U.S. patent number RE30,077 [Application Number 05/862,218] was granted by the patent office on 1979-08-21 for surface for boiling liquids.
This patent grant is currently assigned to Union Carbide Corporation. Invention is credited to Alfred M. Czikk, Leslie C. Kun.
United States Patent |
RE30,077 |
Kun , et al. |
August 21, 1979 |
Surface for boiling liquids
Abstract
A boiling surface layer is formed on a thermally conductive wall
comprising a plurality of ridges separated by grooves provided at
microscopic density, with outer sections of the ridges partly
deformed into adjacent grooves to provide sub-surface cavities with
restricted openings to the outer surface and sub-surface, openings
between some of the cavities.
Inventors: |
Kun; Leslie C. (Williamsville,
NY), Czikk; Alfred M. (Williamsville, NY) |
Assignee: |
Union Carbide Corporation (New
York, NY)
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Family
ID: |
27115395 |
Appl.
No.: |
05/862,218 |
Filed: |
December 19, 1977 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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634403 |
Apr 7, 1967 |
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414755 |
Nov 30, 1964 |
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Reissue of: |
751321 |
May 14, 1968 |
03454081 |
Jul 8, 1969 |
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Current U.S.
Class: |
165/133;
122/367.3; 165/911; 62/527 |
Current CPC
Class: |
F22B
37/101 (20130101); F28F 13/187 (20130101); F28F
13/18 (20130101) |
Current International
Class: |
F22B
37/00 (20060101); F28F 13/00 (20060101); F28F
13/18 (20060101); F22B 37/10 (20060101); F28F
013/18 (); F28F 019/02 () |
Field of
Search: |
;165/133,74,105,185,184,DIG.14 ;122/367R,367C ;62/527 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Zuber; N., Recent Trends in Boiling Heat Transfer Research Part 1:
Nucleate Pool Boiling, Applied Mechanics Reviews, 9/1964, pp.
663-672..
|
Primary Examiner: Davis, Jr.; Albert W.
Attorney, Agent or Firm: Fritschler; Alvin H.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This is a continuation-in-part of application Ser. No. 634,403
filed Apr. 7, 1967 and entitled, "Surface for Boiling Liquids," and
now abandoned which in turn is a continuation-in-part of
application Ser. No. 414,755 filed Nov. 30, 1964 and entitled,
"Heat Exchange System," and now abandoned.
Claims
What is claimed is: .[.1. A thermally conductive metal wall for
transferring heat to a boiling liquid in a heat exchange apparatus
which comprises a boiling surface layer formed from the wall having
a plurality of ridges in said wall separated by grooves provided at
microscopic density, with outer sections of said ridges partly
deformed into said grooves such that a plurality of sub-surface
cavities are formed therein with at least some of said cavities
adapted to entrap vapor bubbles to provide boiling nucleation
sites, the nucleation site cavities opening to the outer surface of
said boiling surface layer through restricted openings having
smaller cross-sectional area than the largest cross-sectional area
of the cavity interiors with said restricted openings providing
communication between the interiors of said cavities and the outer
surface of said boiling surface layer for vapor egress; and said
grooves and cavities being formed to provide sub-surface openings
between at least some adjacent cavities providing fluid
communication with the outer surface of said boiling surface layer
for liquid ingress to sustain growth of entrapped vapor bubbles as
vapor is expelled from said
restricted openings..]. 2. A thermally conductive wall according to
claim .[.1.]. .Iadd.17 .Iaddend.in which said ridges and grooves
are parallel to each other. .[.3. A thermally conductive wall
according to claim 1 in which the grooves are provided at density
of greater than about 20 grooves
per inch..]. 4. A thermally conductive wall according to claim
.[.1.]. .Iadd.17 .Iaddend.in which the grooves are provided at
density of greater than about 80 grooves per inch. .[.5. A
thermally conductive wall according to claim 1 in which said ridges
are formed by metal displaced from said grooves..]. .[.6. A
thermally conductive wall according to claim 1 in which a plurality
of second grooves in spaced relation to each other and at
microscopic density are superimposed on said ridges in intersecting
relation therewith..]. .[.7. A thermally conductive wall according
to claim 1 in which a plurality of depressions in spaced relation
to each other and at microscopic density are superimposed on
said
ridges in intersecting relation therewith..]. 8. A thermally
conductive metal wall for transferring heat to a boiling liquid in
a heat exchange apparatus which comprises a boiling surface layer
formed from the wall having a plurality of ridges in said wall
separated by first grooves provided at .[.microscopic.]. density
.Iadd.of from about 45 to about 225 grooves per inch .Iaddend.with
outer sections of said ridges partly deformed into said first
grooves, and a plurality of second depressions in rows spaced from
each other provided at .[.microscopic.]. density of .Iadd.from at
least about 45 to about 225 depressions per inch .Iaddend.and
superimposed on said ridges in intersecting relation therewith,
said ridges, first grooves and second depressions being shaped such
that a plurality of sub-surface cavities are formed in said first
grooves with at least some of said cavities adapted to entrap vapor
bubbles to provide boiling nucleation sites, the nucleation site
cavities opening to the outer surface of said boiling surface layer
through restricted openings having smaller cross-sectional area
than the largest cross-sectional area of the cavity interiors with
said openings providing communication between the interiors of said
cavities and the outer surface of said boiling surface layer for
vapor egress, and said first grooves, second depressions and
cavities being formed to provide sub-surface openings between
.Iadd.said .Iaddend.at least some adjacent cavities providing fluid
communication with the outer surface of said boiling surface layer
for liquid ingress to sustain growth of entrapped vapor
bubbles as vapor is expelled from said restricted openings. 9. A
thermally conductive wall according to claim 8 in which said ridges
and first
grooves are parallel to each other. 10. A thermally conductive wall
according to claim 8 in which said ridges are formed by metal
displaced
from said first grooves and second depressions. 11. A thermally
conductive wall according to claim 8 in which said second
depressions are grooves
oriented parallel to each other. 12. A thermally conductive wall
according to claim 8 in which the depth of said first grooves is
greater than the
depth of said second depressions. 13. A thermally conductive wall
according to claim 8 formed of aluminum in which said ridges and
first grooves are parallel to each other with said ridges formed by
metal displaced from said first grooves and second depressions,
said second depressions are grooves oriented parallel to each other
and 90 degrees from said first grooves, and said first grooves and
second depressions are
provided at density of 140- 200 per inch. 14. A thermally
conductive wall according to claim 8 in which said ridges and first
grooves are parallel to each other, said second depressions are
oriented parallel to each other and 90 degrees from said first
grooves, and said first grooves are provided at density of
.Badd..[.20-120.]..Baddend. .Iadd.45- 120
.Iaddend.per inch. 15. A thermally conductive wall according to
claim 8 in which said ridges and first grooves are parallel to each
other, said second depressions are parallel to each other and
oriented 90 degrees from said first grooves. .[.16. A thermally
conductive metal wall for transferring heat to a boiling liquid in
a heat exchange apparatus which comprises a boiling surface layer
formed from the wall having a plurality of ridges in said wall
separated by grooves provided at density of greater than about 80
grooves per inch, with outer sections of said ridges partially
deformed into said grooves such that a plurality of sub-surface
cavities are formed therein with at least some of the cavities
adapted to entrap vapor bubbles to provide boiling nucleation
sites, the nucleation site cavities opening to the outer surface of
said boiling surface layer through restricted openings having
smaller cross-sectional area than the largest cross-sectional area
of the cavity interiors providing communication between the
interiors of said cavities and the surface of said boiling surface
layer for vapor egress, and said grooves and cavities being formed
to provide sub-surface openings between at least some adjacent
cavities for communication between the interiors of said adjacent
cavities and the outer surface of said boiling surface layer for
liquid ingress to sustain growth of entrapped vapor bubbles as
vapor is expelled
from said restricted openings..]. 17. A thermally conductive metal
wall for transferring heat to a boiling liquid in a heat exchange
apparatus which comprises a boiling surface layer formed from the
wall having a plurality of ridges in said wall separated by first
grooves with outer sections of said ridges partially deformed into
said first grooves, and a plurality of second grooves superimposed
on said ridges at an angle to the orientation of said ridges, said
ridges and first and second grooves being shaped such that a
plurality of sub-surface cavities are formed in said first grooves
with at least some of the cavities adapted to entrap vapor bubbles
to provide boiling nucleation sites, the nucleation site cavities
opening to the outer surface of said boiling surface layer through
restricted openings having smaller cross-sectional areas than the
largest cross-sectional area of the cavity interiors providing
communication between the interiors of said cavities and the
surface of said boiling surface layer for vapor egress, and said
first and second grooves and cavities being formed to provide
sub-surface openings between .Iadd.said .Iaddend.at least some
adjacent cavities for communication between the interiors of said
adjacent cavities and the outer surface of said boiling surface
layer for liquid ingress to sustain growth of entrapped vapor
bubbles as vapor is expelled from said restricted openings .Iadd.,
with said first and second grooves each provided at density of from
about 45 to about 225 grooves per inch.Iaddend.. .[.18. A thermally
conductive metal wall for transferring heat to a boiling liquid in
a heat exchange apparatus which comprises a boiling surface layer
having a pluraity of ridges separated by grooves provided at
microscopic density with said ridges shaped to form a plurality of
sub-surface cavities with at least some of said cavities adapted to
entrap vapor bubbles and constitute boiling nucleation sites, the
nucleation site cavities communicating with the outer surface of
said boiling layer through restricted openings between outer
sections of said ridges and having smaller cross-sectional area
than the largest cross-sectional area of the cavity interiors for
vapor egress, and said grooves and cavities being formed to provide
sub-surface openings between at least some adjacent cavities
providing fluid communication with the outer surface of said
boiling surface layer for liquid ingress to sustain growth of
entrapped vapor bubbles as vapor
is expelled from said restricted openings..]. 19. A thermally
conductive wall according to claim .[.18.]. .Iadd.17 .Iaddend.in
which the limiting dimension of said restricted openings is less
than about 5 mils. .[.20. A thermally conductive wall according to
claim 1 in which the limiting
dimension of said restricted opening is less than about 5 mils..].
