Heat Pipe Wick Restrainer

Abstract

This invention relates in general to heat pipes and more particularly to a tubular heat pipe wick restrainer which includes a tubular member with an outside diameter equal to the desired inside diameter of the wick structure. The tubular member has circumferentially placed holes which allow the heat pipe working fluid to evaporate and condense at the evaporator and condenser sections respectively.

Description

BACKGROUND OF THE INVENTION

This invention pertains to heat pipes and more particularly to a new heat pipe wick support.

In accordance with the present state-of-the-art, heat pipe wicks are manufactured by various methods such as by fabricating channels into the heat pipe walls by a broaching process. The channels, which permit unrestricted fluid flow from the heat pipe condenser to the evaporator section, are covered by fine mesh screen to establish greater capillary wicking forces. Composite wicks have also been manufactured by placing layers of heavy mesh screen (30 to 60 mesh) beneath the wick/vapor interface layers of fine mesh screen (200 to 400 mesh). Another technique comprises the fabrication of open annulus wicks by swaging several turns of screen wound between two copper tubes. The copper tubes are then etched away and the porous rigid wick is sinter bonded. Upon insertion into a heat pipe with an open annulus between the wick and heat pipe walls, this wick presents an optimum arrangement for liquid metal charged heat pipes.

While these fabrication techniques have yielded heat pipes which are both homogeneous in pore size and porosity and chemically compatible with the heat pipe working fluids, they have not provided wicks that maintain uniform contact with the heat pipe inner surfaces. Such uniform contact is a necessary pre-requisite for optimum heat pipe operation. Thus, retaining the wick against the heat pipe inner walls has been a problem. The prior art has employed several methods to correct this deficiency. One such method comprises threading a drawn helical spring through the wick material and releasing the spring so that the coils press the wick against the heat pipe inner walls. The problem with this method is that the spring provides non-uniform support over the wick surface area causing constrictions at the spring/wick interfaces which inhibit the capillary action of the wick. This method has particularly proved to be impractical in high temperature heat pipes where at elevated temperatures the spring loses its tension and sags, thereby releasing the wick from the heat pipe walls and creating gaps which result in hot spots, which may burn through the heat pipe walls. Another method employed has been to produce wicks by the aforementioned swaging process. The resulting wicks are tight, homogeneous, free standing, channeled structures. These wicks, however, can soften after a time at elevated temperatures and sag, or be pushed away from the walls by the working fluid. Other wicks have been fabricated by seam welding strips of wick material into tubes and inserting them into heat pipe tubes. Again no uniform restraining device presses the wick to the tube wall.

In order to maintain optimum heat pipe operation the wick support structure must also be able to prevent fluid loses at the vapor-fluid interface caused by the high velocity vapors traveling between the evaporator and condenser sections (entrainment). None of the aforementioned techniques have been able to cope with this problem and the result has been to starve the evaporator region of the heat pipe.

SUMMARY OF THE INVENTION

Briefly, this invention overcomes the difficulties experienced by the prior art by utilizing a tubular heat pipe wick restrainer which comprises a tubular member with an outside diameter approximately equal to the desired inside diameter of the wick structure. The tubular member has circumferentially placed holes which permit the heat pipe working fluid to evaporate and condense at the evaporator and condenser sections respectively. The resulting restrainer is both porous and retains good circumferential strength. This circumferential rigidity permits uniform pressure to be exerted on the wick structure.

The wick structure is tightly rolled onto the restrainer tube, possibly tack welded to it, and inserted into the heat pipe tube. A tight fit can be employed, or a sliding fit can be used if the wick assembly is to be swaged. Thus the restrainer tube: (1) uniformly presses the wick onto the heat pipe inner walls; (2) permits easier heat pipe assembly and processing; and (3) presents a non-homogeneous surface at the liquid/vapor interface to break up the vapor flow path along the wick liquid/vapor interface. This last property prevents the rapid vapor flow from pulling fluid out of the wick (entrainment) and thus starving the evaporator region.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of an exemplary embodiment of this invention, reference may be had to the accompanying drawings, in which:

FIG. 1 is an isometric view of a heat pipe embodying the principles of this invention, having a portion thereof cut away for clarity;

FIG. 2 is an isometric view of the heat pipe of FIG. 1, broken away in layers, and is illustrative of the position of the restrainer tube with respect to the other elements of the heat pipe; and

FIG. 3 is a development of a portion of the heat pipe wick restrainer tube of FIG. 2 and is illustrative of a hole pattern which may be utilized with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the heat pipe illustrated in FIGS. 1 and 2, it will be appreciated that a heat pipe 10 constructed in accordance with the principles of this invention includes an evacuated chamber 12 whose inside walls are lined with a capillary structure, or wick 14, that is saturated with a volatile fluid. A tubular support member, or restrainer 30, having an outside diameter approximately equal to the desired inside diameter of the wick structure 14, is positioned within the evacuated chamber 12 against the wick 14 which closely receives the support member 30 in the center thereof. The support member 30 exerts uniform pressure on the inner surface of the wick 14 so as to restrain the wick 14 against the inner walls of the heat pipe 10. The support member 30 has circumferentially placed holes 32, which allow the working fluid to evaporate and condense at the evaporator section 16 and condenser section 18 respectively.

