U.S. patent application number 13/637630 was filed with the patent office on 2013-01-24 for device having nano-coated porous integral fins.
The applicant listed for this patent is Chanwoo Park. Invention is credited to Chanwoo Park.
Application Number | 20130020059 13/637630 |
Document ID | / |
Family ID | 45372010 |
Filed Date | 2013-01-24 |
United States Patent
Application |
20130020059 |
Kind Code |
A1 |
Park; Chanwoo |
January 24, 2013 |
DEVICE HAVING NANO-COATED POROUS INTEGRAL FINS
Abstract
Disclosed herein is an apparatus including a plurality of
nano-coated, porous integral fins and/or grooves on the evaporator
tubes. In some examples, the apparatus is an evaporative cooler,
such as a horizontal-tube, falling-film evaporator. In some
examples, the evaporator tubes are in either a horizontal and/or
tilted and/or vertical position. Also disclosed are methods of
using the disclosed apparatus, such as a cooling device including
as an evaporative cooler.
Inventors: |
Park; Chanwoo; (Reno,
NV) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Park; Chanwoo |
Reno |
NV |
US |
|
|
Family ID: |
45372010 |
Appl. No.: |
13/637630 |
Filed: |
April 1, 2011 |
PCT Filed: |
April 1, 2011 |
PCT NO: |
PCT/US11/31008 |
371 Date: |
September 26, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61320248 |
Apr 1, 2010 |
|
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|
61470999 |
Apr 1, 2011 |
|
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Current U.S.
Class: |
165/133 |
Current CPC
Class: |
F28D 15/046 20130101;
F28D 2021/0064 20130101; F28F 13/003 20130101; F28D 3/02 20130101;
F25B 39/02 20130101; F28F 13/187 20130101; B01D 1/221 20130101;
B01D 1/065 20130101; F25B 2339/0242 20130101 |
Class at
Publication: |
165/133 |
International
Class: |
F28F 13/18 20060101
F28F013/18 |
Goverment Interests
ACKOWLEDGMENT OF GOVERNMENT SUPPORT
[0002] This invention was made with U.S. Government support of
Grant No. DE-EE0003231 from the Department of Energy. The U.S.
Government has certain rights in this invention.
Claims
1. An apparatus comprising a plurality of nano-coated, porous
integral fins and/or grooves on the evaporator tubes.
2. The apparatus of claim 1, wherein the apparatus is an
evaporative cooler.
3. The apparatus of claim 1, wherein the apparatus is a
horizontal-tube, falling-film evaporator.
4. The apparatus of claim 1, wherein the evaporator tubes are in
either horizontal and/or tilted and/or vertical position.
5. The apparatus of claim 1, wherein one or more liquid dispensers
are located either above and/or below the evaporator tubes.
6. A method of using the apparatus of claim 1 as a cooling
device.
7. A method of using the apparatus of claim 1 as an evaporative
cooler.
8. A method of using the apparatus of claim 1 as a falling-film
evaporative cooler.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 61/320,248, filed Apr. 1, 2010 and U.S.
Provisional Application Ser. No. 61/470,999 filed Apr. 1, 2011,
each of which are incorporated by reference herein in their
entirety.
TECHNICAL FIELD
[0003] The present disclosure relates to evaporators, such as
falling-film evaporators.
BACKGROUND
[0004] Falling-film, horizontal-tube evaporators have been widely
used in air-conditioning, refrigeration, chemical, petroleum
refining and desalination industries. It is known that the
falling-film evaporators have several advantages over flooded
evaporators: (1) higher heat transfer coefficients due to jet
impingement on the horizontal tubes, (2) relatively thin film
evaporation (i.e., low thermal resistances resulting in high
evaporator temperatures and thermodynamic cycle efficiencies and
compact/lightweight design), and (3) low refrigerant (evaporant)
charge and less risk associated with a leak. Despite the superior
performance of the falling-film evaporators, problems associated
with liquid maldistribution, surface nonwetting, and droplet
(localized and relatively thick liquid layer) formation on the
plain evaporator tubes make the conventional falling-film
evaporator less efficient in terms of evaporator surface
utilization and thin-film (low thermal resistance) formation.
[0005] Some prior work with falling-film evaporation enhancement
has employed various structured surfaces, such as extended solid
fins, grooves, hydrophilic coating and reentrant nucleation
cavities, to enhance the evaporation performance. "Circumferential
micro-grooves" on horizontal-tube evaporators "partially-immersed
in liquid pools" were used to create a capillary-assisted liquid
distribution within the solid grooves, thus promoting thin film
evaporation. Because liquid pools were used to supply liquid over
the entire length of the evaporator tubes, the liquid dispenser
design creates undesirable design limitations/constraints, such as
requiring multiple liquid dispensers and large tube spacing.
Because of the circumferential grooves, the liquid distribution is
typically allowed only in circumference of the grooves, not in the
axial direction of the tube.
[0006] Surface evaporation using porous-layer coating groove for a
liquid pool was "numerically" analyzed. Some results indicated that
evaporation can be increased in the thin liquid film region near
liquid menisci and that capillary-assisted liquid distribution in a
porous coating on triangular grooves can increase evaporation
(wetting) area. Heat pipes used for commercial electronic cooling
utilize the same capillary pumping for liquid circulation and
thin-film nucleate boiling in the porous wicks. Note that the
capillary pumping can typically self-regulate itself to varying
heat loads, thus forming a thin liquid film within the porous
layer. The capillary pumping is generally inversely proportional to
the pore diameter of the porous wicks.
