Abstract:
A lightweight, high-efficiency alternating channel counter-flow heat exchanger structure is disclosed. A matrix of alternating hot and cold channels defining a heat exchanger structure is provided. A portion of each of the inlets and outlets of each of the hot and cold channels is blocked to prevent fluid flow through the blocked portion, thus creating hot-only and cold-only fluid communication regions on the ends of the heat exchanger structure. Alternating hot and cold headers provided on each end of the heat exchange structure service the respective hot and cold channels. The partial blocking structures on the channel-ends enable a single hot or cold header/plenum to be offset with respect to individual rows of channels and thus service a pair of adjacent rows of alternating hot and cold channels in the matrix of channels. The true alternating channel counter-flow design provides a higher heat transfer rate than a similarly-sized cross-flow design.

Description:
BACKGROUND 
     Field 
     This invention relates generally to a high-efficiency alternating channel counter-flow heat exchanger and, more particularly, to a heat exchanger configured with a matrix of separated hot fluid flow channels and cold fluid flow channels, where the hot channels and the cold channels alternate in each row and each column such that hot channels are adjacent only to cold channels and vice versa, and where the alternating channel counter-flow arrangement is enabled by channel-end flow blockers and a header/plenum for simplifying the plumbing of the hot and cold fluids. 
     Discussion 
     Heat exchangers have been used for decades to transfer heat energy from one fluid to another. In a typical application, a hot fluid is cooled by a secondary cool fluid. The hot fluid flows through a first passage, such as a tube or channel, and the cold fluid can either flow through a second passage or can flow freely over fins which are fixed to the first passage. The fluids can both be liquids, they can both be gases, or one can be a liquid and the other can be a gas, such as air. 
     In constrained-flow heat exchangers, where both fluids flow through channels or passages, there are three primary classifications of heat exchangers, according to their flow arrangement. In a cross-flow heat exchanger, the hot and cold fluids travel roughly perpendicular to one another through the heat exchanger. In parallel-flow heat exchangers, the two fluids enter the heat exchanger at the same end, and travel in parallel to one another to the other end. In counter-flow heat exchangers, the two fluids enter the heat exchanger from opposite ends. The counter-flow design is the most efficient, in that it can transfer the most heat between the fluids due to the fact that the average temperature difference along any unit length is greater. 
     One way of increasing heat exchanger efficiency is to increase the number of channels through which fluid flows, and decrease the size of the channels. Small channel size enables more complete transfer of heat energy from the hot fluid to the cold fluid for a given heat exchanger length. One heat exchanger design is essentially a cubic matrix of channels arranged in rows and columns, with the number of rows and columns in the hundreds, and the number of channels in the tens of thousands. In such a complex and intricate heat exchanger structure, although the efficiency benefits of a counter-flow arrangement would be desirable, it has not been possible or practical to fabricate such a design until now. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a simple two channel counter-flow heat exchanger of a type known in the art; 
         FIG. 2  is an illustration of a simple counter-flow heat exchanger with fins added in each of the two main channels; 
         FIG. 3  is an illustration of a true alternating channel counter-flow heat exchanger, where each channel is adjacent only to channels carrying the other fluid in the opposite direction; 
         FIG. 4  is a first illustration of a true alternating channel counter-flow heat exchanger, showing how channel-end blockers can be used to simplify plumbing of the fluids to the heat exchanger; 
         FIG. 5  is a second illustration of the heat exchanger of  FIG. 4 , showing how a header is used in conjunction with the channel-end blockers; 
         FIG. 6  is a third illustration of the heat exchanger of  FIGS. 4 and 5 ; and 
         FIG. 7  is an illustration of an alternating channel counter-flow heat exchanger scaled up to include many rows and columns of channels. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to an alternating channel counter-flow heat exchanger is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
     Heat exchangers are widely used to transfer heat energy from a first, hot fluid to a second, cool fluid. Heat exchangers are used in a wide range of industries and applications—from automotive radiators, to aerospace applications such as engine oil cooling and jet fuel preheating, to various applications in power generation and computing. The objective in heat exchanger design is to maximize heat transfer efficiency in order to minimize heat exchanger size/weight and required fluid flow rates. 
       FIG. 1  is an illustration of a simple two channel counter-flow heat exchanger  10  of a type known in the art. In counter-flow heat exchangers such as the heat exchanger  10 , the two fluids enter the heat exchanger from opposite ends. The counter-flow design is the most efficient type of heat exchanger, in that it can transfer the most heat between the fluids due to the fact that the average temperature difference along any unit length is greater. 
     The heat exchanger  10  includes a first side wall  12  and a second side wall  14 . The heat exchanger  10  also includes a top plate  16 , a bottom plate  18  and a middle plate  20 . The ends of the heat exchanger  10  are open, thus defining a first (upper) channel  30  and a second (lower) channel  40 . A cold fluid enters the channel  30  at a cold fluid inlet temperature (TC i ) as shown at arrow  32 . The cold fluid exits the channel  30  at a cold fluid outlet temperature (TC o ) as shown at arrow  34 . A hot fluid enters the channel  40  at a hot fluid inlet temperature (TH i ) as shown at arrow  42 . The hot fluid exits the channel  40  at a hot fluid outlet temperature (TH o ) as shown at arrow  44 . The hot fluid and the cold fluid may each be either liquid or gas. In one example, the hot fluid is a liquid and the cold fluid is cool air. The heat exchanger  10  would typically be made of aluminum, or some other material that has both light weight and good conductive heat transfer properties. 
     Each channel of the heat exchanger  10  has a length X, a width Y and a height Z, where the length X is measured from end to end in the direction of fluid flow through the channels  30  and  40 , the height Z is measured in the vertical direction as shown, and the width Y is measured in the direction perpendicular to both X and Z. The total heat transfer in the heat exchanger  10  is proportional to a product of a heat transfer coefficient, the hot-side heat transfer area, and the hot-to-cold temperature differential. That is:
 
