U.S. patent application number 09/945909 was filed with the patent office on 2003-03-06 for non-inverted meniscus loop heat pipe/capillary pumped loop evaporator.
Invention is credited to Phillips, A. L..
Application Number | 20030042009 09/945909 |
Document ID | / |
Family ID | 25483686 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030042009 |
Kind Code |
A1 |
Phillips, A. L. |
March 6, 2003 |
NON-INVERTED MENISCUS LOOP HEAT PIPE/CAPILLARY PUMPED LOOP
EVAPORATOR
Abstract
The invention introduces a two part wick for use in the
evaporator of a two phase loop (LHP/CPL). The primary wick controls
evaporation from the primary heat input area. The secondary or
"distribution" wick separates the liquid and vapor volumes of the
evaporator and feeds liquid to the primary wick. The secondary wick
allows the primary wick to be configured so heat enters from the
liquid side of the wick, this constitutes a non-inverted meniscus
evaporator which can tolerate high heat fluxes without restricting
vapor flow. The secondary wick is removed from the primary heat
flow path lends itself to fabrication in small dimensions
compatible with direct cooling of electronic devices.
Inventors: |
Phillips, A. L.; (Pine
Grove, PA) |
Correspondence
Address: |
DUANE MORRIS, LLP
ATTN: WILLIAM H. MURRAY
ONE LIBERTY PLACE
1650 MARKET STREET
PHILADELPHIA
PA
19103-7396
US
|
Family ID: |
25483686 |
Appl. No.: |
09/945909 |
Filed: |
September 4, 2001 |
Current U.S.
Class: |
165/104.26 ;
165/104.33; 174/15.2; 257/715; 257/E23.088; 361/700 |
Current CPC
Class: |
H01L 23/427 20130101;
F28D 15/046 20130101; H01L 2924/0002 20130101; H01L 2924/0002
20130101; F28D 15/043 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
165/104.26 ;
165/104.33; 257/715; 361/700; 174/15.2 |
International
Class: |
H05K 007/20; H01B
007/42; F28D 015/00; H01L 023/34 |
Claims
What is claimed is:
1. An evaporator for a capillary pumped two phase loop comprising:
a primary wick which controls evaporation from a primary heat input
area; and, a secondary wick which separates liquid and vapor
volumes disposed in the evaporator, and which feeds liquid to the
primary wick.
2. The evaporator of claim 1, wherein heat from the primary heat
input area enters the evaporator on the liquid side of the primary
wick.
3. The evaporator of claim 1, wherein the primary and secondary
wicks are formed of different materials.
4. The evaporator of claim 1, wherein said secondary wick
determines the loop pressure of the heat pipe.
5. The evaporator of claim 1, wherein said primary wick has larger
pores than said secondary wick.
6. An evaporator structure comprising: a first wick structure; and,
a second wick structure disposed adjacent the first wick structure,
wherein a liquid disposed within the evaporator is adjacent said
first wick structure such that said first wick structure lies
between said liquid and said second wick structure.
7. A computer comprising: a heat-producing apparatus; and a heat
pipe disposed adjacent the heat-producing apparatus, said heat pipe
including a primary wick which controls evaporation from a primary
heat input area of the heat producing apparatus, and a secondary
wick which separates liquid and vapor volumes disposed in an
evaporator section of the heat pipe, and which feeds liquid to the
primary wick.
8. A computer comprising: a heat-producing apparatus; and a heat
pipe disposed adjacent the heat-producing apparatus, said heat pipe
including a first wick structure and a second wick structure
disposed adjacent the first wick structure, wherein a liquid
disposed within an evaporator section of the heat pipe is adjacent
said first wick structure such that said first wick structure lies
between said liquid and said second wick structure.
9. A loop heat pipe system comprising: a heat-producing apparatus;
a loop heat pipe with an evaporator section including a wick
disposed on one side thereof, wherein said evaporator section is
coupled to the heat-producing apparatus on the one side thereof
which includes the wick, such that the heat-producing apparatus is
disposed on the convex side of the meniscus of liquid in the
wick.
10. A method for cooling heat-producing equipment, comprising the
step of: disposing a heat-producing apparatus on the convex side of
a meniscus of liquid located in a wick of a loop heat pipe.
11. The method of claim 10, wherein the wick is disposed in an
evaporator section of the loop heat pipe.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a heat pipe system, and in
particular, a two phase loop commonly known as a Loop Heat Pipe
(LHP) or Capillary Pumped Loop (CPL).
DESCRIPTION OF THE RELATED ART
[0002] A basic heat pipe comprises a closed or sealed envelope or a
chamber containing a liquid-transporting wick and a working fluid
capable of having both a liquid phase and a vapor phase within a
desired range of operating temperatures. When one portion of the
chamber is exposed to relatively high temperature it functions as
an evaporator section. The working fluid is vaporized in the
evaporator section causing a slight pressure increase forcing the
vapor to a relatively lower temperature section of the chamber
defined as a condenser section. The vapor is condensed in the
condenser section and returned through the liquid-transporting wick
to the evaporator section by capillary pumping action.
