Abstract:
A method is provided for heat transfer from a surface to a fluid. The method includes directing a first fluid flow towards the surface in a first direction and directing a second fluid flow towards the surface in a second direction. The first and second fluid flows cooperate to cool the surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/613,327 filed Mar. 20, 2012, the content of which is incorporated herein by reference in its entirety. 
    
    
     FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under contract number W31P4Q-09-C-0028 awarded by the U.S. Army Contracting Command. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     The present invention relates to heat transfer. 
     More specifically, the present invention relates to an apparatus and method for improving heat transfer from a heat source to a fluid flow. The rate of heat transfer from a heat transfer surface, such as a heat sink, to a fluid, such as air, is affected by flow conditions at the surface. Turbulent flow generally results in a higher heat transfer rate than laminar flow. 
     SUMMARY 
     In one embodiment, the invention provides a heat transfer apparatus. A surface exchanges heat from a heat source to a fluid. A first fluid driver drives a first portion of the fluid along the surface in a first direction. A second fluid driver drives a second portion of the fluid along the surface in a second direction. A third fluid driver drives a third portion of the fluid along the surface in a third direction. Each of the first direction, the second direction, and the third direction are substantially non-parallel to one another. 
     In another embodiment, the invention provides a heat transfer apparatus. A first wall of the apparatus has a first base portion, a first end portion, and a first surface extending between the first base portion and the first end portion. A second wall has a second base portion, a second end portion, and a second surface extending between the second base portion and the second end portion. The first surface and the second surface at least partially define a channel for heat exchange with a heat source. The heat source is thermally coupled to the first wall and the second wall, and a fluid. A first fluid driver drives a first portion of the fluid through the channel in a first direction. A second fluid driver drives a second portion of the fluid through the channel in a second direction. A third flow fluid driver driving a third portion of the fluid through the channel in a third direction. Each of the first direction, the second direction, and the third direction are substantially non-parallel. 
     In another embodiment, the invention provides a method for heat transfer from a surface to a fluid. The method includes directing a first fluid flow towards the surface in a first direction and directing a second fluid flow towards the surface in a second direction. The first and second fluid flows cooperate to cool the surface. 
     In another embodiment the invention provides a method for heat transfer from a surface to a fluid. The method includes driving a first portion of the fluid along the surface on a first axis that is substantially parallel to the surface. A second portion of the fluid is agitated with an agitator reciprocating on a second axis that is substantially non-parallel with the first axis. A third portion of the fluid is injected along a third axis that is substantially non-parallel with the first axis and second axis. 
     In another embodiment, the invention provides a heat transfer surface. A substrate has. A plurality of surface modification members are coupled to the surface. The surface modification members include a body structure projecting from the surface. The body structure has a base end and a distal end. The base end is coupled to the substrate and the distal end is wider than the base end. 
     In another embodiment, the invention provides a heat transfer surface. A substrate has a surface. An array of surface modification members are coupled to the surface. The surface modification members include a cylindrical body with a base end and a distal end. The base end is coupled to the substrate. A dome-shaped end-cap is coupled to the distal end. 
     In another embodiment, the invention provides a method of fabricating surface modification members on a substrate. The method includes depositing a titanium layer over the substrate and applying a photoresist over the titanium layer. The photoresist is selectively exposed to cure the selected portions of the photoresist. Uncured portions of the photoresist are removed. Portions of the titanium layer exposed when removing the uncured portions of the photoresist are removed, thereby exposing the substrate in a desired pattern. The exposed substrate is plated to form surface modification members. The remaining portions of photoresist and titanium are removed to exposed the surface modifications members. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a heat transfer apparatus according to one aspect of the invention. 
         FIG. 2  is a perspective view of a heat transfer apparatus according to another aspect of the invention. 
         FIG. 3  is a perspective view of a heat sink of the heat transfer apparatus of  FIG. 2 . 
