US20050284612A1

US20050284612A1 - Piezo pumped heat pipe

Info

Publication number: US20050284612A1
Application number: US10/874,488
Authority: US
Inventors: Sridhar Machiroutu
Original assignee: Individual
Current assignee: Intel Corp
Priority date: 2004-06-22
Filing date: 2004-06-22
Publication date: 2005-12-29

Abstract

An arrangement is provided for cooling a heat-generating device (e.g., an integrated circuit chip) in a system such as a laptop computer. The arrangement includes a piezo pumped heat pipe having a piezoelectric device near an evaporator in the heat pipe. The piezoelectric device, when actuated, helps reduce evaporator resistance when the evaporator evaporates a liquid coolant in the heat pipe.

Description

BACKGROUND

1. Field
The present invention relates generally to liquid cooling systems and, more specifically, to heat pipes for dissipating heat generated by integrated circuits.
2. Description
As integrated circuits (e.g., central processing units (CPUs) in a computer system) become denser, components inside an integrated circuit chip are drawing more power and thus generate more heat. Various liquid cooling systems have been used to dissipate heat generated by integrated circuit chips, for example within personal computers, mobile computers, or similar electrical devices. A liquid cooling system circulates a liquid coolant (e.g., water) through a heat sink attached to an integrated circuit chip inside of a device such as a computer. As the liquid passes through the heat sink, heat is transferred from the hot integrated circuit chip to the cooler liquid. The hot liquid (or the vapor of the liquid) then moves out to a radiator at the back (or side) of the case of the device and transfers the heat to the ambient air outside of the case. The cooled liquid then travels back through the system to the integrated circuit chip to continue the process.
A heat pipe is a commonly used form of heat sink in a liquid cooling system to dissipate heat generated by integrated circuits, especially CPUs, inside a computer system. A heat pipe may include an evaporator section and a condenser section. Heat may be transferred from the evaporator section to the condenser section through vapor generated by an evaporator in the evaporator section by evaporating a liquid coolant. The vapor may condense back to liquid form at the condenser section through a heat exchanger coupled to the heat pipe. A heat pipe may also include a wick to act as a pump to bring the liquid coolant back from the condenser section to the evaporator section. The evaporator may again evaporate the liquid coolant, drawing to the evaporator section by the wick, when heated by the heat generated by an integrated circuit chip. The heat transfer rate from the integrated circuit chip into the liquid coolant in the evaporator section depends on evaporation resistance. The lower the evaporation resistance is, the higher the heat transfer rate is. Thus, it is desirable to reduce the evaporation resistance whenever possible.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become apparent from the following detailed description of the present invention in which:
FIG. 1 illustrates an exemplary block diagram of a computer system which may be utilized to implement embodiments of the present invention;
FIG. 2 illustrates an exemplary block diagram of a piezo pumped heat pipe along with a heat exchanger, according to one embodiment of the present invention;
FIG. 3 is a block diagram illustrating an example of a piezo pumped heat pipe, according to one embodiment of the present invention;
FIG. 4 is an internal top view of an example implementation of a piezo pumped heat pipe, according to one embodiment of the present invention; and
FIG. 5 is a side view of the piezo pumped heat pipe whose top view is shown in FIG. 4, according to one embodiment of the present invention.

