Control System For Heat Pipes

Abstract

In preferred form, a heat pipe joining a heat source and a heat sink through an evacuated control section. A reservoir is located in direct fluid communication with the evacuated control section and has a predetermined quantity of control fluid therein. A heat-pipe wick within the control section has its wetness varied in accordance with changes in the condition of the control fluid to vary the rate of heat transport between the heat source and heat sink by the heat pipe itself; "wetness" hereinafter being taken as the fraction of total interconnected void volume of the wick that is filled with liquid. In a test device heat transport in the "off" state was increased by a factor of approximately 6 in the "on" state.

Description

This invention relates to the control of heat-transfer devices and more particularly to the control of heat transport in a two-phase heat-transfer device or heat pipe including a capillary structure for flow of the liquid phase in a first direction and a section for flow of the vapor phase in an opposite direction.

Heat-pipe devices are heat-transfer arrangements that have an evacuated section for the flow of vapor from a heat source to a heat sink and a capillary structure for the return flow of condensed liquid in an opposite direction back to the heat source for evaporation.

The devices are characterized by having a high rate of heat transfer. To date the control of the heat transport rate in such devices is maintained by varying the temperature differential between the heat source and the heat sink within the structure.

An object of the present invention is to improve operation of heat pipes by controlling the heat-transport rate therein by the heat pipe itself without deliberately varying the temperature of a heat source and/or heat sink associated therewith.

A further object of the present invention is to control the rate of heat transfer in a heat pipe having a vapor phase and a liquid phase that returns through a capillary structure from a condenser section to an evaporator section and to do so by including means within the heat pipe itself operative to vary the amount of liquid within the capillary structure for controlling the rate of heat transfer between a heat source and a heat sink associated with the heat pipe.

Still another object of the present invention is to control or shut off the heat transport through a heat pipe without regulating a heat source to the heat pipe and to do so by means of a control reservoir in direct communication with an evacuated control section of the heat pipe and wherein the control reservoir includes a predetermined amount of control fluid which has its condition varied to vary the amount of liquid in a control wick within the evacuated control section.

In one working embodiment of the invention these and other objects of the present invention were attained in a two-phase heat pipe device including an open-ended large-diameter outer housing having end caps in the opposite end thereof. Means are included to evacuate the housing.

Each of the end caps supports a copper pipe concentrically within the first pipe. One of the supported pipes serves as a heat source for the system and the other of the pipes serves as a heat sink. A porous control wick or sleeve has one end thereof fit over the heat input pipe and the opposite end thereof fit over the heat sink pipe to define a capillary flow structure for the return of liquid from the heat sink to the heat source.

A tubular reservoir on the outer housing is located approximately at the midpoint of the ends of the outer housing and serves as a container for a predetermined amount of control fluid which when heated increases the vapor pressure within the outer housing. This causes more liquid in the sleeve wick and an increase in the rate of heat transport between the heat source and the heat sink.

When the reservoir is cooled the control fluid will condense therein and reduce the vapor pressure within the outer housing so as to cause less liquid in the capillary structure of the control wick thereby to reduce the rate of heat transport between the heat source and the heat sink.

The method of heat-pipe control is obtained without regulating the temperature differential between the heat source and the heat sink of the structure. The arrangement makes it possible to readily control the heat-transport rate by the heat pipe itself in that the reservoir and the control fluid therein are part of the heat-pipe device between the heat source and the heat sink therein.

In the working embodiment and to attain the objects of the present invention it is important to note that the thermal conductance of the control section between the heat source and the heat sink, in the absence of the heat-pipe fluid, is small as compared to the heat-transport rate that is present when the heat-pipe control fluid is present in the wick during two-phase heat-pipe transport operation.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown:

In the drawings:

FIG. 1 is a view in front elevation, partly in section, of a heat-pipe system including a controller operated in accordance with the present invention;

FIG. 2 is an enlarged vertical sectional view of the control section in the system of FIG. 1;

FIG. 3 is a diagrammatic view of a two-phase system showing heat-pipe transport principles;

FIG. 4 is a view of a heat-transport system including first and second heat pipes having the rate of heat transport therebetween regulated by the control of the present invention;

FIG. 5 is another embodiment of a controller suitable for association with either the system of FIG. 1 or that of FIG. 4;

FIG. 6 is an embodiment of still another controller for association with the system of the present invention;

FIG. 7 is a chart of the heat transport versus control reservoir temperature;

FIG. 8 is a vertical cross-sectional view taken along line 8--8 of FIG. 9 looking in the direction of the arrows; and

FIG. 9 is a fragmentary sectional view of another embodiment of the invention having first and second heat pipes.

SINGLE HEAT-PIPE ARRANGEMENT

Referring now to the drawings, in FIG. 1 apparatus is illustrated for controlling the rate of heat transport in a two-phase heat-pipe arrangement.

It includes a heat pipe 10 having a large diameter tubular outer containing wall 12 which surrounds a heat source 14 and a heat sink 16. A layer of insulation 15 comprised of three wraps of Refrasil woven insulation covers the wall between its ends.

Within the outer containing wall 12 is a wick 18 that connects the heat source 14 and heat sink 16. The connecting wick 18 and the outer containing wall 12 constitute a heat pipe with an independently controlled atmosphere.

