Abstract:
This invention relates to a double layer acoustic liner for attenuating noise and consisting of a plurality of cells formed in a plate in a manner to form an array of resonators, and a fluid processing device and method incorporating same.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuing application of co-pending parent application Ser. No. 09/745,862 filed on Dec. 21, 2000. 
    
    
     BACKGROUND 
     This invention relates to an acoustic liner of two layers and a fluid pressurizing device and method utilizing same. 
     Fluid pressurizing devices, such as centrifugal compressors, are widely used in different industries for a variety of applications involving the compression, or pressurization, of a gas. However, a typical compressor produces a relatively high noise level which is an obvious nuisance to the people in the vicinity of the device. This noise can also cause vibrations and structural failures. 
     For example, the dominant noise source in a centrifugal compressor is typically generated at the locations of the impeller exit and the diffuser inlet, due to the high velocity of the fluid passing through these regions. The noise level becomes higher when discharge vanes are installed in the diffuser to improve pressure recovery, due to the aerodynamic interaction between the impeller and the diffuser vanes. 
     Various external noise control measures such as enclosures and wrappings have been used to reduce the relative high noise levels generated by compressors, and similar devices. These external noise reduction techniques can be relatively expensive especially when they are often offered as an add-on product after the device is manufactured. 
     Also, internal devices, usually in the form of acoustic liners, have been developed which are placed in the compressors, or similar devices, for controlling noise inside the gas flow paths. These liners are often based on the well-known Helmholtz resonator principle according to which the liners dissipate the acoustic energy when the sound waves oscillate through perforations in the liners, and reflect the acoustic energy upstream due to the local impedance mismatch caused by the liner. Examples of Helmholtz resonators are disclosed in U.S. Pat. Nos. 4,100,993; 4,135,603; 4,150,732; 4,189,027; 4,443,751; 4,944,362; and 5,624,518. 
     A typical Helmholtz array acoustic liner is in the form of a three-piece sandwich structure consisting of honeycomb cells sandwiched between a perforated facing sheet and a back plate. Although these three-piece designs have been successfully applied to suppress noise in aircraft engines, it is questionable whether or not they would work in fluid pressurizing devices, such as centrifugal compressors. This is largely due to the possibility of the perforated facing sheet of the liner breaking off its bond with the honeycomb under extreme operating conditions of the compressor, such as, for example, during rapid depressurization caused by an emergency shut down of the compressor. In the event that the perforated facing sheet becomes loose, it not only makes the acoustic liners no longer functional but also causes excessive aerodynamic losses, and even the possibility of mechanical catastrophic failure, caused by the potential collision between the break-away perforated sheet metal and the spinning impeller. 
     Therefore what is needed is a system and method for reducing the noise in a fluid pressurizing device utilizing a Hemholtz array acoustic liner while eliminating its disadvantages. 
     SUMMARY 
     Accordingly an acoustic liner is provided, as well as a fluid processing device and method incorporating same, according to which the liner attenuates noise and consists of one or more acoustic liners each including a plurality of cells formed in a plate in a manner to form an array of resonators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a portion of a gas pressurizing device incorporating a pair of acoustic liners according to an embodiment of the present invention. 
     FIG. 2 is an enlarged cross-sectional view of one of the acoustic liners of FIG.  1 . 
     FIG. 3 is an enlarged elevational view of a portion of the liner of FIG.  2 . 
     FIGS. 4 and 5 are views similar to that of FIG. 1, but depicting additional acoustic liners disposed at other locations in the fluid pressurizing device. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts a portion of a high pressure fluid pressurizing device, such as a centrifugal compressor, including a casing  10  defining an impeller cavity  10   a  for receiving an impeller  12  which is mounted for rotation in the cavity. It is understood that a power-driven shaft (not shown) rotates the impeller  12  at a high speed, sufficient to impart a velocity pressure to the gas drawn into the compressor via the inlet. 
     The impeller  12  includes a plurality of impeller blades  12   a  arranged axi-symmetrically around the latter shaft for discharging the gas into a diffuser passage, or channel  14  formed in the casing  10  radially outwardly from the chamber  10   a  and the impeller  12 . The channel  14  receives the high pressure fluid from the impeller  12  before it is passed to a volute, or collector,  16 . The diffuser channel  14  functions to convert the velocity pressure of the gas into static pressure which is coupled to a discharge volute, or collector  16  also formed in the casing and connected with the channel. Although not shown in FIG. 1, it is understood that the discharge volute  16  couples the compressed gas to an outlet of the compressor. 