21. A thermally conductive wall according to claim 8 in which the
limiting
dimension of said restricted openings is less than about 5 mils.
22. A thermally conductive wall according to claim 8 in which said
ridges and first grooves are parallel to each other with said
ridges formed by metal displaced from said first grooves and second
depressions, said second depressions are grooves oriented parallel
to each other, said first grooves and second depressions are
provided at density of 140- 200 per inch, and in which the limiting
dimension of said restricted openings is
less than 1.75 mils. 23. A thermally conductive wall according to
claim 8 in which said ridges and first grooves are parallel to each
other, said second depressions are oriented parallel to each other,
said first grooves are provided at density of 45- 120 per inch, and
in which the limiting dimension of said restricted openings is
1.75- 4 mils.
Description
BACKGROUND OF THE INVENTION
This invention relates to the art of improving heat transfer from
heated surfaces to boiling liquids, and particularly to surfaces
which enhance the phenomenon of nucleate boiling. The invention
also relates to a method for forming a layer containing the
surfaces.
The transfer of heat at effective rates from a heated surface to a
boiling liquid in contact therewith ordinarily requires a
substantial temperature difference between the surface and the
liquid which greatly affects the efficiency of heat transfer. One
important factor controlling this efficiency is the nature of the
heated surface in contact with the liquid; it being known, for
example, that smooth boiling surfaces produce low heat transfer
coefficients on the boiling side. Low boiling heat transfer
coefficients often severely restrict the heat transfer capacity of
boiling apparatus. For example, when the heat for boiling is
supplied by a vapor condensing on a smooth-walled heat transfer
surface, the condensing heat transfer coefficient may easily be on
the order of 2,000 B.t.u./hr./sq. ft./.degree.F., while the boiling
heat transfer coefficient against the opposite side of the heat
transfer surface may be only 100 to 200 B.t.u./hr./sq.
ft./.degree.F. According to the familiar method of summing heat
transfer resistances when the boiling and condensing heat transfer
surfaces are of equal area, the overall heat transfer coefficient U
is obtained approximately as follows: ##EQU1## where h.sub.B and
h.sub.C are the boiling and condensing heat transfer coefficients
respectively. It is clear that if h.sub.B is small compared to
h.sub.C, then the value of U approaches h.sub.B and most of the
advantage of a high condensing coefficient is lost.
Principal objects of this invention are: to provide a thermally
conductive wall for transferring heat to a boiling liquid in a heat
exchange apparatus having a boiling surface layer containing a
plurality of cavities adapted to provide boiling nucleation sites
within the surface layer; to provide a thermally conductive wall
with a grooved boiling surface layer of a character which produces
boiling heat transfer coefficients many times as large as those
obtained with conventional smooth or roughened surfaces; and to
provide a cross-grooved boiling surface layer of a character that
is able to transfer to a boiling liquid large quantities of heat at
much lower temperature differences than required in conventional
heat exchange apparatus.
These and other objects and novel features of the invention will
become apparent from the following description and accompanying
drawing.
SUMMARY
According to this invention, there is provided a heat exchange wall
having a boiling surface layer formed thereon with a plurality of
cavities within the boiling surface layer. These cavities are
sub-surface cavities adapted to entrap vapor bubbles within the
boiling surface layer to provide boiling nucleation sites. The
cavities open to the outer surface of the boiling surface layer
through restricted openings which have cross-sectional area smaller
than the largest cross-sectional areas in the cavity interiors and
which provide communication between the interiors of the cavities
and the surface of the boiling surface layer for vapor egress
during boiling and liquid ingress. The cavities also have
sub-surface openings providing communication between the interiors
of the cavities for liquid ingress to sustain growth of the
entrapped vapor bubbles during the boiling process as vapor is
expelled from the restricted openings.
In one embodiment of this invention, the boiling surface layer of a
heat exchange wall is formed by providing a plurality of ridges in
the surface of the wall, each ridge being separated from adjacent
ridges by grooves at microscopic density. The outer sections of the
ridges remote from the wall are partially deformed into adjacent
grooves such that the aforementioned cavities are formed in the
grooves. As used herein the term "microscopic" refers to objects so
small or fine as to be not clearly distinguished without the use of
a microscope. The individual grooves, cavities and ridges of the
low groove density boiling surface layers (e.g. 20 grooves per
inch) are visible to the naked eye. However the restricted openings
from the cavities to the outer surface of even these layers cannot
be readily distinguished without aid of a microscope. Since the
cross-sectional area relationship between the cavity and the
restricted opening is essential to the growth of vapor bubbles, the
layers of this invention may not be clearly identified by the naked
eye. It is in this sense that the grooves are provided at
microscopic density.
In another, and preferred, embodiment of this invention the boiling
surface layer of a heat exchange wall is formed by providing a
plurality of ridges in the surface of the wall, each ridge being
separated from adjacent ridges by grooves, and by providing a
second plurality of depressions or grooves superimposed on the
ridges at an angle to the orientation of the ridges. By
superimposition of depressions or grooves on the ridges, the ridges
are segmented into sections and the extent of segmentation depends
in part on the relative depths of the two sets of grooves. For
example, if the superimposed grooves have the same depth as the
first-formed grooves, the ridge sections will tend to be completely
isolated from adjacent ridge sections. The outer sections of the
ridges are partially deformed into adjacent grooves such that the
aforementioned cavities are formed in the grooves. This embodiment
has a cross-grooved appearance.
In both of the embodiments described above, the grooves perferably
extend substantially completely across the surface of the heat
exchange wall and are preferably of uniform density. These two
preferred conditions enhance the likelihood of uniform boiling
performance across the boiling surface layer. In addition, the
density of the grooves is preferably relatively high, being greater
than 20 grooves per inch, for reasons that will be discussed
subsequently.
Another aspect of this invention relates to a method for forming a
boiling surface layer from a thermally conductive metal wall. The
grooved boiling surface layer embodiments described above are
preferably formed by scoring the surface of the heat exchange wall
such that the wall material is substantially displaced into
adjacent ridges rather than removed. When a scoring tool is used to
form the boiling surface layer, the tool will tend to displace the
wall material upward from the wall surface and outward away from
the tool as the tool moves across the wall surface such that
grooves separated by ridges are formed in the wall material. In
forming the preferred cross-grooved boiling surface embodiment, a
second set of grooves is scored across the first-formed grooves and
ridges, at an angle--preferably 90.degree.--to the orientation of
the latter, such that the first-formed ridges are segmented into
sections. This cross-scoring further displaces the wall
material.
