US20040070941A1 - Compact thermosiphon with enhanced condenser for electronics cooling - Google Patents
Compact thermosiphon with enhanced condenser for electronics cooling Download PDFInfo
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- US20040070941A1 US20040070941A1 US10/271,630 US27163002A US2004070941A1 US 20040070941 A1 US20040070941 A1 US 20040070941A1 US 27163002 A US27163002 A US 27163002A US 2004070941 A1 US2004070941 A1 US 2004070941A1
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F28—HEAT EXCHANGE IN GENERAL
- F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
- F28F3/00—Plate-like or laminated elements; Assemblies of plate-like or laminated elements
- F28F3/02—Elements or assemblies thereof with means for increasing heat-transfer area, e.g. with fins, with recesses, with corrugations
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F28—HEAT EXCHANGE IN GENERAL
- F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
- F28D15/00—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies
- F28D15/02—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes
- F28D15/0266—Heat-exchange apparatus with the intermediate heat-transfer medium in closed tubes passing into or through the conduit walls ; Heat-exchange apparatus employing intermediate heat-transfer medium or bodies in which the medium condenses and evaporates, e.g. heat pipes with separate evaporating and condensing chambers connected by at least one conduit; Loop-type heat pipes; with multiple or common evaporating or condensing chambers
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F28—HEAT EXCHANGE IN GENERAL
- F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
- F28F1/00—Tubular elements; Assemblies of tubular elements
- F28F1/10—Tubular elements and assemblies thereof with means for increasing heat-transfer area, e.g. with fins, with projections, with recesses
- F28F1/12—Tubular elements and assemblies thereof with means for increasing heat-transfer area, e.g. with fins, with projections, with recesses the means being only outside the tubular element
- F28F1/126—Tubular elements and assemblies thereof with means for increasing heat-transfer area, e.g. with fins, with projections, with recesses the means being only outside the tubular element consisting of zig-zag shaped fins
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L23/00—Details of semiconductor or other solid state devices
- H01L23/34—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements
- H01L23/42—Fillings or auxiliary members in containers or encapsulations selected or arranged to facilitate heating or cooling
- H01L23/427—Cooling by change of state, e.g. use of heat pipes
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F28—HEAT EXCHANGE IN GENERAL
- F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
- F28F3/00—Plate-like or laminated elements; Assemblies of plate-like or laminated elements
- F28F3/12—Elements constructed in the shape of a hollow panel, e.g. with channels
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
- H01L2924/0001—Technical content checked by a classifier
- H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
Definitions
- the present invention relates to heat sinks in general, and more particularly to heat sinks for use in dissipating waste heat generated by electrical or electronic components and assemblies.
- heat sinks At the component level, various types of heat exchangers and heat sinks have been used that apply natural or forced convection or other cooling methods.
- the most commonly existing heat sinks for microelectronics cooling have generally used air to directly remove heat from the heat source.
- air has a relatively low heat capacity.
- Such heat sinks are suitable for removing heat from relatively low power heat sources with power density in the range of 5 to 15 W/cm 2 (4 to 13 Btu/ft 2 s).
- Increases in computing speed resulted in corresponding increases in the power density of the heat sources in the order of 20 to 35 W/cm 2 (18 to 31 Btu/ft 2 s) thus requiring more effective heat sinks.
- Liquid-cooled heat sinks employing high heat capacity fluids like water and water-glycol solutions are more particularly suited to remove heat from these types of high power density heat sources.
- One type of liquid cooled heat sink circulates the cooling liquid so that the liquid removes heat from the heat source and is then transferred to a remote location where the heat is easily dissipated into a flowing air stream with the use of a liquid-to-air heat exchanger.
- These types of heat sinks are characterized as indirect heat sinks.
- thermosiphon As computing speeds continue to increase even more dramatically, the corresponding power densities of the devices rise up to 100 W/cm 2 .
- the constraints of the necessary cooling system miniaturization coupled with high heat flux calls for extremely efficient, compact, simple and reliable heat sinks such as a thermosiphon.
- a typical thermosiphon comprises an evaporating section and a condensing section.
- the heat-generating device is mounted to the evaporating section.
- the heat-generating device is affixed to the internal surface of the evaporating section where it is submerged in the working fluid.
- the heat-generating device can also be affixed to the external surface of the evaporating section.
- the working fluid of the thermosiphon is generally a halocarbon fluid, which circulates in a closed-loop fashion between the evaporating and condensing sections.
- the captive working fluid changes its state from liquid-to-vapor in the evaporating section as it absorbs heat from the heat-generating device. Reverse transformation of the working fluid from vapor-to-liquid occurs as it rejects heat to a cooling fluid like air flowing on an external finned surface of the condensing section.
- the thermosiphon relies exclusively on gravity for the motion of the working fluid between the evaporating and condensing sections.
- a fluid moving device like an axial fan is employed.
- the second non-uniformity is attributed to the attachment of the air-moving device like an axial fan attached to the exterior of the thermosiphon.
- Axial fans generally have a large hub which acts as blockage to airflow.
- the airflow entering and exiting from the axial fan is highly concentrated in the peripheral region of the fan blades.
- Typical airflow exit and entry velocity profiles are shown in FIGS. 5 a and 5 b respectively.
- the maximum air velocity is in the tip region of fan blades. The velocity falls off sharply and reaches zero in the central hub region. Under certain flow conditions and blade angle, the local velocity at the root of the fan blade may even become negative, i.e., opposite to the direction of the predominant airflow.
- thermal performance penalty attributed to these non-uniformities can be of the order of 25 to 50% compared to the case with uniform heat flux and uniform airflow.
- thermal solution becomes considerably more challenging when the heat flux as well as the airflow is non-uniform.
- the difficulty is compounded when the available airflow rate is small. Therefore, careful attention must be paid to the fluid flow and heat transfer boundary conditions when developing the thermal solutions for the computer chips.
- thermosiphons intended to fit in a computer case require boiling and condensing processes to occur in close proximity to each other thereby imposing conflicting thermal conditions in a relatively small volume. This poses significant challenges to the process of optimizing the thermosiphon performance.
- thermosiphon optimization process to intensify the processes of boiling, condensation and convective heat transfer at the external surface of the condenser while maintaining low airside pressure drop.
- the heat sink assembly comprises a fan housed in a shroud, the fan having a hub and fan blades extending therefrom for causing an axially directed airflow through the shroud upon rotation of the fan blades.
- a thermosiphon is positioned at one end of the shroud such that the fan is aligned with the condenser for directing the axial airflow therethrough.
- the thermosiphon comprises an evaporator defining an evaporating chamber containing a working fluid therein and further including a condenser mounted thereabove.
- the condenser includes a base having an upper surface and a plurality of fins extending substantially upwardly from the upper surface.
- the condenser also includes a plurality of tubes forming a tube grouping. Each tube having an opening in fluid communication with the evaporator and for receiving and condensing vapor of the working fluid received from the evaporator.
- the tubes are axially aligned with the airflow and are laterally positioned such that a lateral width of the tube grouping is approximately equal to a width of the hub and substantially in lateral alignment thereto.
- the condenser comprises a base having an upper housing affixed thereto wherein the upper housing has open ends.
- the base further includes a plurality of fins extending substantially upwardly from an upper surface of the base and within the upper housing.
- a fan is mounted at one of the open ends, the fan having a hub and fan blades extending therefrom for causing an axially directed airflow through the housing upon rotation of the fan blades.
- a plurality of tubes is positioned within the housing for transmitting therethrough a vapor of a working fluid.
- the tubes define a tube grouping such that the tubes are arranged in axial alignment with the fan and laterally positioned such that a lateral width of the tube grouping is approximately equal to a width of the hub and substantially in lateral alignment thereto.
