FIELD OF THE INVENTION

The invention relates to the field of heat exchangers. Specifically, it relates to microtube heat exchangers for computer cooling systems.

BACKGROUND OF THE INVENTION

During operation of a computer, quickly dissipating the heat generated by the central processing units and other processors is essential to high performance of the computer. The temperature must be kept within the design range to ensure that the components will not be damaged by excessive heat. This problem is well known and only becomes more relevant as increasingly more heat-generating components are included in computers.

Liquid cooling is known as a method that allows efficient cooling due to the more efficient heat transfer of liquids such as water-based cooling liquids compared to gasses such as air. This is often done by having a closed liquid system circulating the cooling liquid past the heat generating components. In such a system, it is necessary to remove the heat from the liquid, while it is away from the heatgenerating components, and before it is recirculated past them. For this purpose, it is common to have the cooling liquid pass through a heat exchanger placed away from the heat-generating units of the computer.

A known type of heat exchangers for cooling the liquid is based on a number of round tubes through which a cooling liquid passes and around which a fluid can pass to transport the heat away from the tubes and the liquid inside. Although this method is known, it is a continuous problem to optimise such heat exchangers for efficient transfer of heat away from the liquid. It is a complex system, where all components influence how efficiently the heat exchange system operates. A larger surface area of the tubes allows a larger area of heat exchange, while less of the cooling liquid can pass through each tube. Similarly, the amount of the external fluid, which can pass around the tubes, and the resistance it meets also affect how well the heat can be exchanged between the two fluids. SUMMARY OF THE INVENTION

In the present disclosure, several potential improvements to heat exchange systems are taught.

In this regard a heat exchanger for transferring heat from a first fluid to a second fluid is taught, the heat exchanger comprising

- one or more manifolds,

- a liquid inlet for the liquid to be cooled to enter the heat exchanger

- a liquid outlet for the liquid having been cooled to exit the heat exchanger

- a plurality of microtubes placed substantially parallel to each other, each microtube having a first and a second end wherein the liquid inlet and liquid outlet are fluidically connected through the plurality of microtubes, and wherein the cross section of each microtubes in the plurality of microtubes is non-circular.

By a microtube is understood a tube with a single channel in which the cooling liquid can be contained and through which it can be passed. By a plurality of such microtubes is understood that there are multiple microtubes, and the amount may vary for different systems and could for example be ten for small systems or several hundred for larger systems. The number of microtubes should not be seen as limiting to the invention.

The plurality of microtubes being substantially parallel should be understood with respect to their length, i.e. in the direction the first fluid can travel through the microtubes. That all micro tubes have a first and second end is also with respect to their length, but all first ends of the microtubes are adjacent to each other, and all second ends are adjacent to each other, and it is thus with respect to the placement of the microtubes and not with respect to direction of travel through the microtubes.

By the fluid inlet and fluid outlet being fluidically connected through the microtubes is understood that the cooling first fluid enters the heat exchanger through the fluid inlet then passes through the one or more microtubes and then reaches the fluid outlet from where it can leave the heat exchanger. The fluid inlet and outlet are connected to the microtubes through the one or more manifolds.

By the cross section of each microtube in the plurality of microtubes being noncircular is understood that independently of the other microtubes in the plurality each microtube has a cross-sectional shape which is not circular. Instead the microtubes have cross-sectional shapes that have been optimised for the fluid flow through and around them. In addition to this, the microtubes in the plurality may be placed in specific patterns that benefit the fluid flow around them

The non-circular cross section of the microtubes is particularly beneficial with respect to the fluid flow around the microtubes. To enable efficient heat exchange and cooling of the first fluid inside the microtubes, the fluid around the microtubes must be able to pass the microtubes with a continuous flow. The more air that passes within a unit of time, the more heat can be transferred from the microtubes, as the heated air moves away, and new cold air is introduced next to a given microtube.

Circular tubes disrupt the flow of air around them, as they create slipstreams behind them, whereby any channels located in the region of the slip stream behind them will not experience a flow of air and thus will be cooled less. The creation of multiple slipstreams is also likely to lead to turbulence in the flow, generally slowing the flow of fluid past the microtubes and further lessening the cooling. The non-circular shape also has the benefit of lowering the counter pressure at the impinging point.

Preferably, the heat exchange system is used with a liquid as a first fluid inside the microtubes and gas as the second fluid surrounding the microtubes. More preferably, the first fluid inside the microtubes is a cooling liquid, e.g. a mixture comprising water and glycol and/or electrolyte inhibitors, and the second fluid surrounding the microtubes is air.

In an embodiment, the cross section of each microtube is an aerofoil.