DETAILED DESCRIPTION

Evaporation resistance in a heat pipe may be affected by many factors such as evaporator structure and flow velocities of liquid around the evaporator. High flow velocities of liquid around the evaporator can make the evaporation mechanism in the evaporator more like flow boiling mechanism than like thin film evaporation mechanism. Typically flow boiling mechanism in the evaporator results in lower evaporation resistance than does thin film evaporation mechanism. According to an embodiment of the present invention, a piezoelectric device may be used to induce flow boiling in the evaporator in a heat pipe. A piezoelectric material can convert between mechanical and electrical energy. An electric potential applied to a piezoelectric material causes a small change in the shape of the material. Likewise, physical pressure applied to a piezoelectric material creates an electrical potential difference between the surfaces of the material. The piezoelectric device may be embedded near the evaporator in the heat pipe. Upon actuation, the piezoelectric device may generate mechanical vibrations, which oscillate liquid in the evaporator section. The oscillating motions generated by the piezoelectric device may increase flow velocities of the liquid in the evaporator section to generate flow boiling characteristics, and thus reduce evaporation resistance.
Reference in the specification to “one embodiment” or “an embodiment” of the present invention means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase “in one embodiment” appearing in various places throughout the specification are not necessarily all referring to the same embodiment.
FIG. 1 illustrates an exemplary block diagram of a computer system which may be utilized to implement embodiments of the present invention. Although not shown, the computer system is envisioned to receive electrical power from a direct current (DC) source (e.g., a battery) and/or from an alternating current (AC) source (e.g., by connecting to an electrical outlet). The computer system comprises a central processing unit (CPU) or processor 110 coupled to a bus 115. For one embodiment, the processor 110 may be a processor in the Pentium® family of processors including, for example, Pentium® 4 processors, Intel's XScale® processor, Intel's Pentium® M processors, etc., available from Intel Corporation. Alternatively, other processors from other manufacturers may also be used.
The computer system as shown in FIG. 1 may also include a chipset 120 coupled to the bus 115. The chipset 120 may include a memory control hub (MCH) 130 and an input/output control hub (ICH) 140. The MCH 130 may include a memory controller 132 that is coupled to a main memory 150. The main memory 150 may store data and sequences of instructions that are executed by the processor 110 or any other device included in the system. For one embodiment, the main memory 150 may include one or more of dynamic random access memory (DRAM), read-only memory (RAM), FLASH memory, etc. The MCH 130 may also include a graphics interface 134 coupled to a graphics accelerator 160. The graphics interface 134 may be coupled to the graphics accelerator 160 via an accelerated graphics port (AGP) that operates according to an AGP Specification Revision 2.0 interface developed by the Intel Corporation. A display (not shown) may be coupled to the graphics interface 134.
The MCH 130 may be coupled to the ICH 140 via a hub interface. The ICH 140 provides an interface to input/output (I/O) devices within the computer system. The ICH 140 may be coupled to a Peripheral Component Interconnect (PCI) bus. The ICH 140 may include a PCI bridge 145 that provides an interface to a PCI bus 170. The PCI Bridge 145 may provide a data path between the CPU 110 and peripheral devices such as, for example, an audio device 180 and a disk drive 190. Although not shown, other devices may also be coupled to the PCI bus 170 and the ICH 140.
The CPU 110, the chipset 120, and other devices in the computer system as shown in FIG. 1 may use a piezo pumped heat pipe to dissipate heat generated by them.
FIG. 2 illustrates an exemplary block diagram of a piezo pumped heat pipe along with a heat exchanger, according to one embodiment of the present invention. The piezo pumped heat pipe 210 comprises a sealed container whose inner surfaces have a capillary material that forms the wick (not shown in FIG. 2). One end (212) of the piezo pumped heat pipe may be coupled to a heat-generating device 230 (e.g., a processor). The heat generated by the device 230 transfers to the working fluid inside the heat pipe by evaporating the working fluid. The section inside the heat pipe (near the heat-generating device) where the working fluid is evaporated is also called an evaporator section. The pressure difference inside the heat pipe may help transport the vapor of the working fluid from the evaporator section to the other end of the heat pipe, which may be coupled to a heat exchanger 240. The heat exchanger 240 transfers the heat from the vapor to ambient air so that the vapor may condense back to liquid. The section inside the heat pipe (near the heat exchanger) where the vapor condenses is also called a condenser section. After the vapor condenses, the liquid then moves to the evaporator section with the help of the wick. This process continues so long as the heat-generating device generates enough heat to evaporate the working fluid in the evaporator section.
