Patent Application
20120261106
The present invention relates to heat exchangers and reactors.
Finned Compact Heat Exchangers.
Heat Exchanger (HEX) size and weight with gas flows are typically limited by the low conductivity of the gas and resulting lower gas side heat transfer coefficients. In these cases, the surface area of plates that separate the fluids, or bound the source of heat (e.g. electronics component) or cooling, is insufficient to meet performance requirements. Fins are added to the separating plate, or primary surface area, to add surface area and reach out into the gas flow. This facilitates the flow of heat from the gas to the separating plate. Fins can increase surface area exposed to the gas by multiple factors. In fact, in some examples, fins represent over 80% of the available surface area. While the fins provide enhanced surface area and heat transfer, the added area also adds weight, volume, pressure drop and cost. Therefore, fin configurations need to be carefully chosen to optimize heat transfer while minimizing volume, weight, pressure drop and cost.
Thermal Efficiency (TE), which is the ratio of the heat transfer coefficient to the friction, or pressure drop, factor, is an important measure of heat exchanger performance, since there is always a tradeoff between heat transfer effectiveness and pumping power losses. Pumping power losses are a serious limitation in many cases. Therefore, a fin configuration that minimizes pressure drop, or pumping power, for a given heat transfer is highly desired. In these cases, the HEX can be made more compact (lower volume and higher face velocity cases), without causing excessive pumping power. Table 1 lists the thermal efficiencies of several conventional fins, including plain plate, perforated plate, wavy plate, and louvered fins. The thermal efficiency (TE) in the table is defined as the heat transfer Stanton (St) number times Prandtl (Pr) number, to the two-thirds power, over the friction (f) coefficient. The non-dimensional St and Pr combination is a measure of the heat transfer for the fin configuration of interest, with the non-dimensional f playing a similar role for pressure drop. Plain plate fins are very simple, and relatively easy to form. The perforated fin requires that small holes be formed in the plain plate fin, which makes this fin more expensive. The wavy fin configuration doesn't require holes, but special tooling is required to form the wavy surfaces that need to be fitted between separation plates, or on tubes. Lastly, louvered fins are the most complex to form and probably the most expensive.
Plain fins simply increase the amount of surface area exposed to the gas, and through heat conduction to the fluid in adjoining tubes or channels, increase the heat transfer. Well-known formulas can be used to define the effectiveness of the increased fin surface area, or fin efficiency. With the plain fin, a boundary layer develops on the plate that has a high heat transfer coefficient at the front of the plate where the boundary layer starts and is very thin. However, the coefficient drops substantially with distance, as the boundary layer thickens. On average, the heat transfer coefficient is then relatively low over the whole plate. With perforated fins, the smooth boundary layer of the plain fin becomes interrupted at the perforations. As the boundary layer restarts at each perforation, the heat transfer coefficient again reaches a locally high level. With the constant restarting of the boundary layer, the average heat transfer coefficient is increased over that for the plain fin. This is very beneficial. However, because of the restarting of the boundary layer, friction, or pressure drop, also increases. However, the net overall effect is beneficial, as noted by the TE value in Table 1. As shown, the perforated plate fin has the best Thermal Efficiency (TE) of all of the cases. Therefore, for a given pressure drop, perforated plates would produce the highest heat transfer.
Wavy wall and louvered fin thermal efficiencies are not as high as that for the perforated fin, as indicated in Table 1. It is speculated that the disruption of the boundary layer in the perforated fin case is modest, and the overall pressure drop, consisting of both form (i.e. fluid separation zones) and surface friction contributions, is not significantly increased versus the plain plate fin case. The net result is a higher heat transfer than a plain fin and only modestly higher pressure drop, giving enhanced thermal efficiency. In contrast, the louvered fins have substantial protrusions into the flow. These create substantial flow disruptions and flow separation. Heat transfer is increased as a result of these disruptions. However, pressure drop is also substantially increased, resulting in a net reduction of thermal efficiency. For the wavy wall case, flow separations can also be induced as the flow moves over the “waves”, resulting in improved heat transfer, but also a reduction in thermal efficiency relative to the perforated plate case. In conclusion, the perforated plate yields the best thermal efficiency, as a result of boundary layer disruption, but not bulk flow disruption. This high thermal efficiency is important to controlling pressure drop in compact HEXs.
