a craft outer skin heat exchanger

The invention relates to a craft outer skin heat exchanger, to the use of such an outer skin heat exchanger in an aircraft, and to a method for manufacturing such a craft outer skin heat exchanger.

Fuel cell systems enable low-emission, highly efficient generation of electric current. For this reason, efforts are currently made to use fuel cell systems to generate electrical energy in various mobile applications, such as for example in automotive engineering or aeronautics. It is, for example, conceivable in an aircraft to replace the generators, which are currently used to supply power on board and are driven by main engines or the auxiliary power unit (APU), with a fuel cell system. A fuel cell system, moreover, might also be used to supply the aircraft with emergency power and replace the ram air turbine (RAT) hitherto used as an emergency power system.

Besides electrical energy, a fuel cell during operation generates thermal energy which has to be removed from the fuel cell with the aid of a cooling system in order to prevent overheating of the fuel cell. A fuel cell system installed in an aircraft, for example for the on-board power supply, therefore has to be designed in such a way that it is capable of meeting a high demand of electrical energy. A fuel cell that has a high capacity for generating electrical energy, however, also generates a large amount of thermal energy and therefore has a high cooling requirement. Moreover, on board of a craft, especially an aircraft, a large number of further technical devices are provided, which generate heat and have to be cooled in order to guarantee reliable operation. For example in an aircraft, these technical devices include, inter alia, the air conditioning unit and the electronic control components of the aircraft.

In the aeronautic field, efforts are therefore being made to employ outer skin heat exchangers in aircraft cooling systems in order to remove heat from devices on board the aircraft which are to be cooled into the surroundings of the aircraft. DE 10 2008 026 536 Al and US 2011/0146957 Al, for example, describe a heat exchanger which is directly integrated into the aircraft outer skin. The heat exchanger comprises a cooling circuit allowing a flow of a heat-carrying fluid therethrough, which is embedded in an aircraft outer skin so as to be thermally coupled to the ambient air. It is further known from WO 2010/105744 A2 to provide a cooler for an aircraft cooling system, which comprises a matrix body designed to form a section of the aircraft outer skin. In the matrix body of the cooler, there is provided a plurality of coolant channels which extend from a first surface of the matrix body to a second surface of the matrix body and allow a flow of coolant through the matrix body.

The object on which the invention is based is to specify a heat exchanger which is suitable for use as a craft outer skin heat exchanger in any desired section of the craft outer skin, and a method for manufacturing such a heat exchanger.

This object is achieved by a heat exchanger having the features of claim 1 and a method for manufacturing a heat exchanger having the features of claim 13.

A heat exchanger according to the invention comprises a plurality of heat transfer modules. The heat transfer modules are arranged side by side so as to define a multilayer body of the heat exchanger. Specifically, in the multilayer body of the heat exchanger the heat transfer modules are arranged such that side surfaces of heat transfer module bodies of adjacent heat transfer modules face each other. The side surfaces of the heat transfer module bodies preferably form the main surfaces of the heat transfer module bodies, i.e. the surfaces of the heat transfer module bodies having the largest area. The heat transfer module bodies may further comprise an inner surface which is adapted to form a section of an inner surface of the heat exchanger and an outer surface which is adapted to form a section of an outer surface of the heat exchanger.

For example, the heat transfer module bodies may be in the form of a flat pipe having a very small thickness (the distance between the side surfaces), a small height (the distance between the inner surface and the outer surface), but a comparatively great length (the distance between end faces of the heat transfer module bodies). The heat transfer module bodies may be manufactured in an extrusion process and may be of any desired material which allows the use of the heat exchanger as a craft outer skin section. Preferably, the material used to manufacture the heat transfer module bodies has good heat transfer properties.

Each heat transfer module is provided with at least one heat transfer medium channel designed to allow a flow of a heat-carrying medium therethrough. The heat- carrying medium flowing through the heat transfer medium channel may be any desired liquid or gaseous fluid which is adapted to discharge heat from a heat generating component. When the heat exchanger is installed in a craft, in particular an aircraft, the heat exchanger may form a part of a cooling system for cooling a heat generating component on board the craft. The cooling system may comprise a conveying unit, such as a pump, so as to convey the heat-carrying medium through the heat transfer medium channels of the heat exchanger.

At least one portion of the multilayer body of the heat exchanger is provided with a curvature which is designed so as to allow the heat exchanger to be used as a curved outer skin section of a craft. That to say, the multilayer body of the heat exchanger is provided with a curvature to a curvature of a craft outer skin section the heat exchanger is intended to form. The term "curvature" in the context of the present application designates a quantitative parameter which is invers to a curvature radius and which may be measured in 1/m.