The container is leak-proof so that it can isolate the inside working fluid from the outside environment. The container maintains the pressure differential across its walls, and enables transfer of heat to take place from and into the working fluid. Selection of the container material depends on many factors such as compatibility (both with working fluid and external environment), strength to weight ratio, thermal conductivity, ease of fabrication, and porosity. The material should be non-porous to prevent the diffusion of the vapor of the working fluid. A high thermal conductivity ensures minimum temperature drop between the heat source and the wick. Although it is shown as a rectangular “L” shape in FIG. 2, the container can be any other shape (e.g., a straight or “L” shape cylinder) and any size so long as the end 212 can be made fit with a heat-generating device and the other end can be made fit to a heat exchanger.
It is desirable that the working fluid can be evaporated by a heat-generating device. In one embodiment, the working fluid may be water, alcohol, glycol, an inert liquid, combinations thereof, surfactants, mixtures thereof, and the like. A high value of surface tension may be desirable in order to enable the heat pipe to operate against gravity and to generate a high capillary driving force. In addition to high surface tension, it is also desirable for the working fluid to wet the wick and the container material. A high latent heat of vaporization is desirable in order to transfer large amounts of heat with minimum fluid flow, and hence to maintain low pressure drops within the heat pipe. The thermal conductivity of the working fluid should preferably be high in order to minimize the radial temperature gradient and to reduce the possibility of nucleate boiling at the wick or wall surface. The resistance to fluid flow will be minimized by choosing fluids with low values of vapor and liquid viscosities.
The capillary structure or the wick over the inner surfaces (not shown in FIG. 2) of the container may be a porous structure made of materials like steel, aluminum, nickel or copper in various ranges of pore sizes, fabricated using metal foams. Fibrous materials, such as ceramics and carbon fiber filaments, may also be used. The main purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser section to the evaporator section. It should also be able to distribute the liquid around the evaporator section to any area where heat is likely to be received by the heat pipe. The selection of the wick for a heat pipe depends on many factors. The maximum capillary head generated by a wick increases with decrease in pore size. The wick permeability increases with increasing pore size. Another feature of the wick is its thickness. The heat transport capability of the heat pipe may be raised by increasing the wick thickness. The overall thermal resistance in the evaporator section also depends on the conductivity of the working fluid in the wick. Other necessary properties of the wick are compatibility with the working fluid and wettability. Types of commonly used wick comprises sintered powder, grooved tube, and screen mesh.
Although it is desirable that the working liquid can be evaporated by the heat from a heat-generating device, in one embodiment, there may be no evaporation process or only a partial evaporation process. The colder liquid may move from one end, which is coupled to a heat exchanger to the other end, which is coupled to a heat-generating device, and is heated there to become hotter liquid (or hotter liquid and vapor mixture), which then moves back to the colder end.
A piezoelectric device (not shown in FIG. 2) may be placed at the end 212 near the evaporator section. The piezoelectric device may be actuated by an oscillating voltage source (not shown in FIG. 2) via wires 220. When actuated, the piezoelectric device may generate oscillating or wavy motions in the liquid in the evaporator section to increase flow velocities and thus generate flow boiling characteristics. The flow boiling characteristics may reduce evaporation resistance and increase the efficiency of heat transfer from the heat-generating device to the liquid. The heat generated by the heat-generating device causes the liquid in the evaporator section to evaporate and enter a vapor state. The vapor, which has a higher specific volume, moves inside the sealed container to the other end of the heat pipe that is coupled to the heat exchanger 240. The heat exchanger 240 may include a fan 242 to provide higher air flow. Heated air 250 may be rejected by the fan 242 into the ambient air. When the vapor condenses to liquid at the heat exchanger end of the heat pipe, it transfers the heat to the heat exchanger walls. The heat exchanger walls may further transfer energy to ambient air with the help of the heat exchanger 240. The liquid then moves back to the evaporator section through the wick.
