Patent Publication Number: US-7712680-B2

Title: Ultrasonic atomizing nozzle and method

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to nozzles and to methods used for forming small drops of liquid. More particularly, the present invention relates to ultrasonic nozzles and to methods of operating such nozzles. 
   BACKGROUND OF THE INVENTION 
   Ultrasonic atomization techniques are currently available for forming drops of liquid that have number median drop sizes (d N,0.5 ) of slightly below 20 microns (i.e., approximately 17 or 18 microns). According to these techniques, a solid surface of a metallic nozzle is vibrated at an ultrasonic frequency. Then, a liquid is introduced onto the surface of the nozzle and forms a liquid film thereon. 
   Since the solid surface vibrates in a direction that is perpendicular to the surface the liquid film, the liquid film absorbs vibrational energy from the solid surface. As a result, standing waves (known as “capillary waves”) form in the liquid film. These capillary waves form a rectangular grid of wave crests and troughs and, at relatively low amplitudes of a given vibrational frequency, the crests and troughs of the standing waves are uniformly distributed and stable. However, as the amplitude of the given vibrational frequency is increased, the distance between the crests and troughs of the capillary waves increases (i.e., the waves grow larger) until, at a critical amplitude, the waves become unstable and collapse. 
   As unstable waves collapse, drops of liquid are ejected from the crests of the waves. These drops are ejected at a low velocity in a direction that is normal to the vibrating, solid surface. The formation and ejection of these drops is referred to as “ultrasonic atomization.” 
   The range of amplitudes over which atomization occurs at a given frequency is limited. As discussed above, when the amplitude of the vibration is below a critical level, the capillary waves are stable and no appreciable amount of liquid is ejected from the crests of the waves. On the other hand, when the amplitude is too far above the critical level, cavitation occurs, wherein relatively large amounts of liquid are ejected at high velocities from the vibrating surface. Since cavitation is undesirable when relatively small drops of liquid are sought, when implementing currently-available ultrasonic atomization techniques, the amplitude of vibration is maintained within a relatively narrow range. 
   The peak-to-peak distance between any two adjacent crests in the above-discussed stable, capillary waves depends upon the frequency at which the solid surface vibrates. For example, adjacent crests form in closer proximity to each other at high frequencies than they do at lower frequencies. As such, when capillary waves become unstable and collapse, waves having adjacent crests that are closer together eject smaller drops of liquid than do waves having adjacent crests that are further apart from each other. Therefore, when the formation of relatively small drops of liquid is sought, it is often desirable to operate an ultrasonic atomization device at a relatively high frequency. 
   One currently-available ultrasonic atomization device that may be used to implement the above discussed techniques includes a nozzle that itself includes three principle active sections: an atomizing section (i.e., a front horn), a rear section (i.e., a rear horn) and an intermediate section. The front horn includes a solid, metallic vibrating surface where atomization takes places. The rear horn is configured to be connected to a source of liquid to allow the liquid to enter the nozzle. The intermediate section, which is positioned between the front horn and the rear horn, includes two piezoelectric transducers. When in operation, these transducers cause the atomizing surface on the front horn to vibrate at an ultrasonic frequency. More specifically, the transducers convert high-frequency electrical energy from an external power source into high-frequency mechanical motion that is transferred to the atomizing surface in order to cause the vibration thereof. 
   The transducers in currently-available ultrasonic atomization devices are disk-shaped and made from zirconate-titanate ceramics. Also, silver-plated or nickel-plated copper electrodes are used to introduce high-frequency electrical energy into the currently-available nozzle. 
   The front and rear horn of the currently-available nozzle are each fabricated from a Ti-6Al-4V titanium alloy. However, like all metal-based nozzles, this alloy has a plurality of shortcomings when it comes to forming small drops of liquid via ultrasonic atomization techniques. For example, the number median drop size (d N,0.5 ) of the drops formed has a lower limit of approximately 17 or 18 microns. Also, the maximum flow rate of the liquid from which such small drops may be formed has an upper limit of approximately 10 gallons per hour (i.e., 600 ml per minute). 
   At least in view of the above, it would be desirable to provide nozzles and methods capable of forming drops of liquid having a number median drop size below 17 or 18 microns. It would also be desirable to provide nozzles and methods capable of forming such drops while maintaining flow rates of above 10 gallons per hour. 
