Patent Description:
Fluid coolers and heat exchangers are used in various locations in gas turbine engines to transfer heat between two or more fluids. Tubes containing the different fluids may be nested to promote the transfer of heat between the fluids. To increase the efficacy of a fluid cooler, the effective length of the tubes may be increased to increase the surface area on which the heat transfer may occur. However, doing so may increase the size and weight of the fluid cooler, requiring more space within the gas turbine engine, and potentially increasing the weight of the fluid cooler, both of which are undesirable in airborne gas turbine engines. In addition, by increasing the weight, additional mounting hardware may be required to support the fluid cooler within the gas turbine engine. Document <CIT> discloses a heat exchanger for heat exchange between a first fluid and a second fluid, comprising a generally cylindrical casing with a first inlet and a first outlet for allowing said first fluid to flow through said casing in a generally axial direction, and at least one helical coil of a finned or corrugated tube arranged inside said casing.

According to a first aspect of the present invention, there is provided a gas turbine engine as set forth in claim <NUM>.

The gas turbine engine as defined above and described herein may further include one or more of the following additional features, in whole or in part, and in any combination.

In certain embodiments, the primary axis of the fluid cooler is a central longitudinal axis of the outer tube.

In certain embodiments, the outer tube helically extends about the primary axis, conforming to an outer shape of the plurality of inner tubes.

In certain embodiments, the common inner tube inlet is at the second end of the fluid cooler and the common inner tube outlet is at the first end of the fluid cooler.

In certain embodiments, a pitch of one or more of the plurality of inner tubes varies along a length of the fluid cooler between the first end and the second end.

In certain embodiments, a taper angle of one or more of the plurality of inner tubes varies along a length of the fluid cooler.

In certain embodiments, the outer tube is a cylindrical outer tube.

In certain embodiments, the plurality of inner tubes include circular cross-sections.

In certain embodiments, the common inner tube inlet is fluidly coupled to the air supply.

In certain embodiments, the common inner tube outlet is fluidly coupled to the compressor section.

In certain embodiments, the outer tube inlet and the outer tube outlet define respective inlet and outlet axis which are oriented in a same direction on an outer surface of the outer tube, the inlet and outlet axis intersecting the primary axis of the fluid cooler.

In certain embodiments, the gas turbine engine comprises a casing of the gas turbine engine, the compressor section within the casing, and the fluid cooler mounted outside the casing.

In certain embodiments, a nacelle surrounds the casing, wherein the fluid cooler is mounted between the nacelle and the casing.

In certain embodiments, a second fluid cooler is fluidly coupled to the fluid cooler in series.

In a further aspect, there is provided a method for operating a fluid cooler in a gas turbine engine as set forth in claim <NUM>.

<FIG> illustrates a gas turbine engine <NUM> of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan <NUM> through which ambient air is propelled, a compressor section <NUM> for pressurizing the air, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section <NUM> for extracting energy from the combustion gases. A shaft <NUM> extending along a central engine axis <NUM> interconnects the fan <NUM>, the compressor section <NUM> and the turbine section <NUM>. A core casing <NUM> surrounds the compressor section <NUM>, combustor <NUM> and turbine section <NUM> to define a main fuel path. A nacelle <NUM> surrounds the fan <NUM> and core casing <NUM> to define an outer bypass duct <NUM> between the nacelle <NUM> and core casing <NUM>. While <FIG> shows gas turbine engine <NUM> to be a turbofan gas turbine engine, it is understood that the present disclosure is applicable to other types of gas turbine engines as well, such as turboprops and turboshafts.

A fluid cooler <NUM>, which is a heat exchanger, is illustratively mounted outside the core casing <NUM> and adjacent the compressor section <NUM>. In the embodiment shown, where the gas turbine engine <NUM> is a turbofan gas turbine engine, the fluid cooler <NUM> may be mounted in the bypass duct <NUM> between the nacelle <NUM> and the core casing <NUM>. Other locations for the fluid cooler <NUM> in other types of gas turbine engines may be contemplated as well. For example, in a turboprop or turboshaft engine, the fluid cooler <NUM> may be mounted to the external casing of the engine. In a particular embodiment, the fluid cooler <NUM> is operable to deliver a flow of cooling air to the compressor section <NUM> after the air has undergone a heat exchange process with a flow of engine oil, as will be discussed in further detail below. In other cases, the fluid cooler <NUM> may be utilized elsewhere in the engine to conduct heat exchange processes between two fluids.