 Q∝h·XY [   T   H   −   T   C   ]  (1)
 
Where h is the net heat transfer coefficient, XY is the hot-side area defined by the length X multiplied by the width Y, and  T H    and  T C    are the hot and cold fluid average temperatures (difference between inlet and outlet temperature), respectively.
 
     While the heat exchanger  10  is a counter-flow design, it is not fully optimized due to the large size of the channels  30  and  40 . A design with smaller channels and more heat exchange surface area can increase efficiency. 
       FIG. 2  is an illustration of a simple counter-flow heat exchanger  50  which is similar to the heat exchanger  10  but with vertical fins added in each of the two main channels. A series of vertical fins  52  are incorporated between the top plate  16  and the middle plate  20 , and the middle plate  20  and the bottom plate  18 , respectively. The fins  52  define a plurality of channels  54  which are much smaller than the channels  30  and  40  of the heat exchanger  10  in  FIG. 1 . It can be seen that heat exchanger  50  is still partially a counter-flow design, in that the upper layer of the channels  54  handles the cold fluid flowing in one direction, and the lower layer of the channels  54  handles the hot fluid flowing in the opposite direction. This fluid flow arrangement is simple and practical from a plumbing connection standpoint, as all of the cold fluid channels are adjacent to each other and all of the hot fluid channels are adjacent to each other. 
     The theoretical heat transfer in the heat exchanger  50  can be defined as:
 
 Q   theoretical   ∝h ( XY+ 10 ZX )[   T   H   −   T   C   ]  (2)
 
Where the hot-side wetted area now includes a term  10 ZX, which represents the area of the fins in the channels  54 . However, the fins  52  in the heat exchanger  50  do not directly conduct heat from hot fluid to cold fluid, so there is a “fin efficiency” to account for. Thus, the actual heat transfer in the heat exchanger  50  can be defined as:
 
 Q   actual   ∝h ( XY+η· 10 ZX )[   T   H   −   T   C   ]  (3)
 
Where η is the fin efficiency factor.
 
     The small size of the channels  54  and the additional heat exchange surface area offered by the fins  52  make the heat exchanger  50  more efficient than the heat exchanger  10 . However, efficiency could be further increased by increasing the degree of counter-flow. 
       FIG. 3  is an illustration of a true alternating channel counter-flow heat exchanger  60 , where each channel is adjacent only to channels carrying the other fluid in the opposite direction. The heat exchanger  60  is identical in construction to the heat exchanger  50 , including the vertical fins  52  and the plurality of channels  54 . The only difference with the heat exchanger  60  is the fluid flow arrangement, where the channels  54  alternate in type of fluid carried and direction of flow, in both the lateral and vertical direction. That is, each of the channels  54  has only counter-flowing channels adjacent to it. For example, consider channel  62 , which is near the middle of the bottom layer of channels and which has a hot fluid inlet at the right-hand end of the heat exchanger. It can be seen in  FIG. 3  that the channel  62  has a counter-flowing cold fluid channel as its neighbors above, to the left and to the right. Thus, the heat exchanger  60  is a true alternating channel counter-flow design. 
     In the heat exchanger  60 , there is no longer an “effective” fin area, as all of the fin surfaces now provide direct conduction from the hot fluid to the cold fluid. Thus, the actual heat transfer is equal to the theoretical heat transfer in the heat exchanger  60 , as follows:
 
 Q   actual   =Q   theoretical   ∝h ( XY+ 10 ZX )[   T   H   −   T   C   ]  (4)
 
That is, the fin efficiency η is equal to one.
 