     Due to centrifugal action of the impeller blades  12   a,  gas can be compressed to a relatively high pressure. The compressor is also provided with conventional labyrinth seals, thrust bearings, tilt pad bearings and other apparatus conventional to such compressors. Since this structure is conventional, it will not be shown or described in any further detail. 
     A mounting bracket  20  is secured to an inner wall of the casing  10  defining the diffuser channel  14  and includes a base  22  disposed adjacent the outer end portion of the impeller and a plate  24  extending from the base and along the latter wall of the casing. 
     Two one-piece, unitary, annular acoustic liners  28  and  30  are mounted in a groove in the plate  24  of the bracket  20  in a abutting relationship and each is annular in shape and extends around the impeller  12  for 360 degrees. The upper section of the liner  28  is shown in detail in FIGS. 2 and 3, and is formed of an annular, relatively thick, unitary shell, or plate  32  preferably made of steel. The plate  32  is attached to the bracket plate  24  in any conventional manner, such as by a plurality of bolts, or the like. 
     A series of relatively large cells, or openings,  34  are formed through one surface of the plate  32  and extend through a majority of the thickness of the plate but not through its entire thickness. A series of relatively small cells  36  extend from the bottom of each cell  34  to the opposite surface of the plate  32 . Each cell  34  is shown having a disc-like cross section and each cell  36  is in the form of a bore for the purpose of example, it being understood that the shapes of the cells  34  and  36  can vary within the scope of the invention. 
     According to one embodiment of the present invention, each cell  34  is formed by drilling a relative large-diameter counterbore through one surface of the plate  32 , which counterbore extends through a majority of the thickness of the plate but not though the complete thickness of the plate. Each cell  36  is formed by drilling a bore, or passage, through the opposite surface of the plate  32  to the bottom of a corresponding cell  34  and thus connects the cell  34  to the diffuser channel  14 . 
     As shown in FIG. 3, the cells  34  are formed in a plurality of annular extending rows along the entire annular area of the plate  32 , with the cells  34  of a particular row being staggered, or offset, from the cells of its adjacent row(s). A plurality of cells  36  are associated with each cell  34  and the cells  36  can be randomly disposed relative to their corresponding cell  34 , or, alternately, can be formed in any pattern of uniform distribution. 
     With reference to FIG. 1, the liner  30  is similar to the liner  28  and, as such, is formed of an annular, relatively thick, unitary shell, or plate  42  (FIG.  1 ), preferably made of steel, and is attached to the liner  28  in any conventional manner such as by a plurality of bolts, or the like. A series of relatively large cells, or openings,  44  are formed through one surface of the plate  42  and a series of relatively small cells  46  extend from the bottom of each cell  34  to the opposite surface of the plate  32 . Since the cells  44  and  46  are similar to the cells  34  and  36 , respectively, they will not be described in further detail. Although not shown in the drawings, it is understood that the liners  30  and  28  can be of different thickness. 
     The liners  28  and  30  are mounted in the bracket plate  24  with the surface of the liner  28  through which the cells  34  extend abutting the surface of the liner  30  through which the cells  46  extend. Also, the cells  34  of the liner  28  are in alignment with the cells  44  of the liner  30 . The open ends of the cells  44  of the liner  30  are capped by the underlying wall of the plate  24  of the bracket  20 , and the open ends of the cells  34  of the liner  28  are capped by the corresponding surface of the liner  30 . The cells  34  of the liner  28  and the cells  44  of the liner  30  are connected by the cells  46  of the liner  30 , due to their alignment. 
     Due to the firm contact between the liners  28  and  30 , and between the liner  30  and the corresponding wall of the plate  24  of the bracket  20 , and due to the cells  36  and  46  connecting the cells  34  and  44  to the diffuser channel  14 , the cells work collectively as an array of acoustic resonators in series. As such, the liners  28  and  30  attenuate the sound waves generated in the casing  10  by the fast-rotation of the impeller  12 , and by its associated components, and eliminate, or at least minimize, the possibility that the noise will by-pass the liners and pass through a different path. 