[0007] Pool boiling enhancement using porous-layer coatings with
surface modulations (periodically non-uniform thickness) and
laminated screen meshes has been studied by some researchers.
Generally, the results indicate that the pool-boiling Critical Heat
Flux (CHF) using surface modulations was increased nearly three
times over that of a plain surface because the modulation separates
the liquid and vapor streams, thus reducing the liquid-vapor
counterflow resistances adjacent to the surface and improving the
hydrodynamic stability in the vapor-liquid interface, thus
increasing the pool boiling CHF.
[0008] It has been known that surface roughness and oxidation
(aging) can alter pool boiling characteristics, such as because of
the increased nucleation sites and improved surface wetting and
because surface oxidation can introduce a nano-scale surface
morphology to the original surfaces, and increases surface
wettability. Although metal oxides (e.g., CuO and Al.sub.2O.sub.3)
have lower thermal conductivities than original metals resulting in
higher conduction resistances, increased nucleation sites (new
nano-scale surface morphology) and surface wettability
(hydrophilicity) of the oxide surfaces can significantly alter
liquid distribution and surface wetting characteristics and improve
the overall boiling performance.
[0009] In other studies, the effect of degree of oxidation of the
heater surface on nucleate pool boiling and the surface oxidation
was quantitatively measured by static contact angle of a sessile
water drop. This study attempted to correlate the surface oxidation
to wettability (contact angle) and pool boiling heat flux. It was
reported that as the surface wettability improves by the surface
oxidation, the maximum heat flux increases and a higher surperheat
is required to attain the same heat flux.
[0010] In further studies, it was found that the influence of the
surface roughness is small for the fully-developed film boiling
where vapor film thicknesses are considerably larger than wall
roughness. But as the vapor film gets thinner near the pool-boiling
minimum heat flux condition, the surface roughness elements could
interact with the liquid-vapor interface and the boiling curve
could be significantly altered. The surface oxidation was reported
to increases the pool-boiling minimum heat flux where the oxide
surface (hydrophilic) easily spreads the liquid as the liquid
contact begins to occur. As the pool-boiling CHF is approached, the
influence of surface roughness wanes because the CHF is thought to
depend primarily on the hydrodynamics of vapor removal and gets
stronger on the nucleate boiling portion of the boiling curves.
[0011] Surface nano-coating was used to influence surface
wettability from hydrophilicity (SiO.sub.x, TiO.sub.2, Pt and
Fe.sub.2O.sub.3) to hydrophobicity (SiOC and Teflon) in a further
study. Particles of very small size (less than 100 nm) called
"nanoparticles" were deposited on the heated surface using various
deposition techniques. Another recent report describes using a
nanofluid with Al.sub.2O.sub.3 nanoparticle for the pool boiling.
The study describes that the boiling critical heat flux increases
compared to those of the pure fluids. The enhanced heat flux was
partially attributed to increased surface wettability of the
nanofluids due to the deposition of the nanoparticles on the heater
surface, called Nanofluid Nucleate Boiling Deposition (NNBD).
SUMMARY
[0012] In one embodiment, the present disclosure provides
capillary-assisted evaporation using nano-coating, porous integral
fins on horizontal-tube, falling-film evaporators.
[0013] In wicked heat pipes used as an electronic cooling device,
capillary liquid distribution, and consequently, thin film
evaporation/boiling are typically the underlying heat and mass
transfer processes. According to one aspect of the present
disclosure, using capillary (gravity-insensitive) liquid
distribution in porous integral fins for falling-film,
horizontal-tube evaporators mitigates inherent liquid
maldistribution and surface nonwetting problems found in the
conventional plain surface evaporators and helps approach ideal
evaporation conditions, i.e., thin-film evaporation with
low-thermal resistance over increased wetted surface area even with
less liquid supply. In a particular implementation, nano-coating
(nano-particle deposition), including oxidation on the porous fins
of the falling-film evaporators, is used to further enhance surface
wettability (hydrophilicity), thus increasing evaporation.
[0014] In various embodiments, the present disclosure uses the
following properties/variables/conditions in falling-film
evaporation to improve performance: micro-structural properties
(porosity, permeability, pore size(10-100 .mu.m), pore
size/distribution, and specific surface area) and geometrical
properties (fin height (.about.1 cm), width and pitch),
thermo-physical properties (effective thermal conductivity) of
porous integral fins and surface wettability change (oxidation and
nano-coating), evaporator design variables (tube
diameter/length/spacing and liquid dispenser) and operating
conditions (flow rate and temperature of feed liquid and vapor
pressure).
[0015] There are additional features and advantages of the subject
matter described herein. They will become apparent as this
specification proceeds.
[0016] In this regard, it is to be understood that this is a brief
summary of varying aspects of the subject matter described herein.