         FIG. 4  is a perspective view of an agitator assembly of the heat transfer apparatus of  FIG. 2 . 
         FIG. 5  is a perspective view of an agitator actuator of the heat transfer apparatus of  FIG. 2 . 
         FIG. 6  is a side view of the agitator actuator of  FIG. 5 . 
         FIG. 7  is a perspective view of a synthetic jet assembly of the heat transfer apparatus of  FIG. 2 . 
         FIG. 8  is a cross-sectional view of a portion of the synthetic jet assembly of  FIG. 7 . 
         FIG. 9  is a planar view of various nozzle configurations of a synthetic jet assembly. 
         FIG. 10  is a perspective view of a heat transfer apparatus according to another aspect of the invention. 
         FIG. 11  is a detailed cutaway view of a portion of  FIG. 10 . 
         FIG. 12  is a perspective view of a dual heat transfer apparatus according to another aspect of the invention. 
         FIG. 13  is a perspective view of a heat transfer apparatus according to another aspect of the invention. 
         FIG. 14  is an exploded view of the heat transfer apparatus of  FIG. 13 . 
         FIG. 15  is a cross-sectional view of a heat transfer apparatus according to another aspect of the invention. 
         FIG. 16  is a detailed view of a portion of  FIG. 15 . 
         FIG. 17  is a cross-sectional view of a heat transfer apparatus according to another aspect of the invention. 
         FIG. 18  is a detailed perspective view of a portion of  FIG. 3 . 
         FIG. 19  is a detailed view of a portion of  FIG. 11 . 
         FIG. 20  is a perspective view of an array of surface modification members according to one aspect of the invention. 
         FIG. 21  is a perspective view of an array of surface modification members according to another aspect of the invention. 
         FIG. 22  illustrates a process for manufacturing surface modification members on a substrate. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
     In various embodiments, the invention includes methods and apparatus for improving heat transfer from a surface. The methods and apparatus include modifications of the surface as well as the use of multiple directions of fluid flaw in a cooperating manner to improve heat transfer. Without being limited as to theory, the methods and apparatus disclosed herein are believed to improve heat transfer by interfering with laminar flow at the surface, for example by inducing turbulent air flow. 
       FIG. 1  shows an embodiment of the invention in which multiple cooperating air flows are used to improve heat transfer from a surface. More specifically,  FIG. 1  illustrates the combined, simultaneous operation of a first fluid driver (e.g., a bulk air mover such as a fan or blower), a second fluid driver (e.g., an agitator assembly), and a third fluid driver (e.g., a synthetic jet assembly) around a pair of heat transfer surfaces  12  defining a primary flow channel  16 . The first fluid driver causes a bulk airflow  20  to flow along (e.g. substantially parallel to) a primary flow axis  24 . The second fluid driver generates secondary flow  28  along a secondary flow axis  32  that is different from (e.g. substantially perpendicular to) the primary flow axis  24 . The third fluid driver generates tertiary airflow  36  along axes  40  that are different from (e.g. substantially perpendicular to) the primary flow axis  24  and the secondary flow axis  32 . 
     As stated above, it is believed that the secondary flow  28  and tertiary flow  36  over the heat transfer surfaces  12  increase heat transfer to the bulk airflow  20  along the primary flow axis  24  by substantially reducing laminar flow conditions along the heat transfer surfaces  12 . In various alternative embodiments, the first, second, and third axes may be at varying angles with respect to one another, for example in a range of 45-90 degrees apart, although the three axes do not have to all be at the same angle with respect to the others. 
     Referring to  FIG. 2 , a heat transfer apparatus  44  is illustrated. The heat transfer apparatus  44  includes a heat sink assembly  48 , an agitator assembly  52 , synthetic jet assemblies  56 , and a blower  60 . 
     The heat sink assembly  48  includes a base wall  64  having an engagement surface  68 . The base wall  64  is oriented along a base plane  72 . The engagement surface  68  may be coupled to a heat source, such as a printed circuit board (PCB), a micro-processor, a flat-screen display, or other device that generates heat during operation. 