Adjacent heat transfer modules of the at least one portion of the multilayer body are arranged with a tilt angle of their central axes towards each other such that each heat transfer module is aligned towards the center of a local osculating circle defined by an outer surface of the heat exchanger. A cross-sectional shape of a heat transfer module body of the heat transfer modules and/or a sequence of the heat transfer modules in the heat exchanger multilayer body, may be selected so as to adjust the curvature of the heat exchanger multilayer body as desired. The heat transfer modules of the heat exchanger multilayer body may have identical or different heat transfer module bodies.

The modular design of the heat exchanger allows a tailoring of the shape, i.e. the curvature of the heat exchanger as desired so as to enable the heat exchanger to be employed as a craft outer skin heat exchanger in any desired section of the craft outer skin, while using only a limited number of different heat transfer modules. Hence, the heat exchanger may be installed as a craft outer skin heat exchanger in any desired section of the craft outer skin.

The heat exchanger multilayer body of the heat exchanger may comprise heat transfer modules having a heat transfer module body with a rectangular cross- section. To provide the heat exchanger multilayer body of the heat exchanger with the desired curvature, the heat exchanger multilayer body preferably further comprises at least one heat transfer module having a heat transfer module body with a cross-sectional shape tapering towards the center of the osculating circle defined by the outer surface of the heat exchanger.. Heat transfer modules having a heat transfer module body a cross-sectional shape of which tapers in a direction from an outer surface to an inner surface of the heat transfer module body may be employed in a heat exchanger with a convex curvature, while heat transfer modules having a heat transfer module body with a frustro-conical cross section which tapers in a direction from an inner surface to an outer surface of the heat transfer module body may be employed in a heat exchanger with a concave curvature.

The cross-sectional shape of the heat transfer module bodies of the heat transfer modules in the heat exchanger multilayer body may vary in a direction parallel to a curvature axis of the heat exchanger. The variation of the cross-sectional shape should, however, not result in significant change of the flow rate of the heat-carrying medium along the the heat transfer medium channels provided in the heat transfer modules. The heat exchanger multilayer body of the heat exchanger may be defined exclusively by heat transfer modules having a heat transfer module body with a cross-sectional shape tapering towards the center of the osculating circle defined by the outer surface of the heat exchanger so as to obtain a heat exchanger with a strong curvature, i.e. a small curvature radius around a curvature axis. By employing heat transfer modules having a heat transfer module body with a rectangular cross- section and heat transfer modules having a heat transfer module body with a cross- sectional shape tapering towards the center of the osculating circle defined by the outer surface of the heat exchanger in the heat transfer module body, a heat exchanger with a slight curvature, i.e. a large curvature radius around a curvature axis may be obtained.

In a heat transfer module having a heat transfer module body with a cross-sectional shape tapering towards the center of the osculating circle defined by the outer surface of the heat exchanger a tapering angle may correspond to the tilt angle of the central axis the heat transfer module towards the central axis of an adjacent heat transfer module. As a result, side faces of adjacent heat transfer modules are oriented parallel to each other. Heat transfer modules having a heat transfer module body with a cross-sectional shape tapering towards the center of the osculating circle defined by the outer surface of the heat exchanger with a large tapering angle can be used for manufacturing a heat exchanger with a strong curvature, i.e. a small curvature radius around a curvature axis. Contrary thereto, heat transfer modules having a heat transfer module body with a cross-sectional shape tapering towards the center of the osculating circle defined by the outer surface of the heat exchanger with a small tapering angle can be used for manufacturing a heat exchanger with a slight curvature, i.e. a large curvature radius around a curvature axis.

The at least one heat transfer medium channel provided in the heat transfer modules preferably is designed to allow a flow of a heat-carrying medium therethrough in a direction parallel to a curvature axis of the heat exchanger. When the heat

exchanger is installed in a craft so as to form a section of the craft outer skin, ambient air flowing over the craft outer skin serves to discharge heat from the heat- carrying medium flowing through the heat transfer medium channel provided in the heat transfer modules. When the heat exchanger is installed in an aircraft, the heat transfer medium channels preferably extend in a direction parallel to a longitudinal axis of the aircraft and hence parallel to the direction of flow of the ambient air over the aircraft outer skin in flight operation of the aircraft. A heat transfer medium may be supplied to the heat transfer medium channels via a supply manifold and discharged from the heat transfer medium channels via a discharge manifold. The heat-carrying medium flow through the heat transfer medium channels may be unidirectional or bidirectional. If desired, the heat exchanger may be designed so as to allow at least one diversion by 180° of the heat-carrying medium flow through the heat transfer medium channels so that the heat-carrying medium flow meanders through the heat exchanger multilayer body.