More particularly, in the illustrated arrangement the heat pipe 10 is shown in association with a glass water cooler 20 which has an inlet tube 22, a convoluted flow section 24 that is surrounded by a water jacket 26 and an outlet 28 that is connected directly to the interior of the outer containing wall 12. The sleeve 26 includes an inlet fitting 30 and an outlet fitting 32 for the flow of cooling water through the sleeve 26 and in heat-transfer relationship with the convoluted flow section 24.

The set up includes a top cock valve 34 and a bottom cock valve 36.

When the valves 34, 36 are opened a predetermined quantity of heat-pipe fluid can be directed into the interior chamber 38 of the outer containing wall 12.

In the illustrated arrangement the outer containing wall 12 is made of glass and has opposite ends that are closed by stainless steel end caps 40, 42. The cap 40 includes a peripherally mounted O-ring seal 44 which seals against the outer containing wall 12 continuously around the cap 40. A like continuously formed O-ring seal element 46 is supported on the periphery of the end cap 42 to seal the opposite end of the outer containing wall 12.

The heat source 14 more particularly, includes a copper pipe 48 directed through a central opening 50 in the end cap 40 so as to locate a closed end section 52 on the pipe 48 concentrically with respect to the outer containing wall 12 within the interior 38 of the wall 12 axially inwardly of the cap 40 a distance of approximately one-third of the length of the outer containing wall 12.

The outer surface of the copper pipe 48 is sealed with respect to the cap 40 by an O-ring seal element 54 which is located within an annular groove 56 formed in a small diameter outer extension of the cap 40.

A 200-watt cartridge heater 58 is inserted into the closed end 52 of the pipe 48 at a point within the interior 38 of the outer containing wall 12. The heater 58 includes lead lines 60, 62 connected to a variable source of power such as a Variac unit 64 so as to establish a range of thermal inputs through the pipe 48 into the interior 38 of the heat pipe 10.

The heat sink 16 more particularly, includes an elongated copper pipe 66 which is directed through a central opening 68 in the end cap 42. The pipe 66 is thereby supportingly received by the cap 42 so as to locate a closed end section 70 of the pipe 66 axially inwardly of the end cap 42 to approximately one-third of the length of the containing wall 12, coaxially of the pipe 48 and concentrically of the interior 38 as defined by the wall 12.

An inlet pipe 72 of a water cooler 74 extends coaxially and inwardly of the pipe 66 and includes an outlet 76 located in close-spaced relationship with the end section 70 of the pipe 66 where the cooling water picks up heat prior to return thereof through the open end of the pipe 66 which is in communication with an interior 78 of a header 80. The heated return fluid is directed through an outlet fitting 82 on the header 80 to suitable drain means (not shown).

In the illustrated arrangement the pipe 66 is sealed with respect to the end cap 42 by an annular O-ring 84 which is received within an annular groove 86 in a small diameter end extension on the cap 42.

A glass tube 88 or control reservoir is formed integrally with the glass outer containing wall 12 at a point intermediate the ends thereof so as to extend downwardly from the container 12 as is best illustrated in FIG. 2. The tube 88 thereby defines a sump volume 90 into which condensed liquid can flow and be collected from within the interior space 38 of the containing wall 12.

In FIG. 2 the tube 88 is shown partially filled with a predetermined quantity of control fluid 92. A heater 94 is wound around the outside surface of the tube 88 for purposes to be discussed.

In accordance with certain principles of the present invention the porous wick 18 is in the form of an elongated tubular sleeve member 95 formed of three turns of 60-mesh screen with an outside turn of 1,400-by -200 SS screen. The tubular sleeve 95 has one end 96 thereof fit over the outer surface of the closed end section 52 of the pipe 48 that houses the cartridge heater 58 and is bonded thereto by suitable means such as brazing.

Likewise the opposite end 98 of the sleeve 95 fits over the closed end section 70 of the pipe 66 that makes up part of the heat sink 16 and is bonded thereto by a suitable means such as brazing.

The intermediate section of sleeve 95 serves to bridge the end sections 52, 70 and defines a control section path between the heat source 14 and the heat sink 16 that has a thermal conductance when the pipe is "off" which is small compared to the thermal conductance when the heat pipe is operative.

To obtain an operative heat pipe the control-section chamber 38 is evacuated of air by applying a vacuum to the inlet tube 22 and maintaining the cock valves 34, 36 open. Following air evacuation, a predetermined charge of liquid is directed through the inlet tube 22 to flow through the convoluted flow section 24 and the outlet tube 28 into the interior 38 of the outer containing wall 12 making sure there is still an excess of liquid still in inlet 22 above valve 34 when it is shut to ensure no entrance of air. Valve 36 is then closed and the only heat transfer medium within the chamber 38 is that of the predetermined quantity of heat-pipe liquid.

When the chamber 38 is so evacuated the heat transfer medium remaining therein is in a two-phase state of the type shown in FIG. 3 which is a representative illustration of the type of fluid dynamics found in heat pipes.

More particularly, as illustrated in FIG. 3, the heat pipe includes a capillary structure having a controlled porosity upon which the principle of operation of a closed heat-pipe transfer circuit is dependent.

The capillary structure finds its counterpart in the porous wick 18 in the embodiment of FIGS. 1 and 2.

The sealed evacuated pipe finds its counterpart in the outer containing wall 12 and the tube container end caps 40, 42. The evaporator section in the illustrated arrangement is at the heat source 14 and a condenser section is at the heat sink 16.

