Patent Publication Number: US-6700338-B2

Title: Tubular acoustic pressure wave generator

Description:
FILED OF THE INVENTION 
     The present invention relates to a device for generating pressure variations of large amplitude in an acoustic tube without forming shock waves in the fluid contained in the acoustic tube. 
     BACKGROUND OF THE INVENTION 
     Already known as pressure wave generators in a gas fluid are acoustic compressors which comprise an acoustic tube with an inlet and an outlet for the fluid provided at one end thereof, and a driving device connected to the other end of the acoustic tube [see, for example, JP-A No. 11-303800(1999), No. 8-219100(1996), No. 4-224279(1992), etc.]. The driving device produces pressure variations within the acoustic tube which cause the fluid to be discharged from the tube through the outlet while taking the fluid into the tube through the inlet. The fluid is compressed by a pressure difference between in the intake fluid and discharge fluid. 
     In the conventional acoustic compressors, however, shock waves generally appear as the pressure variations become large. This not only imposes limitations on the magnitude of amplitude of pressure variations in the fluid to limit the pressure difference between in the intake fluid and discharge fluid, i.e., the compression ratio of the fluid, but also causes heat generation in the fluid, and accordingly in the compressor itself, to a high temperature and producing loud noise. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to produce shock-free pressure variations of larger amplitude than the ones produced by conventional acoustic compressors or like pressure wave generators. 
     The present inventors previously made a theoretical analysis of the propagation of nonlinear acoustic waves in a tunnel provided with an array of Helmholtz resonators (“Propagation of nonlinear acoustic waves in a tunnel with an array of Helmholtz resonators,” J. Fluid Mech. (1992), vol. 244, pp. 55-78). In consequence, the inventors found that the shock wave which emerges from the pressure waves generated by entry of a high-speed train into the tunnel can be effectively suppressed by a suitable array of Helmholtz resonators connected to the tunnel, as arranged axially thereof. The present inventors have conceived the idea of applying this theory to the suppression of shock waves in pressure wave generators, and substantiated the effect thereof to accomplish the present invention. 
     The present invention provides a pressure wave generator which comprises a closed acoustic tube  1 , and a driving device  3  generating vibration mounted as directed toward the interior of the tube  1  at the resonance frequency of a fluid in the acoustic tube  1  or at a frequency close to the resonance frequency, a plurality of Helmholtz resonators  2  each having a channel which connects the cavity of the resonator with the interior of the acoustic tube  1  and being arranged on a periphery of a tubular wall of the acoustic tube  1  with suitable axial spacing (see FIG.  1 ). 
     Each of the Helmholtz resonators  2  comprises a throat  21  having a narrower channel in diameter than the acoustic tube  1  and joined at a base end thereof to the tubular wall of the acoustic tube  1 , and a closed cavity of suitable volume  22  joined at one end of the throat. 
     As will be apparent from the experimental result to be described later, the pressure wave generator of the present invention produces the same effect (geometrical dispersion as will be described later) as that involved in the propagation of pressure waves in the tunnel provided with an array of Helmholtz resonators, whereby the generation of shock waves in the tunnel is suppressed. 
     Stated more specifically, the acoustic tube  1  connects thereto an intake pipe  13  and a discharge pipe  14 , whereby an acoustic compressor is provided, in which the gas taken in through the intake tube  13  is compressed and discharged from the discharge pipe  14 . 
     According to another specific embodiment, the acoustic tube  1  is in the form of a straight tube or loop, and a regenerator  41  in the form of stack of flat plates or a porous regenerator  44  disposed inside the acoustic tube  1 , and the channel of the acoustic tube  1  is provided with a high-temperature heat exchanger  42  and a low-temperature heat exchanger  43  (see FIG. 8) connected respectively to the high-temperature end and the low-temperature end of the regenerator  41  or  44 , whereby an acoustic refrigerator is provided to execute radiation and absorption of heat through the two heat exchangers  42 ,  43 . 
     The Helmholtz resonators  2  can be replaced by a plurality of closed side-branch  2   a  from the acoustic tube  1  serving as the resonators (see FIG.  11 ). Usable as the driving device  3  in place of a linear motor is a device wherein a plate  32  as attached to bellows  31  is driven to reciprocate by a piezoelectric vibrator  35 , or a device wherein a diaphragm  36  is driven instead of the bellows (see FIG.  10 ). 
     The pressure wave generator of the present invention has a simple construction wherein an array of cavities is arranged along an acoustic tube and by which generation of shock waves is effectively suppressed, generating a shock-free and larger pressure amplitude than the ones conventionally available entailing generation of shock waves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view in section of an acoustic compressor as a first embodiment of the invention; 
     FIG. 2 is a diagram for illustrating the mechanism of generation of shock waves; 
     FIG. 3 is a diagram for illustrating the effect of the geometrical dispersion where the generation of shock waves is inhibited; 
     FIG. 4 is a graph showing pressure variations at a fixed end of a conventional acoustic tube; 
     FIG. 5 is a graph showing pressure variations at a fixed end of an acoustic tube of the invention; 
     FIG. 6 is a graph showing pressure variations at a position away from a driving device by a distance {fraction (7/16)} of the length of the conventional acoustic tube; 
     FIG. 7 is a graph showing pressure variations at a position away from a driving device by a distance {fraction (7/16)} of the length of the acoustic tube of the invention; 
     FIG. 8 is a sectional view of an acoustic refrigerator as a second embodiment of the invention; 
     FIG. 9 is a sectional view showing another example of acoustic tube for use in the second embodiment; 
     FIG. 10 is a sectional view showing a different example of acoustic driving device; and 
     FIG. 11 is a sectional view of an acoustic compressor utilizing side-branches instead of resonators. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The present invention as embodied into acoustic compressors and acoustic refrigerators will be described below in detail with reference to the drawings. 
