This invention relates to heat pipes and components and devices utilizing such heat pipes, for example, nose cones, wing leading edges, engine nacelles, rocket nozzles, turbine engine stators, combustion chambers, turbine engine after-burner parts and other gas turbine engine parts. Heat pipes have many advantages in the transmission of heat while at the same time having a basic constructional simplicity, great flexibility of use and the ability to transport heat at a high rate over substantial distances with very small temperature drops while at the same time requiring no external source of power other than the heat which is to be transported. However, heat pipe technology is a young technology and present designs, particularly in high temperature heat pipes useful in high heat flux environments such as those experienced in structures in certain parts of hypersonic craft encountering the atmosphere, are far from the theoretical and economic potential of the technology.

In its conventional form (see Fig. 1) , the heat pipe is a closed tube or chamber 1 whose inner surfaces are lined with a wick 2. The wick is saturated with the liquid phase of a working fluid and the remaining volume 3 of the tube, in operation, contains the vapor phase. Heat applied at an evaporator section 4 by an external source 5 vaporizes the working fluid in that section. The resulting difference in pressure drives vapor from the evaporator section to a condenser section 6 where it

condenses releasing the latent heat 7 of vaporization. Depletion of liquid by evaporation causes the liquid-vapor interface in the evaporator section to enter into the wick surface thereby developing a capillary pressure. This capillary pressure pumps the condensed liquid back to the evaporator section. The heat pipe can continuously transport the latent heat of vaporization from the evaporator section to the condenser section without drying out the wick. As the latent heat of vaporization is usually several orders of magnitude larger than the sensible heat that can be transferred in a conventional convective system, the heat pipe can transport a large amount of heat with a small unit size. Because of this, heat pipes have been developed which have thermal characteristics orders of magnitude better than known solids.

Such is their versatility, working fluids which range from cryogenic liquids to liquid metals have been successfully used in heat pipes. The wick 2 must provide the necessary flow passages for the return of the condensed liquid, surface pores at the liquid-vapor interface for the development of the required capillary pumping pressure, and a heat-flow path between the inner wall of the container 2 and the liquid- vapor interface. Generally, an effective wick structure requires small surface pores for large capillary pressure, large internal pores for minimal liquid-flow resistance, and an uninterrupted highly conductive heat-flow path across the wick thickness for a small temperature drop. High temperature heat pipes have been made from

nickel base superalloys and refractory metals. These pipes have been made using layers of fine mesh screen or sintered porous metal as a capillary wicking material to pump the heat pipe fluid back to the heat source. Pores of the sintered porous metal are usually small thereby enabling large capillary pumping pressures to be developed at the liquid-vapor interface. However, such small internal pores often cause a larger pressure drop in the liquid-flow passage than is desired. The maximum heat flux capability of these devices is frequently limited by the undesirable high pressure drop of the heat pipe fluid in the capillary wick structure. If this pressure drop is too great then the pumping capacity will fail to supply fluid at the required evaporation rate at the heat source and the wick will catastrophically "dry out" (see e.g. the test results for a leading edge heat pipe, when a heat flux of 250 watts/cm ² was exceeded, as set forth on pages 6 and 7 of AIAA-88-2679) . One method of improving the supply of fluid to the evaporator is to incorporate "arteries" or "tunnels" within the wick structure thus allowing the fluid a free flowing channel or pipeline back to the evaporator section. Heat pipe wicks with arteries and channels have been made out of high temperature materials, including sintered porous metal powder wicks (see e.g. US Patents Nos. 4,196,504 and 4,565,243). However, they have the following disadvantages:

1. The arteries or channels must be sufficiently small in cross-section so that the working liquid will fill the artery due to capillary forces. The small cross-section of such

arteries requires that fairly intricate and labor intensive means be used to fabricate these small fluid flow channels. This is true regardless of how they are formed, i.e. by the wrapped screen method or sintered metal powder method. 2. Once a heat pipe is formed with arteries, it is not easily inspected for location of possible defects within the arteries. 3. For very high performance heat pipes many arteries or tunnels are required to supply enough fluid to the evaporator. It is difficult to form heat pipes with many arteries or channels using the wrapped screen or sintered metal powder method.