     Moreover, the dominant noise component commonly occurring at the blade passing frequency, or other high frequency can be effectively lowered by tuning the liners  28  and  30  so that the maximum sound attenuation occurs around the latter frequency. This can be achieved by varying the volume of the cells  34  and  44 , and/or the cross-section area, the number, and/or the length of the cells  36  and  46 . The provision of the two liners  28  and  30  enables them to attenuate noise in a much wider frequency range than if a single liner were used, thus enabling a maximum amount of attenuation of the acoustic energy generated by the rotating impeller  12  and its associated components to be achieved. 
     According to the embodiment of FIG. 4, two one-piece, unitary, annular liners  48  and  50  are secured in a groove formed in the internal wall of the casing  10  opposite to the liners  28  and  30 . The liner  48  extends in the bottom of the groove and is connected to the structure forming the groove in any conventional manner, such as by a plurality of bolts, or the like; and the liner  50  extends in the groove in an abutting relationship to the liner  48  and is connected to the liner  48  in any conventional manner, such as by a plurality of bolts, or the like. The liner  50  partially defines, with the liner  30 , the diffuser channel  14 . Since the liners  48  and  50  are similar to, and functions the same as, the liners  28  and  30 , they will not be described in any further detail. 
     Due to the firm contact between the liners  48  and  50 , and between the liner  48  and the corresponding wall of the casing  10 , and due to the arrangement of the respective cells of the liners, the cells work collectively as arrays of acoustic resonators in series. As such, the liners  48  and  50  attenuate the sound waves generated in the casing  10  by the fast-rotation of the impeller  12 , and by its associated components, and eliminate, or at least minimize, the possibility that the noise will by-pass the liners and pass through a different path. 
     Moreover, the dominant noise component commonly occurring at the blade passing frequency, or other high frequency can be effectively lowered by tuning the liners  48  and  50  so that the maximum sound attenuation occurs around the latter frequency. This can be achieved by varying the volume and/or the cross-section area, the number, and/or the length of their respective cells. The provision of the two liners  48  and  50  enables them to attentuate noise in a much wider frequency range than if a single liner were used, thus enabling a maximum amount of attenuation of the acoustic energy generated by the rotating impeller  12  and its associated components to be achieved. 
     Also, two one-piece, unitary, annular liners  54  and  56  are mounted in a groove formed in the casing  10  to the rear of the impeller  12 . The liner  54  extends in the bottom of the groove and is connected to the structure forming the groove in any conventional manner, such as by a plurality of bolts, or the like; and the liner  56  extends in the groove in an abutting relationship to the liner  54  and is connected to the liner  54  in any conventional manner, such as by a plurality of bolts, or the like. The liner  56  partially defines, with the liner  52 , the chamber in which the impeller  12  rotates. 
     The liners  54  and  56  have a smaller outer diameter than the liners  28 ,  30 ,  48  and  50 , but otherwise are similar to, and are mounted in the same manner as, the latter liners. 
     Due to the firm contact between the liners  54  and  56 , and between the liner  54  and the corresponding wall of the casing  10 , and due to the arrangement of the respective cells of the liners, the cells work collectively as arrays of acoustic resonators in series. As such, the liners  54  and  56  attenuate the sound waves generated in the casing  10  by the fast-rotation of the impeller  12 , and by its associated components, and eliminate, or at least minimize, the possibility that the noise will by-pass the liners and pass through a different path. 
     Moreover, the dominant noise component commonly occurring at the blade passing frequency, or other high frequency can be effectively lowered by tuning the liners  54  and  56  so that the maximum sound attenuation occurs around the latter frequency. This can be achieved by varying the volume and/or the cross-section area, the number, and/or the length of their respective cells. The provision of the two liners  54  and  56  enables them to attenuate noise in a broader frequency range than if a single liner were used, thus enabling a maximum amount of attenuation of the acoustic energy generated by the rotating impeller  12  and its associated components to be achieved. 
     Still another preferred location for liners is shown in FIG. 5 which depicts an inlet conduit  60  that introduces gas to the inlet of the impeller  12 . The upper portion of the conduit  60  is shown extending above the centerline C/L of the conduit and the casing  10 , as viewed in FIG.  5 . 