If desired, the heat transfer modules employed in the heat exchanger may also comprise more than one heat transfer medium channel. These heat transfer medium channels may be arranged on top of each other in a direction along an axis of the heat transfer modules, i.e. in a direction substantially parallel to the side surfaces and substantially perpendicular to the inner and outer surfaces of the heat transfer module bodies of the heat transfer modules and extend parallel to a curvature axis of the heat exchanger. A heat transfer medium channel adjacent to the outer surface of a heat transfer module body of a heat transfer module then advantageously serves to guide heat-carrying medium transferring heat from a heat-generating device on board the craft, which has a relatively high cooling power demand, while a heat transfer medium channel adjacent to the inner surface of a heat transfer module body of a heat transfer module advantageously is assigned to heat-carrying medium transferring heat from heat-generating devices on board the craft, which have a lower cooling power demand. The heat transfer module body of at least one heat transfer module may have an inner surface which is adapted to form a section of an inner surface of the heat exchanger and which has a curvature which is adjusted to the curvature of an inner surface of the craft outer skin section the heat exchanger is intended to form. If the craft outer skin section and hence the heat exchanger has a convex curvature, the inner surface of the heat transfer module body preferably has a slight concave curvature. If the craft outer skin section and hence the heat exchanger has a concave curvature, the inner surface of the heat transfer module body preferably has a slight convex curvature.

Similarly, the heat transfer module body of at least one heat transfer module may have an outer surface which is adapted to form a section of an outer surface of the heat exchanger and which has a curvature which is adjusted to the curvature of an outer surface of the craft outer skin section the heat exchanger is intended to form. If the craft outer skin section and hence the heat exchanger has a convex curvature, the outer surface of the heat transfer module body preferably has a slight convex curvature. If the craft outer skin section and hence the heat exchanger has a concave curvature, the outer surface of the heat transfer module body preferably has a slight concave curvature. Preferably, a curvature radius of the inner surface of the heat transfer module is smaller than the curvature radius of the outer surface of the heat transfer module body.

At least one heat transfer module may comprise a rib which forms a protruding section of an outer surface of the heat exchanger multilayer body. Preferably, the rib extends in a direction parallel to a curvature axis of the heat exchanger. When the heat exchanger is installed in an aircraft, the rib preferably extend in a direction parallel to a longitudinal axis of the aircraft and hence parallel to the direction of flow of the ambient air over the aircraft outer skin in flight operation of the aircraft. The rib enhances the cooling performance of the heat exchanger and protects the multilayer body and in particular its outer surface from external influences. The rib, however, increases the aerodynamic drag caused by the heat exchanger when installed in a craft, in particular an aircraft.

The rib may be formed integral with the heat transfer module body of the heat transfer module. Further, the rib may be composed of the same material as the heat transfer module body of the heat transfer module, but also of a different material. For example, the rib may be produced from a metal or plastic material, preferably a fiber-reinforced plastic material. It is especially preferred that the rib is integrally formed with the heat transfer module body of the heat transfer module in an extrusion process. The rib may have a substantially triangular cross-section. Further, the rib may have a rounded tip.

The heat exchanger may comprise heat transfer modules which are disposed immediately adjacent to each other. In another embodiment of the heat exchanger at least two adjacent heat transfer modules in the multilayer body of the heat exchanger may be separated from each other by a separating element. The separating element preferably is composed of a material with good thermal transfer characteristics. Alternatively, the separating element may have isolating

characteristics. In general, the separating element may be used as a spacer between heat transfer modules in the multilayer body of the heat exchanger. As a spacer, the separating element may be designed and arranged so as to prevent the entry of ambient air in the space between two adjacent heat transfer modules. The heat exchanger then has the function of a surface heat exchanger and causes only low aerodynamic losses when employed in a craft, in particular an aircraft.

