Patent Publication Number: US-7224103-B2

Title: Acoustic driver assembly with recessed head mass contact surface

Description:
REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. patent application Ser. No. 10/931,918, filed Sep. 1, 2004 now U.S. Pat. No. 6,958,569. 

   FIELD OF THE INVENTION 
   The present invention relates generally to sonoluminescence and, more particularly, to an acoustic driver assembly for use with a sonoluminescence cavitation chamber. 
   BACKGROUND OF THE INVENTION 
   Sonoluminescence is a well-known phenomena discovered in the 1930&#39;s in which light is generated when a liquid is cavitated. Although a variety of techniques for cavitating the liquid are known (e.g., spark discharge, laser pulse, flowing the liquid through a Venturi tube), one of the most common techniques is through the application of high intensity sound waves. 
   In essence, the cavitation process consists of three stages; bubble formation, growth and subsequent collapse. The bubble or bubbles cavitated during this process absorb the applied energy, for example sound energy, and then release the energy in the form of light emission during an extremely brief period of time. The intensity of the generated light depends on a variety of factors including the physical properties of the liquid (e.g., density, surface tension, vapor pressure, chemical structure, temperature, hydrostatic pressure, etc.) and the applied energy (e.g., sound wave amplitude, sound wave frequency, etc.). 
   Although it is generally recognized that during the collapse of a cavitating bubble extremely high temperature plasmas are developed, leading to the observed sonoluminescence effect, many aspects of the phenomena have not yet been characterized. As such, the phenomena is at the heart of a considerable amount of research as scientists attempt to not only completely characterize the phenomena (e.g., effects of pressure on the cavitating medium), but also its many applications (e.g., sonochemistry, chemical detoxification, ultrasonic cleaning, etc.). 
   Although acoustic drivers are commonly used to drive the cavitation process, there is little information about methods of coupling the acoustic energy to the cavitation chamber. For example, in an article entitled  Ambient Pressure Effect on Single - Bubble Sonoluminescence  by Dan et al. published in vol. 83, no. 9 of Physical Review Letters, the authors describe their study of the effects of ambient pressure on bubble dynamics and single bubble sonoluminescence. Although the authors describe their experimental apparatus in some detail, they only disclose that a piezoelectric transducer was used at the fundamental frequency of the chamber, not how the transducer couples its energy into the chamber. 
   U.S. Pat. No. 4,333,796 discloses a cavitation chamber that is generally cylindrical although the inventors note that other shapes, such as spherical, can also be used. As disclosed, the chamber is comprised of a refractory metal such as tungsten, titanium, molybdenum, rhenium or some alloy thereof and the cavitation medium is a liquid metal such as lithium or an alloy thereof. Surrounding the cavitation chamber is a housing which is purportedly used as a neutron and tritium shield. Projecting through both the outer housing and the cavitation chamber walls are a number of acoustic horns, each of the acoustic horns being coupled to a transducer which supplies the mechanical energy to the associated horn. The specification only discloses that the horns, through the use of flanges, are secured to the chamber/housing walls in such a way as to provide a seal and that the transducers are mounted to the outer ends of the horns. 
   U.S. Pat. No. 5,658,534 discloses a sonochemical apparatus consisting of a stainless steel tube about which ultrasonic transducers are affixed. The patent provides considerable detail as to the method of coupling the transducers to the tube. In particular, the patent discloses a transducer fixed to a cylindrical half-wavelength coupler by a stud, the coupler being clamped within a stainless steel collar welded to the outside of the sonochemical tube. The collars allow circulation of oil through the collar and an external heat exchanger. The abutting faces of the coupler and the transducer assembly are smooth and flat. The energy produced by the transducer passes through the coupler into the oil and then from the oil into the wall of the sonochemical tube. 
   U.S. Pat. No. 5,659,173 discloses a sonoluminescence system that uses a transparent spherical flask. The spherical flask is not described in detail, although the specification discloses that flasks of Pyrex®, Kontes®, and glass were used with sizes ranging from 10 milliliters to 5 liters. The drivers as well as a microphone piezoelectric were simply epoxied to the exterior surface of the chamber. 
   U.S. Pat. No. 5,858,104 discloses a shock wave chamber partially filled with a liquid. The remaining portion of the chamber is filled with gas which can be pressurized by a connected pressure source. Acoustic transducers are used to position an object within the chamber while another transducer delivers a compressional acoustic shock wave into the liquid. A flexible membrane separating the liquid from the gas reflects the compressional shock wave as a dilation wave focused on the location of the object about which a bubble is formed. The patent simply discloses that the transducers are mounted in the chamber walls without stating how the transducers are to be mounted. 
