Acoustic resonator for synthetic jet generation for thermal management

A thermal management system is provided herein which comprises a synthetic jet ejector (201) driven by an acoustic resonator (209).

FIELD OF THE DISCLOSURE

The present disclosure relates generally to synthetic jet ejectors, and more specifically to the use, in thermal management applications, of acoustical resonators in conjunction with synthetic jet ejectors.

BACKGROUND OF THE DISCLOSURE

As the size of semiconductor devices has continued to shrink and circuit densities have increased accordingly, thermal management of these devices has become more challenging. This problem is expected to worsen in the foreseeable future. Thus, within the next decade, spatially averaged heat fluxes in microprocessor devices are projected to increase by a factor of two, to well over 100 W/cm2, with core regions of these devices experiencing local heat fluxes that are several times higher.

In the past, thermal management in semiconductor devices was often addressed through the use of forced convective air cooling, either alone or in conjunction with various heat sink devices, and was accomplished through the use of fans. However, fan-based cooling systems were found to be undesirable due to the electromagnetic interference and noise attendant to their use. Moreover, the use of fans also requires relatively large moving parts, and corresponding high power inputs, in order to achieve the desired level of heat transfer.

More recently, thermal management systems have been developed which utilize synthetic jet ejectors. These systems are more energy efficient than comparable fan-based systems, and also offer reduced levels of noise and electromagnetic interference. Systems of this type, an example of which is depicted inFIG. 1, are described in greater detail in U.S. Pat. No. 6,588,497 (Glezer et al.).

The system depicted inFIG. 1utilizes an air-cooled heat transfer module101which is based on a ducted heat ejector (DHE) concept. The module utilizes a thermally conductive, high aspect ratio duct103that is thermally coupled to one or more IC packages105. Heat is removed from the IC packages105by thermal conduction into the duct shell107, where it is subsequently transferred to the air moving through the duct. The air flow within the duct103is induced through internal forced convection by a pair of low form factor synthetic jet ejectors109which are integrated into the duct shell107. In addition to inducing air flow, the turbulent jet produced by the synthetic jet ejector109enables highly efficient convective heat transfer and heat transport at low volume flow rates through small scale motions near the heated surfaces, while also inducing vigorous mixing of the core flow within the duct.

While the systems disclosed in Glezer et al. represent a very notable improvement in the art of thermal management systems, in light of the aforementioned challenges in the art, a need exists for thermal management systems with even greater energy efficiencies. There is also a need in the art for thermal management systems that are scalable and compact, and that do not contribute significantly to the overall size of the device. These and other needs are met by the devices and methodologies described herein.

SUMMARY OF THE DISCLOSURE

In one aspect, a thermal management system is provided herein which comprises a synthetic jet ejector which is used in combination with an acoustic resonator.

In another aspect, a synthetic jet ejector is provided in combination with an acoustic resonator which is adapted to drive the synthetic jet ejector. The combination comprises (a) a cavity, (b) a partition which divides the cavity into first and second compartments, (c) a diaphragm which extends into the first and second compartments, (d) a transducer which is adapted to vibrate the diaphragm at the resonant frequency of the cavity, and (e) first and second pipes which are in open communication with the first and second compartments, respectively.

In yet another aspect, a method for dissipating heat from a heat generating device is provided. In accordance with the method, a heat generating device is provided which is disposed in a fluid medium. An acoustic resonator is also provided which is adapted to generate a turbulent jet in the fluid medium, and which is positioned such that the turbulent jet will impinge upon the heat generating device. The acoustic resonator is then excited by a suitable transducer.

These and other aspects of the present disclosure are described in greater detail below.

DETAILED DESCRIPTION

It has now been found that the aforementioned needs can be addressed through the use, in a thermal management system, of an acoustic resonator in conjunction with one or more synthetic jet ejectors. Thermal management systems which utilize this combination exhibit significantly enhanced rates of thermal transfer at substantially lower levels of power consumption. Without wishing to be bound by theory, it is believed that the acoustic resonator acts in these systems as an efficient transformer which enables the synthetic jet ejector to operate at higher pressures and with lower movements of ambient fluid mass into and out of the synthetic jet ejector. Consequently, the synthetic jet ejector provides superior heat dissipation and better energy efficiencies. These systems are also scalable and compact, and do not contribute significantly to the overall size of a device which incorporates them. As an additional benefit, a variety of heat sinks can be formed in the thermal management systems described herein by incorporating heat exchangers, or elements thereof, into the acoustic resonator.