The various features described in this section, Appendix I and
below for various embodiments may be used in combination or
separately. Any particular embodiment need not provide all features
noted above, nor solve all problems or address all issues in the
prior art noted above. Additional features of the present
disclosure are described in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] Various embodiments are shown and described in connection
with the following drawings in which:
[0018] FIG. 1A is a photograph of a thin film boiling apparatus
under saturated conditions before insulation. FIG. 1B is a
photograph of a sintered copper wick (porous-layer coating) showing
reddish copper (I) oxide.
[0019] FIGS. 2A-2B are electronic images showing the flow modes and
surface wetting for falling-film water on two-row horizontal copper
tubes with (a) plain surface and (b) porous-layer coating at
various liquid Reynolds numbers (Re.sub.liq=4.GAMMA./.mu..sub.liq).
.zeta. is the surface wetting ratio defined as
.zeta.=A.sub.wet/A.sub.tube,outer.
[0020] FIGS. 3A-3C are digital images showing the contact angle
measurement of 1 mL sessile water droplets on (a) a polished
(fresh) surface (sample 1); (b) copper surface oxidized at room
temperature of two days (sample 4); and (c) copper surface oxidized
at 100.degree. C. for two hours (sample 6).
[0021] FIGS. 4A and 4B are diagrams illustrating a process for
fabricating an integral porous fin.
[0022] FIG. 5A is a schematic diagram illustrating a porous
integral fin on a substrate, showing capillary-driven liquid
flow/distribution and surface evaporation. FIG. 5B is a photograph
of a porous integral fin built on a copper substrate. FIG. 5C is a
schematic diagram illustrating a porous integral fin on a
substrate. FIG. 5D is a diagram of a grooved fin.
[0023] FIG. 6 is a schematic diagram of a system using a
falling-film evaporator.
[0024] FIG. 7 is a diagram illustrating modes of heat transfer and
vapor formation in a porous layer.
DETAILED DESCRIPTION
[0025] Unless otherwise explained, all technical and scientific
terms used herein have the same meaning as commonly understood by
one of ordinary skill in the art to which this disclosure belongs.
In case of conflict, the present specification, including
explanations of terms, will control. The singular terms "a," "an,"
and "the" include plural referents unless context clearly indicates
otherwise. Similarly, the word "or" is intended to include "and"
unless the context clearly indicates otherwise. The term
"comprising" means "including;" hence, "comprising A or B" means
including A or B, as well as A and B together. Although methods and
materials similar or equivalent to those described herein can be
used in the practice or testing of the present disclosure, suitable
methods and materials are described herein. The disclosed
materials, methods, and examples are illustrative only and not
intended to be limiting.
[0026] In wicked heat pipes used as an electronic cooling device,
capillary liquid distribution and consequently, thin film
evaporation/boiling (low thermal resistance of 0.05K-cm.sup.2/W,
and heat flux.ltoreq.200 W/cm.sup.2) are typically the underlying
heat and mass transfer processes. The capillary-assisted
(gravity-insensitive) liquid distribution using the disclosed
porous integral fins can produce enhanced cooling devices, such as
falling-film, horizontal-tube evaporators, where the liquid
maldistribution, poor surface wetting and liquid droplet formation
inherently tends to occur. Surface oxidation (aging) is known to
change pool boiling characteristics. Such changes are likely
because of the increased nucleation sites and improved surface
wetting associated with newly introduced nano-scale surface
morphology onto the original surfaces. Although few studies exist
of surface roughness and oxidation on pool boiling, a systematic
investigation of the effect of micro-structural change (surface
morphology, coating thickness, porosity, pore size, specific
surface area) and wettability change due to surface oxidation and
nano-coating (using nanoparticles) used with porous-layer
structured surface on falling-film evaporation is not believed to
exist.
[0027] The capillary-assisted (gravity-insensitive) liquid
distribution using the disclosed porous integral fins with
nano-coating for surface wettability can be useful in various
cooling devices, such as falling-film, horizontal-tube evaporators,
where the liquid maldistribution, poor surface wetting and
localized liquid droplet inherently tends to occur.
[0028] Porous-layer coatings on a copper substrate were used to
characterize the effect of the wick parameters on boiling
performance. FIG. 1(a) shows the boiling test apparatus under
saturation (closed) conditions. FIG. 1(b) shows a sintered copper
wick (1 inch in diameter and 1 mm thick) tested with water for the
boiling test. The wick was not completely immersed in water, but
rather horizontally positioned slightly (.about.1 mm) higher than
the water level so that water was radially drawn from the side wall
of the wick toward the center by capillary pumping. Considering the
wicking distance (0.5 inches) and heat flux level (.about.100
W/cm.sup.2) of the boiling tests, the wicking (capillary-assisted
liquid distribution) and high heat flux evaporation using the
disclosed porous integral fins (1 inch in height) is achievable.
From the bubbling pressure measurement using a test sample with
water (about 8,718 Pa), the pore diameter of the wick sample is
estimated to be about 17 .mu.m. The heat flux measurement results
using the fully-oxidized copper wick (as shown in FIG. 1(b)) were
in reasonable agreement with the results using another wick design
and similar boiling conditions.
