BACKGROUND OF THE INVENTION Heat exchangers are used in a wide variety of applications for dissipating heat or cold. For example, U. S. Patent No. 5,858,537 describes a heat exchanger for an electronic circuit. A ceramic or diamond substrate is connected to a support by interdigitated fibers 28 and 34 extending from the substrate to the support. Heat is conveyed through the materials along the fibers.

U. S. Patent No. 5,281,479 describes an ordered packing for heat exchange processes. The packing is formed of a carbon fiber-reinforced carbon with carbon fibers or carbon fiber yarns, which are linked with one another by way of textile bindings, as a filler and a matrix carbon. Through-openings are located in the surface of the packing for aiding in the heat exchange process.

U. S. Patent No. 5,542,471 describes a heat transfer element having longitudinally thermally conductive fibers. A composite material has fibers extending longitudinally, and generally parallel to each other, from one side of the material to the other. The fibers have a greater longitudinal thermal conductivity than radial thermal conductivity.

U. S. Patent No. 4,545,429 describes a woven ceramic composite heat exchanger. The composite is made of a set of woven high-modulus high strength fibers embedded in a low-modulus low-strength matrix. The high-modulus fibers form a set of interconnected passageways.

U. S. Patent No. 5,810,075 describes a heat- insulating lining on a heat exchanger surface. A ceramic fiber material is applied as a heat insulating material around the tubes. A ceramic moleskin mat is fastened to the ceramic fiber material.

U. S. Patent No. 5,211,220 describes a fiber coating for a heat exchanger tube. The fiber coating is covered by a polymer.

U. S. Patent No. 5,150,748 describes fibrous material coating for a heat pipe. The fibers may be woven within a support matrix and directly connected to the heat pipe.

U. S. Patent No. 5,077,103 describes a heat shield used to cool leading edges. The heat shield has a thermally conductive solid substrate or filament that has a diamond coating and that is surrounded by a containment matrix.

U. S. Patent No. 5,042,565 describes a fiber reinforced composite leading edge heat exchanger.

Conduits are connected by braided fibers having high thermal conductivity, resulting in a braided preform. The preform is consolidated by introducing a matrix material of high thermal conductivity.

U. S. Patent No. 4,838,346 describes a heat pipe panel used as a hypersonic vehicle leading

edge. A pipe is embedded in a carbon-carbon composite structure. The fibers are then impregnated with a carbonaceous resin system.

U. S. Patent No. 4,832,118 describes a heat exchanger that uses a graphite composite as the thermal conducting medium. The composite may be corrugated to define channels of increased surface area extending in the direction of flow.

U. S. Patent No. 4,813,476 describes a system for radiating heat from spacecraft. Sleeves that may be made of a fiber or composite reinforced elastomer surround a conduit. Fins may be attached to or integral with the sleeve to provide additional radiating surface.

Some of these heat exchangers may have use in high heat flux applications such as the leading edge of hypersonic cruise vehicles, combustion chamber walls of rocket engines, and engine walls of combined-cycle engines which require materials that have high thermal conductivity, good strength at cryogenic and elevated temperatures, and excellent resistance to thermal and mechanical fatigue. In such applications, weight is a factor.

Often, the heat exchanger configurations for these applications involve an array of intricate passages or tubes that have a high pressure coolant flowing within them. The size of the tubes and their spacing will, of course, affect the weight.

Fiber reinforced composite heat exchangers typically are constructed of fiber reinforcement materials embedded in matrix materials. Through the use of such reinforcement materials which ultimately become a constituent element of the

completed heat exchanger, the desired characteristics of the reinforcement materials, such as very high strength and good thermal conductivity are imparted to the completed heat exchanger. The constituent reinforcement materials may be in any one or more of the following physical forms: fibers per se, monofilaments, multifilaments, yarns, twisted tow or untwisted tow or sliver produced from fibers and/or other forms of continuums. The reinforcement materials used in heat exchangers ordinarily are formed of woven fibers or yarns, but could also be formed into batts, arrays or other groupings, and/or they may be woven, braided, knitted or otherwise oriented into desired configurations and shapes for the heat exchangers.

