Abstract:
A low velocity, laminar jet of air is directly impinged on the surface of heat producing components such as a microprocessor chip. A laminar jet air flow has a very low mass flow rate and has a high convection heat transfer rate at the stagnation region of the flow. A fan or an actuator coupled to a tube can be used to provide laminar jet air on the heat producing component. For any apparatus that places a premium on conserving power, delivery of a low velocity, laminar jet provides an effective way to cool the components of the apparatus while only minimally draining its power source.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Managing heat dissipation of electronic components such as those in portable systems continues to be a challenge for engineers and designers. Portable computers pose a significant thermal management problem because of the need to increase their computational capabilities as well as their portability. Portable computers such as laptops or hand-held computers have higher power densities than desktop computers that use the same processor chips. Forced convection cooling schemes used in desktop computers are not generally applicable to portable computers because of space and battery power constraints.  
         SUMMARY OF THE INVENTION  
         [0002]    The present invention relates to laminar jet cooling of heat producing components in an apparatus such as a portable computer. A low velocity, laminar jet of air is directly impinged on the surface of a heat producing component such as a microprocessor chip. A laminar jet flow has a very low mass flow rate and has a high convection heat transfer rate at the stagnation region of the flow. An air driving device such as a small energy efficient fan, an acoustic resonance device, or a small electromagnetically driven actuator assembly can be used to provide laminar jet air flow on the heat producing component. For any apparatus that places a premium on conserving power, delivering laminar jet air flow provides a way to dissipate heat while only minimally draining the power source of the apparatus.  
           [0003]    In one embodiment of the invention, an apparatus for cooling heated bodies includes a heat producing component and an air driving device such as a fan, an acoustic resonance device or an electromagnetically driven actuator assembly, which produces a laminar jet of air having a Reynolds number below 2000, to cool the heat producing component. The laminar air flow preferably has a Reynolds number between 1500 and 60 and more preferably between 1500 and 500. The heat producing component can be a microprocessor.  
           [0004]    The apparatus further includes a directing device such as a tubing for impinging the laminar air from the air driving device directly on the heat producing component.  
           [0005]    In another embodiment of the invention, a laminar jet air cooling device includes a fan, an acoustic resonance device, or an electromagnetically driven actuator device for impinging a laminar jet of air directly on a heat producing electronic component mounted on a circuit board and a tubing coupled to the air driving device and having an outlet above the heat producing electronic component. The outlet directs the laminar jet of air on the heat producing electronic component at a predetermined angle, 90 degrees or less. The laminar air flow has a Reynolds number below 2000, preferably between about 1500 and 60, and more preferably between about 1500 and 500.  
           [0006]    In another embodiment of the laminar jet air cooling device, the tubing bifurcates over and below the circuit board such that the tubing over the circuit board directs the laminar jet of air over the heat producing electronic component and the tubing under the circuit board forces the air away from the circuit board to dissipate additional heat.  
           [0007]    The laminar jet air cooling device can further include a heat pipe mounted on the underside of the circuit board and connected to a keyboard of a portable system containing the circuit board for transferring heat from the circuit board to the keyboard.  
           [0008]    In another embodiment of the invention, a portable computer system includes a circuit board containing a heat producing electronic component such as a microprocessor, and an air driving device such as a fan, an acoustic resonance device, or an electromagnetically driven actuator device for impinging a laminar jet of air directly on the heat producing electronic component.  
           [0009]    The portable computer further has a tubing coupled to the air driving device and an outlet above the heat producing electronic component. The outlet directs the laminar jet of air on a surface of the heat producing electronic component at a predetermined angle, 90 degrees or less, over the heat producing electronic component.  
           [0010]    In another embodiment of the portable computer system, a tubing bifurcates over and below the circuit board such that the tubing over the circuit board directs the laminar jet of air over the heat producing electronic component and the tubing under the circuit board forces the air out through a vent located close to the heat producing electronic component.  
           [0011]    The portable computer system can further have a heat pipe mounted on the underside of the circuit board and connected to a keyboard for transferring heat from the circuit board to the keyboard.  
           [0012]    The portable computer system can also have a card and a power supply positioned above the circuit board such that flowing air from the laminar jet of air carries heat away from the card and the power supply through the outlet vent.  
           [0013]    In still another embodiment of the invention, a method of cooling a portable computer includes delivering a laminar jet of air to a heat producing electronic component mounted on a circuit board disposed within a portable computer. The laminar jet of air is directed over the heat producing electronic component at a predetermined angle over the heat producing electronic component.  
           [0014]    A laminar jet of air can be directed on the heat producing electronic component from over the circuit board and also under the circuit board for letting the air escape through a vent disposed in the wall of the portable computer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1A shows a top view of a portable computer with a tubing providing a laminar jet of air over a microprocessor according to an embodiment of the invention.  
