Patent Application
20070028588
This invention relates to devices for efficient heat transfer. More particularly, this invention relates to the use of heat transfer surfaces having low surface energy to promote stable dropwise condensation, and devices incorporating these surfaces.
Condensation of a liquid phase from a vapor phase generally occurs when the vapor comes into contact with a surface having a temperature below the saturation temperature of the vapor, as commonly occurs in condenser devices used in power generation and refrigeration systems. The latent heat of vaporization is released during the condensation process, and this heat is transferred to the surface.
Two alternate mechanisms may govern a condensation process. In most cases, the condensing liquid (“condensate”) forms a film covering the entire surface; this mechanism is known as filmwise condensation. The film provides a considerable resistance to heat transfer between the vapor and the surface, and this resistance increases as the film thickness increases. In other cases, the condensate forms as drops on the surface, which grow on the surface, coalesce with other drops, and are shed from the surface under the action of gravity or aerodynamic forces, leaving freshly exposed surface upon which new drops may form. This so-called “dropwise” condensation results in considerably higher heat transfer rates than filmwise condensation, but dropwise condensation is generally an unstable condition that often becomes replaced by filmwise condensation over time.
Efforts to stabilize and promote dropwise condensation over filmwise condensation as a heat transfer mechanism in practical systems have often required the incorporation of additives to the condensing medium to reduce the tendency of the condensate to wet (i.e., form a film on) the surface, or the use of low-surface energy polymer films applied to the surface to reduce film formation. These approaches have drawbacks in that the use of additives may not be practical in many applications, and the use of polymer films may insert significant thermal resistance between the surface and the vapor. Polymer films may also suffer from low adhesion and durability in many aggressive industrial environments.
Therefore, advances in technologies that promote and stabilize dropwise condensation would be most welcome in the art, particularly if these technologies provided durability and did not substantially inhibit heat transfer between a surface and a vapor.
Embodiments of the present invention meet these and other needs. One embodiment is an apparatus for the transfer of heat. The apparatus comprises a textured heat transfer surface disposed to promote condensation of a vapor medium to a liquid condensate, the surface comprising a plurality of surface texture features disposed on the heat transfer surface. The plurality of features has a median size, a median spacing, and a median height displacement such that the force exerted by the surface to pin (that is, to hold in contact) a drop of condensate to the surface is equal to or less than an external force acting to remove the drop from the surface.
Another embodiment is an apparatus for the transfer of heat. The apparatus comprises a textured heat transfer surface disposed to promote condensation of a vapor medium to a liquid condensate, the surface comprising a plurality of holes disposed in the surface. The plurality of holes has a median hole size, a, of up to about 10 micrometers, a median spacing, b, and a median height displacement, h, such that the ratio b/a is up to about 6 and the ratio h/a is in the range from about 0.5 to about 10. The heat transfer surface comprises a material having an inherent wettability sufficient to generate, with a condensate liquid, a contact angle of at least about 70 degrees.
Another embodiment is an apparatus for the transfer of heat. The apparatus comprises a textured heat transfer surface disposed to promote condensation of a vapor medium to a liquid condensate, the surface comprising a plurality of elevations disposed on the surface. The plurality of holes has a median hole size, a, of up to about 10 micrometers, and a median spacing, b, and a median height displacement, h, such that the ratio b/a is up to about 6 and the ratio h/a is in the range from about 0.5 to about 10. The heat transfer surface comprises a material having an inherent wettability sufficient to generate, with a condensate liquid, a contact angle of at least about 70 degrees.
Another embodiment is a heat pump. The heat pump comprises a working fluid capable of undergoing a phase change; and a condenser capable of receiving the working fluid. The condenser comprises a textured heat transfer surface disposed to promote condensation of a liquid condensate from the working fluid, and the surface comprises a plurality of surface texture features disposed on the heat transfer surface. The plurality of features has a median size, a, of up to about 10 micrometers, a median spacing, b, and a median height displacement, h, such that the ratio b/a is up to about 10 and the ratio h/a is in the range from about 0.5 to about 10, and the heat transfer surface comprises a material having an inherent wettability sufficient to generate, with the condensate liquid, a contact angle of at least about 70 degrees.
