US20050284614A1

US20050284614A1 - Apparatus for reducing evaporator resistance in a heat pipe

Info

Publication number: US20050284614A1
Application number: US10/874,554
Authority: US
Inventors: Sridhar Machiroutu; Jo-Young Chang
Original assignee: Intel Corp
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 heat pipe having an evaporator made of porous materials with substantially uniformly sized pores and a wick made of finely porous material. The evaporator is thinner, on average, than the wick.

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 generating more heat. Various 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 heat pipe is a commonly used in a 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 heat pipe along with a heat exchanger;
FIG. 3 is a block diagram illustrating an example of a heat pipe, according to one embodiment of the present invention;
FIG. 4 is an internal top view of an example implementation of a heat pipe, according to one embodiment of the present invention; and
FIG. 5 is a side view of the 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 depend on the evaporation/boiling process in the evaporator section of the heat pipe and also on the ability to wet the evaporator section, which in turn depends on the structure of the evaporator and the wick. The evaporation/boiling process depends negatively on the thickness of the evaporator structure, while the ability to wet the evaporator section by the wick through its capillary pumping depends positively on the thickness of the wick structure. The evaporator and the wick are normally made of porous materials. According to an embodiment of the present invention, a thin porous evaporator structure made of a substantially uniform pore sized porous material and a thick wick structure made of a finely porous material may be used to help reduce evaporation resistance. A thin porous structure for the evaporator helps reduce thermal conduction losses. A substantially uniform pore sized material used for the evaporator allows for minimum vapor flow resistance, small and substantially uniform spaces for vapor generation, and substantially uniform wicking in the evaporator section. All of these lead to a low evaporation resistance. On the other hand, a thick wick structure made of finely porous material may improve capillary pumping of the wick and thus result in the better ability of the wick to wet the evaporator section, which helps reduce the 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 heat pipe to dissipate heat generated by them.
FIG. 2 illustrates an exemplary block diagram of a heat pipe along with a heat exchanger. The 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 heat pipe may be coupled to a heat-generating device 220 (e.g., a processor). The heat generated by the device 220 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 230. The heat exchanger 230 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 a decrease in 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 comprise 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.
FIG. 3 is a block diagram illustrating an example of a heat pipe. An attach block 310 may attach the heat pipe 330 to a heat-generating device 320. 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 320 to the evaporator section of the heat pipe 330. 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.
Inside the heat pipe 330 there may be a vapor area 332 and a wick 334. The vapor generated in the evaporator section may transport through the vapor area 332 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 evaporator section is located, the other end of the heat pipe may be coupled to a heat exchanger 340. The heat exchanger may comprise a fan 342 and a plurality of fins 344. The fan 342 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 344 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 334.
FIG. 4 is an internal top view of an example implementation of a heat pipe, according to one embodiment of the present invention. The heat pipe may include outer walls 410, a wick 420, and an evaporator 430. Although not explicitly illustrated in FIG. 4, the heat pipe may also include a liquid coolant. An evaporator section of the heat pipe may be located near evaporator 430, and a condenser section of heat pipe may be spaced apart from evaporator 430 (e.g., including the far end of heat pipe). The heat pipe may be of any size. Outer walls 410 may enclose wick 420, evaporator 430, and the coolant. Outer walls 410 may contact 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, or a screen material, or a mixture of any of the above) that covers an inner surface of the 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. 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 made of a thick layer of finely porous material. The porous material may include particles that have an average diameter in a range from 2 μm to approximately 100 μm. The average thickness of wick may range from about 0.5 mm to approximately 1 mm. Wick 420 may be designed to have a relatively high capillary pumping efficiency to hydrate evaporator 430. Typically, the smaller the pore size is, the better the capillary pumping is. Good capillary pumping of the wick helps improve the ability of the wick to bring the liquid coolant from the condenser section to the evaporator section and the ability of the wick to wet the evaporator, which may in turn improve the evaporation/boiling process in the evaporation section and thus reduce evaporation resistance.
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 top surface of the heat-generating device to be cooled. In one embodiment, the porous material in evaporator 430 may be the same as the porous material in wick 420. In another embodiment, the evaporator may use different porous material from that used in the wick. Evaporator 430, by virtue of its geometry and material, may have a relatively low thermal resistance. Because of the evaporator's low thermal resistance, the material of wick 420 may have a somewhat higher thermal resistance without adversely affecting the heat transfer efficiency of heat pipe.
The porous material of evaporator 430 may be formed on the inner surface of the heat pipe with an average thickness which may be less than the average thickness of wick 420. A thin structure for the evaporator may help reduce thermal conduction losses. The porous material of evaporator 430 may include, for example, copper particles whose average size is greater than the average particle size of the wick to improve vapor flow. A substantially uniform pore sized material may be used for the evaporator to reduce vapor flow resistance, to create small and substantially uniform spaces for vapor generation, and to form substantially uniform wicking in the evaporation section. In one embodiment, evaporator 430 may comprise one to two layers of porous material with substantially uniformly sized pores. The average diameter of the particles in the evaporator may be from approximately 25 μm to 150 μm. The thickness of a layer of porous material is approximately the same as the diameter of the particles in the material. In general, a thin porous structure with substantially uniform pore sized material may help reduce evaporation resistance and improve the ability to wet the evaporation section.
FIG. 5 is a side view of the 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, and evaporator 430, which are discussed above with regard to FIG. 4, the heat pipe may also include a liquid coolant 510 and a vapor space 520. The liquid coolant 210 may include water, methanol, ethanol, acetone, heptane, Freon, or another refrigerant, or a mixture of any of the above. 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, the amount of coolant 510 should be sufficient to ensure continuous 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 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 heat 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 certain example numerical ranges are given above for thickness, sizes, and values, these ranges are purely exemplary and may vary according to design needs. The values given may vary, for example, 10-30% above and below the respective endpoints of the ranges given above.
Furthermore, 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 heat pipe to transfer heat from a first section to a second section, the heat pipe having an evaporator made of substantially uniform pore sized material; and

