U.S. patent application number 12/523579 was filed with the patent office on 2010-01-21 for heat exchanger unit.
This patent application is currently assigned to NEC CORPORATION. Invention is credited to Sakae Kitajo, Kazuyuki Mikubo, Hitoshi Sakamoto.
Application Number | 20100012299 12/523579 |
Document ID | / |
Family ID | 39644301 |
Filed Date | 2010-01-21 |
United States Patent
Application |
20100012299 |
Kind Code |
A1 |
Sakamoto; Hitoshi ; et
al. |
January 21, 2010 |
HEAT EXCHANGER UNIT
Abstract
According to the present invention, there is provided a heat
exchanger unit having, over a base, a surface-modified portion
composed of a metal, the surface-modified portion being brought
into contact with a flow path provided for a liquid refrigerant,
wherein the liquid refrigerant is a liquid having a surface tension
smaller than that of water, and the surface-modified portion has a
porous structure, in which a plurality of recesses are provided on
the flow path side thereof, each recess has an introduction path
having a cross-section area gradually reduced from the inlet of the
recess, and a cavity communicated with the introduction path while
placing an inflection portion in between, and the shortest distance
between the inflection portion and the flow path is larger than the
shortest distance between the cavity and the flow path.
Inventors: |
Sakamoto; Hitoshi; (Tokyo,
JP) ; Mikubo; Kazuyuki; (Tokyo, JP) ; Kitajo;
Sakae; (Tokyo, JP) |
Correspondence
Address: |
Mr. Jackson Chen
6535 N. STATE HWY 161
IRVING
TX
75039
US
|
Assignee: |
NEC CORPORATION
Tokyo
JP
|
Family ID: |
39644301 |
Appl. No.: |
12/523579 |
Filed: |
January 15, 2008 |
PCT Filed: |
January 15, 2008 |
PCT NO: |
PCT/JP2008/000023 |
371 Date: |
July 17, 2009 |
Current U.S.
Class: |
165/104.21 ;
165/185 |
Current CPC
Class: |
H01L 23/427 20130101;
H01L 2924/0002 20130101; F28F 13/18 20130101; H01L 2924/3011
20130101; H01L 2924/00 20130101; H01L 2924/0002 20130101; F28D
15/046 20130101 |
Class at
Publication: |
165/104.21 ;
165/185 |
International
Class: |
F28F 13/02 20060101
F28F013/02; F28D 15/02 20060101 F28D015/02; F28F 7/00 20060101
F28F007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2007 |
JP |
2007-014062 |
Claims
1. A heat exchanger unit having, over a base, a surface-modified
portion composed of a metal, said surface-modified portion being
brought into contact with a flow path provided for a liquid
refrigerant, wherein said liquid refrigerant is a liquid having a
surface tension smaller than that of water, and said
surface-modified portion has a porous structure, in which a
plurality of recesses are provided on the flow path side thereof,
each recess has an introduction path having a cross-section area
gradually reduced from the inlet of said recess, and a cavity
communicated with said introduction path while placing an
inflection portion in between, and the shortest distance between
said inflection portion and said flow path is larger than the
shortest distance between said cavity and said flow path.
2. The heat exchanger unit as claimed in claim 1, wherein said
liquid refrigerant is an organic refrigerant, and said organic
refrigerant is a hydrofluoroether or fluorine-containing inert
liquid.
3. The heat exchanger unit as claimed in claim 1, wherein said
recess has a pore size of 1 .mu.m to 10 .mu.m.
4. The heat exchanger unit as claimed in claim 1, further
comprising a flow path of micro-channel type as a forced-convection
boiling refrigerant type cooling unit, and said surface-modified
portion is provided along said flow path.
Description
TECHNICAL FIELD
[0001] The present invention relates to a semiconductor heat
exchanger unit, and in particular to a semiconductor heat exchanger
unit making use of phenomenon of boiling.