By an aerofoil is understood a shape optimised for directing fluids such as air around the shape having a higher air pressure on one side of the structure than on the other. Such structures are known for example from aircrafts used for example for wings and propellers. Using aerofoil cross sections for the microtubes provides the possibility of minimising turbulence and slip streams behind each microtube thereby allowing a continuous flow of air reaching subsequent microtubes with respect to the direction of fluid flow. Furthermore, using aerofoil shapes allows the fluid flow around the microtubes to be steered and directed based on the angles of the microtubes with respect to each other. Thereby, the flow of the fluid can be directed to achieve different speeds and different cooling in various regions of the heat exchanger to achieve the optimum overall distribution of fluid-flow around the microtubes. Furthermore, the control and knowledge of fluid-flow around aerofoils makes it possible to optimise the placement of the microtubes to minimise wear on them thereby increasing the structural integrity of the system. It is also possible to design for increased strength in regions of higher stress.

In an embodiment of the invention, the cross section of each microtube has a leading edge and a trailing edge, said leading edge being rounded and said trailing edge being pointed.

A rounded leading edge provides the benefit of maintaining a smooth airflow at multiple angles of incidence. Thereby, the same cross section can be used for the microtubes throughout the heat exchanger even though there may be slight variations in the angle of incidence of the second fluid on the micro tubes depending on their position. This will minimise the disruption of the flow while allowing a simple distribution of the microtubes throughout the heat exchanger.

A pointed trailing edge allows a laminar fluid flow around the structure, as the changes in flow direction becomes less abrupt following a small angle. Less disruption of the fluid flow around the microtubes ensures fewer zones with no or little presence of the second fluid or regions wherein the second fluid moves very slowly both of which would lower the efficiency of the heat exchanger.

Within the disclosure, the radius of curvature of the leading edge may be in the range of 0.5 mm to 3 mm.

Within the disclosure, the angle of the pointed trailing edge may be in the range of 5 degrees to 45 degrees. It is to be understood that pointed does not necessarily mean that the edge is sharp as such. The structural tip may still be rounded due to processing and handling.

In an embodiment of the invention, the cross section of each microtube has a leading edge and a trailing edge, said leading edge being pointed and said trailing edge being pointed.

A pointed leading edge will be shaped similarly to a pointed trailing edge. Such a pointed leading edge will decrease the local contact pressure between the flow of the second fluid, where it impinges on the microtube. A pointed leading edge is most beneficial when the angle of attack between the second fluid surrounding the microtubes can be kept low. This is possible to achieve in a static system of a heat exchanger, where the orientation of each microtube can be chosen with respect to the direction of the fluid flow around them.

In an embodiment of the invention, the cross section of each microtube is asymmetric.

By having asymmetric microtube structures, the fluid flow around each structure may be directed to continue in a direction not directly behind itself. Thereby the flow path through the heat exchanger can be controlled. In this manner, it is possible to ensure that any regions of low fluid flow speed or volume will be directed around rather than at surrounding microtubes thereby increasing the efficiency of the heat exchanger.

Within the disclosure, an asymmetric microtube cross section could for example have a flat side causing minimum disruption of airflow on one side of the microtube. Another example of an asymmetric cross section has a lower curvature on one side of the cross section than the other, despite neither side being completely flat. In yet another asymmetric variant of the cross section, both sides of the microtube may curve in the same direction.

In an embodiment of the heat exchanger, the width of each microtube is no greater than 5 mm. By the width is understood the longest axis of the cross-sectional area of the microtube. By keeping the cross section of the microtube small, the surface area to inner volume is increased which increases the amount of heat transfer that can take place.

In an embodiment of the heat exchanger, the width of each microtube is at least one and a half times larger than the height.

By the height of the microtube is understood the dimension of the cross section of the microtube perpendicular to the width at the point where it is largest. By having the microtube wider than it is tall, it is possible to achieve aerodynamic benefits of minimising the disruption of laminar fluid flow around the microtube based on the orientation of the microtube with respect to the direction of fluid flow around the microtube.

In an embodiment of the heat exchanger, subsets of the plurality of microtubes are connected thermally by tube sheets.

By subsets of the microtubes is to be understood that a number of microtubes are thermally connected to each other but not to other microtubes that are not part of that subset. In some of the disclosed variants, there will be multiple subsets of thermally connected microtubes such that all microtubes belong to one of those subsets. In other variants, some microtubes may be connected thermally by tube sheets, while other microtubes remain unconnected.

By connected thermally is to be understood that the microtubes of subset are connected structurally by a sheet in contact with each microtube of the subset. While the sheets provide a mechanical and thermal connection, they do not provide a fluid connection between the microtubes, and the first fluid cannot move from one microchannel to another through the tube sheets.

The tube sheets have the benefit of directing the flow of the second fluid around the microtubes. A microtube which is not connected to a tube sheet will cause a flow of a second fluid to split, when said second fluid impinges on the leading edge of the microtube. Once the flow of the second fluid has passed the microtube, it will remerge at the trailing edge of the microtube. This remerging of the second fluid can cause turbulence which disturbs the flow of the second fluid and creates slipstreams. If the microtube is connected to other microtubes in the tube sheet by a sheet connector at the trailing edge of the microtube, that sheet connector blocks the flow of the second fluid from remerging at the trailing edge. Instead, the flow of the second fluid will continue along the sheet connector to the next microtube in the tube sheet. Thus, the tube sheets minimise disruption of laminar flow around the microtubes.

