US3666005A

US3666005A - Segmented heat pipe

Info

Publication number: US3666005A
Application number: US52249A
Authority: US
Inventors: Robert David Moore Jr
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-07-06
Filing date: 1970-07-06
Publication date: 1972-05-30
Anticipated expiration: 1989-05-30

Abstract

An improved heat pipe capable of conveying a greater heat flux than a conventional heat pipe is provided in practice of this invention. A heat pipe transfers heat from a heat source to a heat sink in the form of latent heat of vaporization of a fluid within the heat pipe. Hot vapor transfers heat from the heat source to the heat sink. Condensed liquid is returned from heat sink to the heat source through porous capillary material due to surface tension forces. The heat flux obtainable is limited by the available flow of returning liquid. In the improved heat pipe, the flow paths for liquid and vapor are serially segmented by impermeable barriers transverse to the direction of heat flow so that the distance of liquid flow in each segment is minimized. In zero gravity the heat flux obtainable is approximately proportional to the number of serial segments N into which the heat pipe is divided, that is, if the heat is transferred serially through N segments approximately N times the heat flux is possible as compared with a conventional heat pipe of the same overall dimensions. When operating against a gravity head, the maximum heat flux is about N2 times the heat flux of a conventional heat pipe. Thus a heat pipe segmented into 10 serial segments has approximately 10 to 100 times the maximum heat flux capacity of an unsegmented heat pipe of the same cross section and total length.

Description

[451 May 30,1972

llnited States Patent Moore, Jr.

flux than a conventional heat pipe is provided in practice of this invention. A heat pipe transfers heat from a heat source to a heat sink in the form of latent heat of vaporization of a fluid within the heat pipe. Hot vapor transfers heat from the heat SEGWNTED FEAT PIPE Robert David Moore, Jr., 817 West [22] Filed: July 6, 1970 source to the heat sink. Condensed liquid is returned from heat sink to the heat source through porous ca due to surface tension forces. The heat flu pillary material x obtainable is [21] Appl.No.: 52,249

limited by the available flow of returning liquid. In the improved heat pipe, the flow paths for liquid and va por are serially segrnented by impermeable barriers transverse to the direction of heat flow so that the distance of liquid flow in 3 3 3 m3 5 6 1 M m 5 0E6 1 u 5 6 1 [52] US. [51] Int. [58] Field each segment is minimized. in zero gravity the heat flux ob- References Cited tainable is approximately proportional to the number of serial segments N into which the heat pipe is divided, that is, if the UNITED STATES PATENTS heat is transferred serially through N segments approximately 9/1969 N times the heat flux is possible as com 8/1970 165/105 X pared with a conven- 165/105 X tional heat pipe of the same overall dimensions. When operating against a gravity head, the maximum heat flux is about N 3,465,813 Bromberg et a]. 3,525,670 Brown...................

Primary Examine,. A1ben w Davis, JR times the heat flux of a conventional heat pipe. Thus a heat Atwmey ChriStie Parker & Hale pipe segmented into 10 serial segments has approximately 10 to 100 times the maximum heat flux capacity of an unseg- ABSTRACT mented heat pipe of the same cross section and total length.

An improved heat pipe capable of conveying a greater heat 2Clairm, 3Drawing Figures SEGMENTED HEAT PIPE BACKGROUND In recent years so-called heat pipes have become of considerable interest for transfer of heat because of a variety of highly useful characteristics such as substantially isothermal operation, low weight per unit heat transferred, high heat flux density, and, probably most significantly, they are self owered and have no moving parts. In addition, heat pipes can be made in a broad variety of shapes and sizes for special heat transfer situations.

In a conventional heat pipe, a closed chamber is charged with a material that coexists in vapor and liquid phases at the operating temperature of the heat pipe. Other gases are excluded from the heat pipe. Although a variety of heat transfer fluids have been employed, water and liquid metals are among the most successful in the appropriate temperature ranges. In

the conventional heat pipe, a permeable, liquidwettable,

capillary wick material is provided between the heat sink where liquid condenses and the heat source region where the liquid is vaporized. Since vapor at the heat source is at a higher temperature than vapor at the heat sink, a pressure differential in the vapor is present in the heat pipe and a substantial mass flow of vapor occurs to carry heat from the heat source to the heat sink, principally as the latent heat of vaporization of the material within the heat pipe. The condensed liquid is returned from the heat sink to the heat source by capillary flow through the wick. The nature of conventional heat pipes, heat transfer fluids and construction details are included in an article entitled The Heat Pipe" by G. Yale Eastman in Scientific American, May 1968, at page 38.

