Abstract:
An apparatus providing high efficiency heat exchange between two fluids is disclosed. The apparatus most commonly comprises a flat panel with microchannels directing the flow of the two fluids, specifically: with a small hydraulic diameter in order to increase the heat transfer effect; while, at the same time, the flow length and cross-section of the microchannels are controlled to reduce the pressure losses normally associated with such small hydraulic diameters.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This Application is a continuation-in-part of co-pending application Ser. No. 10/003,882, filed Oct. 25, 2001 and published as U.S. patent application 2002/0125001 A1, the disclosure of which is incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002]     The work leading to the invention described herein was made with Government support under Grant No. DABT63-95-C-0020 awarded by the Defense Advanced Projects Research Agency. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention relates to heat exchangers, particularly very high efficiency flat panel microchannel heat exchangers.  
       SUMMARY OF THE INVENTION  
       [0004]     Heat exchangers are used in a great many mechanical and electrical systems. Some of the most commonly known applications include the condenser of a refrigerator or the radiator of an automobile. Increasingly, customers have demanded high efficiency heat exchangers for cooling computer chips and other electronic components. This demand, among others, produces a continuing need for increased efficiency in a smaller volume.  
         [0005]     Compact heat exchangers are used in applications which demand low pressure drops in the working fluids and low overall mass or weight. Until recently, such a heat exchanger was incapable of transferring the amounts of heat necessary for certain applications, such as an automobile. It is desirable to reduce the size of the necessary heat exchanger while maintaining the rate of heat exchange.  
         [0006]     The rate of heat transfer for a given heat exchanger is related to the surface area-to-volume ratio of the fluid channels. Advances in microfabrication allow the design of a heat exchanger to increase the amount of surface area in relation to the overall volume. As is well understood in the field, the overall heat transfer of a given fluid channel increases when the hydraulic diameter of that channel decreases. So-called microchannels provide the reduction in hydraulic diameter necessary to produce the required heat transfer performance.  
         [0007]     In prior heat exchangers, the hydraulic diameter of at least one set of fluid channels is reduced. In exchange for this increase in heat transfer performance, however, prior heat exchangers impose a steep penalty in lost pressure (pump losses) in the working fluids. That pressure loss has been the limit on reduction in size.  
         [0008]     Typically, the prior art defines a microchannel as any fluid flow channel with a smallest dimension less than 2.0 mm. See, e.g., WO 2004/017008 A1.  
         [0009]     In further illustration, U.S. patent application 2002/0125001 A1 discusses, in detail, the history of so-called micro heat exchangers and highlights the high pressure drops created in those designs. The heat exchanger disclosed in that application consists of two sets of microchannels. The length of the first fluid channels, claimed for the first time, is less than 6.0 mm and preferably less than 1.0 mm. At the same time, that heat exchanger shows that the first fluid channels number at least 50 per square centimeter, preferably as much as 1000 per square centimeter.  
         [0010]     We have discovered a better design which captures the benefits of microchannels through increased heat transfer efficiency and limits the pressure drop to a range which makes the heat exchangers preferable in terms of performance, weight, size and cost. Instead of maximizing the number of fluid channels, we have optimized the cross-section of the microchannels to maintain the increased heat transfer performance without suffering pumping losses to the extent the previous design requires.  
         [0011]     The current invention uses microchannels for the first fluid flow (typically a liquid such as water).  
         [0012]     The current invention also uses microchannels for the second fluid flow (typically a gas such as air), substantially reducing the required thermal diffusion lengths for the first fluid. The reduced fluid flow length provides a much greater value for heat transfer per unit volume or per unit mass than has been achieved with traditional heat exchanger design. At the same time, the reduction in thermal diffusion length of the microchannels offsets the pressure loss caused by the reduction in hydraulic diameter.  
         [0013]     The fluid channels in the first set may have a greater flow length than that provided by the first fluid channels, but tend to have a smaller nominal cross-sectional area.  
         [0014]     With both fluids contained in microchannels, the current invention does not require secondary surfaces such as fins or pins to achieve high thermal efficiency. This allows for simpler designs and ease of manufacturing.  
         [0015]     As discussed in U.S. patent application 2002/0125001 A1, the innovation which provides intersecting flows between the first fluid channels and the second fluid channels serves to allow relatively short flow lengths in the gas-side channels. It is well-understood in the art that increased flow length in a microchannel provides very little benefit in heat transfer but produces a greater pressure drop.  