FIG. 2illustrates a first particular, non-limiting embodiment of a synthetic jet ejector made in accordance with the teachings herein. The synthetic jet ejector201depicted therein comprises a housing203which encloses a cavity205. The cavity205, which is in open communication with the ambient environment by way of an orifice207, is equipped with an actuator209. The actuator comprises a diaphragm which is vibrated by a transducer or by other suitable means. In the particular embodiment depicted, the cavity205is divided into a plurality of channels211through a series of partitions213such that an open, though convoluted, pathway is formed between the actuator209and the orifice207.

The diaphragm associated with the actuator209is adapted to vibrate at the resonance frequency of the cavity205. The resulting oscillations cause a portion of the mass of fluid disposed within the cavity205(or adjacent to the orifice207) to be alternately expelled from, and withdrawn into, the cavity205via the orifice207. These oscillations produce adiabatic rarefactions and compressions of the ambient fluid mass within the cavity205, which generate an alternating pressure wave at the orifice207as indicated by the arrow. If the orifice207and the pathway within the cavity205have appropriate dimensions, the fluidic motion created by the pressure wave will induce the formation of a turbulent jet in the ambient fluid. This jet may be effectively utilized as a thermal management element by directing it at a heat source, where it serves to dissipate, in a highly efficient manner, any unwanted thermal energy generated by the heat source.

The synthetic jet ejector201depicted inFIG. 2has a number of advantages over other synthetic jet ejectors as a result of the actuator209which drives it. Significantly, and in contrast with conventional synthetic jet ejectors, the synthetic jet ejector201ofFIG. 2displaces only a small portion of the fluid resident within the cavity205. In particular, when the vibrations of the diaphragm associated with the actuator are properly tuned to the resonance frequency of the cavity205so that the cavity205functions as an acoustic resonator, an acoustical pressure wave is generated in the ambient fluid that induces fluid motion at the orifice207in the form of a turbulent synthetic jet. Since the synthetic jet ejector201ofFIG. 2requires relatively small levels of fluid displacement from the actuator in comparison to conventional synthetic jet actuators, its input power requirements are correspondingly smaller. Partially as a result of this, synthetic jet ejectors of this type offer increased reliability and lifetimes. At the same time, synthetic jet ejectors of the type depicted inFIG. 2offer many of the same benefits as conventional synthetic jet ejectors, including a 10-fold increase in flow rate in the ambient fluid (when the ambient fluid is air) and a 2.5 fold increase in heat transfer.

Another unique attribute of the synthetic jet ejector201depicted inFIG. 2is that the pressure wave is only generated (and hence the synthetic jet is only produced) when the resonance of the transducer is tuned to the resonance of the cavity205. This feature may be used advantageously as a control mechanism for the synthetic jet ejector201.

The principles by which the synthetic jet ejectors (and in particular, their component acoustical resonators) described herein operate, and the advantages of these devices over conventional synthetic jet ejectors and resonators, may be further understood with respect toFIGS. 3-14.

FIG. 3depicts a Helmholtz resonator301which may be used in the thermal management devices described herein. The Helmholtz resonator301is driven by an actuator303. The actuator (an example of which is shown inFIG. 18) comprises a diaphragm which is caused to vibrate at a desired frequency by an electromagnetic coil. The actuator303is disposed at one end of a cavity305that terminates in a pipe307. An optional enclosure309may be provided at the rear of the actuator303, as indicated by the dashed lines. The Helmholtz resonator301transforms smaller volume velocities (movements) and higher pressures at the actuator303(and more specifically, at the diaphragm of the actuator303) to higher velocities and lower pressures at the external opening of the pipe307. The velocity at the opening of the pipe307will be more or less the same as the velocity throughout the length of the pipe307. Notably, there is very little movement of the ambient fluid within the volume of the cavity305.