     Referring to  FIG. 5 , opposite the engagement surface  68 , a plurality of fin walls  76  extend from the base wall  64  in cantilever fashion. Each fin wall  76  extends from a base end  80 , coupled to the base wall  64 , to a distal end  84 . Heat transfer surfaces  88  are defined between the base end  80  and the distal end  84  on two sides of each fin wall  76 . Each fin wall  76  further defines a first agitator cutout  90 , a second agitator cutout  92 , and a central cutout  96  disposed between the first agitator cutout  90  and the second agitator cutout  92 . The first agitator cutouts  90  of the fin walls  76  are substantially aligned, and define a first agitator channel  102 . The second agitator cutouts  92  of the fin walls  76  are substantially aligned, and define a second agitator channel  106 . The central cutouts  96  are substantially aligned, and collectively define central cavity  110 . 
     The central cavity  110  divides the fin walls  76  into two opposing groups  114  and  118 . Primary airflow channels  122  are defined between adjacent fin walls  76  of each group  114  and  118 , with opposing directions of airflow corresponding to the opposing groups. The primary airflow  122  channels terminate in the central cavity  110 . In some embodiments, a flow director may be disposed within the central cavity  110  for redirecting flow from the primary airflow channels  122  towards the blower assembly  60  ( FIG. 2 ). 
     Referring back to  FIG. 2 , the blower assembly  60  draws bulk airflow  126  through the heat sink assembly  48  along the primary airflow channels  122  defined between the fin walls  76 . In the central cavity  110  ( FIG. 3 ), the bulk airflow is redirected along a blower axis  130  ( FIG. 2 ) that is substantially perpendicular to the primary airflow channels  122 . The blower assembly  60  may be an axial-type blower or a centrifugal-type blower or other mechanism for inducing bulk air flow. While the illustrated embodiments show the blower pulling bulk air flow  126  through the fin assembly, in some embodiments, hulk air flow may be pushed through the fins. 
       FIG. 4  illustrates the agitator assembly  52 . The agitator assembly  52  includes an agitator carrier  134 , an agitator actuator  138 , a first group of agitator members  142 , and a second group of agitator members  146 . The agitator carrier  134  includes a first carrier arm  150  and a second carrier arm  154 . Referring to  FIG. 2 , the carrier arms  150  and  154  are spaced apart a distance  158  corresponding to a separation distance between the first agitator channel  102  and the second agitator channel  106 , respectively, such that the carrier arms  150  and  154  may reciprocate without interference from the fin walls  76  of the heat sink assembly  48 . Referring to  FIG. 4 , the first and second carrier arms  150  and  154  are coupled to a connecting portion  162 . The connecting portion  162  is coupled to the agitator actuator  138 . 
     The first group of agitator members  142  is coupled to the first carrier arm  150 , and the agitator members  142  are spaced along the first carrier arm  150  to reciprocate between adjacent fin walls  76  of the heat sink  48  ( FIGS. 2 and 3 ). Referring to  FIG. 4 , the second group of agitator members  146  is coupled to the second carrier arm  154 , and the agitator members  146  are spaced to reciprocate between adjacent fin walls  76  of the heat sink  48  ( FIGS. 2 and 3 ). Each agitator member  142  or  146  includes a substantially rectangular body  160 , although other shapes are also possible, for example square, curved, triangle, or other shapes that promote movement of gas (e.g. air). In some embodiments, the body  160  of the agitator member  142  or  146  is approximately the same size and shape as the fin walls  76  (e.g.  FIG. 3 ) while in other embodiments the body  160  is smaller (e.g.  FIG. 4 ). In the latter case, the body  160  may be placed at a location where air or other gas enters the space between adjacent fin walls  76  to disrupt laminar air flow as air enters the space, where the disrupted flow pattern continues downstream of the body  160 . 
