US20060060331A1

US20060060331A1 - Apparatus and method for enhanced heat transfer

Info

Publication number: US20060060331A1
Application number: US11/205,665
Authority: US
Inventors: Ari Glezer; Samuel Heffington; Marc Smith
Original assignee: Individual
Current assignee: Georgia Tech Research Corp; Nuventix Inc
Priority date: 2004-08-20
Filing date: 2005-08-17
Publication date: 2006-03-23
Also published as: WO2006023737A2; WO2006023737A3

Abstract

One embodiment of the system is implemented as a device for two-phase heat transfer. This device comprises a chamber containing a fluid, where a heated wall makes up a portion of the chamber. The device also comprises an actuator that emits pressure vibrations. The pressure vibrations dislodge vapor bubbles that form at the heated wall due to the heat in the wall.

Description

CLAIM TO PRIORITY

The present application claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 60/603,436, filed on Aug. 20, 2004, which is hereby incorporated by reference herein.

BACKGROUND

1. Technical Field
The present invention is generally related to thermal management technology and, more particularly, is related to an apparatus and method for cooling heat-producing bodies or components using a two-phase cooling heat transfer device based on a vibration-induced bubble ejection process.
2. Description of the Related Art
Cooling of heat-producing bodies is a concern in many different technologies. Particularly in microprocessors, the rise in heat dissipation levels accompanied by a shrinking thermal budget has resulted in the need for new cooling solutions beyond conventional thermal management techniques. In the microelectronics industry, for example, advances in technology have brought about an increase in transistor density and faster electronic chips. As electronic packages increase in speed and capability, the heat flux that must be dissipated to maintain reasonable chip temperatures has also risen. Thermal management is recognized as a major challenge in the design and packaging of state-of-the-art integrated circuits in single-chip and multi-chip modules.
One method for effective heat transfer is so-called “two-phase” heat transfer. Two-phase heat transfer involves, generally, the evaporation of a liquid in a hot region and the condensation of the resulting vapor in a cooler region. This type of cooling is a highly effective cooling strategy for at least three reasons. First, the liquid to vapor phase change greatly increases the heat flux from the heated surface. Second, the high thermal conductivity of the liquid medium, as opposed to that of air, enhances the accompanying natural or forced convection. A third reason for the efficient heat transfer that occurs during two-phase heat transfer is that buoyancy forces remove the vapor bubbles generated at the heated surface away from the heated surface.
Two-phase, or “boiling,” heat transfer is known and has been studied for a number of years. Heat pipes and thermosyphons are examples of efficient heat transfer devices that have been developed to exploit the benefits of two-phase heat transfer. Immersion cooling, which involves the pool boiling of a working fluid on a heated surface, is another example of a two-phase cooling technology.
There are limitations to the current state of the art in two-phase cooling. First, two-phase heat transfer systems have traditionally been viewed as incompatible with microelectronic packages. This is largely due to the fact that liquid is involved in the process.
Second, two-phase heat transfer systems are constrained by a phenomena that manifests itself most noticeably in microgravity environments. When the heat flux from the surface is increased past a critical level, a large, potentially catastrophic increase in temperature occurs. This critical heat flux marks the transition from nucleate boiling to what is known as film boiling. In film boiling, a thin insulating layer of vapor completely covers the heated surface, which then produces a large temperature increase. This transition occurs at much lower heat fluxes in a microgravity environment because buoyancy forces are almost negligible. Thus, the performance of immersion cooling in this environment is drastically reduced.
A heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.