Conventional fiber reinforced composite heat exchangers constructed of woven fibers require the heat to pass from the hot surface into the composite material by flowing traverse to length direction of the fibers (i. e., through the cross section of the fibers). The fibers will then conduct the heat to the cooling tubes. See Figure 1. The difficulty that arises from this arrangement is that the thermal conductivity of the fibers is substantially lower for transverse flow, i. e., through the fiber cross-section, than it is for flow along the length of the fibers. Since composite materials are ordinarily constructed in layers, this difficulty is compounded because the low conductivity path is extended by requiring the heat to pass through a greater depth in order to reach fibers which will then carry the heat to the

cooling tubes. In essence, the performance of the current state of the. art heat exchanger could be improved if a greater amount of heat traveled along the length of the fibers, at the expense of the amount of heat that traveled transverse to the length of the fibers.

Also, the spacing of the coolant channels or tubes, if too great, may result in undesired hot spots. While it may be desired to space the tubes at a greater distance, if too great, the effectiveness of the exchanger to remove hot spots diminishes which may have a deterious effect on the supportive composite.

SUMMARY OF THE INVENTION It is therefore a principal object of the invention to provide for a heat exchanger which is efficient whilst addressing applications where weight is a concern.

It is a further object of the invention to provide for a heat exchanger which addresses the occurrences of hot spots in a simple yet effective manner.

It is a yet further object of the invention to provide for a heat exchanger that has improved operation particularly when used in high heat applications.

The concept behind the invention is to weave a fiber preform in such a manner that the conductive fibers are positioned at the surface of the preform. This results in an improvement in thermal conductivity for the heat exchanger. The present invention achieves this by weaving a fabric wherein

the conductive fibers pass through the surface of the preform or terminate at the surface thereof.

One method of fabricating the preform is to weave conductive fibers into a weave design referred to as a multi-warp. Typical multi-warp weave designs consist of layers of yarns or fibers that are at 0° (warp direction) and 90° (weft direction) plus additional warp yarns or fibers that pass through the layers so as to hold the layers together. The layered yarns in the warp direction are in columns and all of the yarns in a column were in the same dent of the reed during the weaving process. The layered yarns in the weft direction may be in a repeating pattern such as columns but the arrangement may be more complex than simple columns.

The invention is based on manipulating some of the layered warp yarns in such a manner that they changed direction and pass through the surface of the preform at a location that would correspond to hot areas of the heat exchanger. By diverting the yarns out of the layers and up to the hot surface, the resistance of heat conduction is lowered because the transverse flow of the heat through the layers is reduced. Those yarns that reach the surface provide a direct flow path for the heat from the surface to the coolant tubes.

Due to considerations such as strength, it may be that not all of the layered warp yarns are diverted to the surface. To achieve this end, yarns that are in the same dent can be diverted to the surface of the fabric and yarns in an adjacent dent can remain in plane and not be diverted to the

surface. Another possibility is to divert only selected yarns within a dent. The skilled artisan would readily understand how to weave a fabric in which at least some of the thermally conductive yarns of the fabric pass through the surface or terminate thereat. The details of yarn selection would be based on the structural and thermodynamic objectives of the specific case. Yarns that are brought through the surface may be trimmed after they come through the surface but in some cases, it may be considered desirable to leave them untrimmed.

BRIEF DESCRIPTION OF THE DRAWINGS Thus by the present invention its objects and advantages will be realized, the description of which should be taken in conjunction with the drawings wherein: Figure 1 is a sectional view of a heat exchanger having coolant tubes within a composite material incorporating a layer of warp yarns in a conventional weave design; Figure 2 is a sectional view of a heat exchanger having layered warp yarns that are exposed to the surface of the composite; Figure 3 is an enlarged sectional view of the warp and weft yarns in the CD at area A of Figure 2; and Figure 4 is an enlarged sectional view of a portion of the warp and weft yarns in MD of Figure 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now more particularly to the drawings, in Figure 1, there is shown a portion of heat exchanger 10 which utilizes coolant tubes 12 and 14 about which a composite material 16 is positioned.