- FIG. 1 is a perspective view of a heat sink assembly embodying the present invention, wherein an axial fan is arranged to draw cooling air through a thermosiphon.
- FIG. 2 is an elevational cross-section view of an embodiment of the thermosiphon shown in FIG. 1 and taken along the line 2 - 2
- FIG. 3 is an enlarged segment of the cross-sectional view of the boilerplate shown in FIG. 2.
- FIG. 4 is a typical heat flux distribution of an electronic device requiring cooling.
- FIG. 5A is a typical air velocity distribution just downstream of the axial fan in push mode used in conjunction with a thermosiphon.
- FIG. 5B is a typical air velocity distribution just upstream of the axial fan in pull mode used in conjunction with a thermosiphon.
- FIG. 6 is an elevational cross-section view of a second embodiment thermosiphon.
- FIG. 7 is an elevational cross-section view of a third embodiment thermosiphon.
- FIG. 8 is an elevational cross-section view of a fourth embodiment thermosiphon.
- FIG. 1 shows an air-cooled thermosiphon heat sink 10 , which is one of the preferred embodiments of the present invention and illustrates its various components.
- a single axial fan 14 is housed in shroud 16 and coupled to thermosiphon 12 through duct 18 .
- the fan 14 could be a pull or push type fan, however, a pull type of fan is preferred to minimize shadowing of the thermosiphon 12 by the fan hub 15 .
- the shadowing effect of hub 15 occurs over a lateral width 55 denoted by dimension “H” and substantially at a center of thermosiphon 12 .
- the shadowing effect of hub 15 is greater with a push type fan than a pull type fan and reduces the airflow behind the hub and thereby interferes with the heat transfer from thermosiphon 12 to the cooling air stream.
- FIGS. 4, 5A and 5 B illustrate the differences between the heat distribution of the device 8 to be cooled and the areas of maximum airflow of fan 14 .
- the heat distribution of device 8 approximates a bell curve with the greatest heat at the area above the center of device 8 .
- the area of maximum airflow in push mode appears as an inverse of the heat distribution, namely minimal airflow in the middle and maximum airflow at the outmost portion of the fan.
- FIG. 5B illustrates the airflow in pull mode being similar to the push mode airflow illustrated in FIG. 5A. Therefore, without any enhancements, the fan generates maximum airflow over the minimum heat regions and low airflow over the regions of maximum heat.
- FIG. 2 shows a sectional view of a preferred embodiment of the thermosiphon 12 .
- Thermosiphon 12 comprises an evaporator 20 and a condenser 22 mounted thereabove.
- the evaporator 20 comprises a baseplate 26 having a thickness 25 denoted by dimension “t” and sidewalls 24 about a periphery of baseplate 26 .
- the thickness 25 “t” of the evaporator base plate 26 is suitably chosen based on an analysis of the particular boiling and heat transfer considerations for a desired application.
- Electronic device 8 having a mean width 9 denoted by dimension “z” is attached to a bottom surface 27 of baseplate 26 using a heat conductive adhesive, also known as “thermal grease”. Bottom surface 27 is preferably polished for attachment of electronic device 8 to enhance the thermal contact from device 8 to baseplate 26 .
- An upper surface of baseplate 26 defines boiling surface 31 and can have a plurality of stud fins 28 formed thereon. Stud fins 28 are preferably machined as an integral part of baseplate 26 for maximum heat transfer. As illustrated in FIG. 3, the boiling surface 31 of baseplate 26 can also have a surface coating 30 deposited thereon to enhance the boiling properties of boiling surface 31 .
- Surface coating 30 can comprise a sintered metal powder of aluminum or copper.
- Sidewalls 24 have a height 37 denoted by dimension “h” and have a bottom affixed to baseplate 26 . Sidewalls 24 can also be integrally formed with base 26 as a single structure to minimize the number of joints requiring a fluid seal.
- An upper surface of sidewalls 24 defines an upper horizontal flange 29 about the periphery of evaporator 20 to which the base 32 of condenser 22 is attached thereby defining evaporating chamber 36 .
- the height of evaporating chamber 36 is also represented by dimension “h”.
- Base 32 is preferably affixed to flange 29 by one of brazing, welding or diffusion bonding to form a leak-proof chamber 36 .
- the flange joint between the sidewalls 24 and base 32 can be enhanced by means of a trunion groove type mating of the protruding and recessed side of the flange prior to brazing or welding.
- a good joint can also be enhanced by means of peripheral screws (not shown) fastening base 32 to sidewalls 24 .
- the screws provide additional reinforcement and prevent leakage at high pressure.
- Evaporation chamber 36 is charged with a working fluid 38 through charging port 40 in base 32 . Chamber 36 also functions as a manifold to distribute saturated or super-heated vapor into the hairpin condenser tubes 44 .
- Condenser 22 comprises base 32 and housing 34 mounted thereon. Housing 34 is open at both ends when viewed axially with respect to fan 14 thereby permitting the airflow induced by fan 14 to flow therethrough.
- Base 32 includes a plurality of condensing fins 33 extending downwardly from bottom surface 35 into evaporation chamber 36 .
- Vertical fins 47 are parallel one to the other and are axially aligned with the airflow induced by fan 14 to permit the flow of air between adjacent ones of fins 47 .
- fins 47 are integrally formed upon base 32 such as by machining, forging, or extrusion methods known in the art.
- the height “d”, thickness and linear fin density (fin spacing) are determined based upon such factors as fin efficiency and with consideration to the pressure drop of the airflow induced by fan 14 .
- Fins 47 are advantageously positioned in the high airflow region of fan 14 in the area approximately below fan hub 15 and therefore serve to dissipate heat extremely well to the passing airflow.
- a condenser tube 44 is formed in an inverted “U” shape wherein each leg thereof has a respective inlet end 43 extending through base 32 into evaporation chamber 36 . Inlet ends 43 are open and place an interior of tubes 44 in fluid communication with evaporation chamber 36 . In this manner, working fluid vapor formed as a result of boiling on the boiling surface 31 can enter either end 43 of hairpin tube 44 and rise therein for the ultimate dissipation of heat.
- Hairpin tube 44 has a width 45 denoted by the dimension “a” and is positioned above the area of high heat flux q′′ of device 8 .
- Tube 44 is formed such that its respective legs form a tube grouping behind fan hub 15 within hub diameter 55 as denoted by dimension “H”. Thus, hairpin tube 44 resides in the wake of hub 15 in the middle of the thermosiphon 12 , and serves primarily as a conduit for vapor flow between the evaporator 20 and the condenser 22 .
- First fins 50 having a height 51 are placed outside of tube 44 and are substantially inline with the fan blades of axial fan 14 where the airflow is high.
- Second fins 52 having a height 53 are placed in the low flow region directly behind the hub 15 between the legs of hairpin tube 44 .
- Fins 50 and 52 are generally of a convoluted accordion configuration and have their apexes bonded to the surface of tubes 44 or housing 34 to which they contact in the area above fins 47 .
- Housing 34 encases tubes 44 , vertical fins 47 , first fins 50 and second fins 52 to direct and maintain the airflow from fan 14 thereover.
- Fins 33 extending downwardly from surface 35 of base 32 facilitate the condensation and drainage of the condensed working fluid 38 within evaporator 20 .
- the close proximity of fins 33 to base plate 26 and the pool of working fluid 38 permits a very small temperature differential between the two plate, since the buoyancy force required to maintain the boiling-condensation loop for height “h” 37 is very small.
- the condensation loop within tube 44 requires a higher thermal potential.
- the combination of the low thermal potential of condensation within the evaporator by fins 33 and vertical fins 47 and therefore a reduced volume of evaporated and condensed fluid flowing within tubes 44 , permit a smaller height L of condenser 22 than typical thermosiphons that do not employ fins 33 .
- condensation fins 33 within evaporator 20 helps to enhance the design and performance of condenser 22 .
