis Ac 3 . The exit flow path Fc is connected to the radial outside of the compression flow path Pc. 
     The exit flow path Fc includes a diffuser flow path Fc 1  and an exit scroll Fc 2 . The diffuser flow path Fc 1  is provided to recover the static pressure of the fluid introduced from the compression flow path Pc. The diffuser flow path Fc 1  is formed in an annular shape which extends from the exit of the compression flow path Pc toward the outside in the radial direction. In the cross-sectional view including the axis Ac 3 , the flow path width of the diffuser flow path Fc 1  is constant over the entire area in the extending direction. The plurality of diffuser vanes  304  are provided in the diffuser flow path Fc 1 . The plurality of these diffuser vanes  304  are arranged at intervals in the circumferential direction. 
     The exit scroll Fc 2  is connected to the radially outer exit of the diffuser flow path Fc 1 . The exit scroll Fc 2  is formed in a swirl shape extending in the circumferential direction of the axis Ac 3 . The exit scroll Fc 2  has a circular flow path cross-section. Part of the exit scroll Fc 2  is provided with an exhaust hole for guiding a high-pressure fluid to the outside (not shown). 
     (Configuration of Speaker Wall) 
     The speaker wall  305  is provided on the wall surface at the other side in the direction of the axis Ac 3  in the diffuser flow path Fc 1 . The speaker wall  305  is embedded in this wall surface to face the diffuser flow path Fc 1 . The speaker wall  305  has an annular shape centered on the axis Ac 3 . The speaker wall  305  is provided to reduce noise caused by the fluid flowing through the diffuser flow path Fc 1 . 
     As shown in  FIG.  12   , the speaker wall  305  has a plate shape and includes a plurality of speaker elements  351  arranged in the radial direction and the circumferential direction. The computing device  90  is connected to each speaker element  351  via a signal line. Also, a pressure sensor Sp is provided below these speaker elements  351  (that is, the upstream side of the speaker wall  305  in the diffuser flow path Fc 1 ). The pressure sensor Sp detects noise in the diffuser flow path Fc 1  as pressure fluctuations and sends the detection result to the computing device  90  as an electrical signal. 
     (Configuration of Computing Device) 
     The computing device  90  sends a signal to the speaker element  351  to emit a sound (canceling sound) having a frequency that cancels out a target sound (that is, a sound having a frequency to be reduced) on the basis of the detection value of the pressure sensor Sp. 
     As shown in  FIG.  13   , the computing device  90  is a computer including a CPU  91  (Central Processing Unit), a ROM  92  (Read Only Memory), a RAM  93  (Random Access Memory), an HDD  94  (Hard Disk Drive), and a signal transmission/reception module  95  (I/O: Input/Output). The signal transmission/reception module  95  receives the value of the pressure of the diffuser flow path Fc 1  detected by the pressure sensor Sp as an electrical signal. Also, the signal transmission/reception module  95  transmits an electrical signal for outputting the canceling sound to the speaker element  351 . Additionally, the signal transmission/reception module  95  may transmit and receive an amplified signal via, for example, a charge amplifier or the like. 
     As shown in  FIG.  14   , the CPU  91  of the computing device  90  includes a pressure acquisition unit  81 , a frequency analysis unit  82 , an opposite-phase generation unit  83 , and a signal oscillation unit  84  by executing a program stored in advance in the device itself. The pressure acquisition unit  81  receives a sound as the pressure value detected by the pressure sensor Sp. The frequency analysis unit  82  analyzes the frequency of the input sound and determines the frequencies to be reduced. The opposite-phase generation unit  83  generates a sound (canceling sound) having a frequency opposite in phase to that of the target sound. The signal oscillation unit  84  outputs an electrical signal to the speaker element  351  to output the canceling sound to each speaker element  351 . 
     (Operation and Effect) 
     Next, the operation of the compressor  300  will be described. When operating the compressor  300 , the rotation shaft  301  is first rotated around the axis Ac 3  by an external drive source. As the rotation shaft  301  rotates, the impeller  302  also rotates, so that an external fluid is introduced to the compression flow path Pc. The fluid guided to the blades  322  of the impeller  302  in the compression flow path Pc is compressed by a centrifugal force to a high pressure state. This high-pressure flow path is taken out to the outside through the diffuser flow path Fc 1  and the exit scroll Fc 2 . 
