WO2001000391A1

WO2001000391A1 - Heat exchanger using high conductivity yarn insertions

Info

Publication number: WO2001000391A1
Application number: PCT/US2000/017073
Authority: WO
Inventors: Keith Edward Burgess
Original assignee: Albany International Techniweave, Inc.
Priority date: 1999-06-29
Filing date: 2000-06-21
Publication date: 2001-01-04
Also published as: AU7982400A

Abstract

A heat exchanger (10) having a fiber preform embedded in a matrix with a second discreet group of fibers (30) extending through the fiber preform to the surface of the matrix with the fibers being made of a heat conductive material and extend to cooling channels or tubes (12, 14) so as to improve the heat exchange therebetween.

Description

HEAT EXCHANGER USING HIGH CONDUCTIVITY YARN

INSERTIONS

FIELD OF THE INVENTION

The objective of this invention is to improve the performance of heat exchangers using fiber reinforced composite materials.

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.

A problem in the fabrication of the heat exchangers is that the fibers are woven into preforms that are then densified with a matrix material to complete the composite material. The rigors of the weaving process restricts the fiber selection or yarns to only those fibers or yarns tough enough to withstand the weaving process. In effect, certain fibers that provide high thermal conductivity, but which are not tough enough, are eliminated from consideration. For example, fibers such as very high modulus pitch based graphite have excellent thermal conductivity but are very fragile and thus difficult to weave. This typically forces the selection of fibers that are of lower conductivity. In essence, the performance of the current state of the art heat exchanger is not what it could be because of design considerations that preclude the use of some of the most thermally conductive materials. 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 insert high thermal conductivity fibers in such a manner as to provide better thermal conductivity and thus better heat exchanger performance. The performance of the heat exchanger can be substantially improved by inserting such very high conductivity fibers into the previously woven preform so that the fiber extends from the hot surface of the heat exchanger directly to the surface of the cooling tube(s). In other words, thermally efficient passageways are created which extend from a location on the surface of the fiber reinforced composite to a location on the cooling tube, with the passageway being defined by a fiber or yarn having a very high thermal conductivity which has been inserted into the fiber reinforced composite. The highly conductive fibers extend from the surface of the fiber reinforced composite, through the previously woven preform, to a location so that the fibers are in physical contact with the cooling tube(s). Thus, the heat exchangers of the present invention are constructed of a first group of fibers formed into a woven preform, and a second group of fibers having a very high thermal conductivity which are inserted through the fibers of the woven preform. With respect to the first group of fibers, the second group of fibers are discreet, distinct from the fibers of the woven preform through which they pass . This construction improves the performance by two mechanisms. First, the yarn can conduct the heat directly from the surface without requiring the heat to flow through the low conductivity path transverse to the fibers. Second, it allows very high conductivity fibers to be used as the primary heat conductors as the yarns are not subjected to the weaving process. The difference in the thermal conductivity of the yarns can be as much as three times as high as yarns used for a weaving process.

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 inserted warp yarns that are exposed to the surface of the composite; and Figure 3 is a sectional view of a yarn insertion device for inserting yarn into a yarn preform.

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.

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 Figure 2, the present invention addresses these disadvantages by inserting yarns, particularly the yarns having very high conductive properties such as yarns of the type K1100, manufactured by BP Amoco Chemicals which may be too brittle to be woven, 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, the inserted yarns 30 are diverted to the surface of the matrix 24. Yarns 30 are inserted into a woven preform and extend from the surface thereof 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. This allows the heat to be pulled out of the hot spot (midway between the cooling tubes 12 and 14) more efficiently, and allows the tubes to be spaced further apart. The type of yarn 30 selected for the preform would depend upon the structural and thermodynamic objections of the particular application. Yarns of the type K321 graphite manufactured by Mitsubishi Chemical are suitable for the preformed weave since they also are heat conductors. 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 conductive yarns to form the preform.

Note, that as to the inserted yarns 30 that extend to the surface, depending upon the circumstances, they may or may not be trimmed.

A suitable device for making the composite materials of the present invention is the yarn insertion mechanism shown in Figure 3 and is described in co-pending U.S. Patent Application Serial No. 09/438,242 filed November 12, 1999 entitled "Yarn Insertion Mechanism" commonly assigned herein, the disclosure of which is incorporated herein by reference. Note that yarn Y in this case is taken to include any textile yarn, monofilaments or coated yarns.