4. A heat pipe having arteries or tunnels in the sintered wick structure cannot be bent to a final shape without collapsing the arteries unless some internal support is provided during bending (e.g. US Patent No. 4,565,243 uses a sintered metal wick with arteries serially joined to a screen wick located in a region where bending is required) . It should be noted here that the bending method referred to on page 8 of AIAA-88-2679 uses internal support for the arteries during bending. Another method of manufacturing heat pipes is to form axial capillary grooves inside the vapor chamber in the inside wall of the heat pipe housing without the presence of a conventional porous wick. This method has two

disadvantages:

1. The width of the grooves must be sufficiently small so that adequate capillary pumping pressures can be developed at the liquid/vapor interface.

2. Machining or forming of these capillary grooves is very difficult because of their small size and the large number of grooves required.

A proposal for the utilization of heat pipes in leading edges and nose cones, utilizing a carbon-carbon composite structure within which are disposed a plurality of heat pipes suggested to be suitable for use at temperatures of about 3,000°F in an oxidizing environment and above 3,000°F in an inert or vacuum environment, is disclosed in US Patent No. 4,838,346. In this patent, the heat pipe panel is made from an array of refractory-metal pipes spaced from one another and embedded in a carbon- carbon composite structure. A similar construction is set forth in NASA Tech Brief of July 1989, pages 64 and 65. It is an object of the present invention to overcome the various shortcomings of prior art heat pipes and to provide heat pipe designs that are capable of operating at very high heat fluxes such as those to be expected during aerother al heating at hypersonic velocity. It is a further object of the present invention to provide heat pipe designs capable of withstanding heat fluxes ranging from 100 to over 500 watts/cm ² using conventional high temperature materials such as refractory materials or nickel base superalloys. It is a further object of the present invention to

provide such heat pipe wick designs which can be readily fabricated by powder metallurgy methods.

It is a further object of the present invention to provide heat pipe designs which are capable of being formed in complex shapes previously thought to be impossible to make.

It is a further object of the present invention to provide wing leading edge structures and nose cones suitable for use in hypersonic aircraft operational in the Mach 10 to Mach 20 range where these parts will have to withstand aerothermal heating capable of producing temperatures of 5,000 to 10,000°F.

It is a further object of the present invention to provide components and structures utilizing such heat pipes in the form of, for example, engine nacelles, rocket motor nozzles, gas turbine engine stators, combustion chambers, turbine engine after-burner parts, other turbine engine parts.

In accordance with the present invention, heat pipes are not shaped as traditional pipes but rather have vapor chambers specifically shaped for each application with porous wicks having integral capillary grooves to ensure high performance.

According to the present invention there is provided a heat pipe comprising a housing having an interior surface defining a closed chamber, the chamber having an evaporator section and a condenser section, a working fluid contained in the chamber, a wick of porous material intimately engaging the interior surface, extending from the condenser section to the evaporator section and

defining, at least in part, a vapor space extending from the evaporator section to the condenser section and a plurality of capillary grooves formed in the wick, open to the vapor space and extending from the condenser section to the evaporator section, the grooves being sized to convey, by capillary action, the working fluid when in a liquid phase.

Also, according to the present invention there is provided a heat pipe structure for use in a heat pipe comprising a housing having an interior surface defining a closed chamber, a wick of porous wick material intimately engaging the interior surface and defining, at least in part, a space within the chamber and a groove, for conveying a working liquid by capillary action, formed in the wick and open to the space.

Also, according to the present invention there is provided a sandwich heat pipe comprising a housing having two substantially parallel superimposed laminae, the housing, including one surface of each laminae, defining an interior surface defining a closed chamber, a working fluid within the chamber, a wick of porous material intimately engaging at least a substantial portion the interior surface of at least one of the laminae and defining at least in part, a vapor space within the chamber, the wick and the vapor space extending between an evaporator section and a condenser section of the heat pipe and a plurality of grooves extending from the condenser section to the evaporator section and sized to convey the working fluid, when in a liquid phase, by capillary action, from the condenser section to the

evaporator section, the grooves being formed in the wick and open to the vapor space.

Also, according to the present invention there is provided a high temperature high heat flux heat pipe comprising a housing, resistant to high temperatures, defining a closed chamber having a wick therein of porous high temperature resistant metal, in intimate contact with the housing defining, at least in part, a vapor space in the housing and defining grooves open to the space and extending from a condenser section to an evaporator section of the heat pipe to convey, by capillary action, a liquid phase of a working metal fluid in the chambers, whereby the heat pipe can be formed in any desired shape.