- the condensation induced by fins 33 and vertical fins 47 reduces the vapor loading in tube 44 , since some of the vapor is condensed on fins 33 .
- splitting the total vapor load reduces the number of tubes 44 or equivalently using a similar number of tubes but with a shorter length 57 denoted by dimension ‘L’.
- the shorter length of tube 44 facilitates a compact design.
- the reduced vapor flow rate in tube 44 reduces the accelerating vapor velocity entering tube 44 .
- the reduced flow rate reduces the negative impact of vapor drag on the condensing working fluid 38 draining down the walls of tube 44 .
- vapor drag has the potential to significantly impact condenser performance in an adverse manner by impairing the return of condensed working fluid 38 to chamber 36 especially where there is high wattage and high heat flux.
- a further advantage resulting from the requirement of few tubes 44 is the corresponding few number of joints requiring brazing or fusing to provide a vapor tight environment in thermosiphon 12 .
- a significant advantage resulting from the inclusion of fins 47 is the avoidance of airflow from fan 14 bypassing any of the cooling fins.
- a minimum standoff distance is required to be maintained between base 32 and fins 50 and 52 .
- the required standoff distance results from the difference in thermal mass of base 32 and fins 50 and 52 during the brazing process and is approximately 5-6 millimeters. The standoff distance prevents the first few blades of fins 50 and 52 most proximate to base 32 from melting and collapsing onto each other.
- the standoff distance therefore provides a minimum resistance to the airflow induced by fan 14 and consequently permits a significant volume of the airflow to pass through the condenser without realizing any of the potential heat transfer to the airflow from the structure of condenser 22 .
- Fins 47 therefore obviate the need for a standoff distance by permitting fins 50 and 52 to be bonded to the upper edges of fins 47 and thus eliminate the aforementioned inefficient airflow bypass.
- thermosiphon 12 is a fluid such as demineralized water, methanol or a halocarbon such as R134a (C 2 H 2 F 4 ).
- R134a C 2 H 2 F 4
- both the evaporator and condenser can be fabricated out of aluminum.
- an aluminum evaporator or condenser cannot be used when water is the working fluid in view of the corrosive effect of water on aluminum over time.
- an all-aluminum construction has the benefit of reduced manufacturing costs. Because of its low thermal conductivity, aluminum presents a higher thermal resistance in comparison to copper. Therefore, an evaporator 20 constructed from aluminum is not suitable when the heat flux generated by the electronics device 8 is very high.
- copper is the preferred material of construction for evaporator 20 when the heat flux generated by the electronics device 8 is very high. Copper also has the benefit of usability for both R134a and water based working fluids 38 , while aluminum is generally suitable only for an R-134a working fluid.
- thermosiphon 12 Based on theoretical and experimental study, the following dimensions of thermosiphon 12 were found to be optimal: the ratio of the width 45 of tube 44 to hub diameter 55 of fan 14 is expressed by the relationship 0.08 ⁇ a/H ⁇ 0.25; the ratio of the height 53 of second fins 52 to hub diameter 15 of fan 14 is expressed by the relationship 0.2 ⁇ p/H ⁇ 0.5; the ratio of the height 51 of first fins 50 to diameter 55 of hub 15 of fan 14 is expressed by the relationship 0.15 ⁇ q/H ⁇ 0.375; the ratio of the height 49 of vertical fins 47 to hub diameter 55 of fan 14 is expressed by the relationship 0.2 ⁇ d/H ⁇ 0.375; the thickness t f of fins 47 is expressed by the dimension 0.1 ⁇ t f ⁇ 0.3 mm; and the ratio of the height 37 of evaporating chamber 36 to the height 57 of tubes 44 is expressed by the relationship 0.075 ⁇ h/L ⁇ 0.25.
- the linear fin density of each fin strip ranges from 8 fins per inch to 20 fins per inch.
- Axial fan 14 causes cooling air to flow primarily through vertical fins 47 and convoluted first fins 50 and secondarily through second fins 52 , convectively drawing heat therefrom.
- the vapor is cooled below its condensation temperature and condenses on fins 33 in chamber 36 and on the interior walls of tubes 44 .
- the condensed liquid congregates and with the aid of gravity falls back to the pool of working fluid in vapor chamber 36 whereupon the process is repeated.
- thermosiphon 112 of FIG. 6 another embodiment 112 of a thermosiphon is illustrated wherein like features according to the previous embodiment are identified with like numbers preceded by the numeral “1”.
- thermosiphon 112 of FIG. 6 only the components that differ from the components of thermosiphon 12 of FIG. 2 will be described below since the common components are already described with reference to FIG. 2.
- a single central stem tube 196 has been added between the legs of tube 170 of thermosiphon 112 .
- Tube 170 is similar to tube 44 of thermosiphon 12 .
- the single tube 196 reduces the number of brazing joints and thereby further reduces the potential for leakage of the working fluid from the thermosiphon 112 compared to a thermosiphon having multiple tubes configured as tube 170 since tube 196 has only one inlet 195 extending through base 132 into evaporating chamber 136 .
- Thermosiphon 112 utilizes different tube and different fin sizes.
- the central stem tube 196 has a width 197 denoted by dimension “c” of wider cross-section than tube 170 .
- Central stem tube 196 is placed centrally behind fan hub 115 and directly above the high heat flux region of device 108 .
- Tube 196 is sealed at its top.
- Tube 170 has a width 171 denoted by dimension “b” and is formed to have a substantially flat top over the top of central stem tube 196 .
- First fins 150 at the periphery have a height 151 , denoted by dimension “q”, and are substantially in line with the airflow from fan 114 .
- First fins 150 are generally of the same height or taller than second fins 152 having a height 153 denoted by dimension “p”.
- Thermosiphon 112 is particularly suited for high heat flux and very concentrated heat loads, and where spreading of heat is difficult and the vapor side pressure drop requirement is low. Additionally, central stem tube 196 significantly enhances heat transfer performance of the evaporator as a result of condensate dripping into the liquid pool 138 directly over the center of device 108 . This improves the performance of the boiling surface at very high heat flux.
- thermosiphon 112 For the embodiment illustrated in FIG. 6 as thermosiphon 112 , and through careful design and test iterations, it was established that the benefits of the present embodiment are best realized within the following ranges of the key dimensions.
- the ratio of the width 197 of tube 196 to hub diameter 155 of fan 114 is expressed by the relationship 0.08 ⁇ c/H ⁇ 0.35.
- the width 171 of tube 170 to hub diameter 155 is expressed by the relationship 0.125 ⁇ b/H ⁇ 0.3.
- the ratio of the height 153 of second fins 152 to hub diameter 155 is expressed by the relationship 0.08 ⁇ p/H ⁇ 0.3.
- the ratio of the height 151 of first fins 150 to diameter 155 of hub 115 is expressed by the relationship 0.2 ⁇ q/H ⁇ 0.4.
- the ratio of the height 149 of vertical fins 147 to hub diameter 155 of fan 114 is expressed by the relationship 0.2 ⁇ d/H ⁇ 0.4.
- the thickness t f of fins 147 is expressed by the dimension 0.1 ⁇ t f ⁇ 0.3 mm.
- the ratio of the height 137 of evaporating chamber 136 to the height 157 of wide tube 170 is expressed by the relationship 0.075 ⁇ h/L ⁇ 0.25.
- the linear fin density of each fin strip ranges from 8 fins per inch to 20 fins per inch.
- thermosiphon 212 of FIG. 7 another embodiment 212 of a thermosiphon is illustrated wherein like features according to the previous embodiment are identified with like numbers preceded by the numeral “2”.
- thermosiphon 212 of FIG. 7 only the components that differ from the components of previous embodiments will be described below since the common components are already described with reference to previous embodiments.