   SUMMARY OF THE INVENTION 
   The foregoing needs are met, to a great extent, by certain embodiments of the present invention. According to one embodiment, a nozzle is provided. The nozzle includes an interface section configured to allow introduction of a liquid into the nozzle. The nozzle also includes an atomizing section that itself includes a ceramic material. The atomizing section is configured to form drops of the liquid having number median drop sizes of less than approximately 20 microns. The nozzle further includes an intermediate section positioned between the rear section and the atomizing section. The intermediate section is configured to promote ultrasonic-frequency mechanical motion in the atomizing section. 
   According to another embodiment of the present invention, a method of atomizing a liquid is provided. The method includes coating a portion of a ceramic surface with a liquid. The method also includes mechanically moving the surface at an ultrasonic frequency. The method further includes forming drops of the liquid having number median drop sizes of less than approximately 20 microns. 
   According yet another embodiment of the present invention, another nozzle is provided. The nozzle includes means for interfacing with a source of a liquid. The nozzle also includes means for forming drops of the liquid having number median drop sizes of less than approximately 20 microns, wherein the means for forming includes a ceramic material. The nozzle further includes means for promoting ultrasonic-frequency mechanical motion in the atomizing means, wherein the means for promoting is positioned between the means for interfacing and the means for forming. 
   There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
   In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
   As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal cross-sectional view of a ceramic-containing ultrasonic atomizing nozzle arrangement according to a first embodiment of the present invention. 
       FIG. 2  illustrates a radial cross-section of the ultrasonic atomizing nozzle arrangement illustrated in  FIG. 1  taken along line A-A. 
       FIG. 3  is a longitudinal cross-sectional view of a ceramic-containing ultrasonic atomizing nozzle arrangement according to a second embodiment of the present invention. 
       FIG. 4  is a side view of a ceramic-containing ultrasonic atomizing nozzle arrangement according to a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout.  FIG. 1  is a longitudinal cross-sectional view of a ceramic-containing ultrasonic atomizing nozzle arrangement  10  according to a first embodiment of the present invention. However, before further discussing the drawing figures any further, a few scientific principles related to ultrasonic atomization are briefly reviewed below. 
   Ceramic materials (e.g., SiC and Al 2 O 3 ) differ from metals (e.g., titanium and titanium alloys) in a number of ways. For example, in some ceramic materials, such as silicon carbide (SiC) and aluminum oxide (Al 2 O 3 ), the characteristic velocity at which sound waves propagate through these materials is considerably greater than the characteristic velocity at which sound waves propagate through any metallic material that is practical for use in constructing an ultrasonic atomizing nozzle. For example, SiC can be manufactured such that the characteristic velocity of sound therein is between 2.3 and 2.7 greater than the characteristic velocity of sound in a Ti-6Al-4V titanium alloy. 
   When implementing an ultrasonic atomization method according to certain embodiments of the present invention, capillary waves are produced in a liquid coating that is present on a solid surface that is vibrating at an ultrasonic frequency. Under such conditions, the number median drop size (d N,0.5 ) of the drops formed is calculated as follows:
 
 d   N,0.5 =0.34(8π s/ρf   2 ) 1/3 ,
 
where f=the operating frequency of the nozzle, ρ=the density of the liquid coating the surface and s=the surface tension of the liquid. Hence, as the operating frequency, f, increases, the number median drop size (d N,0.5 ) decreases.
 
   In order to form capillary waves that are suitable for ultrasonic atomization, it is desirable to suppress the formation of waves that are not perpendicular to the solid surface from which the liquid film absorbs vibrational energy. In order to suppress the formation of such non-perpendicular waves, the largest diameter of any active nozzle element is limited. More specifically, the diameter is limited to a length that is below one-fourth of the wavelength, λ, of an acoustic wave in the material from which the atomizing surface is formed. 
   The wavelength, λ, of an acoustic wave in such a material is calculated as follows:
 
λ= c/f,  
 
where c=the characteristic velocity at which sound waves propagate through a ceramic material. Thus, for a given operational frequency, materials having higher characteristic velocities, c, at which sound waves propagate therethrough correspond to longer wavelengths. Hence, such materials allow for a larger nozzle diameter at a given frequency.