[0029] Flow Mode and Surface Wetting Experiment using Falling-Film
Water on Horizontal Copper Tubes
[0030] The results of a preliminary study was conducted to
investigate the effect of a porous-layer coating of horizontal
tubes on surface wetting using falling-film liquid flow. FIG. 2
shows the flow modes and surface wetting for falling-film water on
two-row horizontal copper tubes with (a) plain surface and (b)
porous-layer coating at various liquid Reynolds numbers
(Re.sub.liq=4.GAMMA./.mu..sub.liq). .zeta. is the surface.sub.q
wetting ratio defined as .zeta.=A.sub.wet/A.sub.tube,outer. It was
observed from the figure that the flow mode between the evaporator
tubes [top (second row) and bottom (third row) tubes] was changed
from the droplet to the droplet/column mode at the lower Reynolds
number than the transitional Reynolds number (Re.sub.liq=352) known
for plain tubes. The early flow mode transition might attribute to
the liquid dispenser design such as hole interval and diameter used
for this experiment. Note that the flow mode between the liquid
dispenser and the top tube is quite different than one between the
top and bottom tubes. The surface wetting ratio (.zeta.) for the
plain tubes as shown in FIG. 2(a) was determined by counting the
unwetted areas from the photo images. As shown in the figure, there
was significant unwetted area on the plain tubes and the wet area
was increased as the liquid Reynolds number was increased. On the
contrary, there was observed no unwetted area on the porous-layer
coating tubes as shown in FIG. 2(b). The complete wetting on the
porous-layer coating tubes was confirmed by touching the tube with
a dry paper tissue at various locations and finding the paper wet
even at the lowest liquid Reynolds number of 6.1.
[0031] Contact Angle Measurement of Oxidized Copper
[0032] Contact angle measurement is often used to measure the
affinity of a liquid with surfaces. FIG. 3 shows computer images of
the contact angle measurement. As an attempt to confirm the
well-known hydrophilic property of the metal oxides (copper (I)
oxide), a contact angle measurement was performed using an optical
contact angle and surface tension meter (model: CAM-100,
manufacturer: KSV) based on the sessile drop method equipped with a
high speed digital camera and an image analyzing software.
Distilled water was used as the testing liquid. For the contact
angle measurement, six test coupons (W.times.L=25 mm.times.15.2 mm)
made of Oxygen Free High thermal conductivity Copper (OFHC) were
prepared. Each coupon was prepared by first polished (only in one
direction) by a very fine sandpaper (1500 grit), deoxidized with a
commercial cleaner (TARN-X), dried, and then rinsed with acetone.
Note that water and copper is chemically compatible and the copper
oxidation is mainly affected by oxygen concentration in environment
and accelerated at elevated temperatures.
[0033] For mild oxidation, two test coupons (samples #4 and 5) were
left for two days in open air. Note that in Reno Nev., where the
tests were performed, the air is dry (.about.45% humidity) and the
atmospheric pressure is about 0.89 atm. Another test coupon (sample
#6) was heated at 100.degree. C. for 2 hours on an electrical hot
plate to accelerate the oxidation in open air. All the samples were
tested within 5.about.10 minutes after the sample preparation.
[0034] The contact angle measurement results are also listed in
Table 1. The polished surfaces (oxidation-free, FIG. 3(a)) have the
contact angles between a range of 75.degree..about.80.degree.,
whereas the surfaces oxidized at 100.degree. C. for two hours
(accelerated oxidation, FIG. 3(c)) show a significantly decreased
contact angle of 62.78.degree.. However, the results show that the
oxidized samples (FIG. 3(b)) at room temperature for two days have
a negligible change in the contact angle from those of the polished
surfaces. The contact angle measurement results support that aging
(oxidation) improves surface wettability (hydrophilicity) and
oxidation temperature plays a role in the oxidation level. It
implies that monitoring of the aging effect may be beneficial
because an improper aging procedure can cause inconsistent results
for two-phase heat transfer experiments (boiling, condensation and
evaporation).
[0035] Typical boiling studies are performed in closed systems
after degassing of noncondensible gases (e.g., air) which could be
intentionally or accidentally introduced during a series of
assembling and liquid charging or already exist in working fluids
as a dissolved gas. Note that air contains oxygen (20.9% vol). The
aging (oxidation) is mainly affected by an exposure time of metal
samples to oxygen, its concentration and temperature. The detailed
oxidation mechanism is discussed below. After degassing, the low
concentration air (or oxygen) in the systems could cause a slow
oxidation. By measuring oxygen concentration in the system, the
aging can be systematically monitored and controlled for consistent
and time-efficient boiling studies. It is common to observe that
most of open literatures on two-phase experimental works loosely
use the aging term to explain the conditions used in their
experiments. The present disclosure establishes a rigorous testing
protocol for the two-phase experiments which are greatly affected
by oxidation (aging).
TABLE-US-00001 TABLE 1 Surface contact angle measurement results
Polished Mild Oxidation Surfaces (at Accelerated Oxidation
(Oxidation-Free) Surfaces room temperature for two Surface (at
100.degree. C. for (using Grit 1500) days in ambient) two hours in
ambient) Sample-1 Sample-2 Sample-3 Sample-4 Sample-5 Sample-6
75.43.degree. 78.25.degree. 79.57.degree. 72.97.degree.
82.39.degree. 62.78.degree.
