Abstract:
Apparatus for cooling bleed air on an aircraft may include a source of cooling fluid driven by an engine of the aircraft, a source of bleed air driven by the engine and a heat exchanger configured allow the cooling fluid to pass over tubes through which the bleed air flows. The heat exchanger may have a high-temperature zone constructed from material with a first density, and a low-temperature zone constructed from material with a second density lower than the first density.

Description:
GOVERNMENT RIGHTS 
       [0001]    This invention was made with Government support under Contract FA8650-09-d-2922, program GE AETD Ti HX Demo awarded by U.S. Air Force. The Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to heat exchangers and, more particularly, to heat exchangers that may be employed in an aircraft to cool bleed air from an engine or on ground vehicles to cool exhaust gas or to heat compressed air. 
         [0003]    In a typical turbine-engine powered aircraft, bleed air may be extracted from one or more engines and employed to operate various ancillary systems of the aircraft. For example, bleed air may be employed to drive an environmental control system (ECS), wing ice protection systems, fuel tank inerting and the like. In order to effectively use bleed air in such systems the bleed air must first be cooled after being extracted from an engine compressor. 
         [0004]    Bleed air is usually discharged from an engine compressor at a temperature of 1200° F. or higher. The bleed air may be cooled, in one or more heat exchangers, to a temperature of about 300° F. or lower before being introduced into an ancillary system of the aircraft. Heat exchangers that are capable of withstanding inlet temperatures of 1200° F. are typically constructed from dense materials such as stainless steel or nickel based alloy. 
         [0005]    Bleed air cooling may be performed by passing the bleed air through one or more heat exchangers which may employ ambient air as a cooling medium. In some instances, the ambient cooling air may be by-pass air propelled with a by-pass fan of the engine or ram air or air from an external fan. In this context, a heat exchanger capable of reducing temperature from 1200° F. to 300° F. must be large enough to allow for sufficient residence time of the bleed air to achieve the requisite reduction of temperature. Such a heat exchanger may be quite heavy when constructed from dense stainless steel or nickel based alloy. 
         [0006]    As can be seen, there is a need for a system for reducing bleed air temperature without incurring a weight penalty associated with a heat exchanger constructed entirely from dense high-temperature tolerant materials. 
       SUMMARY OF THE INVENTION 
       [0007]    In one aspect of the present invention, apparatus for cooling bleed air on an aircraft may comprise: a source of cooling fluid driven by an engine of the aircraft; a source of bleed air driven by the engine; a heat exchanger configured to allow the cooling fluid to pass over tubes through which the bleed air flows, the heat exchanger having, a) a high-temperature zone constructed from material with a first density, and b) a low-temperature zone constructed from material with a second density lower than the first density. 
         [0008]    In another aspect of the present invention, a heat exchanger may comprise: a high-temperature zone constructed from material with a first density, and a low-temperature zone constructed from material with a second density lower than the first density. 
         [0009]    In still another aspect of the present invention, a method for cooling hot fluid may comprise the steps: passing the hot fluid through first tube segments of a heat exchanger; passing the hot fluid through second tube segments of the heat exchanger directly from the first tube segments, the second tube segments having a lower temperature tolerance than the first tube segments; passing cooling fluid over the first segments to cool the hot fluid to a temperature within a range of temperature tolerance of the second tube segments; and passing the cooling fluid over the second segments to further cool the hot fluid. 
         [0010]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic diagram of a cooling system in accordance with an embodiment of the invention; 
           [0012]      FIG. 2  is a perspective, partially cut-away view of a heat exchanger in accordance with a second embodiment of the invention; 
           [0013]      FIG. 3  is exploded view of a tube of the heat exchanger of  FIG. 2  in accordance with an embodiment of the invention; and 
           [0014]      FIG. 4  is a flow chart of a method for cooling a hot fluid in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0016]    Various inventive features are described below that can each be used independently of one another or in combination with other features. 