If needed, according to the method of this invention, cutting
techniques other than scoring may be used to form the first set of
grooves as for example milling. This novel method also contemplates
forming a second set of depressions or grooves using other metal
displacement techniques such as rolling or knurling.
BRIEF DESCIPTION OF THE DRAWING
FIG. 1 is a photomicrograph, magnification -20 fold, of the top
surface of a cross-grooved boiling surface layer embodiment of this
invention.
FIG. 2 is a photomicrograph, magnification -75 fold, of a
cross-section of the boiling surface layer of FIG. 1 taken in a
vertical plane approximately along the lines 2--2 in FIG. 1.
FIG. 3 is a photomicrograph, magnification -40 fold, of a
cross-section of a boiling surface layer similar to that shown in
FIGS. 1 and 2 taken in a vertical plane in the same manner as FIG.
2.
FIG. 4 is a photomicrograph, magnification -20 fold, of the top
surface of another cross-grooved boiling surface layer embodiment
of this invention.
FIG. 5 is a photomicrograph, magnification -40 fold, of a
cross-section of still another boiling surface layer embodiment
taken in a vertical plane.
FIG. 6 is a photomicrograph, magnification -20 fold, of the top
surface of a single direction grooved boiling surface layer
embodiment of this invention.
FIG. 7 is a photomicrograph, magnification -40 fold, of a
cross-section of the boiling surface layer of FIG. 6 taken in a
vertical plane approximately along the lines 7--7 in FIG. 6.
FIG. 8 is a photomicrograph, magnification -40 fold of a
cross-section of a boiling surface layer similar to that shown in
FIGS. 6 and 7 taken in a vertical plane in the same manner as FIG.
7.
FIG. 9 is a graph showing pool boiling performance data in water
for a smooth aluminum surface, dashed line, and for aluminum
surfaces of this invention.
FIG. 10 is a graph showing pool boiling performance data in liquid
nitrogen for a smooth aluminum surface, dashed line, and for
aluminum surfaces of this invention.
FIG. 11 is a schematic view taken in cross-sectional elevation, of
single-direction scoring apparatus suitable for practicing the
method of this invention for forming boiling surface layers.
FIG. 12 is a schematic view taken in cross-sectional elevation of
two-direction scoring apparatus.
FIG. 13 is a schematic view taken in cross-sectional elevation of
apparatus suitable for simultaneously scoring several grooves using
a circular tool.
FIG. 14 is an end view of the FIG. 13 apparatus.
FIG. 15 is a schematic view taken in cross-sectional elevation of
single-direction milling apparatus.
FIG. 16 is a schematic view taken in cross-sectional elevation
fold, apparatus suitable for milling the boiling surface layer
according to the method of this invention using a rotary
cutter.
FIG. 17 is a schematic view taken in cross-sectional elevation of
apparatus suitable for cross knurling the first groove set.
FIG. 18 is an end view of the FIG. 17 apparatus.
.[.FIG. 19 is a graph showing pool boiling performance data in
water for a smooth aluminum surface, dashed line, and for single
direction scored surfaces of this invention having between 29 and
230 grooves per inch..].
.[.FIG. 20 is a graph showing pool boiling performance data in
liquid nitrogen for a smooth aluminum surface, dashed line, and for
single direction scored surfaces of this invention having between
29 and 230 grooves per inch..].
FIG. .[.21.]. .Iadd.19 .Iaddend.is a graph showing pool boiling
performance data in water for smooth metallic surfaces, dashed
line, and for cross-scored surfaces formed from a variety of
different metal surfaces with between 45 and 225 grooves per
inch.
FIG. .[.22.]. .Iadd.20 .Iaddend.is a graph showing the relationship
between the heat transfer coefficient and the limiting dimension of
restricted openings in the instant boiling surface layers for
water, 30% ethylene glycol in water, and nitrogen.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Vapor generation in nucleate boiling requires the presence of
nuclei of vapor at active boiling sites. In ordinary surfaces these
sites consists of micropits or scratches which can retain gas or
vapor because of their shape and their small size. However, these
micropits are relatively few in number and are not dependable
because they will intermittently lose their vapor content. It is
believed that the cross-grooved boiling surface of this invention
performs so well because the cavities formed within the grooves
dependably trap bubbles of vapor which are much larger than those
found in active sites of ordinary smooth or mechanically roughened
surfaces.
The essential features of the boiling surface layer of this
invention are believed to be that during the boiling process vapor
bubbles are permanently trapped within the cavities which serve
continuously as nuclei for the formation of vapor; that a thin
liquid layer is maintained between a trapped vapor bubble and the
adjacent metal surface defining the cavity constituting a very low
heat transfer resistance between the metal and the liquid-vapor
interface; and that this liquid film is replenished to sustain
growth of the entrapped vapor bubbles as vapor escapes from the
cavities through the restricted openings of the cavities.
The extreme thinness of the liquid film within the cavities formed
within the first set of grooves is thought to account in large part
for the striking improvement in the boiling heat transfer
coefficient achieved with this invention. The combination of the
well-known Gibbs and Clapeyron equations which relate the thermal
potential required for growth of a bubble (in terms of the
superheat of the liquid surrounding the bubble) to the size of the
bubble states: ##EQU2## where r.sub.c =cavity radius
.sigma.=surface tension, lbs. force/ft.
T=temperature of liquid surrounding a bubble, .degree. R.
T.sub.s =saturation temperature of boiling liquid corresponding to
the vapor pressure of the liquid, .degree. R.
.rho.l=density of liquid, lbs. mass/ft..sup.3.
.rho.V=density of vapor, lbs. mass/ft..sup.3.
.lambda.=latent heat of boiling liquid, B.t.u./lb.
The value of T must be greater than T.sub.s by an amount sufficient
to cause a bubble of radius r.sub.c to grow against surface
tension. Hence T-T.sub.s is the minimum superheat required to
sustain the boiling process. According to the Gibbs-Clapeyron
equation, the superheat necessary for bubble growth is reduced,
i.e. T-T.sub.s is minimized, by increasing r.sub.c.
However, if one wishes to improve overall boiling performance, it
will not be sufficient merely to reduce the superheat required to
sustain bubble growth. The superheat .DELTA.T, T-T.sub.s,
correlated by the Gibbs-Clapeyron equation has been discovered to
be only one of the resistances to the overall boiling process. When
boiling proceeds by the formation of bubbles within cavities of a
surface which comprises a heat source, a second .DELTA.T exists
across the liquid film between the wall temperature T.sub.W and the
superheated vapor-liquid interface temperature T, and in effect
this film .DELTA.T is in series with the superheat .DELTA.T of the
Gibbs-Clapeyron equation. This film .DELTA.T, i.e. T.sub.W -T, has
unexpectedly found to increase as that increases, an effect
opposite to tht observed for the superheat .DELTA.T, T-T.sub.s.
Neglecting the thermal resistance of the material of which the
surface is composed, the overall boiling process is dependent upon
the total .DELTA.T, T.sub.W -T.sub.s, which is approximately the
sum of the film and superheat .DELTA.T's. The boiling surface layer
of this invention represents a marked improvement in that it
effectively reduces the film .DELTA.T and is therefore
characterized by performance where the film .DELTA.T begins to lose
its dominant influence.
A boiling surface layer, as described above, in operation provides
a multitude of partially liquid filled, sub-surface cavities which
act as nuclei for the growth of many bubbles of the boiling liquid.
As the bubbles grow, vapor emerges from the cavities through the
restricted openings therein due to continued generation of vapor
therein, breaks away from the boiling surface, and rises through
the liquid. The liquid continues its flow into the cavities through
sub-surface openings to replenish the thin liquid films. The high
boiling coefficient results from the fact that the heat leaving the
base metal surface does not have to travel through an appreciable
liquid layer before meeting a vapor-liquid surface producing
evaporation.
