TECHNICAL FIELD
The following disclosure relates generally to heat exchangers and, more particularly, to radiators and other heat exchangers containing carbon nanotubes, as well as to methods for manufacturing carbon nanotube-containing heat exchangers.
BACKGROUND
Heat exchangers are well-known devices utilized to promote heat transfer between materials, such as fluids. In the case of a vehicular radiator, for example, heat may be transferred from a liquid coolant to airflow directed through the radiator's core to dissipate excess heat generated by an internal combustion engine or other heat source onboard the vehicle. As have heat exchangers generally, vehicular radiators have been subject to extensive engineering efforts resulting in significant improvements in the physical characteristics (e.g., durability, pressure capabilities, weight, and size) and performance characteristics (e.g., heat rejection capabilities) of modern radiators. Vehicular radiators previously manufactured utilizing expanded tube constructions and copper-based or brass-based materials (e.g., copper-brass alloys) have now been largely superseded by lighter, more efficient radiators fabricated utilizing brazed constructions and aluminum-based materials. Such advancements notwithstanding, there exists a continued demand for still further improvements in the physical and performance characteristics of vehicular radiators, vehicular heater cores, and heat exchangers generally.
BRIEF SUMMARY
Vehicular radiators and other heat exchangers containing carbon nanotubes (CNTs) are provided. In one embodiment, the CNT-containing or “nanotube” heat exchanger includes a coolant flow passage, an airflow path, and a heat exchanger core bounding at least a portion of the coolant flow passage and the airflow path. The heat exchanger core contains a plurality of CNTs through which heat is transferred from a coolant conducted through the coolant flow passage to airflow directed along the airflow path during operation of the heat exchanger. The CNTs can be, for example, single walled CNTs or other CNTs incorporated into one or more regions of the heat exchanger core by applying a nanotube coating to selected surfaces of the heat exchanger core, by producing the heat exchanger core to include one or more CNT-impregnated components, or by otherwise producing the heat exchange core to include the CNTs. The thermal performance and other characteristics of the nanotube heat exchanger may be significantly enhanced by integrating CNT-containing structures into the core or body of the heat exchanger in this manner.
In another embodiment, the nanotube heat exchanger includes a coolant-conducting tube and a CNT-containing structure, such as a CNT-coated or CNT-containing turbulator, disposed within the coolant-conducting tube. During operation of the nanotube heat exchanger, the CNT-containing structure is contacted by coolant flowing through the coolant-conducting tube to promote heat transfer from the coolant to airflow in contact with the nanotube heat exchanger. The CNT-containing structure can be, for example, a nanotube coating applied to an interior surface of the coolant-conducting tube and/or to in-tube structures (e.g., turbulators) mounted within the coolant-conducting tube. In certain cases, the nanotube coating may contain a directional or anisotropic single wall CNT (SWCNT) array containing SWCNTs oriented to extend, at least in substantial part, along the length of the coolant-conducting tube. In other embodiments, the CNT-containing structure may be a sintered component or other body in which a plurality of CNTs is embedded.
Methods for manufacturing nanotube heat exchangers are further provided. In one embodiment, the method includes producing a heat exchanger core having a plurality of air-contacted surfaces and a plurality of coolant-contacted surfaces. CNTs are integrated or incorporated into one or more regions of the heat exchanger core, which are thermally coupled between the plurality of air-contacted surfaces and the plurality of coolant-contacted surfaces. The CNTs can be incorporated into the heat exchanger core by, for example, applying a nanotube coating to selected surfaces of coolant-conducting tubes (e.g., the interior and/or exterior tube surfaces), of one or more fin structures, of one or more structures disposed within the coolant-conducting tubes (e.g., turbulators and/or honeycomb-shaped inserts), and/or of other components included within the heat exchanger core. Additionally or alternatively, the CNTs can be embedded within sintered components contained within the radiator core.
BRIEF DESCRIPTION OF THE DRAWINGS
At least one example of the present invention will hereinafter be described in conjunction with the following figures, wherein like numerals denote like elements, and:
FIG. 1 is an isometric view of a vehicular radiator containing Carbon Nanotubes (CNTs) dispersed throughout one or more regions of the radiator body or core, as illustrated in accordance with an exemplary embodiment of the present disclosure;
FIG. 2 is a partial isometric view of the radiator core shown in FIG. 1 identifying several exemplary locations at which CNTs are usefully integrated into the radiator core, as illustrated in accordance with an exemplary embodiment of the present disclosure;
FIGS. 3 and 4 are simplified cross-sectional views illustrating nanotube coatings (partially shown) applied to selected surfaces of one or more radiator components included in the vehicular radiator shown in FIG. 1, as illustrated in accordance with further embodiments of the present disclosure;
FIG. 5 is a simplified cross-sectional views illustrating a sintered, CNT-containing radiator component (partially shown) included in the vehicular radiator shown in FIG. 1, as illustrated in accordance with a further embodiment of the present disclosure;
FIG. 6 is a cutaway view of a coolant-conducting tube containing a number of honeycomb-shaped in-tube structures to which nanotube coatings have been applied, as illustrated in accordance with a further exemplary embodiment of the present disclosure;
FIG. 7 is an isometric view of a vehicular radiator including a number of CNT-containing structures, as illustrated in accordance with a still further exemplary embodiment of the present disclosure; and
FIG. 8 is a flowchart setting-forth a heat exchanger fabrication method, as illustrated in accordance with an exemplary embodiment of the present disclosure.
DETAILED DESCRIPTION
The following Detailed Description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. There is no intention to be bound by any theory presented in the preceding Background or the following Detailed Description. The term “exemplary,” as appearing throughout this document, is synonymous with the term “example” and is utilized below to emphasize that the following description should not be interpreted to limit the scope of the invention, as set-out in the appended Claims. The term “coating,” as further appearing herein, refers to a single layer or multi-layer structure disposed over and at least partially covering the surface of a component included within a radiator or other heat exchanger. Finally, the term “carbon nanotube” is defined as a generally tube-shaped structure composed of carbon and having a diameter between 0.1 nanometer (nm) and 1000 nm.