   U.S. Pat. No. 5,994,818 discloses a transducer assembly for use with tubular resonator cavity rather than a cavitation chamber. The assembly includes a piezoelectric transducer coupled to a cylindrical shaped transducer block. The transducer block is coupled via a central threaded bolt to a wave guide which, in turn, is coupled to the tubular resonator cavity. The transducer, transducer block, wave guide and resonator cavity are co-axial along a common central longitudinal axis. The outer surface of the end of the wave guide and the inner surface of the end of the resonator cavity are each threaded, thus allowing the wave guide to be threadably and rigidly coupled to the resonator cavity. 
   U.S. Pat. No. 6,361,747 discloses an acoustic cavitation reactor in which the reactor chamber is comprised of a flexible tube. The liquid to be treated circulates through the tube. Electroacoustic transducers are radially and uniformly distributed around the tube, each of the electroacoustic transducers having a prismatic bar shape. A film of lubricant is interposed between the transducer heads and the wall of the tube to help couple the acoustic energy into the tube. 
   PCT Application No. US00/32092 discloses several driver assembly configurations for use with a solid cavitation reactor. The disclosed reactor system is comprised of a solid spherical reactor with multiple integral extensions surrounded by a high pressure enclosure. Individual driver assemblies are coupled to each of the reactor&#39;s integral extensions, the coupling means sealed to the reactor&#39;s enclosure in order to maintain the high pressure characteristics of the enclosure. 
   SUMMARY OF THE INVENTION 
   The present invention provides an acoustic driver assembly for use with any of a variety of cavitation chamber configurations, including spherical and cylindrical chambers as well as chambers that include at least one flat coupling surface. The acoustic driver assembly includes at least one transducer, a head mass and a tail mass. The end surface of the head mass is shaped so that only a ring of contact is made between the outer perimeter of the head mass of the driver assembly and the cavitation chamber to which the driver is attached. The area of the contact ring is controlled by shaping its surface. 
   Any of a variety of head mass end surface shapes can be used to achieve the desired contact ring. In one embodiment the head mass end surface is concave. In another embodiment the head mass end surface is stepped such that the inner portion of the end surface is recessed relative to the perimeter of the end surface. 
   In one embodiment the driver assembly is attached to the exterior surface of the cavitation chamber with a threaded means (e.g., all-thread/nut assembly, bolt, etc.). The same threaded means is used to assemble the driver. In an alternate embodiment, a pair of threaded means is used, one to hold together the driver assembly and one to attach the driver assembly to the cavitation chamber. In another alternate embodiment, a threaded means is used to assemble the driver, the threaded means being threaded into the head mass. The driver assembly is attached to the cavitation chamber by forming a permanent or semi-permanent joint between the head mass of the driver assembly and a cavitation chamber wall. The permanent or semi-permanent joint can be comprised of an epoxy bond joint, a braze joint, a diffusion bond joint, or other means. In yet another alternate embodiment, the head mass is comprised of a pair of head mass portions that are coupled together with an all-thread. The driver assembly is held together by coupling the driver components to one of the head mass portions using a threaded means. The second head mass portion is attached to the cavitation chamber wall with either an all-thread or a joint (e.g., bond joint, braze joint, diffusion bond joint, etc.). 
   In at least one embodiment, the transducer is comprised of a pair of piezo-electric transducers, preferably with the adjacent surfaces of the piezo-electric transducers having the same polarity. 
   In at least one embodiment, a void filling material is interposed between one or more pairs of adjacent surfaces of the driver assembly and/or the driver assembly and the exterior surface of the cavitation chamber. 
