Patent Application
20070023169
The present disclosure relates generally to synthetic jet ejectors, and more specifically to the use of synthetic jet ejectors to augment the flow of liquid in a pumped liquid loop cooling system.
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. One such system is depicted in
The system shown in
While the system disclosed in Glezer et al. represents 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 heat transfer efficiencies, and which can handle even greater heat flux loads. There is also a need in the art for such a system that is scalable and compact, and that does not contribute significantly to the overall size of the device. These and other needs are met by the devices and methodologies described herein.
In one aspect, a thermal management system is provided which comprises (a) a liquid medium, (b) a heat generating device disposed in said medium, (c) a heat exchanger in thermal contact with said heat generating element, said heat exchanger comprising a thermally conductive material and having a channel defined on a surface thereof, and (d) an actuator adapted to direct a jet of the liquid medium along said channel.
In 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 to be cooled, the heat generating device being in thermal contact with a heat exchanger which is immersed in a liquid medium and which has a channel defined in a surface thereof. A synthetic jet ejector is also provided which is positioned to direct a jet of the liquid into said channel, and the synthetic jet ejector is activated.
These and other aspects of the present disclosure are described in greater detail below.
It has now been found that the aforementioned needs can be addressed through the provision of a pumped liquid loop cooling system which utilizes one or more synthetic jet ejectors, in combination with vibration induced boiling enhancement (VIBE), to cool semiconductor die and other heat generating devices by augmenting the flow of liquid coolant through the system. In such a system, the heat generating device may be thermally coupled with a heat exchanger which comprises a plurality of channels, and each of the synthetic jet ejectors may be positioned to direct a jet of the liquid coolant along one of the channels. When energized, each of the synthetic jet ejectors provides one or more high momentum synthetic jets directed in the same direction as the pumped coolant flow, and along the longitudinal axis of one of the channels.
The use of focused jets in liquid loop cooling systems is found to have several advantages. First of all, while the pumps utilized in these systems can provide a suitable global flow of the liquid coolant through the system, the flow rate of the liquid coolant within the channels of the heat exchanger is 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 pumped liquid loop cooling systems. The use of focused jets to direct a stream of liquid into the channels overcomes this problem by reducing this pressure drop, and hence facilitates increased entrainment of the flow of the liquid coolant into the channels.
The use of focused jets in the thermal management systems described herein also significantly improves the efficiency of the heat transfer process. Under conditions in which the liquid coolant 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 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, as explained in greater detail below, the synthetic jets may also be utilized to create beneficial nucleation sites to enhance the boiling process.
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 the main pump (or pumps) used to circulate the liquid coolant.
The principles described herein can be further understood with reference to
Referring again to
While the cooling of a semiconductor die has been specifically illustrated herein, one skilled in the art will appreciate that the devices and methodologies described herein may be applied to the thermal management of a wide variety of heat generating devices. These include, without limitation, printed circuit boards and the components thereof, memory devices, processors, and the like.
It will be appreciated that the shape of the synthetic jet ejector 301, as well as its overall dimensions and the relative size of its components, can vary considerably. For example, any of the various synthetic jet ejector designs disclosed in U.S. Pat. No. 6,588,497 (Glezer et al.), which is incorporated herein by reference, may be incorporated into the thermal management systems described herein.
Moreover, the actuators in these devices may be adapted to operate at ultrasonic or non-ultrasonic frequencies. In some applications, the use of actuators operating at non-ultrasonic frequencies may be preferred, due to the additional nucleation sites, in the form of vapor bubbles, which may be generated at such frequencies. The formation of these vapor bubbles is induced by local accelerations of the liquid coolant in the vicinity of the transducer. These accelerations result in extremely high local velocities in the coolant, and a corresponding reduction in pressure. When the reduction in pressure is sufficiently high, the coolant undergoes localized phase changes at ambient temperatures, thus resulting in cavitation of the coolant. As the transducer oscillates, the cavitation bubbles alternately form and collapse, thereby entraining the surrounding fluid and generating a synthetic jet. As depicted in
The details of the heat exchanger 409 may be appreciated with reference to
Referring again to
In use, cooled liquid coolant flows into the chamber 419 by way of inlet 421. After entering the chamber 419, the coolant flows through the microchannels 417 (see
As seen in
The use of a stacked heat exchanger 409 of the type shown in
Various modifications are possible to the liquid loop cooling system depicted in
The systems and methodologies described herein, and the synthetic jet ejectors utilized in these systems and methodologies, can be implemented in various sizes and dimensions. Thus, for example, at the millimeter scale, synthetic jet ejectors can be integrated into liquid loops using commercially available piezoelectric transducers. At the micron scale, synthetic jet ejectors can be incorporated into the system utilizing conventional semiconductor fabrication techniques. At the nanometer scale, synthetic jet ejectors can be created using nano-scale lithography.
The synthetic jet ejectors utilized in the systems and methodologies described herein may operate on a continuous basis, or on a non-continuous basis. For example, the synthetic jet ejectors may be utilized on an on-demand basis, where they are activated when a temperature sensing probe disposed on a die or other heat-generating device reaches a prescribed temperature limit. The use of the synthetic jet ejectors on an on-demand basis may be advantageous in some applications from the standpoint of improving the reliability of the synthetic jet ejector, while maintaining the heat generating device within prescribed temperature limits.
The synthetic jet ejectors may also be configured to be driven at various frequencies, and the frequency at which a particular synthetic jet ejector is driven in a device of the type described herein may differ from the frequencies at which other synthetic jet ejectors in the device are driven. However, ultrasonic driving frequencies are preferred in many applications, since they reduce acoustic emissions in the audible region of the spectrum. Since actuator frequencies increase with decreasing size, ultrasonic operation becomes easier to implement as device sizes decrease. Hence, the systems and methodologies described herein are favored by Moore's law.
Various liquids may be utilized as the liquid coolant or medium in the devices and methodologies described herein. These include, without limitation, 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 systems and methodologies described herein may be used advantageously in a wide variety of applications where thermal management or boiling enhancement is desired. Such applications include, but are not limited to, single phase cooling enhancement applications (such as pool boiling applications), multiphase forced flow boiling applications, heat pipe applications, and thermosyphon applications.
One skilled in the art will also appreciate that the systems and methodologies described herein may be readily adapted for use in refrigeration applications. In such applications, the synthetic jet actuators described herein may be used, for example, to augment the flow of a refrigerant through the coils or surfaces of a heat exchanger. The use of synthetic jet actuators in these applications is especially suitable for use in miniaturized refrigeration systems, due to their ability to compensate for the pressure drop of refrigerant as it flows through the channels of a heat exchanger.
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.
This application claims the benefit of priority from U.S. Provisional Application No. 60/704,049, filed Jul. 29, 2005, having the same title, and having the same inventors, and which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60704049 | Jul 2005 | US |