Referring to <FIG>, a fluid cooler <NUM> for a gas turbine engine includes an outer tube <NUM> through which a first fluid flows, and a plurality of inner tubes <NUM> (see <FIG>) that pass through the outer tube <NUM> and through which a second fluid flows. The first and second fluids may be different fluids, for instance oil and air. In other cases, the first and second fluids may be the same type of fluid but at different temperatures. The inner tubes <NUM> follow a helical or spiral-like pattern through the outer tube <NUM>, as will be discussed in further detail below.

In the present embodiment, the first fluid flowing through the outer tube <NUM> is engine oil for the gas turbine engine <NUM> and the second fluid flowing through the plurality of inner tubes <NUM> is air, such as but not necessarily pressurized air bled off at a location upstream or downstream from a compressor of the compressor section <NUM> of the gas turbine engine <NUM>. Other fluid types may however be contemplated as well. In one particular embodiment, the fluid cooler <NUM> is operable to promote heat transfer from the second fluid, i.e. air, flowing through the plurality of inner tubes <NUM> to the first fluid, i.e. oil, flowing through the outer tube <NUM>, such that the air is cooled after having passed through the fluid cooler <NUM>. In other cases, the fluid cooler <NUM> is operable to promote heat transfer from warm engine oil to a cooling flow of air. In other cases, the direction of heat transfer may be reversed, i.e. the fluid flowing through the outer tube <NUM> may transfer heat to the fluid flowing through the plurality of inner tubes <NUM>. Regardless, it will be appreciated the heat will transfer from the hotter of the two fluids to the cooler of the two fluids, regardless of which fluid is directed through the inner tubes <NUM> and which is directed through the outer tube <NUM>. The relative direction of each flow may be in the same direction, or the two fluid flows may flow in opposite directions through the fluid cooler <NUM>. Fluids flowing in opposite directions through the fluid cooler <NUM>, referred to as a 'counter-flow' configuration, may increase the fluid cooler's <NUM> overall heat transfer efficiency relative to a 'parallel flow' configuration where the different fluids travel in a same direction through the fluid cooler <NUM>. The fluid cooler <NUM> of the depicted embodiment as depicted is a "single-pass" heat exchanger, in that the fluids in the outer tube <NUM> and the inner tubes <NUM> pass each other once. In alternate embodiments, however, multiple passes are possible, wherein one or both of the fluids travels back and forth two or more times within the fluid cooler before exiting.

In the embodiment shown, the outer tube <NUM> is a cylindrical outer tube with a circular cross-section extending from a first end <NUM> of the fluid cooler <NUM> to a second end <NUM> of the fluid cooler <NUM>. Other cross-sectional shapes, for instance rectangular or oval-shaped, for the outer tube <NUM> may be contemplated as well. The fluid cooler <NUM> includes an outer tube inlet <NUM> positioned towards the first end <NUM> of the fluid cooler <NUM> and an outer tube outlet <NUM> positioned towards the second end <NUM> of the fluid cooler <NUM>. Alternatively, the outer tube inlet <NUM> may be positioned towards the second end <NUM> of the fluid cooler <NUM> while the outer tube outlet <NUM> may be positioned towards the first end <NUM> of the fluid cooler <NUM>. As such, the fluid passing through the outer tube <NUM>, for instance oil for cooling the air flowing through the plurality of inner tubes <NUM>, may be delivered to the fluid cooler <NUM> from either the first end <NUM> or the second end <NUM>.

In the depicted embodiment, the outer tube inlet <NUM> and the outer tube outlet <NUM> are positioned along an outer surface of the outer tube <NUM>. In addition, in the embodiment shown, the outer tube inlet <NUM> and the outer tube outlet <NUM> are oriented in a same direction, for example both defining inlet and outlet axis that intersect the primary axis of the fluid cool. This configuration may, for instance, ease of installation and removal. Other positions and directions may be contemplated as well. In the embodiment shown, both the outer tube inlet <NUM> and outer tube outlet <NUM> include two-bolt flange connections, although other arrangements may be contemplated as well. Various end fittings for fluidly connecting to the outer tube inlet <NUM> and the outer tube outlet <NUM> may be contemplated. For instance, <FIG> shows a B-nut with a nipple <NUM> mounted to the outer tube inlet <NUM> and a spigot fitting with an O-ring <NUM> mounted to the outer tube outlet <NUM>. Various combinations of these end fittings, along with other types of end fittings, such as a <NUM> degree cone and nipple (not shown), may be contemplated as well. A primary axis <NUM> of the fluid cooler <NUM> is defined within the outer tube <NUM> between the first end <NUM> and the second end <NUM>. In the shown embodiment, although not necessarily the case in all embodiments, the primary axis <NUM> is a longitudinal axis for the outer tube <NUM>. Although this longitudinal axis may be centrally located, as shown, it may also be off-center. In still other cases, the primary axis <NUM> may be non-linear, for example it may be curved, S-shaped, etc. As will be discussed in further detail below, the primary axis <NUM> serves as the axis of rotation for the helical or spiraling plurality of inner tubes <NUM>.