 
   When the diameter of the nozzle becomes so small that the nozzle becomes impractical to make or use, the practical operating frequency of the nozzle is reached. As such, in metallic nozzles according to the prior art (i.e., in nozzles where the vibrating surface is metallic), the practical upper limit of the operating frequency, f, is approximately 120 kHz. However, in nozzles according to embodiments of the present invention where ceramics are used, the upper limit of the operating frequency, f, is raised to approximately 250 kHz. Thus, for a given liquid, d N,0.5  is reduced by a factor of (120/250) 2/3 =0.61. 
   Keeping in mind the above-mentioned characteristics of ceramic materials, one of skill in the art will appreciate that, at a given operating frequency, f, ceramic nozzles can be operated at a greater flow rate than their metallic counterparts. In other words, the diameter of the nozzle can remain larger in a ceramic nozzle than in a metallic nozzle, as can stems, the area of the atomizing surface, and/or liquid feed orifices that may be included to lead liquid to the nozzle. 
   As mentioned above,  FIG. 1  is a longitudinal cross-sectional view of an ultrasonic atomizing nozzle arrangement  10  according to a first embodiment of the present invention. The nozzle  10  illustrated in  FIG. 1  includes a rear horn  12  that functions as an interface section. As such, the rear horn  12  is configured to allow the introduction of a liquid into the nozzle  10 . 
   The rear horn  12  illustrated in  FIG. 1  is directly connected to a liquid inlet  14 . However, the rear horn  12  may be directly or indirectly connected to any component that will allow for flow of a liquid into the nozzle  10 . The liquid inlet  14  may be affixed to the rear horn  12  in any manner that would become apparent to one of skill in the art upon practicing the present invention (e.g., a pressure seal or an adhesive). Although not illustrated in  FIG. 1 , the liquid inlet  14  is typically connected, either directly or indirectly, to a source of liquid such as, for example, a tank containing water or an organic solvent. 
   According to certain embodiments of the present invention, the rear horn  12  is either made entirely from a ceramic material or portions of the rear horn  12  are made from a ceramic material. However, according to other embodiments of the present invention, the rear horn  12  is fabricated either partially or entirely from a metal. For example, the rear horn  12  may be made from silicon carbide (SiC) or aluminum oxide (Al 2 O 3 ). 
   The nozzle  10  illustrated in  FIG. 1  also includes a front horn  16  that is configured to function as an atomizing section. The front horn  16 , according to certain embodiments of the present invention, can include one or more portions made from a ceramic material (e.g., SiC or Al 2 O 3 ) or can be made entirely from one or more ceramic materials. The front horn  16  is configured to form drops of the liquid introduced into the nozzle  10  through the rear horn  12 . These drops can, according to certain embodiments of the present invention, have number median drop sizes (d N,0.5 ) of less than approximately 20 microns (e.g., approximately 17 microns), although larger drop sizes are also within the scope of certain embodiments of the present invention. Also, according other embodiments of the present invention, the front horn  16  is configured to form drops of liquid having number median drop sizes of between approximately 7 microns and approximately 10 microns. 
   One of the advantages of the nozzle  10  illustrated in  FIG. 1  is that it increases the rate at which a liquid introduced into the nozzle  10  may be atomized. As discussed above, because the ceramic material used in embodiments of the present invention have higher characteristic velocities at which sound waves propagate therethrough, a larger front nozzle diameter is allowable for a given frequency. Therefore, according to certain embodiments of the present invention, the front horn  16  is configured to allow the liquid introduced into the nozzle  10  to flow through the nozzle  10  at a rate above approximately 600 ml per minute (10 gallons per hour). According to other embodiments of the present invention, the front horn  16  is configured to allow the liquid to flow through the nozzle  10  and the front horn  16  at a rate of approximately 1200 ml per minute (20 gallons per hour). 
   In the nozzle  10  illustrated in  FIG. 1 , the rear horn  12  and the front horn  16  have substantially equal lengths. However, according to other embodiments of the present invention, the rear horn  12  and the front horn  16  have different lengths. According to certain embodiments of the present invention, a ceramic nozzle operates at 250 kHz and the rear horn  12  and front horn  16  both have lengths equal to, for example, 3λ/4, since horns of such length are substantially easier to manufacture than horns having lengths of λ/4. According to certain other embodiments of the present invention, a ceramic nozzle operates at 120 kHz and both horns  12 ,  16  have lengths of λ/4, which are relatively practical to manufacture. 
