U.S. patent application number 13/462442 was filed with the patent office on 2012-11-08 for heat transfer device and system.
This patent application is currently assigned to COMMISSARIAT A L'ENERGVIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES. Invention is credited to Fabien Bonnet, Philippe Gully.
Application Number | 20120279682 13/462442 |
Document ID | / |
Family ID | 46001022 |
Filed Date | 2012-11-08 |
United States Patent
Application |
20120279682 |
Kind Code |
A1 |
Bonnet; Fabien ; et
al. |
November 8, 2012 |
HEAT TRANSFER DEVICE AND SYSTEM
Abstract
Heat transfer device including: a first reservoir (R1) for
storing a diphasic fluid (LC), equipped with first heating means
(MC1) and connected to a cold source (SF) via a first thermal
resistance (RTH1); a second reservoir (R2) for storing said
diphasic fluid, equipped with second heating means (MC2) and
connected to said cold source or to another cold source via a
second thermal resistance (RTH2); and a fluidic pipe (CF) through
which said diphasic fluid may pass, connecting said first and
second reservoirs, said pipe including at least: an evaporator (EV)
that may be thermally connected to a hot source (PC, O) at a
temperature higher than that of said cold source; a first condenser
(C1) and a second (C2) condenser situated on either side of said
evaporator and adapted to be thermally connected to said cold
source. Heat transfer system including at least one such device.
Method for cooling or precooling an object by means of such a
device or system.
Inventors: |
Bonnet; Fabien; (Grenoble,
FR) ; Gully; Philippe; (Saint Egreve, FR) |
Assignee: |
COMMISSARIAT A L'ENERGVIE ATOMIQUE
ET AUX ENERGIES ALTERNATIVES
Paris
FR
|
Family ID: |
46001022 |
Appl. No.: |
13/462442 |
Filed: |
May 2, 2012 |
Current U.S.
Class: |
165/62 |
Current CPC
Class: |
F28D 15/043 20130101;
F28D 15/0266 20130101 |
Class at
Publication: |
165/62 |
International
Class: |
F25B 29/00 20060101
F25B029/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 2, 2011 |
FR |
1153726 |
Claims
1. A heat transfer device including: a first reservoir for storing
a diphasic fluid, equipped with first heating means and connected
to a cold source via a first thermal resistance; a second reservoir
for storing said diphasic fluid, equipped with second heating means
and connected to said cold source or to another cold source via a
second thermal resistance; and a fluidic pipe able to be traversed
by said diphasic fluid, connecting said first and second
reservoirs, said pipe including at least: an evaporator able to be
thermally connected to a hot source at a temperature higher than
that of said cold source; a first condenser and a second condenser
situated on either side of said evaporator and able to be thermally
connected to said cold source; said first and second heating means
and said fluidic pipe being arranged in such a manner that
activation of the first heating means causes expulsion of said
diphasic fluid from said first reservoir toward said second
reservoir via said fluidic pipe and activation of the second
heating means causes expulsion of said diphasic fluid from said
second reservoir toward said first reservoir via said fluidic
pipe.
2. The heat transfer device claimed in claim 1, containing a
diphasic fluid in an amount at least sufficient, in the liquid
state, to fill said fluidic pipe and part of the volume of one of
said first and second reservoirs, but insufficient, in the liquid
state, to fill both reservoirs and said fluidic pipe.
3. The heat transfer device claimed in claim 1, wherein said and
second reservoirs have a capacity greater than that of the fluidic
pipe.
4. The heat transfer device claimed in claim 1, wherein said first
and second reservoirs have the same capacity.
5. The heat transfer device claimed in claim 1, wherein said
diphasic fluid is a cryogenic fluid having a critical temperature
less than or equal to 200K.
6. The heat transfer device claimed in claim 1 further including at
least one cold source including cooling means adapted to bring it
to a temperature enabling the existence of a liquid phase of said
fluid inside said reservoirs.
7. The heat transfer device claimed in claim 1 wherein said fluid
pipe is connected to said first and second reservoirs via
respective bleeds produced at the lower ends thereof.
8. The heat transfer device claimed in claim 1 wherein each of said
first and second reservoirs contains a thermally conductive porous
material, wettable by the liquid phase of said diphasic fluid, in
thermal contact with said heating means.
9. The heat transfer device claimed in claim 1 wherein said fluidic
pipe is connected to a pressure reduction reservoir.
10. The heat transfer device claimed in claim 1 further including a
control device adapted to activate alternately the first heating
means and the second heating means in such a manner as to cause a
transfer of said diphasic fluid from said first reservoir to said
second reservoir and vice versa.
11. The heat transfer system claimed in claim 10 including two
devices (Da, Db) thermally connected between said hot source and
said cold source, or respective cold sources, wherein said control
devices are configured to activate the respective heating means
periodically and in phase quadrature.
12. The heat transfer system claimed in claim 1 including a device
and a heat transfer passive diphasic device, such as a fluidic loop
heat pipe or a pulsating heat pipe, thermally connected between
said hot source and said cold source, or respective cold
sources.
13. The heat transfer system claimed in claim 12 wherein said heat
transfer passive diphasic device is connected to said first and
second reservoirs via a system of valves enabling it to be filled
with diphasic fluid.