Within the boiling surface layer, a multitude of thin-filmed
bubbles are grown so that the heat, in order to reach a
vapor-liquid boundary, need travel only through an extremely thin
liquid layer having a thickness considerably less than the width of
the confining cavity interior. Vaporization of liquid takes place
entirely within the cavities and substantially no superheating of
the bulk liquid is required or can occur.
With a smooth metal surface, however, only a few bubble points
exists and the initiation of bubble growth requires a larger degree
of superheat due to the compressive force of liquid surface tension
on a very small bubble. The heat for bubble growth must be
transferred by convection and conduction from the smooth base metal
to the distant vapor-liquid interface of a bubble which is almost
completely surrounded by bulk liquid.
The above-described performance of the boiling surface layer of
this invention is not merely the result of increasing the surface
area by, for example, mechanically roughening the surface. This
fact was shown by a test comprising immersing a cross-grooved
boiling surface layer of this invention bonded to a copper block
containing embedded heating coils to boil a fluid such as liquid
nitrogen. At very low heat fluxes insufficient to activate the
cavities with vapor, the boiling heat transfer coefficient and the
visual phenomena of bubble ponts were quite similar to those
obtained with a smooth surface copper block. However, at higher
heat fluxes producing vapor activation of the cavities, extremely
high boiling coefficients were obtained which are impossible to
achieve with the smooth block or with a block having thoroughly
mechanically roughened surfaces. The following test results in
boiling nitrogen illustrate the effect of cross-grooved boiling
surfaces of this invention compared to typical prior smooth
surfaces and mechanically roughened surfaces.
TABLE I
__________________________________________________________________________
.DELTA.T required for- Q/A=1,000 Q/A=10,000 Heat transfer
coefficient B.t.u./hr.ft..sup.2 B.t.u./hr./ft..sup.2
B.t.u./hr./ft..sup.2 /.degree.F.) Type surface (.degree.F.)
(.degree.F.) At Q/A=1,000 At Q/A=10,000
__________________________________________________________________________
(1) Smooth aluminum 5 25 200 400 (2) Mechanically roughened by
mill- ing in one direction with neither restricted or sub-surface
openings 2.5 5 400 2,000 (3) Mechanically roughened alumi- num by
cross-milling at right angles with neither restricted or sub-sur-
face openings 1.5 2.25 670 4,440 (4) Aluminum prepared according to
this invention to produce sub-sur- face cavities having both
restricted openings and sub-surface openings by single-direction
scoring, (FIGS. 6 and 7 embodiment) 0.5 1.9 2,000 5,260 (5)
Aluminum prepared according to this invention to produce sub-sur-
face cavities having both restricted openings and sub-surface
openings by cross-scoring at right angles, -(FIGS. 1 and 2
embodiment) 0.3 1 3,300 10,000
__________________________________________________________________________
The cross-grooved boiling surface layer shown in FIG. 1 was formed
by a scoring tool. The tool was first drawn relative to and across
the surface from left to right beginning at the top of the figure.
The tool was advanced relative to the surface from the top toward
the bottom of the figure after formation of each groove such that a
first plurality of parallel ridges separated by grooves was scored
into the base material. The tool was then drawn relative to the
surface across the first plurality of ridges to form cross-wise
grooves. The cross-wise grooves were formed by drawing the tool
relative to and across the base material at substantially
90.degree. to the orientation of the ridges, from the top to the
bottom of the figure and the tool was advanced from the right
toward the left of the figure after formation of each cross-wise
groove such that a second plurality of grooves cross-wise to the
first formed ridges to segment the ridges were produced as shown in
the figure.
To understand the structure of the boiling surface layer, a
photomicrograph of the surface such as shown in FIG. 1 is not
completely adequate because the sub-surface structure is not
evident. To more clearly show the contour of the boiling surface
layer shown in FIG. 1 as an example, the boiling surface layer was
impregnated in a plastic resin, vertically cross-sectioned at a
very slight oblique angle to the orientation of the first-formed
grooves, and one of the sectioned edges polished and
microphotographed.
The boiling surface layer was first impregnated with a plastic
resin so that the subsequent cutting and polishing will not deform
the structure of the boiling surface layer.
The boiling surface layer was vertically sectioned at a very slight
oblique angle to the orientation of the first formed grooves so
that the vertical edges will depict the structure of the boiling
surface layer in different vertical planes. The FIG. 2 section, for
example, was taken at a 95.degree. oblique angle to the orientation
of the first-formed grooves. The projections which are shown in
FIG. 2 are cross-sections of the ridges formed by drawing a
somewhat blunt-ended scoring tool across the surface of a heat
exchange wall after the ridges have been segmented by cross-wise
scoring. Reference point 1 in FIG. 2 indicates a cross-section of
one of such ridge in a plane located between two adjacent grooves
formed, or superimposed, cross-wise to the ridges. Reference point
2 in FIG. 2 indicates a cross-section of another ridge in a plane
located at the middle of one of the second formed cross-wise
grooves. Notice should be taken that the depth of the cross-wise
grooves is very shallow compared to the depth of the grooves
between the ridges and therefore the ridges are not segmented into
discrete sections. The depth of the cross-wise grooves can be
measured by the difference in elevations of the top of the ridge at
point 1 and of the top of the ridge at point 2. Reference point 3
in FIG. 2 indicates a cross-section of still another ridge in a
plane located along the back edge of one of the second-formed
cross-wise grooves. Thus, the cross-section in FIG. 2 shows, from
right to left, the structure of the ridges of the boiling surface
layer in a series of planes beginning between adjacent cross-wise
grooves (at the right) and progressing into an adjacent groove (in
the center) and through that groove to the back wall thereof (at
the left). This structure repeats itself from one edge of the
boiling surface layer to the opposite edge. Therefore the structure
shown in FIG. 2 permits one to construct a mental image of the
structure of each ridge by mentally arranging the ridge
cross-sections one atop the other from left to right. This is the
advantage of making a vertical cross-section at a very slight
oblique angle to the orientation of the first-formed grooves. The
sub-surface cavities in the boiling surface layer of FIGS. 1 and 2
consist of almost completely enclosed tunnels along the bottom of
the first-formed grooves. The second set of cross-wise grooves is
very superficial and the shape of the first-formed grooves and
ridges appears to be almost independent of the superimposition of
the second set of grooves on the ridges. However, at point 2 in
FIG. 2 corresponding to the location of the bottom of a
second-formed groove, the structure of the upper section of the
ridges appears to be modified such that the first-formed groove is
almost completely closed. Near points 1 and 3 of FIG. 2
corresponding to locations between adjacent cross-wise or
second-formed grooves, the upper sections of the ridges do not
appear to have been affected by the cross-wise grooves and there
exists an opening to the surface that is larger than the space
between the ridges near point 2. It also appears that the
tunnel-like cavity is larger near point 2 beneath the cross-wise
groove than at points 1 and 2.
FIG. 3 is a cross-section of a cross-grooved boiling surface layer
which was taken in the same manner as the FIG. 2 cross-section but
from a separate boiling surface layer and was formed with a
sharper-ended scoring tool. The repeating nature of the
cross-sectioned structure is more evident in FIG. 3 than in FIG. 2.
The ridge at point 6 is located at a plane between adjacent
cross-wise grooves and the bottoms of these adjacent cross-wise
grooves are readily noted. The depth of the cross-wise grooves is
approximately one-half the depth of the grooves between the
first-formed ridges.
The sub-surface cavities shown in FIG. 3 do not appear to be
tunnel-like as in FIG. 2. Rather, the cavities of FIG. 3 appear to
extend from a point between two cross-wise grooves, beneath an
adjacent cross-wise groove, to a point between another two
cross-wise grooves. Furthermore, unlike the cavities of FIG. 2, the
cavities of FIG. 3 appear smallest beneath the cross-wise grooves.
The superimposition of the second-formed cross-wise grooves appears
to have a significant effect on the shape of the cavities. Thus,
the cavities of FIG. 3 extend in an enclosed form from either side
of a point beneath cross-wise grooves to points on opposite sides
between such cross-wise grooves where they open to the surface. The
cavities do not appear to be continuous tunnels as in FIG. 2.