BACKGROUND ART
[0002] For the purpose of conducting a large energy of heat
generated by semiconductor devices, there has been developed a
method of obtaining a high level of cooling effect based on latent
heat of vaporization of a refrigerant capable of boiling at a
temperature not higher than the upper limit temperature of
operation of the semiconductor devices. In recent years,
investigations have been focused on influences of surface
conditions of a boiling surface and thermo-physical properties of
refrigerant, which govern the size and density of generated vapor
bubbles, aiming at stabilizing and optimizing the heat conduction
effect obtainable by boiling of the refrigerant.
[0003] It has been known that the heat conduction characteristics
may be improved in pool boiling on a flat plate, by forming an
irregularity of several micrometers or smaller on the surface of
the flat plate. The first possible reason may be such that fine
projections and notches contribute to increase the area of contact
between the heat radiation surface and the refrigerant.
[0004] The second possible reason may be such that the
micro-structure of the flat plate contributes to formation of
nuclei of vapor, which is the initial stage of boiling bubbles.
[0005] Patent Document 1 describes formation of a surface
irregularity of a nanometer scale. This document describes that any
surface structure "larger than" several micrometers may be "less
likely to ensure effective nuclei for bubbling", with respect to a
refrigerant having a small surface tension. It has, however, been
known from theoretical examinations into simple micro-notches that
physical properties of the refrigerant may he important parameters
with respect to the minimum diameter of a point of bubbling, but
the thickness of a liquid layer super-heated beyond the boiling
point in the vicinity of a heat conduction surface may be
predominant with respect to the maximum diameter of the point of
bubbling, as taught by literatures and so forth. This is because,
in the process of growth of boiling bubbles, supply of the vapor
necessary for growing the bubbles may be suppressed, as soon as the
vapor composing the bubbles is brought into contact with a vapor
not super-heated yet.
[0006] More ideal geometry of the micro-structure may be such as
allowing the nuclei to stay on the heat radiation surface even
after the grown bubbles dissociate from the surface, so as to
promote growth of the next bubbles, and may further be such as
avoiding disappearance of the nuclei by condensation, even when
they are brought into contact with a liquid, cooled to a
temperature slightly lower than the boiling point, flowing
thereinto as a result of dissociation of the bubbles.
[0007] As explained in Non-Patent Document 1, any conventional
efforts for assimilating such ideal geometry have resulted in
thickening over the entire surface geometry (100 .mu.m to 1 mm or
thicker), and any efforts have failed in obtaining the ideal
geometry of the recesses.
[0008] The thickness and the recesses of several hundred
micrometers to 1 mm or around may result in higher thermal
resistance as compared with smaller structures, and may raise a
problem in retention property of the vapor nuclei, when a
refrigerant having a surface tension smaller than that of water is
used.
[0009] Furthermore, while the geometry shown in Non-Patent Document
1 is not given with a scale, the size and so forth of the opening
in an actual structure may vary, so that it may be very easy to
presume that this raises negative effects, depending on physical
properties of the refrigerant.
[0010] Although various micro-structures have been experimented, no
invention has been successful to provide a surface geometry
obtained by optimizing an ideal double-inlet structure with respect
to an actual refrigerant. Also no invention has been made on a heat
exchanger unit having a surface modification assimilating an ideal
structure described in Non-Patent Document 1, provided in contact
with the flow path for liquid refrigerant, so as to be optimized
with respect to physical properties of the refrigerant.
[0011] It has widely been known that an effect of heat conduction
is enhanced by boiling, but the degree of effect actually
obtainable may be affected by density of bubbles generated in the
process of boiling, frequency of dissociation, and size of bubbles
in the process of dissociation, and may largely be affected also by
surface conditions of the boiling surface, which may be supposed to
be an important factor governing these parameters. Various trails
have been made on the surface conditions, only to fail in achieving
the ideal geometry, because thermal resistance may increase as the
thickness of the surface increases.
[0012] The surface conditions ideal for boiling may be such as
promoting generation of vapor bubbles and allowing growth thereof,
wherein retention of "nuclei" which provide origins of growth of
the bubbles may be indispensable. The nuclei are fine vapor
bubbles, and remain on the surface even after the grown bubbles
dissociate therefrom, so as to facilitate growth of next bubbles.