In some cases, thermal connection of microtubes may be beneficial to distribute the heat exchange along several microtubes. In other systems, it is beneficial to have as little heat exchange as possible taking place directly between the microtubes, as having a temperature gradient across the heat exchanger allows efficient distribution of the cooling effects. In such cases, the amount of material is minimised, i.e. the sheet connections are made as thin as possible. In some preferable variants, the sheet connectors are thinner than the thickness of the tube walls of the microtubes which they connect.

In a preferred variant, the tube sheets connect the end points with respect to the width of the microtubes to the endpoints of the adjacent microtubes. In other words, they connect the trailing edge of one microtube to the leading edge of another.

Tube sheets also allow engineering of the flow of the second fluid around the microtubes, as they hinder the first fluid passing from one side of a tube sheet to another, thereby making it possible to restrict and direct the fluid flow through the heat exchanger.

In an embodiment of the heat exchanger, the microtubes of the plurality of microtubes are placed in a staggered manner with respect to each other.

Staggered positions of the microtubes have the benefit of increasing the distance between microtubes placed directly behind each other which is the region where there is the greatest disruption in the fluid flow of the second fluid. Thereby, it is possible to have a large number of densely packed microtubes which are placed away from regions of low flow speed and turbulence, thus increasing the efficiency of the heat exchanger. In an embodiment of the heat exchanger, at least one microtube is placed at an angle with respect to another microtube of the plurality of microtubes

By a microtube being placed at an angle with respect to another microtube is understood that the long axes of the cross section of the respective non-circular microtubes are not parallel. However, it is to be understood that the microtubes are still substantially parallel with respect to their longitudinal axes.

Aside from minimising the disruption of laminar flow, non-circular microtubes may also direct the flow of the second fluid around them. Angling the non-circular microtubes allows further engineering of the fluid flow and directing the second fluid to specific regions of the heat exchanger, e.g. to regions where a larger fluid flow volume is beneficial.

In an embodiment of the heat exchanger, the plurality of microtubes comprises microtubes with differing cross sections.

Different cross-sectional shapes of the microtubes direct the flow of the second fluid differently. By having various different cross-sectional shapes of the microtubes, the flow of the second fluid around the microtubes may be engineered further. For example, the cross-sectional shapes of adjacent microtubes may be different such that the shape of one microtube may mitigate the disruptions caused by the neighbouring microtube. As another example, the cross-sectional shape of the microtubes at the edges of the heat exchanger may differ from those of the central microtubes to direct the flow of the second fluid inwards and maximise the amount of fluid passing through the heat exchanger.

In an embodiment of the heat exchanger, a first manifold comprises the fluid inlet, and a second manifold comprises the fluid outlet.

By having separate manifolds connected to the fluid inlet and the fluid outlet, it is possible to keep the first fluid coming into the heat exchanger from the first fluid exciting the heat exchanger and thereby minimising the risk of the warmer fluid entering the heat exchanger and heating the cooled fluid exciting the heat exchanger. Having the fluid inlet and fluid outlet connected to a first and a second manifold also allows different geometries of the heat exchanger. In one variant of the heat exchanger, the first and second manifold may be spaced apart significantly thus having the liquid flow through the heat exchanger in one direction only.

In another variant, the heat exchanger may have the first and second manifold arranged adjacent to each other, whereby the fluid inlet and fluid outlet are located at the same side of the heat exchanger. In such a variant, the microtube geometry ensures that the first and second ends of the microtube are adjacent to each other, e.g. by each microtube having a U-shape.

In an embodiment of the heat exchanger, it comprises three manifolds.

By having three manifolds in the heat exchanger, it becomes possible to both the fluid inlet and the fluid outlet to be located at the first end of the microtubes, while the second end of the microtubes is connected to a third manifold. Having the fluid inlet and fluid outlet at the same end of the microtubes allows the heat exchange system to be mounted in systems with limited space on one side and introduces more flexibility in the mounting of the heat exchanger. By employing a third manifold rather than a U-shape of the tubes, the first fluid inside is allowed to mix in the third manifold rather than being confined to a specific microtube throughout the entirety of the traversal from the fluid inlet to the fluid outlet. Mixing the liquids allows a further exchange of heat which may be beneficial, if the cooling capacity is not distributed evenly throughout the heat exchanger. Conversely, having confined paths through a U-shaped microtube may be beneficial, where it is the intent to control regions of differentiated cooling across the heat exchanger.

In an embodiment of the heat exchanger having three manifolds, approximately half of the plurality of microtubes is connected to the first manifold and the remainder of the plurality of microtubes is connected to the second manifold, and all of the plurality of microtubes is connected to a third manifold.