As noted above, for optimal thermal efficiency, the boundary layer along the fin should be disrupted, but large scale flow disruptions should be avoided. The greater the frequency of boundary layer disruption, the higher the average heat transfer coefficient, for a nearly fixed thermal efficiency. Therefore, a plate with many perforations might be best. However, it is difficult to form many perforations, and fin cost could substantially increase.
Foam-Based Heat Exchangers.
As noted above, compact finned heat exchangers are well developed and proven, but they do not offer heat transfer and pressure drop performance that can meet advanced cooling or heating requirements. To achieve goals for these applications, substantial advances are required in heat exchanger materials and configurations. As a significant departure from compact finned heat exchangers, open cell metal and graphite foams have been put forward as advanced thermal management solutions for challenging applications, such as fusion reactors. An open cell foam structure viewed in close-up shows small structures in the open cell foam that adds substantial surface area for heat transfer. While offering orders of magnitude increases in surface area and heat transfer capability, these materials have correspondingly much higher pressure drop than is desired for many applications. Also, these materials have very thin ligaments that connect with the adjoining tubes or channels that contain the heat transfer fluids. This limits the effectiveness of the high surface area by bottle-necking the flow of heat to the fluid. The result is a lower thermal efficiency compared to the fin configurations listed in Table 1. In addition, these materials are very expensive.
What is needed is a new material that has the heat transfer performance of open-celled foams, with a pressure drop that is much lower per heat transferred, as well as a lower volume, weight, and a much lower cost.
As indicated above, high performance compact heat exchangers and reactors need substantial surface area in contact with the fluid. This is typically provided by fins that extend out into the flow and provide extra area that augments heat flow to or from the separating plate, or boundary, that is the heat sink or heat source, respectively. While heat flow is augmented, the design of the fins can constrain the flow of heat as a result of conduction limits through the fins. This is quantified by fin effectiveness, which is equal to the ratio of the heat flow per area through the fin surface divided by that achieved at the fin and separating plate contact area. Unless the fin effectiveness can be maintained at high levels, fin area will be excessive, resulting in excessive weight, pressure drop and cost to achieve a given heat transfer.
An innovative and low-cost approach to fin manufacture has been discovered, called Non-Isotropic (or anisotropic) Structure for a Heat Exchanger (NISHEX), that uses a non-isotropic fin structure to simultaneously maximize heat transfer and weight, while minimizing pressure drop and cost. In this approach, small scale fin structures that have high heat transfer are implemented near the surface, where distance from the surface is limited and fin effectiveness is high. With distance away from the surface, larger structures are utilized to maintain high effectiveness throughout the structure. By ordering the structure in this way, optimal use of materials and maximum heat transfer are achieved for the minimum pressure drop and cost. Because the needed non-isotropic features can be achieved by a variety of construction methods and materials, the process is very flexible and addresses many applications. Heat sink, radiator, condenser, evaporator and many other applications can be considered. Also, by inclusion of wash coat and catalysts, simultaneous heat transfer and reaction can be considered.
A NISHEX is a fin having a non-isotropic structure to optimize heat transport properties. The fins of a NISHEX are formed by a first structure and a second structure interconnected to, and arranged parallel to the first structure. A NISHEX structure is characterized, at least in part, by frequent boundary layer restarts and low pressure differences across a fin surface, and avoidance of heat conduction bottlenecks near a heat sink, heat source or separation plate surface, while at the same time maintaining an optimal fin effectiveness due to the novel non-isotropic properties of fin structures constructed in accordance to the invention.