The pore surfaces in the capillary material support the liquid-vapor interface. The surface tension in the interface supports a pressure differential across it between the liquid and vapor phases. This pressure differential, with the vapor leaving the liquid-vapor interface at a higher pressure than the liquid approaching the interface in the capillary, provides the motive force for liquid and vapor flow, overcoming friction losses and any gravity head that may be present. As the liquid flows through the capillary material there is a resistance to liquid flow dependent on the properties of the liquid and'the channel size through which the liquid must flow. The resistance to liquid flow is proportional to the distance that the liquid must travel between the heat sink and the heat source and inversely dependent on the square of the pore sizeof the capillary matrix material. Since the pore size must be small enough to support the gravity head and friction losses the resistance to liquid flow becomes greater with both the length and the gravity head, which is also proportional to length. Thus the flow rate of liquid to the heat source region may be reduced to the point that insufficient liquid flow is available to accept all of the heat put into the heat source region. If this occurs, the capillary material dries out, causing cessation of operation of the heat pipe and possibly damage due to an excessive temperature rise. It is, therefore, desirable to provide a heat pipe having minimized resistance to liquid flow so that a high heat flux can be transferred.

Reference is hereby made to copending US. Pat. application Ser. No. 52,609, entitled HEAT TRANSFER SURFACE STRUCTURE, by Robert David Moore, Jr. The subject matter of this copending patent application is hereby incorporated by reference for full force and effect as if set forth in full herein.

BRIEF SUMMARY OF THE INVENTION Thus in practice of this invention according to a preferred embodiment there is provided an improved heat pipe having a plurality of serial closed chambers between a heat source and a heat sink, each of the chambers having vapor flow means for transferring heat from 'a heat source to a heat sink and capillary flow means for returning liquid from a heat sink to a heat source.

DRAWINGS DESCRIPTION In operation of heat pipes, a significant element limiting the heat flux obtainable and the distance over which heat can be transferred is the flow resistance of the capillary material for conveying liquid into the heat source region of the heat pipe.

In a conventional heat pipe, a capillary material or wick extends all the way from the heat sink to the heat source. In the embodiment hereinafter described, the heat pipe is segmented transverse to the heat flow so that liquid is conveyed in serial segments of much shorter length. The shorter segments have less resistance to liquid flow and, under most conditions a lower gravity head and thus a higher heat flux capability. Thus, for example, as shown hereinafter, if the heat pipe is divided into 10 serial segments, approximately ten to one hun dred times as much heat can be transferred as in a conventional heat pipe without segmenting.

In order to make liquid flow from one point in the system to another point, a pressure differential must exist across the liquid to cause it to flow. In a heat pipe the driving pressure difierential exists across the liquid-vapor interface in the pores of the capillary material. The interface is supported across its area by the surface tension between the liquid and vapor phases and around its periphery by the wetted pore walls.

In order to obtain a large prasure gradient relatively small pore sizes are required in the capillary. The pressure differential is directly proportional to the surface tension of the liquid and inversely proportional to the diameter of the capillary (assuming it is cylindrical, which is a reasonable simplifying assumption, and factors can be readily applied for other cross-sectional shapes). Therefore, a small pore size gives a large pressure gradient for a given liquid and a large pore size gives a small pressure gradient. In order to overcome a high vapor pressure gradient or gravity head a small pore size is needed.

The flow of liquid through a cylindrical passage or conduit, such as a capillary pore, is given by r PF)/8"n 1) where R is the radius of the conduit, AP; the pressure difference along the effective length L of the conduit, and 1; is the viscosity of the liquid. Since the radius of the capillary enters the formula as the fourth power, it has a very pronounced effect upon flow, and ideally the capillary pore size should be large in order to obtain a high liquid flow rate. The pore size cannot, however, be made too large or the pressure differential obtained will be insufi'rcient for overcoming the gravity head gradient and pumping the liquid and vapor against friction losses. In an operable heat pipe, the pore size must be selected to provide a balance between a high pressure differential to induce flow and a large pore size to minimize resistance to flow. As seen hereinafter, the gravity head greatly influences the pore size and the resulting maximum heat capacity of the heat pipes.

With a selected pore size in a capillary material, the liquid flow obtainable in the heat pipe is governed by the length L of flow path. Previously it has been the practice to make the flow path the full distance from the heat sink to the heat source,

which may be a substantial distance and, therefore, cause a high resistance to flow. The maximum heat flux that can be obtained is directly determined by the maximum quantity of liquid flowing through the capillary for delivery to the heat source for vaporization.

In practice of this invention, the length of the liquid flow path is decreased by segmenting the heat pipe so that liquid in each segment need flow over only a reduced length, and the maximum heat flux obtainable is thereby increased by a factor approximately proportional to the number of segments into which the heat pipe is divided under zero gravity, and by a factor about proportional to the square of the number of segments when operating with a high gravity head and an optimized pore size. It might be noted that this invention is principally concerned with transfer of heat along the length of the heat pipe and not particularly with the problems of bringing heat into the warmer end or removing it from the cooler end. Thus, hereafter it is assumed that heat merely enters the flat end of a heat pipe whereas in practice it usually is found that heat enters the end and also a portion of the length adjacent the end. How the heat enters or leaves the heat pipe is not of great significance in practice of this invention. Thus the ends may be enlarged or of other specialized geometry to obtain a desired heat transfer.