The heat-transfer medium sealed within the chamber 38 is found in both the liquid state and the vapor state; the porous capillary structure seen in FIG. 3 is substantially saturated with the liquid phase of the heat-transfer medium while the remainder of the chamber 38 is filled with the heat-transfer medium in the vapor state under evacuated conditions.

The Variac 64 is operated to turn on the cartridge heater 58 so as to produce a heat input at the heat source 14 and to the chamber 38. Concurrently, water is flowed through the water cooler 74 in the pipe 66 for removing heat from the control section chamber 38.

In the embodiment of FIGS. 1 and 2 the vapor at the evaporator end at the heat source 14, will be at a pressure P.sub.2 which is higher than a pressure P.sub.1 of the vapor at the right end of the device represented by the heat sink 16. This is because the vapor is produced in the heat-source area of the chamber 38 and is at a higher temperature. The result is that vapor flows from the heat source or evaporator end of the device to the right as viewed in FIGS. 1 and 2 to the condenser or heat sink 16 thereof.

The pressure balance equation for a heat pipe (gravity free condition) is:

.DELTA.P.sub.M =.DELTA.P.sub.L +.DELTA.P.sub.V

Where .DELTA.P.sub.M is the capillary driving force term, .DELTA.P.sub.L is the liquid pressure drop through the wick, and .DELTA.P.sub.V is the pressure drop through the vapor or P.sub.2 -P.sub.1, as previously expressed.

For the gravity-free condition the capillary pumping pressure is generated almost entirely at the vapor-liquid meniscus interface in the evaporator section, i.e.,

where .gamma. is the surface tension of the liquid, .theta. is the contact angle and r.sub.2 is the radius of curvature of the meniscus in the pores.

The maximum pumping pressure

where r.sub.c is the effective pore radius of the wick and represents the minimum value for r.sub.2, i.e., r.sub.c r.sub.2 .infin.and .DELTA.P.sub.M .DELTA.P.sub.c.

The strain on the meniscus from evaporation puts the liquid under tension to provide for the liquid pressure drop .DELTA.P to return the liquid from the condenser to the evaporator. Also this strain compresses the vapor in the evaporator to provide for the vapor pressure drop .DELTA.P.sub.v.

The total maximum force available for pumping (.DELTA.P.sub.C) can be regulated by varying the effective pore radius (r.sub.c) of the wick.

The pressure drop for liquid flow through the annular wick 95 is given by the following expression for laminar flow:

where

.mu. = liquid viscosity

v.sub.1 = volume flow rate

.epsilon. = void fraction (screen porosity)

l = length of heat pipe wick

A = cross-sectional area of annular wick

b = tortuosity factor

r.sub.c = effective pore radius of the wick

For a homogeneous wick, a small value of effective pore radius r.sub.c would increase the available pumping force .DELTA.P.sub.c but it would also increase the resistive pressure drop term .DELTA.P.sub.1. Thus, there is an optimum value of r.sub.c depending upon the heat load, heat-pipe size and fluid used, and these optimum equations are available in the heat-pipe literature.

When the liquid reaches the heat source 14 end of the annular screen 95 it is heated by the heater 58 causing the liquid to evaporate and flow through the chamber 38 as a vapor. The vapor pressure at this point is at a pressure P.sub.2 which as stated above is greater than that of the opposite end of the device thereby to maintain a continuous left-to-right flow of vapor in the device which is counterflow to the path of liquid flow through the porous wick 18.

The above-described dynamics of the two-phase heat-transfer system is sufficient for purposes of describing the present invention. In order to control the dynamics of the two-phase system, heretofore, temperatures at the evaporator and condenser ends of the closed evacuated pipe portion of the heat pipe have been varied to control the differential vapor pressures therein which in turn regulates the rate of vapor transfer as well as the rate of the counterflow liquid phase.

In accordance with certain principles of the present invention heat transport through a heat pipe is controlled without regulation of the heat-source temperature or that of the heat sink. In order to evaluate the results in the working embodiment of the invention illustrated in FIGS. 1 and 2 the following instrumentation is included.

It includes a thermocouple 100 on the water input of the cooler 74 and a thermocouple 102 on the output thereof. Further, a thermocouple 104 is located on the outer surface of the containing wall 12 adjacent the end cap 42 and beneath the layer of thermal insulation 15 surrounding the outer surface of the containing wall 12. Another thermocouple 106 is located adjacent the end cap 40 on the outer surface of wall 12 adjacent cap 40.

Diametrically opposite the thermocouple 104 on the wall 12 is located a thermocouple 108. Another thermocouple 110 is located at a point on the wall 12 diametrically opposite the thermocouple 106.

The temperature of the outer skin of the reservoir tube 88 is measured by a thermocouple 112.

An additional thermocouple 114 is located on the end of the pipe 48 located outside of the end cap 40. In the illustrated arrangement the thermocouple 114 is intended to detect the temperature of the heat source represented by the cartridge heater 58.

Since the thermocouple is located outside the heat pipe it is obvious that it does not measure the true heat-pipe evaporator temperature but is a value which is greater than that temperature by an amount proportional to the heat-input rate.

Also, the thermocouple 112 on the reservoir does not necessarily represent the true vapor temperature in the heat pipe because in the working embodiment, to prove feasibility, the heater 94 is a hot air fan directed against the reservoir and in some cases against the bottom of the outer containing wall 12 in the vicinity of the reservoir tube 88.

