Abstract:
A heat exchanger has a fluid passage sharing a wall with a cooling fluid passage adjacent to the passage. The thermally conductive wall allows heat to be transferred from the fluid into the cooling fluid passage. The passage additionally has a set of at least one airfoil pin extending into the passage.

Description:
BACKGROUND OF THE INVENTION 
     The present application is related to a pin fin heat exchanger with pins having an airfoil profile. 
     Heat exchangers capable of drawing heat from one place and dissipating it in another place are well known in the art and are used in numerous applications where efficiently removing heat is desirable. One type of heat exchanger used in fluid cooling systems dissipates heat from two parallel fluid passages into a cooling fluid passage between the passages. A cooling fluid (such as air) is then passed through the cooling fluid passage. Heat from the parallel fluid passages is drawn into the cooling fluid passage and is expelled at the opposite end of the heat exchanger with the cooling fluid. Heat exchangers of this type are often used in vehicle applications such as aircraft engines or car engines. 
     Devices constructed according to this principle transfer heat from the surface area of the parallel passages into the fluid flowing through the cooling fluid passage. In order to increase the surface area which is capable of dissipating heat, some heat exchangers have added pins extending from the walls of the parallel fluid passages into the air gap. The pins are thermally conductive and thus heat can be conducted from the passages into the pins and dissipated into the cooling fluid. The pins can be held in place using crossed ligaments. A device according to the above described design is referred to as a pin fin heat exchanger. The ligaments also provide more surface area which the fluid being forced through the cooling fluid passage is exposed to, and thereby allow a greater dissipation of heat. Some designs in the art utilize pins where each pin is connected to both of the parallel fluid passages resulting in a post running perpendicular to the parallel fluid passages through the gap. Current heat exchangers using pins have a symmetrical pin profile such as a circular or diamond profile. 
     SUMMARY OF THE INVENTION 
     Disclosed is a heat exchanger having pins connecting extending from a wall of a fluid passage into a cooling fluid passage. The pins conduct heat from the fluid passage into a cooling fluid passage adjacent to the wall. A cooling fluid flows through the gap and heat is dissipated from the pins and the wall into the fluid. The pins have an airfoil profile. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a cut-out side view of an example heat exchanger. 
         FIG. 2  is an illustration of an airfoil profile in an example heat exchanger. 
         FIG. 3  is an array of pins and ligaments for an example heat exchanger. 
         FIG. 4  is an isometric view of an example construction of a pin and ligament array. 
         FIG. 5  is an example array of pins and ligaments where the angles of attack of the pins are arranged to control the flow of a cooling fluid. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A simplified heat exchange system according to the present application is illustrated in  FIG. 1 . Two parallel fluid passages  102 ,  104  have facing outer walls  106 ,  108  and a cooling fluid passage  110  between the facing outer walls  106 ,  108 . A cooling fluid such as air, which is initially cooler than the facing outer walls  106 ,  108 , passes through the cooling fluid passage  110 . While traveling through the cooling fluid passage  110  the cooling fluid absorbs heat from the exposed surface area of the facing outer walls  106 ,  108  thereby cooling the fluid traveling through the parallel fluid passages  102 ,  104 . 
     In order to increase the surface area exposed to the cooling fluid in the cooling fluid passage  110 , and thereby increase the heat transfer potential of the heat exchanger, thermally conductive pins  112  connect the facing surfaces  106 ,  108  of the fluid passages  102 ,  104 . The pins  112  conduct heat from the facing surfaces  106 ,  108  into the cooling fluid passage  110 , thereby exposing more surface area to the cooling fluid flowing through the cooling fluid passage  110 . Since the amount of heat dissipated in the heat exchanger is proportional to the surface area exposed to the cooling fluid, and the pins generate more exposed surface area, the efficiency of the heat exchanger is increased. 