A feature that is common to the cavities of both FIG. 2 and FIG. 3
is that the structure is such that there exists restricted openings
through which vapor could escape during boiling without losing
vapor bubbles entrapped within the cavities. Another feature is
that there are also other openings through which liquid can enter
to replenish the thin liquid films between the entrapped vapor
bubbles and the enclosing material of the boiling surface layer.
Another characteristic that is common to the cavities of both FIG.
1 and FIG. 2 is that not all of the cavities will be capable of
entrapping vapor bubbles, or of releasing vapor, or the like
because of imperfections caused during formation of the boiling
surface layers. However, boiling surface layers formed in the
manner of those of FIGS. 1-3 contain many tens-of-thousands of
potentially active sites and the actual percentage of active sites
may indeed be relatively small in order to account for the
phenomenal boiling heat transfer capability, that these surfaces
have.
The cross-grooved boiling surface layer shown in FIG. 4 was also
formed by a scoring tool. The tool was first drawn across the
surface from right to left and the tool advanced from bottom to top
to form a first plurality of ridges separated by grooves. The tool
was then drawn across the first plurality of ridges, at
substantially 90.degree. to the orientation of the ridges, from top
to bottom and the tool advanced from right to left to form a second
plurality of grooves cross-wise to the first-formed ridges to
segment the ridges as shown in the figure. The microphotograph was
taken at an angle to the plane of the surface in the direction of
tool advance during formation of the first-formed ridges to more
clearly show the structure. The cavities formed in the boiling
surface layers of FIG. 4 have a considerably different appearance
than in the embodiments of FIGS. 1-3. The cavities of FIG. 4 appear
to be partially cup-shaped. It appears that one can ascertain a
multitude of cavities shaped roughly like halves of cups.
FIG. 5 is a vertical cross-section of a boiling surface layer
viewed in the same manner as the FIG. 2 cross-section but taken
from still another boiling surface layer. The first set of grooves
is clearly much deeper than the second set, as previously shown in
the FIGS. 1-2 surface. A distinguishing feature of the FIG. 5
surface is that the restricted openings to the outer surface are
formed by the overlapping outer deformed sections of the ridges. In
the FIGS. 1-2 embodiment the restricted openings are formed by
lateral deformation of the entire ridges.
FIG. 6 shows a single-direction grooved surface embodiment of a
boiling surface layer formed by a scoring tool. The tool was drawn
across the surface from right to left and the tool advanced from
bottom to top to form ridges separated by grooves.
FIG. 7 shows a vertical cross-section of the boiling surface layer
of FIG. 6 at right angles to the ridges and grooves. The FIG. 7
cross-section was taken in the same manner as the FIG. 2
cross-section.
FIG. 8 is a vertical cross-section of another single direction
grooved-surface embodiment of a boiling surface layer taken in the
same manner as the FIG. 7 cross-section but from another boiling
surface layer.
Restricted-opening cavities are most clearly evident in FIG. 7 in
the cavities just to the left of point 1. In these tunnel-like
cavities, the upper sections of the ridges are deformed into
adjacent grooves leaving a restricted opening to the surface. In
FIG. 8, tunnel-like cavities are also apparent but are more similar
to those of FIG. 2 than of FIG. 7.
The boiling surface layer of FIG. 8 was formed by the scoring tool
advancing from right to left with the last groove being formed at
the left. Whereas in FIG. 8 the scoring tool was held normal to the
work, in FIG. 7 the tool was inclined away from the grooves toward
the direction of advancement at an angle of 10.degree. from
perpendicular, called herein for convenience a plus (+) angle. The
structure shown in FIG. 8 was caused by the scoring tool deforming
the near (toward the direction of advancement) ridge of an adjacent
preceeding groove into that preceding groove during the scoring of
the next succeeding groove. The various boiling surface layer
structures shown in FIGS. 1-8 were formed in the same manner. The
differences in appearance are due to such variables as groove
density, depth of the scored grooves, angle of tool inclination,
speed at which the tool is moved through the material, the type of
lubricant-- if any--employed during scoring, configuration of the
tip of the scoring tool. For example, scoring to a greater depth at
a particular groove density will result in a greater degree of
deformation. Likewise, inclining the scoring tool over the
just-scored material at a minus (-) angle, or at a largest plus (+)
angle, will also result in a greater degree of deformation.
Further, a given high performance can be obtained with a lower
groove density by cross-grooving since there will tend to be more
deformation than by single-direction grooving.
Although the boiling surface layers shown in FIGS. 1-8 were
produced on a flat heat exchange wall, these boiling surface layers
could be produced on a curved surface such as a surface of a heat
exchange tube.
A preferred method of constructing a boiling surface layer of this
invention is to form sets of parallel grooves by cross-scoring such
that the metal is displaced rather than removed. Thus, in forming
the parallel grooves of a first set, the scoring tool will tend to
displace metal upward and outward. When the parallel grooves of a
second set are then superimposed on the first set, the previously
displaced metal in the first set of grooves will be displaced again
resulting in the partial closure of the first set of grooves to
form cavities which are enlarged below the metal surface and have
restricted openings in their outer portions.
To further enhance the capacity of the grooves of the first set to
transport liquid to the cavities and to aid in the formation of the
cavities themselves, it is also preferred to produce the first set
of grooves with a greater depth than the depth of the second set of
grooves. This tends to result in cavities produced at the bottom of
the first set of grooves which are partially closed at the top by
metal which is displaced when the second set of grooves are
superimposed thereon. This is believed to provide undisturbed
channels interconnecting the cavities below the surface and to
contribute to an undercut geometry conducive to the entrapment of
vapor within the cavities.
It is also preferred to produce the grooves of the first set with a
tool inclined to the boiling surface such that the grooves so
formed are inclined. The second set of grooves are then preferably
produced by a tool moving in a direction relative to the
inclination of the first set of grooves to aid in the formation of
the cavities having restricted openings thereto.
The location of the cavities depends at least in part on the
relative depths of the two sets of grooves. It has been observed
that the superimposition of a second set of grooves across a first
set at about the same depth by means of a scoring tool produces
cavities within the grooves of the first set and between the
grooves of the second set. It has also been observed that the
superimposition of a second set of grooves at a relatively shallow
depth by means of a scoring tool produces cavities within the
grooves of the first set and beneath the grooves of the second set.
Also, by scoring the grooves of the second set at an intermediate
relative depth, cavities have been formed within the grooves of the
first set beneath and between the grooves of the second set. In
each of the above examples, the superimposition of the second set
of grooves by means of the scoring tool appeared to constitute an
important factor in the production of the restricted openings to
the outer portions of the multiple cavities. This formation of the
restricted openings appeared to result from the displacement of the
side walls of the grooves of the first set as the scoring tool
moved across the first set of grooves to form the grooves of the
second set.
An important variable in the preferred structure of this invention
is the groove density. A relatively high groove density aids in the
formation of smaller cavities which function better in boiling
liquids having relatively low surface tensions such as liquid
oxygen and nitrogen. For these two liquids, and for liquids having
similar surface tensions, a groove density of between 140 and 200
grooves per inch is preferred. A relatively low groove density aids
in the formation of larger cavities which function better in
boiling liquids having relatively high surface tensions such as
water. For this liquid, and for liquids having similar surface
tensions, a groove density of .Iadd.greater than 20, i.e.
.Iaddend.between .Badd..[.20.]..Baddend. .Iadd.45 .Iaddend.and 120
grooves per inch is preferred. Cavities for boiling liquids having
surface tensions intermediate water and liquid oxygen and nitrogen
would be preferably formed in surfaces having groove densities
intermediate the foregoing values.
The foregoing groove density values are merely preferred inasmuch
as optimum sized cavities are formed therein. However, the boiling
surfaces of this invention designed for example to boil liquid
nitrogen could be used to boil water and the performance in water
would be considerably superior to a smooth surface even though an
optimum cavity size was not employed.
As a general rule the lowest groove density should be chosen which
will provide or achieve the desired performance. This is because
lower groove densities tend to provide more massive and rugged
ridges less prone to damage by corrosion or erosion with lowest
machining costs.
Table II lists the physical data for boiling surface layers shown
in FIGS. 1-8.
TABLE II ______________________________________ Angle.sup.2 of Tip
angle Depth of scoring of Depth of second. tool Grooves.sup.1
scoring first- formed inclination per tool formed cross-wise + or -
FIG. inch (deg.) grooves (deg.)