   A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a driver assembly; 
       FIG. 2  is a cross-sectional view of an embodiment of the invention in which a driver assembly is attached to a flat cavitation chamber wall; 
       FIG. 3  is a cross-sectional view of a driver assembly similar to that shown in  FIG. 2  with an increased ring of contact area between the driver head mass and the flat cavitation chamber wall; 
       FIG. 4  is a cross-sectional view of an embodiment of the invention in which a driver assembly is attached to a cylindrically shaped cavitation chamber, the view presented in  FIG. 4  being along the axis of the cylindrical cavitation chamber; 
       FIG. 5  is an orthogonal cross-sectional view of the embodiment shown in  FIG. 4 ; 
       FIG. 6  is a cross-sectional view of a driver assembly similar to that shown in  FIG. 4  with an increased ring of contact area between the driver head mass and the cylindrical cavitation chamber wall; 
       FIG. 7  is an orthogonal cross-sectional view of the embodiment shown in  FIG. 6 ; 
       FIG. 8  is a perspective view of a head mass similar to the head mass of the head mass shown in  FIGS. 4–7 ; 
       FIG. 9  is a cross-sectional view of a driver assembly in which the area of the contact ring between the driver head mass and the flat cavitation chamber wall is controlled by varying the area of a stepped contact surface; 
       FIG. 10  is a cross-sectional view of an embodiment of the invention in which a driver assembly similar to that of  FIG. 9  is attached to a cylindrically shaped cavitation chamber, the view presented in  FIG. 10  being along the axis of the cylindrical cavitation chamber; 
       FIG. 11  is an orthogonal cross-sectional view of the embodiment shown in  FIG. 10 ; 
       FIG. 12  is a cross-sectional view of an embodiment of the invention in which a driver assembly similar to that of  FIG. 9 , except for the use of a shaped contact surface, is attached to a cylindrically shaped cavitation chamber, the view presented in  FIG. 12  being along the axis of the cylindrical cavitation chamber; 
       FIG. 13  is an orthogonal cross-sectional view of the embodiment shown in  FIG. 12 ; 
       FIG. 14  is a cross-sectional view of an embodiment of the invention in which a driver assembly similar to that of  FIG. 9  is attached to a spherically shaped cavitation chamber; 
       FIG. 15  is a cross-sectional view of an embodiment of the invention in which a driver assembly similar to that of  FIG. 9 , except for the use of a shaped contact surface, is attached to a spherically shaped cavitation chamber; 
       FIG. 16  is a cross-sectional view of an assembly illustrating an alternate means of attaching any of the driver assemblies of  FIGS. 2–15  to a cavitation chamber wall; 
       FIG. 17  is a cross-sectional view of an assembly illustrating an alternate means of attaching any of the driver assemblies of  FIGS. 2–15  to a cavitation chamber wall; 
       FIG. 18  is a cross-sectional view of an assembly illustrating an alternate means of attaching any of the driver assemblies of  FIGS. 2–15  to a cavitation chamber wall; and 
       FIG. 19  is a cross-sectional view of an assembly illustrating an alternate means of attaching any of the driver assemblies of  FIGS. 2–15  to a cavitation chamber wall. 
   

   DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
     FIG. 1  is a perspective view of a driver assembly  100 . Preferably piezo-electric transducers are used in driver  100  although magnetostrictive transducers can also be used, magnetostrictive transducers typically preferred when lower frequencies are desired. A combination of piezo-electric and magnetostrictive transducers can also be used, for example as a means of providing greater frequency bandwidths. 
   Although driver assembly  100  can use a single piezo-electric transducer, preferably assembly  100  uses a pair of piezo-electric transducer rings  101  and  102  poled in opposite directions. By using a pair of transducers in which the adjacent surfaces of the two crystals have the same polarity, potential grounding problems are minimized. An electrode disc  103  is located between transducer rings  101  and  102  which, during operation, is coupled to the driver power amplifier  105 . 
   The transducer pair is sandwiched between a head mass  107  and a tail mass  109 . In the preferred embodiment both head mass  107  and tail mass  109  are fabricated from stainless steel and are of equal mass. In alternate embodiments head mass  107  and tail mass  109  are fabricated from different materials. In yet other alternate embodiments, head mass  107  and tail mass  309  have different masses and/or different mass diameters and/or different mass lengths. For example tail mass  109  can be much larger than head mass  107 . 
   Preferably driver  100  is assembled about a centrally located all-thread  111  which is screwed directly into the wall of the cavitation chamber (not shown). A cap nut  113  holds the assembly together. In a preferred embodiment, all-thread  111  does not pass through the entire chamber wall, thus leaving the internal surface of the cavitation chamber smooth. This method of attachment has the additional benefit of insuring that there are neither gas nor liquid leaks at the point of driver attachment. In an alternate embodiment, for example with thin walled chambers, the threaded hole to which all-thread  111  is coupled passes through the entire chamber wall. Typically in such an embodiment all-thread  111  is sealed into place with an epoxy or other suitable sealant. Alternately all-thread  111  can be welded or brazed to the chamber wall. It is understood that all-thread  111  and cap nut  113  can be replaced with a bolt or other means of attachment. An insulating sleeve, not viewable in  FIG. 1 , isolates all-thread  111 , preventing it from shorting electrode  103 . 