FIG. 3 is a block diagram illustrating an example of a piezo pumped heat pipe, according to one embodiment of the present invention. An attach block 310 may attach the heat pipe 340 to a heat-generating device 330. In one embodiment, the attach block may be a part of the heat pipe as shown in FIG. 2. In another embodiment, the attach block may be coupled to the heat pipe so that the attach block may efficiently transfer heat from the heat-generating device 330 to the evaporator section of the heat pipe 340. For example, when the container of the heat pipe is a cylinder and a heat-generating device has a flat surface, an attach block may be used to attach the heat pipe to the heat-generating device. Alternatively, the cylinder-shaped heat pipe may be made to have a flat end to serve as the attach block. The attach block 310 may comprise a piezoelectric device 315. The piezoelectric device is located inside the heat pipe 340 near the evaporator section (in the case that the attach block is separate from the heat pipe, the piezoelectric device 315 is in the heat pipe). The actuating device 320 applies an oscillating voltage to the piezoelectric device so that the piezoelectric device can generate wavy motions. In one embodiment, the actuating source may be located outside the heat pipe. In another embodiment, the actuating source may be located inside the heat pipe.
Inside the heat pipe 340 there may be a vapor area 342 and a wick 344. The vapor generated in the evaporator section may transport through the vapor area 342 to the colder end of the heat pipe because of pressure difference between the colder end and the hotter end where the evaporator section locates. Opposite to the end where the piezoelectric device is located, the other end of the heat pipe may be coupled to a heat exchanger 350. The heat exchanger may comprise a fan 352 and a plurality of fins 354. The fan 352 helps increase air circulation to generate higher air flow so that heat carried by the vapor inside the heat pipe may be dissipated faster. The plurality of fins 354 increase the contact area between the heat exchanger and the ambient air to improve efficiency of heat transfer from the vapor inside the heat pipe to the ambient air. When the vapor transfers heat inside it to the ambient air through the heat exchanger, the vapor condenses and returns to the liquid state. The liquid then moves back to the evaporator section (not shown in FIG. 3) through capillary actions of the wick 344.
FIG. 4 is an internal top view of an example implementation of a piezo pumped heat pipe, according to one embodiment of the present invention. The piezo pumped heat pipe may include outer walls 410, a wick 420, an evaporator 430, and a piezoelectric device 440. Although not explicitly illustrated in FIG. 4, the piezo pumped heat pipe may also include a liquid coolant. An evaporator section of the heat pipe of may be located near evaporator 430, and a condenser section of heat pipe may be spaced apart from the evaporator (e.g., including the far end of heat pipe). The piezo pumped heat pipe may be of any size. Outer walls 410 may enclose wick 420, evaporator 430, piezoelectric device 440, and the coolant. Outer walls 410 may be coupled to a heat-generating device (e.g., an integrated circuit chip), and they may include a highly thermally conductive material, such as copper or another material. Outer walls 410 may be formed in a roughly rectangular shape, as illustrated in FIG. 1, or any other geometry that facilitates access to evaporator 430 by the coolant and facilitates contact between outer walls and surfaces of the heat-generating device. Outer walls may also be formed to prevent the escape of vapor or liquid.
Wick 420 may include a porous material (e.g., sintered spherical copper particles, sintered metal powder, a fiber material, and/or a screen material), or a porous material with a grooved surface, which covers an inner surface of the piezo pumped heat pipe, except for the area occupied by evaporator 430. Wick 420 may, by virtue of its porous structure, bring coolant from the condenser section of heat pipe to the evaporator section at or near evaporator 430. In this manner, wick 420 may act to hydrate evaporator 430. In other implementations, wick 420 may include axial grooves that act to bring coolant from the condenser section of heat pipe to the evaporator section. Other types of homogenous structures for wick 420 may include an open annular structure, an open artery structure, and/or an integral artery structure. In still other implementations, various composite structures may be used for wick 420 that may include one or more of the homogeneous structures noted above (e.g., sintered particles, screen, fibers, grooves, etc.). Wick 420 may be designed to have a relatively high capillary pumping efficiency to hydrate evaporator 430.