A graph of the characteristic pressure (or velocity) response of the Helmholtz resonator301ofFIG. 3is illustrated inFIG. 4. As shown therein, the response is symmetrical and is centered about the characteristic frequency f0of the resonator301.

FIG. 5is an illustration of a dual Helmholtz resonator401which may be used in the thermal management devices described herein. The Helmholtz resonator401is driven by an actuator403. The actuator403is disposed within a cavity405that is partitioned into first407and second409compartments. The first compartment407is equipped with a first pipe411terminating in a first orifice419, and the second compartment409is equipped with a second pipe413terminating in a second orifice421. The combination of the actuator403, the first compartment407, and the first pipe411define a first resonator415, while the combination of the actuator403, the second compartment409, and the second pipe413define a second resonator417.

The characteristic pressure (or velocity) response of the Helmholtz resonator401ofFIG. 5is illustrated inFIG. 6. As seen therein, the response451of the first Helmholtz resonator415is symmetrically centered about its characteristic frequency f1, while the response453of the second Helmholtz resonator417is symmetrically centered about its characteristic frequency f2. The aggregate response455of the dual Helmholtz resonator401is thus the sum of the individual responses of the first415and second417resonators. Typically, the ratio f2/f1will be in the range of about 4:3 to about 5:2, and more typically will be approximately 2:1, to achieve a more or less uniform output over a frequency span of approximately 1.5 octaves. At these ratios, the relative phase of the two outputs from each side of the diaphragm causes them to interfere in a constructive manner, thus increasing the output of the resonator.

FIG. 7illustrates a single-sided tuned pipe resonator501which may be used in the thermal management devices described herein. The resonator501is driven by an actuator503which is disposed at one end of a pipe505. The actuator may optionally be enclosed by a housing507. As explained below, the distance L1from the actuator503(and more specifically, from the diaphragm thereof) to the end of the pipe505has a significant impact on the resonance frequency of the resonator501.

The characteristic pressure (or velocity) response of the resonator501ofFIG. 7is illustrated inFIG. 8. As shown therein, the resonator501has a number of harmonic resonance frequencies f2, f3, . . . , f3, in addition to its primary resonance frequency f1. The primary resonance frequency f1and the harmonic resonance frequencies f2, f3, . . . , fnare determined by length L1(seeFIG. 7). In particular, the relationship between the kthresonance frequency fkand the length L1is given by EQUATION 1 below:

fk=(2⁢k-1)⁢c4⁢L1(EQUATION⁢⁢1)
where c is the speed of sound in the ambient fluid.

FIG. 9depicts a dual tuned pipe resonator601which may be used in the thermal management devices described herein. The resonator601is driven by an actuator603which is disposed at the joined ends of first605and second607pipes. The distance between the actuator (and more specifically, the diaphragm thereof)603and the end of the first pipe605is L1, while the distance between the actuator (and more specifically, the diaphragm thereof)603and the end of the second pipe607is L2.

The characteristic pressure (or velocity) response of the resonator601ofFIG. 9is illustrated inFIG. 10. As shown therein, the characteristic response651of the resonator601is a combination of the responses653of the first605and second607pipes, including their respective primary and harmonic resonances. Typically, the ratio L2/L1of the length L1of the first pipe605to the length L2of the second pipe607will be approximately 3:1 to achieve a more or less uniform (although combined) output653over a frequency span of 3 octaves or more. Resonators of the type depicted inFIG. 9are not typically used in audio applications, due to the poor transient response (time domain behavior) inherent in their design.

FIG. 11illustrates a first particular, non-limiting embodiment of a preferred Helmholtz resonator701useful in thermal management systems and devices of the type described herein. The resonator701is driven by an actuator703which is disposed within a cavity705that is partitioned into first707and second709compartments. The first compartment707is equipped with a first pipe711that terminates in a first orifice719, and the second compartment709is equipped with a second pipe713that terminates in a second orifice721. The combination of the actuator703, the first compartment707, and the first pipe711define a first resonator715, while the combination of the actuator703, the second compartment709, and the second pipe713define a second resonator717.

