Patent Description:
Heat exchangers for transfer of heat between different fluids are very widely used and exist in various forms. Typically heat exchangers are arranged for flow of a primary fluid and a secondary fluid with heat being transferred between the two fluids as they flow through the device. Multi-stream heat exchangers for exchanging heat between more than two fluids also exist in the prior art. Heat exchangers are required within aircraft structures to regulate temperatures of working fluids as well as to scavenge heat from one system for use in another. Every heat exchanger consumes significant space within an aircraft structure, and in certain areas of the aircraft structure space is at a premium and it is therefore desirable to optimise the size of each component fit them together in a way that minimises the space taken, while also maintaining sufficient levels of heat transfer.

Some heat exchangers have a layered structure with a large number of parallel flow paths between plates that separate the flow paths. There may be <NUM>-<NUM> plates, or more, in this type of heat exchanger, typically with alternating hot/cold fluid flow paths either side of each plate. Such heat exchangers can also be referred to as laminate heat exchangers.

In laminate heat exchangers, the flow paths will typically be square or rectangular in cross-section meaning that there is very limited primary heat transfer surface area. In circumstances where the entire space between two adjacent plated forms the flow path, the primary heat transfer surface is only the top and bottom surface. There is therefore a desire to provide heat exchanger cores with increased primary transfer surfaces.

In certain heat exchangers, instead of having a core comprising multiple plates defining flow paths alternating between hot and cold fluid, it is possible to provide a heat exchanger core comprising multiple flow path tubes in contact with each other arranged in a plurality of rows and columns. The fluid flow path tubes in each row alternate between hot fluid and cold fluid thus providing a larger primary heat transfer surface.

Heat exchanger cores typically require large headers to supply the hot fluid to the heat exchanger core. These significantly increase the weight and size of the overall heat exchanger which can lead to a reduction in working efficiency.

There is therefore a need to provide an inlet and outlet arrangement that can supply the fluid in an alternating fashion to so many flow path tubes within the heat exchanger core.

Recent developments in additive manufacturing allow for more complex geometries of heat exchangers to supply fluid to multiple flow path tubes. One area of this emerging technology utilises fractal-type connecting structures whereby heat exchanger channels divide into two or more sub-channels at divergence points along the length of the channel.

These small sub-channels which feed flow path tubes in the heat exchanger core can be subject to vibration damage. In typical heat exchangers, these small sub-channels will also be positioned in the coldest part of the flow which can lead to high thermal gradients resulting in high thermal stress peaks.

There is therefore a need to provide a heat exchanger which can overcome the above problems, such as preventing any vibration damage, heat transfer stresses and increasing the working efficiency of the heat exchanger.

<CIT> discloses a two-fluid heat exchanger comprising a geometry based on sequential branching of nearly circular passages.

The invention is a heat exchanger as defined by claim <NUM>. The heat exchanger comprises: a heat exchanger core comprising a plurality of channels in a close-packed configuration, the plurality of channels comprising a plurality of first channels and a plurality of second channels wherein the heat exchanger core comprises a homogeneous block of material having a plurality of bores extending therethrough defining the plurality of channels; a first fractal channel for conveying a fluid to the plurality of first channels of the heat exchanger core; a second fractal channel for conveying the fluid from the plurality of first channels of the heat exchanger core; and an outer wall extending from the heat exchanger core and defining a second fluid inlet and a second fluid outlet for a second fluid to flow through the plurality of second channels, wherein the second fluid inlet and the second fluid outlet are axially aligned with each other along a longitudinal axis of the heat exchanger to allow the outer wall to replace part of a duct carrying a second fluid in use; and a first fluid inlet conduit connected to the first fractal channel and a first fluid outlet conduit connected to the second fractal channel, the first fluid inlet and outlet conduits both extending through the outer wall; wherein the first fractal channel and the second fractal channel each comprises at least one divergence point along its length where a parent channel splits into a plurality of sub-channels which diverge away from each other.

This combines the use of a fractal channel with a heat exchanger core comprising a plurality of channels fed by each sub-channel of the fractal channel. The use of a fractal channel allows fluids to be distributed between a large number of close-packed first and second fluid channels, giving a large heat transfer surface to maximise heat transfer efficiency within a compact volume.