Another embodiment is a system for the generation of power. The system comprises a power generator unit configured to emit an exhaust fluid, and a condenser in fluid communication with the power generator unit, the condenser comprising a textured heat transfer surface disposed to promote condensation of a liquid condensate from the exhaust fluid. The surface comprises a plurality of surface texture features disposed on the heat transfer surface. The plurality of features has a median size, a, of up to about 10 micrometers, a median spacing, b, and a median height displacement, h, such that the ratio b/a is up to about 10 and the ratio h/a is in the range from about 0.5 to about 10, and the heat transfer surface comprises a material having an inherent wettability sufficient to generate, with the condensate liquid, a contact angle of at least about 70 degrees.
Another embodiment is a distillation system. The system comprises an evaporator configured to produce a vapor from a source liquid; and a condenser in fluid communication with the evaporator. The condenser comprises a textured heat transfer surface disposed to promote condensation of a liquid condensate from the vapor, and the surface comprises a plurality of surface texture features disposed on the heat transfer surface. The plurality of features has a median size, a, of up to about 10 micrometers, a median spacing, b, and a median height displacement, h, such that the ratio b/a is up to about 10 and the ratio h/a is in the range from about 0.5 to about 10, and the heat transfer surface comprises a material having an inherent wettability sufficient to generate, with the condensate liquid, a contact angle of at least about 70 degrees.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. Furthermore, whenever a particular feature of the invention is said to comprise or consist of at least one of a number of elements of a group and combinations thereof, it is understood that the feature may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.
To promote and maintain desirable dropwise condensation behavior, the condensation surfaces of heat transfer equipment should have a high specific surface area to provide a high density of sites for droplet nucleation; should have low wettability for the condensing liquid (often water, for example) to inhibit condensate film formation; and should promote rapid shedding (“roll-off”) of nucleated drops to maintain a high area of direct surface-vapor contact. In addition, the condensation surface should achieve the above while maintaining an acceptable level of thermal conductivity so that the temperature of the surface can be maintained at suitably low temperatures to sustain efficient condensation. Translating the above performance specifications into a workable design involves the resolution of certain trade-offs, which have been addressed by embodiments of the present invention.
Referring to the drawings in general and to
Surface 120 comprises a plurality of surface texture features 130 disposed on surface 120. In some embodiments the plurality of features 130 comprises at least one hole 140 disposed in surface 120, and in some embodiments, the plurality of features 130 comprises at least one elevation 150 disposed on the surface. As used herein, the term “hole” refers to any depression disposed in surface 120, including naturally occurring holes (e.g., pores) and artificially occurring holes (e.g. drilled holes). Features 130 comprise a height dimension (h), which represents the height of an elevation 150 above the surface base plane 160 or, in the case of holes 130, the depth to which the holes extend below the surface base plane 160. Features 130 further comprise a width dimension (a), referred to herein as feature “size.” Features 130 are disposed in a spaced-apart relationship characterized by a spacing dimension (b). Spacing dimension b is defined as the distance between the edges of two nearest-neighbor features. The plurality of features 130 has a median size, a median spacing, and a median height displacement such that the force exerted by the surface 120 to pin a drop of condensate of a pre-selected size to the surface 120 is equal to or less than an external force acting to remove the drop from the surface 120. The drop thus will be shed from the surface when it grows beyond the predetermined size, thereby clearing the surface 120 for more drops to nucleate. In this way, stable dropwise condensation may be maintained at surface 120, providing for markedly increased heat transfer efficiency over equipment that must rely on filmwise condensation. In some embodiments, the external force comprises the force of gravity acting on the drop, which may be readily calculated based on the value of the pre-selected drop size and density of the liquid. In other embodiments, the external force comprises a force exerted on the drop by a fluid (such as a fluid comprising air) in relative motion with respect to the surface, which force may be readily calculated using standard fluid dynamics techniques. Other force components, such as electromagnetic forces and the like, may be present depending on the nature of the application and of the liquid being condensed. Moreover, in some embodiments, the external force comprises a mechanical force. Such mechanical forces may be generated by vibrating the surface or by application of mechanical actuators to wipe drops from the surface, for example.