[0036] One aspect of the present disclosure is to provide
capillary-assisted evaporation in nano-coating, porous integral
fins on falling-film, horizontal-tube evaporators. The same
capillary (gravity-insensitive) liquid distribution, and
consequently, thin film evaporation (low thermal resistance of 0.05
K-cm.sup.2/W) used in wicked heat pipes can be used to mitigate
inherent liquid maldistribution and surface nonwetting problems of
the conventional plain surface evaporators and create an ideal
evaporation condition, i.e., thin film evaporation with low-thermal
resistance over increased surface wetting even with less supply of
liquid. The disclosed porous integral fins can further enhance the
evaporation due to the increased evaporation area.
[0037] The present disclosure allows the effects of the following
variables/conditions to be used to enhance falling-film
evaporation: physical variables (porosity, permeability, pore size
(10.about.100 .mu.m), pore size/distribution, and specific surface
area) and geometrical properties (fin height (.about.1 cm), width
and pitch), thermo-physical properties (effective thermal
conductivity) of porous integral fins and surface wettability
(nano-coating), evaporator design (tube diameter/length/spacing and
liquid dispenser) and operating conditions (flow rate and
temperature of feed liquid and vapor pressure).
[0038] Porous Integral Fin Fabrication
[0039] The present disclosure, in one specific embodiment, uses
sintered (diffusion bonded) powders (e.g., copper and metal oxides)
to create porous coating/integral fins using physical properties of
porous media (porosity, permeability, pore size/distribution, and
specific surface area), geometrical variables of porous integral
fins (fin pitch, height, and thickness; fin base thickness) and
evaporator design variables (tube diameter, length, number of tubes
and nozzle height). In some implementations, sintering techniques
for sintered metallic (copper, and nickel) powder/screen composite
for two-phase heat sinks (e.g., heat pipes) for high performance
electronic cooling can be applied to achieve the disclosed
purposes.
[0040] In one example, the porous integral fins made of metal
powder and/or screen meshes are built on a substrate by diffusion
bonding (sintering) in a mold using a multi-step procedure. A
rendering showing the multi-step sintering process is illustrated
in FIG. 4. The molds (stainless steel or graphite), in a more
specific example, are fabricated to provide a 3-D negative image of
desired surface structures on the substrate. The dimensions of the
porous structures can be chosen as desired can be determined, for
example, by the fabrication method and machining tolerance of the
molds.
[0041] The process, in a particular example, proceeds as follows.
First, the metal powders are filled into a mold through fill ports
and are shaken compact. Then, the powder-laden molds are placed
into a high temperature, quartz glass tube furnace and heated up to
sintering temperatures (.ltoreq.1000.degree. C. for copper powder),
which is lower than melting temperature of the metal, in an
inert/reducing atmosphere such as a nitrogen/hydrogen mixture
(forming gas) over a preset time (<1 hour for copper). At
elevated sintering temperature, the metal particles are
diffusion-bonded (sintered) to each other and to the copper
substrate due to increased mobility of metal atoms creating a
desired 3-D porous structure. The sintering duration can vary with
metal powder kind, particle size, surface condition and size of
sintering sample. Particles with nominal diameters as small as 50
.mu.m are considered suitable for sintering.
[0042] This sintering technique has been used, for example, for
fabrication of micro-scale pore structures used for heat pipes.
Other fabrication methods can be used, such as for higher
production rates for the modulated coatings. The surface morphology
of the micro-scale porous coating can be visually inspected and or
measured using suitable techniques, such as SEM and AFM.
[0043] FIG. 5(a) shows a porous integral fin illustrating a
capillary-driven liquid flow/distribution and surface evaporation
of the liquid drip from a liquid dispenser. The porous layer
adjacent to the heating substrate provides liquid flow conduits to
fins built on the substrate. The geometric optimization of the
porous fins (spacing (pitch), height, thickness, and shape) allows
for low pressure drops for liquid flow. FIG. 5(b) shows a porous
integral fin fabricated using a sintering furnace.
[0044] Falling-Film Evaporation
[0045] To verify the enhancement mechanism of falling-film
evaporation using the porous integral fins, the porous integral
fins (as shown in FIG. 5 as an illustrative example) of sintered
metal powder on horizontal tubes are tested in a
hermetically-sealed chamber under saturated or reduced pressure
conditions. A plain tube evaporator is tested to establish the
baseline results of evaporation heat transfer coefficient and
liquid distribution/wettability and number of droplet sites. To
establish a validity of the evaporation studies, the baseline
results are compared with published data based on the same
conditions.
[0046] A schematic of an example of a hardware setup for
falling-film evaporation is shown in FIG. 6. The hardware setup
based on a closed-system design consists of at least three basic
components: (i) an evaporator with a heater/degassing system; (ii)
a condenser with a chiller system in a hermetically-sealed chamber
with a vacuum pump; and (iii) a measurement/control system. Some
evaporation methods are carried out using high surface tension
fluids (e.g., water) and low surface tension fluids (e.g.,
refrigerants) as the working fluids. In a specific example, the
evaporation chamber is constructed of a corrosion-resistant
stainless steel (SS 316) which is compatible to the working fluids
and be designed to endure high pressures for the vapor pressure of
the working fluids. The wet components (e.g., gasket, valve and
fitting) are typically chosen depending on the material
compatibility level with the working fluids.