SUMMARY

Embodiments of the present invention provide a system and method for cooling heated bodies and environments by using a vibration-induced bubble injection system, method, and device.
A cooling cell based on the submerged vibration-induced bubble ejection (VIBE) process in which small vapor bubbles attached to a solid surface are dislodged and propelled into the cooler bulk liquid capitalizes on the benefits of two-phase cooling while improving on traditional methods of implementing two-phase heat transfer. The VIBE device described below exceeds the performance of conventional immersion cooling devices because it delays the onset of the critical heat flux. By forcibly removing the attached vapor bubbles with pressure instabilities, the VIBE device and method dissipate more energy for a given surface temperature than previous immersion coolers.
Briefly described, in architecture, one embodiment of the VIBE device described herein, among others, can be implemented as a device for two-phase heat transfer. This one embodiment comprises a chamber containing a fluid. This embodiment also comprises a heated wall making up a portion of the chamber. Finally, the embodiment comprises an actuator that emits pressure vibrations. The pressure vibrations dislodge vapor bubbles forming at the heated wall due to the heat in the wall.
Embodiments of the present invention can also be viewed as providing methods for cooling. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: (i) providing a chamber with a fluid; (ii) generating heat in a wall of the chamber; (iii) causing the formation of vapor bubbles at the heated wall; and (iv) emitting pressure vibrations into the fluid, wherein the vapor bubbles dislodge from the heated wall due to the pressure vibrations.
Other devices, systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional devices, systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
FIG. 1 is a cut-away side view of a first embodiment of a two-phase heat transfer device.
FIG. 2 is a cut-away side view of an alternative embodiment of an actuator used in a two-phase heat transfer device.
FIG. 3 is a cut-away side view of a second embodiment of a two-phase heat transfer device.
FIG. 4 is a cut-away side view of a third embodiment of a two-phase heat transfer device.
FIG. 5 is a cut-away side view of a fourth embodiment of a two-phase heat transfer device.
FIG. 6 is a cut-away side view of a fifth embodiment of a two-phase heat transfer device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present disclosure is directed to a method and apparatus for heat transfer. The cooling method and apparatus described herein generally use a two-phase cooling heat transfer device based on a vibration-induced bubble ejection (“VIBE”) process.