In preferred embodiments, first and second elongate fin structures are provided by commercially available woven wire meshes, examples of which are illustrated in
All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. To the extent there are any inconsistent usages of words and/or phrases between an incorporated publication or patent and the present specification, these words and/or phrases will have a meaning that is consistent with the manner in which they are used in the present specification.
Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
Theory
Through recent investigations of foams and other enhanced heat transfer methods, it has been concluded that a primary limitation of foam is a result of its isotropic nature. Heat transfer from a sink to air has to occur via heat flow from the source through structures that reach out into the air-flow. These structures can be a bottleneck to heat transfer, which is commonly termed “low fin effectiveness”.
As shown in
For optimal air heat transfer, small structures are beneficial to take advantage of the greater surface area per volume and the inverse relationship of the heat transfer coefficient to small scales.
For an isotropic structure attached to adjoining tubes or channels containing heat transfer fluids, there is then a basic conflict between optimal heat removal to the air and the bottlenecking of heat flow through the structure. Shrinking the height of the heat exchanger fin structure (e.g. isotropic foam), and thereby forcing the air to flow close to the surface of the heat source, can better balance this conflict. However, gas flow velocity through the structure and thereby pressure drop, which is a power function of velocity, increases beyond acceptable levels. In contrast, using a much taller isotropic structure, to stay within the gas flow pressure drop requirement then results in the addition of significant material that has diminished heat transfer contribution, but a significant contribution to pressure drop and weight. Given these limitations, an approach was found to make a non-isotropic material structure at low cost that is a significant advance beyond isotropic structures, such as foams. This approach can be used with plate type fins, described in
Non-Isotropic Wire Mesh for Fins
To achieve the same effect as a highly perforated plate fin at low cost, fins may be formed using a woven wire material. Examples of this type of material are illustrated in
In addition, the diameter of the wire for the weaves illustrated in
The wire mesh material can be corrugated into channels that are then bonded to flattened tubes or channel, which contain fluid, or to a boundary plate, to which a heat generating component (e.g. electronic component) is attached. In one embodiment a NISHEX 10 uses a highly anisotropic wire mesh, as illustrated in
Almost the entire mass of the wire mesh used to form NISHEX 10 is in the wires 12 that extend perpendicular to the fluid separation, heat source, source plate or boundary plate 15, with comparatively few number of, and thinner wires 14 parallel to the mean flow direction, and adequate to hold the wires 12 together ahead of bonding to the separation plates. These smaller, parallel wires 14 act like fins-on-fins, and provide structural stability, which has benefit. However, if wires 14 are equal in number to the perpendicular wires 12 and had the same diameter as wires 12, i.e., the weight and pressure drop for a given heat transfer will not be as optimal. Therefore, the anisotropic approach of using fewer connecting wires and/or wires of smaller diameter (compared to the perpendicular wires) has better heat transfer, pressure drop and volume and weight characteristics than a uniform mesh. Also, this structure will be superior to open cell isotropic metal foam based fins. The ligaments, or wires in metal foams, are isotropic in three dimensions; that is, they all have similar heat transport capacity in three directions. Therefore, foams have many “fins-on-fins”, relative to the structure shown in
According to the disclosure, a NISHEX utilizes a non-isotropic material configuration that yields a higher level of performance than conventional fins or foams.