- a hairpin tube 244 has a width 245 denoted by dimension “a” and is bent to a small radius 248 denoted by dimension “R”. Hairpin tube 244 is placed substantially above the highest heat flux q′′ region (the center) of device 208 . The intervening space between the innermost tube segments of hairpin tube 244 is filled with third convoluted fins 272 having a height 273 denoted by dimension “n”. Wide tube 270 has a height slightly greater than hairpin tube 244 and is formed to envelop the hairpin tube 244 within its inverted U-shape.
- Tube 270 extends through base 232 such that an interior of tube 270 is in fluid communication with vapor chamber 236 through either end 269 .
- Wide tube 270 has a width 271 denoted by dimension “b” which is generally larger, and thus less restrictive, than width 245 of tube 244 .
- Second fins 252 having a height 253 denoted by the dimension “p” extend between adjacent legs of tubes 244 and 270 . Enveloping the tube 244 by tube 270 in this fashion helps to maintain structural integrity at high internal pressure and also facilitates manufacturing.
- third convoluted fin 272 having a height 273 and tube 244 having a small bend radius 248 wide tube 270 can be kept relatively close to device 208 .
- the top of wide tube 270 can also be angled from the horizontal to prevent condensate build up and thus, always ensure the condensate return from the top of tube 270 to the chamber 236 .
- the size of the hairpin tube 244 having bend radius 245 and the short height 273 of fins 272 is selected specifically to utilize the low airflow in the region of hub 215 . Strategic placement of wide tube 270 on the outside of tube 244 , but within the width 255 of fan hub 215 , enables heat dissipation through first fins 250 .
- First fins 250 are bonded to wide tube 270 and shroud 234 and are positioned in the wake of the fan blades of fan 214 , therefore ensuring good airflow and lower overall airside pressure drop. In this fashion, fins 250 are placed in the periphery of thermosiphon 212 and are utilized to dissipate the majority of the latent heat from the vapor carried by tube 270 .
- the condenser 222 employs a convoluted fin 247 in lieu of the integral fins of previous embodiments.
- Convoluted fin 247 is oriented at right angles to fins 250 , 252 , and 270 and is bonded to the top surface of base 232 of condenser 222 .
- Those practiced in the art will realize that all embodiments can optionally include either the integrally formed fin 47 and 147 or the convoluted fins 247 shown in FIG. 7 and perform the same operational function.
- thermosiphon 212 For the embodiment illustrated in FIG. 7 as thermosiphon 212 , and through careful design and test iterations, it was established that the benefits of the present embodiment are best realized within the following ranges of the key dimensions:
- the ratio of the width 245 of tube 244 to hub diameter 255 of fan 214 is expressed by the relationship 0.08 ⁇ a/H ⁇ 0.25.
- the width of wide tube 270 to hub diameter 255 is expressed by the relationship 0.08 ⁇ b/H ⁇ 0.3.
- the ratio of the height 253 of third fins 252 to hub diameter 255 of fan 214 is expressed by the relationship 0.1 ⁇ n/H ⁇ 0.3.
- the ratio of the height 251 of first fins 250 to diameter 255 of hub 215 is expressed by the relationship 0.1 ⁇ q/H ⁇ 0.4.
- the ratio of the height 253 of second fins 252 to diameter 255 of hub 215 is expressed by the relationship 0.2 ⁇ p/H ⁇ 0.3.
- the ratio of the height 249 of vertical fins 247 to hub diameter 255 of fan 214 is expressed by the relationship 0.075 ⁇ d/H ⁇ 0.375.
- the ratio of the height 237 of evaporating chamber 236 to the height 257 of tubes 244 is expressed by the relationship 0.075 ⁇ h/L ⁇ 0.25.
- the linear fin density of each fin strip ranges from 8 fins per inch to 20 fins per inch.
- thermosiphon 312 of FIG. 8 another embodiment 312 of a thermosiphon is illustrated wherein like features according to the previous embodiment are identified with like numbers preceded by the numeral “3”.
- thermosiphon 312 of FIG. 8 only the components that differ from the components of previous embodiments will be described below since the common components are already described with reference to previous embodiments.
- Condenser 322 comprises base 332 and two hairpin condenser tubes 344 .
- Hairpin tubes 344 are formed in an inverted “U” shape wherein each leg thereof has a respective inlet end 343 extending through base 332 into evaporation chamber 336 .
- Inlet ends 343 are open and place an interior of tubes 344 in fluid communication with evaporation chamber 336 . In this manner, working fluid vapor formed as a result of boiling on the boiling surface 331 can enter either end of hairpin tubes 344 and rise therein for the ultimate dissipation of heat.
- Each of hairpin tubes 344 has a width 345 denoted by the dimension “a”; a bend radius 348 at an upper end thereof denoted by the dimension “R”; and is positioned above the area of high heat flux q′′ of device 308 .
- Radius 48 (R) is selected such that tubes 344 and their respective legs form a tube grouping behind fan hub 315 within hub 315 diameter 355 as denoted by dimension “H”.
- hairpin tubes 344 reside in the wake of hub 315 in the middle of the thermosiphon 312 , and serve primarily as conduits for vapor flow between the evaporator 320 and the condenser 322 .
- Tubes 344 have a minimal lateral tube spacing 346 denoted by dimension “e”.
- condenser 322 employs convoluted fins 347 in lieu of the integral fins of the embodiments of FIGS. 2 and 6, which are bonded to the top surface of base 232 of condenser 222 .
- First fins 350 having a height 351 are intentionally placed outside of tubes 344 and are substantially inline with the fan blades of axial fan 314 where the airflow is high.
- Second fins 352 having a height 353 are placed in the low flow region directly behind the hub 15 between the legs of each hairpin tube 344 .
- Third fins 360 having a height 346 are bonded to the facing sides of laterally spaced tubes 344 .
- Fins 350 , 352 , and 360 are generally of a convoluted accordion configuration and have their apexes bonded to the surface of tubes 344 or housing 334 they contact. Housing 334 encases tubes 344 , first fins 350 , second fins 352 , and third fins 360 to direct and maintain the airflow from fan 314 thereover.
- thermosiphon 312 For the embodiment illustrated in FIG. 8 as thermosiphon 312 , and through careful design and test iterations, it was established that the benefits of the present embodiment are best realized within the following ranges of the key dimensions.
- the ratio of the width 345 of tube 344 to hub diameter 355 of fan 314 is expressed by the relationship 0.125 ⁇ a/H ⁇ 0.3.
- the ratio of the height 353 of second fins 352 to hub diameter 315 of fan 314 is expressed by the relationship 0.1 ⁇ p/H ⁇ 0.325.
- the ratio of the height 351 of first fins 350 to diameter 355 of hub 315 of fan 314 is expressed by the relationship 0.08 ⁇ q/H ⁇ 0.3.
- the ratio of the height 349 of vertical fins 347 to hub diameter 355 of fan 314 is expressed by the relationship 0.2 ⁇ d/H ⁇ 0.375.
- the ratio of the height 337 of evaporating chamber 336 to the height 357 of tubes 344 is expressed by the relationship 0.1 ⁇ h/L ⁇ 0.25.
- the linear fin density of each fin strip ranges from 8 fins per inch to 20 fins per inch.
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Abstract
Description
- The present invention relates to heat sinks in general, and more particularly to heat sinks for use in dissipating waste heat generated by electrical or electronic components and assemblies.
- Research activities have focused on developing heat sinks to efficiently dissipate heat from highly concentrated heat sources such as microprocessors and computer chips. These heat sources typically have power densities in the range of about 5 to 35 W/cm2 (4 to 31 Btu/ft2s) and relatively small available space for placement of fans, heat exchangers, heat sinks and the like.