     As shown above, the heat exchanger  60  is ideal from a heat transfer efficiency standpoint. Unfortunately, as a practical matter, it would be extremely labor intensive to build the heat exchanger  60  with all of the requisite hot and cold fluid plumbing connections. This is particularly apparent when it is considered that many real-world applications require heat exchangers with hundreds of rows and hundreds of columns of channels. Clearly, there is no practical way to build such a device. Thus, the benefits of an alternating channel counter-flow heat exchanger have been unobtainable until now. 
       FIG. 4  is a first illustration of a true alternating channel counter-flow heat exchanger  80 , including design features which make it possible to construct and route fluids to the heat exchanger  80 . The heat exchanger  80  starts with the same geometry as the heat exchanger  60 , with two layers of the channels  54 . However, in the heat exchanger  80 , partial channel-end blockers are added on each end of the device, with a purpose and function that will become apparent in the following discussion. A plurality of hot channel-end blockers  82  is positioned over part of each end of each hot fluid channel. Specifically, the blockers  82  block the upper half of each of the hot fluid channels in the upper layer, and the blockers  82  block the lower half of each of the hot fluid channels in the lower layer. A corresponding set of the blockers  82  is also included at the opposite end (not visible in  FIG. 4 ) of the heat exchanger  80 . As a result of the blockers  82 , all of the hot fluid openings are clustered together in a narrow vertical band, as seen in  FIG. 4 . 
     Similarly, a plurality of cold channel-end blockers  84  is positioned over part of each end of each cold fluid channel. Specifically, the blockers  84  block the lower half of each of the cold fluid channels in the upper layer, and the blockers  84  block the upper half of each of the cold fluid channels in the lower layer. A corresponding set of the blockers  84  is also included at the opposite end (not visible in  FIG. 4 ) of the heat exchanger  80 . As a result of the blockers  84 , all of the cold fluid openings are clustered together in two narrow vertical bands—one at the top and one at the bottom of the heat exchanger  80 . 
     It is emphasized here that each of the channels  54  in the heat exchanger  80  still has a full height Z, just as in the heat exchanger  60  of  FIG. 3 . It is only the end openings which are partially blocked by the blockers  82  and  84 . The blockers  82  and  84  are shown in  FIG. 4  as blocking a little more than half of each of the channel openings, as would be necessary to facilitate subsequent fabrication steps discussed below. It should be noted that the blockers  82  and  84  do not necessarily have to block half of the channel-end. For example, if the hot fluid is a liquid with a fairly low flow rate and the cold fluid is air with a high flow rate, it may be desirable to make the hot channel blockers  82  larger (for example, ⅔ height) and the cold channel blockers  84  smaller (for example, ⅓ height), so that the cold fluid experiences less of a flow obstruction. The opposite configuration is also possible—where the hot channel blockers  82  are made smaller and the cold channel blockers  84  are made larger. 
       FIG. 5  is a second illustration of the heat exchanger  80  of  FIG. 4 . In  FIG. 5 , a plenum or header  90  has been added (shown semi-transparent), and is used in conjunction with the channel-end blockers  82  and  84  to greatly simplify the external plumbing. The header  90  has an open end  92 , into which the hot fluid is inlet. From inside the header  90 , the hot fluid can only flow into hot fluid channels, due to the presence of the blockers  84  on the cold fluid channels. After passing through the six half-height inlets, the hot fluid will fill the entire vertical height of each of the hot fluid channels. In fact, the half-height inlets may increase turbulence in the channels, with a beneficial increase in heat transfer coefficient. 
       FIG. 6  is a third illustration of the heat exchanger  80  of  FIGS. 4 and 5 . In  FIG. 6 , the header  90  is shown with solid walls and with the hot fluid flowing in at the open end  92 . A second header  100  is also added, which receives the hot fluid exiting the heat exchanger  80  and delivers it through a single hot fluid outlet as shown at the left. Thus, it can be seen in  FIG. 6  that the hot fluid plumbing to and from the heat exchanger  80  can be handled through a single inlet to the header  90  and a single outlet from the header  100 . This is much simpler than the multiple hot fluid inlets and multiple hot fluid outlets required for the heat exchanger  60  of  FIG. 3 . 