The NISHEX 20 has five stacked wire mesh fin layers 22, 24, 26, 28 and 30 with smaller diameter wire fins used closer to the boundary plate 15. Each of the layers 22-30 are corrugated in a direction perpendicular to the flow direction, as in
The wire meshes 22, 24, 26, 28, 30 are placed on top of each other and bonded to the source plate 15 at portion 21b. The portions 21b (
The wires 22, 24, 26, 28 and 30 of the fin element 21a conduct the majority of the heat perpendicular to the source plate 15, with a few and/or smaller diameter wires (not shown) of the respective wire meshes holding wires 22, 24, 26, 28 and 30 together, as with the wires 14 of the wire mesh that holds wires 12 together in
Accordingly, NISHEX 20 may be constructed of anisotropic wire meshes, e.g., slotted wire meshes, which are folded or corrugated to achieve a multiplicity of parallel channels of predominantly wires perpendicular to the plate 15, as described above, to carry heat into, or remove heat from gas flowing through the channels of the HEX (flow direction being indicated in
In the above example, the multiple slotted wire mesh layers 22, 24, 26, 28 and 30 (as depicted in
Compared to Foams and Wall Fins
While boundary layer restarts are optimized with a NISHEX, the configuration also optimizes thermal efficiency, or minimizes pressure drop for a given heat transfer. Louvered and wavy wall fins have been shown to produce high heat transfer. However, because they produce large-scale flow disturbances, including separated flow regions, and block the flow and increase local velocity, they promote pressure drop more aggressively than heat transfer. Therefore, thermal efficiency is low. In contrast, NISHEX channels are parallel to the flow and avoid large-scale flow disturbances and flow blockage. Also, because the wire mesh used for fin layers has many open spaces between wires, the fin layers cannot support pressure differences across the plane of the material. Therefore, large-scale separation regions that create high pressure drop cannot be formed with NISHEX 20. In contrast, solid plate type conventional fins can act like aircraft wings under stall conditions, when the entering heat exchanger flow is at an angle of attack, and large separation regions, flow blockage, high velocity local flows and associated pressure drops, can be created. Since all practical flow situations have some non-parallel flow, pressure drops will be higher with solid plate fins. Pressure drops are reduced using the NISHEX 20 structure of
Conventional open cell foams have high heat transfer by having a large number of cell ligaments in contact with the gas flow. However, they also have high-pressure drop and low thermal efficiency. In one respect, foams may be considered as isotropic structure with ligaments equally distributed in three dimensions, since the material configuration is similar in any direction. Only a limited number of ligaments are in contact with the heat sink. Those attached to the plate will readily channel heat to the cooling air. Ligaments branching from these can be considered “fins-on-fins”. While providing some benefit, “fins-on-fins” effectiveness is constrained because of the bottleneck of heat transfer at the plate attachment point. Unfortunately, besides adding more weight per heat transfer, these “fins-on-fins” contribute equally to pressure drop relative to ligaments attached to the plate. Therefore, conventional isotropic foams will have high-pressure drop per heat transfer, or a low thermal efficiency.
A NISHEX having one or more mesh wire layers forming fin elements optimizes material use to maximize heat transfer while minimizing pressure drop, by having each of the fin element 21a in direct contact with the boundary plate 15. Furthermore, since it represents the optimal use of material per heat transfer, it reduces material weight and thereby cost. For example, NISHEX 20 may be constructed of an anisotropic woven-wire mesh that is folded into needed shapes by conventional and cheap fin-forming equipment. Therefore, forming costs are low. Relative to bonding, well proven similar alloy fin brazing techniques can be utilized to bond all mesh layers to the plate 15. In some applications, a non-metal bonding agent with good conductivity could be utilized. Bonding of wire mesh can be no more costly than typical conventional fin bonding costs. Moreover, given the broad use of woven metal wire mesh in many filtration and separation-type applications, mesh fabrication costs are low. For example, high manufacturing volume meshes are cheaper than solid plates of the same thickness. For example, a typical stainless steel wire mesh would be $0.76/ft2 versus $0.85/ft2 for a thin plate of the same thickness. Since the mesh will have approximately 50% more actual surface area than the plate, and a much higher heat transfer coefficient, heat transfer performance is superior to a plate fin. Moreover, material weight of the mesh is substantially less than a plate with the same thickness. The heat transfer per weight of a mesh fin is therefore many times higher than that of a plate fin. Given that much less material is required to achieve a given heat transfer, a NISHEX is considerably lower in cost than conventional fins that provide the equivalent heat transfer. Also, compared to foam approaches, costs are orders of magnitude lower.