Where drop size is assumed to be much greater (for example, at least about 10 times) than the size (a) of features 130, an analysis of the physics of the interaction between a drop and surface 120 reveals that the ratios b/a and h/a have a significant effect on wetting behavior. See, for example, N. A. Patankar, Langmuir 2004, 20, 7097-7102. In this regime, the presence and configuration of features 130 on surface 120 have a significant effect on, for example, the wettability of surface 120, the wetting state of drops on the surface 120, and, under some circumstances, on the nucleation behavior at the surface 120. In some embodiments, the plurality of features has a median size, a, that is up to about 100 micrometers, to ensure that drops having a size of at least about 1 mm are at least about 10 times the median feature size. In particular embodiments, a is up to about 10 micrometers. A smaller median feature size may be desirable in some embodiments to inhibit fouling of the surface by the lodging of foreign particles within or upon features 130, for example.
The “liquid wettability”, or “wettability,” of a solid surface is determined by observing the nature of the interaction occurring between the surface and a drop of a given liquid disposed on the surface. A surface having a high wettability for the liquid tends to allow the drop to spread over a relatively wide area of the surface (thereby “wetting” the surface). In the extreme case, the liquid spreads into a film over the surface. On the other hand, where the surface has a low wettability for the liquid, the liquid tends to retain a well-formed, ball-shaped drop. In the extreme case, the liquid forms nearly spherical drops on the surface that easily roll off of the surface at the slightest disturbance.
In some embodiments of the present invention, features 130 have a size (e.g., a), shape (including, e.g. aspect ratio, h/a), and orientation (including, for example, spacing parameter b/a) selected such that the surface 120 has a low liquid wettability. One commonly accepted measure of the liquid wettability of a surface 120 is the value of the static contact angle 165 (
The texture features 130 also affect nucleation, in that they provide an increase in nucleation sites for droplets condensing on the surface. In general, this increase in sites is attributable to the increased surface area relative to a surface without texture. An analysis of surface area as a function of b/a and h/a indicates that the surface area is most strongly affected by feature geometry where b/a (the relative spacing between features) is relatively low. For example,
In particular embodiments, where the size of the features 130 is very small, the type of feature 130 present at surface 120 also plays a significant role in the promotion of nucleation. The critical drop nucleation radius, r*, is defined as that radius a nucleating drop must attain in order to remain as a stable liquid drop. This value is generally less than about 5 nanometers (nm) for water condensation under typically observed conditions, for example. Where feature size is less than about ten times r* (or less than about 50 nm, for example), convex features present an increased energy barrier to nucleation compared to the energy required for nucleation on macro-scale features, while concave features (such as holes, pores, and other depressions) present a lower energy barrier compared to macro-scale features. Thus, at small size scales for features 130, depressions present a more energetically favorable nucleation site than, for instance, convex elevations (e.g. cylindrical posts) do; in certain embodiments, the plurality of features a plurality of holes 140 having a feature size (i.e., hole diameter) of less than about 100 nm, such as less than about 50 nm, or in some particular embodiments, less than about 20 nm.
In addition to a high drop nucleation rate, effective heat transfer by dropwise condensation relies upon the continual shedding, or roll-off, of condensate drops from surface 120 so that surface 120 is continually exposed to vapor. Where density of features 130 is comparatively high, the desirable condition based purely on nucleation concerns, the area of contact between the drop and surface 130, and hence the forces pinning the drop to surface 130, will also be comparatively high. If gravity, aerodynamic drag, and other forces acting to dislodge the drop are exceeded by the pinning force, the drop will not be shed easily from surface 130. As described above, surface 120 is designed to allow rapid shedding of drops; that is, the surface 120 is designed such that force exerted by the surface 120 to pin a drop of condensate of a pre-selected size to the surface 120 is equal to or less than an external force (such as, for example, gravity, aerodynamic drag, and combinations of these) acting to remove the drop from the surface 120. Consideration of this point is aided by an understanding of how a drop of condensate interacts on surface 120.
A drop of liquid resides on a textured surface typically in any one of a number of equilibrium states. In the “Cassie” state, a drop sits on the peaks of the rough surface, trapping air pockets between the peaks, as is depicted by drop 175 in
Further consideration of
The considerations described above represent a set of competing factors that are accounted for in designing a surface 120 in accordance with some embodiments of the present invention. For example, nucleation rate concerns urge for the use of the highest possible surface area: a high density of high aspect ratio features. However, the desire for rapid shedding generally urges the use of features having comparatively high relative spacing, and the desire for at least some Cassie-state drops urges the use of low aspect ratio features. Thus, the particular values selected for h/a and b/a represent the results of an analysis of competing mechanisms to arrive at an acceptable configuration.