         [0016]    [0016]FIG. 1B shows a side view of FIG. 1A.  
         [0017]    [0017]FIG. 2 shows a flow pattern of an impinging jet of air.  
         [0018]    [0018]FIG. 3A shows a top view of a portable computer with an angled tube outlet directed to a microprocessor according to another embodiment of the invention.  
         [0019]    [0019]FIG. 3B shows a side view of FIG. 3A.  
         [0020]    [0020]FIG. 4A shows a top view of a portable computer with a tubing extending over a circuit board and a branched tubing underneath the circuit board according to another embodiment of the invention.  
         [0021]    [0021]FIG. 4B shows a side view of FIG. 4A.  
         [0022]    [0022]FIG. 5 shows a microprocessor chip facing downward and a air driving device directly facing the chip from below in a portable computer according to another embodiment of the invention.  
         [0023]    [0023]FIG. 6 shows a circuit board positioned below a power supply and a card according to another embodiment of the invention.  
         [0024]    [0024]FIG. 7 shows a hybrid structure of a laminar jet cooling device and a heating pipe in a portable computer according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The present invention relates to a method and apparatus for cooling a heat producing body by a laminar jet of air. The heat producing body can be an electronic component in an apparatus such as a laptop, a hand-held computer, or a mobile phone. The heat producing body could be a part of any system, requiring specific and selective cooling.  
         [0026]    Referring to FIGS. 1A and 1B, in one embodiment of the invention, a portable computer  10  includes a microprocessor chip  30  mounted on a circuit board  20 . The chip  30  includes a substrate and a silicon die. A top cover with a keyboard and selected side walls of the computer  10  are omitted to better reveal the inner configuration. Other components of the portable computer  10  such as a power supply, a disk drive, a CD drive, and PCMCIA cards are also omitted in FIGS. 1A and 1B and later figures for clarity of presentation.  
         [0027]    The circuit board  20  is surrounded by walls  40 , one of which includes an outlet vent  50 . A jet outlet  60  located directly above the microprocessor chip  30  is connected to a tubing  70  of appropriate diameter that leads to a fan  80 , serving as an air driving device, located near or at the edge of one of the walls  40 . The fan  80  can be a self-contained axial fan less than 25 mm in diameter to accommodate the compact internal spacing of the portable computer  10 . Instead of the fan  80 , other air driving devices may be used such as an acoustic resonance device or other electromagnetically driven actuators that produce the necessary laminar jet air flow.  
         [0028]    The tubing  70  can be made of flexible or rigid plastic or other lightweight material in accordance with the particular structural requirements of the portable computer  10 . For example, a fixed geometry conduit such as a molded plastic tubing can be designed and integrated into the portable computer  10 .  
         [0029]    In this embodiment, the tubing  70  is straight. However, it can be arranged to turn in a manner necessary to accommodate the design of the portable computer  10  so long as any bends or curves of the tubing do not significantly disrupt the laminar jet flow at the outlet.  
         [0030]    Referring to FIG. 2, a typical flow pattern of impinging jets of air can be subdivided into three regions: free jet region  100 , stagnation flow region  110 , and the wall jet region  120 . The free jet region  100  includes a potential core  130 . The length of the potential core  130  is generally 5-8 times the jet width. Much of the heat transfer from a heat producing component to the air occurs at the stagnation flow region  110 . After impingement, the flow deflects in the transverse direction and begins to accelerate outwardly, creating a wall of jet flow in the wall jet region  120 .  
         [0031]    The Reynolds number is proportional to the average jet velocity times the diameter of the jet flow. To be in the laminar regime, the Reynolds number should be less than 2000, and preferably less than or equal to 1500. A jet with a Reynolds number of 1500 is expected to cool a processor chip with a heat load of approximately 10 Watts to a temperature of about 100° C. For a jet with the same Reynolds number, a processor with a heat load of approximately 5 Watts can be cooled to about 70° C. Furthermore, even a low laminar jet flow with a very low Reynolds number can still effectively cool heated components. For example, a jet with a Reynolds number of  63  is expected to cool a processor chip with a heat load of approximately 5 Watts to a temperature of about 90° C. Hence, a low laminar jet air with a Reynolds number less than 500 may be useful for cooling moderately powered components in portable systems, specifically, components with power dissipations that are just high enough to make cooling by natural convection unreliable.  
         [0032]    The thermal characterization of the portable computer  10  is next considered in terms of (i) jet to chip spacing expressed as a ratio of height of the jet from the chip and width of the jet diameter (H/W), (ii) location and size of the vent, and (iii) cross-section of the jet flow.  