     First Embodiment 
     FIG. 1 shows this embodiment, i.e., an acoustic compressor, which comprises an acoustic tube  1  provided at one end thereof with a gas inlet and a gas outlet. An intake pipe  13  and a discharge pipe  14  are connected respectively to the inlet and the outlet, with check valves  11 ,  12  provided between the tube  1  and the pipes. The acoustic tube  1  is connected at the other end thereof to a driving device  3  for producing pressure variations in the tube  1 . 
     The acoustic tube  1  is provided on the tubular wall thereof with a plurality of Helmholtz resonators  2  arranged at a suitable spacing axially of the tube and each having a channel through which the resonator is connected to the interior of the acoustic tube  1 . Each of the Helmholtz resonators  2  comprises a throat  21  having a smaller diameter than the ones of the acoustic tube  1  and joined at a base end thereof to the tubular wall of the acoustic tube  1 , and a closed cavity  22  joined at one end of the throat  21  and having a suitable volume. 
     The acoustic driving device  3  comprises bellows  31  attached to the above-mentioned other end of the acoustic tube  1 , a vibration plate  32  attached to the outer end of the bellows  31 , a linear motor  33  generating reciprocation of the plate  32 , and a spring  34  for balancing an interia force working on the vibrating part of the linear motor  33 . 
     The driving device  3  produces in the interior of the acoustic tube  1  pressure variations with a large amplitude having antinodes at opposite ends of the tube as indicated in broken lines. The pressure variations makes the gas discharge through the discharge pipe  14  while taking in gas through the intake pipe  13  to compress the gas with the pressure difference between in the intake gas and discharge gas. 
     With the acoustic compressor described, the pressure variations occurring inside the acoustic tube  1  are transmitted also to the inside of the Helmholtz resonators  2 . Since the Helmholtz resonators  2  are arranged in an axial array along the acoustic tube  1 , the resonators  2  respond differently to higher harmonic wave components produced by nonlinearity, i.e., the wave components having frequencies of multiples of the driving frequency, to give rise to geometrical dispersion to the pressure waves which would suppress the formation of shock waves. 
     FIG. 2 illustrates the mechanism of formation of shock waves in a conventional acoustic tube. In the pressure wave, the portion of high pressure is higher than the portion of low pressure in the speed of propagation, so that the waveform which is initially sinusoidal deforms with time t to steepen as shown in FIG.  2 . As a result, marked slope in pressure variations occurs to form shock waves. 
     In the acoustic tube  1  of the present invention which is provided with Helmholtz resonators  2  arranged in an array, the resonators  2  respond differently to higher harmonic wave components which are different in frequency as mentioned above, so that these components become gradually dispersed with time t as illustrated in FIG.  3 . Thus, the gas which has originally no dispersion in itself as a medium is given dispersion to avoid steepening of the pressure waveform, whereby the formation of shock waves is suppressed. 
     FIGS. 4 to  7  show an experimental result obtained to substantiate the effect in the acoustic compressor of the present invention. Used for the experiment was an acoustic tube  1  which was 3.2 m in length and 80 mm in inside diameter and which was provided with 64 Helmholtz resonators  2  which were 50 cm 3  in the volume of each cavity portion, 238 Hz in Helmholtz resonance frequency and 50 mm in axial spacing between the resonators  2 . The driving device  3  was driven approximately at the resonance frequency of the acoustic tube  1  thereof and was so adjusted that the pressure variations (maximum-minimum) at the fixed end of the acoustic tube  1  were 15% of the atmospheric pressure. The resonance frequency was 53 Hz for the conventional acoustic tube having no Helmholtz resonators, or 48 Hz for the acoustic tube  1  of the invention provided with the array of Helmholtz resonators  2 . 
     FIGS. 4 and 5 show the pressure variations at the fixed end (the end opposite to the driving device) of the acoustic tube, as determined for the conventional acoustic tube and the acoustic tube of the invention, respectively. Shock waves with a discontinuity in waveform were formed in the conventional acoustic tube having no Helmholtz resonators as seen in FIG. 4, whereas the acoustic tube of the invention provided with Helmholtz resonators produced a smooth waveform as shown in FIG. 5, obviously with no shock waves. 
