Abstract:
This invention relates to an 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:
This invention relates to an acoustic liner 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 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 and an acoustic liner according to an embodiment of the present invention. 
     FIG. 2 is an enlarged cross-sectional view of the acoustic liner of FIG.  1 . 
     FIG. 3 is an enlarged elevational view of a portion of the liner of FIGS. 1 and 2. 
     FIG. 4 is a view similar to that of FIG. 1, but depicting additional acoustic liners disposed at other locations in the fluid pressurizing device. 
     FIG. 5 is a view similar to that of FIG. 1, but depicting another acoustic liner disposed around the inlet duct of 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. The impeller has openings, or flow passages, formed therethrough, one of which is shown by the reference numeral  12   a . A diffuser channel  14  is provided in the casing  10  radially outwardly from the chamber  10   a  and the impeller  12 , and receives the high pressure fluid from the impeller before it is passed to a volute, or collector,  16  for discharge from the device. 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 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. 
     A one-piece, unitary, annular acoustic liner  30  is mounted to the bracket  20  with its upper section being shown in detail in FIGS. 2 and 3. The liner  30  is formed of an annular, relatively thick, unitary shell, or plate  32  which is secured to the plate  24  of the bracket  20  in any known manner. The plate  32  is preferably made of steel, and is attached to the bracket plate  24  by a plurality of equally-spaced bolts, or the like. The liner  30  is annular in shape and extends around the impeller  12  for 360 degrees. 
     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 shown 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. 
     The liner  30  is installed on the inner wall of the plate  24  of the bracket  20  so that the open ends of all the cells  34  are capped by the underlying wall of the plate. Due to the firm contact between the plate  32  of the liner and the bracket plate  24 , and due to the cells  36  connecting each cell  34  to the diffuser area, the cells work collectively as array of Helmholtz acoustic resonators. Thus, the sound waves generated in the casing  10  by the high-rotation of the impeller  12 , and by its associated components, are attenuated as they pass by the liner  30 . 
     Moreover, the dominant noise component commonly occurring at the blade passing frequency, or other high frequency can be effectively lowered by tuning the liner  30  so that its maximum sound attenuation occurs around the latter frequency. This can be achieved by varying the volume of the cells  34 , and/or the cross-section area, the number, and/or the length of the cells  36  to tune the liner. Thus, a maximum amount of attenuation of the acoustic energy generated by the rotating impeller  12  and its associated components can be achieved. 
     According to the embodiment of FIG. 4, an additional one-piece, unitary, annular liner  40  is provided on the internal wall of the casing  10  opposite the bracket plate  24  and defining, with the bracket plate, the diffuser channel  14 . To this end, the latter wall is cut out as shown to accommodate the liner  40 , which is identical to the liner  30  and therefore will not be described in detail. The liner  40  functions in an identical manner as the liner  30  as discussed above, and thus also contributes to a significant reduction of the noise generated by the impeller  12  and its associated components. 
     FIG. 4 also depicts two additional one-piece, unitary, annular liners  52  and  54  located at other preferred locations in the casing  10 , i.e., to the front and the rear of the impeller  12 . To this end, the corresponding portions of the internal walls of the casing  10  that houses the impeller  12  are cut out as shown to accommodate the liners  52  and  54 . The liners  52  and  54  have a smaller outer diameter than the liners  30  and  40  and otherwise are identical to the liners  30  and  40 . The liners  52  and  54  thus function in an identical manner as the liner  30  as discussed above, and thus contribute to a significant reduction of the noise generated in the casing  10 . 
     The above-described preferred locations of the liners  30 ,  40 ,  52 , and  54  enjoy the advantage of optimum noise reduction, since the liners are relatively close to the source of the noise, and therefore reduce the possibility that the noise will by-pass the liners and pass through a different path. 
     Still another preferred location for a liner 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 in shape, is disposed in a cut-out recess of the inner surface of the conduit  60 , and is attached in the recess in any known manner. Since the liner  64  is otherwise identical to the liners  30 ,  40 ,  52 , and  54 , it will not be described in further detail. The liner  64  also functions in an identical manner as the liner  30  as discussed above, and contributes to a significant attenuation of the noise in the casing  10 . 
     It is understood that the liners  40 ,  52 ,  54  and  64  can be tuned to the impeller blade passing frequency to increase the noise reduction as discussed above in connection with the liner  30 . 
     There are several advantages associated with the foregoing. For example, the liners  30 ,  40 ,  52 ,  54 , and  64  are located to attenuate a maximum amount of noise near its source. Also, due to their one-piece, unitary construction, the liners  30 ,  40 ,  52 ,  54 , and  64  have fewer parts and are mechanically stronger when compared to the composite designs discussed above. Also, given the fact that the frequency of the dominant noise component varies with the compressor speed, the number of the smaller cells  36  per each larger cell  34  can be varied spatially across the liners  30 ,  40 ,  52 ,  54 , and  64  so that the entire liner is effective to attenuate noise in a broader frequency band. Consequently, the liners  30 ,  40 ,  52 ,  54 , and  64  can efficiently and effectively attenuate noise, not just in constant speed machines, but also in variable speed compressors, or other fluid pressurizing devices. The liners  30 ,  40 ,  52 ,  54 , and  64  also provide a very rigid inner wall to the internal flow. Further, relative to the three-piece sandwich structure used in the traditional configuration of conventional Helmholtz array acoustic liners, as discussed above, the liners according to the above embodiments of the present invention have less or no deformation when subject to mechanical and thermal loading. Therefore, the liners  30 ,  40 ,  52 ,  54 , and  64  have no adverse effect on the aerodynamic performance of a centrifugal compressor, even when they are installed in the narrow passages such as the diffusor channels, or the like, of a centrifugal compressor. 
     VARIATIONS 
     The specific arrangement and number of liners  30 ,  40 ,  52 ,  54 , and  64  utilized are not limited to the number shown in FIGS. 1,  4  and  5 . Thus, one or both of the liners  30  and  40  could be used in the diffuser channel  14 , one or both of the liners  52  and  53  could be used around the impeller  12 , and/or the liner  64  could be used around the inlet conduit  60 , depending on the particular application. 
     The specific technique of forming the cells  34  and  36  can vary from that discussed above. For example, a one-piece liner can be formed in which the cells  34  and  36  are molded in the plate  32 . 
     The relative dimensions and shapes of the cells  34  and/or  36  can vary within the scope of the invention, 
     The number and the pattern of the cells  34  and  36  in the plate  32  can vary. 
     The liners  30 ,  40 ,  52 ,  54 , and  64  are not limited to use with a centrifugal compressor, but are equally applicable to other relatively high pressure gas pressurizing devices. 
     Each liner  30 ,  40 ,  52 ,  54  can extend for 360 degrees around the axis of the impeller  12 , and the liner  64  can extend for 360 degrees around the axis of the conduit  60 ; or each liner can be formed into segments which extend an angular distance less than 360 degrees. For example, each liner  30 ,  40 ,  52 ,  54  and  64  could be formed by two or four segments each of which extends for 180 degrees or 90 degrees, respectively, with each segment having the unitary, one piece cross-section as described. 
     The spatial references used above, such as “bottom”, “inner”, “outer”, 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.