     Two modes of handling the cold fluid are readily apparent in viewing  FIG. 6 . In a first mode where the cold fluid is a liquid, and closed-loop plumbing of the cold fluid is desired, then additional headers can be added—above and below the hot fluid headers  90  and  100 —to handle the cold fluid. The cold fluid headers could have their inlets and outlets on the same side of the heat exchanger  80  as the hot fluid headers (that is, the “near side” in  FIG. 6 ), or on the opposite side. In a second mode where the cold fluid is air, and the heat exchanger  80  can be placed in a cold air stream flowing in the X direction, then no plumbing or headers are needed for the cold fluid. In this case, the air will freely flow through the cold fluid channels, and will be blocked from entering the hot fluid channels by the headers  90  and  100 . 
     The heat exchanger  80  can be made with two layers and many columns of very tall, narrow channels—thus offering tremendous hot-to-cold counter-flow surface area, but requiring only a single set of hot fluid headers. Such a design could be useful for many different applications. In one exemplary embodiment, the heat exchanger  80  has two layers and hundreds of columns of channels, with each channel being 4.5″ tall and 0.03″ wide. 
       FIG. 7  is an illustration of an alternating channel counter-flow heat exchanger  120  as it could be scaled up to include many rows and columns of channels. As mentioned previously, some real-world applications require heat exchangers with hundreds of rows and hundreds of columns of channels. The heat exchanger  120  of  FIG. 7  shows just a small portion of such a device, which would continue on for many more rows (downward in the Z direction) and many more columns (in the Y direction). In either of the heat exchangers  80  or  120 , the length of the channels (in the X direction) can be whatever is necessary for the application. In one exemplary embodiment, the heat exchanger  120  is a nine inch cube (9″×9″×9″), with 200 rows and 200 columns of channels, for a total of 40,000 channels, with each channel being square in cross-section. 
     In the heat exchanger  80 , which included only two layers (rows) of channels, only a single hot fluid inlet header  90  and hot fluid outlet header  100  were needed. In the heat exchanger  120 , it can be seen that many hot fluid inlet and outlet headers will be needed. Specifically, the hot fluid inlet and outlet headers would need to be placed over the 2 nd  and 3 rd  rows of openings from the top of the heat exchanger  120  (which equate to the bottom of the first row of channels and the top of the second row of channels), over the 6 th  and 7 th  rows of openings, etc. Similarly, if cold fluid headers are needed, they would be placed over the 1 st  row of openings, the 4 th  and 5 th  rows of openings, the 8 th  and 9 th  rows of openings, etc. 
     The heat exchangers  80  and  120  shown in  FIGS. 4-6 and 7  represent an innovative design which offers a great simplification of external plumbing, but which would be difficult to build using traditional fabrication techniques. In particular, the brazing or welding of the blockers  82  and  84  onto the ends of the fins  52  and the plates  16 / 18 / 20  would be difficult, especially considering that the materials involved are very thin, the dimensions are very small, and the seams would all have to be leak-proof. However, the heat exchangers  80  and  120  could be readily built using additive manufacturing techniques (also known as 3D printing). Additive manufacturing can be used with metals such as aluminum, and the number of faces and joints is essentially irrelevant; the geometry can simply be modeled as shown in the preceding figures, and the heat exchanger  80  or  120  would be reliably constructed. 
     In the case of the heat exchanger  80 , it would be possible to construct the heat exchanger channel matrix via additive manufacturing, and manually fabricate the headers  90  and  100  and braze/weld them to the heat exchanger  80  in a subsequent step. In the case of the heat exchanger  120 , with the large number of headers required, it would be preferable to construct the entire heat exchanger assembly—including all of the headers—via additive manufacturing. It is also noteworthy that, using additive manufacturing, the channels need not be straight. The entire heat exchanger can take on almost any arbitrary shape—including bends, twists, warping, etc.—as may be needed for heat exchanger packaging. 
     The use of additive manufacturing techniques enables production of the alternating channel counter-flow heat exchangers  80  and  120 , where it may not have previously been practical. The alternating channel counter-flow design offers maximum heat exchanger efficiency, which allows heat exchanger size and mass to be minimized and fluid flow rates to be reduced, both of which are beneficial in any heat exchanger application. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.