     Referring to  FIG. 5 , the agitator actuator  138 , includes a piezo stack  164 . The piezo stack  164  is disposed within an amplification body  166 . Referring to  FIG. 6 , the amplification body  166  includes an oval loop shell  170  having a primary displacement axis  176  and an agitation axis  180  that is substantially perpendicular to the primary displacement axis. A support leg  184  and an actuation leg  188  are substantially aligned with the agitation axis  180 . The support leg  184  is fixedly coupled to a rigid support  192 . The rigid support  192  may be the heat sink assembly  48  ( FIG. 2 ), a surrounding cabinet, bulkhead, or other fixed structure. The actuation leg  188  is fixedly coupled to the connecting portion of the agitator carrier  150  ( FIG. 4 ). 
     Referring to  FIG. 6 , expansion and contraction of the piezo stack  164  along the primary displacement axis  176  results in an amplified displacement along the agitation axis  180 . Reciprocating displacement of the agitator carrier  134  ( FIG. 4 ) along the agitation axis  180  results in the agitator members  142 ,  146  reciprocating between their corresponding, adjacent fin walls  76  ( FIG. 2 ), generating secondary flow similar to that illustrated in  FIG. 1 . Other means for moving the agitators could also be used, including, for example, a rotating cam driving a piston or a linear actuator. 
     Synthetic jets are generated by creating a closed chamber with a flexible diaphragm and one or a limited number of openings to act as a nozzle when the diaphragm is moved, moving air through the nozzle(s). Several different mechanisms can be used to move the flexible diaphragm, as described below. Referring to  FIG. 7 , the synthetic jet assembly  56  includes a jet actuator  194 , a diaphragm  196 , and a jet body  198 . The jet actuator  194  includes a piezo stack  202  disposed within an amplification body  204 . The amplification body  204  includes an oval shell  206  having a primary displacement axis  210  and jet axis  214  that is substantially perpendicular to the primary displacement axis  210 . A support leg  218  and an actuation leg  222  are substantially aligned with the jet axis  214 . The support leg  218  is fixedly coupled to a rigid support, such as the blower assembly  60  ( FIG. 2 ), a surrounding cabinet, bulkhead, or other fixed structure. The actuation leg  222  is fixedly coupled to the flexible diaphragm  196 . Expansion and contraction of the piezo stack  202  along the primary displacement axis  210  results in an amplified displacement of the amplification body  204  along the jet axis  214 . Other means for driving the synthetic jet diaphragm  196  could also be used, including, for example, a rotating cam driving a piston or a linear actuator. 
     Referring to  FIG. 8 , the jet body  198  includes a diaphragm surface  226  and a nozzle surface  230 . A cavity  234  is defined within the jet body  192  between the diaphragm surface  226  and the nozzle surface  230 . An array of nozzles  238  is defined by the nozzle surface  230 . Referring to  FIG. 9 , in various embodiments the nozzles  238  can have an opening with one or more shapes including circular, square, plus- or star-shaped. 
     Referring to  FIGS. 7 and 8 , when the amplification body  204  expands and contracts, the diaphragm  196  reciprocates along the jet axis  214 . Movement of the diaphragm  196  at the diaphragm surface  226  creates pressure transients with the cavity  234 , causing air or other fluids to be rapidly drawn into the cavity  234  and ejected from the cavity  234  through the nozzles  238  in a direction substantially parallel to the jet axis  214 . The nozzles  238  are spaced apart such that the airflow is discharged upon the distal ends  84  of the fin walls  76  ( FIG. 5 ), in a manner similar to the tertiary flow illustrated in  FIG. 1 . 
       FIG. 10  illustrates a heat transfer apparatus  242  according to another embodiment of the invention. The heat transfer apparatus  242  includes a heatsink  246  with a base wall  250  and fin walls  254  extending from the base wall  250 . The base wall  250  is coupled to a heat spreader  258 . A microchip  262  is sandwiched between a chip carrier  266  and the heat spreader  258 . 