     Previous pin fin heat exchanger designs used a circular, diamond, or other symmetrical shape for the pin  112  profile. In previous designs, when a cooling fluid flowing through the cooling fluid passage  110  in one direction hits the side of a symmetrical pin, the cooling fluid is naturally forced around the pin. It is well known in the art that the flow path can be either attached to surface, whereby the flow path near the wall is moving parallel to the wall and provides effective heat transfer, or separated from the surface, whereby the flow path is not necessarily parallel to the wall and does not provides effective heat transfer. In the process of flowing around the pin, the cooling fluid flow path becomes separated from the surface of the pin, resulting in the cooling fluid flow remaining attached to as little as half of the pin&#39;s surface area. Consequently, only the portion of the surface area of the pin contacting the flow path can provide heat dissipation and the remainder of the pin&#39;s surface area is wasted. 
       FIG. 2  illustrates a profile of a pin  112  design where the profile is airfoil. Airfoil profiles are well known in the field of aircraft design, where they are used to control airflow over the wings and thereby generate lift. It is also known that the curvature of the wing shape may be altered to reduce or adjust the flow separation of an airflow flowing over the wing of an aircraft. In addition to the curvature of the wing, aircraft designs utilize an angle of attack. The angle of attack is the angle of the wing with respect to the fluid flow. Determining the proper angle of attack in order to avoid stalling is well known in aircraft design. The profile illustrated in  FIG. 2  applies these features of aircraft wing design to the pin profile design in a heat exchanger. 
     The airfoil pin  112  profile in  FIG. 2  has an upper acceleration region  210 , an upper deceleration region  220 , a lower acceleration region  212 , and a lower deceleration region  222 . When a cooling fluid flows over the upper acceleration region  210  and the lower acceleration region  212  of the pin, the cooling fluid flow will accelerate. Once the fluid enters the upper deceleration region  220  and the lower deceleration region  222  of the pin, the cooling fluid flow begins to decelerate. Flow separation typically only occurs on an airfoil profile when the cooling fluid flow is in the deceleration regions  220 ,  222  near the trailing edge  230 . Since the surface area of the trailing edge  230  is a smaller portion of the surface area of the pin  112  than the flow separation region of a circular or other symmetrical profile, the airfoil profile allows the pin  112  to more efficiently utilize its surface area, thereby dissipating a larger amount of heat. 
       FIG. 3  shows an example embodiment of a heat exchanger using airfoil pins  112  that also incorporates ligaments  306  connecting a portion of the pins  302 ,  304  in a pin array  300  together. The ligaments  306  are connected between the lower deceleration region  222  of a first pin  302  and the upper deceleration region  220  of a second pin  304 . The ligament  306  attaches multiple pins  302 ,  304  to each other in a similar manner, resulting in an array  300  of pins  302 ,  304  and ligaments  306 . It is additionally possible to connect each end of the ligaments  306  to a frame  200  which holds the ligaments  306  and the pins  302 ,  304  in place. The frame  200  and the ligaments  306  can be constructed out of a single unit. Alternately, the ligaments  306  can be connected to the frame  200  using any other known method, depending on design constraints. The frame  200  can have four sides as depicted in  FIG. 3 , or can be created without flow facing sides  202 ,  204 . In an embodiment without flow facing sides each of the ligaments would be connected to at least one of the sides  206 ,  208  which are parallel to cooling fluid flow. 
     An additional advantage realized by the placement of the ligaments  306  in the cooling fluid passage  110  arises from the natural interference with the cooling fluid flow caused by the ligaments  306 . When the cooling fluid flow contacts the ligaments  306  a wake zone is created behind the ligament  306 . The wake zone causes turbulence in the cooling fluid which mixes the cooling fluid which was directly in the cooling fluid flow path with cooling fluid that was not directly in the cooling fluid flow path. 
     Mixing the cooling fluid in the cooling fluid flow path with the cooling fluid not directly in the cooling fluid flow path provides a beneficial dispersal of the heated cooling fluid from the direct flow path into the unheated cooling fluid not directly in the cooling fluid flow path. The mixing effect thereby increases the efficiency of the heat exchanger as it allows the cooling fluid directly in the fluid flow path to have a reduced temperature farther into the cooling fluid passage  110  than previous designs. 