FIG. 1 illustrates in longitudinal cross section a heat pipe constructed in serial segments according to principles of this invention. As illustrated in this presently preferred embodiment, a first cylindrical cup is arranged adjacent a heat source. The cylindrical side walls of the cup 10 are lined with a porous capillary material 11 which is substantially the same as capillary material employed in conventional heat pipes. The bottom end of the cup (top of the heat pipe) is also lined with a capillary material 12 which is in contact with or a continuation of.the side wall capillary material 1 l. The open end of the cup 10 is enlarged to provide an internal shoulder 13 into.

which the closed end of a second cup 14 is inserted and the two

cups

10 and 14 are welded or otherwise sealed together. A porous capillary material 16 is on the outside of the bottom of the cup 14, so that the porous capillary material 16 is in contact with or a continuation of the capillary material 11 on the sidewalls of the first cup 10. Thus, the closed cavity formed by the first cup 10 and the bottom of the second cup 14 is substantially completely lined with porous

capillary material

11, 12 and 16.

The second cup 14 is substantially the same as the first cup 10 and has a porous capillary material 17 along the side walls and a porous capillary material 18 on the inside of the bottom of the cup. The bottom of the second cup forms an impervious barrier between the capillary material 16 in the cavity formed by the first cup and the porous material 18 in the bot tom of the second cup 14. A third cup 19 is inserted and sealed in the open end of the second cup 14 so that the bottom 20 of the third cup 19 forms an impervious barrier between a layer of capillary material 21 within the cavity formed by the second cup 14 and a porous capillary material 22 in the bottom of the third cup 19. The walls of the third cup 19 are lined with a porous capillary material 23 in the same manner as the first and second cups.

Such a series of cups continues to an m" cup 190 just like the third cup 19. The open end of the m'" cup is closed by the impervious end 24 of a closed can or n" cup 25 inserted in the end of the m'" cup. THe end 24 of the n' can serves as an impermeable barrier between a porous capillary material 26 within the cavity formed by the m" cup 19 and a porous capillary material 27 within the end 24 of the can. The side walls of the n'" can 25 are lined with a porous capillary material 28 and a layer of porous capillary material 29 on the opposite end 30 of the can is a continuation of the capillary material 28 on the side walls. The end 30 of the can is in thermal contact with a heat sink for passing heat out of the heat pipe.

Each of the cups, 10, 14, 19, and 19a, and the can 25 are charged with a single heat transfer material that coexists in liquid and vapor phases at the operating temperature of the heat pipe. During operation the capillary wick material is saturated with liquid and the space within the central region is substantially filled with vapor. The same material (e.g. water) can be employed in each of the closed chambers and when the heat pipe is in operation, the internal pressure in the warmer portion will be higher than in the cooler portion. If desired different liquids can be used in the different segments.

Within each of the closed chambers of the heat pipe there is provided a reservoir 32 in the form of a ring of porous material in contact with the capillary material fonning the liquid flow path within the respective closed chamber. Thus, for example, in the first can the ring shaped reservoir 32 is in contact with the porous capillary material 11 on the walls of the chamber. The reservoir is formed of a porous material having a pore size larger that the pore size of the other capillary material within the closed chamber.

The porous ring 32 serves as a reservoir of excess liquid in the heat pipe to avoid having bulk liquid greater than suffcient to saturate the capillary materials standing in the end of the heat pipe. Bulk liquid in the vapor way of the closed chamber can interfere with vapor flow. This is particularly likely to occur at the heat sink end of the chamber and is particularly serious for a vented capillary vaporizer as hereinafter described. The porous reservoir 32, in effect, acts as a sponge" to soak up any excess liquid in the heat pipe. The pore size in the reservoir is made larger than the pore size in the other capillary material, so that liquid is preferentially in the smaller pore size material where liquid flow is occurring. The liquid will preferentially saturate the finer pore material because of the greater curvature of an interface therein than in the larger pore material or, equivalently, it is energetically more stable due to the greater effective surface to volume ratio 8 of the finer pored material. Thus the larger pored reservoir acts as a liquid reservoir of variable volume, absorbing excess liquid but releasing liquid when it is required to fill the finer pores of the wick material, thus compensating for the varying amount of fluid in the vapor state and for small variations in wick capacity and initial liquid charge. The reservoir may also serve as an additional liquid flow path or wick over a considerable length of the heat pipe by lengthening it to extend to or nearly to the ends, thus further increasing the liquid flow rate.

ln operation the segmented heat pipe accepts heat from the heat source which thereby vaporizes liquid from the porous capillary material 12 in the end of the first cup 10. The resultant vapor travels to the capillary material 16 at the 0pposite end of the cup, which during operation is at a lower temperature than the capillary material 12 adjacent the heat source. The vapor condenses on the cooler capillary material and the resultant liquid is conveyed from the capillary material 16 adjacent the bottom 15 of the second cup back to the vaporizer capillary 12 by way of surface tension forces.