In the depicted embodiment, the outer tube inlet <NUM> and the outer tube outlet <NUM> extend perpendicularly from the outer tube <NUM> relative to the primary axis <NUM>. Alternatively, the outer tube inlet <NUM> and outer tube outlet <NUM> may extend at different angles from the outer tube <NUM>, for instance to form acute angles with the outer tube <NUM>. Such acute angles may minimize the pressure losses in the fluid travelling through the outer tube <NUM>. In the depicted embodiment where the cross-sectional shape of the outer tube <NUM> is circular, the outer tube inlet <NUM> and the outer tube outlet <NUM> are aligned along a length of the outer tube <NUM> parallel to the primary axis <NUM>. In other cases, the outer tube inlet <NUM> and the outer tube outlet <NUM> may be offset relative to the length of the outer tube <NUM>, i.e. the outer tube inlet <NUM> and outer tube outlet <NUM> are at different radial positions relative to the circular cross-section of the outer tube <NUM>. Such offset positioning may, for instance, induce an additional swirl in the fluid traveling through the outer tube <NUM>. Other positions and angles for the outer tube inlet <NUM> and outer tube outlet <NUM> may be contemplated as well, for instance based on the positioning of the inlet and outlet tubes of the various fluids in a given gas turbine engine <NUM>.

As shown in <FIG>, the plurality of inner tubes <NUM> extend through the outer tube <NUM> between the first end <NUM> and the second end <NUM> of the fluid cooler <NUM>. The plurality of inner tubes <NUM> have a common inner tube inlet <NUM>, illustratively at the second end <NUM> of the fluid cooler <NUM>, and a common inner tube outlet <NUM>, illustratively at the first end <NUM> of the fluid cooler <NUM>. As such, in the embodiment shown, the fluid flowing through the plurality of inner tubes <NUM>, for instance air to be cooled by the oil flowing through the outer tube <NUM>, flows from the second end <NUM> to the first end <NUM>. The reverse arrangement may be contemplated as well. In the embodiment shown, the plurality of inner tubes <NUM> meet at either ends thereof at a respective inner tube plenum <NUM> before a respective inner tube inlet <NUM> or inner tube outlet <NUM>. Additionally, in a particular embodiment, the inner tubes <NUM> are radially inwardly spaced from an inner surface of the outer tube <NUM>. In other cases, the fluid cooler <NUM> may include multiple inner tube inlets and multiple inner tube outlets to allow for different fluids in need of cooling to pass through different inner tubes <NUM> simultaneously, as will be discussed in further detail below.

Referring to <FIG> and <FIG>, the plurality of inner tubes <NUM> extend or wrap helically about the primary axis <NUM>, creating a spiral or helix-like shape through the inside of the outer tube <NUM>. As such, the effective length of the plurality of inner tubes <NUM> may be increased relative to comparable inner tubes passing straight through the outer tube <NUM>. Thus, the effective length of the plurality of inner tubes <NUM> may be increased without increasing the overall length of the fluid cooler <NUM>. This increase in effective length may increase the overall rate of heat transfer between the fluid traveling through the plurality of inner tubes <NUM> and the fluid travelling through the outer tube <NUM>. In addition, the helical nature of the plurality of inner tubes <NUM> may induce mixing or swirling of the fluid within the plurality of inner tubes <NUM>. The fluid flowing within the outer tube <NUM> also may endure increased mixing due to its interaction with the spiraling inner tubes <NUM>. Such factors may further contribute to the effectiveness of the overall cooling process.