   For purposes of illustration only, a typical driver assembly is approximately 2.5 inches in diameter with a head mass and a tail mass each weighing approximately 5 pounds. Both the head mass and the tail mass may be fabricated from 17-4 PH stainless steel. Suitable piezo-electric transducers are fabricated by Channel Industries of Santa Barbara, Calif. If the driver assembly is attached to the chamber with an all-thread, the all-thread may be on the order of a 0.5 inch all-thread and the assembly can be tightened to a level of 120 ft-lbs. If an insulating sleeve is used, as preferred, it is typically fabricated from Teflon. 
   The cavitation chamber to which the driver is attached can be of any regular or irregular shape, although typically the cavitation chamber is spherical, cylindrical, or rectangular in shape. Additionally, it should be appreciated that the invention is not limited to a particular outside chamber diameter, inside chamber diameter or chamber material. 
     FIGS. 2–19  illustrate embodiments of the invention in which the end surface of the head mass is shaped so that only a ring of contact is made between the driver and the cavitation chamber to which the driver is attached.  FIG. 2  is a cross-sectional view of a driver  200  attached to a flat cavitation chamber wall  201 . For illustration simplicity, only a portion of the cavitation chamber is shown. It should be understood that driver assembly  200  is attached to the exterior surface  203  of chamber wall  201 . It should also be understood that chamber wall  201  may correspond to a square chamber, rectangular chamber, or other chamber shape which includes at least one flat wall. In addition to shaped head mass  205 , driver assembly  200  includes a tail mass  207 , one or more transducers (e.g., a pair of piezo-electric transducers  209 / 211  are shown), and means such as an electrode ring  213  for coupling the transducer(s) to a driver amplifier  215 . In the illustrated embodiment, an all-thread  217  and a nut  219  are used to mount driver assembly  200  to chamber wall  201 . Alternately a bolt or other means can be used to mount driver assembly  200  to wall  201 . An insulating sleeve  220  isolates all-thread  217 . 
   Due to the curvature of surface  221  of head mass  205 , instead of the entire end surface  221  being in contact with the cavitation chamber, there is only a ring of contact  223  between the two surfaces. To improve the contact between the driver and the chamber, in a preferred embodiment illustrated in  FIG. 3  the contact area is increased by shaping (e.g., chamfering) the outer edge  301  of end surface  303  of the head mass  305 . As in the previous embodiment, this approach limits the contact area to a ring while maintaining a centrally located cavity  307  between the head mass and the chamber surface. 
     FIGS. 4 and 5  are cross-sectional views of a driver assembly similar to that shown in  FIG. 2 , but in which the cavitation chamber surface is cylindrically shaped.  FIG. 4  is a view along the axis of the cylindrical cavitation chamber while  FIG. 5  is a view perpendicular to the chamber&#39;s axis. As illustrated in these figures, head mass  401  is shaped so that there is a ring of contact  403  between the head mass and the outer surface  405  of cavitation chamber wall  407 . If desired, the contact area can be increased by shaping the outer edge  601  of the end surface  603  of the head mass  605  as shown in  FIGS. 6 and 7  of driver assembly  600 . As with the prior embodiment,  FIG. 6  is a view along the axis of the cylindrical cavitation chamber and  FIG. 7  is a view perpendicular to the chamber&#39;s axis. 
     FIG. 8  provides a perspective view of a head mass  800  similar to either head mass  401  or head mass  605 , thus suitable for use with a cylindrical cavitation chamber. In this view, however, the curvature of the end surface  801  is exaggerated, thereby aiding visualization of the shape of the head mass. It will be appreciated that if the cavitation chamber diameter is sufficiently small relative to the diameter of the driver assembly, end surface  801  is not exaggerated. 
   In addition to the curved surface (e.g., surface  221 ) of the head mass shown in the previous embodiments, the inventors also envision that the surface of the head mass that is adjacent to the chamber external surface can utilize other shapes to achieve the desired ring of contact between the chamber wall and the driver assembly. For example, the surface of the head mass can be stepped as shown in  FIGS. 9–15 . 
     FIG. 9  is a cross-sectional view of an embodiment of the invention in which driver assembly  900  is attached to flat exterior surface  203  of flat cavitation chamber wall  201 . As in the previous illustrations, only a portion of the cavitation chamber is shown. As previously noted, chamber wall  201  may correspond to a square chamber, rectangular chamber or other chamber shape which includes at least one flat wall. The end surface of head mass  901  includes at least two different surfaces  903  and  905 , surface  905  recessed relative to surface  903 , thereby providing the desired ring of contact  907  between head mass  901  and chamber external surface  203 . 