When the vapor condenses at the cooler end of the first can 10, the latent heat of vaporization is released and the heat flows through the impermeable barrier 15 into the porous capillary material 18 in the bottom of the second can 14. This, in turn, causes vaporization of heat transfer liquid within the second cup 14 in the same manner as in the first cup 10. The heat is thereby transferred by vaporization and condensation through the second cup to the third cup and through the m"' cup to the end can 25. Heat is transferred through the end can 25 in the same manner by vaporization of heat transfer fluid from the porous capillary material 27 and condensation in the porous capillary material 29 at the heat sink end of the can. In this manner, heat is transferred from the heat source end to the heat sink end of a heat pipe through the n serial segments thereof.

The maximum heat flow through a heat pipe is limited by the rate which the condensed fluid, i.e. the liquid, will flow from the cooler end through the capillary material to the warmer end under the influence of surface tension forces. A somewhat simplified model of the capillary material through which the liquid flows is assumed herein for the purpose of demonstrating the improvement obtained by segmenting the heat pipe. Thus, a cylinder of porous material of length L and having a parallel array of uniform cross section pores of radius R and length L equal to the length of the cylinder is assumed for liquid flow. The total cross-sectional area of the pores (not of the porous cylinder) is A. For simplicity, it is assumed that heat is applied directly to one end for a cylinder and that vapor can escape directly from that face without restriction. Likewise, the other end of the cylinder is directly cooled and is easily reached by the vapor for condensation. This is an approximation not quite accurate in a practical system but the diminution in total heat flux is not significant enough to affect the significant improvement in maximum heat flux available by segmenting. it is also assumed that the liquid of viscosity 1; and surface tension 0- wets the pore walls completely with a zero contact angle. If the contact angle 6 is not zero, then the calculations are corrected merely by replacing 8 in all formulas with a cosine 0.

Equation (1) hereinabove set forth the flow through one capillary pore, and the total liquid flow is obtained merely by substituting the total pore area A for rrR' on the right side of that same equation. The maximum pressure differential AP obtainable from surface tension forces (also called the hub ble pressure is AP -/R (2) In the more general case where the pore geometry is irregular 9 thatt sre isnqsr i stmrad ss s artfifiseti capillary surface to volume ratio 8 is defined as 6= AP (0)/o')/ where AP,;(o-) is the measured bubble pressure with a liquid having a surface tension 0-. This measured bubble pressure AP is proportional to the surface tension 0'. By dividing by a, the resulting effective surface to volume ratio 8 is dependent only on the capillary size and shape and not on the surface tension of the liquid. Thus AP, 0-8 2 In order to get the ressure differential AP; available to drive the liquid through the pores, the gravitational head AP, and pressure drop in the returning vapor stream AP must be subtracted from the bubble pressure.

APF=APB APU APR The gravitational head AP, gz(p p,,) where g is the acceleration of gravity, z is the height of the hot end of the heat pipe above the cool end, and p and p, are the liquid and vapor densities, respectively. The pressure drop clue to vapor flow in the system AP CLF where C is a constant dependent on the vapor density and viscosity and the cross section of the vapor return passage. Thus, one can solve for the flow as follows liquid flow F and thus the maximum possible heat transfer rate, one can take the derivitive of F with respect to R, set the derivitive equal to zero, and solve for R to get Equation (7) for R may be substituted into Equation (6) for the maximum flow F to yield a rather complicated expression for the maximum possible fluid flow rate; however, such a substitution is not necessary for demonstrating the significant advantages obtained by segmenting a heat pipe. From Equation (7) it is seen that R,,,,, is independent of the length L of the porous body through which the liquid flows. Since R is independent of length L, the entire portion of Equation (6) within the brackets is also independent of the length L so that the maximum possible flow rate F "m I is proportional to l/L.

