Abstract:
A muffler includes an annular flow path for the gas with the center of the annulus having a plurality of resonators which are in open communication with the downstream end of the annular flow path. The flow path is at least partially lined by an absorptive material overlain by a perforate material.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    In positive displacement compressors, discrete volumes of gas are trapped and compressed with the trapped, compressed volumes being discharged from the compressor. The trapping of the volumes at suction pressure and their discharge at discharge pressure each produce pressure pulsations and the related noise generation. While mufflers can be made to attenuate noise in a particular frequency range, or ranges, variable speed compressors may operate over ranges beyond the effective range(s) of conventional absorptive mufflers. This may be due to operating at rotational speeds outside the peak performance region of the absorptive device or at speeds where absorptive techniques are inadequate e.g. at frequencies well below the quarter wave thickness of the absorptive material. Accordingly, there would be no effective attenuation of a variable speed positive displacement compressors over some ranges of normal operation where conventional absorptive mufflers are employed.  
           [0002]    The flow of gas through a muffler is along a flow path defined by the pressure differential across the muffler. The direction of noise generation is not dictated by the flow direction. Reflected sound energy is generated each time there is a change in the cross section of the flow path with some of the sound energy being reflected in the opposite direction to that of the gas flow. It is through this mechanism that “reactive” type mufflers are designed to attenuate specific frequencies. In an absorptive muffler a portion of the flow path is defined by an absorptive material overlain by perforate metal, or the like. There is a trade off between flow resistance and noise reduction, with respect to the length and cross section of the flow path, in designing the muffler. Typical performance is limited by the relationship of the flow passage length to its height/minimum spacing in an absorptive device with peak attenuation occurring at a frequency related to the depth and impedance characteristics of the liner material.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention is directed to an absorptive/reactive muffler including a central cylindrical section having an opening, preferably, at the downstream end and containing a plurality of Helmholtz resonators, a mix of quarter and half wave resonators with each of the resonators being turned to a slightly different frequency to provide wider bandwidth attenuation characteristics or a combination of Helmholtz and quarter and/or half wave resonators. The central cylindrical section is serially overlain by an absorptive material and a first perforate material. The perforate material defines the inner surface of the flow path. A second perforate annular surface is underlain with an absorptive material and is spaced from the first perforate material and coacts therewith to define the fluid flow path. Noise traveling along the fluid flow path reflects between the two surfaces of absorptive material overlain by the perforate material and is attenuated by the absorptive material. Upon reaching the end of the annular flow path, the impedance discontinuity defined by the change in flow cross section directs some of the generated noise into the central cylindrical section containing the resonators. If necessary, or desired, the outer annular surface partially defining the annular flow path may be smooth rather than lined with absorptive material overlain by perforate material.  
           [0004]    It is an object of this invention to provide performance enhancement over conventional absorptive mufflers.  
           [0005]    It is a further object of this invention to provide a muffler having enhanced performance in a plurality of narrow frequency bands. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.  
           [0006]    Basically, the preferred muffler includes an annular flow path for the gas with the center of the annulus having a plurality of resonators which are in open communication with the downstream end of the annular flow path. The flow path is at least partially lined by an absorptive material overlain by a perforate material. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    For a fuller understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying drawings wherein:  
         [0008]    [0008]FIG. 1 is a sectional view of a PRIOR ART absorptive muffler;  
         [0009]    [0009]FIG. 2 is a sectional view of an absorptive/reactive muffler made according to the teachings of the present invention; and  
         [0010]    [0010]FIG. 3 is a sectional view of a modified absorptive/reactive muffler made according to the teachings of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]    In FIG. 1, the numeral  10  generally designates a PRIOR ART absorptive muffler. Muffler  10  includes an outer hollow cylindrical housing portion  12  and an inner portion  14  which is suitably supported in said housing portion  12  and radially spaced therefrom so as to provide an annular flow path  20  therebetween. Inner portion  14  includes an inner cylindrical portion  14 - 1  closed at the upstream end by disc  14 - 2  which extends radially outward of the inner cylindrical portion  14 - 1 . Annular disc portion  14 - 3  is located at the downstream end of cylindrical portion  14 - 1  and extends radially outward therefrom. Cylindrical portion  14 - 1  and disc  14 - 2  coact to define cylindrical chamber C which is open at its downstream end to the flow path  20  but does not form a part of the flow path. Acoustical lining  16  surrounds inner cylindrical portion  14 - 1  and is held in place axially by discs  14 - 2  and  14 - 3 . Acoustical lining  17  lines a portion of the inner surface  12 - 1  of housing portion  12  and is held in place axially by annular discs  12 - 2  and  12 - 3 . Acoustical linings  16  and  17  may be of any suitable material such as foam or fiberglass. Acoustical linings  16  and  17  are overlain by perforate members  18  and  19 , respectively, which may be any suitable material such as plastic or metal.  
         [0012]    In operation of muffler  10 , gas flow and sound enter annular flow path  20  at the left side of FIG. 1 and exit at the right side of FIG. 1. The primary mechanism for reducing sound is the absorptive elements  16  and  17  located beneath perforate annuli  18  and  19 , respectively, which form the outer surface of inner portion  14  and the inner surface of housing portion  12 . In going through muffler  10  the sound reflects between the surface defined by perforate member  18  and the surface defined by perforate member  19  with sound passing through the perforations  18 - 1  of perforate member  18  and the perforations of  19 - 1  of perforate member  19  thereby being attenuated by absorptive elements  16  and  17 , respectively. Chamber C, which is an empty volume, acts as a one quarter wave resonator which attenuates the sound in a narrow frequency range.  