The heat exchanger may comprise a separating element which is generally U-shaped and has two substantially parallel legs extending between side surfaces of the heat transfer module bodies of adjacent heat transfer modules. Further, the separating element may comprise a connecting bar extending between the legs in a direction parallel to a curvature axis of the heat exchanger. The connecting bar prevents the entry of ambient air in the space between two adjacent heat transfer modules and allows the formation of a smooth outer surface of the heat exchanger. An outer surface of the connecting bar may extend parallel to the outer surfaces of the heat transfer module bodies of the adjacent heat transfer modules separated from each other by the separating element and may be flat or curved, as desired.

Alternatively or additionally thereto, the heat exchanger may comprise a separating element including airside fins extending between side surfaces of the heat transfer module bodies of adjacent heat transfer modules. The airside fins may be offset fins or louvered fins and may be designed in accordance with the heat transfer requirements of the heat exchanger. When the heat exchanger is installed in a craft, in particular an aircraft, a separating element of this kind allows ambient air to enter the space between two adjacent heat transfer modules and to thus enhance the cooling capacity of the heat exchanger. So as to keep the additional aerodynamic drag caused by the design of the separating element as low as possible, the separating element may comprise sharp-edged fine grooves which, when the heat exchanger is installed in a craft, in particular an aircraft, are oriented parallel to flow lines of the ambient air flowing over the outer surface of craft, when the craft is moving. Such a surface structure brings about a so-called "shark skin effect", i.e. it brings about a reduction of the frictional drag caused by the heat exchanger.

The heat exchanger is in particular suitable for use in an aircraft. An aircraft cooling system thus may comprise at least one heat exchanger as described above which may be integrated into the aircraft outer skin, preferably in a lower region of aircraft fuselage so as to protect the heat exchanger from solar radiation.

In a method for manufacturing a heat exchanger a plurality of heat transfer modules is arranged side by side so as to define a multilayer body of the heat exchanger, wherein each heat transfer module is provided with at least one heat transfer medium channel designed to allow a flow of a heat-carrying medium therethrough, wherein at least one portion of the multilayer body of the heat exchanger is provided with a curvature which is designed so as to allow the heat exchanger to be used as a curved outer skin section of a craft, and wherein adjacent heat transfer modules of the at least one portion of the multilayer body are arranged with a tilt angle of their central axes towards each other such that each heat transfer module is aligned towards the center of a local osculating circle defined by an outer surface of the heat exchanger.

Preferably, the heat transfer modules are arranged side by side in a manufacturing form and fixed relative to one another, while being arranged in the manufacturing form.

A biasing force may be applied to the heat transfer modules arranged side by side the in a direction substantially perpendicular to side surfaces of the heat transfer module bodies of the heat transfer modules until the heat transfer modules are fixed relative to one another.

Preferred embodiments of the invention are now explained in more detail with reference to the appended schematic drawings, of which Figures 1A to ID show illustrations of four different embodiments of a heat

transfer module designed to form a layer of a heat exchanger multilayer body,

Figure 2 shows an illustration of a heat exchanger having a multilayer body after manufacturing and before integration into a craft outer skin,

Figure 3 shows a cross-sectional view of a heat exchanger having a

multilayer body when integrated into a craft outer skin,

Figure 4 shows a cross-sectional view of an alternative heat exchanger when integrated into a craft outer skin,

Figure 5 shows a three-dimensional view of the alternative heat

exchanger according to Figure 4, and

Figure 6 shows an illustration of a heat exchanger when accommodated in a manufacturing form during its manufacturing process.

Figures 1A to ID show four different embodiments of heat transfer modules 10, 20, 30, 40, each of which may form a layer of a multilayer body 102, 202, 302 of a heat exchanger 100, 200, 300 as, for example, shown in one of Figures 2 to 5. Each heat exchanger multilayer body 102, 202, 302 comprises a plurality of heat transfer modules 10, 20, 30, 40 as shown in Figures 1A to ID. If desired, only one type of heat transfer modules 10, 20, 30, 40 might be employed in the heat exchanger multilayer body 102, 202, 302. Alternatively, it is, however, also conceivable to equip the heat exchanger multilayer body 102, 202, 302 with at least two different types of heat transfer modules 10, 20, 30, 40 as shown in Figure 1A, IB, 1C or ID.

The heat transfer module 10 shown in Figure 1A comprises a heat transfer module body 10a and a rib or fin 12 which is formed integrally with the heat transfer module body 10a. The rib or fin 12 and the heat transfer module body 10a, however, also may be formed as separate components which are connected to one another so as to form the heat transfer module 10 shown in Figure 1A. The heat transfer module body 10a of the heat transfer module 10 has a generally rectangular cross-section and is provided with four heat transfer medium channels 14. The heat transfer medium channels 14 also have a generally rectangular cross-section and are designed to allow a flow of heat transfer medium therethrough. Specifically, the heat transfer medium channels 14 are designed so as to allow the heat transfer medium to flow through the heat transfer module body 10a of the heat transfer module 10 in a direction perpendicular to an axis A of the of the heat transfer module 10.