     Here, noise occur while the impeller  302  rotates in the compressor  300 . Among such noise, especially the noise called NZ sound is likely to cause resonance with each part of the compressor  300 . As a result, it is important to reduce and suppress the noise. The NZ sound is noise (discrete frequency sound) of a frequency based on the sum of the number of blades (that is, the number of blades  322 ) N of the impeller  302  and the number of revolutions Z of the rotation shaft  301 . 
     For the purpose of reducing and suppressing such NZ sound, the speaker wall  305  is provided in the diffuser flow path Fc 1  in this embodiment. According to the above-described configuration, the speaker wall  305  emits a sound having a frequency that cancels out the sound as the pressure fluctuation detected by the pressure sensor Sp. This sound can cancel the noise in the exit flow path Fc. Also, even if the frequency of the noise changes with time, the pressure sensor Sp immediately detects this change, and the computing device  90  generates a sound for having a frequency canceling another sound having different frequency. Accordingly, the noise reduced state can be maintained autonomously regardless of the operating state of the compressor  300 . 
     According to the above-described configuration, the frequency analysis unit  82  determines a target sound based on the detection value of the pressure sensor Sp, and the opposite-phase generation unit  83  generates a sound having a frequency opposite in phase to that of the target sound. The signal oscillation unit  84  transmits a signal to the speaker wall  305  to emit the opposite-phase sound. Accordingly, noise of a specific frequency can be selectively and effectively reduced. 
     Seventh Embodiment 
     Next, a seventh embodiment of the present disclosure will be described with reference to  FIG.  15   . Additionally, the same reference numerals will be given to the same configurations as those of the above-described sixth embodiment and detailed description thereof will be omitted. As shown in the same drawing, in this embodiment, canceling sounds of which frequencies are different from each other are emitted from the plurality of speaker elements  351   b  of the speaker wall  305   b . The opposite-phase generation unit  83  generates the canceling sounds having frequencies opposite in phase to those of a plurality of target sounds. The signal oscillation unit  84  sends signals to the speaker elements  351   b  to emit the canceling sounds. For example, a particular speaker element  351   b  emits a canceling sound of which frequency is 2 kHz, and a different speaker element  351   b  emits another canceling sound of which frequency is 2.1 kHz. 
     According to the above-described configuration, the opposite-phase generation unit  83  generates the canceling sounds having frequencies opposite in phase to those of the target sounds. The signal oscillation unit  84  makes the speaker elements  351   b  emit the canceling sounds having frequencies opposite in phase to those of the target sounds. Thus, it is possible to reduce the noise which occurs in the exit flow path Fc in every frequency bands. As a result, it is possible to significantly suppress the noise of the compressor  300 . 
     Other Embodiments 
     The embodiments of the present disclosure have been described above. Additionally, various changes and modifications can be made to the above-described configuration without departing from the gist of the present disclosure. For example, it is also possible to apply a combination of different types of vortex suppressors  6  described in the first to third embodiments and the configuration of the introduction hole h 2  described in the third embodiment to one acoustic liner  5 . 
     For example, it is also possible to apply a combination of different types of sound reducers  205  (the recessed portion  205 R and the passage  205 P) described in the fourth and fifth embodiments to one vane body  241 . 
     APPENDIX 
     The compressors  100 ,  200 , and  300  described in the embodiments are understood as follows, for example. 
     (1) The compressor  100  according to a first aspect includes: the rotation shaft  1  which is allowed to rotate around the axis Ac 1 ; the impeller  2  which is configured to pressure-feed a fluid from one side in the direction of the axis Ac 1  toward the outside in the radial direction by rotating together with the rotation shaft  1 ; the casing  3  surrounding the rotation shaft  1  and the impeller  2  and in which exit flow path Fa guiding the fluid pressure-fed from the impeller  2  is formed; and the acoustic liner  5  which is installed to face the inside of the exit flow path Fa in the casing  3 , wherein the acoustic liner  5  includes the acoustic space V which is formed inside the acoustic liner  5 , the introduction hole h communicating the acoustic space V with the exit flow path Fa, and the vortex suppressor  6  which is placed in a connection area between the introduction hole h and the acoustic space V and is configured to suppress vortexes which occur in the connection area. 