In certain embodiments, where features 130 comprise elevations 150, the ratio b/a is up to about 10. In particular embodiments, b/a is up to about 6. In other embodiments, where features 130 comprise holes 140, b/a is up to about 20 and in particular embodiments is up to about 10. Selecting a relative spacing within these ranges puts the design in a range where, depending on the selection of h/a, the beneficial characteristics described above are readily achieved without unduly sacrificing performance. Regardless of whether features 130 are holes 140 or elevations 150, in some embodiments h/a is in the range from about 0.1 to about 100, and in particular embodiments h/a is in the range from about 0.5 to about 10. Note that at h/a less than 0.5, there is generally very little enhancement of surface area (a nucleation issue) while at h/a greater than 10 less than about 2% of the nucleation area is available for Cassie-state drops.
It should be noted that embodiments of the present invention contemplate any range contained within the respective ranges specified herein, regardless of whether the particular endpoints of the range are explicitly stated as viable endpoints. Moreover, embodiments of the present invention include any combination of parameter range limitations explicitly or implicitly set forth herein. For example, in particular embodiments, a is up to about 100 micrometers, b/a is up to about 6 and h/a is in the range from about 0.5 to about 10, in order to exploit more fully the advantages described above.
Numerous varieties of feature shapes are suitable for use as features 130. In some embodiments, at least a subset of the features 130 has a shape selected from the group consisting of a cube, a rectangular prism, a cone, a cylinder, a pyramid, a trapezoidal prism, and a segment of a sphere (such as a hemisphere or other spherical portion). These shapes are suitable whether the feature is an elevation 150 or a hole 140. As an example, in particular embodiments, at least a subset of the features comprises nanowires, which are structures that have a lateral size constrained to tens of nanometers or less and an unconstrained longitudinal size. Methods for making nanowires of various materials are well known in the art, and include, for example, chemical vapor deposition onto a substrate. Nanowires may be grown directly on surface 120 or may be grown on a separate substrate, removed from that substrate (for example, by use of ultrasonication), placed in a solvent, and transferred onto surface 120 by disposing the solvent onto the surface and allowing the solvent to dry.
In some embodiments, all of the features 130 in the plurality have substantially the same respective values for h, a, and b (“an ordered array”), though this is not a general requirement. For example, the plurality of features 130 may be a collection of features, such as nanowires, for instance, exhibiting a random distribution in at least one parameter such as feature size, feature shape, or feature spacing. In certain embodiments, moreover, the plurality of features is characterized by a multi-modal distribution (e.g., a bimodal or trimodal distribution) in h, a, b, or any combination thereof. Such distributions may advantageously provide reduced wettability in environments where a range of drop sizes is encountered. Estimation of the effects of h, a, and b on wettability are thus best performed by taking into account the distributive nature of these parameters. Techniques, such as Monte Carlo simulation, for performing analyses using variables representing probability distributions are well known in the art. Such techniques may be applied in designing features 130 for use in articles of the present invention.
In certain applications, the presence of multiple size-scale features amplifies the low-wettability effects obtained on surfaces textured as described above, allowing for a broader acceptable range of feature size, shape, and orientation. As shown in
Features 130 are disposed on surface 120 so as to maintain an acceptable degree of heat transfer between the surface 120 and a contacting vapor. In certain embodiments, features comprise a metal, such as, for instance one or more of the metals described above as suitable for fabrication of surface 120. However, if heat transfer performance requirements allow, other materials such as, for example, ceramics, semi-metals, and polymers, may be used in fabricating features 130. Anodized metal oxides are one example of a class of ceramics, and anodized aluminum oxide is a particular example of a potentially suitable material for use in embodiments of the present invention. Anodized aluminum oxide typically comprises columnar pores, and pore parameters such as diameter and aspect ratio may be closely controlled by the anodization process. If the thickness of the porous anodized metal oxide layer is kept sufficiently small, the thermal penalty may be negligible compared to the benefits offered by the presence of porous features.