The heat transfer module body 10a of the heat transfer module 10 further comprises two substantially parallel side surfaces 16 as well as an inner surface 18. The inner surface 18 of the heat transfer module body 10a is disposed opposite from the rib or fin 12 and may either be flat, as shown in Figure 1A, or may be provided with a desired curvature. When the heat transfer module 10 is installed in a heat exchanger multilayer body 102, 202, 302, the inner surface 18 of the heat transfer module body 10a forms a section of an inner surface 104, 204, 304 of the heat exchanger 100, 200, 300. Contrary thereto, the rib or fin 12 forms a protruding section of an outer surface 106, 206, 306 of the heat exchanger 100, 200, 300. As becomes apparent from Figure 1A, the rib or fin 12 has a substantially triangular cross-section, i.e. a cross-section which tapers in a direction parallel to the axis A of the heat transfer module 10 from a base of the rib or fin 12 which is disposed adjacent to the heat transfer module body 10a to a tip of the rib or fin 12 which is disposed distal from the heat transfer module body 10a. The tip of the rib or fin 12 has a rounded shape.

The heat transfer module 20 of Figure IB differs from the heat transfer module 10 shown in Figure 1A in that it does not have a rib or fin attached to or formed integrally with a heat transfer module body 20a. Again, the heat transfer module body 20a of the heat transfer module 20 has a generally rectangular cross-section and is provided with four heat transfer medium channels 24, which also have a generally rectangular cross-section. The heat transfer module body 20a of the heat transfer module 20 comprises two substantially parallel side surfaces 26 as well as an inner surface 28, wherein the inner surface 28 of the heat transfer module body 20a is adapted to form a section of an inner surface 104, 204, 304 of a heat exchanger 100, 200, 300, when the heat transfer module 20 is installed in the heat exchanger multilayer body 102, 202, 302. The inner surface 28 may either be flat, as shown in Figure IB, or may be provided with a curvature.

The heat transfer module body 20a of the heat transfer module 20 further comprises an outer surface 22 which is disposed opposite from the inner surface 28. When the heat transfer module 20 is installed in a heat exchanger 100, 200, 300, the outer surface 22 of the heat transfer module body 20a forms a section of an outer surface 106, 206, 306 of the heat exchanger 100, 200, 300. Like the inner surface 28, the outer surface 22 may either be flat, as shown in Figure IB, and extend substantially parallel to the inner surface 28, or may be provided with a curvature.

The heat transfer module 30 of Figure 1C generally corresponds to the heat transfer module 20 shown in Figure IB, its heat transfer module body 30a, however, has a cross-sectional shape tapering towards the center of an osculating circle defined by an outer surface 106, 206, 306 of a heat exchanger 100, 200, 300 including the heat transfer module 30. Correspondingly, each of the heat transfer medium channels 34 also has a cross-sectional shape tapering towards the center of the osculating circle defined by the outer surface 106, 206, 306 of the heat exchanger 100, 200, 300 including the heat transfer module 30. Two side surfaces 36 of the heat transfer module body 30a are inclined so as to approach each other in a direction parallel to the axis A of the heat transfer module 30 from the outer surface 32 to the inner surface 38 of the heat transfer module body 30a. The side surfaces 36 may be inclined so as to define a tapering angle of the cross-sectional shape of the heat transfer module body 30a of approximately 1 to 2°, in particular 1.6°.

The inner surface 38 of the heat transfer module body 30a is adapted to form a section of the inner surface 104, 204, 304 of a heat exchanger 100, 200, 300, when the heat transfer module 30 is installed in the heat exchanger multilayer body 102, 202, 302 of the heat exchanger 100, 200, 300. The inner surface 38 has a concave shape. The outer surface 32 of the heat transfer module body 30a is disposed opposed from the inner surface 38 and is adapted to form a section of an outer surface 106,206, 306 of the heat exchanger 100, 200, 300, when the heat transfer module 30 is installed in the heat exchanger multilayer body 102, 202, 302 of the heat exchanger 100, 200, 300. The outer surface 32 has a convex shape. It is, however, also conceivable to provide the heat transfer module body 30a of the heat transfer module 30 with flat inner and outer surfaces 38, 32, or with a convex inner surface 38 and a concave outer surface 32.