- At the component level, various types of heat exchangers and heat sinks have been used that apply natural or forced convection or other cooling methods. The most commonly existing heat sinks for microelectronics cooling have generally used air to directly remove heat from the heat source. However, air has a relatively low heat capacity. Such heat sinks are suitable for removing heat from relatively low power heat sources with power density in the range of 5 to 15 W/cm2 (4 to 13 Btu/ft2s). Increases in computing speed resulted in corresponding increases in the power density of the heat sources in the order of 20 to 35 W/cm2 (18 to 31 Btu/ft2s) thus requiring more effective heat sinks. Liquid-cooled heat sinks employing high heat capacity fluids like water and water-glycol solutions are more particularly suited to remove heat from these types of high power density heat sources. One type of liquid cooled heat sink circulates the cooling liquid so that the liquid removes heat from the heat source and is then transferred to a remote location where the heat is easily dissipated into a flowing air stream with the use of a liquid-to-air heat exchanger. These types of heat sinks are characterized as indirect heat sinks.
- As computing speeds continue to increase even more dramatically, the corresponding power densities of the devices rise up to 100 W/cm2. The constraints of the necessary cooling system miniaturization coupled with high heat flux calls for extremely efficient, compact, simple and reliable heat sinks such as a thermosiphon. A typical thermosiphon comprises an evaporating section and a condensing section. The heat-generating device is mounted to the evaporating section. In some thermosiphons, the heat-generating device is affixed to the internal surface of the evaporating section where it is submerged in the working fluid. Alternatively, the heat-generating device can also be affixed to the external surface of the evaporating section. The working fluid of the thermosiphon is generally a halocarbon fluid, which circulates in a closed-loop fashion between the evaporating and condensing sections. The captive working fluid changes its state from liquid-to-vapor in the evaporating section as it absorbs heat from the heat-generating device. Reverse transformation of the working fluid from vapor-to-liquid occurs as it rejects heat to a cooling fluid like air flowing on an external finned surface of the condensing section. The thermosiphon relies exclusively on gravity for the motion of the working fluid between the evaporating and condensing sections. As for the motion of the cooling fluid on the external surface of the condensing section, a fluid moving device like an axial fan is employed.
- Most electronics devices have high degree of non-uniformity built into them. Thermal management of these devices is subject to two constraints that the thermal engineer must address. First, the heat flux generated by the electronics device is highly non-uniform. Second, the air circulated by the air-moving device like an axial fan is very non-uniformly distributed. Most computer chips have their heat generation concentrated in a very small region in the core of the chip. For example, a typical 40×40 mm2 computer chip has almost 80% of its total heat flux concentrated in its central 10×10 mm2 surface. The heat flux distribution in a typical electronics device is shown schematically in FIG. 4. The second non-uniformity is attributed to the attachment of the air-moving device like an axial fan attached to the exterior of the thermosiphon. Axial fans generally have a large hub which acts as blockage to airflow. The airflow entering and exiting from the axial fan is highly concentrated in the peripheral region of the fan blades. Typical airflow exit and entry velocity profiles are shown in FIGS. 5a and 5 b respectively. The maximum air velocity is in the tip region of fan blades. The velocity falls off sharply and approches zero in the central hub region. Under certain flow conditions and blade angle, the local velocity at the root of the fan blade may even become negative, i.e., opposite to the direction of the predominant airflow.
- The non-uniformity of airflow is far more pronounced in push mode (FIG. 5a) wherein the fan blows relatively cooler ambient air into the heat exchanger. In pull mode (FIG. 5b), on the other hand, the fan sucks relatively hotter air from the heat exchanger. For a high heat load push mode is advantageous when airflow rate is low. In order to attain flatter airflow profile entering the heat exchanger face a standoff distance of at least three times the hub diameter is preferable between the fan and the heat exchanger. However, because of packaging constraints only about one-fifth to one-quarter of the hub diameter standoff distance is typically available between the fan and heat exchanger. This is because the airflow at the heat exchanger face is non-uniform.
- A limitation of the axial fan relating to smallness of the pressure rise across the fan needs to be borne in mind. The curve of the pressure head developed by the fan falls off very rapidly as the volumetric flow rate of air increases. In other words, the air exiting an axial fan cannot sustain a high-pressure drop through the fins. Therefore, managing the airflow through the heat sink at a low-pressure drop is a very important consideration in the design of a thermosiphon.
- It is apparent from the foregoing considerations that from a system's point of view, the computer chip, heat sink and fan assembly are constrained not only by very non-uniform heat flux but also by non-uniform airflow capable of sustaining small pressure drop across the heat exchanger. Ideally, the airflow should be high in regions of high heat flux and low in regions of low heat flux. Overlaying FIGS. 4 and 5 in push mode clearly reveals that the airflow distribution is opposite to that ideally desired for better heat transfer. This is detrimental to the functioning of a computer chip, as the chip junction temperature becomes high because of inadequate heat removal locally from the core of the chip. The thermal performance penalty attributed to these non-uniformities can be of the order of 25 to 50% compared to the case with uniform heat flux and uniform airflow. Thus thermal solution becomes considerably more challenging when the heat flux as well as the airflow is non-uniform. The difficulty is compounded when the available airflow rate is small. Therefore, careful attention must be paid to the fluid flow and heat transfer boundary conditions when developing the thermal solutions for the computer chips.
- The compact thermosiphons intended to fit in a computer case require boiling and condensing processes to occur in close proximity to each other thereby imposing conflicting thermal conditions in a relatively small volume. This poses significant challenges to the process of optimizing the thermosiphon performance.
- Thus, what is desired is a thermosiphon optimization process to intensify the processes of boiling, condensation and convective heat transfer at the external surface of the condenser while maintaining low airside pressure drop.
- One aspect of the present invention is a heat sink assembly for cooling an electronic device. The heat sink assembly comprises a fan housed in a shroud, the fan having a hub and fan blades extending therefrom for causing an axially directed airflow through the shroud upon rotation of the fan blades. A thermosiphon is positioned at one end of the shroud such that the fan is aligned with the condenser for directing the axial airflow therethrough. The thermosiphon comprises an evaporator defining an evaporating chamber containing a working fluid therein and further including a condenser mounted thereabove. The condenser includes a base having an upper surface and a plurality of fins extending substantially upwardly from the upper surface. The condenser also includes a plurality of tubes forming a tube grouping. Each tube having an opening in fluid communication with the evaporator and for receiving and condensing vapor of the working fluid received from the evaporator. The tubes are axially aligned with the airflow and are laterally positioned such that a lateral width of the tube grouping is approximately equal to a width of the hub and substantially in lateral alignment thereto.
- Another aspect of the present invention is a condenser for a heat sink assembly for cooling an electronic device. The condenser comprises a base having an upper housing affixed thereto wherein the upper housing has open ends. The base further includes a plurality of fins extending substantially upwardly from an upper surface of the base and within the upper housing. A fan is mounted at one of the open ends, the fan having a hub and fan blades extending therefrom for causing an axially directed airflow through the housing upon rotation of the fan blades. A plurality of tubes is positioned within the housing for transmitting therethrough a vapor of a working fluid. The tubes define a tube grouping such that the tubes are arranged in axial alignment with the fan and laterally positioned such that a lateral width of the tube grouping is approximately equal to a width of the hub and substantially in lateral alignment thereto.
- These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings.
- FIG. 1 is a perspective view of a heat sink assembly embodying the present invention, wherein an axial fan is arranged to draw cooling air through a thermosiphon.
- FIG. 2 is an elevational cross-section view of an embodiment of the thermosiphon shown in FIG. 1 and taken along the line2-2
- FIG. 3 is an enlarged segment of the cross-sectional view of the boilerplate shown in FIG. 2.
- FIG. 4 is a typical heat flux distribution of an electronic device requiring cooling.
- FIG. 5A is a typical air velocity distribution just downstream of the axial fan in push mode used in conjunction with a thermosiphon.
