Patent Application
20120279068
Disclosed embodiments relate to phase change based heat and heat exchange devices, such as heat pipes and reflux boilers.
Heat pipes use successive evaporation and condensation of a working fluid to transport thermal energy, or heat, from a heat source to a heat sink. Because most working fluids have a high heat of vaporization, heat pipes can transport large amounts of heat in a vaporized working fluid. Further, the heat can be transported over relatively small temperature differences between the heat source and heat sink. Heat pipes generally use capillary forces through a porous wick to return condensed working fluid, or condensate, from a heat pipe condenser section (where transported thermal energy is given up at the heat sink) to an evaporator section (where the thermal energy to be transported is absorbed from the heat source).
To reveal details, one end cap for sealed container 112 is not shown. Saturated inside wick 114 is a liquid working fluid (or coolant) 118, which typically comprises ammonia, methanol, water, sodium, lithium, fluorinated hydrocarbons or other fluid selected for its high heat of vaporization and acceptable vaporization temperature and other transport properties in a preselected temperature range within which the heat pipe 100 will operate. Heat pipe 100 typically includes an evaporator section 120, an optional adiabatic section 122 and a condenser section 124.
In operation, the evaporator section 120 of the heat pipe is placed into thermal contact with a heat source 126 and the condenser section 124 is placed into thermal contact with a heat sink 128. As thermal energy from heat source 126 is supplied to evaporator section 120, liquid working fluid 118 impregnating the wick 114 absorbs the thermal energy and begins to vaporize, undergoing a phase change from liquid to vapor. The vapor pressure of the heated working fluid 118 in the evaporator forces the vapor through vapor space 116 toward condenser section 124 of the heat pipe 100. Because condenser section 124 is at lower temperature than evaporator section 120 and the vaporization temperature of working fluid 118, the vapor condenses back into a liquid, giving up to heat sink 128 its latent heat of vaporization, which was acquired in evaporator section 120. The now again liquid phase working fluid 118 is absorbed by wick 114 in condenser section 124 and capillary action wicks the liquid back toward evaporator section 120 where it is again available for evaporation. This process rapidly reaches equilibrium and operates continuously as long as heat is supplied.
The type of working fluid 118 generally influences and limits the performance of heat pipe 100 in several ways. These are usually related to the “transport properties” of the working fluid 118, which is generally defined by a Figure of Merit known as the Liquid Transport Factor (M), given by the following equation:
M=(ρL*σ*λ*cosθ)μL
Where M=Liquid Transport Factor; ρL=Liquid Density; σ=Surface Tension; λ=Enthalpy of Vaporization, θ is the wetting angle, and μL=Liquid Viscosity. As known in the art, ρL, σ, and λ all decrease with increasing temperature (T), and μL increases with increasing T. Provided the working fluid is operable within the desired temperature range, the working fluid is often selected based on its Enthalpy of Vaporization (λ) in an attempt to maximize M and thus increase the heat transfer efficiency of the heat pipe or other heat transfer device. Another class of evaporating/condensing heat transfer device is the reflux boiler, which utilizes gravity rather than wick capillary pumping used by typical heat pipe 100 to return liquid from the condenser to the evaporator. Reflux boilers are also known as wickless heat pipes, or two-phase closed thermosyphons.
Disclosed embodiments include methods for manufacturing heat pipes. Activated particles or particle clusters are formed, such as by chemically or thermally removing surface impurities from a plurality of particles. The activated particles or particle clusters are nanosize or micronsize. The activated particles or particle clusters are contacted with a working fluid in a non-oxidizing environment to form a chemisorbed layer of the working fluid thereon to generate chemisorbed working fluid surfaced activated hydrophilic particles or activated hydrophilic particle clusters which provide a solid-liquid contact angle to working fluid when subsequently added of <30 degrees. This solid-liquid contact angle is thus between the chemisorbed layer on the surface of the particles or clusters and the liquid (working fluid) outside of the chemisorbed layer.
The chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters are vacuum transferred and filled inside the heat pipe along with an additional volume of working fluid. The heat pipe is then sealed.
Disclosed embodiments thus provide nanosize or micronsize chemisorbed working fluid surfaced activated hydrophilic particles or activated hydrophilic particle clusters on the inner surface of reflux boilers or within heat pipe wicks. As used herein the term “heat pipe” includes both heat pipes having wicks and wickless heat pipes. Disclosed embodiments can also be applied to a class of heat pipes referred to as “variable conductance” heat pipes where a controlled amount of inert gas (e.g., argon) is introduced intentionally during operation to moderate the active heat transfer length of the condenser, because an inert gas does not affect the wetting properties of the wick.