[0003] Because it operates on the principle of phase changes rather
than on the principles of conduction or convection, a heat pipe is
theoretically capable of transferring heat at a much higher rate
than conventional heat transfer systems. Consequently, heat pipes
have been utilized to cool various types of high heat-producing
apparatus, such as electronic equipment (See, e.g., U.S. Pat. Nos.
5,884,693, 5,890,371, and 6,076,595).
[0004] Because conventional heat pipes must transport liquid
through the capillary wick, they incur a large flow pressure drop
if they are made very long. Also, because liquid and vapor flow in
opposite directions, vapor can entrain liquid at high power rates
and limit the operation of the device; this is commonly known as
the flooding limit. To overcome these limitations and transport
high thermal power over long distances, the Loop Heat Pipe (LHP)
and Capillary Pumped Loop (CPL) were developed. (See notably U.S.
Pat. No. 4,515,209.)
[0005] In conventional heat pipes, heat almost always enters the
heat pipe from the liquid (i.e., convex) side of the meniscus. As
is known in the art, the meniscus is the curved shape of the
surface of a liquid in a container, caused by the cohesive effects
of surface tension (capillary action).
[0006] Alternatively, in capillary pumped two phase loop heat
pipes, such as loop heat pipes (LHPs) and capillary pumped loops
(CPLs), heat enters the device (e.g., LHP, CPL, etc.) from the
vapor (i.e., concave) side of the meniscus. This is known as an
inverted meniscus arrangement.
[0007] Because of the `inverted meniscus` arrangement, devices such
as LHPs and CPLs have relatively high thermal resistance in the
evaporator area, and are typically not capable of operating at high
heat fluxes without drying out. Thus, conventional LHPs/CPLs can
dissipate approximately only 10 W/cm.sup.2.
[0008] Some have utilized a method of filling the vapor spaces of
the evaporator portion of the LHP/CPL with bidispersed wick in
order to achieve higher heat dissipation figures. LHPs/CPLs with
bidispersed wicks have achieved approximately 100 W/cm.sup.2 of
heat dissipation, however, at the expense of constricting vapor
flow and maximum power capacity, as well as introducing
considerable complexity and cost.
[0009] The nature of a two phase loop requires that the temperature
difference (often referred to as `delta T` or `.DELTA.T`) from the
vapor to the liquid side of the wick correspond to the capillary
pressure being produced by the wick. If the .DELTA.T is
insufficient, then boiling will occur on the liquid side of the
wick and the loop will deprime (i.e., stop operating). This
.DELTA.T relationship becomes increasingly more difficult to
maintain as the wick dimensions are made smaller. Miniature
evaporators for two phase loops are thus very difficult to design
and build, and sub-miniature evaporators of a size that would
permit integration with a semiconductor chip have so far not been
feasible.
[0010] Therefore, there is currently a need for a two phase loop
system which can accommodate high heat flux inputs without also
restricting vapor flow. There is also a need for means to maintain
a suitable .DELTA.T in evaporators scaled to permit integration
with semiconductor chips.
SUMMARY OF THE INVENTION
[0011] The present invention is a evaporator including a primary
wick which controls evaporation from a primary heat input area and,
a secondary wick which separates liquid and vapor volumes disposed
in the evaporator, and which feeds liquid to the primary wick. The
present invention also includes a method for cooling heat-producing
equipment by disposing a heat-producing apparatus on the convex
side of a meniscus of liquid located in a wick of a loop heat
pipe.
[0012] The above and other advantages and features of the present
invention will be better understood from the following detailed
description of the exemplary embodiments of the invention which is
provided in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a perspective view showing an evaporator section
of a loop heat pipe according to a first exemplary embodiment of
the present invention.
DETAILED DESCRIPTION
[0014] The present invention comprises an evaporator for a two
phase loop which does not employ an inverted meniscus, and which
thereby allows both high heat flux and large vapor flows. This is
accomplished by providing at least two wicks within the evaporator.
The first or "primary" wick controls evaporative cooling from the
heat input area. This primary wick functions in the same manner as
the wick in a conventional heat pipe. This primary wick does not
employ an inverted meniscus, and thus can tolerate high heat
fluxes. This primary wick can be used in the evaporator of a two
phase loop because of the addition of a secondary or "distribution"
wick into the evaporator. The distribution wick provides two
essential functions; it separates the liquid volumes from the vapor
volumes within the evaporator, and it feeds liquid to the primary
wick. In order for the distribution wick to feed liquid to the
primary wick, the primary wick must have a smaller capillary pore
radius than the distribution wick. The capillary pore radius of the
distribution wick determines the pressure drop which may be
incurred in the external portions of the two phase loop.
[0015] Note that the term "secondary wick" is commonly used in LHP
technology in reference to a wick which provides a capillary
connection between the compensation chamber and the evaporator. The
secondary or distribution wick used in this application is in
addition to the wick associated with the compensation chamber.
