Patent Application
20070103034
The present invention relates generally to sonoluminescence and, more particularly, to an apparatus for increasing the displacement amplitude in a cavitation chamber coupled to an acoustic driver.
Sonoluminescence is a well-known phenomena discovered in the 1930'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.
U.S. Pat. No. 6,956,316 discloses an acoustic driver assembly for use with a spherical cavitation chamber. The surface of the driver's head mass that is coupled to the chamber has a spherical curvature greater than the spherical curvature of the external surface of the chamber, thus providing a ring of contact between the acoustic driver and the cavitation chamber. The area of the contact ring can be controlled, for example by chamfering a portion of the head mass such that the chamfered surface has the same curvature as the external surface of the chamber.
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's integral extensions, the coupling means sealed to the reactor's enclosure in order to maintain the high pressure characteristics of the enclosure.
The present invention provides an acoustic driver horn that is integral to a wall of a cavitation chamber. The horn design is applicable to any of a variety of cavitation chamber configurations, including spherical, cylindrical, and rectangular chambers. Although a variety of driver assemblies can be coupled to the driver horn, preferably the acoustic driver assembly includes a head mass, a tail mass, and at least one transducer, typically a piezoelectric transducer, and preferably a pair of piezoelectric transducers.
The external surface of the cavitation chamber wall to which the acoustic driver assembly is attached includes a groove, the groove surrounding a portion of the wall which acts as a horn for the attached acoustic driver assembly. Due to the thinning of the wall around the horn region, the head mass is capable of greater displacement than would otherwise be possible for a given driver energy. As a result, the driver assembly is able to more effectively couple its energy into the cavitation fluid within the chamber.
In one embodiment of the invention, the head mass of the acoustic driver is separate from the integral horn region which is defined by the groove within the external surface of the chamber. As a result of this configuration, the head mass can be shaped and/or have a different diameter from that of the horn region. Alternately the head mass and the horn region can be combined into a single structure.
In one embodiment the acoustic driver assembly is attached to 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. If the acoustic driver head mass is separate from the horn region, a pair of threaded means can be used, one to hold together the head mass and the horn, and another to couple the remaining portions of the driver assembly to the head mass. Alternately a single threaded means can be used, both to hold together the head mass and the horn and to assemble the driver.
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.
Preferably piezoelectric 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 piezoelectric and magnetostrictive transducers can also be used, for example as a means of providing greater frequency bandwidth.
Although driver assembly 100 can use a single piezoelectric transducer, preferably assembly 100 uses a pair of piezoelectric 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. Head mass 107 and tail mass 109 can be fabricated from the same material and be of equal mass. Alternately head mass 107 and tail mass 109 can be fabricated from different materials. In yet other alternatives, head mass 107 and tail mass 109 can 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.
Typically driver assembly 100 is assembled about a centrally located all-thread 111 which is screwed directly into the wall 201 of the cavitation chamber. A nut 113 holds the assembly together. If all-thread 111 does not pass through the entire chamber wall as shown, the internal surface of the cavitation chamber remains smooth, thus insuring that there are neither gas nor liquid leaks at the point of driver attachment. It is understood that all-thread 111 and nut 113 can be replaced with a bolt or other means of attachment. An insulating sleeve 203 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 piezoelectric transducers are fabricated by Channel Industries of Santa Barbara, California. 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.
As previously noted, attaching the driver assembly to the outside of the cavitation chamber is advantageous as it eliminates a potential source of gas and fluid leaks, assuming that the means used to couple the driver to the chamber does not protrude through the chamber wall. A disadvantage, however, of this approach is that the energy produced by the driver is dampened by the chamber wall, the degree of dampening being directly proportional to the thickness of the wall. Accordingly even though thick walls can handle higher pressures and are generally better from a fabrication and assembly point of view, for example providing a convenient mounting location for drivers, such walls can significantly decrease the driver coupling efficiency.
Although the chamber shown in the embodiment of
The cavitation chamber of the invention can be fabricated from any of a variety of materials, or any combination of materials, although the surface to which the cavitation driver (or drivers) is attached should be of a machinable material, thus allowing the region of the wall surrounding the wall portion to which the driver is attached to be thinned in accordance with the invention. Other considerations for material selection are the desired operating pressure and temperature of the chamber and system. In addition, preferably the material or materials selected for the cavitation chamber are relatively corrosion resistant to the intended cavitation fluid, thus allowing the chamber to be used repeatedly.