The foregoing description often referred to a wire material, or wire mesh to form the fin elements. However, the disclosure contemplates, in the alternative, using layers of perforated or slotted sheet material. It will be appreciated that with the appropriate perforations/slots formed in this sheet material similar results can be achieved as in the case of a wire mesh. Furthermore, through the use of non-isotropic molds and casting of metal a structure and results similar to the wire mesh case can be achieved. Lastly, while reference has been made to metal construction, it is easy to envision non-metal wire, mesh, plates and bonding materials used to fabricate NISHEX articles.
A subscale version of a NISHEX, consisting of a single, multi-layer fin element 21a was assembled and tested (NISHEX 1). The test article was constructed of five wire mesh layers, similar to what is shown in
A broad range of wire mesh sizes and/or wire density may be used to construct a NISHEX. As such, the wire sizes and densities shown in Table 2 should not be viewed as limiting on the embodiments for a NISHEX. Moreover, in other embodiments a NISHEX may use more than five layers (e.g. eight layers may be used) or less than five layers.
The five layers of mesh wire channels that formed the NISHEX test article (NISHEX1) was formed using dies. These dies created different mesh fin shapes and heights, depending on wire diameter, similar to what is shown in
The test article was constructed using wire mesh weave with all wires oriented at 45° to the flow direction, as compared to perpendicular/parallel to the mean flow direction. This orientation of the heat conducting mesh wires may not be optimal. NISHEX 20, which may be more optimal, has wires arranged perpendicular and parallel, respectively, to the mean flow direction. The wires arranged parallel to the mean flow direction have a smaller diameter and/or are fewer in number than the diameters that are arranged perpendicular to the mean flow direction. However, for convenience, an isotropic wire mesh was used in a multiple layer non-isotropic configuration with the wires aligned at 45 degrees to the flow direction to ensure that each wire had contact with the base plate 15 in
During the bonding operation for NISHEX1, the furnace was operated at 670 C, to ensure a good bond. Once cooled down, the mesh material at the sides of the bar were trimmed to a total width of 0.5-inches for the test. To simulate the electronics heat load, a 0.125-inch diameter cartridge heater was inserted in the center of the copper bar, or heat sink plate simulator.
Given the small heat input, the test article needed to be heavily insulated to prevent heat loss from impacting the test results. A 3-inch diameter Microtherm insulation, plus low-density, fiber insulating blanket, was used to minimize heat loss effects. Airflow into the single fin test article was monitored and controlled, as well as the heater input.
To determine heat transfer performance, the inlet air, bar and outlet air temperatures were measured. Also, the pressure drop across the heat exchanger was measured. During testing, single fin heater inputs of 10 to 40 Watts and airflows from 0.33 to 1.42 CFM were tested.
The results shown in
Pressure drop performance was also very good for the test article, as shown in
As another example of NISHEX1 capability, DARPA has recently identified a State-of-the-Art (SOA) and Microtechnologies for Air Cooled Exchangers (MACE) performance targets for a typical DOD 1000 W heat dissipation 4 inch×4 inch×1 inch high air cooled heat exchanger application. Using the single fin results in
The good performance of NISHEX1 versus DARPA SOA and MACE targets further supports that a NISHEX can also be effective in many applications. Importantly, a NISHEX is very compact, with the height of heat exchanger and manifold being less than one inch. The NISHEX concept uses 2-inch length segments that would be 0.28-inches high, fed by a manifold that is similarly 0.28-inches high that distributes air to the various segments. The air is then exhausted upward through slots in the structure. The 2-inch segments are probably not optimal. Nevertheless, the result of 4 to 8 kW potential heat dissipation, summarized above, shows that a NISHEX could readily extract the needed heat, yielding a low resistivity and pressure drop, in a very compact package.