Evaporator 430 may include a porous material (e.g., spherical metal particles of various sizes sintered onto the inner surface of outer wall 410) that roughly corresponds in area and orientation to a surface of the heat-generating device to be cooled. In one embodiment, the porous material used for evaporator 430 may be the same as the porous material used for wick 420. In another embodiment, the evaporator may use different porous material from that used in the wick. The porous material of evaporator 430 may include, for example, copper particles. In one embodiment, the evaporator may include a grooved surface. The grooved surface may be made of the same material as the container of the heat pipe.
The piezoelectric device 440 may be located near the evaporator. When actuated by an oscillating voltage source, the piezoelectric device generates wavy motions in the liquid in the evaporator section. The liquid is brought to the evaporator section from the condenser section of the heat pipe by capillary pumping of the wick. Without the piezoelectric device 440, the liquid flow in the evaporator section is driven by capillary actions and flow velocities of the liquid in the evaporator section may be smaller than those with wavy motions generated by piezoelectric device 440. As a result, evaporation resistance may be higher without piezoelectric device 440. Evaporation resistance depends on the evaporation/boiling process in the evaporator section of the heat pipe. Lower evaporation resistance may result in higher heat transfer efficiency for the heat pipe. Typically, a thin film evaporation process results in higher evaporation resistance than a flow boiling process for the same heat flux. Without the piezoelectric device, the boiling process in the evaporator section resembles thin film evaporation heat transfer. Wavy motions generated by piezoelectric device 440 may enhance pumping of liquid into the evaporator section. The wavy motions in the liquid in the evaporator section may result in high local velocities in the liquid. The high local velocities in turn make the boiling process similar to the flow boiling process. Therefore, piezoelectric device 440 may help reduce evaporation resistance and thus increase heat transfer efficiency.
FIG. 5 is a side view of the piezo pumped heat pipe whose top view is shown in FIG. 4. The heat pipe shown in FIGS. 4 and 5 has an approximately rectangular cross-sectional shape. In addition to outer walls 410, wick 420, evaporator 430, and piezoelectric device 440, which are discussed above with regard to FIG. 4, the piezo pumped heat pipe may also include a liquid coolant 510 and a vapor space 520. The liquid coolant 510 may include water, methanol, ethanol, acetone, heptane, Freon, or another refrigerant, or a mixture of two or more types of liquids. The liquid coolant may pool on the surface of the evaporator, as illustrated in FIG. 2, and may also permeate wick 420. The liquid coolant may be evaporated by boiling over the evaporator. In one embodiment, wick 420 may extend vertically above the evaporator to improve wetting of evaporator by the liquid coolant. In another embodiment, wick 420 may not extend vertically above the evaporator. In such an embodiment, however, it is desirable that the amount of coolant 510 be sufficient to ensure continuous wetting of the evaporator. In either embodiment, wavy motions generated by piezoelectric device 440 may improve wetting of the evaporator.
Vapor space 520 may be located between wick 420 and the top one of outer walls 410. When liquid coolant 510 is evaporated by boiling over the evaporator in the evaporator section, the vapor pressure in the evaporator section becomes higher than that in the condenser section. The pressure difference thus helps transport vapor to the condenser section of the peat pipe via vapor space 520 (and possibly also wick 420), where it cools, becomes liquid, and is transported back to the evaporator section by the wick.
Although an example embodiment of the present disclosure is described with reference to diagrams in FIGS. 1-5, persons of ordinary skill in the art will readily appreciate that many other methods of implementing the present invention may alternatively be used. For example, the order of execution of the functional blocks or process procedures may be changed, and/or some of the functional blocks or process procedures described may be changed, eliminated, or combined.
In the preceding description, various aspects of the present disclosure have been described. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the present disclosure. However, it is apparent to one skilled in the art having the benefit of this disclosure that the present disclosure may be practiced without the specific details. In other instances, well-known features, components, or modules were omitted, simplified, combined, or split in order not to obscure the present disclosure.
While this disclosure has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the disclosure, which are apparent to persons skilled in the art to which the disclosure pertains are deemed to lie within the spirit and scope of the disclosure.