For purposes of evaluating the vapor temperature within the heat-pipe assembly 10 of FIGS. 1 and 2, it is believed that the temperature of any of the couples 104, 106, 108, 110 is best chosen to most accurately reflect the vapor temperature within the control section chamber 38.

The following table is a quantitative listing of heat-pipe control tests. Temperatures are shown for thermocouple locations 104, 108, 112, 114. Thermocouple temperature measurements at 100, 102 and the water flow rate establish a dynamic calorimeter for measuring the rate of heat rejection (Q.sub.2 or P .sub.out ) into the heat sink. Q or P.sub.in is established by ammeter 116 and voltmeter 117 measurements by instrumentation shown in FIG. 2. ##SPC1##

RUN I

This demonstrates that the transport rate of a dry wick 18 is relatively low in the order of 21 watts. The balance of the input heat which was 39 watts is dissipated in the form of thermal conduction through the stainless steel end caps or other connection losses. These losses are present since in the working embodiment there is no insulation used on the end caps or the exposed end of the heater housing. The input of 39 watts is selected to avoid a rapid rise in the temperature of the heat source and burnout of the cartridge heater 58.

In most of the runs illustrated in the summary table, a 10-ml. charge of H.sub.2 0 is included in the reservoir tube 88 and serves as the only heat transfer medium with the control section chamber 38 evacuated. In any case where the amount of control fluid changes appropriate comment will be made. In run I the water is condensed in the reservoir 90.

RUN II

This demonstrates ordinary steady state "on" conditions for heat-pipe operation. Under these circumstances the amount of fluid 92 in the reservoir tube 88 is slightly heated and the heat-source temperature is actually reduced from that where the wick 18 is dry. Under these "on" conditions the 10-ml. water charge is vaporized into chamber 38 and cooled at the end 98 of the wick by the water flow through the closed pipe section 70 within the chamber 38 to condense and return by capillary action through the porous structure represented by wire mesh sleeve 92 to the evaporator end 96. Under these circumstances heat-transport rate is increased by a factor of at least 6 as compared to the first run where the heat pipe was "off" and the 10 ml. of control fluid was collected within the reservoir 88.

RUN III

In this run the operation is not under steady state conditions since the heat-source temperature is increasing. However, it is run at a higher power input level than in run II. At a heat-source temperature of 183.degree. C. the run terminates. This run indicates what the limit of maximum heat-transport ability is for the wick 18. The limit is thought to be due to a radial heat transport limit across the bonded section between the sleeve 95 and the pipes 48, 66 at 96 and 98, respectively. The heat-transport limit of the device at 167 watts as shown in the summary table is not considered to be an axial limit, that is, not due to the maximum heat transport limit according to the previous equation .DELTA.P.sub.c =.DELTA.P.sub.v +.DELTA.P.sub.L, but rather one dependent on the radial conduction characteristics at these points.

RUN IV

This shows that the actual heat input into the system through the control section represented by the reservoir tube 88 is small. In this test, the heat input by the cartridge heater 58 is turned off and full heat by a hot air blast is applied to the reservoir tube 88. As noted, the full wattage output is in the order of 37 watts. This represents the upper bound and the heat supplied to the reservoir 88 during the "on" tests of runs II and III would be less inasmuch as the internal parts were hotter in those runs.

RUNS V THROUGH VIII

These runs show the proportioning control ability of the devices illustrated. Runs V and VI were made to show that the device is able to be turned "on" when the wick is already hot and dry as the case in run I. With the control heat initially off and the wick at a steady state temperature of 122.degree. C., heat is applied to the water reservoir 88 by a hot air blower. The heat-source temperature at thermocouple 114 rapidly decreases to a temperature of 39.degree. C. as heat is transported to the condenser through the now-wetted porous wick represented by the sleeve or wire mesh 95. During this run the input power is then increased to bring the temperatures of thermocouple 114 up to the same level of approximately 122.degree. C. at the wick following wetting of the sleeve 95. By varying the power input it is found that a new steady state condition for maximum input power is possible in the range of --62 watts as shown in run VI. Under these conditions the wattage output even though the wick 95 is damp, drops to 61 watts under steady state conditions. This is a drop from the on output of 130 watts as shown in run II. It is obtained mostly (since T.sub.114(II) >T.sub.114(VI)) by dropping the control temperature at reservoir 88 from 76.degree. C. during run II to 44.degree. C. at run VI. This shows the capability of proportional control in the system.

In any case it is clear that the change in the condition of the control fluid within the sump 90 can cause the sleeve 92 to be dry to effectively stop or turn off the heat pipe. Under conditions the screen 95 is damp or wet the axial heat transport is thereby turned on and modulated. In run VII an intermediate level of heat output is represented. It is attained by varying the heat input to 79 watts for a vapor temperature corresponding to about 58.degree. C. Under this test, sufficient heat could not be supplied through blowing hot air against the reservoir tip to bring the control section up to a temperature in the range of 44.degree. C. at the thermocouple 112. Accordingly, the hot air was, in addition to being blown against the tip, also directed against the underside of the outer container wall in the vicinity of the reservoir tube 88. Also, in the run an appreciable amount of heat was supplied by the hot air blower to the heat pipe cooler (.apprxeq.28 watts) where the measured heat output was a total of 87 watts. The air blower heat at the cooler (.apprxeq.28 watts) in run VII was determined by run VIII which was obtained by shutting off the input of electrical power with the hot air blower still continuously being run in the same location.