     Agitator plates  270  are disposed between the fin walls  254 . The agitator plates  270  are coupled to an agitator carrier  274 . The agitator carrier  274  is coupled to an agitator actuator  278  including a piezo stack  282 . Referring to  FIG. 11 , a synthetic jet assembly  286  includes a jet body  290  and a diaphragm  294 . The jet body  290  includes a diaphragm surface  298  and a nozzle surface  302 . An array of cavities  306  are defined within the jet body  290  between the diaphragm surface  298  and the nozzle surface  302 , with a corresponding array of nozzles  310  (one per cavity  306 ) defined by the nozzle surface  302 . The diaphragm  294  includes an array of piezo bender actuators  314 , with one piezo bender actuator  314  substantially aligned with each cavity  306 . When the piezo bender actuators  314  expand and contract, the diaphragm  294  reciprocates over cavities  306 . This diaphragm movement creates pressure transients with the cavities  306 , causing air or other fluids to be rapidly drawn into the cavity  306  and ejected from the cavity  306  upon the fin walls  254 . 
       FIG. 12  illustrates a dual heat transfer apparatus  318  according to another aspect of the invention. The dual heat transfer apparatus  318  includes a first heat sink  322  and a second heat sink  326 , a first agitator assembly  330  and a second agitator assembly  334 , a first synthetic jet assembly  338  and a second jet assembly  342 , and a first blower  346  and a second blower  350 . Each of the first and second synthetic jet assemblies  338  and  342  includes a diaphragm  354  oriented substantially perpendicular to a synthetic jet body  358 . Manifolds  362  fluidly connect each diaphragm  354  to its corresponding synthetic jet body  358 . 
       FIGS. 13-14  illustrate a heat transfer apparatus  366  according to another aspect of the invention in which the heat transfer surface (e.g. one or more fins) is an outer wall of a heat pipe. The heat transfer apparatus  366  includes a blower assembly  370 , an agitator and synthetic-jet assembly  374 , and a heat pipe heat sink  378 . The heat pipe  378  includes a base portion  382  and rib portions  386  (e.g. fins) extending from the base portion  382 . The base portion  382  and rib portions  386  define a vapor chamber  390 . 
     A heat pipe vapor chamber is a heat-transfer device that combines the principles of both thermal conductivity and phase transition. A liquid within the vapor chamber turns into a vapor by absorbing heat from a first surface (e.g. at the base portion). The vapor condenses back into a liquid at a cold surface (e.g., at the ribs), releasing the latent heat. The liquid then returns to the hot interface through capillary action where it evaporates once more and repeats the cycle. In addition, the internal pressure of the heat pipe can be set or adjusted to facilitate the phase change depending on the demands of the working conditions of the thermally managed system. 
       FIGS. 15-16  illustrates a heat transfer apparatus  394  according to another aspect of the invention. The heat transfer apparatus  394  includes a heat pipe  398  with a vapor chamber  402 , similar to that described with respect to the heat transfer apparatus of  FIG. 13 . The heat transfer apparatus  394  includes a synthetic jet assembly  406  including piezo agitators  410 . The piezo agitators  410  extend from a synthetic jet body  414 , between adjacent nozzles  418  of the synthetic jet body  414 . 
       FIG. 17  illustrates a heat transfer apparatus  422  according to yet another aspect of the invention. The heat transfer apparatus  422  includes a heat sink  426  with a base portion  430  and fin walls  434  extending from the base portion  430 . The fin walls  434  have an inlet end  438  and an outlet end  442 , with flow channels  446  defined between the fin walls  434  from the inlet end  438  to the outlet end  442 . A blower assembly  450  includes a blower housing  454  and a centrifugal blower  458 . The blower housing  454  includes a blower inlet  462  that receives air from the fin wall outlet end  442 . When in operation, the centrifugal blower  458  draws air or other fluids through the flow channels  446  of the heat sink  426 , from the inlet end  438  to the outlet end  442 . The air is then redirected through the blower inlet  462  and into the centrifugal blower  458 . 