Surfaces which exhibit wetting angles <10 degrees are termed “super-hydrophilic” herein. In addition, surfaces which exhibit wetting angles <30 degrees are termed “hydrophilic” herein. For embodiments where the working fluid is a fluid other than water, the terms “hydrophilic” and “super-hydrophilic” can be generalized to their respective contact angle definitions.
For heat pipes having wicks, the wick is generally coated, infused and or intercolated with a plurality of hydrophilic or super-hydrophilic particles. Applied to wickless heat pipes, the hydrophilic particles are generally bonded to the inner wall of the device, such as by electrostatic forces (e.g., van der Waals forces). Other phase change based heat exchange (e.g., such as spray cooling-based) can also benefit from disclosed embodiments.
Disclosed embodiments are described with reference to the attached figures, wherein like reference numerals, are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate subject matter disclosed herein. Several disclosed aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of subject matter in this Disclosure. One having ordinary skill in the relevant art, however, will readily recognize that embodiments of the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring certain detail. This Disclosure not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with this Disclosure.
The Inventors have recognized that hydrophilic particles when infused into phase change-based heat exchange devices reduce the wetting angle θ. As described in the Background above, the liquid Transport Factor (M) is directly proportional to cosine of the wetting angle θ. Under conditions of zero or near zero contact angle, for example, the working fluid spreads across the surface of a solid as a thin film, as opposed to beading up on the surface. As a result, enhancements to wetting (i.e., reduced wetting angle θ) of the working fluid at the wick surface for heat pipes having wicks or the inner surface of wickless heat pipes has been found by the Inventors to provide significant performance improvements.
In one embodiment the wick comprises a porous metallic wick having a plurality of pores, where the internal pore space of the plurality of pores defines an interstitial pore volume. The Inventors have recognized that adding too many chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters 215 will have the undesirable effect of reducing the flow area within the pores thus increasing the pressure loss.
However, adding chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters 215 to occupy a small % of the interstitial pore volumes between the large pores has minimal impact on pressure drop within the pores. These particles generally should occupy only small portion of the interstitial pore volume, such as 2 to 30%, and 2 to 10% in one embodiment.
Conventional heat pipe and reflux boiler performance operating envelopes are known to be governed by distinct thermophysical limits. At steady state, there are 5 different limits that control the amount of heat transfer during the heat pipe or reflux boiler operation:
The addition of chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters as disclosed herein to a heat pipe such as a conventional high performance bi-porous sintered metallic or grooved wick or a reflux boiler, can improve all five these limits. This can be explained as a reduction of overall thermal resistance of the device by incorporation of the hydrophilic particles. Reducing the thermal resistance allows operation at lower temperature for a given heat load, thereby lowering the operating, pressure, mass flow rate and velocity and increasing liquid capillary pumping pressure.
Heat transfer in the evaporator is mainly by thermal conduction through the liquid filled wick and evaporation at the liquid vapor interface at low heat fluxes. At high heat flux the liquid in the porous structure exceeds saturation conditions, nucleates vapor bubbles within the wick, and consequently starts to boil. The formation of vapor bubbles in the wick can disrupt the capillary flow in both the radial and axial directions and may lead to an effect analogous to film boiling in the wick porous media. If the bubbles do not exit the porous media quickly enough, a vapor blanket forms at the heated wall, preventing the liquid from re-wetting the heated “evaporator” wall. Evaporator performance can be improved significantly as the evaporator approached dry out in compound multilayer screen wicks with concentric inner axial liquid-in-groove resupply configuration.
The chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters disclosed herein promote hydrophilic wetting of the pores. This improvement is achieved in part by increasing the meniscus surface area and total interline heat transfer area within individual pores, and improved rewetting speed characteristic of hydrophilic surfaces as described below.
In conventional heat pipe wicks, the maximum capillary pumping pressure (ΔPcap) is related to the surface tension of the liquid (σL), the contact angle between the liquid and vapor at the pore's solid surface (θ) and the effective pore radius (reff pore), and is given by:
ΔP
cap=2σcos θ/reff pore
Offsetting this liquid capillary pumping is the pressure loss due to fluid friction in the wick, given by Darcy's Law. For a given heat pipe length (L) and wick cross sectional area (Aw) and fluid mass flow rate (m′L), the pressure drop (ΔP) in the wick is:
ΔP=μ
L
m′
L
/κA
wρLL
where μL is the liquid viscosity, ρL is the liquid density, and κ is the wick permeability given by: κ=εDh2/32 so the viscous pressure loss (ΔPliq) can be written as:
ΔP
liq=32μLm′L/εDh2AwρLL
where for a circular pore Dh=2 rpore.