The plurality of inner tubes <NUM> may be positioned at different radii from the primary axis <NUM>, leading to a variety of possible rotational patterns. Such radii may be taken from the outer edge of a given inner tube <NUM> to the primary axis <NUM>. The plurality of inner tubes <NUM> may be grouped into different groups of inner tubes <NUM>, each group being at a different radius from the primary axis <NUM>. The size and/or shape of the inner tubes <NUM> may vary between groups, as well as within a given group. In the case shown in <FIG>, a first group includes a single inner tube 34a disposed at a first radius R1 from the primary axis, while a second group includes six inner tubes 34b disposed at a second radius R2 from the primary axis <NUM>. As such, the inner tube 34a is disposed at a distance R1 from the primary axis <NUM>, while the inner tubes 34b are disposed at a distance R2 from the primary axis <NUM>. Other numbers of groups, for instance three or more groups at different radii from the primary axis <NUM>, as well as number of inner tubes <NUM> in each group, may be contemplated as well. For instance, the type of fluid, their mass flow rate, their target temperatures, etc. may factor into the number of inner tubes <NUM> and their respective rotation rates.

In another embodiment, as discussed above, the fluid cooler <NUM> may include multiple inner tube inlets and multiple inner tube outlets to allow for multiple fluids to be cooled at once. For instance, referring to <FIG>, a given fluid cooler <NUM> may include two inner tube inlets 50a, 50b and two inner tube outlets 52a, 52b. The first inner tube inlet 50a and the first inner tube outlet 52a may be joined by a first group of inner tubes 34a, illustratively a single inner tube 34a, while the second inner tube inlet 50b and the second inner tube outlet 52b may be joined by a second group of inner tubes 34b via the inner tube plenums <NUM>. As such, a first fluid to be cooled may pass through the first inner tube inlet 50a, the first group of inner tubes 34a and the first inner tube outlet 52a, while a second fluid to be cooled may pass through the second inner tube inlet 50b, an inner tube plenum <NUM>, the second group of inner tubes 34b, another inner tube plenum <NUM> and the second inner tube outlet 52b. Both of these fluids would thus be simultaneously cooled by the fluid traveling through the outer tube <NUM>. Other modes of use may be contemplated as well, such as flowing the two fluids to be cooled sequentially through their respective inner tubes. Other arrangements and numbers of inner tube inlets and inner tube outlets may be contemplated as well.

<FIG> show cross-sectional views of the inner tubes <NUM> within the outer tube <NUM> taken at different points along a length of the outer tube <NUM>, illustrating the spiraling nature of the inner tubes' path. The pitch of the inner tubes' <NUM> may vary, for instance to increase or decrease the number of spirals and therefore the effective length of the inner tubes <NUM>. The rotation rate of the inner tubes <NUM> about the primary axis <NUM> may also vary along the length of the fluid cooler <NUM>. For instance, in an embodiment a fluid cooler <NUM> may include more densely packed inner tubes <NUM> towards the first and second ends <NUM>, <NUM> with more spacing between the inner tubes <NUM> towards the middle of the fluid cooler <NUM>. As such, the rotational pattern of the inner tubes <NUM> about the primary axis <NUM> may be uniform or non-uniform, as will be discussed in further detail below. Other geometric considerations may be contemplated, as will be discussed in further detail below.

Various end fittings for fluidly connecting to the inner tube inlet <NUM> and inner tube outlet <NUM> may be contemplated. For instance, <FIG> shows flat two bolt flanges with C-seals <NUM> at both the inner tube inlet <NUM> and inner tube outlet <NUM>. Other end fittings for the inner tube inlet <NUM> and outlet <NUM> may be contemplated as well, for instance the B-nut and nipple-type fitting <NUM> as mounted to the outer tube inlet <NUM> as per <FIG>. Various combinations of these end fittings, along with other types of end fittings such as a <NUM> degree cone and nipple (not shown), may be contemplated as well.

Referring to <FIG> and <FIG>, in an exemplary embodiment, warm engine air in need of cooling may enter the fluid cooler <NUM> through the inner tube inlet <NUM>, pass through an inner tube plenum <NUM> before being diverted through the plurality of inner tubes <NUM>. For instance, in one particular embodiment, this warm engine air is air bled from the exit of the compressor (sometimes referred to as "P2. <NUM>" air), or downstream of the exit of the compressor <NUM>. Other sources for the warm engine air may be contemplated as well. Concurrently, relatively cool engine oil may enter the fluid cooler <NUM> via the outer tube inlet <NUM> and into the outer tube <NUM>. As the two fluids flow through their respective tubes, heat is transferred from the air to the oil, effectively cooling the air. In the embodiment shown, the air and oil enter the fluid cooler <NUM> at opposite ends of the fluid cooler <NUM>. In other cases, the air and oil, or other fluids selected for a heat transfer operation, may enter the fluid cooler <NUM> at the same end, for instance at the first end <NUM> or at the second end <NUM>. The fluid cooler <NUM> may be reversible as well. In other cases, the outer tube <NUM> may transport the warm air to be cooled and the plurality of inner tubes <NUM> may transport the cooling engine oil. The fluid cooler <NUM> may therefore be used to transfer heat between at least two other fluids within the engine <NUM>. The now-cooled air exiting the inner tube outlet <NUM> may be transported to, for instance, a location further upstream, such as the inlet of the compressor section <NUM>, to cool the compressor section <NUM>. The now-cooled air may accordingly be used to cool the bore and/or disc of a compressor rotor (such as an impeller or an axial compressor disc), or for cooling other parts of the engine <NUM>.