The following describes embodiments of nanotube heat exchangers, which leverage the unique properties of Carbon Nanotubes (CNTs) to bring about significant improvements in the thermal performance and physical characteristics of the heat exchanger. These attributes may render embodiments of the nanotube heat exchanger particularly well-suited for usage as a radiator, such as a radiator included within a Heating, Ventilation, and Air Conditioning (HVAC) system onboard a vehicle. For this reason, the following description focuses primarily on the integration of CNTs or CNT-containing structures into vehicular radiators and other vehicular heat exchangers, such as vehicular heater cores. This notwithstanding, the nanotube heat exchanger need not assume the form of a vehicular radiator in all embodiments. Instead, embodiments of the nanotube heat exchanger can be beneficially utilized in any application or platform wherein it is desired to promote the transfer of heat between at least two materials or fluids. For example, embodiments of the nanotube heat exchanger can be utilized within stationary, non-vehicular platforms, such as in chemical treatment facilities and residential buildings. Furthermore, in instances wherein the nanotube heat exchanger assumes the form of a radiator onboard a vehicle, the vehicle need not necessarily assume the form of a passenger motor vehicle and may alternatively be an aircraft, a spacecraft, a watercraft, or another mobile platform.
Potential Improvements in the Thermal Performance and Size/Mass Reduction of the Nanotube Radiator
As a primary advantage, embodiments of the nanotube radiator can provide enhanced heat dissipation capabilities in a reduced-size or a reduced-mass package as compared to a conventional radiator lacking CNTs. The magnitude by which the heat dissipation capabilities are enhanced will vary amongst embodiments of the nanotube radiator. In general, however, the thermal transfer properties of the nanotube radiator can be optimized through usage of single walled CNTs (SWCNTs) having average diameters between 1-20 nm and arranged in an organized network or array. Numerical analysis indicates that substantial improvements in thermal performance can be achieved through the incorporation of such SWCNT arrays into targeted regions of the radiator core. To provide a relatively simple, but instructive example, consider the thermal conductivity of a radiator fin structure included within a conventional radiator (hereafter the “baseline fin structure”) as compared to a SWCNT array of similar dimensions. The heat transfer coefficient of the baseline fin structure (hfin) may be calculated as follows (EQ. 1):
where Afin is the surface area of the baseline fin structure in square meters (assumed to be 0.292 m2 in this example), ΔT is the temperature differential between an engine cooled by the radiator containing the fin structure and its surrounding environment in Kelvin (assumed to be 167 K in this example), and Qdis is the heat transfer quantity in Watts (assumed to be 70,730 W in this example). The heat transfer coefficient of the baseline fin structure (hfin) is thus about 1,453 Watts per square meter per kelvin (W/m2k) in the present example.
By comparison, experimental results indicate that a SWCNT array containing CNTs having an average diameter between 1-20 nm can, at least in theory, achieve a markedly higher thermal transfer coefficient (hCNT_max), which may approach or exceed 111×105 W/m2K. The SWCNT array, then, is capable of providing superior thermal transfer capabilities as compared to the baseline fin structure of equivalent size. Moreover, this disparity in thermal transfer capabilities is sufficiently large to enable a fin structure including such a SWCNT array to be miniaturized relative to the baseline fin structure, while still providing superior thermal transfer capabilities. This may be appreciated by calculating the ratio of the heat transfer coefficient of SWCNT array taken over the heat transfer coefficient of the baseline fin structure, as indicated by EQ. 2 below:
As can be seen, the dimensions of the SWCNT array can theoretically be minimized in a manner providing about a 75 fold reduction in surface area, while still achieving substantially equivalent thermal transfer capabilities to the baseline fin structure. A similar increase in the overall thermal performance and/or a similar decrease in the dimensions of the nanotube radiator can be extrapolated from this numerical example.
The numerical example presented above is an analytical approximation of the interaction between an idealized SWCNT array and its gas/fluid environment. In actual implementations, the nanotube heat exchanger may contain multi-arrayed SWCNTs, multi-walled CNTs, or CNT particulates dispersed throughout a larger structure, such as a sintered coolant-conducting tube, fin structure, turbulator, or end tank. Thus, in such implementations, the heat transfer coefficient of the nanotube radiator will be lower than the theoretical value presented above. It is generally anticipated, however, that a substantial (e.g., about 10 to 50 fold) increase in the heat transfer capabilities of the nanotube radiator can be realized relative to a conventional radiator of comparable dimensions. The enhanced heat rejection capabilities of the nanotube radiator may thus enable substantial reduction in radiator size and weight, while still satisfying the heat rejection needs of a vehicle (or other platform) on which the radiator is deployed. The enhanced heat rejection capabilities of such a nanotube radiator may also enable a reduction in the size, weight, and/or power requirements of overflow tanks, fans, or other components associated with the nanotube radiator. The overall weight, cost, and complexity of the vehicle's HVAC system can be reduced as a result. This is highly desirable.
Embodiments of the nanotube radiator and heat exchangers described herein may also provide other notable benefits including, for example, increased heat exchanger stability at higher temperatures and pressures. Additionally, CNTs and CNT arrays typically possess relatively high tensile strengths (ultimate strengths), which may approach or exceed about 63 Gigapascal (GPa) in at least some instances. As this value exceeds the tensile strengths of metals and alloys commonly utilized in radiator production, the integration of CNTs into the below-described nanotube radiators (or other heat exchangers) may increase the tensile strength and pressure capabilities of the nanotube radiator in certain embodiments. A first exemplary embodiment of the nanotube radiator will now be described in conjunction with FIGS. 1-5.
Examples of Nanotube Radiators
FIG. 1 is an isometric view of a nanotube radiator 20, illustrated in accordance with an exemplary embodiment of the present invention. To provide an exemplary context, nanotube radiator 20 is schematically illustrated in FIG. 1 as included within a larger HVAC system 16 deployed onboard a vehicle 18, such as a motor vehicle. This notwithstanding, nanotube radiator 20 can be utilized in various other applications and platforms without limitation.