14. The heat transfer system claimed in claim 1, including a device
wherein a pulsating heat pipe thermally connected between said hot
source and cold source is integrated into said fluidic pipe.
15. A method of cooling or precooling an object by means of the
device claimed in claim 1, including the following steps: a.
Thermally connecting said object to the evaporator of said device
so that it functions as a hot source; b. Thermally connecting said
first and second reservoirs and said first and second condensers to
said cold source or to respective cold sources in such a manner as
to cause at least partial filling of at least said first reservoir
with a liquid phase of said diphasic fluid; c. Activating said
first heating means so that said liquid phase of said diphasic
fluid flows toward said second reservoir via said evaporator, in
which it evaporates at least partially, cooling said object, and
said second condenser, where the vapor formed in this way returns
to the diphasic state; d. Deactivating said first heating means
when the first reservoir is substantially empty of said liquid
phase; e. Activating said second heating means so that said liquid
phase of said diphasic fluid flows toward said first reservoir via
said evaporator, where it is evaporated at least in part, cooling
said object, and said first condenser, where the vapor formed in
this way returns to the diphasic state; and f. Deactivating said
second heating means when the second reservoir is substantially
empty of said liquid phase; the steps c. to f. being repeated
cyclically.
16. The method claimed in claim 15 of cooling an object, including
a precooling step followed by a step of thermal stabilization by
means of a heat transfer passive diphasic device.
Description
[0001] The invention relates to a device and a system, in
particular a cryogenic device and a cryogenic system, for transfer
of heat and to a method of cooling or precooling an object by means
of such a device or such a system. The device and the system of the
invention notably enable a thermal link of high thermal
conductivity to be produced between a cold source and an object to
be cooled (hot source).
[0002] Controlling the temperature of an object to be cooled and/or
giving off heat necessitates the use of a thermal link connecting
the object to a "cold source" which, as a function of various
technological constraints, may be far removed. At low temperature,
i.e. at a temperature below ambient temperature, controlling the
temperature of the object necessitates two essential thermal
functions, namely:
[0003] to enable cooling of the object, initially at ambient
temperature, to its operating temperature, i.e. the temperature
required for the correct functioning of the object; this is the
"precooling" function;
[0004] to maintain its temperature by a transfer of heat to the
cold source at the operating temperature; this is the "thermal
stabilization" function.
[0005] In some applications, in particular in the space field, it
is of benefit also to have an additional "thermal switch" function
that consists in interrupting the transfer of heat by an external
action.
[0006] Transfer of heat over a distance that may reach several
meters may be effected thanks to various devices.
[0007] The simplest of these and the most widely used is the metal
braid operating by simple conduction of heat in a solid of high
thermal conductivity. The major drawback of such a device is that
it has a high mass, especially if a low thermal resistance is
required. Moreover, a thermal gradient between the cold source and
the object to be cooled cannot be avoided. Furthermore, the
"thermal switch" function cannot be assured.
[0008] Convection devices, exploiting the circulation of a fluid
driven by a pump, therefore appear much more attractive in numerous
applications, notably cryogenic applications. In most cases, for
reasons of reliability, mass and overall size, one wishes to avoid
the use of mechanical pumps in which mechanical elements move
relative to each other. Pumping with no moving parts is thus
preferred. Moreover, diphasic systems are generally employed in
which the transfer of heat is effected by evaporating a liquid on
the object side and condensing the vapor on the cold source side.
In principle this type of system operates with no temperature
difference and enables high heat transfer by way of liquid/vapor
phase change because of the high latent heat of the phase change,
which accounts for its benefit. Four principal types of thermal
link operate on this principle: the thermosiphon, the "simple" heat
pipe, the fluid loop and the pulsating heat pipe. These are passive
devices, i.e. devices in which the fluid circulates without
external action.
[0009] In a thermosiphon the diphasic fluid circulates because of
the effects of gravitational forces induced by the density
difference between the liquid and the vapor. The system may be a
simple passage in which the descending liquid and the rising vapor
circulate in counterflow, or a loop with a liquid passage and a
vapor passage. In all cases, the object must be situated lower down
than the cold source, which constitutes a constraint that is
sometimes unacceptable. Moreover, given its operating principle,
the thermosiphon does not work in a microgravity situation and is
therefore not suitable for space applications.
[0010] The "simple" heat pipe and the fluid loop make it possible
to avoid these constraints by exploiting "capillary pumping"
generated by the curvature of a liquid meniscus evaporating within
a capillary structure.
[0011] The "simple" heat pipe is a tube containing a capillary
structure, commonly called a "wick", in thermal contact with the
internal walls of the tube. The liquid is located only in the wick
and flows from the condenser to the evaporator. The vapor produced
returns in contraflow to the condenser, in a passage provided at
the center of the tube.
[0012] Just as in the case of the thermosiphon, the "thermal
switch" function cannot be provided in a simple way; on the other
hand, reliable operation in a microgravity situation is possible.
Used more with a rectilinear geometry to homogenize the temperature
of an object, its integration into a complex thermal control system
may be problematic, because of its necessitating non-rectilinear
geometries. Moreover, the heat pipe is not suitable for
transporting heat over long distances (several meters) because of
the high head losses generated by the flow of the liquid in the
porous wick and by viscose interactions between the liquid and the
vapor (driving losses).
