BACKGROUND OF THE INVENTION 
     This invention relates generally to turbine engines, and more particularly to a system and method using heat pipes for transferring heat within a gas turbine engine. 
     Gas turbine engines use pressurized oil to lubricate and cool various components (e.g. bearings, etc.). The oil picks up significant heat in the process which must be rejected to maintain the oil temperature within acceptable limits. Prior art gas turbine engines often employ heat exchangers to cool the engine oil using a relatively cool air stream such as fan discharge air. In turbofan engines, this heat exchanger is often located in the fan duct flow path. This configuration results in a pressure loss and hence a significant fuel burn penalty. It has been estimated that the specific fuel consumption (SFC) penalty associated with this type of configuration can be as high as 1%. There are also cost and weight penalties associated with this configuration. 
     In addition, in some engines, outlet guide vanes (OGVs), fan struts, or other strut-like members in the fan duct downstream of the fan accrete ice under certain environmental conditions. Ice accumulation within the engine and over exposed engine structures may be significant. The accreted ice may lead to partial blocking of the OGV passages and fan instability. The accumulated ice can also be suddenly shed, for example through continued operation of the engine, a throttle burst from lower power operation to higher power operation, or vibrations due to either turbulence or asymmetry of ice accretion. 
     Various prior art methods exist for anti-icing, for example, running the engine with an increased operating temperature, directing high temperature bleed air from the engine compressor to the exposed surfaces, spraying the engine with a deicing solution prior to operation, and electric resistance heating. However, all of these methods have various disadvantages. The increased operating temperature and the bleed systems may decrease engine performance. Such systems may also require valves to turn off the flow of the high temperature air during take-off and other high power operations to protect the engine. Deicing fluid provides protection for only a limited time. Electrical heating requires large quantities of electricity for performing the de-icing operation and may require additional electrical generators, electrical circuits and complex interaction logic with the airplane&#39;s computers with the attendant increased cost, weight and performance penalties. 
     BRIEF SUMMARY OF THE INVENTION 
     The above-mentioned shortcomings in the prior art are addressed by the present invention, which provides a heat transfer system that removes waste heat from the engine lubrication oil and transfers that heat to engine components that require heating, for example for anti-icing or de-icing purposes. This heat is transferred using heat pipes which are lightweight, sealed, and passive, requiring no valves or pumps. Furthermore, the heat pipes may use a working fluid which is non-flammable to avoid creating a fire hazard within the engine. 
     According to one aspect, the invention provides a heat transfer system for a turbine engine of the type including an annular casing with an array of generally radially-extending strut members disposed therein. The heat transfer system includes: at least one primary heat pipe disposed at least partially inside a selected one of the strut members; and at least one secondary heat pipe disposed outside the fan casing and thermally coupled to the at least one primary heat pipe and to a heat source, such that heat from the heat source can be transferred through the secondary heat pipe to the primary heat pipe and to the selected strut member. 
     According to another aspect of the invention, a gas turbine engine includes: an annular fan casing; an array of generally radially-extending guide vanes disposed therein, each guide vane having an airfoil cross-section defined by first and second sides extending between spaced-apart leading and trailing edges; a plurality of primary heat pipes, each primary heat pipe being disposed at least partially inside one of the guide vanes, so as to define a first array of primary heat pipes; and a secondary heat pipe disposed outside the fan casing and thermally coupled to the first array of primary heat pipes and to a heat source, such that heat from the heat source can be transferred through the secondary heat pipe to the primary heat pipes and to the guide vanes. 