Nanotube radiator 20 includes a first end tank 22, a second end tank 24, and a radiator core 26 positioned between tanks 22, 24. In the illustrated example, radiator core 26 is produced from a plurality of coolant-conducting tubes 28(1)-(4) and a plurality of fin structures 30(1)-(3). Coolant-conducting tubes 28(1)-(4) and fin structures 30(1)-(3) are interspersed or stacked in an alternating relationship to form a generally rectangular structure or unit. In one embodiment, coolant-conducting tubes 28(1)-(4) are produced from metallic (e.g., aluminum) sheets, which are rolled into tubular bodies (e.g., D-style folded tubes) and welded along their longitudinal edges to form fluid-tight conduits. Fin structures 30(1)-(3) may likewise be produced from metallic (e.g., aluminum) sheets, which are formed (e.g., by rolling or stamping) into undulating or sinusoidal shapes. Fin structure 30(1)-(3) may be retained between coolant-conducting tubes 28(1)-(4) by physical contact (e.g., press-fit or interference fit) with tubes 28(1)-(4), end tank 22, and end tank 24; or by the application of an adhesive. In further embodiments, radiator core 26 can include a greater or lesser number of coolant-conducting tubes 28(1)-(4) and fin structures 30(1)-(3), which can vary in size, shape, composition, and relative disposition. For example, in further embodiments of nanotube radiator 20, coolant-conducting tubes 28(1)-(4) can be round or B-style rolled tubes.
Coolant-conducting tubes 28(1)-(4) define or bound a plurality of coolant flow passages, as further represented in FIG. 1 by arrows 36. Coolant flow passages 36 extend through radiator core 26 to fluidly couple end tank 22 (the “inlet tank”) to end tank 24 (the “outlet tank”) in a single pass design. The terminal ends of coolant-conducting tubes 28(1)-(4) extend into mating openings provided in end tanks 22, 24 to retain tubes 28(1)-(4) in their desired positions. Coolant-conducting tubes 28(1)-4) can be secured in place by press-fit or interference fit in an embodiment. The joints or interfaces between the terminal ends of tubes 28(1)-(4) and end tanks 22, 24 can be sealed by brazing (e.g., in embodiments wherein nanotube radiator 20 is produced as a brazed radiator), by application of an epoxy or utilizing gaskets (e.g., in embodiments wherein radiator 20 is produced as a “mechanically-attached” radiator), or in another manner. In further embodiments, coolant-conducting tubes 28(1)-(4) may have other geometries and constructions providing that tubes 28(1)-(4) are suitable for fluidly coupling end tanks 22, 24, as described below. During operation of nanotube radiator 20, a first conduit 38 (partially shown) directs a liquid coolant into inlet tank 22, which serves as a manifold directing the coolant flow into each of coolant-conducting tubes 28(1)-(4). After flowing through coolant-conducting tubes 28(1)-(4), the coolant is then received by outlet tank 24, consolidated into a single stream, and delivered to a second conduit 40 (partially shown). Conduits 38, 40 can be pipes, hoses, or other fluid-conducting lines. Conduits 38, 40 constitute a portion of a larger flow circuit, which is included within HVAC system 16 containing nanotube radiator 20. The various other components included in HVAC system 16 are not shown in FIG. 1, but are well-known within the industry and may include one or more pumps, overflow tanks, heater cores, and associated plumbing features.
As indicated FIG. 1 by arrows 32, airflow paths extend through radiator core 26. Airflow paths 32 are at least partially defined or bound by the outer surfaces of coolant-conducting tubes 28(1)-(4) and fin structures 30(1)-(3) such that airflow directed along airflow paths 32 contacts or impinges upon the external surfaces of radiator core 26. One or more fans 34 may be positioned either upstream or downstream of nanotube radiator 20 (relative to the direction of airflow) to direct forced airflow through radiator core 26 along airflow paths 32 during operation of radiator 20. As generally shown in FIG. 1, coolant-conducting tubes 28(1)-(4) are usefully imparted with substantially ovular or rectangular “flat tube” geometries to increase the surface area of tubes 28(1)-(4) in contact with the airflow directed along airflow paths 32; however, as previously noted, the shape and construction of tubes 28(1)-(4) can vary amongst embodiments. Nanotube radiator 20 may also include other structural features or devices in addition to those described above. For example, nanotube radiator 20 may also include an overflow port 42, which is fluidly connected to an overflow tank (not shown) to accommodate thermally-induced changes in coolant volume. In further embodiments, multiple discrete radiators similar or identical to nanotube radiator 20 can be positioned at different locations along the coolant flow circuit included within HVAC system 16, whether the discrete radiators are fluidly coupled in series, in parallel, or in a series-parallel combination.
FIG. 2 is a simplified isometric view of a portion of radiator core 26 illustrating coolant-conducting tube 28(4) and an end segment of fin structure 30(3) in greater detail. While only a single coolant-conducting tube 28(4) and fin structure 30(3) are shown for clarity, coolant-conducting tubes 28(1)-(3) and fin structures 30(1)-(2) may have identical or similar structural features; thus, the following description is equally applicable thereto. Here, it can be seen that a number of in-tube structures 44(1)-(4) are disposed within coolant-conducting tube 28(1)-(4). In-tube structures 44(1)-(4) can be, for example, concentric tubes or conduits extending within coolant-conducting tube 28(4). In-tube structures 44(1)-(4) can extend the entire length of tube 28(4) or, perhaps, only a portion thereof. In-tube structures 44(1)-(4) can be mounted within tube 28(4) by physical contact with the interior tube walls. Alternatively, as indicated in FIG. 2, in-tube structures 44(1)-(4) can be suspended within coolant-conducting tube 28(4) by radially-extending members or “radial tie bars” 46, which are provided at the inlet end of tube 28(4). Similar radial tie bars may also be present at the outlet end of tube 28(4). Radial tie bars 46 mount in-tube structures 44(1)-(4) within coolant-conducting tube 28(4), while also providing thermally-conductive paths connecting structures 44(1)-(4) to the body of tube 28(4). In one implementation, radial tie bars 46 and in-tube structures 44(1)-(4) are slid into their desired position tubes 38(1)-(3) during assembly of radiator 20 and then secured in place by, for example, a spring force exerted by radial tie bars 46 against the interior of tubes 28(1)-(3) in radially outward directions.