______________________________________ 1 and 2 140 30 16 2.5 +10 3
208 30 11 8 +10 4 160 45 +10 5 60 30 23 8 +10 6 and 7 230 30 8 0
+10 8 230 30 12.5 0 0 ______________________________________ .sup.1
Groove density the same to both sets of grooves in cross-grooved
boiling surface layers. .sup.2 A plus (+) angle indicates that the
scoring tool was inclined in the direction of tool advance; a minus
(-) angle indicates that the scoring tool was inclined opposite to
the direction of tool advance.
FIGS. 9 and 10 exemplify the pool boiling heat transfer performance
of boiling surface layers of this invention compared to a smooth
surface. FIG. 9 plots test results in water and FIG. 10 plots test
results in liquid nitrogen.
In FIG. 9, the performance of the single-direction grooved aluminum
boiling surface layer shown in FIGS. 6-7-- curve 1, is shown as
well as the performance of a typical cross-grooved aluminum boiling
surface layer-- curve 2. The latter was scored with the first
grooves 15 mils deep and with the second grooves 11 mils deep in a
direction normal to the first grooves, both sets being provided at
a density of 100 grooves per inch. Curve 3 represents performance
of the copper boiling surface illustrated in FIG. 5 and described
in Table II. In terms of the temperature difference required as a
driving force to transfer any particular quantity of heat, the two
aluminum boiling surface layers shown by curves 1 and 2 are over 5
times superior to a smooth aluminum surface. Furthermore, the
greater slope of curves 1 and 2 indicate the surfaces of this
invention become more active faster than a smooth surface.
In FIG. 10, the performance of: the single-direction grooved
aluminum boiling surface layer shown in FIGS. 6-7-- curve 1; and
the cross-grooved aluminum boiling surface layers shown in FIG.
4--55 curve 2, FIG. 3-- curve 3, and FIGS. 1 and 2-- curve 4 are
compared to a smooth aluminum surface. In terms of the temperature
difference required as a driving force to transfer any particular
quantity of heat, the boiling surface layers of this invention are
well over 50 times superior to a smooth aluminum surface.
Cross-grooving at relatively deep penetrations at high groove
densities results in an extreme degree of groove interference and
metal displacement. The boiling surface layers so produced exhibit
random rather than uniform orientation of the boiling surface
layers and the metal forming this surface layer is somewhat
fragile. Nevertheless, the performance of such boiling surface
layers is very high as shown in curve 5 in FIG. 10. This aluminum
boiling surface layer has a cross-groove density of about 230
grooves per inch for both sets of grooves and a groove depth of
about 10 mls. for both sets of grooves. Such surfaces have especial
utility in boiling low surface tension fluids such as the
cryogens.
The vast improvement in boiling performance of this invention in
liquid nitrogen is applicable to all cryogens in general and hence
the boiling surface layers of this invention will have especial
utility in heat transfer processes involving the boiling of
cryogenic liquids. The more modest--but nevertheless
signficant--improvement in boiling water indicates that this
invention will have significant utility in such processes as the
desalination of sea water by distillation where small temperature
differentials and high efficiency are desired.
According to a method aspect of this invention a grooved surface
boiling layer is formed by cutting a series of parallel grooves at
microscopic density in a thermally conductive metal wall so as to
form first ridges separating adjacent grooves. Another series of
parallel second grooves are then cut at microscopic density in the
wall at an angle to the orientation of the first ridges so as to
form second ridges separating adjacent second grooves. During the
second cut a plurality of sub-surface cavities are formed in the
first groove communicating with the outer surface through
restricted openings having smaller cross-sectional area than the
largest cross-sectional area of the cavity interiors. Also during
this second cut, sub-surface openings are formed between at least
some adjacent cavities providing vapor communication
therebetween.
As previously indicated, the grooved boiling surface layers of this
invention are preferably formed by scoring--the cutting method in
which the tool "slices" through the work with a sharp edge thereby
displacing rather than removing metal from its path. Stated
otherwise, with scoring the metal is displaced outwardly and
approximately normal to the direction of relative movement between
the tool and work. Scoring produces grooves which may not be
clean-cut but instead may contain many burrs, metal fragments and a
high degree of surface roughness. It has been found that best
results are obtained when the scored surface is simply cleaned in
an appropriate solution such as acetone to remove only loose metal
particles. The firmly attached rough projections should not be
romoved as they frequently constitute part of the reentry cavity
configuration producing the characteristically high
performance.
An attractive use for scored boiling surfaces is in plate-and-fin
heat exchangers where the surface treatment is applied on the flat
parting sheets which separate the narrow passages. Flat plates and
sheets are conveniently scored by holding them firmly down against
the flat bed of a planing mill and mounting the tool in
conventional fashion on a fixed gantry above the work. The surface
is scored as the bed moves the work horizontally under the tool.
After each scoring stroke of the machine, the tool is indexed
laterally a few thousandths of an inch into position for the next
adjacent groove. For accurate grooving a tight machine is required
with very little vibration in the tool holder and indexing
mechanism.
An alternative machine for scoring flat sheets is a shaper, on
which the work is held firmly against the fixed bed and the tool
moves horizontally over the work. After each scoring stroke, the
work is indexed into position for the next groove.
FIGS. 11-14 illustrate scoring tools suitable for preparing the
grooved boiling surfaces. FIG. 11 shows tool 11 with a single
cutting edge 12 which scores in one direction only to form groove
13 in wall 14. When in use, the tool scores a full-length groove
across the wall, and then lifts or swings free while the tool or
work is returned to the original starting position. The next
adjacent groove is then scored.
FIG. 12 shows a double bit tool 11 with two symmetrical cutting
edges 12a and 12b arranged to score in either direction. The tool
11 scores one groove 13 as the machine moves in one direction and
immediately scores an adjacent groove on the return stroke of the
machine. In this manner, the scoring operation is less costly and
time consuming.
FIGS. 13 and 14 illustrate another tool arrangement designed for
faster scoring. A group of tools 11a-11f are mounted on shaft 15
and accurately separated by cylindrical spacers 16a-16e. Each tool
is disk-shaped with a cutting edge 12a-12f provided around its full
periphery. All tools are keyed or locked to the shaft 15. The
latter is held in bearing 17 which in turn is mounted on the tool
holder-and-indexing mechanism 18 of a machine similar to a planning
mill. A disk or gear 19 is keyed to the end of shaft 15, and is
provided with pin 20 insertable through holes or teeth in disc 19
and into a recess provided in the rigid tool holder 18. In this
manner the tools are fixed against rotation and as the bed 20 moves
the work 14 under the tool assembly, multiple grooves 13 are scored
in the surface on each scoring stroke. Since the too cutting edges
are symmetrical toward each direction of work movement a second
multiplicity of grooves may be scored on the return stroke of the
machine.
When the cutting edges of the tool assembly become worn at one
radial position, pin 20 is removed and the assembly is rotated a
few degrees to present a fresh set of cutting edges to the work.
The assembly is then locked again by reinserting pin 20 through
another hole now aligned with the pin recess in member 18.
There are two ways that the tool can strongly influence the type of
surface generated during scoring. One is the angle of tool
inclination (discussed previously) and the other is the included
angle of the tool's tapered cutting edge. A large bit angle tends
to produce wide grooves while a small bit angle produces narrow,
slit-like grooves. The bit angle also determines the extent to
which metal will be displaced into an adjacent parallel groove.
Aside from its influence on the groove, the bit angle also affects
tool life; tools with very small angles are prone to chip with the
result that tool life is prohibitively short. It has been found
that the included angle of the scoring tool bit should be between
20.degree. and 45.degree. with 20.degree.-30.degree. preferred.
The grooved boiling surface layers of this invention may
alternatively be produced by other cutting methods, as for example
milling. In contrast to the preferred scoring method, milling
removes at least a portion of the metal from the groove in the form
of chips or shavings. This metal is removed in a direction parallel
to the direction of relative movement between the tool and work.
The cutting edge of the tool which bites into the metal is blunt,
so as to remove the metal from the groove as cleanly as
possible.