         [0033]    In terms of the jet to chip spacing, moving the laminar jet closer to the chip  30  in the portable computer  10  is not expected to change the cooling performance significantly. Generally, H/W spacing of about 5.0 or less is desired for the potential core  130  of the laminar jet to reach the target for effective cooling. However, a portable computer can have a shallow compartment width of approximately 25 to 30 mm, which translates roughly into H/W spacing of about 2.5 or less with a given jet width of about 10 mm or less. Therefore, within the confines allowed by a thin portable computer, a laminar jet potential core of even a very low Reynolds number is expected to reach the target surface.  
         [0034]    Changing the cross sectional area of the vent  50  is not expected to have much effect on the temperature of the chip  30 . However, the location of the vent  50  is expected to have some significance on the chip temperature. It would be preferable to have the vent  50  located next to or as close as possible to the chip  30  so that the heated air can escape from the compartment rapidly. Furthermore, it would be preferable to have the vent  50  located at the side of the compartment rather than on the top surface of the compartment because a top surface vent is more likely to become inadvertently obstructed by the user. Furthermore, the top vent can be a target for spilled liquids or other foreign matter.  
         [0035]    For a given Reynolds number, the cross-section of the jet flow, i.e. jet size, is expected to have some effect on the cooling of a processor. Larger jets result in cooler bulk air escaping the compartment because of the increased mass flow rate for a given Reynolds number. Smaller jets result in slightly cooler processor temperatures because of the increased heat transfer rate for a given Reynolds number. For example, for a given Reynolds number, a jet size smaller than the dimension of the microprocessor chip  30  would have a greater heat transfer rate that a jet size equal to the dimension of the microprocessor chip. The effect of jet size is expected to be more noticeable at higher Reynolds numbers, e.g. 1500 or greater, than at low Reynolds numbers, e.g. less than 500.  
         [0036]    However, it is not the jet size but the Reynolds number that is the dominant thermal control parameter. A test was conducted to show the above concept. A conventional laptop was modified by removing its fan/heat sink assembly and positioning a round plastic tubing above the CPU through which low flow air was directed. The original fan/heat sink assembly had an airflow rate of 6.3×10 −4  m 3 /s. Three jet diameters, 4.3 mm, 6.4 mm, and 9.5 mm were employed at the Reynolds numbers (Re) of 500, 1000, and 1500. This testing range yielded the volumetric flow rates summarized in Table 1.  
                                                                             TABLE 1                           Volumetric Flow rates (10 −4  m 3 /s)                Jet Diameter                        4.3 mm   6.4 mm   9.5 mm                            Re   500   0.19   0.28   0.35               1000   0.38   0.56   0.84               1500   0.76   0.84   1.25                      
 
         [0037]    The modified laptop was connected with Type T thermocouples to measure the CPU temperature. Steady state CPU surface temperatures above the ambient temperature are shown in Table 2.  
                                                                             TABLE 2                           CPU Temperature (C. above ambient).                Jet Diameter                        4.3 mm   6.4 mm   9.5 mm                            Re   500   88 ± 4   83 ± 1   82 ± 2               1000   69 ± 1   72 ± 2   69 ± 2               1500   59 ± 3   62 ± 1   60 ± 1                      
 
         [0038]    The data in Table 2 indicate that Re, not the jet size, is the dominant thermal control parameter. The measured CPU temperatures for all jet diameters at the same Re are shown to be in agreement within experimental uncertainty. A jet with Re=1500 produced cooling similar to the original design with approximately one-eighth the volumetric flow rate. This would translate to a fan power consumption reduction of over 50% and a commensurate noise reduction.  
         [0039]    In FIGS. 1A and 1B, the jet outlet  60  bends in a right angle from the tubing  70  to impinge a laminar jet of air on the surface of the chip  30 . In other embodiment, the bend can be made with a smooth curve to further promote the laminar jet flow.  
         [0040]    Referring to FIGS. 2A and 2B, in another embodiment of the invention, a jet outlet  65  is angled such that the laminar jet strikes the microprocessor chip  30  at an angle less than 90 degrees from the horizontal. This configuration can be suitable for a portable system with a very narrow compartmental width. The angle can be designed to provide the greatest amount of laminar flow within the structural constraint of the portable computer  10 .  
         [0041]    Referring to FIG. 4A and FIG. 4B is a side view of FIG. 4A, a tubing  75  extends over the microprocessor chip  30  from the fan  80 . The tubing  75  has a hole  77  cut in the tubing at a position over the chip  30  to allow the laminar jet flow to directly impinge on the chip. The lack of a protruding outlet allows the configuration to be closer to the circuit board  20 . The tubing  75  also has a second branched tubing extending underneath the circuit board  20  to push the air out to the outlet vent  50  for efficient heat dissipation.  