[0013] The fluid loop, commonly called a Loop Heat Pipe (LHP) or
Capillary Pumped Loop (CPL), makes it possible to overcome the
aforementioned drawbacks of the "simple" heat pipe. It operates on
the basis of the same principle as the latter, but the capillary
structure is located uniquely at the level of the evaporator in
order to generate the capillary pumping necessary for the
circulation of the fluid; moreover, the evaporator and the
condenser are connected by two separate pipes for the liquid and
the vapor. In this way, head losses are lower, heat transfer may be
effected over long distances, and recourse to non-rectilinear
geometries proves less problematic. The "thermal switch" function
may be effected by simple localized heating on the liquid line that
causes the liquid to boil and brings about unpriming of the
loop.
[0014] A more detailed description of the operation of a fluid loop
may be found in the paper by Jentung Ku "Operating Characteristics
of Loop Heat Pipes", 29th International Conference on Environmental
Systems, 12-15 Jul. 1999, Denver, USA.
[0015] The use of fluid loops at cryogenic temperatures gives rise
to the problem of precooling of the object ("hot source") in
thermal contact with the evaporator. For capillary pumping to be
primed, it is necessary for the wick to be engorged with the
cryogenic fluid in the liquid state. However, in a fluid loop, the
wick is located only in the evaporator, i.e. in the "hot" part of
the device. Precooling is therefore necessary to enable wetting of
the wick. A number of solutions have been proposed for producing
cryogenic LHP or CPL.
[0016] A first cryogenic CPL concept known as the Cryogenic
Capillary Pumped Loop (CCPL) is described in the paper by D. Bugby
and B. Marland "Flight results from the Cryogenic Capillary Pumped
Loop (CCPL) Flight Experiment on STS-95" SAE paper No. 981814, 28th
International Conference on Environmental Systems, Jul. 13-16,
1998, [Danvers], USA. In this device, precooling is effected by the
expulsion toward the evaporator a cryogenic liquid contained in a
cold reservoir via a precooled reservoir line in a heat exchanger
("condenser spool"). This expulsion is effected by electrical
heating on the reservoir. The arrival of liquid at the evaporator
causes cooling thereof and finishes by filling it with liquid. The
system is then ready to be primed.
[0017] Another concept consists in inserting into the principal
loop a secondary capillary pump hydraulically connected to a
secondary condenser thermally connected to the cold source.
Application of electrical heating to the secondary pump generates
the circulation of the fluid and consequently the feeding with
liquid of the principal evaporator, which leads to its precooling.
This concept is described in the following publications: [0018] D.
Khruslatev, "Cryogenic loop heat pipes as flexible thermal links
for cryocoolers", Proc. 12th Cryocoolers Conference, pp. 709-716
(2003); [0019] Q. Mo and J. T. Liang, "A novel design and
experimental study of a cryogenic loop heat pipe with high heat
transfer capability" IJHMT 49, pp 770-776 (2006); and [0020] Q. Mo,
J. T. Liang and C. Jinghui, "Investigation of the effects of three
key parameters on the heat transfer capability of a CLHP",
Cryogenics 47 pp. 262-266 (2007).
[0021] A further solution consists in using a secondary circuit
including a secondary fluid line, a condenser, a diphasic reservoir
and a secondary capillary pump. By simple application of electrical
power to the secondary pump, this circuit enables precooling of the
loop, in particular filling of the principal evaporator with
liquid. This solution is described in the paper by J. Yun, E.
Kroliczek and L. Crawford "Development of a Cryogenic Loop Heat
Pipe (CLHP) for Passive Optical Bench Cooling Applications", 32nd
ICES 2002, SAE paper n.degree. 2002-01-2507, San Antonio, Tex.,
2002, and in U.S. Pat. No. 7,004,240.
[0022] A similar concept is that of the cryogenic advanced LHP: see
T. T. Hoang, D. Khruslatev and J. Ku, "Cryogenic advanced loop heat
pipe in temperature range of 20-30K" Proc. 12th International heat
pipe conference, (2002), pp. 201-205; US 2003/0159808; WO
03/054469, 3 Jul., 2003.
[0023] A further possibility consists in using gravity to prime the
loop. The paper by H. Pereira, F. Haug, P. Silva, J. Wu, and T.
Koettig, "Cryogenic loop heat pipe for the cooling of small
particle detectors at CERN", Cryogenic Engineering Conf., 28 Jun.-2
Jul. 2009, Tucson, USA, describes an LHP in which the liquid line
is placed above the evaporator; in this way, "gravity pumping"
enables priming and assists capillary pumping in normal operation.
This principle is not suited to space applications, of course.
[0024] Another diphasic passive heat transfer device is the
Pulsating Heat Pipe (PHP). This device is constituted by a simple
tube, with a diameter less than the capillary length, forming a
plurality of loops or undulations and filled with a diphasic fluid
constituted by liquid, forming "liquid plugs", and vapor, forming
bubbles. One end of each loop or undulation is brought into thermal
contact with a hot source and the opposite and with a cold source.
Under these conditions instability is created causing oscillatory
movement of the liquid plugs or bubbles. The result of this is
extremely efficient thermal transfer. The capillary may be closed
at both ends ("open" PHP) or be looped on itself ("closed" PHP,
more effective).