In-tube structures 44(1)-(4) increase the surface area within coolant-conducting tube 28(4) across which heat is convectively removed from the coolant passing through radiator core 26. Further, in the context of nanotube radiator 20, in-tube structures 44(1)-(4) provide additional opportunities for the introduction of CNTs into the interior of coolant-conducting tube 28(4). To this end, nanotube coatings can potentially be applied to in-tube structures 44(1)-(4) or structures 44(1)-(4) may be impregnated with CNTs, as described more fully below in conjunction with FIGS. 3-5. In-tube structures 44(1)-(4) may serve as turbulators during radiator operation; that is, structures or bodies located within in-tube structures 44(1)-(4) designed to induce a controlled amount of localized turbulence into coolant flow through coolant-conducting tube 28(4). The localized turbulence induced by in-tube structures 44(1)-(4), when serving as turbulators, may further promote convective heat transfer from the coolant to the respective bodies of coolant-conducting tube 28(4). In further embodiments, nanotube radiator 20 may include different types of in-tube structures, which may or may not serve as turbulators. In still further embodiments, nanotube radiator 20 may lack in-tube structures within any or all of coolant-conducting tube 28(1)-(4).
During operation of nanotube radiator 20, heated coolant is introduced into an inlet end 47 of coolant-conducting tube 28(4) (indicated in FIG. 2 by arrows 48). As the coolant flows through coolant-conducting tube 28(4), heat is removed from the coolant by convective heat transfer to the tube body (identified by reference numeral “49”). Specifically, heat is convectively transferred to coolant-conducting tube 28(4) by direct contact with an interior surface 51 thereof, as well as by convective transfer to in-tube structures 44(1)-(4) and subsequent conductive transfer to tube 28(4) through radial tie bars 46. From coolant-conducting tube 28(4), the heat is then conductively transferred to fin structure 30(3). Finally, the heat is convectively transferred from fin structure 30(3) to the ambient airflow directed over radiator core 26 and across fin structures 30(3), as indicated in FIG. 2 by arrows 50. The airflow directed across fin structures 30(3) may be ram airflow received through the vehicle grille (not shown), forced airflow provided by fan 34 (FIG. 1), or a combination thereof. Through this process, heat is removed from the coolant circulated through coolant-conducting tube 28(4) and dissipated to the ambient environment surrounding radiator 20. Notably, and as further indicated in FIG. 1 by cutaway 52, a liquid coolant containing particulate CNT (shown in an enlarged state) can be circulated through nanotube radiator 20. The usage of such a CNT-containing coolant may further enhance thermal performance of nanotube radiator 20 and may favorably interact with the below-described CNT-containing structures integrated into radiator 20. In further embodiments, different types of materials can be conducted through nanotube radiator 20 including, for example, a different liquid- or gas-phase coolant.
Nanotube radiator 20 further includes a number of CNT-containing structures, such as freestanding nanotube coatings, deposited nanotube coatings, CNT-impregnated bodies, or other static structures that contain or carry CNTs. The CNT-containing structures can be introduced into nanotube radiator 20 at any number of selected locations suitable for promoting heat transfer from the coolant flowing through coolant-conducting tubes 28(1)-(4) to the airflow directed along airflow paths 32 (FIG. 1). Generally, the thermal performance of nanotube radiator 20 can be optimized when CNT-containing structures are integrated into radiator core 26 at each (or at least one) of the following interfaces: (i) the coolant-contacted surfaces within coolant-conducting tubes 28(1)-(4) across which convective heat transfer occurs, (ii) the interface between tubes 28(1)-(4) and fin structures 30(1)-(3) across which conductive heat transfer occurs, and (iii) the air-contacted surfaces of fin structures 30(1)-(3) across which convective heat transfer occurs. With respect to romanette (i), in particular, the CNT-containing structures can be applied to or integrated into coolant-conducting tubes 28(1)-(4) and/or integrated into in-tube structures disposed within tubes 28(1)-(4), such as any or all of in-tube structures 44(1)-(4). In further embodiments, CNTs can also be introduced into nanotube radiator 20 at other locations in addition or in lieu of those listed above. For example, in certain implementations, end tanks 22, 24 (FIG. 1) can be produced as sintered, CNT-containing structures or the interior of end tanks 22, 24 can be lined with nanotube coatings of type described below.
Thermal transfer may be enhanced through the usage of directional or anisotropic CNTs arrays; that is, ordered layers or groupings of CNTs that are predominately oriented to extend in a common direction relative to the centerline, longitudinal axis, or other spatial reference point of the CNT-containing structure. When the CNT arrays are provided at or adjacent a fluid-contacted surface of nanotube radiator 20, the orientation of such directional CNT arrays can affect the rate of convective heat transfer. Generally, heat transfer is enhanced by orienting the directional CNT arrays (when present) such that the CNTs making-up the arrays extend substantially perpendicular to a primary direction of airflow when the arrays are located at or adjacent the air-exposed surfaces of radiator 20. Conversely, heat transfer may be enhanced by orienting the CNTs of the directional CNT arrays to extend substantially parallel to a primary direction of coolant flow when the arrays are located at or adjacent the coolant-contacted surfaces of radiator 20. This may be more fully appreciated by referring to FIG. 2 wherein a striated or lined pattern 53 is shown on the air-exposed surfaces of fin structure 30(3) to indicate the manner in which the CNTs included within directional CNT arrays can extend along length of fin structure 30(1)-(3) and, therefore, substantially perpendicular to the direction of airflow along airflow paths 50. A similar striated pattern 55 is shown on the coolant-contacted surfaces of coolant-conducting tube 28(1)-(4) and the coolant-contacted surfaces of in-tube structures 44(1)-(4). Striated pattern 55 further indicates the manner in which the CNTs of directional CNT arrays can extend along the respective lengths of tubes 28(1)-(4) and fin structures 30(1)-(3) and, therefore, substantially parallel to the direction of coolant flow 48 in at least some embodiments of nanotube radiator 20.