   The nozzle  10  illustrated in  FIG. 1  also includes a transducer portion  18  that includes a pair of transducers that are positioned in an intermediate section of the nozzle  10  that is located between the rear horn  12  and the front horn  16 . The transducers in the transducer portion  18  are piezoelectric transducers and are configured to promote ultrasonic-frequency mechanical motion in the front horn  16 . In other words, the transducers in the transducer portion  18  provide the mechanical energy to cause the atomizing surface  20  located on the front horn  16  illustrated in  FIG. 1  to vibrate at an ultrasonic frequency with sufficient amplitude to result in atomization. Although two transducers are discussed above as being included in the transducer portion  18  illustrated in  FIG. 1 , a single transducer and/or any other component or system that can be used to cause ultrasonic-frequency mechanical motion in the front horn  16  is also within the scope of the present invention. 
   The rear horn  12  and the front horn  16  each include a flange  22 . A cover, in the form of a ring  24 , is positioned adjacent to each of the flanges  22  illustrated in  FIG. 1 . A plurality of fasteners, in the form of bolts  26 , are also illustrated in  FIG. 1  and connect the two rings  24 . 
   The above-discussed bolts  26  and rings  24  are components of a clamping mechanism that is positioned adjacent to the exterior surfaces of the rear horn  12  and front horn  16 , respectively. This clamp is configured to keep the front horn  16  and the rear horn  12  adjacent to the transducer portion  18 . In addition, this clamp is also configured to apply predetermined compressive forces to the transducer/horn assembly, thereby assuring proper mechanical coupling amongst the various elements of the assembly. 
   By using the clamp arrangement illustrated in  FIG. 1 , the rear horn  12  and the front horn  16 , one or both of which may be made from a ceramic material, do not need to include threaded holes that directly accommodate the bolts to be kept adjacent to each other. This reduces the likelihood that either the rear horn  12  or the front horn  16  will crack as threaded holes are formed therein or that the ceramic threads formed in such holes will lack the shear strength to sustain the amounts of pressure to which they may be subjected (e.g., over 10,000 psi). 
   Also illustrated in  FIG. 1  are a front shroud  11 , a rear shroud  13  and a plurality of O-rings  15 . Together, the front shroud  11  and the rear shroud  13  provide a housing for the nozzle  10  and the O-rings  15  provide a plurality of seals within this housing. 
     FIG. 2  illustrates a radial cross-section of the ultrasonic atomizing nozzle arrangement  10  illustrated in  FIG. 1  taken along line A-A. As illustrated in  FIG. 2 , the rear horn  12  has a fluid inlet  28  at the center thereof. This fluid inlet  28  is part of the liquid conduit  30  illustrated in  FIG. 1  that allows liquid to travel from the liquid inlet  14  all the way to the atomizing surface  20  on the front horn  16 . 
   As also illustrated in  FIG. 2 , the ring  24  extends around the rear horn  12  and has a plurality of bolts  26  positioned at various locations about the circumference thereof. Although a ring  24  is illustrated in  FIG. 2  as making up a portion of the above-discussed clamp, other components may be positioned adjacent to the flanges  22  illustrated in  FIG. 1 . For example, square or rectangular plates may be used. Also, although six regularly spaced bolts  26  are illustrated around the periphery of the ring  24  in  FIG. 2 , other distributions of one or more bolts  26  or other fasteners may be used according to other embodiments of the present invention. 
     FIG. 3  is a longitudinal cross-sectional view of an ultrasonic atomizing nozzle arrangement  32  according to a second embodiment of the present invention. Like the nozzle  10  illustrated in  FIG. 1 , the nozzle  32  illustrated in  FIG. 3  includes a liquid inlet  34 , a rear horn  36  and a front horn  38 , each having a flange  40 . The front horn  38  also includes an atomizing surface  42  that is positioned at one end of a liquid conduit  44 . In addition, the nozzle  32  illustrated in  FIG. 3  includes a clamp arrangement that includes a plurality of rings  46  and bolts  48 . Further, the nozzle  32  also includes a transducer portion  49  that includes a pair of transducers that are positioned in an intermediate section of the nozzle  32  that is located between the rear horn  36  and the front horn  38 . Also illustrated in  FIG. 3  are a front shroud  33  and a rear shroud  35  that, together, provide a housing for the nozzle  32  and a plurality of O-rings  37  that provide a plurality of seals within this housing. 