Having approximately half of the microtubes connected to the first manifold and the remainder of the microtubes connected to the second manifold, while they are all connected to the third manifold, enables packing the microtubes closely in two dimensions. The third manifold can be used as a connection between multiple layers of microtubes allowing flow of the first fluid in multiple directions, while every microtube may be straight, i.e. requires no bends, which simplifies the production of the microtubes.

In some variants of the disclosure, more manifolds may be present enabling further changes of flow direction of the first fluid.

In an embodiment of the invention, the heat exchanger is used in connection with the cooling system of a computer.

The heat exchanger is particularly suitable for use with computer systems, as it is compact and provides efficient cooling. It may be connected to known liquid cooling systems for personal computers to cool the liquid which has been used to transport heat away from heat-generating processing units, i.e. this liquid being the first fluid in the heat exchanger system.

SHORT LIST OF THE DRAWINGS

In the following, example embodiments are described according to the invention, where

Figs. 1 a and 1 b are overviews of a heat exchanger with microtubes shown in different perspectives.

Figs. 2a-2e illustrate various non-circular microtube cross sections.

Figs. 3a-3f illustrate various non-circular microtube cross sections.

Fig. 4 illustrates staggered positioning of microtubes with a non-circular cross section.

Fig. 5 is a schematic illustration of how microtubes with different cross sections may be used together in a heat exchanger.

Figs. 6a-6c are illustrations of various types of tube sheets.

Fig. 7 is a conceptual illustration of a heat exchanger microtube configuration with tube sheets and unconnected microtubes. DETAILED DESCRIPTION OF DRAWINGS

In the following, the invention is described in detail through embodiments thereof that should not be thought of as limiting to the scope of the invention.

Figs. 1a and 1 b disclose a variant of a heat exchanger 10 shown in different perspectives. The heat exchanger 10 comprises a plurality of microtubes 100. Each microtube 100 has a first end 101 and a second end 102. In the illustrated embodiment, the heat exchanger comprises a first 51 , second 52 and third manifold 53. In this variant, the second end 102 of all microtubes are connected to the third manifold 53. The first manifold comprises a fluid inlet 20, and the second manifold comprises a fluid outlet 25. Approximately half of the microtubes 100 have their first end 101 connected to the first manifold 51 , while the remainder of the plurality of microtubes 100 has the first end 101 connected to the second manifold 52. In this embodiment, the heat exchanger 10 is designed such that the fluid inlet 20 and fluid outlet 25 are both located at the first end 101 of the microtubes 100 thereby allowing the remainder of the cooling system to be connected only at one end of the heat exchanger 10 thus making it possible to make the connection, where there is limited space surrounding the heat exchanger 10. Although the embodiment illustrated in Figs. 1a and 1 b has the fluid inlet 120 and fluid outlet 125 oriented such that connection to the heat exchanger is from the same side of the heat exchanger 10, in other embodiments of the heat exchanger 10 the fluid inlet 120 and fluid outlet 125 may be oriented such that the connection to the tubing of the cooling system (not shown) is from opposing sides of the heat exchanger 10.

As best seen in Fig. 1 b the first 51 and second manifold 52 are placed substantially parallel to and aligned with each other. The flow path of the first fluid (not shown) through the heat exchanger is illustrated by the dashed arrows L in Fig. 1 b: The first fluid enters the heat exchanger at the fluid inlet 20 and through the first manifold 51 it is distributed in a first subgroup of microtubes 100’ comprising approximately half of the plurality of microtubes 100. The first fluid passes from the first manifold 51 through the microchannels 110 (see Fig. 2a) of the microtubes 100 to the third manifold 53. From the third manifold 53, the first fluid enters the second subgroup of microtubes 100” and flows through the microchannels from the third manifold 53 to the second manifold 52. The flow of the first fluid in the first subgroup 100’ and the flow of the first fluid in the second subgroup of microtubes 100” are parallel but in opposing directions. Once having reached the second manifold 52, the first fluid exits the heat exchanger 10 through the fluid outlet 25.

In the embodiment illustrated in Figs. 1 a and 1 b exactly half of the microtubes 100 belongs to the first subgroup of microtubes 100’ while the other half belongs to the second subgroup of microtubes 100”. In other embodiments, there might be slight variations in the division of the microtubes into the subgroups, as the microtubes 100 may be arranged in different structures than making patterns that direct the flow of the second fluid (not shown) around them. In yet other embodiments of the disclosure, there may be more subgroups than two, e.g. in variants where there are more than three manifolds 50.

As shown in Figs. 1 a and 1 b, the heat exchanger may further comprise a first end plate 61 and a second end plate 62. The end plates are located at the first 101 and second ends 102 of the microtubes, respectively, and the microtubes may be connected to the one or more manifolds 50 through the endplates 61 ,62. The end plates 61 ,62 ensure the seal of the connection between the microtubes, and the manifolds ensure that the heat exchanger is leak-tight.