         [0017]    The present invention generally provides a system by which hot fluid such as bleed air may be cooled in a heat exchanger that has some portions of its structure comprised of low density materials. 
         [0018]    Turning now to the description and with reference first to  FIG. 1 , a schematic diagram may illustrate a cooling system  100  constructed in accordance with an exemplary embodiment of the invention. An engine  102  of an aircraft (not shown) may be provided with a high-pressure bleed air port  104 , an intermediate-pressure bleed air port  106  and a by-pass air port  108 . A heat exchanger  110  may be positioned to receive hot fluid  111 , such as bleed air, from a hot-fluid inlet line  112  and to receive cooling fluid  113 , such as engine by-pass air, from a cooling-fluid inlet line  114 . Cooled bleed air may be discharged through a hot-fluid outlet line  116  and by-pass air may be discharged through a cooling-fluid outlet line  118 . 
         [0019]    Referring now to  FIG. 2 , the heat exchanger  110  is illustrated in a simplified cut-away format. In an exemplary embodiment, the heat exchanger  110  may be constructed with multiple zones. In the embodiment of  FIG. 2 , three zones are illustrated: a high-temperature zone  120 ; a medium-temperature zone  122 ; and a low-temperature zone  124 . Hot fluid  111  or hot bleed air may enter the heat exchanger  110  at an inlet end  126 . The hot fluid  111  may pass through a plurality of tubes  128  as cooling air  113  passes over the tubes  128 . Within the high-temperature zone  120  each of the tubes  128  may comprise a high-temperature segment  132 . Similarly each of the tubes  128  may comprise a medium-temperature segment  134  in the medium temperature zone  122  and a low-temperature segment  136  in the low-temperature zone  124 . 
         [0020]    In an alternate exemplary embodiment, the heat exchanger  110  may comprise only two zones, the high-temperature zone  120  and the low-temperature zone  124 . In such a two zone configuration, the tubes  128  may include only the high-temperature segment  132  and the low-temperature segment  136 . 
         [0021]    In an exemplary embodiment the tubes  128  may be constructed as brazed assemblies. As shown in  FIG. 3 , the segments  132  and  136  may be provided with at least one bell end  140 . Non-bell ends  142  of the segments  134  may be inserted and brazed into the bell ends  140  of the segments  132 . Similarly, the non-bell ends  142  of the segments  134  may be inserted and brazed into the bell ends  140  of the segments  136 . The segments  132  may be constructed from stainless steel or a nickel-based alloy so that they are capable of withstanding high inlet temperature of 1200° F. or higher. As shown in  FIG. 2 , the segments  134  may be a distance D 1  away from the inlet end  126  of the heat exchanger  110 . The distance D 1  may be great enough so that temperature of the hot fluid  111  may be reduced from about 1200° F. to about 1000° F. The segments  134  may be constructed from titanium or a titanium alloy if exposed to temperatures of 1000° F. or less. The segments  136  may be a distance D 2  away from the inlet end  126  of the heat exchanger  110 . The distance D 2  may be great enough so that temperature of the hot fluid  111  may be reduced to about 600° F. The segments  136  may be constructed from aluminum or an aluminum alloy if exposed to temperature of 600° F. or less. 
         [0022]    A multiple step brazing operation may be employed to construct the heat exchanger  110 . In a first brazing step, the high-temperature segments  132  may be inserted into holes  151  of hot-fluid inlet manifold  150 . The inlet manifold  150  may be constructed from material that can tolerate exposure to inlet temperatures of the hot fluid  111  of 1200° F. or higher. For example, the inlet manifold  150  may be constructed from the same type of material as that used for the segments  132 . High-temperature brazing filler  152  may be employed to produce brazed connections between the segments  132  and the manifold  150 . The brazing filler  152  must be capable of maintaining a solid connection when exposed to hot-fluid inlet temperatures of 1200° F. 