     According to another aspect of the invention, a method is provided for transferring heat in a turbine engine having an annular casing with an array of generally radially-extending guide vanes disposed therein. The method includes: providing a plurality of primary heat pipes, each primary heat pipe being disposed at least partially inside one of the guide vanes; providing a secondary heat pipe disposed outside the fan casing and thermally coupled to the first array of primary heat pipes and to a heat source; receiving heat from the heat source in the secondary heat pipes and transferring the heat to the primary heat pipes; and receiving heat from the secondary heat pipes in the primary heat pipes and transferring the heat to the guide vanes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a side cross-sectional view of a fan section of a gas turbine engine including a heat transfer system constructed in accordance with an aspect of the present invention; 
         FIG. 2  is a view of an outlet guide vane taken along lines  2 - 2  of  FIG. 1 ; 
         FIG. 3  is a schematic perspective view of a portion of the fan section of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view of a pair of heat pipes connected in an alternative configuration of a coupler; 
         FIG. 5  is a cross-sectional view of a pair of heat pipes connected in another alternative configuration of a coupler; and 
         FIG. 6  is schematic perspective view of a portion of the fan section of  FIG. 1 , showing the connection of heat pipes to a heat exchanger. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates a portion of a fan section of a gas turbine engine, including an inner housing  10  with a forward-facing splitter  12 , and an annular fan casing  14  with inner and outer surfaces  16  and  18 , which is connected to the inner housing  10  by an array of radially extending fan struts  20 . A plurality of outlet guide vanes (OGVs)  22  extend between the inner housing  10  and the fan casing  14 . Each of the OGVs  22  (also shown in  FIG. 2 ) has a root  24 , a tip  26 , a leading edge  28 , a trailing edge  30 , and opposed sides  32  and  34 . The OGVs  22  are airfoil-shaped and are positioned and oriented to remove a tangential swirl component from the air flow exiting an upstream fan (not shown). In the illustrated example, the fan struts  20  and the OGVs  22 , both of which are “strut members” extending in a generally radial direction, have different functions, the fan struts  20  providing structural support while the OGVs  22  serve an aerodynamic purpose. However, in other engine configurations, these functions may be combined in a single row of generally radially-extending strut members. 
     The OGVs  22  may be constructed from any material which has adequate strength to withstand the expected operating loads and which can be formed in the desired shape. In the illustrated example, the OGVs  22  are formed from a nonmetallic composite material including a matrix with reinforcing fibers disposed therein, such as glass-reinforced plastic, carbon-carbon, or carbon-epoxy. These materials are strong and lightweight, but have a relatively low thermal conductivity as compared to metal alloys. Metals could also be used for the OGVs  22 . Examples of suitable metals include aluminum-, iron-, nickel- or titanium-based alloys. 
     Primary heat pipes  36  are disposed inside one or more of the OGVs  22 . In the illustrated example, a primary heat pipe  36  is placed within the cross-section of the individual OGV  22  near the leading edge  28  and extends parallel to the leading edge  28 . These forward-placed primary heat pipes  36  collectively form a forward array  38  of primary heat pipes  36  (see  FIG. 3 ). Another primary heat pipe  36  is also placed within the cross-section of the OGV  22  in the rear half of the OGV  22 , closer to the trailing edge  30 , and extends parallel to the stacking axis “S” (which in this case is swept rearward from a radial direction). These aft-placed primary heat pipes  36  collectively form an aft array  40  of primary heat pipes  36 . As shown in  FIG. 2 , the portion of the primary heat pipes  36  that lie within the OGV  22  may be formed into an oval, flatted, or other non-circular cross-sectional shape to accommodate a desired cross-sectional area while fitting within the thickness of the OGV  22 . Although not shown, it is also possible that primary heat pipes  36  could be laid into open grooves formed in the sides  32  or  34  of the OGV  22 , in which case the primary heat pipes  36  would form a part of the surface of the sides  32  or  34 , respectively. It is also possible that primary heat pipes  36  could be placed within the fan struts  20  if desired. 