         [0016]     In its preferred embodiment, the current invention is a unitary body, flat panel heat exchanger. The fluid channels described are contained in the panel. This embodiment allows heat transfer through primary surfaces between two fluids flowing in substantially perpendicular (or at least non-parallel) channels. One fluid, typically water or a water/ethylene glycol mixture, flows in the plane of the flat panel heat exchanger. The other fluid, typically air or some other gas, flows in channels perpendicular to the plane of the coolant flow—typically across the shortest dimension of the flat panel.  
         [0017]     In its preferred embodiment, the shortest dimension of the flat panel, and the fluid flow length for the gas, is less than 8.0 mm. Other than the heat exchanger described in U.S. patent application 2002/0125001 A1, it is believed no prior gas-fluid cross-flow heat exchangers have been thinner than about 2.0 cm in this dimension.  
         [0018]     In the majority of its current applications, the current invention provides heat transfer from a gas to a liquid. The fluid channels for the gas usually provide very short thermal diffusion lengths. The current invention uses microchannels with a typical width from 0.200 mm to 1.0 mm. As is well understood in the art, this reduction in size causes increased pressure gradients along the length of the channels. To combat that increase, the current invention uses a large number of these microchannels, providing parallel flow paths for the gas which intersect the plane of the coolant flow. As opposed to the competing design, the current invention does not maximize the number of these cross flow channels.  
         [0019]     Instead, the current invention provides the cross flow channels in the shape of substantially flat slots. Flat slots provide a small hydraulic diameter but allow for greater flow area to the fluid, resulting in reduced pressure drop without a loss of heat transfer efficiency. At the same time, flat slots are easier to manufacture than numerous smaller features.  
         [0020]     For the same reasons, the preferred embodiment of the current invention provides substantially flat slots for the coolant flow.  
         [0021]     It has been discovered that the use of flat slots allows the current invention to provide a highly increased open area to the gas flow. That is, the area of open channels divided by the total frontal area seen by the gas side flow is more than 25%, and potentially as high as 50%.  
         [0022]     Finally, the flat slot geometry used allows for much reduced wall thicknesses in both sets of microchannels. Prototypes of the current invention result in wall thicknesses as low as 75 μm (0.075 mm). As is well understood in the art, the thermal resistance of a solid is proportional to the length of the conduction path. With conduction paths as short as 75 μm, even materials with poor thermal conductivity are effective in the construction of the current invention. This allows the use of polymers, ceramics, and other materials previously thought inappropriate for use in heat exchangers.  
         [0023]     A further advantage of the current invention is that it is a primary surface heat exchanger. This means that the surfaces are all transferring heat from one fluid to the other. The use of features such as pins, fins and the like require material which has a low thermal resistance. Just as with the reduced wall thickness created by our design, materials with poor thermal conductivity are appropriate if indicated by other concerns, such as strength, corrosive materials or weight.  
         [0024]     As an added bonus, once the flat panel cross flow micro heat exchanger is created, a person having ordinary skill in the art would be able to design heat exchanger systems which incorporate multiple panels in various alignments, such as stacking panels. One example of a heat exchanger system incorporates several flat panels in a corrugated pattern to increase thermal transfer as a factor of frontal area.  
         [0025]     These and other embodiments and features of the present invention will become even more apparent from the following detailed description of preferred embodiments, the accompanying figures, and the appended claims. As used in this description, the term “microchannel” is used to mean a channel with at least one dimension on the scale of 2.0 mm or less. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  illustrates schematically a cross section of an embodiment of a flat panel cross flow micro heat exchanger in accordance with the present invention.  
         [0027]      FIG. 2  depicts a Scanning Electron Microscope view of a high efficiency flat panel micro-channel heat exchanger.  
         [0028]      FIG. 3  is a picture showing a complete view of the high efficiency flat panel micro-channel heat exchanger panel, including tubes which provide the coolant-side flow to the panel.  
         [0029]      FIG. 4  is a photograph of an heat exchanger system employing dual high efficiency flat panel microchannel heat exchangers to increase heat transfer as a function of frontal area.  