In order to fully understand the proportioning control aspect of these runs it should be kept in mind that the "on" data listed on table I are the maximum steady state values for the electrical power input. For a particular vapor pressure or "control temperature" at reservoir 88, the heat pipe would operate properly at any value of power P.sub.in <P.sub.in max.

FIG. 7 shows a chart of the control of vapor transport of heat by control of the fluid vapor pressure in a control section of the heat pipe.

The chart shows the net heat out of the embodiment of the heat pipe illustrated in FIGS. 1 and 2. The net heat transported by two-phase fluid flow or Q.sub.(net) is obtained by subtracting from the measured heat output an estimated value for Q (metal conduction) and the Q (from reservoir) or the heat that was placed into the system through the reservoir tube 88. These points are plotted versus the estimated vapor temperatures as obtained by thermocouples 104, 108.

MULTIHEAT PIPE ARRANGEMENTS

In another embodiment of the invention it is proposed that a plural system of heat pipes be arranged in association with a heat-pipe controller of the type previously discussed.

Thus, in FIG. 4 of the drawings an arrangement is illustrated that includes a first heat pipe 120 joined to a control section 122 which is in turn joined to a second heat pipe 124.

In this arrangement a fluid charging unit 126 is included that has an inlet 128 through a cock valve 130 to a convoluted flow section 132 surrounded by a cooling-water jacket 134. The convoluted section 132 communicates through an outlet 136 and a cock valve 138 to an inlet opening 140 through a glass tube 142 having the opposite ends open.

The water jacket 134 includes an inlet 144 and an outlet 146 therefrom.

In this arrangement the control section 122 is formed by the glass tube 142 in cooperation with end caps 148, 150 therein which seal the opposite open ends thereof. An annular O-ring 152 on the outer periphery of the cap 148 seals to the glass tube 142. A like annular O-ring 154 in the end cap 150 seals against the inside diameter at the opposite ends of the glass tube 142.

Together the glass tube 142 and sealed end caps 148, 150 define an evacuated chamber 156 into which liquid can be directed through the unit 126.

The heat pipe 120 more particularly includes an elongated pipe 158 which is closed at its opposite ends by end plates 160, 162. The pipe 158 is directed through a central opening 164 in the end cap 148 to be supportingly received and located with a condenser section 166 of the heat pipe 120 interiorly of the chamber 156 about one-third the length of tube 142.

An evaporator section 168 of the heat pipe 120 is located within the interior 170 of a heat source representatively shown as hot air plenum 172 which includes a hot air inlet 174 and an air return 176.

In this embodiment of the invention the heat pipe 120 includes a tubular porous sleeve 178 that extends throughout the length of the pipe 158 and is bonded in good thermal-conductive relationship with the inside surface of the pipe 158 throughout the lengths of the evaporator and condenser sections of heat pipe 120.

The sleeve 178 can be made of a wire mesh porous wick material to have a predetermined degree of capillary action of the type described with respect to the wick section in the first embodiment of the invention.

The heat pipe 124 includes an elongated tubular section 180 directed through a central opening 182 in the end cap 150 to be supported thereby and directed into coaxial alignment with the pipe 120 at a point generally concentrically of the tube 142 and axially inwardly thereof about one-third the length of tube 142.

The tubular section 180 is closed at its opposite end by end caps 184, 186.

The heat pipe 124 also includes an interiorly located tubular wick 188 which is formed of a porous material having a suitable capillary action like that of the tubular sleeve 178.

Both of the heat pipes 120, 124 are sealed with respect to their supporting end caps 148, 150 by O-ring elements 190, 192 respectively.

In this arrangement the heat pipe 124 includes an evaporator section 194 located within the control chamber 156 axially inwardly of the cap 150 which takes up heat input from the heat pipe 120.

The heat pipe 124 further includes a condenser section 196 that is located within the interior 198 of a water-cooled jacket 200 having a water inlet 202 and a return fitting 204.

Additionally, in this embodiment of the invention a control reservoir 206 is formed as a depending tubular member from the underside of the glass tubular 208 in 142 to define a sump or liquid collection volume 208 in which is located a predetermined charge of control fluid 210.

The tip or control reservoir 206 is selectively heated by suitable heating means representatively illustrated as being a resistance wire element 212.

As was the case in the first embodiment, the control section 122 includes a tubular wick element or wire sleeve 214 having one end 216 thereof fit over the outer surface of the pipe 158 at the end thereof located within the chamber 156.

More particularly, the end 216 can be bonded to the outer surface of the pipe 158 at this point to obtain good thermal conductive transfer therebetween.

The opposite ends 218 of the wick 214 fits over the end of the pipe 180 located within the control chamber 156 and is bonded thereto for like thermal-conduction properties.

The sleeve wick 214 thereby serves to bridge between the condenser section 166 of the heat pipe 120 and the evaporator section 194 of the heat pipe 124.

In this embodiment of the invention, each of the heat pipes 120, 124 include a two-phase fluid system of the type described with respect to FIG. 3 above. Hence, in the case of the heat pipe 120 heat input at the hot air plenum 172 will produce an evaporator section at 168 form whence the heat-transfer medium will vaporize to flow to the right through the interior of the pipe 158 into the vicinity of the cooler temperature and represented by the evaporator section 168.