As indicated above, the CNT-containing structures integrated into nanotube radiator 20 (FIGS. 1-2) can be freestanding or deposited coatings in an embodiment. As shown in FIG. 2, striated patterns 53 and 55 further indicate that the below-described nanotube coatings (when present) are usefully, but not necessarily imparted with intentionally roughened, texturized, or non-planar topologies. As appearing herein, the term “non-planar” is defined as a surface topology having feature heights or depths exceeding 1 micron (μm). In certain embodiments, this may be accomplished by arranging the CNTs in macro-scale bundles extending substantially parallel to coolant-conducting tubes 28(1)-(4) (when the below-described nanotube coatings are located within tubes 28(1)-(4)) and/or substantially perpendicular to the direction of airflow along airflow paths 50. Such a texturized or non-planar surface topology may help promote heat transfer by creating controlled regions of localized turbulent flow and/or by increasing the cumulative surface area of the nanotube coating available for convective heat transfer. Furthermore, in certain embodiments, the thermal transfer properties of the CNT-containing structures may be further enhanced by incorporating SWCNTs into nanotube radiator 20 having average diameters between 1 and 100 nm and, preferably, between 1 and 20 nm. The foregoing notwithstanding, embodiments of the nanotube radiator and heat exchanger can contain other types of CNTs (e.g., multi-walled CNTs) having diameters greater than or less than the aforementioned ranges.
In further embodiments, nanotube radiator 20 can include other unique structural features further enhancing the thermal performance and/or mechanical properties of radiator 20. For example, in embodiments wherein nanotube radiator 20 includes fin structures 30(1)-(3) interspersed with coolant-conducting tubes 28(1)-(4), a thermally-conductive adhesive can be applied at the junctures or interfaces between fin structures 30(1)-(3) and coolant-conducting tubes 28(1)-(4) to enhance conductive heat transfer at the tube-fin interfaces. Further emphasizing this point, FIG. 2 illustrates a body of thermally-conductive adhesive 54 applied between fin structure 30(3) and the exterior of coolant-conducting tube 28(4). Although not shown in FIGS. 1-2 for illustrative clarity, similar bodies of thermally-conductive adhesive can likewise be applied at the interfaces between coolant-conducting tubes 28(1)-(3) and fin structures 30(1)-(2). Thermally-conductive adhesive 54 can be, for example, a carbon allotrope-filled epoxy, such as a CNT- or graphene-filled epoxy. In other embodiments, a different type of thermally-conductive epoxy can be utilized, such as a copper-filled or silver-filled epoxy.
As noted above, CNTs can be integrated into radiator core 26 and, more generally, nanotube radiator 20 in various different locations or regions to promote heat transfer between the airflow contacting radiator 20 and the coolant circulated therethrough. More specifically, CNTs can be introduced into selected regions or targeted locations within radiator core 26, such as those regions identified in FIG. 2 by circles I-IV. Here, circle I (FIG. 2) denotes or identifies the regions of coolant-conducting tubes 28(1)-(4) and/or fin structures 30(1)-(3) across which conductive heat transfer occurs. Circle II denotes the regions of in-tube structures 44(1)-(4) across which convective heat transfer occurs with the coolant flow. Circle III denotes the regions of coolant-conducting tubes 28(1)-(4) and fin structures 30(1)-(3) across which conductive heat transfer occurs and, specifically, the fin-tube interface regions formed between coolant-conducting tubes 28(1)-(4) and fin structures 30(1)-(3). For example, a CNT-containing can be applied at this interface and within thermal transfer paths extending from interior surfaces of tubes 28(1)-(4) to exterior surfaces of fin structure 30(1)-(3). Finally, circle IV denotes the portions of fin structures 30(1)-(3) across which convective heat transfer occurs. In further embodiments, CNT-containing structures can be integrated into other portions of nanotube radiator 20, as well. For example, as indicated in FIG. 1 by circle V, CNTs can also be applied to or integrated into the surfaces or bodies of end tanks 22 and 24 in certain embodiments. Manners in which the CNT-containing structures can be integrated into the aforementioned regions of nanotube radiator 20 and/or other regions of radiator 20 will now be described in conjunction with FIGS. 3-5.
Examples of Integration of Cnts into the Nanotube Radiator or Other Heat Exchanger
FIGS. 3-5 depict simplified cross-sectional views of exemplary CNT-containing structures, which are partially shown and which may be included in nanotube radiator 20 (FIGS. 1-2). With initial reference to FIG. 3, there is shown a simplified cross-sectional view of a region of a CNT-containing structure in the form of a nanotube coating 60. Nanotube coating 60 is applied over a selected surface 68 of a radiator component 70. As illustrated, radiator component 70 may be representative of any one of coolant-conducting tubes 28(1)-(4), of fin structures 30(1)-(3), of in-tube structures 44(1)-(4), or of end tanks 22, 24. Furthermore, selected surface 68 is intended to represent any principal surface of radiator component 70 to which nanotube coating 60 is desirably applied. For example, if radiator component 70 is one of coolant-conducting tubes 28(1)-(4), surface 68 can be interior or exterior surface of the tube. If, instead, radiator component 70 is one of fin structures 30(1)-(3), surface 68 can be either the upper or lower principal surface of the fin structure (referring to the orientation shown in FIGS. 1-2). Nanotube coating 60 is described in this way to emphasize that coating 60 (and the other nanotube coatings described herein) can be applied to any one of a number of different surfaces included within radiator 20. In at least some embodiments of nanotube radiator 20 (FIGS. 1-2), nanotube coating 60, other nanotube coatings (e.g., nanotube coating 80 described below in conjunction with FIG. 4), and the CNT-impregnated structures (e.g., structure 90 described below in conjunction with FIG. 5) can be contained within the same radiator or heat exchanger. The CNT-coatings and CNT-impregnated structures shown in FIGS. 3-5 are therefore not mutually exclusive in the context of the present document.
With continued reference to the illustrated example shown in FIG. 3, nanotube coating 60 includes at least three layers: (i) a CNT array layer 62, (ii) a substrate layer 64, and (iii) a bond layer 66. In this particular example, nanotube coating 60 is a freestanding thin film coating. Nanotube coating 60 may be obtained from a supplier as a freestanding thin film coating in a sheet- or strip-form and attached to surface 68 of radiator component 70 by a bond layer 66. Bond layer 66 can be an adhesive backing provided on the freestanding thin film or, instead, an epoxy dispensed onto surface 68 prior to application of CNT array layer 62 and substrate layer 64. The manner in which nanotube coating 60 is produced will vary amongst embodiments, as will the composition of coating layers. However, by way of non-limiting example, a physical and chemical vapor deposition technique can be utilized to deposit CNT array layer 62 on substrate layer 64, which can be composed any material on which CNT array layer 62 can be deposited including carbon (C), silicon (Si), and germanium (Ge) based substrates.