     According to the above-described configuration, since the vortex suppressor  6  is provided, the occurrence of the vortexes of the fluid in the acoustic space V is suppressed. Accordingly, the resistance (acoustic resistance) generated when the sound wave introduces to the introduction hole h is reduced. As a result, it is possible to more efficiently absorb and attenuate the sound wave by the acoustic liner  5 . 
     (2) In the compressor  100  according to a second aspect, the vortex suppressor  6  is formed of the foam metal m and covering the introduction hole h inside the acoustic space V. 
     According to the above-described configuration, the vortex suppressor  6  formed of the foam metal m is installed to cover the introduction hole h. Since the vortexes are dispersed by the foam metal m, it is possible to reduce the resistance (acoustic resistance) generated when the sound wave introduces to the introduction hole h. 
     (3) In the compressor  100  according to a third aspect, the vortex suppressor  6  includes a plurality of plate members  7  extending from the introduction hole h toward the acoustic space V and between which a plurality of slits are formed. 
     According to the above-described configuration, since the vortexes are dispersed when passing through the slits as the vortex suppressor  6 , it is possible to reduce the resistance (acoustic resistance) when the sound wave introduces to the introduction hole h. 
     (4) In the compressor  100  according to a fourth aspect, the cavity C having an opening area larger than that of the introduction hole h is formed at a portion of the introduction hole h on the side of the acoustic space V and the vortex suppressor  6  is placed inside the cavity C and includes a plurality of plate members  7   b  extending from the introduction hole h toward the acoustic space V and between which a plurality of slits are formed. 
     According to the above-described configuration, since the vortexes are dispersed when passing through the slits as the vortex suppressor  6 , it is possible to reduce the resistance (acoustic resistance) when the sound wave introduces to the introduction hole h. Also, since the plate members  7   b  forming the slits are placed inside the cavity C, it is possible to secure for cavity C a large effective volume as the acoustic space V. Accordingly, it is possible to significantly reduce noise. 
     (5) The compressor  100  according to a fifth aspect includes: the rotation shaft  1  which is allowed to rotate around the axis Ac 1 ; the impeller  2  which is configured to pressure-feed a fluid from one side in the direction of the axis Ac 1  toward the outside in the radial direction by rotating together with the rotation shaft  1 ; the casing  3  surrounding the rotation shaft  1  and the impeller  2  and in which the exit flow path Fa guiding the fluid pressure-fed from the impeller  2  is formed; and the acoustic liner  5   d  which is installed to face the inside of the exit flow path Fa in the casing  3 , wherein the acoustic liner  5  includes the acoustic space V which is formed inside the acoustic liner  5  and the plurality of introduction holes h 2  communicating the acoustic space V with the exit flow path Fa, and the plurality of introduction holes h 2  are formed to communicate with the acoustic space V while coming closer to each other as the introduction holes are extended from the exit flow path Fa toward the acoustic space V and perform as the vortex suppressor  6  suppressing vortexes which occur in a connection area between the plurality of introduction holes h 2  and the acoustic space V. 
     According to the above-described configuration, the sound waves from the plurality of introduction holes h 2  interfere with each other when introducing to the acoustic space V. Accordingly, the occurrence of the vortexes of the fluid is suppressed and thereby the resistance (acoustic resistance) generated when the sound wave introduces to the introduction hole h 2  is reduced. As a result, it is possible to more efficiently absorb and reduce the sound wave by the acoustic liner  5   d.    