The arrangement of the outer wall allows the outer wall to replace part of a duct carrying a second fluid, for example. By positioning the heat exchange core within a duct the heat exchanger can fit within a relatively confined space. In particular this heat exchanger can be placed in the area where a duct is normally present. This eliminates the need for additional space to mount a conventional plate-fin heat exchanger with ducts connected to the second fluid inlet and outlet.

By positioning the first fractal channel and the second fractal channel within said duct, the fractal channels also take part in the heat transfer as the second fluid surrounds the fractal channels. This means that heat transfer will take place along the entire length of the fluid flow path, from the first fractal channel, through the heat exchanger core and to the second fractal channel. Thus, the heat transfer efficiency will be increased compared to conventional heat exchanger cores.

The first fluid inlet conduit and the first fluid outlet conduit may be at an angle of between <NUM> and <NUM> degrees with respect to the longitudinal axis of the body of the heat exchanger.

The fluid conveyed by the first fractal channel and the second fractal channel may be a first fluid. The first fluid may be the "hot flow" and the second fluid may be the "cold flow". Alternatively, the first fluid may be the "cold flow" and the second fluid may be the "hot flow".

Each sub-channel of the first fractal channel and second fractal channel may be directly connected to every alternating channel of the heat exchanger core. Each alternating channel may therefore be supplied with secondary fluid. The other channels of the heat exchange core may be supplied with the primary fluid from the duct. This arrangement means that the block is less prone to vibration damage than if unsupported tubes were used for the channels.

The first fractal channel and second fractal channel may comprise multiple fractal stages wherein each fractal stage may comprise a parent channel and its subsequent sub-channels.

The sub-channels may be rotationally symmetrical about the corresponding parent channel. For example, in the case where there are four sub-channels, each sub-channel may be offset by <NUM> degrees from neighbouring sub-channels.

The sub-channels may have smaller diameters than a diameter of a corresponding parent channel. The sub-channels may diverge away from a central axis of the parent channel.

The sub-channels of each fractal stage may be shaped such that at least a portion of each sub-channel is parallel to a common axis.

The sub-channels of each fractal stage may be shaped so that a portion of the sub-channels are diverging away from the common axis. Each sub-channel may form a parent channel of a subsequent fractal stage.

The sub-channels of each fractal stage may be distributed in a grid configuration.

A cross-sectional area of each parent channel may be equal to a total cross-sectional area of its corresponding sub-channels. This results in the overall cross-sectional area of the first fractal channel and the second fractal channel being constant along their respective length. This helps to prevent any significant pressure drops in the flow of the fluid.

Each channel of the heat exchanger core may have any shape cross section. For example the channels may be rectangular, square, circular, round or diamond shape.

The heat exchanger device may be for use with any required combination of fluids, such as liquid-liquid, liquid-gas or gas-gas heat exchange. The heat exchanger may be configured to use air as the second fluid for heating or cooling of the first fluid.

In some examples the heat exchanger is for aerospace use and an embodiment of the invention may provide an aircraft including the heat exchanger device described above. In context of aerospace use the first and second fluids could include any combination of two of: atmospheric air, cabin air, engine oil, generator oil, coolant, fuel and so on. Any combination of these fluids can be used within the same heat exchanger device, it is not limited to two types of fluid. The fluid used depends on the requirements of the heat exchanger as different fluid will have different thermal and fluidic properties. Some fluid will move with a lower/higher velocity than others which may be preferable in certain situations to provide the necessary thermal transfer.

The invention is further a method for manufacturing a heat exchanger according to the first aspect in one piece by a process of additive manufacturing.

The method may include providing the heat exchanger with the features discussed above in connection with the first aspect.

The heat exchanger may be printed starting from a first end of the duct in a longitudinal direction to a second end of the duct.

This allows for rapid production and for producing the complex shapes or the irregular geometries required. Additive manufacturing allows for the complex configuration described by the first aspect of the invention to be manufactured quickly in one printed part.

Example embodiments of the invention are described below by way of example only and with reference to the accompanying drawings.

<FIG> shows a heat exchanger <NUM>. The heat exchanger <NUM> is arranged to exchange thermal energy between a first fluid <NUM> and a second fluid <NUM>, whilst preventing the first fluid <NUM> and second fluid <NUM> from mixing with one another.