         [0023]    In a second brazing step, the segments  134  may be brazed into a sub-assembly that includes the manifold  150  and the segments  132 . As shown in  FIG. 3 , the ends  142  of the segments  134  may be inserted into the bell ends  140  of the segments  132 . Medium-temperature brazing filler  154  may be employed to produce brazed connections between the segments  132  and the segments  134 . The brazing filler  154  must be capable of maintaining a solid connection when exposed to hot-fluid temperatures of about 1000° F. 
         [0024]    In a third brazing step, the segments  136  may be brazed into a sub-assembly that includes the manifold  150 , the segments  132  and the segments  134 . As shown in  FIG. 3 , the ends  142  of the segments  134  may be inserted into the bell ends  140  of the segments  136 . Low-temperature brazing filler  156  may be employed to produce brazed connections between the segments  134  and the segments  136 . The brazing filler  156  must be capable of maintaining a solid connection when exposed to hot-fluid temperatures of about 600° F. 
         [0025]    During the third brazing step, the segments  136  may be brazed into holes  161  of a hot-fluid outlet manifold  160 . The outlet manifold  160  may be constructed from the same type of material as the segments  136 . The brazing filler  156  may be employed to perform brazing of the outlet manifold  160 . 
         [0026]    The heat exchanger  110  may weigh less than a high-temperature heat exchanger constructed completely from stainless steel or nickel-based alloy. The segments  134 , which may be constructed from titanium or titanium alloy, may be less dense than equivalent sections of tubing constructed from stainless steel or nickel-based alloy. Similarly, the segments  136  and the outlet manifold  160  which may be constructed from materials less dense than equivalent elements constructed from stainless steel, nickel-based alloy. For example, in one of the heat exchangers with only two temperature zones, the segments  136  and the outlet manifold  160  may be titanium or titanium based alloy. In one of the heat exchangers  110  that is constructed with three temperature zones, the low-temperature segments  136  and the outlet manifold may be aluminum or aluminum based alloys. Through employment of these lower density materials the light-weight heat exchanger  110  may be particularly suited for weight-critical applications such as aircraft or other aerospace vehicles. 
         [0027]    It may be noted that the heat exchanger  110  may be vulnerable to damage under conditions in which cooling fluid flow is interrupted while high-temperature fluid passes through the heat exchanger. Under these circumstances, the medium temperature brazing filler  154  and the low-temperature brazing filler  156  may be exposed to hot fluid temperatures of about 1200° F. However, these problematic conditions will not occur when the heat exchanger  100  is employed as an element in the cooling system  100  of  FIG. 1 . In the cooling system  100 , the hot fluid  111 , i.e., bleed air, is produced only when the engine  102  is running. The cooling fluid  113 , in this case by-pass air, is continuously produced whenever the engine  102  is running. Thus cooling fluid flow will cease only when bleed air flow ceases. Consequently, the light-weight heat exchanger  110  may be employed in the cooling system  100  without concern for risk of damage that might result from cessation of cooling fluid flow. 
         [0028]    Referring now to  FIG. 4 , a flow chart  400  may illustrate a method for cooling hot fluid. In a step  402 , hot fluid may be passed through first tube segments of a heat exchanger (e.g., hot fluid  111  may be passed though tube segments  132  of the heat exchanger  110 ). In a step  404 , the hot fluid may be passed through second tube segments of the heat exchanger directly from the first tube segments, the second tube segments having a lower temperature tolerance than the first tube segments (e.g. the hot fluid  111  may be passed directly from the tube segments  132  directly into the tube segments  134 ). In a step  406 , cooling fluid may be passed over the first segments to cool the hot fluid to a temperature within a range of temperature tolerance of the second tube segments (e.g., the cooling fluid  113  may be passed over the tube segments  132 ). In a step  408 , the cooling fluid may be passed over the second segments to further cool the hot fluid. 
         [0029]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.