FIGS. 15 and 16 illustrate preparation of the grooved boiling
surface layers by milling. In FIG. 15, tool 11 with a single blunt
cutting edge 12 mills in one direction only to remove shavings. The
differences between milling and scoring will be apparent by
comparing FIG. 11 with FIG. 15.
Whereas in FIG. 15 the milling tool 11 moves parallel to wall 14,
FIG. 16 shows rotary milling tool 11 with the wall (work) moving
horizontally beneath the wheel. As illustrated the movement of the
cutter blades is opposed to the work movement. Alternatively the
cutter blade may be reversed and move in the same direction as the
work. Both procedures remove metal in the form of clean chips.
This invention also contemplates forming the second set of
depressions or grooves by deforming techniques other than scoring
or milling. In some instances the first set of grooves may be cut
without restricted openings from the cavity interiors to the outer
surface of the wall, or alternatively the openings may not have as
restricted cross-sectional areas as desired for effective vapor
bubble entrapment. In either situation, a second deforming step may
be used to partially smash in the ridge top surfaces and thereby
reduce the cross-sectional area of the openings. This second
deforming step may for example be performed by rolling a smooth
member of circular cross-section across the surface at 90 degree
orientation to the first set of grooves and ridges. It will be
apparent that if the roll contacts all of the ridges with equal
force, the entire top surface of the ridges is depressed and there
will be no distinct second set of depressions. If the roll contacts
only part of the ridge top surfaces, that part will form the second
set of depressions as distinguished from the higher unsmashed ridge
portions.
Still another technique for forming the desired vapor reentry
cavity contour is by knurling the metal wall containing the first
set of grooves and ridges, preferably at 90 degrees orientation
thereto. FIGS. 17 and 18 illustrate the knurling embodiment in the
form of wheel 11 having knurl teeth 12 on at least a portion of its
circumference spaced by valleys 21 and mounted on shaft 15.
The as-formed first grooves 13 in wall 14 are separated by ridges
22, and openings 23 communicate with the outer surface (see left
side of FIG. 18). In the knurling operation wheel 11 is positioned
so that teeth 12 impinge and smash in the top surface 24 of ridge
portions to form second rows of depressions 25 at an intermediate
level between the cavity base of first grooves 13 and the top
surface 24 of the unsmashed ridge portions.
Wheel 11 is preferably aligned perpendicular to the first grooves
13 and ridges 22 (see FIG. 18) so that the resulting second
depression rows 25 are oriented 90 degrees from the first grooves.
When the knurling wheel 11 has completed its run across the surface
from one side to the other, it may be indexed forwardly towards the
opposite end of the work to downwardly smash portions of the next
ridge tops. It will be apparent from the foregoing description and
FIGS. 17 and 18 that the unsmashed portions 26 of ridges 22 fall
beneath the valleys 21 between teeth 12. These unsmashed portions
do not close the openings 23 to the extent that the downwardly
smashed portions 25 form restricted openings.
In all of the embodiments heretofore described, the boiling surface
layer is formed from the heat exchange base wall by metal removal
and displacement. It is also contemplated that such layer could be
partially or completely formed from material attached to the heat
exchange base wall, as by metal bonding. For example, open parallel
grooves might be cut in one direction and a perforated sheet or
screen bonded thereto with the perforations or spaces enclosed by
the screen strands serving as the restricted openings. It might
also be possible to prepare the boiling surface layer by first
positioning a series of parallel spaced strands directly on the
smooth heat exchange wall, the strands being formed of corrodible
or decomposable material. Than a layer of non-corrodible metal
might be cast over and between the aligned strands followed if
necessary by sufficient grinding to expose the top of the oriented
strands. The exposed strands would then be removed as by leaching,
leaving cavities surrounded by the non-corrodible metal layer.
Still another possible method for preparing the boiling surface
layer is by applying multiple layers of screening or wires, perhaps
of different diameters and with different orientations.
It is emphasized that the preparative method must produce a layer
having the previously described essential characteristics if the
remarkably high boiling heat transfer coefficients are to be
obtained. There must be a plurality of ridges separated by grooves
provided at microscopic density to form cavities adapted to entrap
vapor bubbles and provide boiling nucleation sites. The cavities
must communicate to the outer surface of the layer through
restricted openings having smaller cross-sectional area than the
largest cross-sectional area of the cavity interiors. Finally there
must be sub-surface openings between at least some adjacent
cavities providing fluid communication therebetween.
.[.The performance advantages of this boiling surface layer as
compared to smooth surfaces are also demonstrated in the FIGS. 19
and 20 graphs for single direction scored surfaces in boiling water
(FIG. 19) and boiling liquid nitrogen (FIG. 20) for a wide range of
groove densities between 29 and 230 grooves per inch. These
particular fluids are selected as representing those characterized
by low surface tension (liquid nitrogen) and high surface tension
(water) spanning a wide temperature range of between -196.degree.
C. and 100.degree. C..].
.[.The boiling surface layers were all formed from aluminum
sheeting using scoring tools similar to those illustrated in FIGS.
11 and 12, and all parameters were identical except the groove
spacing. The groove depth was nominally 8 mils, that is, the
scoring tool and the wall were positioned to cut grooves at this
depth. The included angle of the scoring tool tip was 30 degrees
and the scoring tool inclined angle to the wall was +10 degrees.
The number of grooves per inch was nominal, in that the scoring
assembly was set to cut the designated number of grooves. This is
the same criteria used to designate the groove density throughout
the disclosure and claims. The single-direction scored aluminum
surfaces used in the water and liquid nitrogen pool boiling test
summarized in FIGS. 19 and 20 are follows..].
.[.Table III ______________________________________ Surface No.:
Grooves per inch ______________________________________ 1 29 2 45 3
70 4 100 5 140 6 230.]. ______________________________________
.[.FIG. 19 demonstrates that with even the relatively low groove
densities of surfaces 1 and 2, a very significant improvement is
afforded over smooth surfaces. For example if a heat flux of
5.times.10.sup.3 B.t.u./hr.-ft..sup.2 is required in a given
system, this level may be achieved with a .DELTA.T of about
6.degree. F. whereas the smooth surface requires a .DELTA.T of
about 13.5.degree. F. The improvement is even greater with higher
heat fluxes, primarily due to the steep slopes of the boiling
surface layers as contrasted with the lower slope of the smooth
surface. It should be noted that although the highest groove
density (surface 6) affords the lowest .DELTA.T values for a given
heat flux, the slope of this surface is appreciably lower than for
the lower groove densities (surfaces 1-5). At relatively high heat
fluxes these curves demonstrate that surface 6 is only marginally
superior to surfaces 4 and 5, and the latter would be preferred for
water boiling due to their lower fabricating costs and higher
durability. A probable explanation for this phenomenon is the
relatively high surface tension of water, whereby vapor bubbles may
be retained in relatively large cavities characteristic of
relatively low groove densities..].
.[.FIG. 20 demonstrates a very substantial improvement for the
single-direction scored aluminum surfaces in boiling liquid
nitrogen. For example at a heat flux of 1.times.10.sup.4
B.t.u./hr.-ft..sup.2, the smooth surface requires a .DELTA.T of
about 15.degree. F. whereas the .DELTA.T values for surfaces 1, 4
and 6 are respectively as follows: 5.8, 4.1, and 1.8. It will be
apparent from FIG. 20 that the slope for surface 6 (highest groove
density) is about the same as for surface 1 (lowest groove density)
along with the former's substantially superior performance,
reflecting a preference for smaller reentrant cavities for
relatively low surface tension liquids such as nitrogen..].
.[.FIGS. 19 and 20 (in addition to FIGS. 9 and 10) demonstrate that
the boiling surface layer affords outstanding performance for
boilable liquids of any surface tension, whether relatively low or
high..].
FIG. .[.21.]. .Iadd.19 .Iaddend.illustrates the performance of ten
different cross-score surfaces with between 45 and 225 grooves per
inch for boiling .[.liquid nitrogen at -196.degree. C..].