A discussed above, the fluid cooler <NUM> and its associated mounting hardware may be mounted external to the engine's core casing <NUM>, i.e. in the bypass duct <NUM> between the nacelle <NUM> and the core casing <NUM>. Other locations for the fluid cooler <NUM> may be contemplated as well. In the case of engine air being cooled, the source and destination of the air may vary. For instance, the cooled air may be subsequently delivered to the compressor section <NUM>. In the case of engine oil providing the cooling to the other fluid, the source and destination of the oil may vary as well. For instance, the oil may arrive from the engine's primary oil tank and, after the cooling process at the fluid cooler <NUM>, be delivered to the engine's primary fuel-oil heat exchanger. Other sources and destinations for the various fluids may be contemplated as well.

Referring to <FIG>, another embodiment of a fluid cooler <NUM> according to the present disclosure is shown. <FIG> does not show the first or second ends of the fluid cooler, nor the inlets or outlets of the outer tube <NUM> and inner tubes <NUM>. However, it is understood that the shown fluid cooler <NUM> includes an outer tube <NUM> extending between a first end and a second end of the fluid cooler with a plurality of inner tubes <NUM> passing through the outer tube <NUM>. As in the above case, a first group of inner tubes 34a are positioned at a first radius R1 from the primary axis <NUM>, in this case the central longitudinal axis of the outer tube <NUM>. The size and/or shape of the inner tubes <NUM> may vary between groups, as well as within a given group. Three inner tubes 34a illustratively form this first group, although other numbers of first group inner tubes 34a may be contemplated as well. A second group of inner tubes 34b are positioned at a second radius R2 from the primary axis <NUM>. Three inner tubes 34b illustratively form this second group, although other numbers of second group inner tubes 34b may be contemplated as well. Other numbers of groups may be contemplated as well at various radii from the primary axis <NUM>.

The various inner tubes 34a, 34b are extended helically about the primary axis <NUM>, as shown in <FIG> and <FIG>, forming a spiral or helical shape. The pitch of rotation may vary, as discussed above. In the embodiment shown, the outer tube <NUM> includes a cross-sectional shape consisting of peaks 32a and valleys 32b (see <FIG>) and is extended helically or twisted around the inner tubes <NUM> to form an outer twisting or spiral shape that follows the contour of the inner tubes <NUM>. By tightly wrapping the outer tube <NUM> around the inner tubes <NUM> such that the outer tube <NUM> conforms to the outer shape of the inner tubes <NUM>, the overall efficacy of the heat transfer process may improve. For instance, the heat transfer coefficient on the outer surface of the inner tubes <NUM> may increase due to the lower cross-sectional area of the outer tube <NUM>. In addition, the spiral-like shape of the outer tube <NUM> may contribute to further mixing of the fluid contained in the outer tube <NUM>. The number of peaks 32a and valleys 32b may vary, for instance based on the length of the fluid cooler, the number of inner tubes <NUM> and their respective pitch.

Referring to <FIG>, in various cases, the inner tubes <NUM> may have a variety of shapes and/or geometric features to promote heat exchange between the fluids. While a straight inner tube <NUM>(<NUM>) may be used, i.e. with a circular cross-section, other shapes and features may be contemplated as well. For instance, the inner tubes <NUM> may include a square step inner tube <NUM>(<NUM>), an inner tube with a spring-feature <NUM>(<NUM>), a twisted-tape inner tube <NUM>(<NUM>), a solid inner core tube <NUM>(<NUM>), a corrugated inner tube <NUM>(<NUM>), an inner tube with inward dimples <NUM>(<NUM>), an inner tube with outward dimples <NUM>(<NUM>), an inner tube with helical grooves <NUM>(<NUM>), and/or an inner tube with a butterfly twisted-tape <NUM>(<NUM>). Various combinations of the above inner tubes <NUM> may be contemplated. Inner tubes <NUM> with other shapes and/or features may be contemplated as well.