CNT array layer 62 may be the outermost layer within nanotube coating 60, in which case array layer 62 may include a fluid-contacted surface 72; that is, an outer principal surface contacted by either coolant flow or airflow during operation of nanotube radiator 20. Alternatively, as shown in phantom in FIG. 3, an outer terminal layer 74 may be disposed over surface 72 of CNT array layer 62 such that the outer surface of layer 74 (rather than surface 72 of array layer 62) is intimately contacted by fluid flow during radiator operation. It may be particularly useful to include outer terminal layer 74 when nanotube coating 60 is contacted by coolant flow and coating 60 is applied to an interior surface of a coolant-conducting tube (e.g., tubes 28(1)-(4) shown in FIGS. 1-3) and/or to a surface of an in-tube structure (e.g., in-tube structures 44(1)-44(4) shown in FIG. 2). Outer terminal layer 74 may help reduce the likelihood of CNT dislodgement or shedding in the presence of the coolant flow carrying abrasive contaminants, which may tend to gradually erode and/or corrode material from coating 60 over time. In this manner, outer terminal layer 74 may serve as a sacrificial liner decreasing the likelihood of CNT shedding, contamination of the coolant flow, and undesired interactions with other coolant-contacted components within the coolant flow circuit, such as gaskets and seals. When present, outer terminal layer 74 is ideally formed to be relatively thin to minimize interference with the thermal interaction between the coolant flow and CNT array layer 62. Outer terminal layer 74 can be formulated from any material suitable for performing this function including, for example, an aluminide. In further embodiments, nanotube coating 60 may lack such a sealant layer or sacrificial liner such direct contact is permitted between CNT array layer 62 and the airflow or coolant flow conducted through nanotube radiator 20.
In the exemplary embodiment shown in FIG. 2, CNT array layer 62 contains an organized array or network of CNTs, such as an array of SWCNT aligned to extend in a common direction. With reference to the orientation of the cross-sectional view shown in FIG. 3, specifically, the CNTs contained within CNT array layer 62 extend into and out of the page or, stated differently, along the Y-axis identified by coordinate legend 76. In embodiments wherein nanotube coating 60 is contacted by airflow, thermal transfer may be enhanced orienting the CNTs within CNT array layer 62 to extend perpendicular to the direction of airflow; that is, along the X-axis identified in FIG. 3 by coordinate legend 76. Conversely, in embodiments wherein nanotube coating 60 is contacted by coolant flow, thermal transfer may be enhanced orienting the CNTs within CNT array layer 62 to extend substantially parallel to the direction of coolant flow; e.g., along the Y-axis identified by coordinate legend 76. In further embodiments, CNT array layer 62 may be isotropic or non-directional such that the CNTs contained within layer 62 are randomly oriented.
As described above, nanotube coating 60 can be applied to selected surface 68 of radiator component 70 (e.g., any one of coolant-conducting tubes 28(1)-(4), fin structures 30(1)-(3), in-tube structures 44(1)-(4), and/or end tanks 22 and 24) as a freestanding thin film. In further embodiments, the nanotube coating can be deposited directly on one or more selected surfaces of any number of radiator components contained within nanotube radiator 20. Further emphasizing this point, FIG. 4 is a cross-sectional view illustrating a second exemplary nanotube coating 80 deposited directly on selected surface 68 of radiator component 70. Nanotube coating 80 includes a metallic body or matrix 82 and a plurality of CNTs 84, which are embedded within metallic matrix 82 and may be (but are not necessarily) dispersed substantially evenly throughout matrix 82. In this embodiment, metallic matrix 82 can be composed of a metal or alloy selected for chemical compatibility with the parent material of radiator component 70. For example, if radiator component 70 is composed of an aluminum-based alloy, matrix 82 may also be composed of an aluminum-based alloy having a formulation similar or identical to radiator component 70. CNTs 84 may be embedded within (e.g., dispersed throughout) matrix 82 utilizing any of a number of different techniques, a few examples of which are described below.
In one embodiment, nanotube coating 80 is applied over surface 68 of radiator component 70 utilizing a wet state application technique. In this case, a flowable or wet state coating precursor material is first obtained, which may include CNTs 84 and metallic particles dispersed within an organic binder. The coating precursor material may also contain a solvent or liquid carrier (e.g., a high molecular weight alcohol) transforming the precursor material to a wet or flowable state. The volume of solvent or liquid carrier contained within the coating precursor material can be adjusted to tailor of the viscosity of the precursor material to the selected wet state application technique. The coating precursor material may contain sufficient liquid to create a paste, slurry, or paint, depending upon the application technique used. The coating precursor material is then applied by, for example, screen printing, dipping, spraying, or doctor blade application. After application of the wet state coating material, a drying process is carried-out to remove excess liquid from the coating material. A sintering, cladding, brazing, or other powder consolidation process is then carried-out to consolidate and densify the metallic particles within the coating precursor layer and thereby yield matrix 82 containing CNTs 84, as generally shown in FIG. 4.
In further embodiments, nanotube coating 80 can be deposited utilizing a spray process, such as a thermal spray, cold spray, or electrostatic spray process. An electrostatic spray process may be useful in embodiments wherein CNTs 84 are desirably aligned or imparted with a controlled directionality to, for example, promote convective thermal transfer by orientating CNTs 84 to, for example, extend predominately perpendicular to the direction of airflow along an airflow-contacted surface or to extend predominately parallel to the direction of coolant flow along a coolant-contacted surface, as previously described. Alternatively, nanotube coating 80 can be deposited directly on surface 68 of radiator component 70 utilizing a physical or chemical vapor deposition technique, such as electron beam physical vapor deposition (EB-PVD). If desired, nanotube coating 80 can be produced such that the concentration of CNTs 84 varies, as taken through the thickness of coating 80. In this case, and as indicated in FIG. 4, nanotube coating 80 is formed such that the nanotube concentration of coating 80 decreases with increasing proximity to a fluid-contacted surface 86. This may decrease the likelihood of CNT shedding in implementations wherein surface 86 contacted in by coolant flow, albeit with a corresponding penalty in heat transfer efficiency. In such embodiments, the CNT content of the nanotube coating 80 may vary in a gradual manner or, instead, in a more pronounced, stepwise manner. Suitable processes for producing nanotube coating 80 to include such a graded CNT content include, but are not limited to, spray processes wherein the feed rate of different powder feed sources having different CNT contents are varied to achieve the desired gradient. In still further embodiments, the CNT concentration with nanotube coating 80 can increase when moving from the center of the coating 80 toward either or both of the opposing principal surfaces (e.g., inner and outer surfaces) of the coating 80.