[0047] A quartz glass viewport in the chamber can be used to view
and record the falling-film evaporation and surface wetting
condition on the horizontal tubes using a long-focal point
microscope/high speed video camera and an infrared thermometer
camera. The visual observation using the high speed camera can
provide information on liquid film/droplet distribution on the tube
surface and liquid drip pattern in the tube bundle. The infrared
thermal image can provide an accurate estimation of the wetted
surface area by measuring a sharp change in the surface temperature
distribution due to surface evaporation. The infrared thermal image
may also be used to measure the surface temperature distribution
for evaporation heat transfer calculation, in addition to the
direct temperature measurement using thermocouples embedded in the
evaporator tubes.
[0048] In some methods, the working liquid is degassed before use
using a heater/degassing tank with an ultrasonic stirrer as shown
on the left side of FIG. 6. Such a system can, with, for example,
FC-72. Fluorocarbons can absorb a significant degassing of air at
atmospheric conditions. Depending on the loop construction, this
separate degassing system might be omitted, in some examples, such
as for water, and sufficient degassing could be obtained by running
the evaporation/boiling loop for a significant time before official
operation (this would separate any dissolved gases in the system)
and evacuating the collected gases from the upper part of the
chamber using a vacuum pump. Degassing helps control the
evaporation/condensation affected by non-condensable gases and if
air exists in the system, surface oxidation of evaporator tubes
would alter the evaporation performance over time.
[0049] A temperature-controlled, external cooling/heating loop is
used, in some examples, to control the temperatures of the
evaporator and condenser. The evaporator may be heated by a heating
fluid circulated by an external heating loop. The condenser may be
cooled by a coolant loop connected to an external chiller loop. The
system can be insulated with commercial insulation materials. This
helps to provide quicker heating/cooling to reach a steady-state
condition. A series of copper-constantan (T-type) thermocouples can
be embedded along the evaporator tubes (axially and
circumferentially). The evaporation heat transfer rate may be
estimated using the measured temperature differential of the
heating fluid at the inlet and outlet of the evaporator tubes.
Similarly for the condenser heat transfer measurement. Evaporation
studies can be performed to generate heat transfer coefficient
curves by gradually varying wall superheat of the evaporator and
ambient pressure (by varying condenser temperature).
[0050] When a liquid film flows from one horizontal tube to another
below it, according to an increasing flow rate order, the flow may
take the form of droplets, circular jets or continuous sheet.
[0051] Both porous integral fins (tube diameter, fin height, width,
pitch) and porous medium properties (porosity, permeability,
specific surface area, pore size/distribution, and effective
thermal conductivity) can be optimized for improved system
performance.
[0052] Surface Oxidation Kinetics
[0053] The oxidation layer thickness ( ) of metals under a high
temperature oxidizer can be generally estimated by the following
exponential oxidation law
d = 2 k o exp ( - Q o RT ) t ( 1 ) ##EQU00001##
[0054] where k.sub.o is the oxidation constant [m.sup.2/s] and
Q.sub.o is the activation energy [J/mol] and T is the oxidation
temperature [K] and t is the elapsed time [sec]. k.sub.o and
Q.sub.o are empirically determined. For a steel (grade: MS 1200),
k.sub.0=0.076 m.sup.2/s and Q.sub.o=239 kJ/mol. The effect of
oxygen concentration is not considered in the above equation. The
oxygen concentration, along with the metal specimen temperature,
can affect the oxidation rate. If oxidation occurs in a closed
system under saturation or reduced pressure (i.e., very low oxygen
concentration), the oxidation rate is typically slower than under
ambient condition (20.9% vol. oxygen). Since the oxidation layers
increase the thermal resistance of the conduction heat transfer
because of low thermal conductivity than the fresh surface, the
thickness of the oxidation layer may influence heat transfer
reduction.
[0055] Nano-Scale Surface Morphology of Oxidation Layer
[0056] Copper is often used for boiling surfaces because of its
high thermal conductivity and good chemical compatibility with many
working fluids, including water. But copper becomes easily oxidized
forming copper oxides (Cu.sub.2O and CuO) even at room temperature
while exposed to oxygen in air while preparing samples The reddish
copper oxidation (copper (I) or cuprous oxide, Cu.sub.2O) is
naturally formed in ambient conditions over an extended time.
Accelerated formation of the surface oxidation is usually achieved
at elevated temperatures. With further heating, the copper (I)
oxide is converted into a blackish copper oxide (copper (II) or
cupric oxide, CuO).
[0057] Since many applications, such as heat pipes, are closed
systems, the oxidation can be significantly reduced, but not
completely removed, depending on preparation conditions and
assembling procedure.
[0058] Surface morphology changes due to surface oxidation can be
characterized by AFM and/or SEM using test coupons oxidized under
various temperature and oxygen-concentration over different
oxygen-exposure times and quantitatively measured in terms of
coverage area and thickness of the oxidation layer, and the effect
of the oxidation level on thin falling-film evaporation
investigated by the contact angle measurement. To quantitatively
measure the change of the surface morphology due to surface
oxidation, test coupons free of oxidation can be prepared (cleaned
in a tube furnace using a reducing environment or chemically) as a
baseline surface condition and used for surface roughness
measurement using AFM in an environmental chamber or a glove box
using temperature-controlled and inert cover gases (e.g., Ar or
N2).