The present invention also covers, for example, nose cones, leading edges, panels, rocket motor nozzles, gas turbine stators, etc. when constructed using the heat pipe constructions of the present invention as set forth herein.

More specifically, in the present invention a heat pipe is formed with a porous powder metal wick. Within the wick structure a large number of capillary grooves are either molded or cut into the powder metal wick. The grooves are open to the vapor space of the pipe. Some of the advantages of this design are as follows: 1. The maximum capillary pressure that can be developed is determined by the powder particle size which controls the effective pore size. 2. Many capillary grooves in the porous wick can supply more returning fluid to the evaporator with very little pressure drop as

compared to a non-grooved porous wick.

3. All of the grooves can be readily accessed for quality inspection using fiber optic methods.

4. A grooved wick structure can be formed in substantially any shape or bent quite severely without pinching off or closure of the capillary grooves and thus impeding the fluid flow in the grooves.

5. Groove capillary patterns can be superimposed on each other to form complex fluid flow patterns to "cross channels" to further improve resistance to "dry out" by providing multiple fluid supply grooves. For example capillary grooves can be formed in a 90 degree grid pattern. Grooves can be specifically aligned to promote directional heat flow as required for each application.

6. Using powder metallurgy methods capillary grooves can be formed on very complex shapes with very intricate patterns not possible by conventional machining. Several examples of this type of heat pipe design are hereinafter given to demonstrate with specific examples of how this type of heat pipe design can be effectively utilized.

The complex shapes of the porous powder metal wick structures suitable for accomplishing the present invention can be produced by a relatively simple powder metallurgy method capable of producing a wide variety of complex heat pipe shapes having various pore

sizes and densities of up to 100%.

In accordance with this method a simple shape such as niobium based alloy tube having an interior surface lined with porous niobium can be produced as follows. The niobium based alloy tube is first inserted into a thin wall iron tube. Inside the niobium tube is placed an iron rod which is centrally located. The space between the iron rod and tube is filled with niobium powder. After welding iron end plugs in the outer iron tube and evacuating the air within the particle interstices, an evacuation stem is sealed shut. This entire assembly is then subjected to moderate heat, less than 2,000°F, and pressure between 5,000 and 10,000 psi, in a gas inert to niobium, for a predetermined time period sufficient to densify the powder and diffusion bond the particles to each other and the inner wall of the niobium tube. All of the iron is then removed by leaching in boiling 20% H ₂ S0 ₄ . For more complex shapes, the inner iron rod or form can be machined or formed into a variety of shapes. Likewise groove shapes can be produced with various combinations of expendable tooling all of which can be removed by choosing the right combination of tooling material and solutions which will selectively dissolve only the expendable tooling. In other embodiments the gas, pressure temperature and time period will be chosen to suit the materials involved and the core may be any material chosen to withstand the environment of densification and diffusion and then to be desolved by leaching, etching, melting, vaporizing etc. without detrimentally effecting the diffusion bonded powder metal.

A number of embodiments incorporating the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic illustration of a conventional prior art heat pipe;

Figure 2 is a longitudinal cross-section of a hypersonic nose cone utilizing the heat pipe concepts of the present invention;

Figure 3 is a cross-section on section line 3-3 of Figure 2;

Figure 4 is an enlarged detail of the portion of Figure 3 shown circled and identified by arrow A;

Figure 5 is an enlarged longitudinal elevation of the outer portion of a hypersonic nose cone as illustrated in Figure 2;

Figure 6 is a cross-section on section line 6-6 of Figure 5 with the inner portion of that construction omitted;

Figure 7 is an enlarged detail of the portion of Figure 6 shown circled and identified by arrow B;

Figure 8 is an end elevation of a hypersonic leading edge incorporating heat pipes utilizing the heat pipe concepts of the present invention;

Figure 9 is a fragmentary cross-section on section line 9-9 of Figure 8;

Figure 10 is a transverse cross-section of an alternative design of hypersonic leading edge utilizing the heat pipe concepts of the present invention;

Figure 11 is a fragmentary cross-section on section line 11-11 of Figure 10;

Figure 12 is a fragmentary plan view of panel utilizing the heat pipe concepts of the present invention;

Figure 13 is a fragmentary cross-section on section line 13-13 of Figure 12; Figure 14 is a fragmentary plan view of another form of panel utilizing the heat pipe concepts of the present invention;