[0016] Referring to FIG. 1, there is shown an evaporator section
100 of a capillary pumped two phase loop according to a first
exemplary embodiment of the present invention. Two phase loops
typically include an evaporator section and a condenser section
which are connected by vapor and liquid lines in a loop
arrangement, as is well known in the art. For ease of discussion,
the condenser section and the remainder of the tubing are not shown
in FIG. 1.
[0017] The evaporator section 100 includes a housing 110, primary
wick structure 120, secondary or distribution wick structure 130,
and working fluid 140. The housing 110 includes respective sides
101, 102, 103 and 104 as well as a liquid inlet port 111 and vapor
outlet port 112. The working fluid is present in both liquid 141
and vapor 142 states. The primary and secondary wick structures
120, 130 and the liquid 140 are all disposed within the housing
110, as is well known in the art. The housing 110 includes a liquid
inlet port 111 for permitting liquid condensed in a condenser
section of the loop heat pipe to return to the evaporator section
100, and a vapor outlet port 112 for permitting vapor created in
the evaporator section to escape to the condenser section to be
condensed. The liquid 140 preferably comprises water, but may
comprise any suitable liquid.
[0018] As shown in FIG. 1, the secondary wick structure 130 (or
`distribution` wick) consists of separation structure 132 which
separates the liquid and vapor states of the working fluid, and a
number of post structures 131 which supply fluid to the primary
wick 120. The secondary wick may also contain support structures
133 which provide physical support to the assembly. Separation
structure 132 is typically of planar form, while structures 131 and
133 are typically posts of square or cylindrical form. However, it
will be noted that the entire secondary wick structure 130 may be
formed in any desired shape to provide the above functions.
[0019] The primary wick structure 120 (or `primary` wick) is
substantially thinner in cross section than the first wick
structure. Although the primary wick structure 120 is shown in FIG.
1 as an additional layer, it may be formed as etched grooves on the
surface of the housing 110. The primary wick structure may comprise
sintered powder, etched grooves, or any other equivalent structure.
However, the primary wick structure 120 should have a smaller pore
radius than the distribution wick structure 120 so that it can
syphon liquid from the distribution wick structure.
[0020] In operation, heat (produced by some heat-producing
apparatus) enters the evaporator section 100 from the first side
104 of the housing 110. The heat is transferred into the liquid
contained within the primary wick structure 120 causing it to
evaporate. This evaporation takes places in the primary wick
structure 120 as would take place in the wick of a conventional
heat pipe. As explained above, the distribution wick structure 130
serves a dual purpose: it separates the liquid in the evaporator
section 100 from the vapor, and it assists in feeding liquid to the
primary wick structure 120 via posts 131. Under some conditions,
the secondary wick structure may contribute evaporation, especially
from the posts 131.
[0021] It will be noted that in the evaporator section 100 heat
(from some heat-producing apparatus) enters from the liquid (i.e.,
convex) side of the meniscus of the liquid 140 in the primary wick
120, thus creating a `non-inverted meniscus` arrangement. Such an
arrangement is now known for use in loop heat pipes, and provides
significant advantages over conventional loop heat pipe
arrangements.
[0022] In the evaporator structure described above, the second wick
structure 130 (`distribution` wick) is removed from the primary
heat flow path, thereby allowing it to be readily maintained at or
near the temperature of the liquid, minimizing any propensity for
the liquid to boil and deprime the device. In addition, the
arrangement readily allows the distribution wick to be fabricated
from materials different from the primary wick or the housing,
thereby contributing further to maintaining a large thermal
resistance (.DELTA.T), and providing additional structural
strength.
[0023] The design of the evaporator section 100 described above
allows high heat fluxes without sacrificing vapor flow through the
heat pipe or increasing thermal resistance. Furthermore, the
evaporator section 100 may be easily manufactured in planar
configurations, and allows fabrication in Silicon (Si), which
allows on-chip cooling with the added ability to transport heat
off-chip.
[0024] In an alternative embodiment of the present invention, the
evaporator section 100 described above may be formed entirely of
Silicon. In this alternative embodiment, the primary wick structure
120 may be formed as an etching on the inner surface of the first
wall 104. An all-Silicon evaporator section is particularly useful
in embodiment where the apparatus being cooled is made of Silicon
as well (e.g., microchip).
[0025] The above-described evaporator section 100 of a heat pipe
may be used to cool heat-producing apparatus contained within a
desktop or laptop computer or server. For example, the evaporator
section 100 may be disposed on or near a heat-producing apparatus
such as a microchip or Central Processing Unit (CPU).
[0026] Although the invention has been described in terms of
exemplary embodiments, it is not limited thereto. Rather, the
appended claims should be construed broadly, to include other
variants and embodiments of the invention which may be made by
those skilled in the art without departing from the scope and range
of equivalents of the invention.
* * * * *