   One way in which the nozzle  32  illustrated in  FIG. 3  differs from the nozzle  10  illustrated in  FIG. 1  is that the front horn  38  illustrated in  FIG. 3  is approximately 3 times a long as the rear horn  36  illustrated therein. This is particularly representative of the fact that the rear horn  12  and front horn  16  may, according to certain embodiments of the present invention, have different lengths. In fact, according to certain other embodiments of the present invention, the respective lengths of the rear horn and front horn in a given nozzle are multiples or fractions of each other. As mentioned above, under certain operating conditions (e.g., high frequencies), it becomes impractical to manufacture horns having lengths equal to λ/4. Therefore, horns having lengths equal to multiples of λ/4 are often used under such circumstances. 
     FIG. 4  is a side view of a ceramic-containing ultrasonic atomizing nozzle  50  arrangement according to a third embodiment of the present invention. Although only a front horn  52  and an atomizing surface  54  are illustrated in  FIG. 4 , the nozzle  50  illustrated in  FIG. 4  also includes a rear horn, flanges, transducers and other components analogous to the components included in the nozzles  10 ,  32  illustrated in  FIGS. 1-3 . However, the nozzle  50  illustrated in  FIG. 4  sits in a nozzle holder  56  that is positioned adjacent to a probe adjuster and holder  58 . In turn, the probe adjuster and holder  58  is connected to a liquid delivery probe  60  that delivers liquid from a liquid input  62  to the atomizing surface  54 . In other words, whereas liquid in the nozzles  10 ,  32  illustrated in  FIGS. 1-3  traveled through the centers thereof before reaching the atomizing surfaces  20 ,  42 , the nozzle  50  illustrated in  FIG. 4  has liquid delivered directly to the atomizing surface  54  from a source exterior to the nozzle  50  (i.e., liquid delivery probe  60 ). 
   Typically, an exit point  64  of liquid delivery probe  60  is positioned within a few thousandths of an inch and to the side of atomizing surface  54 . However, according to certain embodiments of the present invention, particularly those used to atomize liquid metals, the exit point  64  is located substantially directly above the atomizing surface  54 . 
   According to yet another embodiment of the present invention, a method of atomizing a liquid is provided. The method includes coating a portion of a ceramic surface (e.g., the atomizing surface  20  illustrated in  FIG. 1 ) with a liquid. According to certain embodiments of the present invention, this coating step includes introducing the liquid onto the surface at a rate of between approximately 600 ml/minute (i.e., 10 gal/hour) and approximately 1200 ml/minute (i.e., 20 gal/hour). 
   The method also includes mechanically moving (i.e., vibrating) the surface at an ultrasonic frequency. According to certain embodiments of the present invention, this mechanically moving step includes mechanically moving the surface at a frequency of between approximately 120 kHz and approximately 250 kHz. According to other embodiments of the present invention, the mechanically moving step includes mechanically moving the surface at a frequency of between approximately 25 kHz and less than approximately 120 kHz (e.g., approximately 60kHz). 
   The above-discussed method also includes forming drops of the liquid having number median drop sizes of less than approximately 20 microns. According to certain embodiments of the present invention, the coating step comprises selecting liquids containing an organic solvent. According to these embodiments, the number median drop size of the drops formed during the above-discussed forming step is between approximately 7 microns and approximately 10 microns. 
   The above-discussed method also includes passing the liquid through an interface section that includes a ceramic material before performing the coating step. This passing step may be performed, for example, by passing liquid through either the rear horn  12  or the front horn  16  illustrated in  FIG. 1 , so long as at least one of these horn  12 ,  16  has a ceramic material incorporated therein. 
   According to other embodiments of the present invention, the above-discussed method includes clamping the interface section to an atomizing section that includes the ceramic surface. This clamping step is typically an alternative to having to use fasteners that would have to be screwed directly into components of a nozzle used to implement the above-discussed method. 
   The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.