Figure 15 is a fragmentary cross-section on section line 15-15 of Figure 14;

Figure 16 is a fragmentary cross-section on a section line 16-16 of Figure 15; Figure 17 is a fragmentary plan cross-section taken half way through the thickness of a third form of panel utilizing the heat pipe concepts of the present invention;

Figure 18 is a fragmentary cross-section on section line 18-18 on Figure 17 showing the entire cross-sectional thickness of the panel;

Figure 19 is a longitudinal cross-section of a rocket nozzle utilizing the heat pipe concepts of the present invention;

Figure 20 is a cross-section on section line 20-20 of Figure 19;

Figure 21 is an enlarged detail of the portion of Figure 20 shown circled and identified by arrow C;

Figure 22 is a fragmentary sectional elevation of a gas turbine engine turbine section construction utilizing the heat pipe concepts of the present invention;

Figure 23 is a cross-section on section line 23-23 of Figure 22; and

Figure 24 is an enlarged detail of the portion of Figure 23 shown circled and identified by arrow D. With reference to Figure 2-7, a hypersonic nose cone 10 which typically may have a length of 1-2 feet (c. 30- 60cm) is constructed, in the form of a heat pipe housing, of two concentric frusto-conical portions 12, 14 welded to hemispherical end caps 16, 18 located at the small ends of the frusto-conical portions with the space between the

large ends of the frusto-conical portions 12, 14 being sealed so that the space between the frusto-conical portions and the hemispherical end caps forms a closed chamber 4 containing lithium or sodium as a working fluid. The frusto-conical portions and the hemispherical end caps together with the associated closure at the large end of the frusto-conical portions may comprise a niobium based alloy having a thickness of .030" (c. 0.8mm). The chamber so formed is lined with a thin coating forming a wick 20 of porous niobium, using the .powder metallurgy methods described above, the wick having a high capillary pumping pressure. The wick 20 may have a thickness of .025" (c. 0.6mm). The wick 20, on the inside of the outer frusto- conical portion 12 and the interior of the outer hemispherical end cap 16, which is to be subjected to aerothermal heating, has formed therein a substantially evenly distributed plurality of longitudinally extending capillary grooves 22, each 0.020" deep by 0.020" wide (c. 0.5mm deep by 0.5mm wide) open to the interior of the nose cone. This interior of the nose cone is a vapor space within the chambers defined by the wick 20. As can be seen from these dimensions, in this construction, a thin layer of the porous metal wick remains between the bottom of each groove and the interior surface of the housing of the nose cone. However, this thin layer at the base of each groove is not essential and the grooves can be formed, as illustrated in Figures 4 and 7, to extend to the interior surface of the housing.

As can be seen clearly in Figure 6, the grooves, which run longitudinally of the nose cone, converge at the

hemispherical end of the outer end cap 16 to form a radial pattern. It is the outside hemispherical end cap, which is subjected to the highest heat flux of the nose cone, that the density of the capillary grooves is at its greatest with the grooves covering approximately 50% of the area of that outer hemispherical end cap. This provides maximum capillary flow expected to be able to deal with heat fluxes in excess of 1,000 watts /cm ² . The requisite density of radial distribution of grooves in the outer hemispherical end cap is provided by progressively terminating various grooves as they approach the longitudinal axis 23 of the nose cone at points where adjacent grooves would start to interfere with one another thus destroying their integrity. In this connection, it should be noted that it is a requirement that individual integrity of the grooves be maintained so as to preserve a dimensional width of each groove which will provide the required capillary pumping action. In this connection, it should also be noted that it is only the width of the grooves which must be maintained at a suitable dimension for the necessary capillary action, the depth of the grooves from the exposed surface of the wick being irrelevant with this respect.

Working fluid which condenses on the porous wick layer 20 on the inner core of the nose cone is returned to the grooved outer wick layer by eight longitudinally extending webs 24 of porous niobium which not only return the working fluid but also act as radial struts which secure the inner frusto-conical portion 14 to the outer frusto-conical portion 12. The webs extend for a

substantial longitudinally extent of the frusto-conical portions and have a transverse cross-section shown in detail in Figure 4. Through the center of each web 24, extending longitudinally the length of the frusto-conical portion, is a passage 26 for working liquid distribution with the end, of each of these passages 26 adjacent the hemispherical end caps, exiting adjacent the grooved outer porous niobium wick 20.