The heat transfer module 40 as shown in Figure ID generally corresponds to the heat transfer module 10 as shown in Figure 1A, its heat transfer module body 40a, however, has a cross-sectional shape tapering towards the center of an osculating circle defined by an outer surface 106, 206, 306 of a heat exchanger 100, 200, 300 including the heat transfer module 40.. Correspondingly, each of the heat transfer medium channels 44 also has a cross-sectional shape tapering towards the center of the osculating circle defined by the outer surface 106, 206, 306 of the heat exchanger 100, 200, 300 including the heat transfer module 40. Two side surfaces 46 of the heat transfer module body 40a are inclined so as to approach each other in a direction parallel to the axis A of the heat transfer module 40 from the outer surface 42 to the inner surface 48 of the heat transfer module body 40a. The side surfaces 46 may be inclined so as to define a tapering angle of the cross-sectional shape of the heat transfer module body 40a of approximately 1 to 2°, in particular 1.6°. The inner surface 48 of the heat transfer module body 40a has a concave shape. It is, however, also conceivable to provide the heat transfer module body 40a of the heat transfer module 40 with a flat or a convex inner surface 38.

The heat exchanger 100 shown in Figure 2 comprises a multilayer body 102 formed of two different types of heat transfer modules 40, 30, namely eleven heat transfer modules 40 as shown in Figure ID and ten heat transfer modules 30 as shown in Figure 1C. In the multilayer body 102 of the heat exchanger 100 according to Figure 2, the different heat transfer modules 40, 30 are arranged such that the side surfaces 46 of a heat transfer module 40 face the side surfaces 36 of two adjacent heat transfer module 30 and vice versa. The heat transfer modules 40, 30 thus define alternating layers of the multilayer body 102. Specifically, adjacent heat transfer modules 40, 30 of the multilayer body 102 are arranged with a tilt angle of their central axes A towards each other such that each heat transfer module 30, 40 is aligned towards the center of a local osculating circle defined by the outer surface 106 of the heat exchanger 100. Thus, a multilayer body 102 of the heat exchanger 100 is defined which is curved around a curvature axis C.

The curvature radius of the multilayer body 102 depends on the shape of the heat transfer module bodies 40a, 30a. For example, a minimum curvature radius of the multilayer body 102 of 500 mm can be obtained by employing in the multilayer body 102 heat transfer modules 40, 30 having heat transfer module bodies 40a, 30a, the side surfaces 46, 36 of which define a tapering angle of the cross sectional shape of the heat transfer module body 40a of approximately 1.6°. The heat transfer medium channels 44, 34 of the heat transfer modules 40, 30 allow a flow of heat transfer medium through the heat transfer module bodies 10a, 30a of the heat transfer modules 10, 30 in a direction parallel to the curvature axis C of the of the heat exchanger 100. The curvature radius of the heat exchanger 100 thus can be tailored by suitably adapting the cross-sectional shape of the heat transfer module bodies 40a, 30a of the heat transfer modules 40, 30. It is, however, also conceivable to tailor the curvature radius of the heat exchanger 100 by installing different types of heat transfer modules, i.e. heat transfer modules, the heat transfer module bodies of which have different cross-sectional shapes in the multilayer body 102 of the heat exchanger 100. For example, in the heat exchanger 100 of Figure 2, instead of heat transfer modules 40, heat transfer modules 10, the heat transfer module bodies 10a of which have a rectangular cross-sectional shape can be employed so as to increase the curvature radius of the heat exchanger 100. Of course, all or only a selected number of heat transfer modules 40 may be replaced by heat transfer modules 10. Similarly, all or a selected number of heat transfer modules 30 of the heat exchanger 100 may be replaced by heat transfer modules 20 so as to increase the curvature radius of the heat exchanger 100.