Construction of the Vibe Device

FIG. 1 depicts a first embodiment 10 of an apparatus for accomplishing the disclosed method through the use of a VIBE cooling apparatus. The VIBE apparatus 10 of the first embodiment generally comprises a chamber 11 for holding a fluid 12.
The chamber 11 could be constructed of any suitable material. Generally, the material used for the chamber 11 will depend to some degree on the particular fluid 12 in the chamber 11 and on the particular heat transfer characteristics desired. The preferred material from which the chamber 11 is to be constructed is a light-weight metallic material from which the chamber 11 can be easily and inexpensively manufactured. For example, the material for the chamber 11 of the present embodiment 10 is aluminum.
In the present embodiment 10, the entire chamber 11 is constructed from an aluminum material. However, in an alternative embodiment, the chamber 11 is manufactured from more than one material. In other words, different parts of the chamber 11 are manufactured from different materials. Such a configuration minimizes heat transfer to certain parts of the chamber 11, while maximizing heat transfer to other parts of the chamber 11.
More specifically, in this alternative embodiment, some parts of the chamber 11 are constructed from a highly thermally conductive material. Other parts of the chamber 11 are constructed from a thermally insulating material. This possibility will be discussed more specifically below.
Generally, the chamber 11 may be manufactured in any shape desired or dictated by the use to which the VIBE apparatus 10 will be put. One of ordinary skill in the art will easily be able to size and shape an appropriate chamber 11 for a given application. In the present embodiment 10 the chamber 11 is cubic. The cubic chamber 11 has a lower wall 13, an upper wall 14, and two side walls 15, 16. Of course, the chamber 11 also comprises a front wall and a back wall. As FIG. 1 is a cut-away side view of the present embodiment 10, the front wall is not depicted in FIG. 1.
As will be explained in more detail below, the fluid 12 in the chamber 11 of the VIBE device 10 will be involved in a heat transfer process. For this reason, the selection of the fluid 12 to be used with the VIBE device 10 may change depending on the particular application of the device 10. As will be readily understood by one of ordinary skill in the art after reading this description, different fluids will exhibit different heat transfer, safety, availability, and other characteristics. After reading the present description, one of ordinary skill in the art would easily be able to make an appropriate fluid selection.
The fluid 12 in the present embodiment 10 is a mixture of methanol and water. The preferred mixture of the present fluid 12 is 70% distilled water and 30% methanol. However, the fluid 12 of the present embodiment 10 does not have to comprise such a mixture.
For example, if more viscosity in the fluid 12 is desired, ethylene glycol, or an ethylene glycol/water mixture, is used as the working fluid 12 of the device 10. Alternatively, 100% distilled water could be used of the working fluid 12 of the present embodiment 10. Almost any fluid could be used in the VIBE device 10, depending on the particular application of the device 10 and the particular performance characteristics desired. Generally, it has been found that lower viscosity fluids are preferred for most applications. Lower viscosity fluids in the VIBE device 10 generally permit greater heat transfer and, thereby, a greater cooling effect. In most applications, greater cooling is desired.
In the present embodiment 10, the chamber 11 is preferably hermetically sealed except for an inlet pipe 17 and an outlet pipe 18. These two pipes 17, 18 permit the fluid 12 to flow into and out of the chamber 11, respectively. Preferably, a fluid flow is established in the chamber 11 by moving fluid into the chamber 11 through the inlet pipe 17, thereby forcing fluid 12 out of the chamber 11 through the outlet pipe 18. Of course, the fluid flow could also be established in the chamber 11 by withdrawing fluid 12 through the outlet pipe 18, thereby creating a pressure gradient that draws fluid 12 into the chamber 11 through the inlet pipe 17. Although described in the present embodiment, a fluid flow in the chamber 11 is not required for the VIBE device 10 to function properly. Alternative embodiments of a VIBE device without a fluid flow will be discussed in more detail below.
The fluid flow described above is created in the present embodiment 10 because the inlet pipe 17 and outlet pipe 18 are both part of a connected fluidic system, as depicted in FIG. 1. In the present embodiment, the pipes 17, 18 are fluidically connected to a fluid reservoir 19 and/or a remote heat exchanger. The fluid 12 is caused to flow into the chamber 11 though the inlet pipe 17, and out of the chamber 11 through the outlet pipe 18. The outlet pipe 18 carries the fluid 12 to the fluid reservoir 19, where the fluid 12 is circulated back into the inlet pipe 17 and carried back to the chamber 11. Of course, the fluid reservoir 19 of the present embodiment 10 is not required for the VIBE device to function. In some embodiments, the fluid reservoir 19 can be omitted.
In an alternative embodiment of the present VIBE apparatus 10, the device includes a process for cooling the fluid 12 while the fluid 12 is in, or passing through, the reservoir 19. This is preferably accomplished by the fluid reservoir 19 taking the form of a container in a refrigerated cabinet. Alternatively, the reservoir 19 is equipped with other means of refrigeration. In either configuration, heat is directly extracted from the fluid 12 in the reservoir 19 by an external cooling mechanism.
In an alternative embodiment, the fluid reservoir 19 takes the form of a heat exchanger remote to the chamber 11. In this alternative embodiment, the fluid 12 is cooled as it moves through the fins of the remote heat exchanger.
Preferably, a pump 21 is affixed at the fluid reservoir 19 in order to move the fluid 12 from the fluid reservoir 19 through the inlet pipe 17 back to the chamber 11. Basically, the pump 21 is the apparatus of the fluid system that actually creates the desired fluid flow in the chamber 11.