     An example construction for the array of pins  112  and ligaments  306  is disclosed in  FIG. 4 . The example embodiment of  FIG. 4  illustrates a pin fin array created using a stamping or etching process to form the ligaments  306  and portions of each pin  112  out of a sheet of metal or other thermally conductive material. The frame may also be formed out of the same sheet using the same method. In the etching process, a profile of the ligaments  306 , the pins  112  and the frame is etched or stamped out of the sheet. Once the profile has been created, the ligament portion  306  is etched to be thinner than the pin  112  portion. By way of example the pin  112  portion could be 1 mm thick, and the ligament  306  portion could be 0.3 mm thick. Additionally the frame can be etched to connect to, or interlock with, other stacked frame portions thereby creating a completed unit. Additional sheets are also created using the same method resulting in multiple stackable sheets  402 ,  404 ,  406 . 
     Once each sheet  402 ,  404 ,  406  has been etched to the proper shape and thickness, the sheets  402 ,  404 ,  406  are stacked on top of each other (illustrated in  FIG. 4 ), with the number of sheets  402 ,  404 ,  406  being stacked depending on the pin height necessary for the particular application. Once stacked, the pin profile portions of the sheet are bonded together using any known bonding method to form solid pins  112  comprising multiple sheets  402 ,  404 ,  406  and connected to multiple ligaments  306 . The stacked array  300  of pins  112  and ligaments  306  is then placed in the cooling fluid passage  110  with the top of the pins  112  contacting the first facing wall  106 , and the bottom of the pins  112  contacting the second facing wall  108 . The array  300  may be held in place using a frame or any other known method. Since the ligament  306  portion of the etched sheet is thinner than the pin  112  profile portion, cooling fluid is allowed to flow between the ligaments  306  and through the cooling fluid passage  110 . 
     In addition to providing more surface area through which heat can be dissipated, including additional ligaments  306  creates a restriction in the flow passage because the ligaments  306  block a portion of the flow. The restriction decreases the space through which the fluid can flow, thus causing flow acceleration and a decrease in flow pressure through the cooling fluid passage  110 . By design, this decrease occurs in the deceleration regions  220  and  222 , thereby this decrease in flow pressure results in less flow separation. A design taking advantage of the lower flow separation could be used in an application where the fluid flow pressure drop is not a significant design constraint. 
     Another example embodiment, illustrated in  FIG. 5 , utilizes the airfoil profile of the pins  112  to control and direct the flow path  504  of the cooling fluid, thereby minimizing the pressure drop, or controlling any other desired attribute. In  FIG. 5 , the ligaments  306  connect the lower deceleration region  222  of a first pin  506  with the lower acceleration region of a second pin  508 . This design also uses different angles of attack for each pin in order to shape the flow of the cooling fluid through the cooling fluid passage  110 . The example method of  FIG. 5  utilizes a pattern where two pins  506 ,  508  are angled in a first direction relative to fluid flow are followed by two pins  510 ,  512  angled in a second direction opposite the first direction relative to fluid flow with the pattern repeating itself. A line illustrates a flow path  504  of the cooling fluid resulting from the angled pin pattern as the cooling fluid flows through the cooling fluid passage  110 . With this flow path  504  the fluid has a farther distance to travel before it hits another pin than a pattern with conventional pin profiles, thereby allowing heated cooling fluid to mix with non-heated cooling fluid longer before hitting another pin. The mixing of the cooling fluid provides for better heat absorption rates of the fluid itself. In order to achieve a desired mixing level, the ligaments can be arranged to interfere with the fluid flow as much or as little as is required for a particular application. 
     Designs utilizing the ligament  306  layout of  FIG. 5  additionally have a lower pressure drop associated with the cooling fluid traveling through the cooling fluid passage  110  than designs constructed according to the example ligament  306  layout of  FIG. 3 . The lower pressure drop is a result of the ligaments  306  having less interference with the fluid flow path  504  thereby reducing the amount of obstruction to fluid flow. The lower pressure drop additionally results in a lower heat transfer. The example embodiment of  FIG. 5  could be used in any application where minimizing the pressure drop is a key design constraint. It is also known that alternate flow paths can be constructed by altering the angle of attack on some or all of the pins  112  in the pin array  300  thereby allowing the cooling fluid flow path to be differently controlled. 
     Although example embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the true scope and content of this invention.