Even more informative, because of their simplicity, are the solutions for pore size R and flow rate F in the two limiting cases, that is, in zero gravity and in a high gravity environment.

in the zero gravity situation, such as in space applications, and when a linear heat pipe is operated in a horizontal posi- 1 tion, the term gz(p, p, is zero so that In high gravity situations, such as earthbound heat pipes operated with the hot end at a substantial elevation above the cooler end, the term gz(p, p,,) is large and the pore radius R must be very small in order to get the pressure differential due to surface tension forces AP, ZU/R greater than the gravity head gZ(p p,,). This causes the term ACR /8-n in Equation (7) to become very small with respect to 1 so that it may be H neglected. Then (r 5: (P *m) (10) The maximum heat flow, as predicted from this simplified model, for a heat pipe divided into N segments of equal length is N times that of the original heat pipe in the zero gravity case and N times that of the similar unsegmented heat ipe in the high gravity case. Thus, a lO-segment heat pipe would be expected to carry between 10 and times the maximum heat flux of an unsegmented heat pipe of the same cross section and total length. Actual performance without other change in the heat pipe is generally somewhat below that indicated by these model calculations since the model neglects the difficulty of simultaneously transferring heat to the hot surface of a porous material while allowing vapor to escape freely from it. An equivalent situation also occurs at the cold heat transfer surface where liquid condenses. Proper design of the capillary vaporizer and condenser surface is described in detail hereinafter, and more extensively set forth in the aforementioned copending patent application entitled HEAT TRANSFER SURFACE STRUCTURE. Such perforated or channeled surfaces providing vapor passages are often necessary in order to take full advantage of the high liquid flow rates available when using short segments in a heat pipe.

Thus, in the illustrated embodiment of FIG. 1, the quantity of heat that can be transferred through the length of the first cup 10 is N to N times the quantity of heat that can be trans ferred along the length of a heat pipe N times as long. The same is true of each of the additional cups l4, 19, 19a, and 25. Thus, if the first segment can carry N to N times the quantity of heat that an unsegmented heat pipe of the same overall length can carry, and so can each of the additional segments connected serially with the first segment, it follows that the total heat flux along the length of the segmented heat pipe is N to N times the total heat flux that can be accommodated by that length of heat pipe without serial segments. If N is 10, that is the heat pipe has 10 serial segments, from 10 to 100 times the heat can be transferred as in a conventional heat pipe.

This increased heat fiux would be obtained in an ideal situation where'no losses occur; however, in any real heat pipe, there is some resistance to heat flow through the impermeable barriers between adjacent segments of the heat pipe and also through the porous capillary material adjacent the impermeable barriers. In addition, the effective length L over which the liquid must flow is not merely the length of the segment but (.iljih) also inclues a certain am ount of radial liquid flow in the porous capillary material at the ends of the segments. Thus, in

practice, if no other changes are made the effect of segmenting a heat pipe into N segments does not actually increase the total heat flux capability by the theoretical factor of N or N I but to a somewhat lesser extent depending mainly on the end i losses.

A sacrifice that must be made in order to obtain the higher heat flux in the serially segmented heat pipe is that the temperature difference between the heat source and the heat sink must be higher than for an unsegmented heat pipe extending therebetween. This is due to the requirement of having a temperature drop across each segment of the heat pipe so that mass flow of vapor occurs between the warmer and cooler l portions, and also because a small temperature drop is required across the impenneable barriers and porous capillary material adjacent the barriers in order to cause heat to flow. In most instances this is not a significant limitation since heat pipes are inherently quite near to being isothermal, and, even in a segmented heat pipe, a temperature difference between the heat source and the heat sink of only a few degrees centigrade will normally suffice for quite large heat fluxes.

The temperature difference between the two ends of a pipe automatically adjusts itself to a value necessary to produce a difference in vapor pressure between the two ends sufficient to just remove the heat from the hot end to the cold end. That is, the pressure gradient in the heat pipe is determined by the volume of vapor flow necessary to transfer the heat from the hot end to the cold end. The vapor pressure at the vaporizing and condensing surfaces is directly determined by the temperatures of these surfaces. The pressure gradient is just sufficient to cause mass transfer of vapor to carry heat from the hot end to the cool end, and this needed pressure gradient detemiines the temperature drop required between the vaporizing surface and the condensing surface.

As mentioned hereinabove, the resistance to liquid flow in the capillary material is determined by capillary pore size. The optimum pore size for a given heat pipe is determined from Equations (7), (8), and (10) as applicable. It has also been pointed out that the available heat flux in a heat pipe at low gravity head was increased by a factor approximately the same as the number of segments into which the heat pipe is divided, less certain losses. This is true when the gravity head on the heat pipe is negligible as, for example, when the heat pipe is employed in a zero gravity environment or when the liquid flow is horizontal. In a situation where the liquid must flow upwardly against a gravity head, an even greater benefit is obtained by segmenting the heat pipe. The vapor pressure difference needed to give sufficient vapor flow is normally relatively small as compared with the gravity head in a vertically oriented heat pipe. That is, the pressure required to lift the liquid against gravity significantly exceeds the pressure needed to overcome the vapor pressure difference. 7 g