[0059] To quantitatively measure the increase of surface
wettability due to the surface oxidation, the contact angle
measurement can be performed in the environmental chamber to
establish a correlation between the contact angle and surface
morphology measurements. To preserve the samples from being further
oxidized before being used for evaporation experiments, a
protective cover and/or coating can be used. This approach can
provide more repeatable and consistent study on the in-situ surface
oxidation.
[0060] Nano-Coating for Surface Wettability Control
[0061] In addition to surface oxidation, surface nano-coating is an
effective way to create various surface wettability from
hydrophilicity (SiO.sub.x, TiO.sub.2, Pt and Fe.sub.2O.sub.3) to
hydrophobicity (SiOC and Teflon). Particles of very small size
(typically less than 100 nm) called "nanoparticles" can be
deposited, such as on heated surface using Metal-Organic Chemical
Vapor Deposition (MOCVD) and Plasma Enhanced Chemical Vapor
Deposition (PECVD) and Nanofluid Nucleate Boiling Deposition
(NNBD). Nanofluids that include metal oxide nanoparticles, such as
Alumina (Al.sub.2O.sub.3), Zirconia (Z.sub.rO.sub.2) and Silica
(SiO.sub.2), for pool boiling indicate that the boiling critical
heat flux increases significantly as compared to those of the pure
fluids. The enhanced heat flux is likely at least partially due to
increased surface wettability of the nanofluids by deposition of
the nanoparticles on the heater surface. In this disclosure, the
effect of the micro-structural variables such as porosity, pore
size, and specific surface area of nano-coating on the wetting
property are used to enhance cooling devices. In some examples,
oxide materials such as Alumina, Zirconia, Silica, and Titania
(TiO2) are used for nano-coatings.
[0062] Tailoring surface nano-coatings to create specific wetting
properties the surface wettability level to be correlated to
evaporation performance. Nano-coatings also allow surface
wettability to be modified without significant changing the
micro/macro-scale surface topology of the porous structured
surfaces.
[0063] Design of Falling-Film Evaporation in Porous Media
[0064] Theoretical considerations can be used to predict and
identify dry-out, maximum heat flux of conditions the falling-film
evaporation using porous media. Theoretical analyses, in one
example, are a combination of analytical and numerical solutions to
the governing equations for two-phase flow and heat transfer in
plain and porous media. Numerical solutions based on the
finite-volume can be performed on the plain and porous media
governing equations. The overall goal is to apply the theory of the
evaporation in structured porous layer (as shown in FIG. 5 as an
illustrative example). This information can be used to enhance the
design of the capillary-driven evaporator systems.
[0065] Falling-Film Evaporation
[0066] The falling liquid flow around a smooth, horizontal tube can
be divided into four regions: stagnation, Jet impingement, thermal
developing and fully-developed. The jet impingement region has the
largest heat transfer coefficient due to a small surface-convection
resistance of jet impingement flow. The stagnation region is often
a very small portion of the circumference of the tube and is often
neglected for heat transfer calculation. In the thermal developing
region, a thermal boundary layer typically develops, resulting in a
large thermal resistance. Thinning the liquid layer in the thermal
developing and fully developed regions can enhance the overall
evaporation performance.
[0067] Using porous fins on evaporators can allow for less liquid
flow, creating a favorable thin liquid film layer resulting in
enhanced evaporation. The heat transfer can be reduced at the jet
impingement region due to the slow liquid flow but increased for
the rest of the regions because of thinner film evaporation.
[0068] The non-boiling heat transfer coefficient for smooth
horizontal tubes is given by
h _ ( v 2 gk 3 ) = C Re 0.15 Pr 0.53 ( 2 ) ##EQU00002##
[0069] where h is the averaged heat transfer coefficient and C is
the constant varying with the tube diameter and k is the thermal
conductivity of the liquid film. The disclosed porous structured
coating made of metallic materials (e.g., copper) provides a higher
effective thermal conductivity than that of the liquid film and the
constant C can increase due to surface wettability increase by the
surface oxidation and nano-coating. As a result, higher evaporation
heat transfer coefficients may be achieved.
[0070] Spreading of Liquid Drops in Porous Layers
[0071] Spreading of liquid drops over thin porous layers (which are
saturated with the same liquid) is a phenomena useable to
understanding the liquid distribution in the disclosed porous
integral fins and the falling-film evaporation. The spreading of
the liquid drops over the porous media is governed by the same
power law as in the case of spreading over a dry solid substrate.
The evolution of the drop profile can be calculated by integrating
the Navier-Stokes equations with boundary conditions considering a
slippage condition over the porous layer. The following Brinkman's
equation can be used to model the liquid flow inside the porous
layer and slippage velocities for the calculation of the evolution
of the drop profile.