     FIGS. 6 and 7 show the pressure variations at a position away from the driving device by a distance {fraction (7/16)} of the entire length of the acoustic tube, as determined for the conventional acoustic tube and the acoustic tube of the invention, respectively. Shock waves with a discontinuity in waveform were formed in the conventional acoustic tube having no Helmholtz resonators as seen in FIG. 6, whereas the acoustic tube of the invention provided with Helmholtz resonators produced a smooth-curved waveform as shown in FIG. 7, thus generating no shock waves. 
     With the acoustic compressor of the present invention, the acoustic tube  1  generates no shock waves therein, consequently achieving a high compression ratio even when a further increased pressure difference is given to the intake gas and the discharge gas. The compressor further eliminates noise and achieves a high energy efficiency. 
     Second Embodiment 
     FIG. 8 shows an acoustic refrigerator embodying the invention and comprising an acoustic tube  1 , a regenerator  41  in the form of a stack of flat plates, and a high-temperature heat exchanger  42  and a low-temperature heat exchanger  43  provided around the acoustic tube  1  and connected respectively to both ends of the regenerator  41 . 
     As in the first embodiment, the acoustic tube  1  is provided on the tubular wall thereof with a plurality of Helmholtz resonators  2  arranged at a suitable spacing axially of the tube and each having a channel through which the cavity is connected to the interior of the acoustic tube  1 . Each of the Helmholtz resonators  2  comprises a throat  21  having a channel smaller in diameter than the acoustic tube  1  and joined at a base end thereof to the tubular wall of the acoustic tube  1 , and a cavity  22  joined at one end of the throat  21  and having a suitable volume. 
     The driving device  3  comprises bellows  31  attached to the above-mentioned other end of the acoustic tube  1 , a vibration plate  32  attached to the outer end of the bellows  31 , a linear motor  33  generating reciprocation of the vibration plate  32 , and a spring  34  for balancing an interia force working on the vibrating part of the linear motor  33 . 
     The driving device  3  produces in the interior of the acoustic tube  1  pressure variations with a large amplitude having antinodes at both ends of the tube as indicated in broken lines. The pressure variations cause the regenerator  41  to absorb heat from the low-temperature heat exchanger  43  close to the node of pressure while causing the regenerator to release heat to the high-temperature heat exchanger  42  close to the antinode of pressure, whereby the object is cooled. 
     With the acoustic refrigerator described, the pressure variations occurring inside the acoustic tube  1  are transmitted also to the inside of the Helmholtz resonators  2 , permitting the internal pressure variations of the acoustic tube  1  and those of the resonators  2  to mutually exert influence. Since the Helmholtz resonators  2  are arranged in an axial array along the acoustic tube  1 , the resonators  2  respond differently to the higher harmonic wave components which have multiples of the driving frequency and included in pressure waves, consequently giving geometrical dispersion to the pressure waves which would otherwise produce shock waves to suppress the generation of shock waves. 
     Thus, without any shock waves generated in the interior of the acoustic tube  1 , the acoustic refrigerator of the invention affords large pressure variations, consequently increasing refrigeration capacity afforded by a high pressure ratio. The refrigerator is free from noise, and a high energy efficiency can be achieved. 
     The apparatus of the invention is not limited to the foregoing embodiments in construction but can be variously modified by one skilled in the art without departing from the spirit of the invention as set forth in the appended claims. For example, the acoustic tube  1  for use in the acoustic refrigerator is not limited to the straight tube shown in FIG. 8 but can be in the form of a loop as shown in FIG.  9 . In this case, a porous regenerator  44  is disposed in the interior of the acoustic tube  1 , with a driving device  3  connected to the acoustic tube  1 . The acoustic refrigerator thus constructed of course has the same effect as the one in the refrigerator shown in FIG.  8 . 
     The Helmholtz resonators  2  for use in the acoustic compressor and acoustic refrigerator can be replaced by a plurality of closed cavities having a resonance frequency, e.g., side branches  2   a  as shown in FIG. 11, whereby the same effects as above can be obtained. 
     Further for use in the driving device  3 , the linear motor  33  can be replaced by a piezoelectric vibrator  35 , or a diaphragm  36 , as shown in FIG. 10, driven reciprocatingly. The driving device  3  is then disposed close to the node of pressure of the acoustic tube  1  so as to give the acoustic tube  1  a length which is about ¼ of the wavelength of the pressure waves to be driven. 
     The pressure wave generator of the present invention is useful not only as a compressor for compressing a gas within a container connected to the discharge pipe  14 , but also for a transport pump for transporting a gas with the pressure difference to be generated, or for a vacuum pump for evacuating a container connected to the intake pipe  13 . 
     The Helmholtz resonators  2  are not only limited to those discretely arranged at a suitable spacing as seen in FIG. 1 but also can be such that a plurality of cavities are formed in the outer tubular wall of an acoustic tube  1  of double tube construction to provide an array of Helmholtz resonators. Further, the resonators can be arranged by lining the outer tubular wall of an acoustic tube  1 . 
     Furthermore a heat engine which is the reverse cycle of the acoustic refrigerator utilizing the phenomenon of resonance can be realized utilizing the phenomenon of spontaneous gas oscillation similarly like the refrigerator by using the high-temperature heat exchanger  42  as a heat input device and the driving device  3  as a mechanical power output device.