The VIBE device 10 of the present description does not require that a pump 21 be used to circulate the fluid 12 through the fluid system. Indeed, if a fluid flow is desired, the fluid 12 may be moved through the pipes 17, 18 and chamber 11 in a variety of ways consistent with the present embodiment 10. For example, fan blades, louvers, or other fluid movement apparatus may be used to move the fluid 12 through the system. In addition, the type and size of pump 21 of the present embodiment 10 may be altered in order to increase or decrease the fluid flow rate as desired for a particular application. One of ordinary skill in the art, upon reading the present description, can readily select and implement a pump 21 of the appropriate size and configuration.
The present embodiment 10 also includes an actuator 22 situated in the chamber 11. The actuator 22 is mounted to the upper wall 14 of the chamber 11. Alternatively, the actuator 22 could be manufactured into the structure of a wall of the chamber 11. This alternative design will be discussed in more detail below.
The actuator 22 of the present embodiment 10 can be of many possible designs. However, the depicted actuator 22 comprises a diaphragm 23 secured to a mounting body 24 (or simply a “mount”).
The diaphragm 23 is preferably constructed of a ceramic material with a copper or brass layer; however, this particular construction is not required. The diaphragm 23 is preferably securely attached to the mount 24. The diaphragm 23 may be attached to the mount 24 by any appropriate means, and the particular method of attachment is not critical to the present embodiment 10.
In the depicted embodiment of the actuator 22, the mount 24 is preferably cubic in shape. The diaphragm 23, therefore, is formed into a square shape such as to form one wall of the mount's cube shape.
Attached to an inner side of the diaphragm 23 is a piezoelectric element 26. The piezoelectric element 26 is preferably attached to the diaphragm 23 by an adhesive, or other means. The piezoelectric element 26 is actuated by a discrete electronic driving circuit 27 of this embodiment that is preferably positioned exterior to the chamber 11. The driving circuit 27 comprises a sinusoidal function generator and an amplification chip (not separately depicted in FIG. 1). The driving circuit 27 is electronically connected to the piezoelectric element 26 by appropriate wiring 30 that passes through the upper wall 14 of the chamber 11.
The mount 24 is preferably constructed of a lightweight metal, such as aluminum. The cubic shape of the mount 24 of the present embodiment is not required. Indeed, the mount 24 could be formed into, for example, a cylindrical shape. In this situation, the diaphragm 23 is manufactured into a circular shape in order to correspond to the cross-section of the mount 24. The shape of the mount 24 and the diaphragm 23 are not critical to the functioning of the VIBE apparatus 10.
An alternative configuration of the actuator 22 positions the driving circuit 27 inside the mount 24. FIG. 2 is a cut-away side view of this alternative actuator 22 configuration. In such a configuration, the driving circuit 27 is placed inside the mount 24 such that the actuator 22 is completely self-contained.
As briefly mentioned above, the actuator 22 of a second embodiment 35 is built into a wall of the chamber 11. See FIG. 3. For example, the diaphragm 23 of the actuator 22 could be positioned flush with, or at least closer to, the upper wall 14 of the chamber 11. This embodiment for a VIBE device 35 is depicted in FIG. 3. In this configuration, the mount 24 is entirely exterior to the chamber 11.
In another alternative embodiment 40, the mount 24 is completely eliminated and the diaphragm 23 forms one of the chamber walls 14. This configuration 40 is depicted in FIG. 4, which is a cut-away side view of this third embodiment 40. In such a configuration 40, the upper wall 14 of the chamber 11 is comprised of a diaphragm 23. The driving circuit 27 is positioned on a side wall 16 of the chamber 11. The diaphragm 23 is still equipped with a piezoelectric element 26 that is driven by the driving circuit 27.
Returning to FIG. 1, the bottom wall 13 of the chamber 11 is adjacent to a heated body or heat-producing body 28. For example, a microelectronic circuit or chip may be situated adjacent to the bottom wall 13 of the chamber 11. Thus, the heat from the heat-producing body 28 travels into the bottom wall 13 of the chamber 11. As will be apparent to one of ordinary skill in the art, the material that forms the bottom wall 13 of the chamber 11 affects the rate of heat transfer into this wall 13. As noted above, the preferred material for all the walls of the chamber 11 is aluminum. Since the preferred bottom wall 13 is constructed of aluminum, the heat transfer into the wall 13 will be at a relatively high rate.
In an alternative embodiment, the bottom wall 13 of the chamber 11 is constructed of a different material from the remainder of the chamber 11 in order to increase heat transfer into the bottom wall 13, but reduce heat transfer into the other walls of the chamber 11. The bottom wall 13 of the chamber 11 in this alternative configuration is constructed of copper, but the other walls of the chamber 11 are constructed of a less thermally conductive material, such as aluminum, brass, or most preferably plastic.
Regardless of the material making up the walls of the chamber 11, the configuration of the VIBE device 10 is modified in other alternative embodiments. For example, in one other alternative embodiment, the bottom wall 13 of the chamber 11 is positioned next to a larger heat sink structure. With such an embodiment, the heat sink absorbs heat from one or more heat-producing bodies. Then, the VIBE device would remove heat from, and consequently cool, the heat sink.
In another alternative embodiment, a heat-producing body actually forms the bottom wall 13 of the chamber 11 itself. Basically, a housing of a microelectronic circuit makes up at least a portion of the bottom wall 13 of the chamber 11. In this alternative embodiment, the VIBE device 10 directly cools the heat-producing device itself.