In other embodiment, the heat exchanger 10 may comprise more or fewer manifolds 50 as long as there is at least one manifold 50. For example, this disclosure also includes a heat exchanger 10 with two manifolds 50, i.e. a first manifold 51 located at the first end 101 of the microtubes 100 and a second manifold 52 located at the second end 102 of the microtubes 100. In embodiments with two manifolds 50, the microtubes 100 need not be divided into subgroups, and the first fluid can flow directly through the full plurality of microtubes 100 from the first manifold to the second manifold. In one embodiment, the microtubes 100 may be straight, and the first fluid may move in only a single direction when traversing from the fluid inlet 20 to the fluid outlet 25. In another embodiment of the invention having two manifolds 50, the first 51 and second manifold 52 may be located adjacent to each other as illustrated in Fig. 1 a, but rather than having a third manifold 53, the microtubes 100 may have a U-shape such that the first 101 and second end 102 of the microtubes 100 are adjacent to each other and connected to the first 51 and second manifold 52, respectively. In Fig. 1 b, the direction of flow G of the second fluid (not shown) is illustrated by the solid arrow indicating that the second fluid flows approximately perpendicular to the direction of flow of the first fluid inside the microtubes 100. However, the direction of flow G of the second fluid may vary depending on the system in which the heat exchanger 10 is installed, and the placement and orientation of the microtubes 100 may be optimised to obtain the most efficient heat exchange between the first and second fluid depending on the flow direction G of the second fluid.

Figs. 2a-2e show examples of non-circular cross sections within the scope of this disclosure. Each of the microtubes 100 comprise a tube wall 120 on the outside which defines the cross-sectional shape of the microtube 100 itself and a microchannel 100 enclosed by the tube wall 120. The microchannel 110 is hollow and is the region of the microtube 100 through which the first cooling fluid may pass.

The tube wall 120 may be made from any material suitable for being in contact with both the first fluid cooling medium to be transferred through the microchannel 110 and the second fluid cooling medium surrounding the outside of the microtube 100. Preferably, the tube wall 120 is made of a material with a high heat conductivity such as polymers or metal. The microtube 100 could for example be made of aluminium, copper, steel, nickel alloy or titanium. Preferably, the thickness of the tube wall 120 is minimised, i.e. it is as thin as possible to minimise the distance between the first and second fluid.

All examples of Figs. 2a-2g show microtubes with a rounded leading edge 105. The trailing edge 107 of the illustrated geometries has either a lower radius of curvature (see Fig. 2a) or is pointed (se Figs. 2b-2e). Although trailing edges 107, which are pointed, are illustrated as sharp, it is to be understood that they may be dull or without a tip due to manufacturing, e.g. through extrusion or compression and forming of a hollow tube to change the cross section.

Fig. 2a shows the cross section of a microtube 100 with a leading edge 105 which is wider than the trailing edge 107. By the leading edge is understood the region of the microtube 100 on which the second fluid is impinging, when the microtube 100 is installed in the heat exchanger 10, and the cooling system is running. The intended direction of flow of the second fluid is illustrated by the arrow G. While this direction of flow G is preferable, it is to be understood that the heat exchanger 10 will still function, when the direction of flow deviates from the one shown, e.g. by being at an angle with respect to the shown arrow G. During use, the direction of flow of the second medium G is likely to vary slightly for different tubes in the plurality of microtubes 100, just as the mounting or factors outside of the heat exchanger 10 may affect the specific angle of incidence of the second medium on the microtubes 100. The trailing edge 107 is located opposite the leading edge 105 and the stream of the second fluid having been divided by impinging on the microtube 100, when flowing in the direction G, will be joined again behind the microtube 100 adjacent to the trailing edge 107.

Both the leading edge 105 and the trailing edge 107 are rounded, while the leading edge 105 has an approximately flat region at the symmetry axis of the microtube 100. In another embodiment, the leading edge 105 may be more rounded having no flat region.

Fig. 2a further shows that the width w of the microtube 100 is the dimension of the microtube 100 measured from the leading edge 105 to the trailing edge in a straight line 107, i.e. measured along the chord line. The height h of the microtube is shown to be the largest dimension of the microtube 100 measured perpendicular to the width.

Fig. 2b illustrates the cross section of a microtube 100 with a shape, where the leading edge 105 is rounded and the trailing edge 107 is pointed. While the width w of the illustrated microtube 100 is more than seven times longer than the height h, in other variants within the disclosure the relation between width w and height h may be either larger or smaller. In some variants, the width w may be as little as 1 .5 times the height h. In yet other variants, the width w may be 10 times larger than the height. The rounded leading edge 105 makes the flow of the second fluid around the microtube 100 less dependent on the angle at which the second fluid impinges on the microtube 100 than a more pointed leading edge would. In other variations, the radius curvature of the leading edge may be larger or smaller than the one illustrated. Such variations may be made to accommodate the system, in which the heat exchanger is to be installed, and the expected variations of direction of flow G of the second fluid. While the trailing edge 107 is shown to be a sharp point, the tip may be rounded as required by the production method (see Fig. 2d for an illustration with a more rounded trailing edge 107). Pointed is considered a feature of the radius of curvature of the tube walls leading towards the trailing edge 107. In various embodiments both the leading 105 and trailing edge 107 may be considered rounded, but the trailing edge 107 has a lower radius of curvature than the leading edge 105.