     (6) The compressor  200  according to a sixth aspect includes: the rotation shaft  201  which is allowed to rotate around the axis Ac 2 ; the impeller  202  which is configured to pressure-feed a fluid from one side in the direction of the axis Ac 2  toward the outside in the radial direction by rotating together with the rotation shaft  201 ; the casing  203  surrounding the rotation shaft  201  and the impeller  202  and in which the diffuser flow path Fb 1  guiding the fluid pressure-fed from the impeller  202  toward the outside in the radial direction is formed; and the plurality of diffuser vanes  204  which are provided in the diffuser flow path Fb 1  at intervals in the circumferential direction of the axis Ac 2 , wherein each of the diffuser vanes  204  includes the vane body  41  extending toward the rotation direction of the rotation shaft  201  as the diffuser vane is extended toward the outside in the radial direction and the sound reducer  205  which is formed on the surface  241 S of the vane body  241 . 
     According to the above-described configuration, the sound reducer  205  is formed on the surface  241 S of the vane body  241 . Accordingly, it is possible to absorb and reduce the noise when the fluid flows along the surface  241 S of the vane body  241 . 
     (7) In the compressor  200  according to a seventh aspect, the sound reducer  205  is the plurality of recessed portions  205 R which are formed on the surface  241 S of the vane body  241 . 
     According to the above-described configuration, the sound wave is trapped in the recessed portion  205 R as the sound reducer  205  and is attenuated inside the recessed portion  205 R. Accordingly, it is possible to reduce the leakage of noise to the outside. 
     (8) In the compressor  200  according to an eighth aspect, the depth of the recessed portion  205 R from the surface  241 S is a quarter length of the wavelength of the target sound. 
     According to the above-described configuration, a non-reflective boundary condition in which Zi is equal to pc is realized at the entrance of the recessed portion  205 R by setting the depth of the recessed portion  205 R from the surface  241 S to a quarter of the wavelength λ of the sound at the frequency to be reduced. Accordingly, the sound wave of the reduction target frequency trapped by the recessed portion  205 R can be attenuated inside the recessed portion  205 R without being reflected to the outside. (9) In the compressor  200  according to a ninth aspect, the cross-sectional area of the portion on the side of the surface  241 S of the recessed portion  205 R is larger than that of the other portion. 
     According to the above-described configuration, the cross-sectional area of the portion on the side of the surface  241 S of the recessed portion  205 R is larger than that of the other portion. In other words, the cross-sectional area of the entrance of the recessed portion  205 R is larger than the cross-sectional area of the bottom portion. Accordingly, it is possible to trap the sound wave in a wider area of the surface  241 S of the vane body  241 . 
     (10) In the compressor  200  according to a tenth aspect, the sound reducer  205  is the plurality of passages  205 P each of which both ends are opened to the surface  241 S of the vane body  241 . 
     According to the above-described configuration, since the sound wave is introduced from one end of the passage  205 P, the sound wave is radiated from the other end while the phase of the sound wave is changed in the passage  205 P. Since the sound wave radiated from the other end interferes with the sound wave incident on the other end, the sound wave can be attenuated. 
     (11) In the compressor  200  according to an eleventh aspect, the length of the passage  205 P from one end to the other end is twice the wavelength of the target sound. 
     According to the above-described configuration, the length of the passage  205 P is twice the wavelength of the sound to be reduced. Accordingly, the sound (having a positive phase) incident from one end to the passage  205 P is emitted as the sound of the negative phase from the other end. Accordingly, it is possible to cancel the sound of the positive phase entering the other end. 
     (12) The compressor  300  according to a twelfth aspect includes: the rotation shaft  301  which is allowed to rotate around the axis Ac 3 ; the impeller  302  which is configured to pressure-feed a fluid from one side in the direction of the axis Ac 3  toward the outside in the radial direction by rotating together with the rotation shaft  301 ; the casing  303  surrounding the rotation shaft  301  and the impeller  302  and in which the exit flow path Fc guiding the fluid pressure-fed from the impeller  302  is formed; the speaker wall  305  which is provided to face the inside of the exit flow path Fc in the casing  303 ; the pressure sensor Sp which is configured to detect a pressure inside the exit flow path Fc; and the computing device  90  which is configured to send a signal to the speaker wall  305  to emit a sound having a frequency for canceling a target sound on the basis of the detected value of the pressure sensor Sp. 