In the embodiment of a heat exchanger 100 shown in Figure 2, the multilayer body 102 further comprises a plurality of separating elements 50. A cross-sectional view of a separating element 50 in the direction of the curvature axis C is shown in the detail view incorporated in Figure 2. The separating elements 50 are provided in between the alternating heat transfer modules 40, 30 and thus serve as spacers for spacing adjacent heat transfer modules 40, 30 apart from each other. Each of the separating elements 50 is generally U-shaped and comprises two substantially parallel legs 52 which extend between the side surfaces 46, 36 of the heat transfer module bodies 40a, 30a of adjacent heat transfer modules 40, 30. Similar to the heat transfer module bodies 40a, 30a of the heat transfer modules 40, 30, the legs 52 of the separating elements 50 may have a cross-sectional shape tapering towards the center of an osculating circle defined by the outer surface 106 of the heat exchanger 100. By employing separating elements 50 having legs 52 with a cross-sectional shape tapering towards the center of an osculating circle defined by the outer surface 106 of the heat exchanger 100 smaller curvature radii of the heat exchanger 100 can be achieved, than by employing separating elements 50 having legs 52 with a rectangular cross-section. It is, however, also conceivable to provide all or a selected number of separating elements 50 with legs 52 having a rectangular cross- section so as to tailor the curvature radius of the heat exchanger 100 as desired.

A connecting bar 53 extends between the legs 52 of each separating element 50 in a direction parallel to the curvature axis C of the heat exchanger 100 and has an outer surface which is designed to form a smooth section of an outer surface of the multilayer body 102. Specifically, the outer surface of the multilayer body 102 is formed by a periodical sequence of the outer surface of a connecting bar of a separating element 50, a rib or fin 12 of a heat transfer module 40, a connecting bar of a further separating element 50, and the outer surface 32 of a heat transfer modules 30. The connecting bars 53 of the separating elements 50 prevents the entry of ambient air in the space between two adjacent heat transfer modules 30, 40. Hence, the heat exchanger 100 has the function of a surface heat exchanger and causes only low aerodynamic losses when employed in a craft, in particular an aircraft.

As described above, the outer surface 32 of the heat transfer modules 30 has a convex shape which is adjusted to the desired curvature radius of the outer surface 106 of the heat exchanger 100 around the curvature axis C. Like the outer surface 32 of the heat transfer modules 30, also the outer surface of the connecting bars 53 of the separating elements 50 may be provided with a convex curvature which is adjusted to the desired curvature radius of the outer surfacel06 of the heat exchanger 100 around the curvature axis C. Further, like the inner surfaces 38, 48 of the heat transfer modules 30, 40, inner surfaces of the legs 52 of the separating elements 50 may be provided with a concave curvature which is adjusted to the desired curvature radius of the inner surface 104 of the heat exchanger 100 around the curvature axis C.

As an alternative to the separating elements 50 shown in Figure 2, the multilayer body 102 of the heat exchanger 100 may also be provided with separating elements 250' which are employed in the heat exchanger 300 of Figures 4 and 5 and which will be described in more detail below.

The heat exchanger 100 as shown in Figure 2 further comprises two connecting elements 60. Each of the connecting elements 60 has a substantially L-shaped cross section and comprises a first leg 62 and a second leg 64. The first leg 62 of each connecting element 60 forms an outermost layer of the multilayer body 102. The second leg 64 extends substantially perpendicular from the first leg 62. The connecting elements 60 are adapted to be connected to an outer skin of a craft in an aerodynamically favourable manner in such a way that the heat exchanger 100 is integrated into the craft outer skin so as to form a section thereof. Figure 3 shows a further heat exchanger 200 having a multilayer body 202, when integrated into a craft outer skin 210 so as to form a section thereof. Connecting elements 260 are provided which serve to integrate the heat exchanger 200 into the craft outer skin 210 in an aerodynamically favourable manner. The multilayer body 202 of the heat exchanger 200 comprises six heat transfer modules 40 as shown in Figure ID, one heat transfer module 10 as shown in Figure 1A, four heat transfer modules 20 as shown in Figure IB and three heat transfer modules 30 as shown in Figure 1C. In the layer sequence of the multilayer body 202, a heat transfer module 20 or 30 as shown in Figure IB or Figure 1C alternates with a heat transfer module 40 as shown in Figure ID and once with a heat transfer module 10 as shown in Figure 1A. As already discussed above, by appropriately selecting the type and the sequence of heat transfer modules in the multilayer body 202 of the heat exchanger 200, the curvature of the heat exchanger 200 can be tailored as desired so as to adapt it to the shape and in particular the curvature of the craft outer skin into which the heat exchanger 200 is to be integrated. Specifically, the curvature of the heat exchanger 200 matches the curvature of the craft outer skin so as to reduce aerodynamic losses caused by the heat exchanger as low as possible.

The heat exchanger 200 of Figure 3 also is provided with separating elements 50 in between the alternating heat transfer modules 10, 20, 30, 40 and also in between the two outermost heat transfer modules 20, 40 and the connecting elements 260. The connecting bars 53 of the separating elements 50 each are provided with an which is designed to form a smooth section of an outer surface of the multilayer body 202.