It will be appreciated that the longitudinal webs could be replaced with a plurality of posts and that the passages for liquid distribution longitudinally of the nose cone could be defined in a porous niobium structure adjacent the grooved outer porous niobium wick. The webs 24 may typically have a thickness of .060" (c. 1.5mm) and the radial spacing between the inner and outer porous metal layers 20 in the region of the frusto-conical portions, may be of the order of 0.412" (c. 10.5mm).

In a nose cone such as that described here, the heat pipe evaporator section is typically within the zone 28 indicated by the line with opposed arrows while the remainder of the nose cone forms the condenser section as indicated by the arrows 30. The evaporator and condenser sections of the hypersonic leading edge illustrated in Figure 8 and the alternative design of Figure 10 are similarly indicated with reference numbers 28 and 30.

It will be appreciated that while the housing of the nose cone hereinbefore described has been referred to as being constructed from a niobium based metal alloy, other refractory alloys could be utilized, for example, titanium, nickel or molybdenum based metal alloys. There

is also no requirement that the metal basis for the alloy of the housing be the same as the metal of the wick. It will also be appreciated that while a primary condensation area is shown to be the exterior of the nose cone as identified by the arrows 30, the interior surface of the nose cone also constitutes a condensation area which might conveniently be used, to increase the performance of the nose cone, where thermal overload might otherwise be experienced, to increase the condensing capacity of the nose cone by the circulation of a cooling fluid over that interior surface to conduct heat therefrom.

In the remaining constructions described with respect to the present invention, elements which are substantially the same as those described with respect to the hypersonic nose cone will retain the same reference numbers.

Referring now to Figures 8 and 9, there is illustrated a hypersonic leading edge structure for the wing (or other aerodynamic body) of a hypersonic aircraft. This leading edge structure is given the general reference numeral 32 and comprises an outer skin 34, for example, a niobium based alloy, which comprises two substantially flat panel portions disposed at an acute angle to one another as is required for aerothermal operation. These flat panel portions are interconnected by a curved leading edge portion 36. Intimately attached on the inside of the outer skin are a plurality of elongate heat pipes 38 having a "D" shaped cross-section (see Figure 9) disposed in close relationship parallel to one another to extend around the leading edge 32 in planes parallel to the elevation illustrated in Figure 8 with the rear or flat

surface of the D-shaped cross-section in intimate contact with the skin. Each heat pipe 38 comprises an outer housing 33, of a niobium based alloy, defining a closed chamber which is lined with a porous niobium wick 20 formed as hereinbefore described with longitudinally extending grooves 22 to form capillaries for the transfer of condensed working fluid from a condensation section remote from the leading edge 36 to the area of the leading edge 36 within the region of the evaporation section 28. Substantially the entire interior of each heat pipe is coated with the porous material and the grooves are distributed throughout substantially the entire perimeter of the cross-section of each heat pipe. The leading edge structure may be formed as a flat panel and then bent around a mandrel to form the nose radius. This can be achieved without significantly affecting the shape or operation of the capillary grooves. Alternatively the individual heat pipes 38 may be formed and then attached to the inside of the outer skin 34. It will also be appreciated that the outer skin may be eliminated in favor of a plurality of the heat pipes individually joined together in a contiguous manner and that the heat pipes need not be of a "D" shape cross-section. Alternatively, they might be of square or any other cross-section which lends itself to the appropriate functionality of a heat pipe in the environment anticipated. It should be noted that the various dimensions referred to with respect to the nose cone construction with respect to the wick and groove .dimensions are applicable here with the leading edge having a dimension transversely of the edge 36 of

approximately of 10 inches (c. 25cm) .

An alternative leading edge construction is illustrated in Figures 10 and 11. In this sandwich construction, substantially flat parallel pairs of inner and outer panels 40 and 42 are disposed at an acute angle to one another with the inner panels being interconnected by a curved inner panel 44 and the outer panels interconnected by a curved leading edge panel 36, the inner and outer curved panels being spaced normal to the leading edge 36 by approximately 3 inches (c. 7.5cm). The flat panels 40, 42 and curved panels 36, 44 together with end closures (not shown) and flat panel closures 46 define the heat pipe housing which, in turn, defines a closed chamber with interior surfaces which are coated with a porous metallic powder, for example, niobium, to form a wick 20 defining an even distribution of grooves 22 extending normal to the leading edge 36 on both the interior and exterior panels 40, 42. The metallic powder wick 20 structure and grooves 22 being dimensioned substantially as described hereinbefore with respect to the nose cone construction. In this construction the panels 40 and 42 are typically spaced apart by 0.5 inches (c. 12.5mm) .