In practice, the effective macroscopic pore radius for a given wick is measured experimentally by wick rise tests and the permeability determined experimentally by liquid flow pressure drop over a given wick length of know cross section and porosity. It is important to note that capillary pumping pressure must be equal to or greater than liquid viscous pressure loss. For high capillary pumping a small radius pore is desired. For low liquid viscous pressure loss, a large hydraulic diameter pore, Dh=2 rpore is required. It should also be noted that the maximum capillary pressure exists for a given fluid and pore radius when the cosine θ is unity, corresponding to a wetting angle of zero degrees. As noted above, surfaces which exhibit wetting angles close to zero defined as <10 degrees are generally termed “super hydrophilic”. In addition, surfaces which exhibit wetting angles <30 degrees are termed “hydrophilic”.
As disclosed above, the Inventors have recognized that adding too many chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters will have the undesirable effects of reducing the flow area within the pore and increasing the pressure loss. However, a limited concentration of chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters to the interstitial regions between the large pores has minimal impact on pressure drop within the pore.
Also disclosed herein are methods of manufacturing high performance heat pipe wicks which incorporate near nanoscale hydrophilic particles infused in microscale porous wicks of relatively low permeability and liquid flow resistance. This can be accomplished by first creating nanoscale clusters of particles by chemisorption activation of the nanocluster particle surface. For example, silicon particles become more hydrophilic after treatment with hydrogen fluoride (HF), which in the aqueous form is hydrofluoric acid. The HF treatment removes the surface silicon oxide layer that is generally contaminated, and exposes a reactive silicon surface that can be re-oxidized to form clean and activated silicon oxide particles. Packing activated silicon particles will generate more hydrophilic silicon clusters that promote super-hydrophilic behavior.
In a non-oxidizing ambient, the chemisorbed hydrophilic nanoparticles or clusters of such nanoparticles are then infused into the macroscale porous wick, and the heat pipe is filled and processed by conventional methods including filling with a working fluid. This results in local near zero contact angle in the vicinity of the infused chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters in the macroscopic wick, and greatly enhancing the surface area of the macroscopic pore and increasing heat flux capability without significantly increasing the liquid pressure drop characteristics of the wick.
Benefits of hydrophilic enhanced heat pipes include at the device level, a significant reduction (e.g., 2×) of heat pipe thermal resistance; thus lowering operating temperature, pressure, mass flow rate and increasing transport distance by increased surface tension. Enhanced evaporator critical heat flux and area (e.g., 2-5×) without burn out. Reduced sensitivity to high g loadings and peak transients via rapid rewetting. As described above, disclosed embodiments apply to reflux boilers and grooved wick heat pipes, not just sintered metallic wick heat or grooved pipes.
Several improvements are enabled by several novel thermo-physical mechanisms described above. For example, reduced contact angle due to super-hydrophilic particle infusion into conventional bi-porous metallic wick structure with minimum impact on overall porosity and permeability. Significantly enhanced interline particle-to-particle surface area and thin film evaporation area. in addition, retardation of boiling incipience via super-hydrophilic rewetting and re-priming large pores and nucleation sites.
Disclosed embodiments include methods of cooling components including electronic components, and heat pipes and related heat transfer devices therefrom. The chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters may be viewed as residing in the inter-pore spaces of the relatively large pore spaces for internally porous heat pipe wicks. In a typical application, during operation the coolant is passively recirculated (i.e. there is no external pump), and particles are stationary [immobile] within the pores or on the surface of the wick. However, actively recirculated arrangements of the particles may also be utilized with embodiments of the invention, such as for zero gravity or near-zero gravity applications.
In the case of a conventional heat pipe, the nanoslurry generally wets the wick of the heat pipe. As described above, although embodiments of the invention are generally described relative to a heat pipe having a wick, a wickless device can also be fashioned according to other embodiments of the invention. A wickless device according to an embodiment of the invention has its wall coated with chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters which are super-hydrophilic and utilizes gravity rather than a capillary wick to accomplish fluid recirculation (e.g. a “reflux boiler” or thermosyphon).
In another embodiment, the chemisorbed layer thickness is in a range between 3 and 5 monolayers. In terms of thickness, a single monolayer is about 0.2 nm thick for water when the sorbent nanoparticles are silicon. The thin chemisorbed layer enables hydrophilic wetting of the pore in the vicinity of the chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters.
