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  
       [0001]     1. Field  
         [0002]     The present invention relates generally to liquid cooling systems and, more specifically, to heat pipes for dissipating heat generated by integrated circuits.  
         [0003]     2. Description  
         [0004]     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.  
         [0005]     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  
       [0006]     The features and advantages of the present invention will become apparent from the following detailed description of the present invention in which:  
         [0007]      FIG. 1  illustrates an exemplary block diagram of a computer system which may be utilized to implement embodiments of the present invention;  
         [0008]      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;  
         [0009]      FIG. 3  is a block diagram illustrating an example of a piezo pumped heat pipe, according to one embodiment of the present invention;  
         [0010]      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  
         [0011]      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  
       [0012]     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.  
         [0013]     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.  
         [0014]      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&#39;s XScale® processor, Intel&#39;s Pentium® M processors, etc., available from Intel Corporation. Alternatively, other processors from other manufacturers may also be used.  
         [0015]     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 .  
         [0016]     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 .  
         [0017]     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.  
         [0018]      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.  
         [0019]     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.  
         [0020]     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.  
         [0021]     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.  
         [0022]     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.  
         [0023]     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.  
         [0024]      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.  
         [0025]     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 .  
         [0026]      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.  
         [0027]     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 .  
         [0028]     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.  
         [0029]     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.  
         [0030]      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.  
         [0031]     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.  
         [0032]     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.  
         [0033]     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.  
         [0034]     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.