Patent Publication Number: US-6659172-B1

Title: Electro-hydrodynamic heat exchanger

Description:
This application claims the benefit of provisional application No. 60/080,728 filed Apr. 3, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to heat exchangers. More specifically, the present invention relates to a heat exchanger that is electro-hydrodynamically enhanced. 
     In a typical heat exchanger, heat from a “hot” fluid is transferred to, and carried away by, a coolant. The typical heat exchanger is made of metal, which facilitates the transfer of heat from the hot fluid to the coolant. A bar and plate type heat exchanger made of metal is described in U.S. Pat. No. 5,183,106, which is assigned to the assignee of the present invention. 
     Heat exchangers can also be made of composite materials. See, for example, U.S. Pat. No. 5,628,363, which describes a plate-fin heat exchanger made of, carbon composite. Such composite heat exchangers also facilitate the transfer of heat from the hot fluid to the coolant. However, composite heat exchangers have lower thermal stresses and better corrosion resistance than heat exchangers made of metal. Composite heat exchangers can also be fabricated into complex geometries more easily than metal heat exchangers. U.S. Pat. No. 5,628,363, also assigned to the assignee of the present invention, is incorporated herein by reference. 
     However, heat transfer efficiency of heat exchangers in general is limited by the thermal conductivity of their structural materials (e.g., metal, composite). Heat transfer efficiency is also limited by the convective coefficient of the fluids flowing through the heat exchanger. 
     Increasing the heat transfer efficiency would allow size and weight of the heat exchanger to be reduced. Smaller, lighter, more efficient heat exchangers would be able to remove more heat than larger, heavier, less efficient heat exchangers. In the aerospace industry, for example, it is extremely desirable to increase the efficiency and reduce the weight of heat exchangers used on board aircraft. Reducing the weight reduces fuel consumption. Reducing fuel consumption, in turn, reduces the cost of operating the aircraft. 
     SUMMARY OF THE INVENTION 
     The present invention can be regarded as a heat exchanger that can be electro-hydrodynamically enhanced to increase heat transfer efficiency. The heat exchanger includes a plurality of plates stacked in a substantially parallel spaced-apart relationship, and a plurality of spacers located between the plates. The spacers and the plates cooperate to define hot-side and cold-side passageways. The plates are thermally and electrically conductive, and the spacers are electrically non-conductive. Such a heat exchanger allows an electric field to be placed across the plates. Applying the electric field causes the heat exchanger to be electro-hydrodynamically enhanced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a counter flow heat exchanger according to the present invention; 
     FIG. 2 is a schematic diagram of the heat exchanger while a voltage is being applied thereto; 
     FIG. 3 is an illustration of a carbon/carbon plate for a composite heat exchanger according to the present invention; and 
     FIG. 4 is a method of operating a heat exchanger according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a heat exchanger  10  including a stack of flat parallel plates  12  and  14  that are thermally conductive. The plates  12  and  14  are stacked in a substantially parallel, spaced-apart relationship. The heat exchanger  10  further includes a plurality of spacers  16  located between the plates  12  and  14 . In addition to separating the plates  12  and  14 , the spacers  16  cooperate with the plates  12  and  14  to define hot-side passageways  18  and cold-side passageways  20 . The passageways  18  and  20  can be arranged in rows and columns. In FIG. 1, the passageways  18  and  20  are shown as being arranged in a counterflow configuration, which provides either counter or parallel flow. The passageways  18  and  20  are of sufficient size to accomplish the desired overall transfer of heat from a hot fluid flowing through the hot-side passageways  18  to a coolant flowing through the cold-side passageways  20 . 
     The plates  12  and  14  are made of a material that is electrically conductive as well as thermally conductive. The plates  12  and  14  can be made of a metal such as aluminum, copper or stainless steel. In the alternative, the plates  12  and  14  can be made as a carbon composite or carbon/carbon. Carbon composite plates include carbon fibers in a resin matrix. Carbon/carbon replaces the resin with carbon deposited by a process such as chemical vapor deposition. Fabrication of the carbon/carbon plates is disclosed in U.S. Ser. No. 08/601,754 filed on Apr. 12, 1996, assigned to the assignee of the present invention, and incorporated herein by reference. 
     The spacers,  16 , which can be bonded to the plates  12  and  14 , are made of a material that is electrically non-conductive. The non-conductivity of the spacers  16  allows a high voltage (but very low current) to be applied to the heat exchanger  10 . The voltage creates a controllable electric field across the heat exchanger  10 . The electric field affects the fluids flowing through the passageways  18  and  20  and provides greater heat transfer from the hot fluid to the coolant. Resulting from the electric field is an electro-hydrodynamically enhanced heat exchanger  10 . 