Although generally described as being constant, the film thickness of the working fluid bonded to the sorbent nanoparticles is generally not constant, although generally still in the range from 2 to 10 monolayers. This is analogous to radius of curvature change in a heat pipe wick under differing heat loads. In the evaporator, the film thickness at a given heat load is different than the film thickness in the condenser because more layers are sent carrying energy from the evaporator to the condenser, so that there is a differential film thickness, or a liquid monolayer “concentration gradient” from one end of the heat pipe to the other. This differential generally remains constant with time during operation, or the layers in the evaporator will be depleted, and axial heat transport would cease, creating the so-called “burnout” event in a conventional heat pipe.
Some exemplary sorbents that can be used with embodiments of the invention can comprise, for example, titania (titanium dioxide), silicon, and activated charcoal. Working fluids can comprise, for example, water, ammonia, and methanol or other fluids which have a high heat pipe fluid figure of merit (i.e. high enthalpy of vaporization, moderate vapor pressure, high surface tension, high density, and low viscosity at the temperature of interest). The working fluid is generally selected based on the desired operating temperature, and there is some overlap between fluids. For example, water is generally appropriate between 50-200° C., and is limited by its high vapor pressure at high T, and high viscosity at low T's. Exemplary combinations of sorbent nanoparticle/working fluid that have been identified by the Inventors as providing a significant heat transfer improvement include titania/water, silicon/water, activated charcoal/ammonia, and activated charcoal/methanol.
Clustering to form nanoclusters has been found by the Inventors to add porosity between the nanoparticles which acts as a porous sponge, so that there is pore space to store several monolayers and confine the working liquid. Depending on the types of sorbent nanoparticles, clustering may or may not occur. Clustering is not required, but can help retain more working fluid. The Inventors have identified candidate sorbent materials including silicon, activated carbon and titanium dioxide when prepared as nanosized clusters are compatible with common heat pipe fluids. Discrete sorbent nanoparticles will generally also adsorb an adequate amount of working fluid which can result in an efficient heat transfer.
The working fluid may also include materials other than the nanoparticles and working fluid. For example, catalysts can be added to enhance the number of sorption sites, generally independent of nanoparticle size. One example is Potassium Chromate K2CrO4, which adds sorption sites or microporosity to activated carbon during thermal pyrolysis of the charcoal. Other common catalysts known for modifying the surface properties of solids may also be used, such as Mo, Co—Mo, Pt, Pd, Ni, ZrO2,V2O5, ZnO, CuO, Al2O3, or Ni—MgO.
Other additives to passivate corrosive reactions between the fluid and container wall are commonly used in some types of heat pipes, the fluid-container and chemical cleaning to reduce potential oxidation-reduction in the heat pipe can be used to prevent poisoning or degradation of the sorbent nanoparticles.
Nanoslurries, according to embodiments of the invention, can be formed using a number of methods. The nanoslurry may be prepared as a solution and introduced into the container (e.g. casing) or by combining pre-loaded dry nanoparticles with fluid in situ in a container. The slurry can be allowed to contact the walls of the container and or wick by fluid distribution dynamics, just as one would generally wet the inside of a bottle with a small amount of liquid. The mass ratio of solid to liquid nanoslurry amounts can be determined experimentally, and is in the range of a few percent of the working fluid fill mass.
Alternatively, nanosized sorbent particles can be added directly into the heat pipe or other heat transfer devices, not as a liquid slurry, but as nanopowder. An appropriate amount of working fluid, such as water, may be added later to complete the heat pipe filling process. Furthermore, a variety of activation approaches can be used to enhance the amount of water or other working fluid adsorbed on the nanoparticles. These approaches can include, but are not limited to, thermal, hydrothermal, magnetic, and chemical approaches.
The active surface of the particles can be highly wettable (e.g., super-hydrophilic), promoting thin film evaporation and condensation, provided the amount of working fluid is small enough to avoid having sufficient liquid to form puddles or to totally saturate [fill or flood] the heat pipe wick pores. Thus, with unbound working fluid (e.g. free water), but not too much working fluid, the particles' surface (e.g. silicon particle surface) through thermal activation can be super-hydrophilic which can significantly enhance spreading of the working liquid thin film. Since this is a free (i.e. unbound) liquid arrangement, the liquid can be capillary pumped analogous to regular liquids, but significantly more efficiently as compared to conventional liquid coolants because the contact angle in the super-hydrophilic condition is nearly zero.