The heat-transfer medium in the control section 122, when the control section 122 is turned "off" is condensed as a predetermined quantity of liquid 210 within the tip or control reservoir 206. The sleeve wick 214 thereby is maintained substantially dry and the heat transfer through the control section 122 from heat pipe 120 to heat pipe 124 is reduced to that of the thermal conductance of the sleeve 214 which is substantially reduced from the rate of heat transport found in heat-pipe assemblies that are "on."

However, when the heat pipe is "on" as when the heater 212 causes the condition of the control fluid or liquid 210 to be in the vapor state, the sleeve wick 214 is wetted so as to produce a normal heat-pipe action of the type described with reference to FIG. 3.

Under these conditions the evaporator section 216 of the controller 122 causes the heat-transfer media within the evacuated sealed chamber 156 to become vaporized and flow through the chamber 156 to the opposite end of the control section 122 where it is condensed on wick end 218. The liquid is returned by capillary action through the sleeve wick element 214 back to the evaporator section to complete the heat-pipe action cycle.

The section 194 of the heat pipe 124 is at a higher temperature than the section 196 thereof. It, thus, serves as an evaporator section in the heat pipe 124 and causes the two-phase heat transfer medium inside pipe 124 to evaporate at the section 194 and flow as a vapor to the cooler end surrounded by the water-jacket housing 200. The vapor condenses at 196 and flows as a liquid in the opposite direction through the sleeve wick 188 back to evaporator 194 to complete the heat-pipe cycle.

The wetted control section 122 has a substantially greater rate of heat transport than when the wick 214 is dry whereby the heat transfer from the heated end of the heat pipe 120 to the cooled end of the second heat pipe 124 is great as compared to when the control section 122 is "off."

In the scheme illustrated in FIG. 4 the rate of transport from the heat input end at the heat source 172 to the heat output or sink end represented by the water jacket 200 is that established by heat pipes of the type illustrated in series and wherein the control section 122 while itself being a heat pipe serves as a means independent of the temperature difference between source 172 and sink 200 for regulating the total transport rate from the heat source to the heat sink.

While the control reservoir and the heat-transfer medium therein used to control the rate of heat transport has been shown as a liquid heated and vaporized to cause a greater vapor pressure within the control chamber, in the embodiments of FIGS. 5 and 6 other kinds of control reservoirs are illustrated having a predetermined amount of control fluid therein and means to change the condition of the control fluid so as to vary the rate of heat transport of an associated heat pipe by changing the degree of dryness or wetness of a wick component in the pipe.

More particularly, in each case a control reservoir is illustrated that is intended for use at a point in a heat-pipe system like that occupied in the embodiments of FIGS. 1 and 2 and FIG. 4 by the tubular reservoir and a surrounding resistance heater.

In the embodiment of FIG. 5 a reservoir housing 213 is illustrated that is adapted to be connected to the outer tubular housing 215 of a control section of a heat-pipe system through a reduced diameter neck 217. Within a large diameter bore 219 of the housing 213 is located a reciprocating piston 221 having a peripheral seal 220 therein that sealingly engages the inside wall at the diameter 219.

A piston rod 222 is connected at one end to the piston 221 and has the opposite end thereof connected to an end plug 224 of a flexible bellows 226. The piston rod 222 is pivotally connected by a pin 228 through one end of a oscillating actuating lever 230. The fulcrum for the lever 230 is defined by a pin 232 fixedly secured to a support base 234.

In the illustrated arrangement the flexible bellows 226 connects to a large diameter open end of the housing 213 and is sealed by the plug 224 to define a sealed, movable link between the actuating lever 230 and the piston rod 222 for reciprocating the piston 218 within the large diameter section 216 so as to vary the effective volume of a control chamber 236 located between the upper face of the piston 221 and the interior 238 of the tubular outer housing 215 of the control section.

In this arrangement the volume of the control chamber is varied by mechanical control and the temperature within the reservoir 236 is maintained less than the temperature of the heat pipe which is controlled by the mechanism. Thus, when the volume of the chamber 236 is reduced by moving the piston 221 upwardly within the section 219 the vapor pressure within the chamber 238 is increased to produce greater wetting of the capillary wick within the heat pipe and will result in increased heat-transport rates. Temperature and heat capacity of housing 215 quickly vaporizes fluid entering reservoir 236.

When the piston 221 is moved downwardly within the section 219 the volume of the control chamber 236 is increased so as to expose more of the cold wall of housing 213 and condense more of the fluid and thereby reduce the amounts of fluid (and wetness) of the wick in the control section 238 which thereby reduce the heat-transport rate through the section.

In the embodiment of FIG. 6 a controller 250 is illustrated adapted to replace a tube reservoir of the type illustrated in the embodiment of FIGS. 1 and 2. In this arrangement the controller 250 includes a movable bellows section 252 having a variable volume interior 254 which is in direct fluid communication through a small diameter neck 256 with the interior 258 of a tubular outer housing a control section like the tubular outer housing 142 in the embodiment of FIG. 4.

In the illustrated arrangement an electrical resistance element 262 is wound around the convolutions of the bellows 252 and is connected across a power source by lead lines 264, 266. A potentiometer 267 is connected between line 266 and element 262. It includes a resistance 268 and a movable contact carrying arm 270. It is operated by a lever 272 pivotally connected to a depending extension 274 on the base of the bellows 252 by a pin 276.