     According to the above-described configuration, the speaker wall  305  emits a sound having a frequency for canceling the sound as the pressure fluctuation detected by the pressure sensor Sp. By this sound, the noise inside the exit flow path Fc can be canceled. Even when the frequency of the noise changes with time, the pressure sensor Sp immediately detects this change and the computing device  90  generates a sound having a frequency for canceling another sound having different frequency. Accordingly, noise can be reduced autonomously regardless of the operating state of the compressor  300 . 
     (13) In the compressor  300  according to a thirteenth aspect, the computing device  90  includes the frequency analysis unit  82  which performs frequency analysis on a detected value of the pressure sensor Sp to be decomposed into a plurality of frequencies, the opposite-phase generation unit  83  which generates a frequency opposite in phase to that of a target sound included in the plurality of frequencies decomposed by the frequency analysis unit  82 , and the signal oscillation unit  84  which sends a signal to the speaker wall  305  to emit a sound of the frequency generated by the opposite-phase generation unit  83 . 
     According to the above-described configuration, the frequency analysis unit  82  determines a target sound on the basis of the detected value of the pressure sensor Sp and the opposite-phase generation unit  83  generates a sound having a frequency opposite in phase to that of the target sound. The signal oscillation unit  84  transmits a signal to the speaker wall  305  to emit the opposite-phase sound. Accordingly, noise of a specific frequency can be selectively and effectively reduced. 
     (14) In the compressor  300  according to a fourteenth aspect, the speaker wall  305  includes the plurality of speaker elements  351   b , the opposite-phase generation unit  83  generates a plurality of frequencies opposite in phase to those of a plurality of target sounds, and the signal oscillation unit  84  sends signals to the plurality of speaker elements  351   b  to emit sounds having opposite-phase frequencies. 
     According to the above-described configuration, the opposite-phase generation unit  83  generates sounds having frequencies opposite in phase to those of a plurality of target sounds. The signal oscillation unit  84  makes the speaker elements  351   b  emit the canceling sounds having frequencies opposite in phase to those of target sounds. Thus, noise which occurs in the exit flow path Fc can be reduced in every frequency bands. 
     INDUSTRIAL APPLICABILITY 
     According to the present disclosure, it is possible to provide a compressor having an excellent noise-reduction property. 
     REFERENCE SIGNS LIST 
     
         
         
           
               100 ,  200 ,  300  Compressor 
               1 ,  201 ,  301  Rotation shaft 
               2 ,  202 ,  302  Impeller 
               3 ,  203 ,  303  Casing 
               4  Return vane 
               5 ,  5   b ,  5   c ,  5   d  Acoustic liner 
               6  Vortex suppressor 
               7 ,  7   b  Plate member 
               21 ,  221 ,  321  Disk 
               21 A,  221 A,  321 A Main surface 
               22 ,  222 ,  322  Blade 
               81  Pressure acquisition unit 
               82  Frequency analysis unit 
               83  Opposite-phase generation unit 
               84  Signal oscillation unit 
               90  Computing device 
               91  CPU 
               92  ROM 
               93  RAM 
               94  HDD 
               95  Signal transmission/reception module 
               204 ,  304  Diffuser vane 
               205  Sound reducer 
               205 R Recessed portion 
               205 P Passage 
               241  Vane body 
               241 S Surface 
               305 ,  305   b  Speaker wall 
               351 ,  351   b  Speaker element 
             Ac 1 , Ac 2 , Ac 3  Axis 
             C Cavity 
             Fa, Fb, Fc Exit flow path 
             Fa 1 , Fb 1 , Fc 1  Diffuser flow path 
             Fa 2 , Fb 2 , Fc 2  Exit scroll 
             h, h 2  Introduction hole 
             in Foam metal 
             M Junction 
             Pa, Pb, Pc Compression flow path 
             S Throat portion 
             Sp Pressure sensor 
             t 1  One end 
             t 2  Other end 
             V Acoustic space