A less compact and lower pressure drop (0.988-inches height) NISHEX test article was constructed of aluminum wire mesh (NISHEX2). The wire characteristics of the three layers are given in Table 4. As with the copper wire mesh case (NISHEX1), these meshes were readily available, but may not be optimal. Custom wire mesh, such as that shown in
For the copy test article, the NISHEX1 was operated as a heat sink, where the high temperature plate dissipates heat to the cooler air through the fin. This is directly applicable to radiator and heat sink problems. By using the data to define a heat transfer coefficient, the test results can be readily adapted to radiator cooling, or any other heat management solution. Therefore, the heat sink heat transfer coefficient results are directly applicable to the radiator cooling problem.
Performance of the single fin, illustrated in
For the less dense NISHEX2 test article, two adjacent fins were created. In this test case, hot air flowed either parallel or perpendicular to the mesh fins that were encased in a rectangular channel that guided the flow. As with the single fin NISHEX1 copper test article, the edges where the mesh is bonded to the plate were trimmed prior to testing. The fin attachment plate was cooled by a flow of water. Therefore, NISHEX2 tests had heat flow opposite to the copper NISHEX1 tests. However, as per standard compact heat exchanger design approaches, if heat transfer results are reduced to heat transfer coefficients, these are applicable to different temperature and heat flow direction conditions.
As noted above, by reducing heat transfer results to heat transfer coefficients, different compact HEX approaches can be directly compared. The conventional Navy DW62 cooling coil results given in
As shown in
In addition to lower volume and weight, NISHEX1 based radiators will have a reasonable pressure drop. At face velocity of 1000 fpm and equal heat transfer, results in Table 6 show that the NISHEX1 pressure drop is low, and comparable to the pin fin case that has a much higher volume. Importantly, the fan power requirement for NISHEX1 is only 1.4 kWe.
Using results in Table 5, NISHEX1 volume and weight advantages versus conventional radiators can be determined. These results are highlighted in Table 7. As shown, the NISHEX1 radiator core is over 90% and over 80% lower in volume and weight relative to alternative conventional radiators.
Method of Making Heat Exchangers
A conceptual side view of a NISHEX assembly apparatus is depicted in
The assembly apparatus allows for the production of NISHEX structures on a continuous basis using dispensing rolls of material. The assembly of the NISHEX proceeds from left to right in
Referring to the case of a NISHEX formed form wire mesh, the mesh layer wire diameters would be smallest for the lowest rollers, and increase at higher rolls, with the top most using the thickest wire. This produces the various fin shapes in
After passing through the integration guide 60 the nested structure 56 then passes through a bonding device 70 having heaters, where the structure is trapped between metal belts on rollers 72. This keeps the layers pressed together as they are heated. There are heating elements 74 above and below a belt guide 72 that guides the nested structure 56 through the heater 70.
For some materials, the flat sheet 55 will have a coating of braze or solder compound that will melt and flow into the area where the layers come together. In other cases, the bonding compound will be added as a foil or paste at the bonding location where the plate and mesh are put in contact. Metal or non-metall bonding materials can be considered, depending on the application. In addition, a fixture could be used to hold the sheet 55 and corrugated mesh/sheet 53 layers together at the bond location in bonding area 70 as the assembly moves through the heaters 74. The heat will activate the bonding agent that will hold the assembly together. The nested layers then are pulled through a cooler area 76, where the bonding agent is cooled, e.g., using a cool air or gas blower 79, resulting in the construction of a strip of NISHEX with a supporting plate or sheet. The strip of NISHEX constructs exiting the cooling area 76 is then cut by a cutter 78 into the desired NISHEX 80 by a laser, or similar type cutter.
The machine shown in
Wires used to form fins may be arranged in different fashions to achieve different varieties of anisotropic fin elements. For example, in the case of NISHEX 10 (
In another example, wire meshes can be used that interleave as they are nested or integrated. The meshes can be arranged so that all wires corrugated to form fin elements directly connect to the plate. Meshes may be integrated so that smaller wire diameter meshes nest between the larger mesh wires. Using this approach, all wires would touch the plate, as shown in
NISHEX Applications
Given the foregoing benefits and flexibility of a NISHEX, several applications are possible.
Heat Sinks.