.Iadd.water. .Iaddend.The surfaces were formed from six different
metals as follows: copper (surface 1), a low copper-high nickel
alloy with 3% Fe, Mn and trace elements (surfaces 2 and 3), nickel
(surface 4), 70% Cu-30% Ni (surface 5), 90% Cu-10% Ni (surface 6)
and aluminum (surfaces 8, 9 and 10). The surfaces were all scored
using tools very similar to those illustrated in FIGS. 11 and 12,
and the second set of grooves were cut at 90 degrees orientation to
the first set of grooves and ridges. The parameters used in
preparing surfaces 1-10 by the method of this invention are
summarized in Table .[.IV.]. .Iadd.III .Iaddend.as follows:
TABLE.[.IV.]..Iadd.III.Iaddend.
______________________________________ Depth of Depth of first-
second- Included Grooves formed formed .Iadd.angle of .Iaddend. per
grooves grooves scoring Surface Material inch (mils) (mils)
tool.sup.1 ______________________________________ 1 Copper 60 23 8
30 2 30% Cu,.sup.2 83 8 6 67% Ni. 3 30% Cu,.sup. 2 200 11 8 67% Ni.
4 Nickel 140 8 8 5 70% Cu, 100 8 4.[.(.sup.3).].
.Iadd..sup.3.Iaddend. 30 % Ni. 6 90% Cu, 140 8 4.[.(.sup.3).].
.Iadd.3.Iaddend. 10% Ni. -7 Aluminum 45 8 8 30 8 " 100 8 8 30 9 "
120 8 4 49 10 " 225 8 8 30 ______________________________________
.sup.1 Angle of scoring tool inclination is +10.degree. for all
surfaces except as noted. .sup.2 Balance of 3% is Fe, Mn and trace
elements. .sup.3 Angle of scoring tool inclination is
0.degree..
Inspection of FIG. .[.21.]. .Iadd.19 .Iaddend.reveals that in
general the higher groove density surfaces performed more
efficiently, compare for example surfaces 5-6 and surfaces 7-8.
.[.This is the same conclusion drawn with the single-direction
scored surfaces of FIG. 20, and again is attributed to the
relatively low surface tension of liquid nitrogen..]. Another
conclusion from FIG. .[.21.]. .Iadd.19 .Iaddend.is that remarkable
improvement in boiling heat transfer efficiency can be achieved
using virtually any type of metal with the surface of this
invention. That is, all surfaces 1-10 demonstrated far higher
effectiveness than the smooth surfaces.
It has been demonstrated that the boiling surface layers of this
invention afford remarkably high heat transfer coefficients in sea
water brine under brine-scaling conditions. Moreover it was found
that when scale-forming conditions were established such that the
boiling surface layers became fouled and the coefficent dropped,
the original heat transfer performance of the surface was
completely restored by adding 0.5% HCl to the sea water. This
hydrochloric acid dissolved the scale on the grooved layer.
More particularly, the cross-scored copper surface illustrated in
FIG. 5 and identified in Table .[.IV.]. .Iadd.III .Iaddend.as
surface 1 having 60 grooves per inch was used in a continuously
circulatory system to boil three different simulated sea water
solutions at a constant heat flux of 5,000 B.t.u./hr.-ft..sup.2.
The alkalinity was about 100 p.p.m. and the concentration factor
was 2.5 to 3.0. Concentration factor is defined as the ratio of the
solution's salinity to that of normal sea water. The first solution
was selected to contain sufficient CaCO.sub.3 scale for
precipitation, the pH value being about 0.5 pH units above the
saturation pH of 6.9. Under these conditions the boiling
coefficient deteriorated from about 3,000 to about 500
B.t.u./hr.-ft..sup.2 -.degree. F. Addition of 0.5% HCl caused the
CaCO.sub.3 scale to dissolve with the evolution of CO.sub.2, and
the coefficient returned to about 3,000. Based on the amount of HCl
added, it was estimated that 2- 5 grams of CaCO.sub.3 deposited on
the boiling surface layer (50-100 grams per sq. ft. of grooved
surface boiling layer) when complete fouling had occurred.
Next the concentration factor was increased to 3.0 where both
CaSO.sub.4 and CaSO.sub.4.1/2H.sub.2 O were above saturation. The
pH of the feed in this test was reduced to prevent the other scale
former from precipitating. The heat transfer characteristics of the
grooved surface boiling layer again deteriorated to a coefficient
of about 750 indicating scaling. Hydrochloric acid was again added
in the same concentration as before, but no carbon dioxide evolved
in this case indicating absence of CaCO.sub.3. The boiling
coefficient again returned to about 3,000 B.t.u./hr.-ft..sup.2
-.degree. F., showing that CaSO.sub.4 scale can be removed by
washing with acid. The system was then operated for several days
under conditions where all of the major scale-forming compounds
(CaCO.sub.3, CaSO.sub.4, and Mg(OH).sub.2) were below their
solubility limits at the boiling point. The boiling coefficient
remained constant at about 3,000, indicating that scaling had
caused no permanent deterioration of the grooved surface layer.
This coefficient is about 10 times greater than achievable with an
equivalent smooth metal surface under the same operating
conditions.
In the foregoing discussion, reference has been made to liquids
such as oxygen and nitrogen comprising a class of fluids
characterized by low surface tension which are preferably boiled on
surface layers having high groove density between 140 and 200
grooves per inch. Similarly, reference is made to liquids such as
water comprising a class of fluids characterized by high surface
tension which are preferably boiled on surface layers having low
groove density .Iadd.of greater than 20, i.e. .Iaddend.between
.Badd..[.20.]..Baddend. .Iadd.45 .Iaddend.and 120 grooves per inch.
In general, as the groove density is creased, the size of the
cavities and the limiting dimension of the restricted openings
therefrom to the outer surface is decreased.
FIG. .[.22.]. .Iadd.20 .Iaddend.correlates the limiting dimension
in mils of the restricted opening and the heat transfer coefficient
in B.t.u./hr.-ft..sup.2 -.degree. F. measured at a heat flux of
20,000 B.t.u./hr.-ft..sup.2. The curves represent data accumulated
from a number of boiling surface layers of different groove
densities and restricted opening limiting dimensions. For each
boiling surface layer, the limiting dimension of the restricted
opening was measured by visually scaling directly from a
microscopic enlargement of the boiling surface layer
cross-sections. As used herein, the "limiting dimension" represents
the largest diameter vapor bubble which may emerge from the cavity
to the outer surface of the boiling layer. For example, the
limiting dimension may be the minor dimension of an ovoid or
elliptically-shaped restricted opening. Limiting dimensions for
certain of the boiling layers illustrated in the photomicrograph
figures may be directly sealed as follows in Table .[.V.].
.Iadd.IV.Iaddend..
TABLE.[.V.]..Iadd.IV.Iaddend.
______________________________________ Limiting dimension of
restricted open- FIG. NO. Grooves per inch ing(mils)
______________________________________ 1 and 2 140 0.5 3 208 0.5 5
60 3.1 6 and 7 230 1.0 ______________________________________
Three different fluids, liquid nitrogen (B.P. -320.degree. F.),
water (B.P. 212.degree. F.), and a 30% by weight ethylene glycol in
water (B.P. 218.degree. F.) were boiled at one atmosphere pressure
in contact with both single and cross-grooved surface layers to
obtain the data for FIG. .[.22.]. .Iadd.20.Iaddend.. The data for
each fluid is plotted separately. It is evident that the surface
exhibiting the highest heat transfer coefficients are rather
precisely defined as having limiting dimensions for restricted
openings of less than about 5 mils. Such surfaces represent a
preferred embodiment of this invention.
Also based on the data summarized in FIG. .[.22.].
.Iadd.20.Iaddend., the surface layers best suited to boiling low
surface tension fluids such as liquid nitrogen are those having
limiting dimensions of restricted openings below about 1.75 mils
(most suitably at the previously discussed 140-200 grooves per
inch), while those best suited to the high surface tension fluids
such as water have limiting dimensions between about 1.75 and 4
mils (most suitably at the previously discussed
.Badd..[.20-120.]..Baddend. .Iadd.greater than 20, i.e. 45-120
.Iaddend. grooves per inch). Such boiling surface layers represent
still more preferred embodiments of this invention.
It is to be expected that for a particular boiling surface layer,
the heat transfer coefficients for the 30% ethylene glycol solution
will be consistently well below those for pure water. This is
because liquid mixtures are characterized by lower coefficients
than their pure constitutents.
* * * * *