Claims

1. A cooling apparatus, comprising:

a piezo pumped heat pipe to transfer heat from a first section to a second section; and

a heat exchanger, coupled to the piezo pumped heat pipe, to dissipate heat from the second section of the piezo pumped heat pipe.

2. The cooling apparatus of claim 1, wherein the first section corresponds to an evaporator section in the piezo pumped heat pipe, and the second section corresponds to a condenser section in the piezo pumped heat pipe.

3. The cooling apparatus of claim 1, wherein the piezo pumped heat pipe comprises:

a liquid coolant;

an evaporator, when heated, to evaporate at least a portion of the liquid coolant;

a wick structure to bring the at least a portion of the liquid coolant to the evaporator; and

a piezoelectric device adjacent to the evaporator to help reduce evaporator resistance when the evaporator evaporates the at least a portion of the liquid coolant.

4. The cooling apparatus of claim 3, wherein the piezo pumped heat pipe further comprises a container to enclose the evaporator, the wick structure, the liquid coolant, and the piezoelectric device.

5. The cooling apparatus of claim 3, wherein the piezoelectric device is actuated by an oscillating voltage source.

6. The cooling apparatus of claim 5, wherein the piezoelectric device, when actuated, generates wavy motions in at least a portion of the liquid coolant, the wavy motions capable of increasing flow velocities in the liquid coolant near the evaporator.

7. The cooling apparatus of claim 1, further comprising an oscillating voltage source to actuate a piezoelectric device in the piezo pumped heat pipe.

8. The cooling apparatus of claim 1, wherein the heat exchanger extracts heat from vapor in the second section to condense the vapor to liquid.

9. The cooling apparatus of claim 8, wherein the heat exchanger comprises a fan to enhance air flow and a plurality of fins to dissipate heat extracted from the vapor to ambient air.

10. A piezo pumped heat pipe, comprising:

a liquid coolant;

an evaporator, when heated, to evaporate at least a portion of the liquid coolant; and

11. The piezo pumped heat pipe of claim 10, further comprising a wick structure to bring at least a portion of the liquid coolant to the evaporator.

12. The piezo pumped heat pipe of claim 10, further comprising a container to enclose the evaporator, the liquid coolant, the piezoelectric device, and a wick structure.

13. The piezo pumped heat pipe of claim 10, wherein the piezoelectric device is actuated by an oscillating voltage source.

14. The piezo pumped heat pipe of claim 13, wherein the piezoelectric device, when actuated, generates wavy motions in at least a portion of the liquid coolant, the wavy motions capable of increasing flow velocities in the liquid coolant near the evaporator.

15. A system, comprising:

a heat-generating device; and

a cooling system to dissipate heat generated by the heat-generating device using a piezo pumped heat pipe.

16. The system of claim 15, wherein the heat-generating device comprises an integrated circuit chip.

17. The system of claim 15, wherein the cooling system comprises:

a piezo pumped heat pipe, including an evaporator section and a condenser section, to transfer heat from the evaporator section to a condenser section;

a heat exchanger, coupled to the piezo pumped heat pipe, to dissipate heat from the condenser section; and

an oscillating voltage source to actuate a piezoelectric device in the piezo pumped heat pipe.

18. The system of claim 17, wherein the piezo pumped heat pipe comprises:

a liquid coolant;

the piezoelectric device adjacent to the evaporator to help reduce evaporator resistance when the evaporator evaporates the at least a portion of the liquid coolant.

19. The system of claim 18, wherein the piezo pumped heat pipe further comprises a container to enclose the evaporator, the wick structure, the liquid coolant, and the piezoelectric device.

20. The system of claim 18, wherein the piezoelectric device, when actuated, generates wavy motions in at least a portion of the liquid coolant, the wavy motions capable of increasing flow velocities in the liquid coolant near the evaporator.

21. The system of claim 17, wherein the heat exchanger extracts heat from vapor in the condenser section to condense the vapor to liquid.

22. The system of claim 21, wherein the heat exchanger comprises a fan to enhance air flow and a plurality of fins to dissipate heat extracted from the vapor to ambient air.