[0025] The PHP principle is described in the paper by M. B. Shafi,
A. Faghri and Y. Zhang "Analysis of heat transfer in unlooped and
looped pulsating heat pipes", Int. Journ. of Numerical Methods for
Heat & Fluid Flow, Vol. 12, No. 5, (2002), pp. 585-609.
[0026] A PHP suited to cryogenic applications is described in the
paper by R. Chandratilleke et al., "Development of cryogenic loop
heat pipe", Cryogenics 38 (1998) pp. 263-269. Although precooling
is necessary, it is not referred to in this publication.
[0027] Diphasic heat transfer devices such as those described above
may generally be qualified as "passive" when they accomplish their
thermal stabilization function; however, when they are used at
cryogenic temperatures, they necessitate precooling means--often
active means--which considerably complicate their structure and
operation and/or that impose constraints that are sometimes
unacceptable (presence of a gravitational field, relative
arrangement of certain components).
[0028] The invention aims to overcome the aforementioned drawbacks
of the prior art by proposing a cryogenic heat transfer device of
particularly simple structure enabling precooling of an object, in
an independent manner and including against gravity, over large
distances, but also evacuation of the heat given off by the object
once cooled. This device may be qualified as "active" because it
employs actuators (heat sources) to assure the circulation of a
diphasic fluid; however, no mechanical moving parts are necessary
(except valves in some embodiments). It is able on its own to
provide the precooling and thermal stabilization functions or be
used only for the step of precooling an object, in which case
thermal stabilization may be assured by a conventional passive
device.
[0029] One object of the invention is therefore a heat transfer
device, notably a cryogenic heat transfer device, including:
[0030] a first reservoir for storing a diphasic fluid, equipped
with first heating means and connected to a cold source via a first
thermal resistance;
[0031] a second reservoir for storing said diphasic fluid, equipped
with second heating means and connected to said cold source or to
another cold source via a second thermal resistance; and
[0032] a fluidic pipe able to be traversed by said diphasic fluid,
connecting said first and second reservoirs, said pipe including at
least:
[0033] an evaporator able to be thermally connected to a hot source
at a temperature higher than that of said cold source;
[0034] a first condenser and a second condenser situated on either
side of said evaporator and able to be thermally connected to said
cold source; said first and second heating means and said fluidic
pipe being arranged in such a manner that activation of the first
heating means causes expulsion of said diphasic fluid from said
first reservoir toward said second reservoir via said fluidic pipe
and activation of the second heating means causes expulsion of said
diphasic fluid from said second reservoir toward said first
reservoir via said fluidic pipe.
[0035] In different embodiments of the invention:
[0036] The device may contain a diphasic fluid in an amount at
least sufficient, in the liquid state, to fill said fluidic pipe
and part of the volume of one of said first and second reservoirs,
but insufficient, in the liquid state, to fill both reservoirs and
said fluidic pipe.
[0037] Said first and second reservoirs may have a capacity greater
than that of the fluidic pipe. These reservoirs may preferably have
the same capacity.
[0038] Said diphasic fluid may be a cryogenic fluid having a
critical temperature less than or equal to 200K, or even 120K. It
may for example be helium, hydrogen, neon, nitrogen or oxygen at
respective temperatures of 4.2K, 20K, 27K, 77K and 90K and the
critical temperatures of which are respectively 5.2K, 33K, 44K,
126K and 154K.
[0039] The device may further include at least one cold source
including cooling means adapted to bring it to a temperature
enabling the existence of a liquid phase of said fluid inside said
reservoirs.
[0040] Said fluid pipe may be connected to said first and second
reservoirs via respective bleeds produced at the lower ends
thereof. This embodiment is suitable for terrestrial applications,
in the presence of a gravitational field.
[0041] Alternatively, each of said first and second reservoirs may
contain a thermally conductive porous material wettable by the
liquid phase of said fluid; by "wettable" material is meant a
material with which said liquid phase forms a contact angle less
than 90.degree.. This embodiment is suitable for space applications
in a microgravity environment.
[0042] Said fluid pipe may be connected to a pressure reduction
reservoir. This is a feature that is advantageously present in most
heat transfer fluidic devices operating at cryogenic
temperatures.
[0043] The device of the invention may further include a control
device adapted to activate alternately the first heating means and
the second heating means in such a manner as to cause a transfer of
said diphasic fluid from said first reservoir to said second
reservoir and vice versa.
[0044] The invention also provides a heat transfer system including
two devices as described above thermally connected between said hot
source and said cold source, or respective cold sources, wherein
said control means are configured to activate the respective
heating means periodically and in phase quadrature.
[0045] The invention further provides a heat transfer system
including a device as described above and a heat transfer passive
diphasic device, such as a fluidic loop heat pipe or a pulsating
heat pipe, thermally connected between said hot source and said
cold source, or respective cold sources. In one particular
embodiment said heat transfer passive diphasic device may be
connected to said first and second reservoirs via a system of
valves enabling it to be filled with diphasic fluid.
[0046] The invention further provides a heat transfer system
including a device as described above wherein a pulsating heat pipe
thermally connected between said hot source and cold source is
integrated into said fluidic pipe.