In contrast to the Helmholtz resonator401depicted inFIG. 5, in the Helmholtz resonator701ofFIG. 11, the tuning is identical on each side of the diaphragm703(that is, the tuning of the first715and second717resonators is the same). This may be accomplished, in part, by ensuring that the volume of the first707and second709compartments is the same. When the first715and second717resonators are tuned in this manner, their output will be essentially identical but will be 180° out of phase, and hence the outputs of the first715and second717resonators will effectively cancel each other out. Preferably, the orifices719and721in pipes711and713will be small relative to the wavelengths of the primary resonances of the first707and second709compartments, respectively. It is also preferred that the spacing between the orifices719and721should be as close together as possible. Preferably, the primary resonances of the first and second compartments occur at the same wavelength λ, and both the orifice diameters and the distance between the orifices are on the order of about ⅕λ or less.

FIG. 12depicts the characteristic response of the Helmholtz resonator701ofFIG. 11. The outputs751of the individual resonators715,717are essentially the same, but are out of phase by 180°. Consequently, the combined output (summed over all space)753of the Helmholtz resonator is very low (a small fraction of the output of either side), and follows the characteristics of a dipole radiator whose dimensions are small relative to the wavelength being emitted.

FIG. 13illustrates a second particular, non-limiting embodiment of a preferred pipe resonator801that is useful in the thermal management devices and methodologies disclosed herein. The particular embodiment depicted has a dual pipe configuration in which the resonator801is driven by an actuator803that is disposed within a cavity805, and wherein the cavity805is partitioned into first807and second809compartments. The first compartment807is equipped with a first pipe811that terminates in a first orifice815, and the second compartment809is equipped with a second pipe813that terminates in a second orifice817. The combination of the actuator803, the first compartment807(including the first pipe811) and the first orifice815defines a first resonator821, while the combination of the actuator803, the second compartment809(including the second pipe813), and the second orifice817defines a second resonator823.

In contrast to the dual pipe resonator depicted inFIG. 9, in the pipe resonator801ofFIG. 13, the tuning is identical on each side of the actuator803(that is, the tuning of the first821and second823resonators is the same). This may be accomplished, in part, by ensuring that the distance L1between the actuator803and the first orifice815is equal to the distance L2between the actuator803and the second orifice817. When the first821and second823resonators are tuned in this manner, their output will be essentially identical but will be 180° out of phase, and hence will effectively cancel each other out. Preferably, the orifices815and817in pipes811and813will be relatively small compared to the wavelengths of the primary resonances of first807and second809compartments, respectively. It is also preferred that the spacing between the first orifice815and the second orifice817should be as close together as possible. As before, it is preferred that L1and L2are about ⅕λ or less, where λ is the wavelength corresponding to the resonance frequency of pipes811and813.

FIG. 14depicts the characteristic response of the dual pipe resonator801ofFIG. 13for the primary resonance and two harmonics thereof. The output851of each of the first815and second817resonators is essentially the same, but is out of phase by 180°. Consequently, the combined output853(summed over all space) of the resonator is very low (a small fraction of the output of either side). The design of the dual pipe resonator801ofFIG. 13offers low acoustic emissions by default, as the response of the device is inherently a low pass filter. This filter reduces the higher frequency sounds emitted by the actuator803, and thus improves the sound quality of the thermal management system.

FIGS. 15-17depict two particular, non-limiting embodiments of highly efficient heat sinks made in accordance with the teachings herein which may be used for the thermal management of heat generating devices. These heat sinks feature acoustically tuned resonators of the type described herein which are coupled with heat exchangers. The heat generating devices that may be thermally managed by these heat sinks include, without limitation, die and other semiconductor devices, printed circuit boards (PCBs), processors, memory chips, graphics chips, batteries, radio-frequency components, and other devices in laptops, PDAs, mobile phones, telecom switches, and other electronic equipment.

FIGS. 15 and 16depict a first particular, non-limiting embodiment of such a heat sink. The heat sink901depicted therein comprises a Helmholtz resonator903which includes a cavity905and a pipe907. The Helmholtz resonator903is driven by an actuator909which vibrates a diaphragm. Although the Helmholtz resonator903is depicted inFIGS. 15-16as a single pipe unit, it will be appreciated that, with appropriate modifications, similar heat sinks could be fabricated using any of the acoustic resonators described herein, including dual or multi-pipe resonators.