In various cases, the fluid cooler <NUM> as per the present disclosure may be lighter and smaller than existing fluid coolers, for instance due to the spiraling nature of the inner tubes <NUM>, the peaks 32a and valleys 32b defined in the outer tube <NUM>, and/or the various shapes and geometric features of the inner tubes <NUM>. As such, the fluid cooler <NUM> may be supported by the various inlet and outlet rigid tubes (not shown) transporting the various fluids to or from the fluid cooler <NUM>. Other supporting features or hardware for the fluid cooler <NUM> may be contemplated as well.

Referring to <FIG>, in various cases, the pitch and taper angle of the inner tubes <NUM> may be varied linearly and/or non-linearly. While <FIG> show the outer tube <NUM> being cylindrical, as per <FIG>, it is understood that the pitch and taper angle of the inner tubes <NUM> may also be varied in the fluid cooler <NUM> shown in <FIG>. <FIG> shows the pitch of the inner tubes <NUM> varied linearly, <FIG> shows the taper angle of the inner tubes <NUM> varied linearly, and <FIG> shows both the pitch and the taper angle of the inner tubes <NUM> varied non-linearly. Other combinations may be contemplated as well.

By varying the pitch of the inner tubes <NUM>, as shown in <FIG>, the number of revolutions of the inner tubes <NUM> relative to the primary axis <NUM> may be altered based on the specific application. By varying the taper angle of the inner tubes <NUM>, as shown in <FIG>, the axial location of the inner tubes' <NUM> center axis with respect to their axis of rotation, i.e. the primary axis <NUM>, may be altered. Various combinations of these alternations may be contemplated, for instance to improve fluid mixing and to increase the overall effectiveness of the fluid cooler <NUM>. For instance, by adjusting both the pitch and taper angle of the inner tubes <NUM>, as shown in <FIG>, the flow of fluid F within the outer tube <NUM> may be forced to follow a sinusoidal pattern around the inner tubes <NUM>. Other flow patterns due to the variations in pitch and taper angle of the inner tubes <NUM> may be contemplated as well.

As discussed above, the fluid cooler <NUM> as described herein may be referred to as a 'single pass' fluid cooler. In various cases, two or more such fluid coolers <NUM> may be connected in series for further cooling of one of the fluids, for instance the fluid passing through the inner tubes <NUM>. Other arrangements may be contemplated as well. In various cases, the fluid cooler <NUM> may be manufactured via 3D printing, i.e. additive manufacturing, although other manufacturing techniques may be contemplated as well. In cases where the fluid cooler <NUM> is manufactured via additive manufacturing, additional mounting hardware may be integrated to the fluid cooler, for instance lugs (not shown) to mount the fluid cooler <NUM> directly to a flange or a supporting bracket within the engine <NUM>.

Claim 1:
A gas turbine engine (<NUM>) comprising:
a compressor section (<NUM>);
an air supply;
an oil supply; and
a fluid cooler (<NUM>) comprising:
an outer tube (<NUM>) having an outer tube inlet (<NUM>) at a first end (<NUM>) of the fluid cooler (<NUM>) and an outer tube outlet (<NUM>) at a second end (<NUM>) of the fluid cooler (<NUM>), a primary axis (<NUM>) of the fluid cooler (<NUM>) defined within the outer tube (<NUM>) between the first end (<NUM>) of the fluid cooler (<NUM>) and the second end (<NUM>) of the fluid cooler (<NUM>); and
a plurality of inner tubes (<NUM>) extending within the outer tube (<NUM>) between the first end (<NUM>) of the fluid cooler (<NUM>) and the second end (<NUM>) of the fluid cooler (<NUM>), the plurality of inner tubes (<NUM>) having a common inner tube inlet (<NUM>) and a common inner tube outlet (<NUM>), the plurality of inner tubes (<NUM>) extending helically about the primary axis (<NUM>) of the fluid cooler (<NUM>), a first group of the plurality of inner tubes (<NUM>) disposed at a first radius (R1) from the primary axis (<NUM>) of the fluid cooler (<NUM>) and a second group of the plurality of inner tubes (<NUM>) disposed at a second radius (R2) from the primary axis (<NUM>) of the fluid cooler (<NUM>), the second radius (R2) different from the first radius (R1), wherein the outer tube inlet (<NUM>) is fluidly coupled to the oil supply.