[0047] The invention further provides a method of cooling or
precooling an object, notably to a cryogenic temperature, by means
of a device as described above, including the following steps:
a. Thermally connecting said object to the evaporator of said
device so that it functions as a hot source; b. Thermally
connecting said first and second reservoirs and said first and
second condensers to said cold source in such a manner as to cause
at least partial filling of at least said first reservoir with a
liquid phase of said diphasic fluid; c. Activating said first
heating means so that said liquid phase of said diphasic fluid
flows toward said second reservoir via said evaporator, where it is
evaporated at least partially, cooling said object, and said second
condenser, where the vapor formed in this way returns to the
diphasic state; d. Deactivating said first heating means when the
first reservoir is substantially empty of said liquid phase; e.
Activating said second heating means so that said liquid phase of
said diphasic fluid flows toward said first reservoir via said
evaporator, where it is evaporated at least in part, cooling said
object, and said first condenser, where the vapor formed in this
way returns to the diphasic state; and f. Deactivating said second
heating means when the second reservoir is substantially empty of
said liquid phase; the steps c. to f. being repeated
cyclically.
[0048] The invention further provides a method of cooling an object
including a precooling step as described above followed by a step
of thermal stabilization by means of a heat transfer passive
diphasic device.
[0049] Other features, details and advantages of the invention will
emerge from a reading of the description given with reference to
the appended drawings provided by way of example and in which:
[0050] FIGS. 1A, 1B and 1C are functional diagrams of three heat
transfer devices of three variants of one embodiment of the
invention;
[0051] FIGS. 2A and 2B are two views in section of a cryogenic
fluid reservoir suitable for use under microgravity conditions;
[0052] FIG. 3 is the functional diagram of a heat transfer
cryogenic system of a first embodiment of the invention employing
two devices of the type represented in FIG. 1A connected in
parallel with a fluid loop between a hot source and a cold
source;
[0053] FIG. 4 is the functional diagram of a heat transfer
cryogenic system of a second embodiment of the invention employing
a device of the type represented in FIG. 1A connected in parallel
between a hot source and a cold source;
[0054] FIG. 5 is the functional diagram of a heat transfer
cryogenic system of a third embodiment of the invention employing a
device of the type represented in FIG. 1A and a closed pulsating
heat pipe connected in parallel between a hot source and a cold
source;
[0055] FIG. 6 is the functional diagram of a heat transfer
cryogenic system of a fourth embodiment of the invention employing
a device of the type represented in FIG. 1A with which an open
pulsating heat pipe is integrated;
[0056] FIGS. 7A-7C are diagrams showing the structure and the
operation of a heat transfer cryogenic system of a fifth embodiment
of the invention employing a device of the type represented in FIG.
1A and an open pulsating heat pipe connected in parallel between a
hot source and a cold source; and
[0057] FIGS. 8A and 8B show experimental results illustrating the
operation of a device shown in FIG. 1A.
[0058] As FIG. 1A shows, a heat transfer device of the invention
essentially comprises two reservoirs R1 and R2, preferably having
the same capacity, interconnected by means of a fluidic pipe CF.
This assembly contains a fluid in the diphasic state, liquid and
vapor. It is constituted, in the direction R1 to R2, of a first
condenser C1 (dark grey), a first section of the fluidic pipe, an
evaporator EV, a second section of the fluidic pipe (light grey),
and a condenser C2.
[0059] The diphasic fluid LC has a critical temperature lower than
the operating temperature of the object O to be cooled, and is
partly in the liquid state at the temperature of the cold source.
As a function of the application concerned, it may for example be
water, ammonia or a cryogenic fluid such as liquid nitrogen,
oxygen, hydrogen, neon or helium. The device of the invention is
particularly suitable for cryogenic applications (temperatures of
the object less than or equal to 200K or even 120K), or more
generally applications in which the object to be cooled must be
brought to an operating temperature less than ambient temperature
(by convention 20.degree. C.). The quantity of diphasic fluid
contained in the device must be sufficient, in the liquid state, to
fill the fluidic pipe and at least part (typically 50% or 75%) of
the internal volume of one of the reservoirs. At the same time, the
device must not be entirely filled with liquid, because in this
case no circulation of the diphasic fluid could occur.
[0060] The first condenser C1, which is in the immediate vicinity
of the first reservoir R1, is in thermal contact with a cold source
SF, having means (for example a bath of cryogenic fluid or a
cryo-refrigerator) able to bring its temperature T.sub.F to a value
less than or equal to the saturation temperature of the diphasic
fluid. Thus this condenser C1 is filled with diphasic fluid in the
liquid state.
[0061] The evaporator EV, located in the central part of the
fluidic pipe CF, is in thermal contact with a "hot plane" PC, which
is an element that is a good conductor of heat that is thermally
connected to an object O to be cooled. The hot plane PC serves as a
"hot source"; its temperature T.sub.C is greater than or equal to
the saturation temperature of the diphasic fluid. Thus the
evaporator EV contains fluid in the liquid state, then diphasic,
then--possibly--entirely in the vapor state.
[0062] The first reservoir R1 is connected to the cold source via a
first thermal resistance RTH1. Similarly, the second reservoir R2
is connected to the cold source via a second thermal resistance
RTH2. The values of these resistances constitute parameters that
are important for the rating of the device of the invention, as
will be discussed hereinafter. Moreover, the two reservoirs are
equipped with respective heating means MC1, MC2, for example
electrical resistances. A control device DC (computer,
microprocessor card, etc.) emits signals sMC1, sMC2 for controlling
the two heating means MC1 and MC2.