Integrally stiffened heat pipe panels of various forms are illustrated in Figures 12 to 18. Heat pipe panels according to the invention can be used for a wide variety of other applications such as vectoring nozzles for gas turbine engines, ram jet panels, rocket nozzles, after burner panels, etc. The internal design of the panels can be configures to transmit heat in the specific

direction for each application. For example, some applications might require, contrary to the direct of transmission in the embodiments hereinbefore described in detail, heat to be transmitted only through the thickness of the panel. Capillary grooves and/or arteries can be oriented to transmit returning fluid in the appropriate directions.

Three forms of panel for applications where heat is to be transmitted only through the thickness of the panel are shown in Figures 12 to 18. These designs incorporate two flat sheets (or laminae) 46, 48 of niobium based alloy which with edge closures define a housing in turn defining a closed chamber. The inside surface of each sheet is coated with a porous niobium powder to form wicks 20. The two sheets 46, 48 of porous metal faced material are connected together by longitudinal webs 24 (or an array of posts 50 seen in Figures 17 and 18) which also act as a wicking structure for returning the condensed heat pipe fluid back to the evaporator section. As shown in these designs, capillary groove patterns can be incorporated in the wicks of both panels to further increase the liquid flow from the condenser to the evaporator section of the panel. In addition, the webs 24 (Figures 15 and 16) and the posts 50 (Figures 17 and 18) can be formed with grooves interconnecting the grooves in the wicks of the two flat sheets to increase the liquid flow between these panels.

Figures 12 and 13 illustrate a panel with grooves extending parallel to the webs. Figures 14 to 16 illustrate a panel with grooves normal and extending

across the webs and Figures 17 and 18 illustrate a panel having a plurality of discrete wick areas with areas of the two sheets superimposed on one another and connected at their centers by posts 50, the grooves of each pair of areas forming a radial pattern centered on the associated post and extending along that post to join together grooves of each area forming a pair.

Figures 19 through 21 illustrate a rocket nozzle 52 which utilizes the basic design concepts of the panels heretofore described, in that the lamina forming the interior of the nozzle is primarily the evaporator section while the lamina forming the exterior of the nozzle is primarily the condenser section. Apart from this, however, the construction is quite similar to that of the truncated cone portion of the nose cone hereinbefore described with grooves formed in the wick material on both the interior and the exterior laminae.

Figures 22 through 24 illustrate a heat pipe . gas turbine engine turbine section stator construction utilizing heat pipe concepts according to the present invention. High temperature gas turbine engines typically use large volumes of bypass air to cool stators and rotating blades in the combustion stream. This ultimately reduces the combustion temperature and overall efficiency of the engine. Using conventional nickel alloy materials, the combustion temperature rarely exceeds 2200°F. If stators and/or other parts in the combustion stream were redesigned as heat pipes, combustion temperatures exceeding 5000°F could become practical with existing materials and overall efficiency significantly improved.

In the arrangement of Figures 22 through 24, each stator 53 is designed as a thin wall hollow shell lined with porous metal wicking material. The wick 20 so formed also defines capillary grooves 22 extending from the condenser section 30 to the evaporator section 28. Externally, the heat pipe stator would appear similar in shape to a conventional nickel base super alloy stator except for the outward radial extension 54 through casing 56 which would protrude into the normal bypass air stream 58. With such a design, the bypass air would extract heat from the heat pipe without the need for mixing bypass air into the combustion gases. The stator would then operate at an intermediate temperature somewhere between the temperature of the combustion gases and the bypass air. The portion of the stator 53 extending into the bypass may be shaped to minimize disturbance of the bypass air flow.

This same concept could be used to extract heat from other parts in the high temperature portion of engines. Combustor nozzles, etc. could all be cooled in this manner using either bypass air or ambient air to cool the condenser portion of the heat pipe design. Lower density titanium alloys could be fabricated into heat pipes that would easily exceed the high temperature capabilities of non-heat pipe nickel superalloy design parts.