     Referring to  FIGS. 18 and 19 , the heat transfer surfaces in some embodiments (e.g.,  FIG. 3 ) include an array of surface modification members  466  or pin fins. As discussed above, the surface modification members  466  are believed to disrupt or substantially reduce laminar flow across the heat transfer surface and are sized and shaped for optimal use with gases (in particular air) as the fluid for heat removal. 
       FIGS. 20 and 21  illustrate two embodiments of the surface modification members.  FIG. 19  illustrates the surface modification members in a cylindrical pin fin configuration  470 . Although cylindrical (i.e. having a circular cross-section) pin fins  470  are illustrated, the pin fins  470  can have a variety of shapes, for example having cross-sections that are oval, triangular, square, or other regular or irregular polygon or curved shapes. The cylindrical pin fins  470  include a cylindrical body  474  with a base end  478  and a distal end  482 . The base end  478  is coupled to the heat transfer surface  88  (the substrate). The cylindrical pin tins have a height from the substrate of H 1  and diameter of the cylindrical body D 1 . In one embodiment, the diameter D 1  is approximately 500 micrometers, with a height H 1  of approximately 250 micrometers. In another embodiment, the diameter D 1  is approximately 75 micrometers, with a height H 1  of 150 micrometers. 
     The spacing of the pin fins can influence the heat removal performance of the surface to which the pin fins are attached. The cylindrical pin fins  470  are separated from each other by a distance S 1 . A ratio of the separation distance S 1  to the diameter D 1  (i.e., S 1 :D 1 ) is approximately 6:1. 
       FIG. 21  illustrates the surface modification members in which the pin fins have an overall shape resembling a mushroom. The mushroom-shaped pin fins  486  include a cylindrical body  490  with a base end  494  and a distal end with a dome-shaped end-cap  498 . The base end  494  is coupled to the heat transfer surface  88  (the substrate). Although the end-caps  498  are shown as being dome-shaped, other end-cap shapes are possible including a pin fin with an overall tapered shape, with the general property that the distal ends of the pin fins have a larger width than the width of the base. The end-cap  498  has a height H 2  from the substrate and a maximum diameter (at the end cap) D 2  and a base diameter (at the base end) of D 3 . In one embodiment, the diameter D 2  is approximately 500 micrometers, the diameter D 3  is approximately 50-65 micrometers, with a height H 2  of approximately 250 micrometers. In another embodiment, the diameter D 2  is approximately 75 micrometers, the diameter D 3  is approximately 50-65 micrometers, with a height of 150 micrometers. A ratio of the separation distance S 2  to the diameter D 2  (i.e., S 2 :D 2 ) is approximately 6:1. 
       FIG. 22  illustrates a method of fabricating surface modification members  466  on a first surface and a second surface of a copper substrate  506  (e.g., a copper fin wall). First, a titanium (Ti) layer  510  is deposited over the fin wall  506 . Next, a high contrast, epoxy based photo resist  514  (e.g., KMPR®) is applied over the Ti layer  510 . The photo resist  514  is selectively cured to establish a mold pattern  518 , and the unexposed photo resist  514  is washed away. After removal of the unexposed photo resist  514 , the Ti layer  514  is washed away from mold areas  522 , selectively exposing the copper substrate  506  in the mold pattern. Copper plating  526  is then applied to the exposed copper substrate  506 , thereby forming the surface modification members  466 . Finally, the remaining cured photo resist  514  and titanium  510  is washed away, leaving only the copper substrate  506  and surface modifications  466 . 
     Thus, the invention provides, among other things, a heat transfer apparatus. Various features and advantages of the invention are set forth in the following claims.