The operation of the heat pipe combines two familiar principles of physics; vapor heat transfer and capillary action. Vapor heat transfer serves to transport the heat energy from the evaporator section 16 at one end of the pipe to the condenser section 18 at the other end. Capillary action returns the condensed working fluid back to the evaporator section 16 to complete the cycle.

The working fluid absorbs heat at the evaporator section 16 and changes from its liquid state to a vapor state. The amount of heat necessary to cause this change of state is the latent heat of vaporization. As the working fluid vaporizes, the pressure at the evaporator section 16 increases. The vapor pressure sets up a pressure differential between the ends of the heat pipe, and this pressure differential causes the vapor, and thus the heat energy, to move towards the condenser section 18. When the vapor arrives at the condenser section 18, it is subjected to a temperature lower than that of the evaporator section 16 and condenses, thereby releasing the thermal energy stored in its heat of vaporization at the condenser section 18 of the heat pipe. As the vapor condenses the pressure at the condenser section 18 decreases so that the necessary pressure differential for continued vapor heat flow is maintained. Movement of the fluid from the condenser section 18 back to the evaporator section 16 is accomplished by capillary action within the wick 14, which connects the condenser 18 to the evaporator 16.

In order to provide optimum heat pipe operation the heat pipe wick 14 must maintain uniform contact with the inner walls of the heat pipe tube 20 and means must be provided for preventing fluid loses in the wick 14 at the wick/vapor interface, due to the high velocity vapor flow along this interface (sometimes approaching sonic velocities).

This invention maintains the aforementioned criteria by utilizing the heat pipe wick restrainer 30, illustrated in FIG. 2. The heat pipe wick restrainer, or support 30, includes a tubular member 34 with an outside diameter approximately equal to the desired inside diameter of the heat pipe wick 14. The tubular member 34 has circumferentially placed holes 32 which allow the heat pipe working fluid to evaporate and condense at the evaporator and condenser sections respectively. The holes 32 are desirably arranged in a pattern, such as the hexagonal pattern illustrated in FIG. 3, and extend over the entire circumference of the restrainer. For closest packing, it is suggested that the outside diameter of the holes 32, should be approximately one-third the restrainer tube outside diameter. The resulting restrainer is approximately 50 percent porous and retains good circumferential strength. This circumferential rigidity permits uniform pressure to be exerted on the wick structure 14.

It is to be understood that the size of the holes and the hole pattern may vary depending upon the heat pipe wick structure and its intended use. For example, the holes may be positioned at the evaporator and condenser sections without extending over the entire length of the restrainer tube, and the size of the holes may vary with the wick surface area required to produce the desired rate of vaporization and condensation of the working fluid.

It is also to be understood that the tubular member 34 need not be of circular cylindrical configuration, but may be desirably designed to conform to any geometric shape the heat pipe wick may form. What has been shown and described is merely an exemplary embodiment of this invention and it is not intended to be limitative thereof.

Thus the restrainer tube: (1) uniformly presses the wick onto the heat pipe inner walls; (2) permits easier heat pipe assembly and (3) presents a non-homogeneous surface to break up the vapor flow path along the wick liquid/vapor interface. This last property prevents the rapid vapor flow from pulling fluid out of the wick and thus starving the evaporator region.

Claims

I claim as my invention:

1. A heat pipe comprising a sealed tubular outer casing, a tubular wick structure positioned in said outer casing and closely received by the inner surface thereof, and a tubular substantially nonresilient, wick restrainer position within said casing and closely received by the inner surface of said wick structure, said wick restrainer being formed to provide uniform support to said wick structure, to maintain the same in engagement with said inner surface of said outer casing.

2. The heat pipe of claim 1 wherein said wick restrainer has an outside diameter approximately equal to the desired inside diameter of the wick structure, said wick restrainer extending over the evaporator and condenser regions of the heat pipe and being perforated along the evaporator and condenser regions to allow the heat pipe working fluid respectively to evaporate at the wick/vapor interface of the evaporator section and condense at the wick/vapor interface of the condenser section.

3. The heat pipe of claim 2 wherein said perforation in said wick restrainer has a diameter approximately equal to one-third of the outside diameter of said wick restrainer.

4. The heat pipe of claim 2 wherein the perforations in said wick restrainer are arranged in a hexagonal pattern.

5. The heat pipe of claim 2 wherein the perforations are arranged in a pattern extending over the entire surface of said wick restrainer.

6. The heat pipe of claim 1 wherein said wick restrainer is rigid.

7. The heat pipe of claim 1 wherein said tubular wick structure comprises wire mesh screen.