- FIG. 5B is a typical air velocity distribution just upstream of the axial fan in pull mode used in conjunction with a thermosiphon.
- FIG. 6 is an elevational cross-section view of a second embodiment thermosiphon.
- FIG. 7 is an elevational cross-section view of a third embodiment thermosiphon.
- FIG. 8 is an elevational cross-section view of a fourth embodiment thermosiphon.
- For purposes of description herein, the terms “upper”, “lower”, “left”, “rear”, “right”, “front”, “vertical”, “horizontal”, and derivatives thereof shall relate to the invention as oriented in FIG. 2. However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise.
- Turning to the drawings, FIG. 1 shows an air-cooled
thermosiphon heat sink 10, which is one of the preferred embodiments of the present invention and illustrates its various components. - As illustrated in FIG. 1 a single
axial fan 14 is housed inshroud 16 and coupled tothermosiphon 12 throughduct 18. Thefan 14 could be a pull or push type fan, however, a pull type of fan is preferred to minimize shadowing of thethermosiphon 12 by thefan hub 15. The shadowing effect ofhub 15 occurs over a lateral width 55 denoted by dimension “H” and substantially at a center ofthermosiphon 12. The shadowing effect ofhub 15 is greater with a push type fan than a pull type fan and reduces the airflow behind the hub and thereby interferes with the heat transfer fromthermosiphon 12 to the cooling air stream. - Although
axial fan 14 is configured as a pull type fan, and thereby minimizes the shadowing effect, FIGS. 4, 5A and 5B illustrate the differences between the heat distribution of thedevice 8 to be cooled and the areas of maximum airflow offan 14. As shown in FIG. 4, the heat distribution ofdevice 8 approximates a bell curve with the greatest heat at the area above the center ofdevice 8. Conversely, the area of maximum airflow in push mode, as illustrated in FIG. 5A, appears as an inverse of the heat distribution, namely minimal airflow in the middle and maximum airflow at the outmost portion of the fan. In like manner, FIG. 5B illustrates the airflow in pull mode being similar to the push mode airflow illustrated in FIG. 5A. Therefore, without any enhancements, the fan generates maximum airflow over the minimum heat regions and low airflow over the regions of maximum heat. - FIG. 2 shows a sectional view of a preferred embodiment of the
thermosiphon 12.Thermosiphon 12 comprises anevaporator 20 and acondenser 22 mounted thereabove. - The
evaporator 20 comprises abaseplate 26 having athickness 25 denoted by dimension “t” and sidewalls 24 about a periphery ofbaseplate 26. Thethickness 25 “t” of theevaporator base plate 26 is suitably chosen based on an analysis of the particular boiling and heat transfer considerations for a desired application.Electronic device 8 having amean width 9 denoted by dimension “z” is attached to abottom surface 27 ofbaseplate 26 using a heat conductive adhesive, also known as “thermal grease”.Bottom surface 27 is preferably polished for attachment ofelectronic device 8 to enhance the thermal contact fromdevice 8 tobaseplate 26. - An upper surface of
baseplate 26 defines boilingsurface 31 and can have a plurality ofstud fins 28 formed thereon.Stud fins 28 are preferably machined as an integral part ofbaseplate 26 for maximum heat transfer. As illustrated in FIG. 3, the boilingsurface 31 ofbaseplate 26 can also have asurface coating 30 deposited thereon to enhance the boiling properties of boilingsurface 31.Surface coating 30 can comprise a sintered metal powder of aluminum or copper. - Sidewalls24 have a
height 37 denoted by dimension “h” and have a bottom affixed tobaseplate 26.Sidewalls 24 can also be integrally formed withbase 26 as a single structure to minimize the number of joints requiring a fluid seal. An upper surface ofsidewalls 24 defines an upperhorizontal flange 29 about the periphery ofevaporator 20 to which thebase 32 ofcondenser 22 is attached thereby defining evaporatingchamber 36. The height of evaporatingchamber 36 is also represented by dimension “h”.Base 32 is preferably affixed toflange 29 by one of brazing, welding or diffusion bonding to form a leak-proof chamber 36. The flange joint between the sidewalls 24 andbase 32 can be enhanced by means of a trunion groove type mating of the protruding and recessed side of the flange prior to brazing or welding. A good joint can also be enhanced by means of peripheral screws (not shown)fastening base 32 to sidewalls 24. The screws provide additional reinforcement and prevent leakage at high pressure.Evaporation chamber 36 is charged with a workingfluid 38 through chargingport 40 inbase 32.Chamber 36 also functions as a manifold to distribute saturated or super-heated vapor into thehairpin condenser tubes 44. -
Condenser 22 comprisesbase 32 andhousing 34 mounted thereon.Housing 34 is open at both ends when viewed axially with respect tofan 14 thereby permitting the airflow induced byfan 14 to flow therethrough.Base 32 includes a plurality of condensingfins 33 extending downwardly frombottom surface 35 intoevaporation chamber 36. A plurality ofvertical fins 47 having avertical dimension 49, denoted by “d”, extend upwardly frombase 32 withinhousing 34.Vertical fins 47 are parallel one to the other and are axially aligned with the airflow induced byfan 14 to permit the flow of air between adjacent ones offins 47. Preferably,fins 47 are integrally formed uponbase 32 such as by machining, forging, or extrusion methods known in the art. The height “d”, thickness and linear fin density (fin spacing) are determined based upon such factors as fin efficiency and with consideration to the pressure drop of the airflow induced byfan 14.Fins 47 are advantageously positioned in the high airflow region offan 14 in the area approximately belowfan hub 15 and therefore serve to dissipate heat extremely well to the passing airflow. - A
condenser tube 44 is formed in an inverted “U” shape wherein each leg thereof has arespective inlet end 43 extending throughbase 32 intoevaporation chamber 36. Inlet ends 43 are open and place an interior oftubes 44 in fluid communication withevaporation chamber 36. In this manner, working fluid vapor formed as a result of boiling on the boilingsurface 31 can enter either end 43 ofhairpin tube 44 and rise therein for the ultimate dissipation of heat.Hairpin tube 44 has awidth 45 denoted by the dimension “a” and is positioned above the area of high heat flux q″ ofdevice 8.Tube 44 is formed such that its respective legs form a tube grouping behindfan hub 15 within hub diameter 55 as denoted by dimension “H”. Thus,hairpin tube 44 resides in the wake ofhub 15 in the middle of thethermosiphon 12, and serves primarily as a conduit for vapor flow between the evaporator 20 and thecondenser 22. - Two additional types of fins are used in
condenser 22 ofthermosiphon 12.First fins 50 having aheight 51, denoted by dimension “q”, are placed outside oftube 44 and are substantially inline with the fan blades ofaxial fan 14 where the airflow is high.Second fins 52 having aheight 53, denoted by dimension “p”, are placed in the low flow region directly behind thehub 15 between the legs ofhairpin tube 44.Fins tubes 44 orhousing 34 to which they contact in the area abovefins 47.Housing 34 encasestubes 44,vertical fins 47,first fins 50 andsecond fins 52 to direct and maintain the airflow fromfan 14 thereover. -
Fins 33 extending downwardly fromsurface 35 ofbase 32 facilitate the condensation and drainage of the condensed workingfluid 38 withinevaporator 20. The close proximity offins 33 tobase plate 26 and the pool of workingfluid 38 permits a very small temperature differential between the two plate, since the buoyancy force required to maintain the boiling-condensation loop for height “h” 37 is very small. The condensation loop withintube 44 requires a higher thermal potential. The combination of the low thermal potential of condensation within the evaporator byfins 33 andvertical fins 47, and therefore a reduced volume of evaporated and condensed fluid flowing withintubes 44, permit a smaller height L ofcondenser 22 than typical thermosiphons that do not employfins 33. - The addition of