         [0042]    Referring to FIG. 5, in another embodiment of the invention, the surface of the microprocessor chip  30  on the circuit board  20  faces down rather than up. This can be achieved either by configuring the circuit board  20  to attach the chip  30  at the underside of the circuit board  20  or by designing the portable computer  10  to have the circuit board  20  inverted. A very thin fan  85 , can be positioned directly under the chip  30  and disposed on the bottom casing of the portable computer  10  to drive a laminar jet of air to the chip from below. In this configuration, a long tube is obviated.  
         [0043]    In an effort to further reduce the chip temperature, components such as PCMCIA cards and a power supply are located above a circuit board to reduce the thermal interaction of the circuit board with these heat producing bodies. For example, in another embodiment of the invention as shown in FIG. 6, the circuit board  20  is separated by about 10 to 20 mm in the vertical direction, relative to PCMCIA cards  140  and a power supply board  150 . Care should be taken to configure the PCMCIA cards  140  and the power supply board  150  so that they do not interfere with the laminar flow to the chip  30 . The laminar flow is indicated by a large arrow above the chip  30 .  
         [0044]    In this configuration, parasitic heating of the circuit board  20  is reduced. Furthermore, since the PCMCIA cards  140  and the power supply board  150  are located on the side of the laminar jet flow, where there is significant air velocity, heat from the PCMCIA cards, the power supply board and the circuit board is carried away from the compartment through the outlet vent  50 . Thus, the thermal interaction between the various assemblies is decreased.  
         [0045]    In addition to providing a laminar jet over the microprocessor, in another embodiment of the invention as shown in FIG. 7, a miniature Copper-water heat pipe  160  can be mounted on the underside of the circuit board  20  to further contribute to the cooling of the microprocessor chip  30 . The portable computer  10  without the walls  40  shows the power supply board  150  and the PCMCIA cards  140  both disposed above the circuit board  20 , a battery  165  disposed beside the circuit board  20 , and a disk drive  167  disposed beside the battery. The heat pipe  160  can provide a low resistance conduction path from the circuit board  20  to a keyboard  170 , with the keyboard serving as a condenser for the heat pipe. The large surface area of the keyboard  170  can be used to spread the heat and provide a medium for heat dissipation by natural convection.  
         [0046]    The heat pipe  160  interfaces the circuit board  20  with a 3.2 mm thick Copper mounting block  180 . The mounting block  180  has the same in plane dimensions as the processor chip  30  and mounts directly below it, on the underside of the circuit board  20 . The overall length of the heat pipe  160  can be 104 mm, and the cross section can be 2 mm×8 mm. The heat pipe  160  can be a solid conductive block with high thermal conductivity or a porous, annulus metal containing a saturated liquid in equilibrium with its own vapor. A thermally conductive adhesive may be used at the following interfaces: mounting block  180 /circuit board  20 , heat pipe  160 /mounting block  180 , heat pipe  160 /keyboard  170 . The thermal resistance due to the adhesive material is assumed to be about 0.5 cm 2 C/W.  
         [0047]    By dividing the heat between the laminar jet of air and the heat pipe  160 , the combined cooling scheme can cool higher power systems. The laminar jet has a low mass flow rates with a high heat transfer rate, and consequently, provides near passive cooling for the circuit board  20  and the microprocessor chip  30 . The heat pipe  160  can be used to efficiently transport heat from the circuit board  20  to the keyboard  170 , effectively reducing the circuit board heat load by the amount of heat that conducts through the heat pipe. By using the large surface area of the keyboard  170  to dissipate heat by natural convection and radiation, active cooling at the condenser may be dispensed with.  
         [0048]    The addition of the heat pipe  160  improves the overall thermal performance of the entire compartment by reducing the average board temperature and the average chip temperature. Transferring heat from the circuit board  20  to the keyboard  170  with the heat pipe  160  is expected to increase the average keyboard temperature. However, such temperature can be controlled to an acceptable level by selecting a heat pipe with appropriate heat conductivity.  
         [0049]    A simulation of the laminar jet/heat pipe system suggests that a reduction of the thermal conductivity of the heat pipe  160  from 5000 W/mK to 1000 W/mK raises the chip temperature by only 3.5% since the thermal resistance of the heat pipe is low compared to heat dissipation by convention or radiation. Thus, even a heat pipe of very low conductivity coupled with laminar jet air cooling can effectively dissipate heat from the circuit board  20 .  
         [0050]    Although all of the embodiments of the invention are described in terms of the portable computer  10 , the invention is applicable to any system that can be effectively cooled by laminar jet air.  
         [0051]    Several embodiments of the invention have been described herein, but it should be understood that various additions and modifications could be made which fall within the scope of the following claims.