The heat exchanger <NUM> comprises a body <NUM> defining a first fluid inlet <NUM> and a first fluid outlet <NUM>. The body <NUM> conveys a second fluid <NUM> from a second fluid inlet <NUM> through a heat exchanger core <NUM> to a second fluid outlet <NUM>. A first and second portion of the body <NUM> comprises an outer wall <NUM> which resembles a duct.

In the illustrated embodiment, the second fluid inlet <NUM> and second fluid outlet <NUM> are axially aligned with one another and are substantially equal in flow area. The body <NUM> is configured so as to replace a section of a fluid duct, or the like, carrying the second fluid <NUM>.

The heat exchanger <NUM> further comprises a first fluid inlet conduit <NUM> and a first fluid outlet conduit <NUM>. The first fluid inlet conduit <NUM> and the first fluid outlet conduit <NUM> enter through a side wall of the body <NUM> of the heat exchanger <NUM> and pass through the flow of the second fluid <NUM>.

The first fluid inlet conduit <NUM> leads into a first fractal channel <NUM> which is directly connected to a first end of the heat exchanger core <NUM> positioned within the body <NUM> of the heat exchanger <NUM>. A second end of the heat exchanger core <NUM> is directly connected to a second fractal channel <NUM> which leads to the first fluid outlet conduit <NUM>. In the illustrated embodiment, the first fluid <NUM> and the second fluid <NUM> are supplied to the heat exchanger core <NUM> in a counter-current fashion. However, the fluid flow directions may alternatively be supplied to the heat-exchanger core in a co-current fashion.

The term fractal channel here describes the repeated diverging structure of the channels, whereby the channel repeatedly splits into two or more smaller sub-channels <NUM> along its length. Such structures are sometimes also known as multi-furcating channels.

<FIG> shows a cut-away view of the first fractal channel <NUM>. The second fractal channel <NUM> in <FIG> can have the same structure as the first fractal channel <NUM>, but is oriented in reverse.

The fractal channel <NUM> comprises a plurality of fractal stages.

The first fractal stage comprises a parent channel <NUM> with the largest diameter, corresponding to the diameter of the first fluid inlet conduit <NUM>. The parent channel <NUM> of the first fractal stage reaches a first divergence point <NUM> where the parent channel <NUM> splits into nine sub-channels <NUM>, the middle three of which are visible in the cut-away of <FIG>, each having a smaller diameter than the parent channel <NUM>. The outer sub-channels <NUM> initially diverge away from each other and a central axis of the parent channel <NUM>, whilst the central sub-channel <NUM> continues along the central axis. The outer sub-channels <NUM> are curved such that after a given length the direction of the sub-channels become parallel to the central axis of the parent channel <NUM>.

The nine sub-channels <NUM> are arranged in a 3x3 grid and are <NUM> degrees rotationally symmetrical.

Each sub-channel <NUM> then forms the parent channel in the next fractal stage. The second fractal stage of the fractal channel <NUM> comprises nine parent channels, corresponding to the nine sub-channels <NUM> of the first fractal stage. Each parent channel of the second fractal stage reaches a second divergence point <NUM> and splits into a plurality of additional sub-channels which diverge and curve in the same way as for the first fractal stage. These nine parent channels divide into <NUM> sub-channels arranged in a 7x7 grid.

The sub-channels <NUM> of the second fractal stage form the parent channels of a third fractal stage, in which those parent channels diverge at a third divergence point to form <NUM> sub-channels arranged in a 15x15 grid.

The sub-channels of the third fractal stage subsequently form the fourth fractal stage which split in the same manner at a fourth divergence point to form <NUM> channels in a 31x31 grid.

Each sub-channel <NUM> is radially offset from the central axis of the corresponding patent channel <NUM>.

The individual sub-channels <NUM> of each fractal stage have a smaller diameter than the individual sub-channels of the preceding fractal stage. The total cross sectional flow area of the sub-channels <NUM> within each particular fractal stage is substantially equal, therefore the total cross-sectional flow area through the fractal channel <NUM> remains substantially constant. This prevents any pressure drop from occurring in the first fluid <NUM>.