Although a larger internal pore surface area is generally used in this embodiment, as described above, micron size chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters infused into 100 micron wick pores are generally sufficient, so that nanoparticles become optional. However, a micron (or several microns) thick layer is well in the operational thicknesses range for thin film evaporation and condensation.
The lower operating temperature provided during heat pipe operation is due to lower temperature resistance from inside to outside of the heat pipe at both the evaporator and condenser regions. Therefore the internal temperatures are lower. And the vapor pressure is therefore lower as well because of the Clausius-Claperon relationship for free water (or other working fluid).
Disclosed embodiments largely overcome deficiencies of the conventional heat pipes, which are usually characterized by operating envelope limits, which include capillary pumping limit, viscous flow pressure drop limit, vapor blockage boiling in the wick, sonic vapor flow limit, and entrainment of returning liquid in the counter flowing vapor, and the freezing point and critical point of the working fluid. Hydrophillic particle enhanced heat pipe/reflux boilers according to disclosed embodiments provide significant performance advantages over conventional heat pipes since they require much lower mass flows in both the vapor and liquid phases. There is also operational temperature advantages by virtue of the vapor pressure suppression effect of the working fluid in embodiments where the working fluid is bound to the sorbant nanoparticle surface as described above.
Disclosed embodiments can be used in a wide variety of applications. For example, as heat pipes, for cooling laptop computers, power electronic devices, and permafrost stabilization in cold climates. Also, embodiments of the invention can be used in some military products. Moreover, embodiments of the invention can be used as a thermal protection system for a high-speed air vehicle. For high volume commercial applications such as laptop cooling, heat pipe devices according to embodiments of the invention can generally be manufactured for a few dollars each.
Embodiments of the invention are further illustrated by the following specific Examples, which should not be construed as limiting the scope or content of embodiments of the invention in any way.
As mentioned above, chemisorbed working fluid surfaced activated hydrophilic particles or hydrophilic particle clusters according to embodiments of the invention can be produced in a number of ways. For example, in the case the working fluid is water, chemisorbed working fluid surfaced activated hydrophilic nanoclusters can be made by direct hydrothermal decomposition (i.e. cracking) of microparticle precursors into nanoparticles while the microparticles are inside the heat pipe. This is analogous to the thermal shock method of cracking a rock with hot water. In this embodiment, in the particular case of silicon, a mixture of silicon microparticles and water can be filled in a heat pipe. After evacuating the air and sealing the ends of the heat pipe, the temperature of the heat pipe is increased to boil the water, which produces high temperature water vapor which reacts with the microparticles. The breakdown of microparticles will produce nanoparticles that have large surface areas.
The thin native oxide monolayers can be removed by suitable treatments, such as by an HF treatment in the case of silicon. Dangling silicon covalent bonds with high reactivity are created at the surface, which can then be re-oxidized (e.g., in the air at room temperature) to form an uncontaminated and thin (e.g., 15 to 20 Angstrom) silicon oxide layer thereon. The reoxidation of the exposed silicon surface allows chemisorption of water molecules on the particle surface to form a chemisorbed layer on the particle surface.
The powder can be processed by conventional chemisorption vapor deposition of water vapor. The resulting highly ordered chemisorbed water monolayers on the Si surface act as hydrophilic surfaces, and generally super-hydrophilic wetting surfaces.
While various embodiments of the invention have been described above, it should be understood that they have been presented by way of example only, and not as a limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of this Disclosure. Thus, the breadth and scope of the invention should not be limited by any of the above-described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.
Although the disclosed embodiments have been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. While a particular feature of the invention may have been disclosed with respect to only one of several implementations, such a feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly-used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
This application is a continuation-in-part (CIP) of U.S. non-provisional patent application Ser. No. 12/755,797 entitled “HYDROPHILIC PARTICLE ENHANCED PHASE CHANGE-BASED HEAT EXCHANGE” filed on Apr. 7, 2010, now U.S. Pat. No. ______, which claims the benefit of Provisional Application Ser. No. 61/167,402 entitled “Particle Enhanced Heat Exchange”, filed Apr. 7, 2009, both of which are incorporated by reference in their entirety into this application.
The U.S. Government has rights to embodiments of the invention based on Air Force Research Laboratory grant #FA8650-09-2-2940 entitled “Dynamic Heat Generation Modeling and Thermal Management of Electromechanical Actuators.”
| Number | Date | Country | |
|---|---|---|---|
| 61167402 | Apr 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12755797 | Apr 2010 | US |
| Child | 13543911 | US |