condensation fins 33 withinevaporator 20 to improve condensation of workingfluid 38 helps to enhance the design and performance ofcondenser 22. The condensation induced byfins 33 andvertical fins 47 reduces the vapor loading intube 44, since some of the vapor is condensed onfins 33. Thus, splitting the total vapor load reduces the number oftubes 44 or equivalently using a similar number of tubes but with a shorter length 57 denoted by dimension ‘L’. The shorter length oftube 44 facilitates a compact design. Additionally, the reduced vapor flow rate intube 44 reduces the accelerating vaporvelocity entering tube 44. The reduced flow rate reduces the negative impact of vapor drag on thecondensing working fluid 38 draining down the walls oftube 44. Fortubes 44 having a thin flat configuration, vapor drag has the potential to significantly impact condenser performance in an adverse manner by impairing the return of condensed workingfluid 38 tochamber 36 especially where there is high wattage and high heat flux. A further advantage resulting from the requirement offew tubes 44 is the corresponding few number of joints requiring brazing or fusing to provide a vapor tight environment inthermosiphon 12. - A significant advantage resulting from the inclusion of
fins 47 is the avoidance of airflow fromfan 14 bypassing any of the cooling fins. In thermosiphon applications whereinfins 47 are eliminated andfins tube 44 along substantially the entire length 57 denoted by dimension ‘L’ ofcondenser 22, for manufacturing considerations, a minimum standoff distance is required to be maintained betweenbase 32 andfins base 32 andfins fins fan 14 and consequently permits a significant volume of the airflow to pass through the condenser without realizing any of the potential heat transfer to the airflow from the structure ofcondenser 22.Fins 47 therefore obviate the need for a standoff distance by permittingfins fins 47 and thus eliminate the aforementioned inefficient airflow bypass. - The preferred working fluid of
thermosiphon 12 is a fluid such as demineralized water, methanol or a halocarbon such as R134a (C2H2F4). For athermosiphon 12 utilizing R134a as workingfluid 38, both the evaporator and condenser can be fabricated out of aluminum. However, an aluminum evaporator or condenser cannot be used when water is the working fluid in view of the corrosive effect of water on aluminum over time. However, an all-aluminum construction has the benefit of reduced manufacturing costs. Because of its low thermal conductivity, aluminum presents a higher thermal resistance in comparison to copper. Therefore, anevaporator 20 constructed from aluminum is not suitable when the heat flux generated by theelectronics device 8 is very high. Therefore, copper is the preferred material of construction forevaporator 20 when the heat flux generated by theelectronics device 8 is very high. Copper also has the benefit of usability for both R134a and water based workingfluids 38, while aluminum is generally suitable only for an R-134a working fluid. - Based on theoretical and experimental study, the following dimensions of
thermosiphon 12 were found to be optimal: the ratio of thewidth 45 oftube 44 to hub diameter 55 offan 14 is expressed by the relationship 0.08≦a/H≦0.25; the ratio of theheight 53 ofsecond fins 52 tohub diameter 15 offan 14 is expressed by the relationship 0.2≦p/H≦0.5; the ratio of theheight 51 offirst fins 50 to diameter 55 ofhub 15 offan 14 is expressed by the relationship 0.15≦q/H≦0.375; the ratio of theheight 49 ofvertical fins 47 to hub diameter 55 offan 14 is expressed by the relationship 0.2≦d/H≦0.375; the thickness tf offins 47 is expressed by the dimension 0.1≦tf≦0.3 mm; and the ratio of theheight 37 of evaporatingchamber 36 to the height 57 oftubes 44 is expressed by the relationship 0.075≦h/L≦0.25. The linear fin density of each fin strip ranges from 8 fins per inch to 20 fins per inch. - In use, as
device 8 generates power and thus, heat, the heat so generated is transferred tobaseplate 26. As baseplate and especiallyfins 28 increase in temperature,surface 30 becomes sufficiently hot to cause the working liquid covering thebaseplate 26 to nucleate or boil. The working fluid vapor rises wherein a portion of thevapor contacts fins 33 andupper surface 35 ofbase 32 and a portion entershairpin condenser tube 44. Withintube 44, the heated vapor contacts the sidewalls oftube 44 and transfers the thermal energy in the vapor to the walls oftubes 44 and thereafter by conduction toconvoluted fins Axial fan 14 causes cooling air to flow primarily throughvertical fins 47 and convolutedfirst fins 50 and secondarily throughsecond fins 52, convectively drawing heat therefrom. By removing thermal energy from the vapor, the vapor is cooled below its condensation temperature and condenses onfins 33 inchamber 36 and on the interior walls oftubes 44. The condensed liquid congregates and with the aid of gravity falls back to the pool of working fluid invapor chamber 36 whereupon the process is repeated. - Turning now to FIG. 6 another
embodiment 112 of a thermosiphon is illustrated wherein like features according to the previous embodiment are identified with like numbers preceded by the numeral “1”. In describingthermosiphon 112 of FIG. 6, only the components that differ from the components ofthermosiphon 12 of FIG. 2 will be described below since the common components are already described with reference to FIG. 2. - As illustrated in FIG. 6 a single
central stem tube 196 has been added between the legs oftube 170 ofthermosiphon 112.Tube 170 is similar totube 44 ofthermosiphon 12. Thesingle tube 196 reduces the number of brazing joints and thereby further reduces the potential for leakage of the working fluid from thethermosiphon 112 compared to a thermosiphon having multiple tubes configured astube 170 sincetube 196 has only oneinlet 195 extending throughbase 132 into evaporatingchamber 136.Thermosiphon 112 utilizes different tube and different fin sizes. Thecentral stem tube 196 has awidth 197 denoted by dimension “c” of wider cross-section thantube 170.Central stem tube 196 is placed centrally behindfan hub 115 and directly above the high heat flux region ofdevice 108.Tube 196 is sealed at its top.Tube 170 has awidth 171 denoted by dimension “b” and is formed to have a substantially flat top over the top ofcentral stem tube 196.First fins 150 at the periphery have aheight 151, denoted by dimension “q”, and are substantially in line with the airflow fromfan 114.First fins 150 are generally of the same height or taller thansecond fins 152 having aheight 153 denoted by dimension “p”. -
Thermosiphon 112 is particularly suited for high heat flux and very concentrated heat loads, and where spreading of heat is difficult and the vapor side pressure drop requirement is low. Additionally,central stem tube 196 significantly enhances heat transfer performance of the evaporator as a result of condensate dripping into theliquid pool 138 directly over the center ofdevice 108. This improves the performance of the boiling surface at very high heat flux. - For the embodiment illustrated in FIG. 6 as
thermosiphon 112, and through careful design and test iterations, it was established that the benefits of the present embodiment are best realized within the following ranges of the key dimensions. The ratio of thewidth 197 oftube 196 tohub diameter 155 offan 114 is expressed by the relationship 0.08≦c/H≦0.35. Thewidth 171 oftube 170 tohub diameter 155 is expressed by the relationship 0.125≦b/H≦0.3. The ratio of theheight 153 ofsecond fins 152 tohub diameter 155 is expressed by the relationship 0.08≦p/H≦0.3. The ratio of theheight 151 offirst fins 150 todiameter 155 ofhub 115 is expressed by the relationship 0.2≦q/H≦0.4. The ratio of theheight 149 ofvertical fins 147 tohub diameter 155 offan 114 is expressed by the relationship 0.2≦d/H≦0.4. The thickness tf offins 147 is expressed by the dimension 0.1≦tf≦0.3 mm. The ratio of theheight 137 of evaporatingchamber 136 to theheight 157 ofwide tube 170 is expressed by the relationship 0.075≦h/L≦0.25. The linear fin density of each fin strip ranges from 8 fins per inch to 20 fins per inch. - Turning now to FIG. 7 another