In <FIG>, the first fractal <NUM> is as described with respect to <FIG>. The sub-channels <NUM> of the fourth fractal stage in the first fractal <NUM> are directly connected to alternative channels of the heat exchanger core <NUM>. The sub-channels <NUM> of the fourth fractal stage in the second fractal channel <NUM> are directly, fluidly, connected to a second end of the heat exchanger core <NUM>.

<FIG> also shows a detailed cut-away of the heat exchanger core <NUM>. The heat exchanger core <NUM> is of a single, solid, construction defining a plurality of parallel channels within it. The plurality of parallel channels comprise a plurality of first fluid channels <NUM> and a plurality of second fluid channels <NUM>. As the heat exchanger core <NUM> is a single, solid, construction it will be more resistant to vibration damage than a heat exchanger comprising unsupported or intermittently supported parallel tubes.

The second fluid channels <NUM> in the heat exchanger core <NUM> are supplied with second fluid <NUM> at a first end of the heat exchanger core <NUM> from the second fluid inlet <NUM> of the body <NUM>. The first fluid channels <NUM> are supplied with first fluid <NUM> at a second end of the heat exchanger core <NUM> by the first fractal channel <NUM>.

The second fluid <NUM> exits the heat exchanger core <NUM> at the second end and travels around the first fractal channel <NUM> to the second fluid outlet <NUM>. The first fluid <NUM> exits the heat exchanger core <NUM> at the first end the second fractal channel <NUM>. Accordingly the first fluid <NUM> is conveyed by the first channels <NUM> and the second fluid <NUM> is conveyed by the second channels <NUM>. It will be appreciated that alternatively, the first fluid <NUM> can be conveyed by the second channels <NUM> and the second channels.

The heat exchanger core <NUM> comprises an approximately equal number of first fluid channels <NUM> and second fluid channels <NUM>. In the illustrated example, the first fluid channels <NUM> are arranged in a 31x31 grid (i.e. with <NUM> first fluid channels <NUM>), whilst the second fluid channels are arranged in a 30x30 grid (i.e. with <NUM> second fluid channels <NUM>).

In the illustrated embodiment, the first and second channels <NUM>, <NUM> within the heat exchanger core <NUM> each have a square diamond cross-sectional shape and are arranged in a grid configuration.

The first and second channels <NUM>, <NUM> are arranged in an alternating fashion such that every side of each first channel <NUM> will act as a primary heat transfer surface with an adjacent second channel <NUM>, and vice versa. It will be appreciated that any cross section shape can be used, for example the cross section of the channels may be rectangular, circular, diamond or any other shape. It is also possible for some of the channels to have different cross section shapes than other channels, this will however require different wall thicknesses to accommodate for it.

The total of the cross-sectional flow area of all the second fluid channels <NUM> is equal to the total cross-sectional flow area of both the first and second fractal channels <NUM>, <NUM>. This prevents pressure fluctuations in the first fluid <NUM>.

The total cross-sectional flow area of the first fluid channels <NUM> is approximately equal to the total cross-sectional flow area of the second fluid channels <NUM>. In the illustrated embodiment, this is less than the cross sectional flow area of the second fluid inlet <NUM> and second fluid outlet <NUM>. However, if it is necessary to avoid pressure variations in the second fluid <NUM> as well as the first fluid <NUM>, then the body <NUM> may be designed such that the total cross-sectional flow area of the second fluid channels <NUM> is approximately equal to the cross-sectional flow area of the second fluid inlet <NUM> and the second fluid outlet <NUM>.

The heat exchanger core <NUM> is arranged so that all of the first fluid <NUM> and all of the second fluid <NUM> passes respectively through the first and second channels <NUM>, <NUM> of the heat exchanger core <NUM>.

The overall cross-section of the heat exchanger core <NUM> is rectangular, however it could be circular or any other cross section. Referring to <FIG>, the first fluid inlet conduit <NUM> and the first fluid outlet conduit <NUM> extend through a side wall of the body <NUM> at an angle to the longitudinal direction of the duct <NUM>. The angle between the ends of first fluid inlet conduit <NUM> and the first fluid outlet conduit <NUM> is approximately <NUM> degrees, however it will be appreciated that the angle can be up to <NUM> degrees. The first parent channel <NUM> of the fractal channel is curved so that the end of the first parent channel <NUM> is parallel to the longitudinal direction of the duct <NUM>.