In a heat pipe the pore size is normally found to be optimum when the pressure differential obtained due to surface tension forces is about twice the pressure differential required to overcome the combined gravity head and the vapor flow pressure differential which is sufficiently small to be neglected in the high gravity head case. Thus under gravitational conditions, about one half of the pressure gradient due to surface tension forces is employed in overcoming the gravity head and the other one half is employed for overcoming the friction due to liquid flow. in a serially segmented heat pipe, the gravity head that needs to be overcome for a vertically oriented heat pipe is only the length of each segment and, therefore, a smaller pressure differential for overcoming the gravity head is required. Since a smaller pressure differential is needed, a larger pore size is acceptable. The larger pore size means that there is less resistance to liquid flow and, hence, a greater quantity of liquid can flow through a given cross-sectional area of capillary material. The resultant larger liquid flow means that the heat pipe segment can accommodate a larger heat flux.

It follows that since each serial segment of the heat pipe can accommodate a larger heat flux due to optimizing the pore size, the total heat flux through the segmented heat pipe is greater than the maximum possible heat flux through a nonsegmented heat pipe of the same length having a pore size optimized for a non-segmented heat pipe. As pointed out hereinabove when a heat pipe operating against a considerable gravity head is segmented and the pore size of the capillary material is optimized, the heat flux obtainable is increased by a factor (neglecting minor end losses) substantially equal to the square of the number of segments into which the heat pipe is subdivided. Thus, in the illustrated embodiment, if the heat pipe is segmented into four segments, the maximum heat flux when operating against a gravity head is increased by a factor of approximately 16, and if divided into segments the increase is by a factor of about 100. Because of the added expense of serially segmenting a heat pipe it is found that, unless the geometry requires it, it is seldom justified to use less than four serial segments. The increase in heat flux of N" times the number of segments becomes significant when N is more than three and it is therefore particularly preferred to employ a segmented heat pipe having more than three serial segments.

FIG. 2 illustrates a fragment of capillary material such as is preferably employed at the ends of the closed chambers or partitions forming the segments for optimum vaporization of liquid in the heat pipe. Thus, for example, the material illustrated in FIG. 2 can be considered to be the capillary vaporizer 18 at the heat source end of the second can 14. The vaporizer 18 is in intimate thermal contact with the impermeable end of the can, and preferably is formed of a porous material such as a sintered metal. lnstead of being just a flat layer of porous material on the end 15 of the can, a plurality of flat bottomed holes or passages 34 are provided normal to the principal extent of the vaporizer so that the vaporizer 18 is relatively thick in some areas between the holes 34 and relatively thin at the bottoms of the holes 34.

Liquid flowing from the heat sink end of the chamber through the capillary material 17 on the side wall of the can reaches the edge of the vaporizer l8 and is conveyed thereacross through the relatively thicker material between the holes 34. Since this material is thick there is relatively low resistance to liquid flow. if desired a layer of finer pored material, that is, having a greater efiective capillary surface to volume ratio 5, can be provided immediately adjacent the wall 15 of the can so as to increase the bubble pressure and allow higher temperature differentials, with correspondingly higher heat fluxes in this region. Liquid is thereby brought into contact with regional areas of the vaporizer 35 at the bottoms of the holes 34, and it is in these regions where the vaporizer is 50 thinner and, hence, hotter that much of the vaporization of liquid occurs. The heat for the vaporization comes through the impermeable wall 15 and heats liquid throughout the vaporizer 18. Vapor so formed can escape from the vaporizer by way of the holes 34 with a minimum resistance to vapor flow. It will be recognized, of course, that the volume of vapor flowing is much higher than the volume ofliquid and provision of the holes 34 substantially reduces frictional resistance to vapor flow as compared with a situation where the vapor is 60 forced to flow through the relatively fine pores of the capillary vaporizer 18. The provision of the vapor passages 34 through the vaporizer 18 therefore also serves to augment the total quantity of heat that can be transferred through the heat pipe. The surfaces of the holes are isothermal, with the same temperature as the escaping vapor. With closely spaced holes the temperature of most of capillary matrix also approaches the vapor temperature with only small regions near the heat source surface becoming hot enough for vapor to displace the liquid in the pores. Thus the liquid transport capacity of the 70 matrix is also retained. Also, the creation of numerous closely spaced regional areas of vaporization 35 by the hole walls and bottoms near the heat source surface greatly lowers the distance through which the heat must be conducted by the matrix.