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

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

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

a liquid coolant;

an evaporator, when heated, to evaporate at least a portion of the liquid coolant, the evaporator having a thin porous structure made of substantially uniform pore sized material; and

a wick to bring the at least a portion of the liquid coolant to the evaporator.

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

5. The cooling apparatus of claim 3, wherein the evaporator comprises at one to two layers of porous material with the thickness of a layer being approximately the average diameter of particles that make the layer.

6. The cooling apparatus of claim 5, wherein the particles in the evaporator comprises approximately substantially uniformly sized particles with an average diameter in an approximate range from 25 μm to 150 μm.

7. The cooling apparatus of claim 3, wherein the wick comprises a porous structure made of finely porous material with average thickness in an approximate range from 0.5 mm to 1 mm.

8. The cooling apparatus of claim 3, wherein the porous structure of the evaporator comprises particles whose average diameter is larger than the average diameter of particles in the porous material in the wick.

9. The cooling apparatus of claim 3, wherein the average thickness of the porous structure of the evaporator is thinner than the average thickness of the porous structure of the wick.

10. A heat pipe, comprising:

a liquid coolant;

an evaporator, when heated, to evaporate at least a portion of the liquid coolant, the evaporator having a porous structure made of substantially uniform pore sized material; and

a wick to bring the at least a portion of the liquid coolant to the evaporator.

11. The heat pipe of claim 10, further comprising a container to enclose the evaporator, the wick, and the liquid coolant.

12. The heat pipe of claim 10, wherein the evaporator comprises one to two layers of porous material with the thickness of a layer being approximately an average diameter of particles that make the layer.

13. The heat pipe of claim 12, wherein the particles in the evaporator comprises approximately uniformly sized particles with an average diameter in an approximate range from 25 μm to 150 μm.

14. The heat pipe of claim 10, wherein the wick comprises a porous structure made of finely porous material with average thickness in an approximate range from 0.5 mm to 1 mm.

15. The heat pipe of claim 10, wherein the porous structure of the evaporator comprises particles whose average diameter is larger than the average diameter of particles in the porous material in the wick.

16. The heat pipe of claim 10, wherein average thickness of the porous structure of the evaporator is thinner than the average thickness of the porous structure of the wick.

17. A system, comprising:

a heat-generating device; and

a cooling system to dissipate heat generated by the heat-generating device using a heat pipe, the heat pipe having an evaporator made of substantially uniform pore sized material.

18. The system of claim 17, wherein the heat-generating device comprises an integrated circuit chip.

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

a heat pipe, including an evaporator section and a condenser section, to transfer heat from the evaporator section to a condenser section, the heat pipe having an evaporator made of substantially uniform pore sized material; and

a heat exchanger, coupled to the heat pipe, to dissipate heat from the condenser section.

20. The system of claim 19, wherein the heat pipe comprises:

a liquid coolant;

an evaporator, when heated, to evaporate at least a portion of the liquid coolant, the evaporator having a porous structure made of substantially uniform pore sized material;

a wick to bring the at least a portion of the liquid coolant to the evaporator, the thickness of the wick being greater on average than the thickness of the evaporator; and

a container to enclose the evaporator, the wick, and the liquid coolant.

21. The system of claim 20, wherein the evaporator comprises one to two layers of porous material with the thickness of a layer being approximately the average diameter of particles that make the layer, the average diameter of particles being in an approximate range from 25 μm to 150 μm.

22. The system of claim 20, wherein the wick comprises a porous structure made of finely porous material with average thickness in an approximate range from 0.5 mm to 1 mm.

23. The system of claim 20, wherein the porous structure of the evaporator comprises particles whose average diameter is larger than average diameter of particles in porous material that makes the wick.

24. The system of claim 19, wherein the heat exchanger extracts heat from vapor in the condenser section to condense the vapor to liquid.

25. The system of claim 24, 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.