The controller of FIG. 6 is intended to be an automatic heat-pipe pressure control. If the pressure within the heat pipe increases beyond a predetermined set point, the bellows 252 will expand to increase the volume of the chamber 254. Concurrently, the potentiometer contact carrying arm 270 is moved with respect to the variable resistance 268 to include greater resistance in the winding or electrical resistance element 262 between lead lines 264, 266. As a result, automatically, the heater current is decreased and the temperature in the chamber 254 will also reduce to produce a reduction in the vapor pressure within the system both to control the rate of heat transport and to serve as a pressure relief within the system.

Another system for controlling the operation of heat pipes is illustrated in FIGS. 8 and 9.

In this embodiment a heat-pipe controller is proposed to prevent any part of the main heat-pipe structure from approaching the heater or heat-source temperature. This is desirable wherein the heat-transfer media is one that can decompose at the temperature of the heat source.

In the illustrated arrangement an evaporator end of a primary heat pipe 300 is illustrated having a tubular pipe 302 closed at its end by means including a plug 304. The heat pipe 300 includes a sleeve wick 306 that is made of a suitable porous capillary flow material. In the illustrated arrangement the wick 306 is suitably bonded to the inner surface of the pipe 302 completely circumferentially therearound and extends throughout the length of the pipe 302.

Located in concentric relationship with the heated end of the primary heat pipe 300 is a control heat pipe 308 that includes an outer tubular housing 310 that is located in spaced-surrounding relationship with the heated end of the primary heat pipe 300 at a point radially outwardly therefrom.

In the illustrated arrangement the housing 310 has an open end closed by a plug 312 and a radially inwardly bent end 314 which is formed as a small diameter neck 316 at the end of the housing 310 fit into close, sealed engagement with the outer circumference of the outer surface of the pipe 302.

On the outer surface of the evaporator end of the primary heat pipe 302 is located a sleeve wick 318 of porous material which extends from the plugged end 304 of the pipe 302 to a point closely adjacent the end 314 of the housing 310 of the control heat pipe 308.

Additionally, a sleeve wick 320 of a larger diameter than sleeve wick 318 is located in surrounding circumferential relationship therewith at a point spaced radially outwardly of the sleeve 318. Furthermore, the sleeve 320 is bonded to the inner surface of the tubular housing 310 between the plug 312 and a point immediately adjacent the end 314 of pipe 308.

Between the sleeve wicks 318, 320 is located an annular space 322 which extends completely throughout the length of the tubular housing 310. The space 322 constitutes a vapor passageway which is in communication with the inlet neck 324 to a control reservoir 326 that defines a sump or liquid collection region 328.

In the illustrated arrangement an electrical resistance element 330 is wound on the outer surface of the reservoir 326 so as to condition a predetermined quantity of control fluid 332 in the reservoir 326 to vary the amount of liquid in the wicks 318, 320 for the purposes to be discussed.

In the illustrated arrangement, a heat source 333 is located around the primary heat pipe 300 and the radially outwardly located control heat pipe 308. It includes a housing 334 located in radially outwardly spaced relationship with the outer surface of the tubular housing 310. The housing 334 includes an end wall 336 and an axially directed continuously formed flange 338 that is located in sealed relationship with the outer surface of the tubular housing 310 at the point which it is bent radially inwardly at 314.

A suitable layer of thermal insulation 440 surrounds the heat source 333.

In this case only the outside components and the vapor of the fluid of the radial control heat pipe 308 ever see the heat-source temperature.

In this design the wick of the radial control heat pipe includes the radially outermost sleeve wick 320, the sleeve wick 318 and a plurality of sandwiched radial sections 335 of wick material which connects the sections 318, 320 as best illustrated in FIG. 9. Each of the sections 335 is located through the axial extent of space 322 and circumferentially spaced from one another as seen in FIG. 8.

Here the vapor flows radially inwardly from the wick section 318 to wick 320 and the condensate of the heat pipe flows radially outward via the wick sections 335. As discussed in the previous embodiments of the invention when the temperature of the control reservoir 326 is increased by the resistance heater 330 the vapor pressure will be increased within the elongated vapor spaces between the axially directed short-length wick strips 335 so as to closely maintain the rate of heat transport from the heat source 333 to the evaporator section of the primary heat pipe 300 at a level where the temperature under steady state conditions in the primary heat pipe 300 will always be maintained less than that of the heat source.

While embodiments of the present invention, as herein disclosed constitute preferred forms, it is to be understood that other forms might be adopted.

Claims

What is claimed is:

1. A heat-pipe control system comprising an outer container having spaced end walls defining a sealed evacuated chamber, a first tubular member having a closed end, said first tubular member being supported on one of said end walls to locate said closed end within said chamber in axial spaced relationship to said end wall, means including said tubular member defining an evaporator section within said chamber, a second tubular member having a closed end, said second tubular member being supported on the other of said end walls to locate its closed end in axial spaced relationship to said other end wall and coaxially of said first tubular member, means including said second tubular member defining a condenser section within said chamber, means including a depending tube on said outer container between the closed ends of said first and second tubular member defining a reservoir having a predetermined amount of control fluid therein, an inlet to said tube directly communicating said reservoir with said sealed chamber, means for selectively heating and cooling said reservoir for causing said control fluid to be conditioned to vary the vapor pressure within said sealed chamber, capillary flow means thermally connected between said condenser and evaporator sections including an unsupported bridge portion overlying said tube inlet wetted when said vapor pressure increases within said chamber to increase the rate of heat transport from said evaporator section to said condenser section, said bridge portion being dried in response to cooling of said reservoir and condensation of fluid therein to reduce the rate of heat transport between said evaporator section and said condenser section.