     Each primary heat pipe  36  has an elongated outer wall  42  with closed ends which defines a cavity  44 . A portion at or near the end of each primary heat pipe  36  that protrudes through the fan casing  14  is designated as the “hot” or “evaporator” portion  45  (see  FIG. 3 ). The portion of the primary heat pipe  36  which is placed within the OGV  22  is designated as a “cold” or “condenser” portion  46  (See  FIG. 1 ). The cavity  44  is lined with a capillary structure or wick (not shown) and holds a working fluid. Various working fluids, such as gases, water, organic substances, and low-melting point metals are known for use in heat pipes. The working fluid may be non-flammable so as to avoid introducing a fire hazard into the area of the fan casing  14 . 
     The primary heat pipes  36  are highly efficient at transferring heat. For example, their effective thermal conductivity is several orders of magnitude higher than that of solid copper. The number, length, diameter, shape, working fluid, capillary structure, and other performance parameters of the primary heat pipes  36  are selected based on the desired degree of heat transfer during engine operation. The operation of the primary heat pipes  36  are described in more detail below. 
     One or more secondary heat pipes  48  are disposed around the exterior of the fan casing  14  adjacent the primary heat pipes  36 . In the illustrated example, a first pair of secondary heat pipes  48 A is provided. Each secondary heat pipe  48 A forms nearly a 180 degree arc around the fan casing  14  adjacent the outer, hot portions  45  of the forward array  38  of primary heat pipes  36 . Another pair of secondary heat pipes  4813  is also provided. Each secondary heat pipe  48 B forms nearly a 180 degree arc around the fan casing  14  adjacent the outer, hot portions  45  of the aft array  40  of primary heat pipes  36 . It is also possible that the secondary heat pipes  48 A and  48 B could be comprised of multiple arc segments each surrounding a portion of the fan casing  14  (e.g. 8, 12, or 16 segments used to cover the complete circumference of the fan casing  14 ). By selectively insulating portions of these arc segments, the circumferential heat distribution can be equalized as desired. 
     The secondary heat pipes  48  are similar in general construction to the primary heat pipes  36 . As shown in  FIG. 1 , each secondary heat pipe  48  has an elongated outer wall  50  with closed ends which defines a cavity  52 . One portion near a terminal end of each secondary heat pipe  48  is designated as the “hot” or “evaporator” portion  54 , while other portions are designated as a “cold” or “condenser” end or portion  56 . It should be noted that terms “hot”, “evaporator”, “cold”, and “condenser”, when used in relation to the primary and secondary heat pipes  36  and  48 , describe the positioning of the heat pipes in areas of relatively high or low temperature, and are not related to any particular aspect of the structure of the heat pipes themselves. The cavity  52  is lined with a capillary structure or wick (not shown) and contains a working fluid. Various working fluids, such as gases, water, organic substances, and low-melting point methods are known for use in heat pipes. The working fluid may be non-flammable so as to avoid introducing a fire hazard into the area of the fan casing  14 . 
     The secondary heat pipes  48  are also highly efficient at transferring heat. For example, their effective thermal conductivity is several orders of magnitude higher than that of solid copper. The number, length, diameter, shape, working fluid, and other performance parameters of the secondary heat pipes  48  are selected based on the desired degree of heat transfer during engine operation. The operation of the secondary heat pipes  48  are described in more detail below. 
     At each location where a primary heat pipe  36  meets a secondary heat pipe  48 , the primary heat pipe  36  extends in a tangential direction, and the two are joined together using couplers  58 . The couplers  58  are made of a material with relatively high thermal conductivity, such as a metal alloy, and are assembled, bonded, molded, or otherwise formed around the primary and secondary heat pipes  36  and  48 . In the example shown in  FIG. 1 , the primary and secondary heat pipes  36  and  48  are of a circular cross-section and contact each other essentially along a line parallel to the length of the coupler  58  in the tangential direction. 
     The joints between the primary and secondary heat pipes  48  may be formed in a number of ways to increase the efficiency of heat transfer. For example,  FIG. 4  depicts a possible configuration in which a filler  60  is disposed inside the coupler  58  in the voids between the two heat pipes. Any material with relatively high thermal conductivity may be used, such as metals, conductive pastes, or plastics. The use of the filler  60  effectively increases the surface area contact between the primary and secondary heat pipes  36  and  48  and thus improves heat transfer. 