Fig. 2c illustrates the cross section of a microtube 100 with the same outer geometry/contour as the one illustrated in Fig. 2b but having a thinner tube wall 120 as well as an inner support wall 120’ dividing the microtube 100 which then has multiple microchannels 100,100’ within it.

The tube walls 120 are not to scale in the figures, but generally it is preferred within the invention to have the thinnest possible tube walls 120, as this will provide better heat exchange between the first fluid within the microchannels 110,110’ and the second fluid surrounding the microtubes 100. Meanwhile the tube walls 120 must have sufficient mechanical strength to withstand the tress of the second fluid passing around the microtubes 100. To enhance the mechanical strength of the microtubes 100, one or more support walls 120’ may be included in the microtube 100. Such support walls will divide the microtube 100 creating multiple microchannels 110,110’. The shape, placement and orientation of the support wall 120’ may differ between embodiment of the microtubes 100 and need to balance several design considerations such as direction of the external stress, strength required, material use, and ease of fabrication.

Fig. 2d shows an asymmetric cross section of a microtube 100. The microtube has a rounded leading edge 105 and a pointed trailing edge 107. One side connecting the leading 105 and the trailing edge 107 is curved, while the other is straight.

Fig. 2e shows the cross section of a microtube with a non-circular and asymmetric shape. In the variant disclosed in Fig. 2e the two long sides of the microtube 100 curve in the same direction but not to the same extent.

Figs. 3a-3f show examples of non-circular cross sections within the scope of this disclosure, wherein both the leading 105 and trailing edge 107 are pointed. The angles of the leading 105 and trailing edge 107 are similar and in the range of 5 to 45 degrees. In other embodiments, the angles of the leading 105 and trailing edge 107 may differ while still being narrow and providing pointed structures. As previously mentioned, pointed is not to be understood in the narrow sense of a sharp tip, but may have some rounding of the structures due to fabrication procedures.

The illustrated cross sections may have been obtained from either extrusion or deformation such as compression of existing tubes with less preferable cross sections. For example, a cross section like the one shown in Fig. 3a may have been obtained by providing pressure to opposite sides of a microtube with a circular cross section thereby obtaining a non-circular cross section with the previously described benefits. Similarly, a cross section as schematically illustrated in Fig. 3b may have been obtained by applying pressure to the sides and/or comers of a rectangular tube thereby changing the angles of the comers and obtaining a more aerodynamic rhomb geometry for use in the heat exchanger 10.

Fig. 3a shows a symmetric cross section of a microtube 100, where both the leading edge 105 and the trailing edge 107 are pointed and the connecting sides are curved. By having both edges pointed, the microtube 100 has minimal impact on the flow of the second fluid around the microtube 100, as long as it impinges on the microtube at a minimal angle of attack, i.e. parallel to the symmetry axis in the case of the symmetric microtube illustrated in Fig. 2d.

Fig. 3b shows a cross section of a microtube which does not have rounded sides, but has four straight edges. While the aerodynamics of the pointed sides may be suboptimal, straight edges may simplify production of the microtubes and thus provide an acceptable trade-off.

Fig. 3c shows a microtube with an outer geometry similar to that of Fig. 3b having four straight sides and both the leading edge and the trailing edge being pointed, there illustrated with slightly rounded point. Furthermore, the middle corners are illustrated as compressed during the shaping process leading to a structure with straight sides while not being a rhomb as such. The microtube 100 of Fig. 3c comprises multiple microchannels 110,110’ inside due to a support wall 120’ lending additional strength to the structure with a thin tube wall 120. Fig. 3d shows a microtube with an outer geometry similar to that of Figs. 3b and 3c while having multiple support walls 120’, 120” leading to the microtube 100 comprising multiple microchannels 110,110’, 110”. Multiple support walls may be beneficial during production and for structural strength.

In general, the wall thickness of the tube wall 120 as well as the support wall 120’ are not to scale. They may be of similar thickness or they may have different thicknesses. For example, it may be beneficial to have a thinner outer tube wall 120 although a thicker tube wall 120 or more support walls 120’, 120” may in some variants be necessary for the mechanical strength of the microtube 100. The support walls shown in the figures should be seen as illustrative of the concept but not limited with regard to size or placement or in which type of outer geometry they may be used. Support walls 120 may be used in any type of microtube and their placement, thickness and number may be chosen to provide the necessary strength necessary for the respective microtubes 100 and heat exchanger 10.

Fig. 3e illustrates the cross section of a microtube with the same outer geometry as the microtube shown in Figs. 3b-3d, while the microchannel inside the microtube is rounded and is not reflecting the outer geometry of the microtube 100. Different microchannel shapes may be preferable for controlling the flow of the first fluid through the microchannel and/or due to manufacturing.