75 Of interest is the construction of a segmented heat pipe wherein the capillary material actually forms the principal structural element at the partitions and the impervious partition is merely a very thin piece of foil secured to the capillary material. The strength of the capillary material sustains stresses and the foil need only be connected at its edges to the walls of the heat pipe to effect a fluid seal. Such a structure is readily fabricated by fitting the capillary pieces and attached foil barriers into the heat pipe, followed by localized brazing or difiusion bonding to seal the several segments from each other. The structure is advantageous in low weight and most particularly in the low temperature drop needed across the very thin foil to sustain the required heat transfer. It might also be noted that the partitions need not be normal to the axis of the heat pipe as in the illustrated embodiment. They can instead be diagonal membranes within the heat pipe, or the seg ments can be on a side in order to increase the heat transfer FIG. 3 illustrates in longitudinal cross section an end portion of a cylindrical heat pipe having a more complex type of vented capillary vaporizer for enhancing liquid flow to the surface from which vaporization occurs, and also permitting vapor to escape from that surface with minimum impedance.

As illustrated in this embodiment, the heat pipe has impervious cylindrical walls 36 and a flat partition 37 adjacent a heat source (not shown). The heat source on the outside of the partition 37 may be a similar structure in an adjacent segment of a heat pipe for vapor condensation or may be an external heat source if this should be an end segment of the heat pipe.

Immediately adjacent the partition 37 are a plurality of closely spaced concentric rings 33 of fine pore size or high effective pore surface to volume ratio 8 porous material having relatively high thermal conductivity defining a plurality of concentric circular passages 39 therebetween. As pointed out in the copending patent application, it is particularly preferred that the passages 39, and the regions of vaporization created by them, be spaced apart by less than about 0.1 inch in order to produce an economical, high efficiency structure. Overlying the concentric rings 38 is a disk-like body 41 of porous material having a somewhat larger pore size or lower effective pore surface to volume ratio and which may have a lower thermal conductivity than the rings 38. A central conical hole 42 in the disk 41 has its larger end in communication with the vapor way 43 along the length of the heat pipe. The smaller end of the conical hole 42 is at, or nearly at, the end wall 37. Six radially extending passages 44 flaring in both width and height are provided in one surface of the disk 41 so as to be in fluid manifolding communication with the circumferential passages 39, and also with the central conical hole 42. The disk 41 is also in contact with, or may be integral with, a hollow cylindrical wick 46 extending along the length of the heat P p a.

When such a structure is employed at the hot end of a heat pipe, liquid flows along the wick 46 into the porous disk 41 which conveys the liquid inwardly and brings it in contact with the finer pored concentric rings 38. Heat coming through the end wall 37 vaporizes liquid from the circular strips or rings 38 at the interface between the rings and the passages 39 which thus form numerous closely spaced regions of vaporization. The vapor so formed passes along a circumferential passage 39 until it reaches an intersection with one of the radial passages 44, each of which has a larger cross section than the passages 39, and is in fluid communication with a plurality of the smaller passages 39. The vapor flows inwardly through the larger passages 44 and empties therefrom into the conical hole 42 from whence it passes into the vapor way along the length of the heat pipe. Such a structure separates the liquid and vapor flow paths in separate fluid manifolds, thereby assuring a high liquid flow rate to the entire surface, and a minimum resistancetoyapor flow from the surface.

llll

heat pipe segments. In such an embodiment the vapor passages allow the vapor to approach the partition or heat pipe end wall closely so as to minimize the distance that the heat released when the vapor condenses must travel through the capillary material. The capillary material transports the liquid away from the condensing surface regions, which are the same as the regional areas forming the vaporizing regions when the vented capillary structures are used as Vaporizers, and are thus also simply tenned regional areas since they often may be used either way. Thus, the heat, vapor, and liquid flows in the vented capillary condenser are similar, but in the opposite directions from those in the vented capillary vaporizer. The result is replacement of the layer of liquid that would be otherwise formed by the condensing vapor on the partition or heat pipe wall, by a number of very thin areas of capillary material which can have a much higher total heat conductance than the liquid layer, particularly when operating with water or organic liquids. The portion of the capillary matrix near the partition or wall should have as high a heat conductivity as feasible while maintaining a reasonably high fluid conductivity.

An essential condition for preventing the buildup of liquid on the condenser surfaces is that the pressure of the liquid in the capillaries adjacent the condensing surface regions be lower than the vapor pressure in the passage just exterior to the condensing surface region. One technique for keeping the liquid at a lower pressure than the vapor is shown in FIG. I where capillary reservoirs 32 having larger pores, that is, a lower effective capillary surface to volume ratio 6, than the other capillary material have been added in each segment. The capillary volume of the reservoirs 32 and the quantity of fluid in the segment are such that the capillary reservoir is always partially filled with liquid under operating conditions, never being either completely dry or completely saturated. It thus tends to soak up" liquid from the vapor passages in the vented capillary heat transfer surfaces, which are larger than the pores in the reservoir, while releasing enough liquid to keep the other capillary material, which has smaller pores, saturated.

Additional description of vented capillary heat transfer surfaces, as illustrated in FIGS. 2 and 3, is contained in the aforementioned copending patent application entitled HEAT TRANSFER SURFACE STRUCTURE.