Metals, ceramics, semi-metals, intermetallic materials, and certain polymers generally have moderate to high wettability, and thus the effect of surface texturing by providing features 130 as described herein may not always suffice to provide desired levels of wettability, absent some means of lowering the inherent wettability (that is, the wettability of a non-textured surface made of the material) of the features 130. The inherent wettability of the material used for surface 120 that will actually contact the liquid condensate, in some embodiments, is sufficiently low to generate, with a static drop of the liquid condensate, a contact angle of at least about 70 degrees; in some embodiments this angle is at least about 90 degrees, and in particular embodiments, the angle is at least about 110 degrees.
In some embodiments, surface 120 further comprises a surface energy modification material (not shown). This material is formed, in one embodiment, by overlaying a layer of material at surface 120, resulting in a coating disposed over features 130. Hydrophobic hardcoatings are one suitable option. As used herein, “hydrophobic hardcoatings” refers to a class of coatings that have hardness in excess of that observed for metals, and exhibit wettability resistance sufficient to generate, with a drop of water, a static contact angel of at least about 70 degrees. Diamond-like carbon (DLC) coatings, which typically have high wear resistance, have been applied to metallic articles to improve resistance to wetting (see, for example, U.S. Pat. No. 6,623,241). As a non-limiting example, fluorinated DLC coatings have shown significant resistance to wetting by water. Other hardcoatings such as nitrides, carbides, and oxides, may also serve this purpose. Particularly suitable materials candidates that have been demonstrated by the present inventors to produce contact angles of about 90 degrees and higher with water when deposited on smooth metal substrates include tantalum oxide, titanium carbide, titanium nitride, chromium nitride, boron nitride, chromium carbide, molybdenum carbide, titanium carbonitride, and zirconium nitride. These hardcoatings, and methods for applying them, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), etc., are known in the art, and may be of particular use in harsh environments. Fluorinated materials, such as fluorosilanes, are also suitable coating materials that exhibit low wettability for certain liquids, including water. Finally, if conditions allow, the coating may comprise a polymeric material. Examples of polymeric materials known to have advantageous resistance to wetting by certain liquids include silicones, fluoropolymers, urethanes, acrylates, epoxies, polysilazanes, aliphatic hydrocarbons, polyimides, polycarbonates, polyether imides, polystyrenes, polyolefins, polypropylenes, polyethylenes or mixtures thereof.
Alternatively, the surface energy modification material may be formed by diffusing or implanting molecular, atomic, or ionic species into the surface 120 to form a layer of material having altered surface properties compared to material underneath the surface modification layer. In one embodiment, the surface energy modifying material comprises ion-implanted material, for example, ion-implanted metal. Ion implantation of metallic materials with ions of boron (B), nitrogen (N), fluorine (F), carbon (C), oxygen (O), helium (He), argon (Ar), or hydrogen (H) may lower the surface energy (and hence the wettability) of the implanted material. See, for example, A. Leipertz et al., “Dropwise Condensation Heat Transfer on Ion Implanted Metallic Surfaces,” http://www.ltt.uni-erlangen.de/inhalt/pdfs/tk_gren.pdf; and Xuehu Ma et. al, “Advances in Dropwise Condensation Heat Transfer: Chinese Research”, Chemical Engineering Journal, 2000, volume 78, 87-93.
In one embodiment, a diffusion hardening processes such as a nitriding process or a carburizing process is used to dispose the surface energy modification material, and thus the surface energy modification material comprises a nitrided material or a carburized material. Nitriding and carburizing processes are known in the art to harden the surface of metals by diffusing nitrogen or carbon into the surface of the metal and allowing strong nitride-forming or carbide-forming elements contained within the metal to form a layer of reacted material or a dispersion of hard carbide or nitride particles, depending on the metal composition and processing parameters. Nitriding processes known in the art include ion nitriding, gas nitriding, and salt-bath nitriding, so named based upon the state of the nitrogen source used in the process. Similarly, a variety of carburizing processes are known in the art. These processes have shown a remarkable potential for lowering metal surface energy. In one example, the contact angle (measured using water as reference liquid) of 403 steel having a surface finish of 32 microinches was increased from about 60 degrees to about 115 degrees by ion nitriding. A preliminary observation of the surface of the nitrided surface applied to mirror-finish specimens suggests that the nitriding process may deposit nano-scale features at the surface in addition to reducing the inherent surface energy of the metal; the presence of such features may amplify the ability of the surface to resist wetting, enhancing the performance of the coating over one having similar composition but a smooth, feature-free structure.