As noted in
Plate and Fin Heat Exchangers and Reactors.
The nested structures produced using the apparatus of
Besides forming plate and fin-type heat exchangers using NISHEX, it is also possible to create reactors that promote simultaneous chemical reactions and heat transfer. This is accomplished by coating the NISHEX structure in one or both channels with appropriate washcoat and catalyst, using standard procedures. The high surface area NISHEX will promote both good reaction and heat transfer. This will be beneficial for reactions that are endothermic or exothermic and require simultaneous heat transfer during reaction.
Radiators and Cooling Coils.
Besides plate and fin heat exchangers, NISHEX structures can be used to create radiators, where the liquid coolant flows in flattened tubes, or thin channels, and NISHEX is used between the tubes or channels to transfer heat between the air and coolant. In addition to the simple radiator configuration, where a water/glycol-type mixture is used as a coolant, a structure using NISHEX could also be used as condensers and evaporators for refrigeration systems, where refrigerant is inside the tubes. Typically, the air side heat transfer limits condenser and evaporator performance. By using NISHEX to substantially enhance air side heat transfer, this limitation is overcome.
Integrating with Phase Change Materials.
Phase Change Materials (PCM), such as paraffinic waxes have a high heat of fusion that can be used to manage transient heat loads produced, for example, by pulsed electronics applications. As the unit is pulsed, a very high heat spike will propagate through the cooling system, leading to the over-temperature of the electronic components, unless the thermal management system is sized for the heat spike. However, by sizing the system for the peak, the weight, volume and cost for the system will be excessive versus a system sized for the average heat load. By including a PCM material in the loop, the PCM can absorb substantial energy as it converts from a solid to a liquid at nearly a fixed temperature. This will shave the peak temperature rise and allow an overall lower volume, weight and cost thermal management system.
While PCMs are very beneficial, those that are effective in the temperature range of interest are relatively poor conductors. In this case, a heat spike may not be absorbed in the time scale needed to prevent the over temperature of a component, due to the bottlenecking of heat transfer through the low conductivity PCM. To eliminate this bottleneck, NISHEX can be used, where one set of channels is filled with PCM and the other set contains the coolant flow. For this case the heat conduction path in the PCM is promoted by the presence of NISHEX in contact with the PCM. This greatly facilitates the thermal response of the PCM mass. By implementing NISHEX with PCM, both heat conductivity and heat capacity are balanced in PCM based thermal management systems. Lastly, while the beneficial case of a solid PCM is considered, NISHEX can also be used to optimize the impact of slurry type PCMs, where fluid heat capacity is enhanced by the addition of micro-encapsulated PCMs.
Other NISHEX Applications.
While the above applications highlighted the heat transfer benefits of NISHEX, this structure could also be used for other applications where a non-isotropic structure is beneficial. Isotropic foams are used as structural, filter and acoustic materials. In structural applications, the non-isotropic nature of NISHEX can be used to tailor crush progress when used to address impact or blast loads. These could be related to accidental impacts or as part of armor shields. For filtration, cross-flow could trap different size particles within the structure, depending on mesh size gradation. An axial flow, possibly combined with a pulsed back-flow, could then be used to periodically clean out the trapped particulate and renew the filtration effectiveness. Depending on the mesh material, layer number and nesting, the material could absorb acoustic waves and cause destructive interference and sound dispersion and damping to control noise. Also, NISHEX structures would also be able to dissipate vibrations. In summary, NISHEX could address all the applications that have utilized isotropic foam, with the added benefit that the NISHEX anisotropic characteristic can provide additional design flexibility to better address some applications.
The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications can be made to the invention in light of the above detailed description. The terms used in the claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the claims, which are to be construed in accordance with established doctrines of claim interpretation.
This application claims priority U.S. provisional application No. 61/475,116 filed on Apr. 13, 2011 (attorney docket no. 100842.5).
| Number | Date | Country | |
|---|---|---|---|
| 61475116 | Apr 2011 | US |