The shape of the microchannel may be made to allow the largest possible stream of the first fluid through the microtube 100, and it may be chosen to minimise the risk of turbulence inside the microchannel, when the first fluid enters the microchannel from a manifold (not shown). In other variants, the shape of the microchannel may be chosen to provide a uniform thickness of the tube wall 120 to ensure uniform heat transfer on all sides of the structure. In other variants, the thickness of the tube wall 120 may be engineered to control where most of the heat is transferred, e.g. where the flow rate of the second fluid around the microtube 100 is highest or where it impinges on the microtube.

Fig. 3f shows the cross section of a microtube with the same outer geometry as the microtube shown in Figs. 3b-3e having multiple rounded microchannels 110,110’, 110” inside it. Providing multiple microchannels 110, 110’, 110” in this manner may be beneficial to ensure a larger volume of the first fluid being able to pass through the microchannel 100. Furthermore, it lessens the region of thick tube walls 120 like the ones near the leading 105 and trailing edge 107 in Fig. 3e.

The structure in Fig. 3f is illustrative of the concept, and the number of microchannels as well as their shape may vary for different embodiments, just as multiple microchannels may be present in microchannel with different outer geometry.

As is clear from the various variations illustrated in Figs. 2a-2g, the thickness of the tube walls 120 may vary between different embodiments. The microchannel 110 may be shaped with a cross section identical to that of the outside of the microtube scaled equally down in all dimensions (see for example Fig. 2d). In other variants the microchannel 110 may be similar in shape to the outside of the microtube 100 but have slight variations in the height relative to the width to increase the volume of the microchannel 110 (see for example Fig. 2b). In yet other variants, the cross- sectional shape of the microchannel 110 may differ significantly from the outer cross-sectional shape of the microtube 100 (see for example Fig. 2g). It is to be understood that these variants in outer cross-sectional shape of the microtube 100 and the cross-sectional shape of the microchannel 110 are not restricted to the shown variants but can be combined independently of the exemplary illustrations. Similarly, there are many other shapes within the scope of this disclosure not illustrated.

Fig. 4 illustrates staggered positioning of microtubes 100 with a non-circular cross section. The microtubes are shown separate from the rest of the heat exchanger, but microtubes 100 may be mounted in such a staggered configuration in the heat exchanger. What is shown is simply a subsection of the microtubes 100 and in many embodiments of the invention, the length of the microtubes will be significantly longer relative to the cross-sectional dimensions of the microtube 100 than is shown in the conceptual sketch of Fig. 4.

The direction of flow of the first fluid through the microchannels 110 is illustrated by the dashed arrow L. The direction of flow of the second fluid entering the microtube structure is marked with the arrow G and is approximately perpendicular to the direction of flow L of the first fluid. Thus, the second fluid impinges on the leading edges 105 of the microtubes 100 which are in the illustrated variant rounded. By staggering the microtubes 100 such that the rows of neighbouring columns of the microtube placement, the in-line distance d between microtubes 100 can be increased while still having a high number of microtubes 100 in the structure. Since the area of lowest flow of the second fluid is directly behind the trailing edge 107 f, the cooling of the microtubes 100 can be made more efficient by avoiding the placement of microtubes 100 directly in the slipstream of other microtubes 100.

Fig. 5 illustrates a configuration of microtubes, where there are multiple different cross sections in different regions of the plurality of microtubes 100. Fig. 5 only illustrates the concept and variants with different numbers of microtubes, and different placements of these are within the present disclosure.

In the illustrated variant, the leading edge 105 of the microtubes 100’ is pointed in the central region and at the leading side of the plurality of microtubes, where the flow G of the second fluid is least likely to have been disturbed. Other subsections of the plurality of microtubes 100” have rounded leading edges, where the angles of the impinging flow G of the second fluid may have been altered by the interaction with other microtubes.

The rows of microtubes 100”’ at the edges of the plurality of microtubes are more closely spaced and angled to direct the flow of the second fluid inwards towards the remaining microtubes 100.

This is simply an illustration of possible parameters that may be engineered when placing the microtubes and choosing different cross sections, e.g. the distance and angle between microtubes and the choice of leading edges. Depending on the system, in which the heat exchanger (not shown) is used, other variations may be preferable to accommodate the space for the heat exchanger and the characteristics of the flow of the second fluid around the microtubes.

Figs. 6a-6c illustrate segments of various types of tube sheets 150, where microtubes 100 are connected by sheet connections 155 such that the second fluid surrounding the microtubes 100 cannot pass from one side of the tubes to each other. Preferably, the sheet connections 155 are made from the same material as the microtubes 100 such that the full tube sheet 150 can be made in one piece. It is, however, within the disclosure to have the microtubes 100 and the sheet connections 155 made from different materials. In some embodiments, it may for example be beneficial to make the microtubes 100 in a material with high conductivity while making the tube sheets 155 in a material with lower conductivity, thereby obtaining the benefits of directing the flow of the second fluid while minimising the thermal exchange between the microtubes 100 of the tube sheet 150.