The surface geometry may therefore be useful to have a structure
capable of retaining the fine bubbles. For stable retention of the
nuclei, it may firstly be necessary that the structure for
retaining the nuclei allows a refrigerant, even if the surface
tension of which being smaller than that of water, to flow therein
only with difficulty. Secondly, it may be necessary that the
structure is not causative of condensation of the vapor bubbles in
the process of dissociation, even if the vapor bubbles are brought
into contact with a liquid flown to the vicinity of the nuclei,
while being cooled below the boiling point. Most of the surfaces at
present are not structurally idealized, and is therefore far from
being optimized for the nuclei, with some exceptions exhibiting the
effect of heat conduction by virtue of the micro-structure.
[0013] FIG. 9 illustrates an exemplary case where a refrigerant
having a small surface tension is adopted to a heat conduction
surface having V-shape notches as described in Patent Document 1.
It may theoretically be supposed that the gas-liquid interface
having a profile concaved towards the liquid 22 side may have a
large radius of curvature, thereby dissociation of the nuclei is
promoted, and retention of the vapor bubbles may be destabilized.
On the other hand, FIG. 10 illustrates an exemplary case where a
refrigerant having a large surface tension such as water was used
on the same heat conduction surface. In this case, the radius of
curvature of the gas-liquid interface concaved towards the liquid
side becomes smaller, so that the nuclei may be made more stable.
It may therefore be understood from the above that the radius of
curvature of the gas-liquid interface is determined by the angle of
contact point of three phases, that are gas, liquid and solid
phases (reference numerals 20 and 21 in FIG. 9 and FIG. 10,
representing the angles on the liquid side), and that use of the
V-shape structure for a refrigerant having a small surface tension
may result in only a poor effect of retention of vapor nuclei.
[0014] [Patent Document 1] Japanese Laid-Open Patent Publication
No. 2002-228389 (p. 3-4, FIG. 2)
[0015] [Non-Patent Document 1] Liquid-Vapor Phase-Change Phenomena
(p. 330, FIG. 8.14)
DISCLOSURE OF THE INVENTION
[0016] According to the present invention, there is provided a heat
exchanger unit having, over a base, a surface-modified portion
composed of a metal, the surface-modified portion being brought
into contact with a flow path provided for a liquid refrigerant,
wherein the liquid refrigerant is a liquid having a surface tension
smaller than that of water, and the surface-modified portion has a
porous structure, in which a plurality of recesses are provided on
the flow path side thereof, each recess has an introduction path
having a cross-section area gradually reduced from the inlet of the
recess, and a cavity communicated with the introduction path while
placing an inflection portion in between, and the shortest distance
between the inflection portion and the flow path is larger than the
shortest distance between the cavity and the flow path.
[0017] In view of raising an effect expected for the case where a
refrigerant having a surface tension smaller than that of water is
adopted, the recess may preferably have a pore size of 1 .mu.m to
10 .mu.m.
[0018] In addition, the liquid refrigerant may preferably be an
organic refrigerant, and the organic refrigerant may preferably be
a hydrofluoroether or a fluorine-containing inert liquid.
[0019] The heat exchanger unit may preferably be configured to have
the surface-modified portion, along a flow path of micro-channel
type as a forced-convection boiling refrigerant type cooling
unit.
[0020] According to the present invention, generation of the
boiling bubbles may he promoted by providing the surface-modified
portion having a multiple-inlet structure to the flow path for a
liquid refrigerant having a surface tension smaller than that of
water.
[0021] According to the present invention, various problems
ascribable to generation of bubbles may be solved in an unified
manner, by adopting the surface-modified portion of the present
invention to other types of boiling refrigerant type cooling
units.
[0022] A surface condition having a structure with cavities will
now be discussed. FIG. 1 illustrates a state of liquid 1 having a
surface tension smaller than that of water, flowing through an
introduction path 2, having a simple inlet structure, into a cavity
3. By virtue of the direction of the upper wall surface of the
cavity 3, the radius of curvature of the gas-liquid interface 4
achieved herein is apparently more closer to that achievable under
a larger surface tension, as compared with that achievable in a
V-shape notch. Recess structures, into which even a refrigerant
having a surface tension smaller than that of water is less likely
to flow, may be those having a double-inlet structure.