2. A heat pipe assembly comprising a sealed evacuated outer housing, means defining an evaporator at one end of said housing, means defining a condenser at the opposite end of said housing, a porous wick located within said housing in spaced relationship thereto, said porous wick having a first segment thereon in heat-transfer relationship with said evaporator section, said porous wick including a second section thereon in heat-transfer relationship with said condenser section, means defining a reservoir having a predetermined amount of control fluid therein, said reservoir having an inlet in direct communication with the interior of said housing, said wick including an unsupported bridge portion between said evaporator and said condenser overlying said tube inlet, control means for varying the condition of said control fluid within said reservoir to vary the vapor pressure within said housing for varying the amount of liquid within said unsupported wick portion thereby to vary the rate of heat transport from said evaporator section to said condenser section, said reservoir being a variable volume bellows, said control means including an electrically energizable resistance heater on said bellows for heating said bellows to vary the vapor pressure in said housing, a potentiometer for controlling current in said resistance heater, means connecting said bellows to said potentiometer for regulating said potentiometer to reduce energization of said heater on the occurrence of a predetermined maximum pressure condition in said bellows.

3. A heat-pipe system including housing means defining a sealed evacuated chamber, first heat-pipe means located within said sealed chamber, second heat-pipe means located within said chamber, a reservoir having a predetermined amount of control fluid therein, means including an inlet in said housing means communicating said reservoir with said sealed chamber, means for varying the amount of control fluid within said reservoir so as to change the vapor pressure within said sealed chamber, capillary flow means located within said sealed chamber having a first portion thereon in heat-transfer relationship with said first heat-pipe means and a second portion thereon in heat-transfer relationship with said second heat-pipe means, said capillary flow means including an unsupported bridge portion between said first and second heat-pipe means overlying said inlet portion increasing in wetness in response to an increase in vapor pressure within said sealed chamber in response to changes in the amount of fluid within said control reservoir for increasing the rate of heat transport between said first and second heat-pipe means, said capillary flow means bridge portion being dried in response to condensation of said control fluid within said reservoir for reducing the amount of heat transport from said first heat pipe means to said second heat pipe means.

4. A heat pipe combination comprising: a first heat pipe having a first tubular housing with the opposite end thereof closed, a porous capillary liner on the interior surface of said housing, an evaporator section on said first heat pipe and a condenser section thereon, a second heat pipe having a second tubular housing with the opposite ends thereof closed, a porous capillary liner on the interior surface of said second housing, an evaporator section on said second heat pipe and a condenser section thereon, a third tubular housing having a diameter greater than that of said first and second tubular housings, said condenser section of said first heat pipe being located within said third housing at one end thereof, said evaporator section of said second heat pipe being located within said third housing at the opposite end thereof, means including said third housing defining a sealed control chamber, a control wick of porous material located within said chamber having a portion thereon in heat-transfer relationship with said condenser section of said first heat pipe and a second portion thereon in heat-transfer relationship with said condenser section of said second heat pipe, means forming a control reservoir having a predetermined amount of fluid therein, means including an inlet in said first tubular housing for communicating said control reservoir with said sealed control chamber, said control wick including an unsupported bridge portion between said first and second portions thereof, said unsupported bridge portion overlying said inlet to be wetted by fluid within said reservoir, means for varying the condition of said fluid within said reservoir for changing the vapor pressure within said control chamber so as to vary the wetness of said unsupported bridge portion of said control wick thereby to vary the rate of heat transport between said first and second heat pipes.

5. A heat pipe device comprising: a large diameter tubular housing having opposite end walls thereon defining an evacuated chamber, a first tubular member directed into said chamber at one end thereof, said first member having a closed end located within said large diameter housing in concentric relationship therewith, a second tubular member directed into said chamber at the opposite end thereof, said second member having a closed end thereon located within said large-diameter pipe in concentric relationship therewith, said first and second tubular members each having a closed end located coaxially and in spaced relationship to each other, a heater located within said first member for directing heat interiorly of said large-diameter housing, heat-sink means located within said second member for removing heat from within said large-diameter housing, a depending tube on said large-diameter housing intermediate the opposite ends thereof forming a control reservoir, an inlet to said tube in direct communication with the interior of said housing, between said spaced, closed ends of said tubular members, a predetermined amount of control fluid within said depending tube, means for varying the condition of said fluid within said tube for controlling the vapor pressure within the interior of said large diameter housing, a sleeve of capillary flow material located within said chamber in spaced relationship to said outer housing have one end thereof fit over the closed end of said first member and having the opposite end thereof fit over the closed end of said second member for returning condensed liquid from the outer surface of the closed end of said second member back to said closed end of said first member for evaporation by said heater, said sleeve of capillary flow material including an unsupported bridge portion overlying said tube inlet having the wetness therein varied in accordance with changes in the condition of fluid within said tube for varying the rate of heat transport from said first member to said second member.