[0063] A pressure reducing "hot" reservoir RRP is connected to the
fluidic pipe CF. This is a conventional feature of cryogenic
systems, intended to prevent an excessive rise in pressure when the
system is at ambient temperature. In other embodiments, the
reservoir RRP may be absent: in this case, the device is
pressurized at ambient temperature, in the supercritical domain;
cooling it then takes a long time, because the fluid must be cooled
first by conduction in the gas, before being condensed in the cold
parts of the system.
[0064] To describe the operation of the FIG. 1A device, there is
considered the initial situation in which the device, apart from
the evaporator EV, is "cold", at the temperature T.sub.F. The two
reservoirs R1 and R2 are partly filled with liquid with vapor
above. The two condensers C1 and C2 are totally full of liquid,
while the rest of the pipe, including the evaporator EV, is filled
with vapor. At the beginning, the heating means MC1, MC2 are off
and the system is "thermalized": the reservoirs R1, R2 and
condensers C1, C2 are at the temperature T.sub.F of the cold source
while the evaporator EV is at the temperature T.sub.C of the hot
plane.
[0065] At the time t=t.sub.0 the first heating means MC1 are
activated to inject heat into the first reservoir R1. This causes
evaporation of a small part of the liquid that is contained
therein, and thus an increase in pressure that causes the expulsion
of another great part of the liquid in the pipe CF to the second
reservoir. The liquid is cooled in the condenser C1. Because of the
effects of the increased pressure induced by heating, the liquid is
then directed toward the evaporator EV where it is heated, and then
evaporates partly or totally. In the latter case, the vapor is
superheated, i.e. its temperature is higher than the saturated
vapor temperature T.sub.SAT at the pressure that reigns in the
fluidic pipe, at the outlet from EV. This being so, the fluid
extracts heat from the hot plane PC and the object O. The vapor (or
the diphasic (liquid/vapor) fluid) that leaves the evaporator
continues to flow toward the second reservoir R2. Before reaching
it, however, it passes through the second condenser C2, where it
gives up heat to the cold source on condensing. At the outlet from
C2, the fluid is diphasic. The vapor phase component of this fluid,
entering the reservoir R2, is condensed thanks to the cold power
passing through the thermal resistance RTH2. The incoming liquid
thus fills the reservoir R2.
[0066] Over time, the first reservoir R1 is emptied of the liquid
component of the diphasic fluid. Given that the outlet bleed from
the reservoir R1 is situated in the lower part, when the liquid
level falls below this bleed, the reservoir is virtually empty.
Pure vapor leaves R1, which is depressurized; consequently, its
temperature falls. This temperature drop is detected and
constitutes the signal that triggers the Deactivating of MC1 and
the turning on of MC2. From this moment, the reservoir R2, which
has been partially filled with liquid, becomes the "source"
reservoir, while R1 becomes the "recovery" reservoir. The flow of
fluid in CF is reversed. R2 is emptied because of the effect of
MC2. The cycle terminates when R2 is virtually empty. A new cycle
may then begin to be repeated as necessary.
[0067] Alternatively, the signal triggering the Deactivating of MC1
and the turning on of MC2 could be the increase in the temperature
of the reservoir R1 that occurs after the latter is completely
emptied of liquid.
[0068] The principle criteria for rating a device of the type shown
in FIG. 1A are as follows:
[0069] A portion Q.sub.RS.sub.--.sub.F of the heating power
Q.sub.RS injected into the source reservoir (R1 or R2, as a
function of the operating phase) is lost via the thermal resistance
RTH1, RTH2. The difference Q.sub.RS-Q.sub.RS.sub.--.sub.F serves to
generates the flow rate {dot over (m)} of fluid in the fluidic
pipe, and to generate by evaporation the vapor replacing the liquid
that leaves the source reservoir. Maximizing the thermal
resistances RTH1 and RTH2 enables limitation of the power lost and
thus improvement of the energy efficiency of the device, but
increases the cooling time, i.e. the time necessary to reach the
initial conditions described above. The power {dot over
(Q)}.sub.R1.sub.--.sub.F, {dot over (Q)}.sub.R2.sub.--.sub.F lost
via the thermal resistors RTH1 and RTH2 has the value:
Q R 1 / 2 _ F = T SAT - T F R TH 1 / 2 . ##EQU00001##
[0070] The power exchanged in the evaporator EV has the value:
Q.sub.EV={dot over
(m)}[c.sub.PL(T.sub.SAT-T.sub.F)+h.sub.LV+c.sub.PV(T.sub.VC-T.sub.SAT)]
where {dot over (m)} is the flow rate in the fluidic circuit,
c.sub.PL and c.sub.PV, respectively, the specific heat of the
liquid phase and the vapor phase at constant pressure, h.sub.LV the
latent heat of evaporation, T.sub.VC the temperature of the fluid
in the pipe on the recovery reservoir side.