[0023] FIG. 2 illustrates a simple double-inlet structure. The
liquid 1 having a surface tension smaller than that of water, which
is ready to go through the introduction path 2, creeps on the wall
surface of the upper portion of the cavity 3 and enters the recess,
wherein the cavity 3 expresses resistance against further intrusion
of the liquid, while making the radius of curvature of the
gas-liquid interface 5 recessed to the vapor side, depending on the
structure of the wall surface of the recess. None of the surface
geometries ever manufactured have such double-inlet structure.
[0024] It may not always be necessary that the actual
micro-structure is completely identical to that illustrated in FIG.
2, wherein a state of allowing stable residence of bubbles by
virtue of the double-inlet structure, and uniform provision of the
structure over the entire heat conduction surface may contribute to
improvement and optimization of the heat conduction effect, based
on boiling as a macroscopic consequence. The vapor bubbles of a
refrigerant having a small surface tension may further downsize
themselves in the process of dissociation, so that micronization of
the recess structure to as small as several micrometers or around
may be effective in view of promoting the downsizing. As a
consequence, the boiling may be expected to start at a temperature
more closer to the boiling point.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] [FIG. 1] A schematic drawing illustrating an exemplary case
where a refrigerant having a surface tension smaller than that of
water enters a cavity.
[0026] [FIG. 2] A recess having a double-inlet, and expressing a
large resistance against intrusion even with respect to a
refrigerant having a small surface tension.
[0027] [FIG. 3] A porous copper plating having a multiple-inlet
structure.
[0028] [FIG. 4] Relations between the radius of bubbles and ambient
liquid temperature in the process of boiling.
[0029] [FIG. 5] A boiling refrigerant type cooling unit as one
example of the present invention.
[0030] [FIG. 6] A heat conduction surface for pool boiling as one
example of the present invention, selectively modified on the
surface at the center thereof.
[0031] [FIG. 7] A perspective view illustrating a parallel straight
pipe micro-flow path heat exchanger as one example of the present
invention.
[0032] [FIG. 8] A thermal siphon-type, boiling refrigerant heat
exchanger making use of boiling on a vertical flat plate, as one
example of the present invention.
[0033] [FIG. 9] A three-phase contact angle and a radius of
curvature of the gas-liquid interface obtainable by a liquid having
a small surface tension in a V-shape notch.
[0034] [FIG. 10] A three-phase contact angle and a radius of
curvature of the gas-liquid interface obtainable by a liquid having
a large surface tension in a V-shape notch.
BEST MODES FOR CARRYING OUT THE INVENTION
[0035] Next, exemplary embodiments of the present invention will be
detailed referring to the drawings.
[0036] FIG. 3 illustrates an electron microphotograph obtained when
the surface of a porous copper plating is observed. Materials of
the plating may preferably be copper characterized by a large heat
conductivity, or may be nickel.
[0037] It is found from the result of observation, from the top of
the plated surface formed on a flat surface, that micro-recesses of
a maximum of approximately 10 .mu.m are formed. These recesses
uniformly arranged are internally communicated with each other, and
contribute to make boiling phenomenon uniform. On the ribs, there
are formed innumerable micro-recesses with a variety of sizes. It
has been confirmed that, by virtue of these micro-recesses arranged
so as to surround the larger ones, the smaller recesses more
earlier help growth of the nuclei, and allow the boiling to start
at temperatures more closer to the boiling point.
[0038] The present invention improves reliability of boiling
refrigerant type cooling unit, by manufacturing a heat conduction
surface capable of retaining a large number of vapor nuclei which
originate growth of boiling bubbles, and thereby making generation
of bubbles uniform, both on the spatial basis and on the temporal
basis. The heat exchanger of the present invention is characterized
by providing a porous modified surface having a multiple inlet
structure with a pore size of 1 .mu.m or larger and 10 .mu.m or
smaller, to the wall surface which serves as the boiling surface of
refrigerant.