In use, the heat exchanger <NUM> uses counter flow so the first fluid <NUM> and the second fluid <NUM> travel in opposite directions. The second fluid <NUM> enters the heat exchanger <NUM> at the second fluid inlet <NUM>. The second fluid <NUM> then enters the heat exchanger core <NUM> at a first end and travels through the first second channels <NUM> of the heat exchanger core <NUM> and leaves via the second end of the heat exchanger core <NUM>. The second fluid <NUM> then leaves the heat exchanger <NUM> at the second fluid outlet <NUM>. The first fluid <NUM> enters the heat exchanger <NUM> via the first fluid inlet conduit <NUM> at an angle between <NUM> and <NUM> degrees to the longitudinal direction off the duct <NUM>. The first fluid <NUM> then travels through the first fractal channel <NUM> and enters the first fluid channels <NUM> at the second end of the heat exchanger core <NUM>. The first fluid <NUM> travels along the first fluid channels <NUM> of the heat exchanger core <NUM> and then enters the second fractal channel <NUM>. The first fluid <NUM> then exits via the first fluid outlet conduit <NUM>.

It will be appreciated that the first fluid <NUM> and the second fluid <NUM> could instead travel in the same direction so that parallel flow is utilised by the heat exchanger <NUM>.

The first and second fractal channels <NUM>, <NUM> are positioned within the body <NUM> and hence the fractal channels <NUM>, <NUM> are submerged in the flow path of the second fluid <NUM>. This means that heat transfer will take place between the first and second fluids <NUM>, <NUM> as they pass around and through the fractal channels <NUM>, <NUM>, respectively, as well as when they flow through the heat exchanger core <NUM>.

As counter flow is utilised, the temperature difference will be more uniform along the length of the duct than if parallel flow is utilised. This prevents the hottest fluid from being in contact with the coldest fluid and hence reduces the thermal stresses on the thin channels and walls of the fractal channels <NUM>, <NUM>.

<FIG> shows how the heat exchanger <NUM> can be manufactured by additive manufacturing. The heat exchanger <NUM> can be manufactured using additive manufacturing as one piece.

By printing the heat exchanger <NUM> from the first fluid outlet <NUM> to the first fluid inlet <NUM>, or vice versa first fluid inlet <NUM> to the first fluid outlet <NUM>, the structure will be self-supporting during the additive manufacturing process.

The illustrated heat exchanger <NUM> is straight, however it will be appreciated that the heat exchanger <NUM> can be curved to accommodate existing systems.

The heat exchanger <NUM> can be printed by additive manufacture from any material suitable for the intended operating conditions. The type of material depends on the specific application of the heat exchanger <NUM>.

Claim 1:
A heat exchanger (<NUM>) comprising:
a heat exchanger core (<NUM>) comprising a plurality of channels (<NUM>, <NUM>) in a close-packed configuration, the plurality of channels comprising a plurality of first channels (<NUM>) and a plurality of second channels (<NUM>), wherein the heat exchanger core (<NUM>) comprises a homogeneous block of material having a plurality of bores extending therethrough defining the plurality of channels (<NUM>, <NUM>);
a first fractal channel (<NUM>) for conveying a first fluid (<NUM>) to the plurality of first channels of the heat exchanger core;
a second fractal channel (<NUM>) for conveying the first fluid from the plurality of first channels of the heat exchanger core;
a first fluid inlet conduit (<NUM>) connected to the first fractal channel (<NUM>) and a first fluid outlet conduit (<NUM>) connected to the second fractal channel (<NUM>);
wherein the first fractal channel and the second fractal channel each comprises at least one divergence point (<NUM>) along its length where a parent channel (<NUM>) splits into a plurality of sub-channels (<NUM>) which diverge away from each other;
characterised in that the heat exchanger (<NUM>) further comprises,
an outer wall (<NUM>, <NUM>) extending from the heat exchanger core (<NUM>) and defining a second fluid inlet (<NUM>) and a second fluid outlet (<NUM>) for a second fluid (<NUM>) to flow through the plurality of second channels, wherein the second fluid inlet and the second fluid outlet are axially aligned with each other along a longitudinal axis of the heat exchanger to allow the outer wall to replace part of a duct carrying a second fluid in use; and
the first fluid inlet and outlet conduits both extending through the outer wall (<NUM>).