       FIG. 5  depicts another possible configuration using modified primary and secondary heat pipes  36 ′ and  48 ′. At least the portions of the primary and secondary heat pipes  36  and  48  that are contained within the coupler  58  are formed in into complementary non-circular shapes, so that the primary and secondary heat pipes  36  and  48  have abutting walls  62  and  64  with substantial conforming contact to enhance heat transfer. 
     As shown in  FIG. 6 , The evaporator portions or ends  54  of the secondary heat pipes  48  are disposed inside a heat exchanger  66 . The heat exchanger  66  is simply a housing with an open interior through which engine oil is circulated via oil conduits  68 . The remainder of the oil storage, circulation, and distribution system connected to the oil conduits  68  is conventional within the gas turbine engine art, and not discussed here. 
     Thermal insulation, which is not illustrated for clarity, may be provided within the anti-icing and oil cooling system wherever it is desired to prevent heat loss. For example, insulation may be placed around the exterior of the heat exchanger  66 , the exterior of the secondary heat pipes  48 , and exposed portions of the primary heat pipes  36  and the couplers  58 . 
     In operation, oil which has absorbed heat from various parts of the engine is circulated into the heat exchanger  66  where it heats the hot or evaporator portions  54  of the secondary heat pipes  48 . The heat removal cools the oil to an acceptable working temperature so that it can be re-circulated through the engine. The working fluid within the secondary heat pipe  48  absorbs that heat and evaporates. The vapor generated then travels through the cavity  52 , and condenses at the cold portions  56  of the secondary heat pipes  48 , thereby transferring heat to the cold portions  56  inside the couplers  58 . A wick that extends from one end of the secondary heat pipe  48  to the other transports the condensed liquid back to the hot portion  54  by capillary action, thereby completing the circuit. The heat from the cold portions  56  of the secondary heat pipes  48  is transferred to the hot portions  45  of the primary heat pipes  36 . 
     The working fluid inside the primary heat pipes  36  absorbs that heat and evaporates. The vapor generated then travels through the cavities  44 , and condenses at the cold portions  46  of the primary heat pipes  36 , thereby transferring heat to the OGVs  22 . Wicks or other capillary structures that extend within the primary heat pipes  36  to the other transport the condensed liquid back to the hot portions  45  by capillary action, thereby completing the circuit. The heat transfer to the OGVs  22  is effective to prevent ice formation (i.e. anti-icing) and/or remove ice which has formed on the OGVs  22  (i.e. de-icing), depending on the heating rate. If necessary, the characteristics of the primary heat pipes  36  may be varied to accommodate their individual orientation. For example, a horizontal primary heat pipe  36 , or a vertical primary heat pipe  36  in which the hot portion  45  is at the top, may require a design providing stronger capillary action to ensure adequate condensate return, than a vertical primary heat pipe  36  with its hot portion  45  at the bottom. 
     The heat transfer system described herein, being passive, needs no valves and is sealed. The number, size, and location of the primary and secondary heat pipes  36  and  48  can be selected to provide heat removal and transfer as needed. Depending upon the exact configuration chosen, the system performance may be used only for anti-icing or de-icing, or for only for oil cooling, or for both purposes. The heat transfer system makes use of heat which is undesired in one portion of an engine and uses that heat where it is needed in another portion of the engine, avoiding both the losses associated with prior art cooling systems and the need for a separate anti-icing heat source. 
     While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.
A heat transfer system is provided for a turbine engine of the type including an annular casing with an array of generally radially-extending strut members disposed therein. The heat transfer system includes at least one primary heat pipe disposed at least partially inside a selected one of the strut members; at least one secondary heat pipe disposed outside the fan casing and thermally coupled to the at least one primary heat pipe and to a heat source. Heat from the heat source can be transferred through the secondary heat pipe to the primary heat pipe and to the selected strut member.