     The voltage can be applied to the plates  12  and  14  by electrical conductors  19  and  21  in such a manner that opposing plates  12  and  14  form anode-cathode pairs (see FIG.  2 ). That is, for each pair of opposing plates, one of the opposing plates  12  collects a positive charge when the voltage is applied, and the other of the opposing plates  14  collects a negative charge when the voltage is applied. The spacers  16  provide electrical insulation between the plates  12  and  14 . 
     The voltage can be applied to edges of the plates  12  and  14 . To make it easier to apply the voltage, the plates  12  collecting the positive charge can have fins  22  extending from one side of the heat exchanger  10 , and the plates  14  accumulating the negative charge can have fins  24  extending from an opposite side of the heat exchanger  10 . 
     The hot fluid circulated through the hot-side passageways  18  and the coolant circulated through the cold-side passageways  20  are also electrically non-conductive. The coolant, for example, can be a two-phase refrigerant. 
     The strength of the electric field depends partly upon the dielectric properties of the hot fluid and the coolant and partly upon the spacing between the plates  12  and  14 . As the voltage is increased, the electro-hydrodynamic effect will be increased. However, the voltage cannot be so high as to cause a dielectric breakdown. 
     A hot-side inlet manifold (not shown) is provided to distribute the hot fluid to the hot-side passageways  18 , and a hot-side outlet manifold (not shown) is provided to collect the fluid leaving the hot-side passageways  18 . A cold-side inlet manifold (not shown) is provided to distribute the coolant to the cold-side passageways  20 , and a cold-side outlet manifold (not shown) is provided to collect the fluid leaving the cold-side passageways  20 . A manifold arrangement is disclosed in U.S. Ser. No. 08/980,122 filed on Nov. 26, 1997, assigned to the assignee of the present invention and incorporated herein by reference. 
     FIG. 3 shows a carbon/carbon plate  12 ′ and spacers  16 ′ for a composite heat exchanger. The carbon/carbon plate  12 ′ might be anisotropic or isotropic, depending upon how its carbon fibers are oriented. The plate  12 ′ is conductive along the direction of the fibers. Isotropic materials are conductive along all three orthogonal axes (x, y and z) while anisotropic materials may have different conductivities along the three axes. If the plate  12 ′ has carbon fibers are oriented in a single direction, heat flow and electrical conductivity in the plate  12 ′ will be unidirectional. 
     For a carbon/carbon plate  12 ′ having fibers oriented in a single direction, an electrode  26 ′ traversing the fibers is attached to the fin  22 ′. The electrode  26 ′ receives the voltage and distributes the voltage to the fibers in the plate  12 ′. For example, the electrode  26 ′ could extend along the y-axis for carbon fibers oriented along the z-axis. The electrode  26 ′ could have a lower profile than the spacers  16 ′. 
     The spacers  16 ′ could be made of a high electrical resistance or insulating material such as fiberglass or a ceramic. In the alternative, the spacers  16 ′ could be made of an electrically non-conductive carbon. Spacers  16 ′ made of non-conductive carbon could be formed integrally with the carbon/carbon plate  12 ′. 
     FIG. 4 shows the operation of the heat exchanger. A coolant is circulated through the cold-side passages  20  (block  100 ), and a hot fluid is circulated through the hot-side passages  18  (block  102 ). When a voltage is applied to the plates  12  and  14  (block  104 ). the resulting electric field across the plates  12  and  14  causes an increase in the efficiency of the heat exchanger  10 . Efficiency of the heat exchanger  10  and, therefore, heat transfer can be controlled by varying the voltage (block  106 ). 
     Thus disclosed is a heat exchanger that can be electro-hydrodynamically enhanced. Electro-hydrodynamic enhancement can increase the heat transfer efficiency of the heat exchanger. Resulting can be a smaller, lighter heat exchanger. 
     The heat exchanger can be made of composite materials. Composite heat exchangers offer certain advantages over metal heat exchangers. Composite heat exchangers offer better corrosion resistance, lower thermal stress and, therefore, a longer operating life. 
     Heat transfer efficiency of the electro-hydrodynamically enhanced heat exchanger can be controlled by varying the voltage applied to the plates. This could eliminate the need for flow control valves and other mechanical flow regulators. 
     The invention is not limited to the specific embodiments described above. For example, the heat exchanger may have a cylindrical, circular or conical configuration. The plates may be made of metal, carbon/carbon or any other material having high thermal and electrical conductivity. The number of plates, spacers and passageways would be selected and sized to provide the required heat transfer or exchange capability for the intended application. Surface enhancements of the plates may be made to further increase turbulence of the hot fluid and/or the coolant. The surface enhancements might take the form of perforations, artificial roughness or louvers. 
     Thus, the invention is not limited to the specific embodiments described above. Instead, the invention is to be construed according to the claims that follow.