The pipe907has a heat exchanger911incorporated therein. The heat exchanger911comprises a base913(seeFIG. 16) having a series of channels915defined thereon (seeFIG. 15), each channel915being bounded by a pair of fins917. The heat exchanger911preferably comprises a highly thermally conductive material, such as a metal, which is in thermal contact with a heat generating device919(seeFIG. 16) that is to be thermally managed.

In operation, the resonator903generates pressure waves which induce the formation of focused turbulent jets (indicated by arrows in the figures) along the longitudinal axes of the channels915of the heat exchanger911. These focused jets effectively dissipate the heat that is transferred to the heat exchanger911from the heat generating device919.

FIG. 17illustrates yet another particular, non-limiting embodiment of a heat sink made in accordance with the teachings herein. This heat sink951again comprises a Helmholtz resonator953, which includes a cavity955with an actuator959disposed on one end thereof. A pipe957is attached to the opposing end of the cavity955. The pipe957has disposed within it a heat exchanger961comprising a series of fins967that are mounted on a base plate963. The base plate963is in thermal contact with a heat generating device969which is to be thermally managed.

The operation of the heat sink951ofFIG. 17is similar to the operation of the heat sink901depicted inFIGS. 15-16. However, in the embodiment depicted inFIG. 17, the cavity955is mounted on top of the pipe957, thereby minimizing the horizontal dimensions of the heat sink951. Such a configuration is especially useful in applications where sufficient vertical room is available, but where lateral real estate is limited.

FIG. 18illustrates on specific, non-limiting embodiment of an actuator1001that may be used in the acoustic resonators described herein. This particular actuator1001is a speaker which includes a diaphragm1003mounted on a basket1005by a resilient suspension1007(also called a surround). The basket1005is in turn supported on a pot1009which houses a permanent magnet1011. A top plate1013, which is typically made of steel or a suitable metal, is mounted over the permanent magnet1011. An annular voice coil1015is suspended from the back of the diaphragm1003and within an annular groove1017formed between the pot1009and the combination of the permanent magnet1011and top plate1013. The voice coil1015is preferably formed from a coil of copper wire which is wound around a spool. The speaker also includes a tinsel lead1019which is connected on one end to the diaphragm1003, and which is connected on the opposing end to a terminal strip1020, the later of which includes a fastener1021and a terminal board1023.

In operation, when the electrical current or signal flowing through the voice coil1015changes direction, the polar orientation of the electromagnetic field created by the voice coil1015reverses, thus altering (by 180° along the longitudinal axis of the voice coil1015) the direction of magnetic repulsion and attraction between the permanent magnet1011and the electromagnet of the voice coil1015. This has the effect of moving the voice coil1015and the attached diaphragm1003back and forth along the longitudinal axis of the voice coil1015, thus inducing physical vibrations in the diaphragm1003. As is well understood to those skilled in the art, the speaker thus serves to translate the electrical signals input into the voice coil1015into physical vibrations in the diaphragm1003, thus generating acoustical waves in the surrounding medium. As has been previously noted, when the actuator1001is used to generate acoustical waves of the proper wavelength or frequency, it generates an acoustical pressure wave in the ambient medium that induces fluid motion at the orifice of the acoustical resonator in the form of a turbulent synthetic jet.

The use of focused jets in the heat sinks and associated thermal management systems described herein is found to have several advantages. First of all, while pumps and fans can be utilized in such systems to provide a suitable global flow of coolant fluid (e.g., air, water, or the like) through the system, the flow rate of the fluid within the channels of a heat exchanger of the type depicted inFIGS. 15-16is typically much slower, due to the pressure drop created by the channel walls. This problem worsens as the system becomes smaller. Indeed, such a pressure drop is one of the biggest obstacles to the miniaturization of such systems. The use of focused jets to direct a stream of fluid into the channels overcomes this problem by reducing this pressure drop, and hence facilitates increased entrainment of the flow of fluid through the channels.