         [0013]    Muffler  100  differs from muffler  10  in replacing a single quarter wave resonator with a series of slightly mis-tuned Helmholtz resonators providing a wide band of sound reduction at problematic frequencies. Inner portion  14 ′ is suitably supported in housing portion  12 . Muffler  100  has all of the structure of muffler  10  except: (1) disc  14 - 2 ′ has a hemispherical or other type of flow loss reducing geometry; (2) annular disc  14 - 3 ′ has a smaller opening than annular disc  14 - 3 ; (3) acoustical lining  16  has been replaced by a plurality of segments  16 - 1  separated by discs  14 - 4 ,  14 - 5 ,  14 - 6  and  14 - 7 ; and (4) acoustical lining  17  has been replaced by a plurality of segments  17 - 1  separated by discs  12 - 4 ,  12 - 5 ,  12 - 6  and  12 - 7 . The subdividing of acoustical lining  16  into segments  16 - 1  by solid disc separators  14 - 4 ,  14 - 5 ,  14 - 6  and  14 - 7  along the complete length of inner portion  14 ′ is such that discs  14 - 4 ,  14 - 5 ,  14 - 6  and  14 - 7  prevent the acoustic wave from traveling the complete length of the material of all of segments  16 - 1  in the flow direction. Rather, acoustic waves are forced to penetrate the material of segments  16 - 1  in directions primarily normal to the flow direction only. This type of absorptive device is termed a “locally reacting” muffler rather than the bulk device of FIG. 1. Additionally, structure is located in the space corresponding to chamber C of muffler  10 . Specifically, perforate cylindrical member  30 , having a plurality of perforations  30 - 1  which may vary in size, extends within inner cylindrical portion  14 - 1  from annular disc  14 - 3 ′ to a point short of the inner surface of end disc  14 - 2 ′. Perforate member  30  has a closed end  30   a  and is supported by annular end disc  14 - 3 ′ and a plurality of inner annular discs with three discs,  14 - 8 ,  14 - 9  and  14 - 10 , being illustrated. Inner cylindrical portion  14 - 1 , perforate member  30  and discs  14 - 3 ′,  14 - 8 ,  14 - 9  and  14 - 10  coact to define chambers C- 1 , C- 2 , C- 3  and C- 4  which define slightly mis-tuned Helmholtz resonators. Mistuning of chambers C- 1  through C- 4  is accomplished by varying the chamber volumes and/or the porosity through the number and/or hole size of perforations  30 - 1  communicating with each of the chambers C- 1  through C- 4 .  
         [0014]    In operation of muffler  100 , the sound passing through the annular path  20  defined by the inner surface of housing portion  12  or perforate member  19  and the underlying absorptive element  17  and the surface defined by perforate member  18  and the underlying absorptive elements  16 - 1  is the same as in the case of muffler  10 . The difference and improvement provided by muffler  100  over muffler  10  is that due to the replacement of the single quarter wave resonator defined by chamber C with the Helmholtz resonators defined by chambers C- 1 , C- 2 , C- 3  and C- 4 . The Helmholtz resonators are similar but not identical and so are able to attenuate a range of frequencies. The attenuated frequencies may be specific frequencies, a wider band of frequency by slight mistuning, or a combination of both.  
         [0015]    Muffler  200  differs from muffler  10  in replacing a single quarter wave resonator with a plurality of quarter and/or half wave resonators. Inner portion  14 ′ is suitably supported in housing portion  12 . Muffler  200  differs from muffler  100  in having a plurality of quarter and/or half wave resonators rather than a plurality of Helmholtz resonators. Muffler  200  has all of the structure of muffler  10  except disc  14 - 2 ′ has a hemispherical or other type of flow loss reducing geometry and annular disc  14 - 3 ″ has a smaller opening than annular disc  14 - 3  and supports tube  40 . In addition to tube  40 , tube  41  supported by annular disc  14 - 11  and tube  42  supported by annular disc  14 - 12  are located in the space corresponding to chamber C of muffler  10 . Tubes  40 ,  41  and  42  are axially spaced and of different lengths. Inner cylindrical portion  14 - 1 , tubes  40 ,  41  and  42  and discs  14 - 3 ″,  14 - 11  and  14 - 12  coact to define chambers C- 1 ′, C- 2 ′ and C- 3 ′ and slightly mis-tuned quarter and half wave resonators. For example, half wave resonators are defined by tubes  40 ,  41  and  42  terminating with open-open end boundary conditions while one quarter wave resonators are defined by open-closed end boundary conditions.  
         [0016]    The operation of muffler  200  is the same as that of muffler  10  and  100  relative to the sound passing through the annular path  20  defined by the inner surface of outer housing portion  12  or perforate member  19  and the underlying absorptive elements  17 - 1  and the surface defined by perforate member  18  and the underlying absorptive element  16 . The difference and improvement provided by muffler  200  over muffler  10  is that due to the replacement of a single quarter wave resonator defined by chamber C with a plurality of quarter and/or half wave resonators which are similar but not identical. The resonators collectively are able to attenuate a range of frequencies which may, for example, be specific frequencies, a wider band of frequencies by slight mistuning of the length of tubes  40 ,  41  and/or  42 , or by a combination of both.  
         [0017]    Although preferred embodiments of the present invention have been illustrated and described, other changes will occur to those skilled in the art. For example, the number and combination of types of resonators and the degree of mistuning will depend upon the specific application of the teachings of the present invention. Also, while segments are preferred, absorptive elements  16 - 1  an  17 - 1  may be made as single elements. It is therefore intended that the scope of the present invention is to be limited only by the scope of the appended claims.