The surface energy modification layer may be applied after features 130 have been provided on surface 120. Alternatively, features 130 may be formed after applying surface energy modification layer to surface 120. The choice of order will depend on the particular processing methods being employed and the materials being used for features 130 and surface 120. It should be noted that the use of surface energy modification material in combination with the use of the textures as described herein may result in surfaces having significantly higher liquid contact angles than those expected where the surface energy modification material is used without the texturing, that is, where the material is applied to a smooth surface. The enhanced resistance to wetting provided by embodiments of the present invention, where texture and surface modification are combined, may promote drop shedding by rolling of the drop, while without texturing the drops may merely slide off the surface. The roll-off of drops is preferable to slide-off because rolling drops are less likely to leave a film of liquid on the surface during the removal process, thereby desirably increasing the direct contact between vapor and surface. These advantages are further illustrated by examples presented herein.
Features 130 can be fabricated and provided to apparatus 100 by a number of methods. In some embodiments, features 130 are fabricated directly on surface 120 of apparatus 100. In other embodiments, features 130 are fabricated separately from surface 120 and then disposed onto surface 120. Disposition of features 130 onto surface 120 can be done by individually attaching features 130, or the features may be disposed on a sheet, foil or other suitable medium that is then attached to the surface 120. Attachment in either case may be accomplished through any appropriate method, such as, but not limited to, welding, brazing, mechanically attaching, or adhesively attaching via epoxy or other adhesive.
The disposition of features 130 may be accomplished by disposing material onto the surface of the apparatus, by removing material from the surface, or a combination of both depositing and removing. Many methods are known in the art for adding or removing material from a surface. For example, simple roughening of the surface by mechanical operations such as grinding, grit blasting, or shot peening may be suitable if appropriate media/tooling and surface materials are selected. For example, grit blasting metal surfaces using media having a mesh size in the range from about 32 to about 220 has produced surfaces having textures sufficient to produce enhanced resistance to wetting by water compared to the resistance exhibited by the surfaces without grit blasting, especially where a surface energy modification material is applied to the roughened (grit blasted) surface, as described above. Such operations will generally result in a distribution of randomly oriented features on the surface, while the size-scale of the features will depend significantly on the size of the media and/or tooling used for the material removal operation. Lithographic methods are commonly used to create surface features on etchable surfaces, including metal surfaces. Ordered arrays of features can be provided by these methods; the lower limit of feature size available through these techniques is limited by the resolution of the particular lithographic process being applied.
Electroplating methods are also commonly used to add features to surfaces. An electrically conductive surface may be masked in a patterned array to expose areas upon which features are to be disposed, and the features may be built up on these exposed regions by plating. This method allows the creation of features having higher aspect ratios than those commonly achieved by etching techniques. In particular embodiments, the masking is accomplished by the use of an anodized aluminum oxide (AAO) template having a well-controlled pore size. Material is electroplated onto the substrate through the pores, and the AAO template is then selectively removed; this process is commonly applied in the art to make high aspect ratio features such as nanorods. Nanorods of metal and metal oxides may be deposited using commonly known processing, and these materials may be further processed (by carburization, for example) to form various ceramic materials such as carbides. As will be described in more detail below, coatings or other surface modification techniques may be applied to the features to provide even better wettability properties.
Micromachining techniques, such as laser micromachining (commonly used for silicon and stainless steels, for example) and etching techniques (for example, those commonly used for silicon) are suitable methods as well. Such techniques may be used to form cavities (as in laser drilling) as well as protruding features. Where the plurality of features 320 includes cavities 300, in some embodiments surface 120 comprises a porous material, such as, for example, an anodized metal oxide. Anodized aluminum oxide is a particular example of a porous material that may be suitable for use in some embodiments. Anodized aluminum oxide typically comprises columnar pores, and pore parameters such as diameter and aspect ratio may be closely controlled by the anodization process, using process controls that are well known to the art to convert a layer of metal into a layer of porous metal oxide.