The alternative heat exchanger 300 of Figures 4 and 5, which forms a section of a craft outer skin 310, differs from the heat exchanger 200 shown in Figure 3 only in that it comprises, instead of separating elements 50, separating elements 250' including airside fins. The separating elements 250' allow cooling medium, in particular ambient air, to enter the spaces between adjacent heat transfer modules 10, 20, 30, 40 and to thus support the heat transfer from a heat transfer medium flowing through the heat transfer channels 14, 24, 34, 44 of heat transfer modules 10, 20, 30, 40 to the cooling medium.

The heat exchangers 100, 200, 300 described above are in particular suitable for integration into an aircraft outer skin and may be used in an aircraft to supply cooling energy to heat generating components on board the aircraft. The heat exchangers 100, 200, 300 shown in Figures 2 to 4 have a convex curvature an thus are adapted to form a section of an aircraft outer skin having a convex curvature, for example in the region of a tail of the aircraft. The heat exchangers, however, might also be provided with a concave curvature so as to be suitable to form a section of an aircraft outer skin having a concave curvature. If desired, the heat transfer module bodies of the heat transfer modules may have a convex inner surface and a concave outer surface. Further, be appropriately selecting the shape of the heat transfer module bodies of the heat transfer modules and/or the sequence of the heat transfer modules, heat exchangers having a varying curvature may be obtained. For example, a heat exchanger may be obtained having a first section with a convex curvature and a second section having a concave curvature.

When the heat exchanger 100, 200, 300 is installed in an aircraft, heat transfer medium flowing through the heat transfer channels 14, 24, 34, 44 provided in the heat transfer module bodies 10a, 20a, 30a, 40a of the heat transfer modules 10, 20, 30, 40 is cooled by heat transfer to the ambient air flowing over the outer surface of the heat exchanger multilayer body 102, 202, 302, in particular during flight operation of the aircraft. Typically, the heat exchanger 100, 200, 300 is installed in the aircraft such that the ribs or fins 12 extend in a direction parallel to a direction of the flow of ambient air over the aircraft outer skin during flight operation of the aircraft. The ribs or fins 12 enhance the cooling performance of the heat exchanger 100, 200, 300, but increase the aerodynamic drag caused by the heat exchanger 100, 200, 300.

The cooling performance of the heat exchanger 100, 200, 300 can further be enhanced by providing the heat exchanger 100, 200, 300 with separating elements 250' which allow ambient air flowing over the aircraft outer skin during flight operation of the aircraft to enter the spaces provided in the heat exchanger multilayer body 102, 202, 302 between adjacent heat transfer modules 10, 20, 30, 40 and to thus directly discharge heat from the heat transfer medium flowing through the heat transfer channels 14, 24, 34, 44 of heat transfer modules 10, 20, 30, 40. Separating elements 250' allowing ambient air to enter the spaces provided in the heat exchanger multilayer body 102, 202, 302 between adjacent heat transfer modules 10, 20, 30, 40, however, also increase the aerodynamic losses caused by the heat exchanger 100, 200, 300. Figure 6 shows a manufacturing form 400 used for manufacturing a heat exchanger 100, 200, 300. A plurality of layers comprising heat transfer modules 10, 20, 30 and/or 40 and separating elements 50, 250 and/or 250' are accommodated in the manufacturing form 400. The type and the sequence of heat transfer modules 10, 20, 30 and/or 40 in the manufacturing form 400 are selected such that a curvature of the heat exchanger 100, 200 or 300 is obtained which matches the curvature of the section of the craft outer skin the heat exchanger 100, 200, 300 is intended to form. When received in the manufacturing form 400, the plurality of layers is fixed relative to one another.

As shown in Figure 6, the manufacturing form 400 is provided with a support 410 for supporting the layers of heat transfer modules 10, 20, 30, 40 in line with the curvature of the section of the outer skin of the craft, the heat exchanger 100, 200 or 300 is intended to form. Further, the manufacturing form 400 comprises a movable element 420 which is pre-stressed by use of two spiral springs 430 and which serves to apply a biasing force to the heat transfer modules 10, 20, 30, 40 in a direction substantially perpendicular to the side surfaces 16, 26, 36, 46 of the heat transfer module bodies 10a, 20a, 30a, 40a of the heat transfer modules 10, 20, 30, 40 until the heat transfer modules 10, 20, 30, 40 are fixed relative to one another.