0 = - .gradient. P 1 - .mu. K u 1 + .mu. e .gradient. 2 u 1 ( 3 )
##EQU00003##
[0072] Evaporation Under Reduced (Vacuum) Pressures
[0073] Evaporation under reduced (vacuum) pressures, which is lower
than the saturated pressure, requires non-equilibrium treatment for
the phase change. For an evaporation process according to the
present disclosure, the pressure of the vapor at the liquid surface
(p.sub.vap) could be less than the saturation vapor pressure
(p.sub.vap) corresponding to the liquid surface temperature
(T.sub.sat). From a kinetic theory of phase change under such low
pressures, the evaporation rate can be calculated by
m . '' = 2 .sigma. 2 - .sigma. [ P sat ( 2 .pi. RT sat ) 1 / 2 - P
vap ( 2 .pi. RT vap ) 1 / 2 ] ( 4 ) ##EQU00004##
[0074] where .sub..rarw.1 is used for water vapor molecules at
non-contaminated surface. The evaporation heat flux is calculated
by q.sub.eva={dot over (m)}''h.sub.fg.
[0075] Two-Phase Heat Transfer Modes in Porous Media
[0076] Two-phase heat transfer (evaporation and boiling) in porous
media is typically more complex than from plain surfaces due to the
existence of porous structures. The heat transfer, liquid film
distribution and vapor formation may change with porous media
properties (material and geometrical), operating conditions and
working fluids. As a result, four major operation modes determined
by the "heat flux or wall superheat" may exist in the
evaporation/boiling and are shown schematically in FIGS. 7A-7D
based on an uniform plain porous layer to help discussion in next
sections.
[0077] (i) Conduction-Convection-Surface Evaporation
[0078] As shown in FIG. 7(A), at the low heat flux conditions, the
entire porous layer is fully saturated with liquid where conduction
occurs across the liquid layer and evaporation takes place from
only the surface (liquid-vapor interface) of the porous layer. The
heat transfer across the porous layer can be calculated by a
conduction model. No boiling occurs within the porous layer.
Natural convection may occur within the porous layer under a
gravitational field. The heat transfer across the wick can be
calculated by a conduction mode given by:
q/(T.sub.s-T.sub.v)=k.sub.wick/L.sub.k (5)
[0079] k.sub.wick is the effective thermal conductivity of the
porous layer. L.sub.k is the thickness of the wick and calculated
based on the averaged thickness of the 3-D structured wick. The
flow and pressure drop through porous media can be modeled using
the following Darcy-Ergun equation:
0 = - .gradient. P 1 + .rho. 1 g - .mu. K u 1 - C E K 1 / 2 .rho. 1
u 1 u 1 ( 6 ) ##EQU00005##
[0080] The capillary pressure
(.DELTA.P.sub.c,max>.DELTA.P.sub.1) can be related to the liquid
saturation, porosity, permeability and wettability using Leverett
J-function.
[0081] (ii) Conduction-Convection-Surface Evaporation
[0082] As shown in FIG. 7(B), as heat flux is gradually increased,
the evaporation at the liquid surface intensifies. The capillary
pumping may not be large enough to feed liquid. Consequently, the
liquid begins to recede into the porous layer. If the receding of
liquid continues, the liquid at the evaporator may completely dried
out. This limit encountered is called as the "capillary (or
hydrodynamic) limit" (not the boiling limit). Before the liquid is
completely depleted, the heat transfer across the liquid layer is
still by conduction, and the liquid vaporization takes place at the
liquid-vapor interface. No boiling occurs within the porous
layer.
[0083] (iii) Nucleate Boiling
[0084] As shown in FIG. 7(C), when the heat flux further increase
and therefore the temperature difference (wall superheat) across
the wick becomes large, nucleate boiling may take place within the
porous layer. Bubbles grow from the nucleate boiling sites in the
porous layer, escape to the liquid surface and burst rapidly. Since
the liquid feed in the porous layer is driven by the capillary
force, nucleate boiling in the porous layer represents a heat
transfer limit (boiling limit).
[0085] (iv) Film Boiling
[0086] As shown in FIG. 7(D), as the temperature difference across
the porous layer is further increased, a large quantity of bubbles
is generated at the heating surface. These bubbles coalesce
together, forming a blanket of vapor adjacent to the heating
surface, which blocks the liquid from reaching the heating surface.
This heat transfer limit is called as the "boiling limit" which is
similar to the critical heat flux condition in pool boiling, and is
the maximum heat transfer limit for the film boiling for the
following reasons: (1) large bubbles bursting may destroy the
menisci at the liquid-vapor interface and interrupt the
capillary-driven liquid flow; and (2) vapor bubbles formed in the
evaporator porous layer may hinder the liquid flow. The boiling
limit is similar to the critical heat flux condition in pool
boiling, and is the maximum heat transfer limit for the boiling in
the porous layer.
[0087] The disclosed falling-film evaporator, in one
implementation, uses low heat fluxes (or low superheats) conditions
to create the surface evaporation condition (as shown in FIGS. 7(A)
and (B)) and to limit the nucleate boiling at high heat fluxes in
porous media. The reduced pressure conditions will be also used to
promote the surface evaporation using sub-cooled liquid.
[0088] It is to be understood that the above discussion provides a
detailed description of various embodiments. The above descriptions
will enable those skilled in the art to make many departures from
the particular examples described above to provide apparatuses
constructed in accordance with the present disclosure. The
embodiments are illustrative, and not intended to limit the scope
of the present disclosure. The scope of the present disclosure is
rather to be determined by the scope of the claims as issued and
equivalents thereto.
* * * * *