         [0030]      FIG. 5  is a representation of a corrugated multi-panel microchannel heat exchanger, one of the applications of the current invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     The schematic illustration of a cross section of an embodiment of a flat panel cross flow micro heat exchanger is shown in  FIG. 1  (not drawn to scale). In  FIG. 1 , the cross-hatched regions denote solid structures through which fluid may not flow. The dotted regions denote channels through which the coolant fluid may flow in the plane of the figure, and the open squares denote cross-sections of the channels through which air, or some other fluid, may flow perpendicular to the plane of the figure.  
         [0032]     The current invention employs microchannels as fluid flow channels for both fluids. Typical dimensions range from 200 μm to 1.0 mm.  FIG. 2  is a SEM view of one embodiment of the high efficiency flat panel microchannel heat exchanger.  
         [0033]     In exchange for the benefits of reduced thermal resistance, microchannels result in an increase in pressure losses compared to macro-scale channels. The current invention alleviates that concern by keeping the total flow length of the microchannels small. In addition, the advanced manufacturing technique provides thinner walls, as well as maximizing the open area available to the fluid.  
         [0034]     Open Area is defined as the sum of the cross-sectional area of all the channels for a given fluid divided by the total frontal area for a given flow. Prior inventions are believed to provide an open area to the gas-side flow of less than 25%. The current innovation allows open area to the gas-side flow above 25% and, perhaps, as high as 50%. As previously mentioned, pressure losses are reduced as open area is increased.  
       FABRICATION OF THE INVENTION  
       [0035]     The invention may be manufactured by any of several methods. An early prototype was manufactured in two halves and bonded together. Each side was made using the LIGA process.  
         [0036]     The LIGA process produces microstructures and is well-known in the art. See, e.g., A. Maner et al., “Mass Production of Microdevices with Extreme Aspect Ratios by Electroforming,” Plating and Surface Finishing, pp. 60-65 (March 1988); W. Bacher, “The LIGA Technique and Its Potential for Microsystems—A Survey,” IEEE Trans. Indust. Electr., vol. 42, pp. 431-441 (1995); and E. Becker et al., “Production of Separation-Nozzle Systems for Uranium Enrichment by a Combination of X-Ray Lithography and Galvanoplastics,” Naturwissenschaften, vol. 69, pp. 520-523 (1982).  
         [0037]     LIGA can be used to create an array of high aspect ratio microstructures on a conductive substrate. Electroplating (often nickel, in the most common embodiments) is performed to fill the “open” volumes in the LIGA array with metal. The array is dissolved through chemical processes known in the art. This leaves a metal mold insert with micro-scale features (here, channels).  
         [0038]     This mold insert is used to mold or emboss a polymer. The resulting polymer negative of the mold insert is covered with a layer of conductive metal, such as gold, through well known processes, such as sputtering. The metal-coated polymer can be electroplated, creating a metal shell around the polymer core. Finally, the polymer can be dissolved by chemical processes known in the art. The remaining metal shell provides a hollow shell with flow paths for two intersecting streams.  
       Application of Flat Panel Heat Exchangers  
       [0039]     Once in possession of the high efficiency flat panel microchannel heat exchanger, a person having ordinary skill in the art will be able to optimize heat exchanger systems designs using the panel. For instance, the size of the microchannels may be changed, the exterior dimensions may all be changed, and the panels may be mounted at some angle other than perpendicular to the gas side flow.  
         [0040]      FIG. 3  is a picture of a high efficiency flat panel microchannel heat exchanger made part of a heat exchanger system which delivers coolant to the panel through tubes. Each tube has a slot in which the panel may be mounted. The panel is brazed or otherwise fixedly connected to the tubes.  
         [0041]      FIG. 4  includes several pictures of a more complex heat exchanger system. In this system, four (4) large high efficiency flat panel microchannel heat exchangers are mounted in a v-configuration. This system allows greater thermal efficiency in a small frontal area than a single panel would allow.  
         [0042]     Finally,  FIG. 5  is a conceptual design using multiple high efficiency flat panel microchannel heat exchangers in a corrugated arrangement. The dimensions shown are illustrative only. Persons having ordinary skill in the art will be able to use these high efficiency flat panel microchannel heat exchangers in a variety of configurations which might take advantage of the unique qualities of the panel.  