     A one-piece, unitary, liner  64  is flush-mounted on the inner wall of the conduit  60  with the radial outer portion being shown. The liner  64  is in the form of a curved shell, preferably cylindrical or conical in shape, is disposed in an annular groove formed in the inner surface of the conduit  60 , and is secured in the groove in any known manner. Since the liner  64  is otherwise similar to the liners  28 ,  30 ,  48 ,  50 ,  52 ,  54 , and  56 , it will not be described in further detail. 
     A one-piece, unitary, liner  66  is also disposed in the latter annular groove and extends around the liner  64  with its inner surface abutting the outer surface of the liner  64 . The liner  66  is in the form of a curved shell, preferably cylindrical or conical in shape having a diameter larger than the diameter of the liner  64  and is secured to the liner  64  in any conventional manner, such as by a plurality of bolts, or the like. Since the liners  64  and  66  are otherwise similar to the liners  28 ,  30 ,  48 ,  50 ,  52 ,  54 , and  56 , and function in the same manner to significantly reduce the noise in the casing  10 , they will not be described in further detail. 
     Due to the firm contact between the liners  64  and  66 , and between the liner  66  and the corresponding wall of the casing  10  defining the latter groove, and due to the arrangement of the respective cells of the liners, and their location relative the inlet conduit  60 , the cells work collectively as arrays of acoustic resonators in series. As such, the liners  64  and  66  attenuate the sound waves generated in the casing  10  by the fast-rotation of the impeller  12 , and by its associated components, and eliminate, or at least minimize, the possibility that the noise will by-pass the liners and pass through a different path. 
     Moreover, the dominant noise component commonly occurring at the blade passing frequency, or other high frequency can be effectively lowered by tuning the liners  64  and  66  so that the maximum sound attenuation occurs around the latter frequency. This can be achieved by varying the volume and/or the cross-section area, the number, and/or the length of their respective cells. The provision of the two liners  64  and  66  enables them to attenuate noise in a broader frequency range than if a single liner were used, thus enabling a maximum amount of attenuation of the acoustic energy generated by the rotating impeller  12  and its associated components to be achieved. 
     Also, given the fact that the frequency of the dominant noise component in a fluid pressurizing device of the above type varies with the compressor speed, the number of the smaller cells per each larger cell of each liner can be varied spatially across the liners so that the entire liner is effective to attenuate noise in a broader frequency band. Consequently, the liners  28 ,  30 ,  48 ,  50 ,  52 ,  54 ,  56 ,  64 , and  66  can efficiently and effectively attenuate noise, not just in constant speed machines, but also in variable speed compressors, or other fluid pressurizing devices. 
     In addition to the attenuation of the acoustic energy and the elimination of by-passing of the latter energy, as discussed above, the one-piece unitary construction of the liners in the above embodiments renders the liners mechanically stronger when compared to the composite designs discussed above. Thus, the liners provide a very rigid inner wall to the internal flow in the fluid pressurizing device, and have less or no deformation when subject to mechanical and thermal loading, and thus have no adverse effect on the aerodynamic performance of a fluid pressurizing device, such as a centrifugal compressor, even when they are installed in the narrow passages such as the diffusor channels, or the like. 
     Variations 
     The specific arrangement and number of liners in accordance with the above embodiments are not limited to the number shown. Thus, the liners to either side of the diffuser channel and/or the impeller and/or the inlet conduit. 
     The specific technique of forming the cells in the liners can vary from that discussed above. For example, a one-piece liner can be formed in which the cells are molded in their respective plates. 
     The relative dimensions, shapes, numbers and the pattern of the cells of each liner can vary. 
     The liners are not limited to use with a centrifugal compressor, but are equally applicable to other fluid pressurizing devices in which aerodynamic effects are achieved with movable blades. 
     Each liner can extend for degrees around the axis of the impeller and the inlet conduit as disclosed above; or each liner can be formed into segments which extend an angular distance less than 360 degrees. 
     The spatial references used above, such as “bottom”, “inner”, “outer”, “side” etc, are for the purpose of illustration only and do not limit the specific orientation or location of the structure. 
     Since other modifications, changes, and substitutions are intended in the foregoing disclosure, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.