[0071] The flow rate {dot over (m)} in the fluidic circuit has the
value:
m = Q RS - Q R 1 / 2 _ F h LV ( .rho. l .rho. v - 1 )
##EQU00002##
where .rho..sub.1 and .rho..sub.v are, respectively, the densities
of the liquid phase and the vapor phase when saturated at the
temperature of the reservoirs, which are assumed equal. This flow
rate is essentially imposed by the exchanged power value Q.sub.EV
required by the application concerned.
[0072] The flow rate {dot over (m)} in CF being given, the fluid
leaving C.sub.1/2 enables cooling of the object. The fluid leaves
the evaporator EV at a temperature T.sub.VC less than but close to
T.sub.O. The fluid, if it is in the vapor state, is cooled on a
very small area and is condensed on virtually all of the cold area
that is at T.sub.F. Almost all of the power is exchanged via this
area. To a first approximation
Q.sub.EV.apprxeq.H.sub.CONDS.sub.C2/1(T.sub.SAT-T.sub.F) where
H.sub.CONL and S.sub.C2/1 are respectively the condensation heat
transfer coefficient and the exchange area S.sub.C2/1 of the
recovery condenser C.sub.2/1. This area is therefore fundamental to
the rating process. It fixes the saturation temperature, i.e. the
temperature of the reservoirs and thus their pressure.
[0073] FIGS. 1B and 1C relate to variants of the FIG. 1A device. In
the case of FIG. 1A, the two condensers are integrated into the
same part, disposed between the reservoirs and the cold source. In
the case of FIG. 1B, the condensers are independent of each other,
but again disposed between the reservoirs and the cold source. In
the case of FIG. 1C, the two condensers are independent of each
other and the reservoirs.
[0074] FIGS. 8A and 8B show experimental results illustrating the
operation of a device of the FIG. 1C type, using helium as diphasic
fluid and a cold source constituted by a cryogenic bath of helium
(T.sub.F=4.3K approx). FIG. 8A shows the evolution of the
temperature T.sub.C of the hot plane, which passes from 7 0K to
4.3K in less than an hour, for a thermal mass of 400 J. FIG. 8B
shows the fluctuations of the temperatures T.sub.R1, T.sub.R2 of
the reservoirs and the much smaller fluctuations of the temperature
T.sub.F of the cold source.
[0075] The FIG. 1A example relates to the case of a device
operating in a gravitational field. Under these conditions, the
heating means MC1 and MC2 are preferably located in the upper part
of each reservoir, while the bleeds connecting the pipe CF to the
reservoirs is located in the lower part of the latter. This
arrangement makes it possible to assure that the increase in
pressure in the reservoir causes an injection of liquid, and not of
vapor, into the pipe CF.
[0076] In the absence of gravity (space applications) there arises
the problem of locating the liquid/vapor interface, which is
necessary to assure that only liquid is injected into the fluidic
pipe CF. The solution shown in FIGS. 2A and 2B consists in using a
porous material MP that can be wetted by the cryogenic liquid to be
engorged thereby, completely (or almost completely) filling each
reservoir R. In the example of FIGS. 2A/2B, the heating means MC
are situated at the center of the reservoir, in contact with the
porous material. When these heating means are activated, a
temperature gradient is created in the porous material, with
temperatures higher than the saturated vapor temperature (i.e. the
boiling or liquefaction point of the fluid) at the center and lower
at the periphery. This imposes that the vapor be at the center,
close to the heating means, and the liquid in the peripheral part
of the reservoir. The increase in the pressure in the vapor, caused
by evaporation in a closed volume, constrains the liquid to escape
via peripheral grooves RP provided for this purpose. The use of a
conductive porous material (for example a metal) is preferable for
the flow of heat to go directly to the liquid/vapor interface,
instead of generating a high temperature gradient that would be of
no utility.
[0077] Of course, other geometries are possible; for example, the
heating means may be disposed at one end of the reservoir and the
bleed for the pipe CF at the opposite end.
[0078] The FIG. 1A device may be used on its own as a thermal link
enabling precooling of the object O from an arbitrary high
temperature toward the temperature of the cold source, and
maintaining it at a low temperature (thermal stabilization). Thanks
to the active character of the device, the thermal switch function
is implemented very simply: it suffices not to activate the
reservoir heating means.
[0079] The device may equally constituent a component of a more
complex heat transfer cryogenic system.
[0080] A first example of such a system is shown in FIG. 3. This
system is constituted by two devices as in FIG. 1A, identified by
the references Da, Db. The various components of these devices are
identified by the letters "a" and "b"; for example, "R1a" is the
first reservoir of the device "a", and so on. The control devices
Dca, DCb transmit signals sMC1a/sMC2a, sMC1b/sMC2b for controlling
the heating means MC1a/MC2a, MC1b/MC2b which are in "phase
quadrature", i.e. offset temporally by one quarter (or three
quarters, which amounts to the same thing) of the duration of a
complete cycle. The two devices are ideally identical and have
equal cycle times.
[0081] If the power Q.sub.EV given off by the objet O is constant
in time, heating must be managed so that the sum of the flow rates
{dot over (m)}.sub.a+{dot over (m)}.sub.b is also constant. The
temperature of the object will then be stable. On the other hand,
if the power Q.sub.EV is not stable in time, it is necessary to
vary the sum of the flow {dot over (m)}.sub.a+{dot over (m)}.sub.b
rates by means of appropriate regulation.