[0039] The minimum pore size of 1 .mu.m herein is determined by
comparing a pore size of a point of bubbling possibly becoming
active, obtained by a theoretical calculation based on a simple
notch model, with a pore size obtained based on a growth model of
vapor bubbles. The minimum pore size (r') capable of actively
generating the vapor bubbles may be obtained by the equation (1)
below, described in Liquid-Vapor Phase-Change Phenomen, p. 183.
r * = 2 .sigma. T sat v l v h l v [ T l - T sat ] ( 1 )
##EQU00001##
[0040] In the equation, a represents surface tension, T.sub.sat
represents saturation temperature, v.sub.lv represents difference
in specific volume between vapor and liquid, and h.sub.lv
represents latent heat of vaporization, all of which being physical
property values of a refrigerant. T.sub.l represents liquid
temperature in the vicinity of bubbles, and indicates that a larger
degree of super-heating (T.sub.l-T.sub.sat) activates notches of
smaller pore size. It has, however, been predicted that a
refrigerant having a smaller value of surface tension .sigma., such
as organic refrigerant, may become active on a smaller surface
structure, because of molecule-dependent nature of surface tension,
contradictory to the discussion given in Patent Document 1.
According to the equation (1), once the refrigerant is given, the
pore size is automatically determined based on difference
(.DELTA.T) between ambient temperature (T.sub.l) and saturation
temperature (T.sub.sat) of the refrigerant. FIG. 4 illustrates a
relation between r' and .DELTA.T by a solid line, referring to an
exemplary case of using an organic refrigerant having a small
surface tension.
[0041] It is predicted that the bubbling may become active making
use of a given difference, if the pore size R (.mu.m) falls in the
region above the solid line in the graph.
[0042] On the other hand, active growth of the boiling bubbles
requires nuclei for originating the growth, so that the pore size
which is supposed to be effective for generation of the nuclei, may
be determined also from the viewpoint of growth process of bubbles.
The process of growth of vapor bubbles may generally be divided
into a first stage allowing formation of non-matured bubbles called
embryos, and allowing the bubbles to grow immediately thereafter
based on difference between the inner and outer pressures; and a
succeeding second stage allowing the non-matured bubbles to grow at
the gas-liquid interface while being promoted by heat conduction.
In the second stage, the bubbles grow just corresponding to
supplied energy, and the grown-up bubbles further grow by absorbing
energy through their enlarged surface area. On the other hand, in
the first stage, the growth of bubbles is necessarily preceded by
accumulation of pressure necessary for the growth as enough as
pushing aside the ambient liquid, so that accumulation of energy
does not directly. result in increase in size of the nuclei. For
this reason, an ideal size of the vapor nuclei which can be
surrounded by the surface structure may be equivalent to a diameter
(r.sub.trans) transiently attainable between the first stage and
the second stage. The transiently-attainable diameter may be
determined by considering the energy balance, as indicated by a
dotted line in FIG. 4. In the region where the pore size R (.mu.m)
falls above the dotted line, the bubbles can grow depending on heat
energy conducting towards the vapor bubbles surrounded by the
surface structure.
[0043] In other words, it may be said from comparison between the
solid line and the dotted line in FIG. 4, that the region above the
dotted line may correspond to a structure assisting the first stage
of growth of bubbles, and the region above the solid line may
correspond to a structure assisting the second stage and
dissociation. Since both regions are necessary for the purpose of
achieving heat conduction effect by boiling, so that the surface
structure which belongs to the region above both lines may be
ideal. Since the temperature of the ambient liquid may vary
depending on the state of operation, and since it may be necessary
to consider the pore size at the point where both lines intersect,
physical properties of an organic refrigerant having a small
surface tension, and the thickness of the modified surface which is
causative of thermal resistance and is necessarily be suppressed,
it may be optimum to adjust the pore size within the range from 1
to 10 .mu.m. The organic refrigerant adoptable herein may be
exemplified by a hydrofluoroether or fluorine-containing inert
liquid.
[0044] A large amount of bubbles may be produced in the process of
boiling, from the surface-modified portion of the present
invention. It may therefore be necessary to keep the state of
supply of the liquid to the heat conduction surface at a high
level, so that a mode of boiling refrigerant type cooling, which
can efficiently expel the bubbles from the heat conduction surface
by forced convection, may be considered as optimum.