embodiment 212 of a thermosiphon is illustrated wherein like features according to the previous embodiment are identified with like numbers preceded by the numeral “2”. In describingthermosiphon 212 of FIG. 7, only the components that differ from the components of previous embodiments will be described below since the common components are already described with reference to previous embodiments. - In the embodiment of FIG. 7, two different tube heights are used in order to utilize the region shadowed by
fan hub 215 for vapor flow. Ahairpin tube 244 has awidth 245 denoted by dimension “a” and is bent to asmall radius 248 denoted by dimension “R”.Hairpin tube 244 is placed substantially above the highest heat flux q″ region (the center) ofdevice 208. The intervening space between the innermost tube segments ofhairpin tube 244 is filled with thirdconvoluted fins 272 having aheight 273 denoted by dimension “n”.Wide tube 270 has a height slightly greater thanhairpin tube 244 and is formed to envelop thehairpin tube 244 within its inverted U-shape.Ends 269 oftube 270 extend throughbase 232 such that an interior oftube 270 is in fluid communication withvapor chamber 236 through eitherend 269.Wide tube 270 has awidth 271 denoted by dimension “b” which is generally larger, and thus less restrictive, thanwidth 245 oftube 244.Second fins 252 having aheight 253 denoted by the dimension “p” extend between adjacent legs oftubes tube 244 bytube 270 in this fashion helps to maintain structural integrity at high internal pressure and also facilitates manufacturing. - By selecting third
convoluted fin 272 having aheight 273 andtube 244 having asmall bend radius 248,wide tube 270 can be kept relatively close todevice 208. The top ofwide tube 270 can also be angled from the horizontal to prevent condensate build up and thus, always ensure the condensate return from the top oftube 270 to thechamber 236. The size of thehairpin tube 244 havingbend radius 245 and theshort height 273 offins 272 is selected specifically to utilize the low airflow in the region ofhub 215. Strategic placement ofwide tube 270 on the outside oftube 244, but within thewidth 255 offan hub 215, enables heat dissipation throughfirst fins 250. The majority of the vapor generated invapor chamber 238 flows through the less restrictivewide tube 270 with larger cross-section and hence with lower flow resistance.First fins 250 are bonded towide tube 270 andshroud 234 and are positioned in the wake of the fan blades offan 214, therefore ensuring good airflow and lower overall airside pressure drop. In this fashion,fins 250 are placed in the periphery ofthermosiphon 212 and are utilized to dissipate the majority of the latent heat from the vapor carried bytube 270. - The
condenser 222 employs aconvoluted fin 247 in lieu of the integral fins of previous embodiments.Convoluted fin 247 is oriented at right angles tofins base 232 ofcondenser 222. Further, as in previous embodiments, there is no requirement for a standoff betweenfin 247 andfins fin convoluted fins 247 shown in FIG. 7 and perform the same operational function. - For the embodiment illustrated in FIG. 7 as
thermosiphon 212, and through careful design and test iterations, it was established that the benefits of the present embodiment are best realized within the following ranges of the key dimensions: The ratio of thewidth 245 oftube 244 tohub diameter 255 offan 214 is expressed by the relationship 0.08≦a/H≦0.25. The width ofwide tube 270 tohub diameter 255 is expressed by the relationship 0.08≦b/H≦0.3. The ratio of theheight 253 ofthird fins 252 tohub diameter 255 offan 214 is expressed by the relationship 0.1≦n/H≦0.3. The ratio of theheight 251 offirst fins 250 todiameter 255 ofhub 215 is expressed by the relationship 0.1≦q/H≦0.4. The ratio of theheight 253 ofsecond fins 252 todiameter 255 ofhub 215 is expressed by the relationship 0.2≦p/H≦0.3. The ratio of theheight 249 ofvertical fins 247 tohub diameter 255 offan 214 is expressed by the relationship 0.075≦d/H≦0.375. The ratio of theheight 237 of evaporatingchamber 236 to theheight 257 oftubes 244 is expressed by the relationship 0.075≦h/L≦0.25. The linear fin density of each fin strip ranges from 8 fins per inch to 20 fins per inch. - Turning now to FIG. 8 another
embodiment 312 of a thermosiphon is illustrated wherein like features according to the previous embodiment are identified with like numbers preceded by the numeral “3”. In describingthermosiphon 312 of FIG. 8, only the components that differ from the components of previous embodiments will be described below since the common components are already described with reference to previous embodiments. -
Condenser 322 comprisesbase 332 and twohairpin condenser tubes 344.Hairpin tubes 344 are formed in an inverted “U” shape wherein each leg thereof has arespective inlet end 343 extending throughbase 332 intoevaporation chamber 336. Inlet ends 343 are open and place an interior oftubes 344 in fluid communication withevaporation chamber 336. In this manner, working fluid vapor formed as a result of boiling on the boilingsurface 331 can enter either end ofhairpin tubes 344 and rise therein for the ultimate dissipation of heat. Each ofhairpin tubes 344 has awidth 345 denoted by the dimension “a”; abend radius 348 at an upper end thereof denoted by the dimension “R”; and is positioned above the area of high heat flux q″ ofdevice 308. Radius 48 (R) is selected such thattubes 344 and their respective legs form a tube grouping behindfan hub 315 withinhub 315diameter 355 as denoted by dimension “H”. Thus,hairpin tubes 344 reside in the wake ofhub 315 in the middle of thethermosiphon 312, and serve primarily as conduits for vapor flow between theevaporator 320 and thecondenser 322. -
Tubes 344 have a minimal lateral tube spacing 346 denoted by dimension “e”. The properties ofbase 332, and the minimum distance permissible for forming slots to receive the tube ends therein governtube spacing 346. - As with
condenser 222 in the previous embodiment,condenser 322 employsconvoluted fins 347 in lieu of the integral fins of the embodiments of FIGS. 2 and 6, which are bonded to the top surface ofbase 232 ofcondenser 222. - Three types of fins are positioned above
convoluted fins 347 incondenser 322 ofthermosiphon 312, and are oriented at right angles thereto.First fins 350 having aheight 351, denoted by dimension “q”, are intentionally placed outside oftubes 344 and are substantially inline with the fan blades ofaxial fan 314 where the airflow is high.Second fins 352 having aheight 353, denoted by dimension “p”, are placed in the low flow region directly behind thehub 15 between the legs of eachhairpin tube 344.Third fins 360 having aheight 346, denoted by dimension “e”, are bonded to the facing sides of laterally spacedtubes 344.Fins tubes 344 orhousing 334 they contact.Housing 334 encasestubes 344,first fins 350,second fins 352, andthird fins 360 to direct and maintain the airflow fromfan 314 thereover. - For the embodiment illustrated in FIG. 8 as
thermosiphon 312, and through careful design and test iterations, it was established that the benefits of the present embodiment are best realized within the following ranges of the key dimensions. The ratio of thewidth 345 oftube 344 tohub diameter 355 offan 314 is expressed by the relationship 0.125≦a/H≦0.3. The ratio of theheight 353 ofsecond fins 352 tohub diameter 315 offan 314 is expressed by the relationship 0.1≦p/H≦0.325. The ratio of theheight 351 offirst fins 350 todiameter 355 ofhub 315 offan 314 is expressed by the relationship 0.08≦q/H≦0.3. The ratio of theheight 349 ofvertical fins 347 tohub diameter 355 offan 314 is expressed by the relationship 0.2≦d/H≦0.375. The ratio of theheight 337 of evaporatingchamber 336 to theheight 357 oftubes 344 is expressed by the relationship 0.1≦h/L≦0.25. The linear fin density of each fin strip ranges from 8 fins per inch to 20 fins per inch. - In the foregoing description those skilled in the art will readily appreciate that modifications may be made to the invention without departing from the concepts disclosed herein. Such modifications are to be considered as included in the following claims, unless these claims expressly state otherwise.
Claims (44)
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