Although but one example of a segmented heat pipe constructed according to principles of this invention has been described herein, many modifications and variations will be apparent to one skilled in the art. Thus, for example, the illus trated embodiment employs a series of cylindrical cups nested end-to-end to form the segmented heat pipe. Many other fabrication arrangements for making a segmented heat pipe can be employed. It will also be apparent that neither a straight path between the heat source and the heat sink nor a circular cross section are required in the segmented heat pipe; many other paths and cross-sectional shapes can be employed. It will also be apparent that the cross-sectional area of different sements of the heat pipe need not be the same, and substantially all of the other variations and applications of conventional heat pipes are equally applicable to the segmented heat pipe hereinabove described.

What is claimed is:

I. An improved heat pipe comprising:

a closed chamber;

a fluid, comprising a liquid and its vapor in the chamber;

a body of capillary material in the chamber for conveying liquid from relatively cooler regions to relatively warmer reg ons;

a vapor way in the chamber for conveying vapor from relatively warmer regions to relatively cooler regions; and

additional capillary material in the chamber having a lower effective capillary surface to volume ratio 8 than a principal portion of the body of capillary material, and wherein the amount of fluid in the chamber is such that said additional capillary material is not completely saturated with liquid during operation.

2. An improved heat pipe comprising:

. c sd s arpest;

a fluid, comprising a liquid and its yapor; in the chamber;

a body of capillary material in the chamber for conveying liquid from relatively cooler regions to relatively warmer regions;

a vapor way in the chamber for conveying vapor from relatively warmer regions to relatively cooler regions;

means for increasing the maximum heat flux capacity of the heat pipe comprising a partition having at least an immgg UNITED STATES PATENT OFFICE Page 1 of 2 Pages CERTIFICATE OF CORRECTION Patent No 3,666 005 Dated May 30, 1972 Inventor(s) Robert David Moore, Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 55, formula (1) should read:

--F =(nR AP /8nL--'.

Column 3, line 6h, "THe" should read --The--.

Column 5, the

formula spanning lines

28 and 29 should read:

Claims 3, 4 and 5 should be included as follows:

--3. An improved heat pipe as defined in Claim 1 further comprising:

a multiplicity of regional areas spaced relatively close to a surface of said body of capillary material adjacent a chamber boundary through which heat passes and to each other as compared to the distance between the regional areas and the vapor way; and

a multiplicity of vapor passages in vapor communication between the regional areas and the vapor way.-

--4. An improved heat pipe as defined in Claim 2 wherein said partition further comprises:

a body of capillary material on at least one side of the impermeable layer and in thermal contact therewith, said body of capillary mate rial including a multiplicity of regional areas relatively close to the impermeable layer as compared to the thickness of the body of capillary material, and a multiplicity of vapor passages in vapor communication between the regional areas and the vapor way.--

(Cont'd /2) Po-ww 1 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Page 2 f 2 Pages Patent No. 3,666,005 Dated May 30, 1972 Inventods) Robert David Moore, Jr.

--5. An improved heat pipe as defined in Claim 4 wherein the partition further comprises:

a second body of capillary material on the opposite side of the impermeable layer from the first described body of capillary material and in thermal contact therewith, said second body of capillar material including a multi licity of regional areas relatively close to the impermeable layer as compared to the thickness of the body of capillary material, and a multiplicity of vapor passages in vapor communication between the regional areas and the vapor way.--

Signed and sealed this 1st day of May 1973.

(SEAL) fittest:

FIIBTCHER, JR. ROBERT GOTTSCHALK attesting 01 float Commissioner of Patents

Claims

1. An improved heat pipe comprising: a closed chamber; a fluid, comprising a liquid and its vapor in the chamber; a body of capillary material in the chamber for conveying liquid from relatively cooler regions to relatively warmer regions; a vapor way in the chamber for conveying vapor from relatively warmer regions to relatively cooler regions; and additional capillary material in the chamber having a lower effective capillary surface to volume ratio delta than a principal portion of the body of capillary material, and wherein the amount of fluid in the chamber is such that said additional capillary material is not completely saturated with liquid during operation.

2. An improved heat pipe comprising: a closed chamber; a fluid, comprising a liquid and its vapor, in the chamber; a body of capillary material in the chamber for conveying liquid from relatively cooler regions to relatively warmer regions; a vapor way in the chamber for conveying vapor from relatively warmer regions to relatively cooler regions; means for increasing the maximum heat flux capacity of the heat pipe comprising a partition having at least an impermeable layer and dividing the chamber into two closed chambers in thermal contact through the partition; and additional capillary material in at least one of the chambers having a lower effective capillary surface to volume ratio delta than a principal portion of the body of capillary material, and wherein the amount of fluid in the chamber is such that said additional capillary material is not completely saturated with liquid during operation.