The materials used to fabricate the cavitation chamber can be selected to simplify viewing of the sonoluminescence phenomena, for example utilizing a transparent material such as glass, borosilicate glass, or quartz glass (e.g., Pyrex®). Alternately the cavitation chamber can be fabricated from a more robust material (e.g., 17-4 precipitation hardened stainless steel) and one which is preferably machinable, thus simplifying fabrication. Alternately a portion of the chamber can be fabricated from one material while other portions of the chamber can be fabricated from one or more different materials. For example, in the preferred embodiment illustrated in
The selected dimensions of the cavitation chamber depend on many factors, including the cost of the cavitation fluid, chamber fabrication issues, operating temperature and frequency, sound speed, and the cavitation driver capabilities. In general, small chambers are preferred for situations in which it is desirable to limit the amount of the cavitation medium or in which driver input energy is limited while large chambers (e.g., 10 inches or greater) are preferred as a means of simplifying experimental set-up and event observation or when high energy reactions or large numbers of low energy reactions are being driven within the chamber. Thick chamber walls are preferred in order to accommodate high pressures.
In order to efficiently achieve high energy density (e.g., temperature) cavitation induced implosions within the cavitation fluid within the cavitation chamber, preferably the cavitation fluid is first adequately degassed of unwanted contaminants. Without sufficient degassing, gas within the cavitation fluid will impede the cavitation process by decreasing the maximum rate of collapse as well as the peak stagnation pressure and temperature of the plasma within the cavitating bubbles. It will be understood that the term “gas”, as used herein, refers to any of a variety of gases that are trapped within the cavitation fluid, these gases typically reflecting the gases contained within air (e.g., oxygen, nitrogen, argon, etc.). In contrast, “vapor” only refers to molecules of the cavitation fluid that are in the gaseous phase.
The present invention is not limited to a particular degassing technique. In the preferred embodiment, degassing is performed with a vacuum pump 321 that is coupled to chamber 301 via conduit 323. In an alternate embodiment, degassing can be performed within a separate degassing reservoir in which the cavitation fluid is degassed prior to filling the cavitation chamber. In yet another alternate embodiment, the cavitation fluid can be degassed initially outside of chamber 301 and then again within chamber 301.
In the embodiment illustrated in
A cavitation fluid filling system, not shown, is coupled to chamber 301 and used to fill the chamber to the desired level. It will be appreciated that the operating level for a particular cavitation chamber is based on obtaining the most efficient cavitation action. For example, while a spherical chamber may be most efficiently operated when it is completely full, a vertically aligned cylindrical chamber (e.g., the chamber shown in
Although not required, the filling system may include a circulatory system, such as that described in co-pending U.S. patent application Ser. No. 11/001,720, filed Dec. 1, 2004, entitled Cavitation Fluid Circulatory System for a Cavitation Chamber, the disclosure of which is incorporated herein for any and all purposes. Other components that may or may not be coupled to the cavitation fluid filling and/or circulatory system include bubble traps, cavitation fluid filters, and heat exchange systems. Further descriptions of some of these variations are provided in co-pending U.S. patent application Ser. No. 10/961,353, filed Oct. 7, 2004, entitled Heat Exchange System for a Cavitation Chamber, the disclosure of which is incorporated herein for any and all purposes.
Although the invention is not limited to a specific number, type, mounting technique or mounting location for the acoustic driver, in the embodiment illustrated in
Under some circumstances, for example when either or both the drive power and the internal cavitation chamber pressure are high, the width of the groove is preferably more narrow than in the previous embodiment. For example, in the embodiment shown in
Although in the embodiments illustrated in
In the previously described embodiments, the head mass of the driver is separate from horn region 315 that is integrated into the chamber wall. In an alternate configuration illustrated in
An advantage of the two-piece head mass and horn assembly shown in
As described above, the present invention is not limited to piezoelectric transducers. For example, the invention can also utilize magnetostrictive transducers, simply replacing the head mass/piezoelectric transducer/tail mass assembly with a magnetostrictive transducer assembly. This aspect of the invention is illustrated in
As previously noted, the present invention is not limited to a particular cavitation chamber configuration, nor is it limited to a particular number or configuration of driver assembly. For example, the embodiment illustrated in
Although not required by the invention, preferably void filling material is included between some or all adjacent pairs of surfaces of the driver assembly and/or the driver assembly and 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.