     FIGS. 10 and 11  illustrate an embodiment of the invention similar to that shown in  FIG. 9  as used with a cylindrically shaped cavitation chamber.  FIG. 10  is a view along the axis of the cylindrical cavitation chamber while  FIG. 11  is a view perpendicular to the chamber&#39;s axis. As shown, head mass  901  of driver assembly  900  contacts external chamber surface  405  along ring of contact  907 . If desired, the area of the ring of contact can be increased by shaping the contacting surface of the head mass. For example,  FIGS. 12 and 13  illustrate a driver assembly  1200  similar to that shown in  FIGS. 10 and 11  except contacting surface  1201  of head mass  1203  is shaped to increase the contact area. In the illustrated embodiment, surface  1201  is shaped to match the curvature of the cylindrical external surface  403  of cylindrical chamber wall  401 . It is understood that surface  1201  can utilize other curvatures in order to achieve the desired contact area. 
     FIG. 14  illustrates the use of driver assembly  900  with a spherically shaped chamber. Due to the symmetry of a spherical chamber, only a single view is required to illustrate the embodiment. As shown, head mass  901  of driver assembly  900  contacts external chamber surface  1401  of chamber wall  1403  along a contact ring of  1405 . If desired, the area of the ring of contact can be increased by shaping the contacting surface  1501  of the head mass as illustrated in  FIG. 15 . Although the curvature of the contacting surface in  FIG. 15  matches the curvature of the spherical surface of the chamber, it will be appreciated that other curvatures can be used, thus providing a relatively simple means of controlling the area of the ring of contact between the driver assembly and the spherical chamber. 
   Although the embodiments described above are shown with either an all-thread/nut or bolt means of attachment, any of these embodiments can also utilize other mounting means. For example,  FIG. 16  is an illustration of a driver assembly  1600  similar to that shown in  FIG. 3 , but in which the driver is assembled about a first threaded means  1601  (e.g., all-thread or bolt) which is threaded into head mass  1603 . Coupling means, for example an all-thread member  1605  as shown, is used to couple head mass  1603  to surface  203  of chamber wall  201 . Alternately and as illustrated in  FIG. 17 , the head mass (i.e., head mass  1701 ) can be semi-permanently or permanently attached to the cavitation chamber at a joint  1703 . Joint  1703  can be comprised of an epoxy (or other adhesive) bond joint, a braze joint, a diffusion bond joint, or other means. As with the embodiment illustrated in  FIG. 16 , the remaining portions of the driver assembly are coupled to the head mass with an all-thread/nut or bolt means. 
   If desired, and as a means of allowing the driver assembly to be assembled/disassembled separately from the chamber/head mass assembly, a two-piece head mass assembly, such as that illustrated in either  FIG. 18  or  FIG. 19 , can be used. As shown in  FIG. 18 , a first head mass portion  1801  is coupled to chamber exterior surface  203  using a first threaded means  1803  (e.g., all-thread) while a second head mass portion  1805  is coupled to the driver assembly via a second threaded means  1807  (e.g., all-thread/nut arrangement or bolt). A third threaded means  1809  couples head mass portion  1801  to head mass portion  1805 . In a slight modification shown in  FIG. 19 , first head mass portion  1801  is semi-permanently or permanently attached to the cavitation chamber at a joint  1901 , joint  1901  comprised of an epoxy (or other adhesive) bond joint, a braze joint, a diffusion bond joint, or other means. The principal benefit of the configurations shown in  FIGS. 18 and 19  is that the driver assembly is independent of the driver-chamber coupling means. As a result, a driver assembly can be attached to, or detached from, a cavitation chamber without disassembling the actual driver assembly. This is especially beneficial given the susceptibility of piezo-electric crystals to damage. 
   Although not required by the invention, preferably void filling material is included between adjacent pairs of surfaces of the driver assembly and/or the driver assembly and the exterior surface of the cavitation chamber, thereby improving the overall coupling efficiency and operation of the driver. Suitable void filling material should be sufficiently compressible to fill the voids or surface imperfections of the adjacent surfaces while not being so compressible as to overly dampen the acoustic energy supplied by the transducers. Preferably the void filling material is a high viscosity grease, although wax, very soft metals (e.g., solder), or other materials can be used. 
   As will be understood by those familiar with the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.