Several different manners have thus been described for transferring, depositing, or otherwise applying CNT-containing coatings to selected surfaces of one or more radiator components included within nanotube radiator 20. In further embodiments, other techniques or approaches can be utilized to integrate CNTs into one or more targeted regions within the nanotube heat exchanger, such as nanotube radiator 20 shown in FIGS. 1-2. For example, in certain implementations, CNTs may be embedded within or impregnated directly into the bodies of one or more radiator components in addition to or in lieu of the application of CNT-containing coatings. In such embodiments, CNTs may be embedded within any of coolant-conducting tubes 28(1)-(4), fin structures 30(1)-(3), in-tube structures 44(1)-(4), and/or end tanks 22, 24, which may then be considered “CNT-containing structures.” Further emphasizing this point, FIG. 5 provides a simplified cross-sectional view of a radiator component 90, which is formed as a structure containing a plurality of CNTs 92 (only a few of which are labeled for clarity) dispersed throughout a metallic matrix 94. Radiator component 90 can be any of coolant-conducting tubes 28(1)-(4), fin structures 30(1)-(3), in-tube structures 44(1)-(4), and end tanks 22, 24. In one implementation, radiator component 90 is produced utilizing a powder metallurgy technique wherein a powder bed containing CNTs 92 in particulate form is consolidated into a desired final shape or a near net shape, which is then machined to a final geometry. Suitable powder metallurgy techniques include, but are not limited to, Hot Isostatic Pressing (HIP), Spark Plasma Sintering (SPS), and other sintering processes. As a still further possibility, radiator component 90 can be produced utilizing Direct Metal Laser Sintering (DMLS) or another additive manufacturing technique. Once again, a relatively thin sealant layer or sacrificial liner 96 can be applied over one or more surfaces of radiator component 90, such as any coolant-contacted surfaces of component 90 that may otherwise be prone to CNT shedding or dislodgement.
Additional Examples of In-Tube Structures Containing Carbon Nanotubes
There has thus been described multiple exemplary embodiments of a nanotube radiator containing CNTs, which are incorporated into one or more regions of the radiator core. In many cases, it may be desirable to introduce CNTs into the coolant-conducting tubes of the radiator. In this regard, nanotube coatings can be applied to surfaces of in-tube structures or dispersed throughout the bodies of in-tube structures when produced as (e.g., sintered) CNT-containing bodies. In further embodiments, other types of in-tube structures can be produced and inserted into the coolant-conducting tubes of the nanotube radiator or heat exchanger. For example, as shown in FIG. 6, further embodiments of the nanotube radiator can be produced to include a coolant-conducting tube 100 within which one or more honeycomb-shaped structures 102 are mounted. As can be seen in FIG. 6, each honeycomb structures 102 includes a honeycomb-shaped body 104 having a number of flow channels, which extend along the length of coolant-conducting tube 100 and which are lined with nanotube coatings 106 of the type described above. In one embodiment, honeycomb-shaped bodies 104 are produced from a number of stamped metal sheets, which are bonded, tack welded, or otherwise joined together to yield bodies 104. In such embodiments, nanotube coatings 106 may be formed on, adhered to, or otherwise applied to the stamped metal sheets prior to assembly into honeycomb structures 102 to create coatings 106 within the longitudinally-extending channels through the bodies 104 of structures 102. In other embodiments, honeycomb structures 104 (or the other in-tube structures described herein) can be produced from a material (e.g., carbon) onto which a CNT layer or array can be directly deposited utilizing one of the deposition processes described above, such as a vapor deposition process. Honeycomb structures 104 serve to increase the surface area available for fluid-solid interaction and convective heat transfer. As was previously the case, coolant-conducting tube 100 can be contained within further embodiments of nanotube radiator 20 (FIGS. 1-5) or another radiator employed within the HVAC system of a vehicle or other mobile platform.
Additional Examples of Nanotube Radiators
FIG. 7 is an isometric view of a nanotube radiator 110, which is suitable for usage within an HVAC system and which is illustrated in accordance with a further exemplary embodiment of the present disclosure. Nanotube radiator 110 includes first and second halves 112, 114, which are shown in an axially-separated relationship to better illustrate an interior portion of nanotube radiator 110. Radiator half 112 includes an end tank portion 116, a core portion 118, and an overflow port 120. Similarly, radiator half 114 includes an end tank portion 122 and a core portion 124, which may matingly engage core portion 118 of radiator half 112 when radiator 110 is fully assembled. A first conduit 126 is joined to and supplies coolant to end tank portion 116 of radiator half 112, while a second conduit 128 is joined to and receives coolant from end tank portion 122 of radiator half 114. Core portion 122 and core portion 124 combine to form a relatively large coolant-conducting tube 122, 124. Multiple in-tube structures can be mounted within coolant-conducting tube 122, 124 by, for example, inserting the in-tube structures and into and expanding the structures within coolant-conducting tube 122, 124. As a more specific example, and as can be seen on the right side of FIG. 7, radiator half 114 contains multiple in-tube structures 130(1)-(5) mounted in a concentric (tube-in-tube) configuration by radial tie bars 132 (only a few of which are labeled for clarity). Radiator half 112 may be substantially identical to radiator half 114; thus, the following description is equally applicable thereto. Core portions 118, 124 of radiator halves 112, 114, respectively, each have a “flat tube” shape in the illustrated example; that is, substantially ovular geometries with their major axes positioned along the direction of airflow. During operation of nanotube radiator 110, coolant flow is circulated through radiator 110 in the direction indicated by arrows 134, while airflow is directed over and against the outer surface of radiator 100, as indicated by arrows 138. As further indicated in FIG. 7, a liquid coolant containing particulate CNTs 136 (enlarged for purposes of illustration) can be circulated through radiator 110 during operation thereof.