Examples
[0045] A heat exchanger unit provided with a surface-modified
portion of the present invention will be explained.
Example 1
[0046] As illustrated in FIG. 5, a heat exchanger unit given with a
flow path 12 for refrigerant, formed so as to allow the refrigerant
to pass over a flat plate 7 provided with a surface-modified
portion, is manufactured. The heat exchanger unit has a heating
element 6, and a heat exchanger block 10 provided over the heating
element 6. The heat exchanger block 10 has a heat exchanger upper
holder 11 and a modified surface 7. The heat exchanger upper holder
11 is provided with an inlet 8 and an outlet 9, allowing
therethrough introduction and discharge of a liquid refrigerant,
respectively. Inside the heat exchanger block 10, there is provided
the flow path 12 allowing therethrough circulation of a liquid
refrigerant. Since the flow path 12 is provided so as to allow the
refrigerant to pass over the modified surface 7, the flow rate of
the refrigerant may be varied, making it possible to avoid adverse
effects such as dry-out ascribable to generation of a large amount
of bubbles.
Example 2
[0047] The heat conduction surface provided with the
surface-modified portion may be used under pool boiling, while
needing a mode capable of avoiding the dry-out. One possible method
may be such as selectively providing a surface-modified portion 14,
rather than providing it over the entire surface of the heat
conduction surface 13. For example, as illustrated in FIG. 6, the
surface-modified portion 14 may possibly be provided only at around
the center of the heat conduction surface 13, so as to induce
natural convection.
[0048] The number of convection cells on the flat plate is an issue
of the Rayleigh-Benard Convection problem, and is determined by a
function of the depth of refrigerant and the area of the bottom
surface. In this Example, by selectively providing surface-modified
portion 14 while making use of the convection cells, the boiling
bubbles may effectively be taken apart from the heat conduction
surface soon after dissociation, and thereby the liquid may be
supplied to the heat conduction surface 13.
Example 3
[0049] The surface-modified portion of the present invention may be
adoptable also to a heat conduction portion having a form generally
called micro-channel, among forced-convection boiling refrigerant
type cooling units, aimed at achieving a heat conduction effect by
forming fine flow paths (FIG. 7). The micro-channel limits the
width of the flow path to as small as the size of the boiling
bubbles or below, in order to extremely expand the contact area
between the heat conduction surface and a refrigerant. As a
consequence, the bubbles may stagnate and the flow may accordingly
be unbalanced among the flow paths, depending on refrigerant and
conditions of flow.
[0050] By providing the surface modification of the present
invention, the boiling bubbles may be allowed to generate uniformly
in the flow paths, and may therefore be reduced in size when they
dissociate from the heat conduction surface. Since small bubbles
are unlikely to stagnate, so that the flow may more readily be
balanced among the flow paths if the stagnation of bubbles may be
avoided.
Example 4
[0051] By further providing the surface for receiving heat and
allowing boiling to proceed thereon in the vertical direction, so
as to allow the vapor bubbles 17 generated in a space having a
micro-thickness to ascend making use of buoyancy, an upflow may
successfully be produced where the surface-modified portion 14 is
provided (FIG. 8). By providing a heat radiation structure such as
a fin 15 to the exterior of the top of the heat exchanger so as to
radiate heat to the ambient air, vapor produced inside the heat
exchanger may be condensed. The condensed liquid refrigerant passes
through a portion provided alongside of the main heater unit,
having no surface-modified portion provided Thereto, and is
supplied to the lower portion of the heat exchanger. In FIG. 8,
reference numeral 16 represents the gas-liquid interface, reference
numeral 18 represents flow of the vapor bubbles 17, and reference
numeral 19 represents flow of the liquid.
[0052] This is a currently-available structure based on the
principle of thermal siphon, but is capable of causing more faster,
and nearly-forced natural convection, from which the heat
conduction effect may more readily be obtained, by virtue of a
marked difference in the generated vapor bubbles depending on
presence or absence of the surface-modified portion 14.
[0053] Applications of the present invention may be exemplified by
a cooling unit for semiconductor, such as CPU, in need of a heat
conduction effect larger than that obtainable by natural air
cooling.
* * * * *