The use of focused jets in the heat sinks and associated thermal management systems described herein also significantly improves the efficiency of the heat transfer process in these systems. Under conditions in which the coolant fluid is a liquid and is in a non-boiling state, the flow augmentation provided by the use of synthetic jet ejectors increases the rate of local heat transfer in the channel structure, thus resulting in higher heat removal. Under conditions in which the coolant fluid is a liquid and is in a boiling state, these jets induce the rapid ejection of vapor bubbles formed during the boiling process. This dissipates the insulating vapor layer that would otherwise form, and hence delays the onset of critical heat flux. In some applications, the synthetic jets may also be utilized to create beneficial nucleation sites to enhance the boiling process. The foregoing considerations make the devices and methodologies disclosed herein particularly suitable for pool boiling applications.

The systems and methodologies described herein further increase the efficiency of the heat transfer process by permitting this process to be augmented locally in accordance with localized thermal loads. For example, the current trend in the semiconductor industry is toward semiconductor devices that generate heat in an increasingly non-uniform manner. This results in the creation of hotspots in these devices which, in many cases, is the first point of thermal failure of the device. Through the provision of directed, localized synthetic jets, these hot spots can be effectively eliminated, thereby reducing the global power requirements of the thermal management system. The reduction in power requirement attendant to the flow augmentation provided by the synthetic jet ejectors also reduces the noise of the system, and improves the reliability of any pumps used to circulate the coolant fluid.

A number of variations are possible in the devices described above. For example, while single pipe and dual pipe acoustical resonators have been specifically described, one skilled in the art will appreciate that devices comprising more than two acoustical resonators can also be created in accordance with the teachings herein. Where noise suppression is a concern, it is preferred that the orifices in these devices are small and are spaced close together, and that the comparative geometries of the individual resonators are such that effective noise suppression can occur through destructive interference.

The synthetic jet ejectors described herein can be implemented at several volume scales and frequencies. The volume of the cavity and the area of the orifice will typically be significant parameters for tuning the actuator and cavity resonances. Typically, other things being equal, as the volume of the cavity decreases, the transducer frequency must increase in order to produce a resonance pressure wave. However, in some embodiments, it may be possible to significantly modify the acoustic performance characteristics of the synthetic jet ejector without changing the cavity dimensions. This may be achieved, for example, by lining the cavity with a fibrous material, in which case both the density and thickness of the fibrous material can affect the acoustic performance characteristics of the synthetic jet ejector. In some applications, such an approach may be utilized to permit reductions in cavity size without an associated increase in resonance frequency.

In many thermal management applications, although the volume of the cavity of the acoustic resonator is significant, the specific dimensions of the cavity are not critical, so long as the appropriate volume is realized. Consequently, the cavity can be implemented in a wide variety of shapes, and may have a plurality of passages. The flexibility in housing design afforded by this feature is a significant advantage over other thermal management devices, such as fan-based units.

In some embodiments of the devices and methodologies described herein, the synthetic jet ejector can be utilized in an on-demand mode. Thus, for example, the synthetic jet ejector may be adapted to be triggered when the device temperature reaches a pre-set limit. Operating the synthetic jet ejector in such a mode can be advantageous, in some instances, in improving the reliability of the thermal management device, while maintaining the prescribed temperature limits on the device being managed.

One skilled in the art will appreciate that the devices and methodologies described herein may be employed in applications wherein the ambient fluid medium is either a gas or a liquid. As a specific, non-limiting example of the former, these systems may be applied where ambient air is utilized as the fluid medium. Of course, it will be appreciated that other gasses could also be advantageously employed, especially if the thermal management system in question is a closed loop system. Specific, non-limiting examples of liquids that could be employed as the fluid medium include, but are not limited to, water and various organic liquids, such as, for example, polyethylene glycol, polypropylene glycol, and other polyols, partially fluorinated or perfluorinated ethers, and various dielectric materials. Liquid metals may also be advantageously used in the devices and methodologies described herein. Such materials are generally metal alloys with an amorphous atomic structure.

The above description of the present invention is illustrative, and is not intended to be limiting. It will thus be appreciated that various additions, substitutions and modifications may be made to the above described embodiments without departing from the scope of the present invention. Accordingly, the scope of the present invention should be construed in reference to the appended claims.