As does nanotube radiator 20 described above in conjunction with FIGS. 1-2, nanotube radiator 110 contains a number of CNT-containing structures, which are incorporated into one or more regions of the heat exchanger core formed by mating core portions 118, 124. In the case of nanotube radiator 110, the CNT-containing structures assume the form of outer nanotube coatings 140 applied over the outer circumferential surfaces 141 of radiator halves 112, 114. Additionally, inner nanotube coatings 142 (only three of which are labeled in FIG. 7) are applied over selected surfaces of in-tube structures 130(1)-(5), respectively. Inner nanotube coatings 142 can be coatings that are deposited directly on in-tube structures 130(1)-(5). Alternatively, nanotube coatings 142 can be freestanding coatings transferred to selected regions of nanotube radiator 110 and bonded thereto utilizing, for example, an adhesive backing or a dispensed epoxy. In the exemplary embodiment shown in FIG. 7, nanotube coatings 140 are freestanding coatings that are acquired in strip or sheet form, trimmed to a desired length, and then applied to the outer circumferential surface of radiator 110. Nanotube coatings 140 are not flush with the surfaces of radiator 110 in this implementation, but instead are formed to have sinusoidal or undulating geometries. In this manner, nanotube coatings 140 may have peak or crest portions 144 of nanotube coatings 140, which serve as fin structures increasing perturbation of the air flowing over coatings 140. This, in turn, may promote convective heat transfer to the airflow directed over the outer surfaces of nanotube radiator 110. When nanotube radiator 110 is utilized as a vehicular radiator, specifically, airflow may be directed over and against the outer surfaces of radiator 110 by one or more fans (not shown) and/or by forward movement of the vehicle (ram airflow).
Example Methods for Manufacturing a Nanotube Radiator and Other Heat Exchangers
While primarily focusing on a nanotube radiator or heat exchanger itself, the foregoing description has also provided methods for manufacturing a nanotube heat exchanger. One such heat exchanger fabrication method 160 is further set-forth by the flowchart illustrated in FIG. 8. Method 160 commences with obtaining the heat exchanger components (STEP 162). For example, referring to nanotube radiator 20 shown in FIGS. 1-2, tubes 28(1)-(4), fin structures 30(1)-(3), and end tanks 22, 24 may be purchased, produced, or otherwise obtained during STEP 162. Next, at STEP 164, a CNT-containing coating may be applied to selected surfaces of the heat exchanger components. For example, freestanding or despotized CNT-containing coatings similar to those described above in conjunction with FIGS. 2-5 can be applied to any or all of the radiator components prior to assembly. In further embodiments, one or more of the radiator components may be produced to contain particulate CNTs (e.g., produced as a CNT-containing, sintered structure), in which case STEPS 162 and 164 may overlap. Finally, at STEP 166, the heat exchanger components are assembled to produce the finished heat exchanger. In the case of nanotube radiator 20, such a manufacturing process may enable high temperature exposure of the CNT-containing structures to be minimized or altogether avoided. In one embodiment, during STEP 166, the nanotube radiator is produced utilizing a non-brazed, mechanically-attached build including joints that are bonded and sealed utilizing physical structures (e.g., gaskets) and/or bonding materials, such as graphene- or CNT-filled epoxies.
The foregoing notwithstanding, the nanotube heat exchangers and radiators can be fabricated utilizing various other manufacturing approaches in further embodiments. Consider, for example, an embodiment wherein the nanotube heat exchanger assumes the form of a radiator containing aluminum-based components, which are assembled utilizing a brazing process during which temperatures can approach the crystallization point of aluminum. In this case, undesired interactions of the CNTs with the aluminum-based components can potentially occur during the brazing process. Should this be of concern, the CNT-containing structures or coatings can be applied after assembly of the radiator and brazing. For example, after brazing, CNT-containing coatings or bodies can be applied to selected surfaces of the radiator by standard painting, electrostatic painting, application of a CNT-filled epoxy, or utilizing another coating application technique of the type described above. In other cases, it may be desirable to apply CNT-containing coatings to selected radiator surfaces that are no longer physically accessible after the brazing process, such as the interior surfaces of the coolant-conducting tubes or surfaces of in-tube structures contained within the coolant-conducting tubes. In this case, the nanotube radiator can be produced as a “mechanically attached” radiator wherein the joints between the end tanks/headers and the coolant-conducting tubes are sealed during or after assembly of the radiator utilizing an epoxy, utilizing gaskets, or in another manner not requiring exposure of the radiator components to highly elevated temperatures associated with brazing. Utilizing such an approach, the CNT-containing coatings can be applied to selected surfaces of the radiator components before, during, or after assembly of the radiator components (or heat exchanger components) into the finished nanotube radiator (or other nanotube heat exchanger). For example, in a further embodiment, a turbulator to which a CNTs has been grown, deposited, or otherwise applied can be inserted into each of the coolant-conducting tubes after partial assembly of the radiator core and secured in place by interference fit, such as by expanding the turbulator once properly positioned within a particular coolant-conducting tube.
CONCLUSION
There have thus been described multiple embodiments of nanotube radiators and, more generally, nanotube heat exchangers into which CNTs are strategically incorporated as, for example, CNT-containing coatings or other CNT-containing structures. In implementations wherein the nanotube heat exchanger assumes the form of a radiator, the enhanced heat rejection capabilities of the nanotube radiator afforded by the CNT-containing structures may enable a reduction in the size and weight of the radiator, while still satisfying the heat rejection needs of the vehicle (or other platform) in which the nanotube radiator is employed. The decreased size and enhanced heat rejection capabilities of the vehicular nanotube radiator may also enable a reduction in the size, weight, and/or power requirements of overflow tanks, fans, and other such components included within a vehicle's HVAC system. Embodiments of the nanotube radiator and, more generally, the nanotube heat exchangers described herein may also have enhanced tensile strengths, increased pressure capabilities, and other desirable characteristics.
While at least one exemplary embodiment has been presented in the foregoing Detailed Description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing Detailed Description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. Various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set-forth in the appended Claims.
Patent Application
20170211899