Operation of the Vibe Device

In operation, the VIBE apparatus 10 functions to cool the heated body 28. As the heated body 28 produces heat, the heat flows into the bottom wall 13 of the chamber 11. The heat is further transferred into the cooler fluid 12. As the fluid 12 absorbs heat, the temperature of the fluid 12 adjacent to the bottom wall 13 rises. At some point in time, the temperature of the fluid 12 adjacent to the bottom wall 13 will reach the boiling temperature of the fluid 12. Upon the fluid 12 reaching its boiling temperature, vapor bubbles 29 will begin to form at the bottom wall 13 of the chamber 11. In essence, the fluid 12 begins boiling.
Initially, the vapor bubbles 29 tend to cling to the bottom wall 13 of the chamber 11. If the VIBE device 10 was is not operating, the vapor bubbles 29 continue to cling to the bottom wall 13 as the temperature of the wall 13 and the adjacent fluid 12 continues to rise. As the temperature of the fluid 12 adjacent to the bottom wall 13 continues to rise, a critical temperature is reached where nucleate boiling of the fluid 12 generally ceases and film boiling begins. This critical point varies depending on the fluid 12 used. In this situation, the vapor bubbles 29 begin to form a thin insulating layer of vapor along the bottom wall 13 of the chamber 11. If this were allowed to continue, there would be a dramatic reduction in cooling of the bottom wall 13, and consequently, the heated body 28.
The present VIBE device 10, however, remedies this potential limitation by causing the actuator 22 to vibrate the diaphragm 23. The vibration of the diaphragm 23 creates a series of pressure waves 31 that emit from the diaphragm 23. The waves 31 strike the bottom wall 13 and cause the vapor bubbles 29 to become dislodged. Once dislodged, the buoyancy of the vapor bubbles 29 carry them up and away from the bottom wall 13 of the chamber 11. At this point, the fluid flow discussed above sweeps the vapor bubbles 29 away from the bottom wall 13 and out of the chamber 11. Once away from the bottom wall 13 of the chamber 11, the vapor bubbles 29 begin to cool. As the bubbles 29 cool, they condense, release their stored heat into the surrounding fluid 12, and are thus reincorporated into the fluid 12. In this manner, the heated bottom wall 13 of the chamber 11 is cooled. In turn, this process cools the heated body 28. Basically, the action of the VIBE device 10 in dislodging the vapor bubbles 29 prevents the formation of the thin insulating layer of vapor discussed above, and prevents reaching the critical heat flux in which the surface is coated with vapor.
Preferably, the diaphragm 23 of the present embodiment 10 is caused to vibrate at its resonant frequency of its first axisymmetric mode of vibration. Nominally, this frequency in the first embodiment is about 1.65 MHz. Vibration of the diaphragm 23 at this frequency produces ultrasonic pressure waves in the fluid 12. It is not necessary to vibrate the diaphragm 23 at its resonant frequency, but this is preferred. This is because ultrasonic pressure waves 31 are also preferred, though not required.
An alternative embodiment of a VIBE apparatus 50 is depicted in FIG. 5. As will be seen in the figure, this embodiment 50 comprises no inflow pipe and no outflow pipe. The chamber 11 is completely sealed. In this embodiment 50 the bubbles 29 that are released from the bottom wall 13 of the chamber 11 move away from the bottom wall 13 and into cooler fluid 12, which causes the bubbles 29 to condense. This embodiment 50 of a VIBE apparatus has the advantage of being self-contained and smaller. This embodiment 50 can be used as a portable device to be attached wherever heat removal and/or cooling is needed. However, the heat removal capacity and rate may not be as efficient as that of the first embodiment 10.
An alternative embodiment of a VIBE apparatus 60 is depicted in FIG. 6. This embodiment 60 is very similar to the previous embodiment 50. However, small synthetic jet actuators 61, 62 have been placed within the chamber 11. Synthetic jet actuators, generally, are described in detail in U.S. Pat. No. 5,758,853 to Glezer et al., entitled “Synthetic Jet Actuators and Applications Thereof,” which is incorporated herein by reference. Basically, the synthetic jet actuators 61, 62 create jets 63, 64 of fluid without net mass injection into the chamber 11. The fluidic jets 63, 64 agitate the fluid 12 in the chamber 11 resulting in more effective heat transfer.
Other alternative embodiments of the VIBE device involve modifications of the actuator 22. One of these alternative embodiments involves using more than one actuator 22 in the chamber 1. An array of actuators is positioned along the upper wall 14 of the chamber 11. In another alternative embodiment, the actuator 22 comprises a mount and a piston system in order to create the pressure waves 31.
It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.