In this example, the heat exchanger 10 has a hot side 18 and a cold side 20. In general, the coolant tubes 12 and 14 are made of a metal material with a coolant (e. g. liquid H2) flowing therethrough. The composite material 16 typically is constructed of fiber reinforcement materials 22 embedded in a matrix material 24. The constituent reinforcement material may take a number of different forms as aforesaid and may comprise layered warp yarns in a conventional weave design.

The fiber reinforcement material 28 may be yarns of the type K321 graphite, manufactured by Mitsubishi Chemical Co. which may be heat treated to raise conductivity.

While the foregoing type of construction of a heat exchanger has provided a certain degree of satisfactory results, there are certain disadvantages as previously discussed which is subject to improvement. In addition to the difficulty in using high conductivity yarns in a preform, weaving creates an array that consist of layers at 0° and 90°, in addition to through thickness yarns. The flow path for the heat is transverse to the 0° and 90° yarns and thus through the 90°s which is perpendicular to the tubes. This path is not efficient due to the poor conductivity in getting into the 90° yarns. The thermal conductivity is high along the yarns lower

transverse to the yarns and poor through the matrix.

In addition, the problem is compounded by the construction in layers which means that the low conductivity path is extended by requiring the heat to pass through a greater depth to reach the fibers or yarns that will then carry the heat to the tubes.

As shown in Figures 2-4, the present invention addresses these disadvantages by manipulating some of the yarns, particularly the yarns having conductive properties, such that they pass through the surface of the composite particularly in the areas of the hot spots. In this regard Figure 2 depicts a heat exchanger 10 similar to that shown in Figure 1 with like parts similarly numbered.

However, in the area designated A, warp yarns are diverted to the surface of the matrix 24. This can be seen most clearly in Figure 2 which shows a somewhat enlarged cross section at area A. Warp yarns 30 in, for example, dent 1 of the reed are manipulated to go through the surface of the preform and ultimately the surface of the matrix 24. The yarns 30 also wrap around coolant tubes 12 and 14. Accordingly, by diverting the yarns out of the layers and up to the hot surface, the resistance to heat conduction is lowered because the transverse flow of the heat through the layers is reduced. A direct flow path from the surface to the coolant tubes 12 and 14 now exists.

In Figures 3 and 4 representative weft yarns 32 are also shown. The pattern of the weave may take on that suitable for purpose. For example, in

Figure 4 there is shown the warp yarn 36 in the adjacent dent 2 which may remain in plane so as to provide strength to the preform. Accordingly, due to strengthened consideration, all of the layered warp yarns 32 may not be diverted to the surface.

As shown (Figure 3), warp yarns 32 in the same dent may, with warp yarns 36 in the adjacent dent (Figure 4), not be diverted. Alternatively, only a certain number of yarns in the same dent may be diverted with the remaining yarns in the dent being maintained in plane.

The type of yarn selected would depend upon the structural and thermodynamic objections of the particular application. While yarns of the type K321 have been mentioned, other type yarns suitable for purpose will be apparent to the skilled artisan. Also, yarns of different types might be intermingled in the weave with the conductive yarns limited to those that are diverted to the surface.

Note, that as to the yarns that are diverted to the surface, depending upon the circumstances, they may or may not be trimmed.

Thus by the present invention its objects and advantages are realized. The present invention addresses the hot spot problem particularly when the coolant tubes are spaced further apart requiring less tubes in a particular application.

This in turn allows for weight reduction which is critical in many applications as aforenoted.

Although a preferred embodiment has been disclosed and described in detail herein, its scope should not be limited thereby rather its scope should be determined by that of the appended claims.