       Theoretical and Experimental Performance of High Efficiency Flat Panel Microchannel Heat Exchangers  
       [0043]     Experimental and theoretical models provide results which highlight the significant impact that the current invention provides. These results demonstrate the potential of high efficiency flat panel microchannel heat exchangers representing substantial improvement over existing systems. In addition to performance factors, high efficiency flat panel microchannel heat exchangers made in accordance with this invention provide advantages in terms of ease of use and ease of manufacturing.  
         [0044]     Theoretical discussion of flow in channels reveals that a fully developed flow occurs after a region of entrance effects. In a fully developed flow, the Nusselt number becomes constant, according to Equation 1:  
             Nu   =         hD   h     k     =     const   .               (   1   )             
 
 Where: 
        Nu=the Nusselt number of the fully developed flow     h=the convective heat transfer coefficient     D h =the hydraulic diameter of the channel     k=the thermal conductivity of the fluid 
 
 Since the thermal conductivity (k) of a given fluid is constant, the heat transfer coefficient (h) must increase when the hydraulic diameter (D h ) of the channel is reduced. These increases are highly valuable for gas streams where the low thermal conductivity of the gases (relative to liquids) leads to low gas-side heat transfer coefficients. 
       
 
         [0049]     The increase in heat transfer coefficient in microchannels allows for a significant reduction of the heat exchanger size with negligible increase in the pressure drop for that stream. Equation 2 approximates the heat transfer to or from a fluid, air for instance, passing through a cross-flow heat exchanger. Equation 3 shows the pressure drop across the ends of the heat exchanger (not including the inlet and exit contraction/expansion losses), assuming fully developed laminar flow, a standard assumption for the analysis of microchannel flows.  
                   T   coolant     -     T     air   ⁢     -     ⁢   exit             T   coolant     -     T     air   ⁢     -     ⁢   inlet           =     exp   ⁡     [       -       β   ⁢           ⁢     k   air           ρ   air     ⁢     C     p   ⁢     -     ⁢   air       ⁢   V         ·     L     D   h   2         ]               (   2   )                 Δ   ⁢           ⁢   P     =     32   ⁢   V   ⁢           ⁢     μ   air     ⁢     L     D   h   2                 (   3   )             
 
 Nomenclature: 
        T coolant =Heat exchanger average wall (also coolant) temperature     T air-inlet =Temperature of the air entering the heat exchanger     T air-exit =Temperature of the air exiting the heat exchanger     ΔP=Air pressure drop across the heat exchanger thickness     β=Constant depending upon the Reynolds number of the air flow     k air =Air thermal conductivity     L=Length of the air microchannels (also, heat exchanger thickness)     ρ air =Air density     C p-air =Air constant pressure specific heat     V=Air velocity through (or across) the heat exchanger     D h =Hydraulic diameter of the micro (air) channels     μ air =Air dynamic viscosity        
 
         [0062]     It can be readily observed that the geometric parameter that controls both the heat transfer and pressure drop of the gas stream is L/D h   2 . Keeping L/D h   2  constant and reducing the hydraulic diameter D h , therefore, allows the reduction in heat exchanger thickness while maintaining the same heat transfer and pressure drop characteristics. That is, there is no pressure drop penalty (to the first order) associated with making a heat exchanger for a given thermal load small by using micro-scaled flow passages.  
         [0063]     Nonetheless, there do not appear to be any competing heat exchangers which are designed to take advantage of this scaling opportunity. Although some heat exchangers operate in the range of L/D h   2  between 5 mm −1  and 30 mm −1 , they do not also use an overall length less than 8.0 mm, as in the current invention.  
         [0064]     Within the parameters discussed above, using long slots as the cross-sectional profile for the channels provides ease of manufacturing compared to smaller features. In addition, long slots provide small hydraulic diameter while leaving a large total area for fluid flow. This selection improves the pressure drop performance without degrading the thermal performance.  
         [0065]     Each and every patent, patent application and printed publication referred to above is incorporated herein by reference in toto to the fullest extent permitted as a matter of law.  
         [0066]     This invention is susceptible to considerable variation in its practice. The forgoing description, therefore, is not intended to limit, and should not be construed as limiting, the invention to the particular embodiments presented hereinabove. Rather, what is intended to be covered is as set forth in the ensuing claims and the equivalents thereof permitted as a matter of law.