Claims

1. A device for cooling a heated object, comprising:

a chamber containing a fluid;

a heated wall comprising a portion of said chamber; and

an actuator, said actuator emitting pressure vibrations for dislodging vapor bubbles from a surface of said heated wall.

2. The device of claim 1, wherein said fluid comprises water.

3. The device of claim 2, wherein said fluid further comprises methanol in addition to water.

4. The device of claim 1, wherein said actuator comprises an ultrasonic actuator and said pressure vibrations comprise ultrasonic pressure waves.

5. The device of claim 1, wherein said actuator comprises:

a diaphragm;

a piezoelectric element attached to said diaphragm; and

a circuit for driving said piezoelectric element, said driving circuit comprising a sinusoidal function generator and an amplification chip.

6. The device of claim 5, wherein said diaphragm comprises a ceramic disk.

7. The device of claim 1, wherein said chamber further comprises:

an inflow pipe at a first wall of said chamber, said inflow pipe in fluid communication with an interior of said chamber;

an outflow pipe at a second wall of said chamber, said outflow pipe in fluid communication with an interior of said chamber; and

wherein said fluid in said chamber moves out from said chamber through said outflow pipe and into said chamber through said inflow pipe.

8. The device of claim 7, further comprising:

a reservoir in fluid communication with said chamber via said inflow pipe and said outflow pipe, said reservoir containing said fluid.

9. The device of claim 1, wherein said heated wall comprises a heat sink material.

10. The device of claim 9, further comprising a heat-producing object adjacent to said heat sink material.

11. The device of claim 10, wherein said heat-producing object comprises a microelectronic circuit.

12. The device of claim 1, wherein said heated wall comprises a portion of a microelectronic circuit.

13. The device of claim 1, further comprising a synthetic jet actuator in said chamber.

14. A method for cooling, comprising the steps of:

providing a chamber with a fluid;

generating heat in a wall of said chamber;

causing the formation of vapor bubbles at said wall of said chamber;

emitting pressure vibrations into said fluid; and

said vapor bubbles dislodging from said wall of said chamber due to the pressure vibrations.

15. The method of claim 14, further comprising the step of circulating said fluid through said chamber.

16. The method of claim 15, wherein said emitting step comprises the steps of:

providing an actuator, said actuator having a diaphragm;

vibrating said diaphragm at a resonant frequency of said diaphragm; and

causing pressure waves to move through said fluid from said diaphragm, said pressure waves impinging upon said wall.

17. The method of claim 14, further comprising the steps of:

providing a fluid reservoir, said fluid reservoir in fluidic communication with said chamber;

moving said fluid from said chamber to said reservoir and back to said chamber; and

cooling said fluid in said reservoir.

18. The method of claim 17, wherein said moving step comprises:

providing a fluidic pump; and

pumping said fluid from said fluid reservoir to said chamber, and from said chamber back to said fluid reservoir.

19. A cooling device, comprising:

a chamber containing a fluid

a heat source supplying heat into said fluid of said chamber; and

an actuator for emitting pressure vibrations into said fluid.

20. The device of claim 19, wherein said heat source comprises a wall of said chamber.

21. The method of claim 20, wherein said actuator comprises a diaphragm vibrating at a resonant frequency of said diaphragm.