The sheet connections 155 connect the leading edge 157 and the trailing edge 107 of adjacent microtubes 100. By blocking the region directly behind a microtube with respect to the flow of the second fluid, e.g. connected to the trailing edge, it is not possible for the second fluid to create a slip stream and cause turbulence at this point which is otherwise the region where that is most likely to happen. The tube sheet 155 prevents streams of the second fluid from either side of the microtube 100 to mix and disrupt the flow of the other stream after passing the microtube 100. Instead, the second fluid will flow along the body of a first microtube and continue its stream along the tube sheet 155 to reach the next microtube 100.

In the preferred configuration, the sheet connections 155 are parallel to the direction of flow G of the second fluid which surrounds the microtubes. In other configurations, though, it may be preferable to have some or all tube sheets 150 angled with respect to the direction of flow G of the second fluid to guide the second fluid to specific regions. For example, tube sheets can be used to funnel the second fluid to specific regions or it may be used to reduce the heat exchange between a first and a second subsection of the microtubes 100 by hindering the direct exchange of the second fluid between the regions in which the subsections are located.

Figs. 6a-6c show slightly different variants of tube sheets 150. All tube sheets comprise microtubes 100 thermally and mechanically connected by sheet connectors 155. Each of the connected microtubes 100 has separate microchannels 110 which are not connected fluid ically to the other microchannels of the tube sheet 150.

The cross-sectional shape of the microtubes 100 in a tube sheet 150 may all be the same shape or they may vary just as they may vary between different tube sheets 150 as illustrated in the different subfigures. The first and/or last microtube 100 of a tube sheet, i.e. microtubes with only a single adjacent microtube, may for example have a different shape than the remaining microtubes of the tube sheet 150 (see Fig. 7 for an example of this). Similarly, the distance d between microtubes may vary both within the same tube sheet 150 or between different tube sheets 150.

Fig. 6a shows a section of a tube sheet 150 with a microtube 100 with a rounded leading edge 105 and a rounded trailing edge 107 with a smaller radius of curvature for the trailing edge than the leading edge. The transition from the microtubes 100 to the sheet connection 155 is sharp leaving the shape of the microtube 100 the same as if it was not connected by sheet connections 155.

Fig. 6b shows a section of a tube sheet 150 with microtubes of a shape similar to those shown in Fig. 6a, the main difference being that the transition from the microtube to the sheet connections 155 is smoothened to make the changes in flow direction G along the tube sheet 150 less sudden.

Fig. 6c illustrates a section of a tube sheet 150 where the microtubes 100 have a cross section with a rounded leading edge and a pointed trailing edge. While the sheet connections 155 impose a limit on how pointed either of the edges can become, the benefits of rounding and pointing edges are similar to the previously described cases, where the microtubes 100 are not connected, e.g. allowing larger angles of attack and minimising the slip stream area.

Fig. 6c further shows a preferred configuration, where the thickness of the sheet connections 155 is smaller than that of the tube walls 120. This is beneficial, as minimising the amount of material in the sheet connections 155 also minimises the rate of heat transfer between the microtubes 100 which may be preferable in some embodiment of the heat exchanger in order to be able to have varying cooling across different regions of the heat exchanger. While thin sheet connections 155 are preferable for low thermal conductivity, a minimum thickness is still required for mechanical strength, as stress will be applied to the sheet connections 155 by the flow of the second fluid. As previously mentioned, it is also within the disclosure to combine different materials for the tube walls 120 and the sheet connections 155 to maintain mechanical strength while lowering the heat conductivity of the sheet connectors 155, e.g. the microtube walls 120 may be made of a metal or metal alloy, while the sheet connections are made of a polymer or another metal with lower heat conductivity. Fig. 7 is a schematic illustration of a segment of the microtubes 100 and tube sheets 150 of a heat exchanger (not shown) seen in a cross-sectional view. As shown, tube sheets 150 and microtubes 100 may be used together in a heat exchanger, where the tube sheets can section off regions of the heat exchanger containing unconnected microtubes 100.

In other variants within the disclosure, a heat exchanger could have no unconnected microtubes 100 and consist of only tube sheets 150.

As illustrated, the first leading edge of the tube sheets 150 may be rounded rather than connected to a sheet connection 155. Similarly, the last trailing edge of the tube sheet 150 may be without a sheet connection 155. In the illustrated variant, the leading edges 105 of the tube sheets 150 have a different rounding than the leading edges of the unconnected microtubes 100, and the same is the case for the trailing edges 107. In other variants, the shape of the leading 105 and trailing edge 107 may be the same. Just as for the situation illustrated in Fig. 5, multiple cross- sectional shapes of microtubes 100 may be present in other variants of the heat exchanger both for the unconnected microtubes 100 and the tube sheets 150.