In short, any of a number of deposition processes or material removal processes commonly known in the art may be used to provide features to a surface. As described above, the features may be applied directly onto surface 120 of apparatus 100, or applied to a substrate that is then attached to surface 120.
Additional aspects of constructing apparatus 100 (
An embodiment of the present invention includes a heat pump 1000 (
Further embodiments of the present invention, as shown in
Another embodiment of the present invention is a distillation system, as shown schematically in
The following example is presented to further illustrate exemplary embodiments of the invention and should not be construed as limiting the invention in any way.
An apparatus for heat transfer is designed. A maximum allowable drop diameter of up to 3 mm prior to roll-off is determined to be allowable to ensure proper levels of heat transfer. An aluminum tube is to be used as a heat transfer surface, and the surface of the tube that will contact the vapor to be condensed is anodized, using a process known in the art, to provide a layer of anodized aluminum oxide (AAO) of 100 micrometer thickness (h). The anodization process selected to perform this work can be manipulated to provide columnar pores having a median pore diameter (a) of about 10 micrometers with a median edge-to-edge spacing (b) of about 30 micrometers. Thus h/a is about 10 and b/a is about 3. Referring to
An experimental test apparatus was designed to measure heat transfer associated with condensation of steam. The test setup consisted of a steam generator, a condensing chamber, and a chill block, one end of which is exposed to the steam and the other end to cooler circulating water. The test sample was mounted onto the chill block so that steam condensed onto the surface of the sample. Heat transfer and associated heat transfer coefficients are determined by measuring the temperatures along the length of the block, the surface of the sample, and the temperature of the steam.
Silicon wafers (4″ diameter) with different surface properties were tested in the above apparatus. Sample A was a regular silicon wafer with water contact angle of about 43 degrees (hydrophilic), and served as a baseline. Sample B was coated with tridecafluoro-1,1,2,2-tetrahydrooctyl-trichlorosilane (fluorosilane) via vapor deposition, to increase its water contact angle to 110 degrees (hydrophobic). Samples C and D had unique surface textures in accordance with embodiments of the present invention, and were fabricated using standard photolithography techniques, followed by deep reactive ion etching. Sample C had rectangular prism post features of width 3 micrometers and spacing of 1.5 micrometers. Sample D had rectangular prism post features of width 3 micrometers and spacing 6 micrometers. The aspect ratio of the posts was about 3 for both samples C and D. The samples were tested under identical conditions in the above apparatus, and each exhibited different condensation behavior. Because of the hydrophilic nature of the surface, filmwise condensation was observed on sample A and the measured heat transfer coefficient was 2.23 kW/m2 K. The condensate on sample B consisted of large drops that slid along the surface; the measured heat transfer coefficient was 2.85 kW/m2 K, only slightly larger than that of sample A. On samples C and D, stable dropwise condensation was observed, and the droplets were observed to roll off the surface rather than sliding. The measured heat transfer coefficients were 4.61 kW/m2 K and 13.48 kW/m2 K, respectively. The enhancement in heat transfer coefficients over the baseline sample (sample A) is about 1.3 for sample B, about 2 for sample C, and about 6 for sample D. This enhancement can be attributed to an increased nucleation area and superior roll-off properties of the textured substrates as discussed above. The average drop size on sample D was observed to be smaller than that of sample C because of its larger relative spacing (b/a). This resulted in an higher heat transfer coefficient for sample D over C.
A pipe composed of 6061 aluminum with a diameter of about one inch was first polished with fine sandpaper and then coated with anodized aluminum oxide (AAO) via an anodization process. The surface consisted of pores that were on average 90 nm in diameter, 500 nm in depth and a typical edge-to-edge spacing of about 10 nm. This specimen was then coated with fluorosilane via vapor deposition as in Example 1. When the surface was exposed to steam, stable dropwise mode of condensation was observed, with droplets being shed from the surface by rolling off.
While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations, equivalents, or improvements therein may be made by those skilled in the art, and are still within the scope of the invention as defined in the appended claims.
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/705,239, filed Aug. 3, 2005.
| Number | Date | Country | |
|---|---|---|---|
| 60705239 | Aug 2005 | US |