Briefly, the concept behind the mechanism 50 is to control the motion of the yarn by using two brakes plus a means of preventing the yarn from buckling when it is "pushed". One of the brakes 52 is attached or moves with the insertion needle 54. As the needle 54 is inserted into the receiving material, which in this case is the woven fiber preform, the brake 52 is in the engaged mode thus causing the yarn to remain fixed relative to the needle position and to be inserted into the receiving material. As the needle is retracted, the brake 52 is in the disengaged mode, thus allowing the yarn to be left in the material as the needle 54 is removed. Due to the friction of the needle against the yarn, the yarn could be dragged out of the receiving material unless some means is used to prevent it. This can be accomplished by using a brake 56 that is at a fixed distance from the receiving material. Alternatively, "drag" can be built into the mechanism at a similar location to the brake in 56 as long as this drag exceeds the needle friction that is pulling the yarn from the receiving material. Drag can be provided by using a tube having a small inside diameter that restricts the yarn, or by engaging a brake pad against the yarn at a force less than what is necessary to prohibit yarn movement. The brake 56 is engaged when the needle is being retracted from the receiving material and is disengaged when the needle is being inserted into the receiving material. The brakes 52 and 56 are engaged when fluid pressure, such as air pressure, is applied in the respective contained areas A and B. In the absence of air pressure, the brakes are disengaged.

When the needles is retracting from the receiving materials, the yarn is in a fixed position so it is essentially being pushed through the needle. As this is akin to "pushing on a string", some means must be used to prevent the yarn form buckling out of column. This can be accomplished by creating a "tube" or "channel" generally designated 58 that contains the yarn between the needle 54 and brake 56. As the needle 56 is retracting and brake 56 is fixed, the distance is not fixed. A means for accomplishing this is to use a telescoping arrangement such as two small tubes (one (60) attached to the needle and one (62) at a fixed distance from the receiving material) where one tube slides within the other but both tubes are small enough to prevent the yarn from buckling. An alternative would be for the large tube to be replaced by a hole in a larger object such that the smaller tube moved inside the hole. Another alternative is to insert the yarn within a spring, with the spring forming a channel that prevents buckling. The skilled artisan will readily appreciate that by inserting high conductivity yarns into the fiber preforms at a preselected angle, it is possible to achieve the aforedescribed structures. Also, it should be readily understood by the skilled artisan that while the present invention is described with respect to woven preforms, this is merely done for exemplary purposes, and that other fiber preforms are suited for use in the present invention, some of which are set forth above. 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 .

Claims

WHAT IS CLAIMED IS:

1. A composite material comprised of a first group of yarns formed in a fiber preform and a second discreet group of fibers extending through the fiber preform from a first preform location to a second preform location.

2. The composite material of claim 1 wherein the fiber preform is formed by weaving.

3. The composite material of claim 1 wherein the second discreet group of fibers have been inserted into the fiber preform.

4. The composite material of claim 1 wherein the composite material has a first surface and a second surface with a subsurface therebetween and second discreet group of fibers extend from the subsurface to the first surface.

5. The composite material of claim 1 wherein the second discreet group of fibers are in contact with a conduit located within the composite material.

6. The composite material of claim 1 wherein the second discreet group of fibers have a thermal conductivity that exceeds thermal conductivity of the first group of fibers.

7. The composite material of claim 1 wherein the fiber preform is embedded in matrix material having a first surface and a second surface.

8. The composite material of claim 7 wherein said second discreet group of fibers which have a portion thereof which are present in the first surface.

9. The composite material of claim 8 wherein said second discreet group of fibers extend from the first surface to a conduit located in the subsurface .

10. The composite material of claim 9 wherein said conduit comprise cooling tubes or channels within the composite material.

11. A heat exchanger comprising a fiber preform embedded in a matrix material having a first and second subsurface and a subsurface therebetween, and a second discreet group of fibers extending through the fiber preform to the first surface.

12. The heat exchanger material of claim 11 wherein the fiber preform is formed by weaving.

13. The heat exchanger material of claim 11 wherein the second discreet group of fibers have been inserted into the fiber preform.

14. The heat exchanger material of claim 11 wherein the second discreet group of fibers are in contact with a conduit located within the composite material .

15. The heat exchanger material of claim 11 wherein the second discreet group of fibers have a thermal conductivity that exceeds thermal conductivity of the first group of fibers.

16. The heat exchanger material of claim 11 wherein said second discreet group of fibers extend from the first surface to a conduit located in the subsurface .

17. The heat exchanger material of claim 16 wherein said conduit comprise cooling tubes or channels within the composite material.