[0082] The two control devices Dca, DCb may be produced in the form
of a single device.
[0083] In systems conforming to other embodiments of the invention,
the FIG. 1A device is used for precooling of the object O and the
hot plane PC, the thermal stabilization function being assured by a
passive device of conventional type connected in parallel between
the cold source SF and said hot plane.
[0084] FIG. 4 shows one such system, in which the thermal
stabilization function is assured by a fluid loop LHP including an
evaporator EVc, in thermal contact with the hot plane PC and
containing the capillary width M, a compensation chamber CC
disposed upstream of said evaporator, a fluidic pipe CFc connected
to a pressure reduction hot chamber PRPc. In the FIG. 4 system, the
device of the invention is used to precool the hot plane to a
temperature enabling the presence of liquid in the compensation
chamber and in the evaporator of the fluid loop LHP. Once primed,
the latter takes over.
[0085] In the FIG. 5 embodiment, the function of thermal
stabilization after precooling is assured by a closed type
pulsating heat pipe PHPF.
[0086] In the embodiments of FIGS. 3 to 5, each device is equipped
with its own pressure reduction reservoir RRP, RRPa, RRPb, RRPc,
RRPd. As a function of the operating regime of the system, these
reservoirs may be at different temperatures. The use of a common
"hot" reservoir would necessitate the use of a complex system of
valves.
[0087] In the system shown in FIG. 6, an open pulsating heat pipe
PHPO is "integrated" into the fluid pipe CF of a device of the
invention. The evaporator EV is thus divided into a plurality of
hot regions of the pulsating heat pipe, alternating with cold
regions thereof. The system operates in the manner explained above
with reference to FIG. 1A during the precooling phase; the heating
means MC1, MC2 are then deactivated and the passive PHP takes over
for the stabilization phase. This concept is beneficial because it
is more compact than that of FIG. 5 (in particular, only one
pressure reduction reservoir is needed), and because the PHP may be
filled directly from the reservoirs R1 and R2. It is also subject
to certain constraints, however:
[0088] firstly, the fluidic pipe CF--or at least its central part
forming the pulsating heat pipe--must be of capillary type
(diameter less than a few times the capillary length of the liquid)
and very long, which increases the head losses. It follows that the
temperature T.sub.RS of the source reservoir must be higher than in
the case of a "simple" device such as that from FIG. 1A, with the
resulting increase in the leakage thermal flow;
[0089] secondly, the pulsating heat pipe must be of the open type,
less efficient than the closed PHP of FIG. 5;
[0090] thirdly, there is no assurance that the volume fraction of
the liquid phase will be close to the optimum value of 50% for
correct operation of the PHP;
[0091] fourthly, the large number of round trips of the fluidic
pipe of the PHPO between the cold source and the hot source imposes
the provision of a large reservoir volume.
[0092] These drawbacks may be avoided, at least in part, thanks to
the system of FIGS. 7A-7C, in which the device of the invention is
an integral part of a closed pulsating heat pipe. The other side of
the coin is the use of two three-port valves V3V1, V3V2, and thus
of mechanical elements having moving parts.
[0093] The system from FIGS. 7A-7C comprises a device D of the type
shown in FIG. 1A and a pulsating heat pipe PHPF' mounted in
parallel between the cold source and the hot plane. The two ends of
the pulsating heat pipe are connected to the first and second
condensers of the device via the three-port valves V3V1, V3V2; in
this way, the heat pipe is looped on itself via the fluidic pipe
CF.
[0094] Initially (FIG. 7A) the valves are in a first position
isolating the pulsating heat pipe, which is filled with vapor. The
device D operates in the manner described above to precool the hot
plane PC and the object O.
[0095] Once precooling has finished, the valves go to a second
position in which they connect the pulsating heat pipe to the two
reservoirs R1, R2 of the device D (FIG. 7B). Thus activation of the
heating means of the source reservoir (R1 in this case) causes
expulsion of liquid therefrom and filling of the pulsating heat
pipe.
[0096] Finally (FIG. 7C), the valves go to a third position in
which the fluidic pipe CF of the device D is connected to the
pulsating heat pipe to form a supplementary loop or undulation
thereof. The heating means are inactive and the system operates in
a passive manner, like a standard pulsating heat pipe.
[0097] In the case of FIGS. 7A-7C, the fluidic pipe CF is
capillary, as shown by the alternating liquid plugs and bubbles
visible in FIG. 7C; however, its length is much less than that of
the FIG. 6 pipe (which of itself forms a pulsating heat pipe), and
consequently the head losses are lower.
[0098] In another embodiment, not shown, the device of the
invention could be used for precooling and filling a fluidic loop
of CPL or LHP type.
[0099] Until now the situation has always been considered in which
there is only one cold source and one hot plane/object to be
cooled. This is not an essential limitation, it is of course
entirely possible to use a separate cold source for each device or
reservoir, for example, although this complicates the control of
the heating means.
[0100] It is also possible to envisage more complex systems,
including one or more devices of the invention cooperating with
each other (as in the case of FIG. 3) and/or with one or more heat
transfer devices of different types (as in the case of FIGS. 4 and
5). Devices more complex than that of FIG. 1 are also conceivable,
including more than two reservoirs and a plurality of fluidic
pipes, condensers and evaporators.
* * * * *