US20100155031A1

US20100155031A1 - Heat pipe and method of making the same

Info

Publication number: US20100155031A1
Application number: US12/491,245
Authority: US
Inventors: Sheng-Lin Wu; Yu-Liang Lo; Sheng-Liang Dai; Nien-Tien Cheng
Original assignee: Furui Precise Component Kunshan Co Ltd; Foxconn Technology Co Ltd
Current assignee: Furui Precise Component Kunshan Co Ltd; Foxconn Technology Co Ltd
Priority date: 2008-12-22
Filing date: 2009-06-25
Publication date: 2010-06-24
Also published as: CN101634532A; CN101634532B

Abstract

A heat pipe includes a casing, a main wick structure received in the casing and attached to an inner surface of the casing, a multi-layered auxiliary wick structure received in the main wick structure and a working fluid contained in the casing and saturating the main wick structure and the auxiliary wick structure. An inner peripheral surface of the main wick structure and an outer peripheral surface of the auxiliary wick structure cooperatively define a vapor channel therebetween. The auxiliary wick structure extends along a longitudinal direction of the casing and defines a liquid channel therein. The auxiliary wick structure is formed by a plurality of layers radially stacked on each other, such that each outer layer is attached around an adjacent inner layer.

Description

BACKGROUND

1. Technical Field
The present invention relates generally to an apparatus for transfer or dissipation of heat from heat-generating components, and more particularly to a heat pipe applicable in electronic products such as personal computers for removing heat from electronic components installed therein and a method for manufacturing the same.
2. Description of Related Art
Heat pipes have excellent heat transfer performance due to their low thermal resistance, and are therefore an effective means for transfer or dissipation of heat from heat sources. Currently, heat pipes are widely used for removing heat from heat-generating components such as central processing units (CPUs) of computers. A heat pipe is usually a vacuum casing containing therein a working medium, which is employed to carry, under phase transitions between liquid state and vapor state, thermal energy from one section of the heat pipe (typically referring to as the “evaporator section”) to another section thereof (typically referring to as the “condenser section”). Preferably, a wick structure is provided inside the heat pipe, lining an inner wall of the casing, for drawing the working medium back to the evaporator section after it is condensed at the condenser section. The wick structure currently available for the heat pipe includes fine grooves integrally formed at the inner wall of the casing, screen mesh or fiber inserted into the casing and held against the inner wall thereof, or sintered powders combined to the inner wall of the casing by sintering process.
In operation, the evaporator section of the heat pipe is maintained in thermal contact with a heat-generating component. The working medium contained at the evaporator section absorbs heat generated by the heat-generating component and then turns into vapor. Due to the difference of vapor pressure between the two sections of the heat pipe, the generated vapor moves and thus carries the heat towards the condenser section where the vapor is condensed into condensate after releasing the heat into ambient environment by, for example, fins thermally contacting the condenser section. Due to the difference in capillary pressure which develops in the wick structure between the two sections, the condensate is then brought back by the wick structure to the evaporator section where it is again available for evaporation.
In order to draw the condensate back timely, the wick structure provided in the heat pipe is expected to provide a high capillary force and meanwhile generate a low flow resistance for the condensate. In ordinary use, the heat pipe needs to be flattened to enable the miniaturization of electronic products, which results in the wick structure of the heat pipe being damaged. Therefore, the flow resistance of the wick structure is increased and the capillary force provided by the wick structure is decreased, which reduces the heat transfer capability of the heat pipe. If the condensate is not quickly brought back from the condenser section, the heat pipe will suffer a dry-out problem at the evaporator section.
Therefore, it is desirable to provide a heat pipe with an improved heat transfer capability, whose wick structure will not be damaged when the heat pipe is flattened.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a longitudinal cross-sectional view of a heat pipe in accordance with a first embodiment of the present invention.

FIG. 2 is a transverse cross-sectional view of the heat pipe of FIG. 1.

FIG. 3 is a flow chart showing a method for manufacturing the heat pipe of FIG. 1.

FIG. 4 is a transverse cross-sectional view of a heat pipe in accordance with a second embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, a heat pipe 10 includes an elongated, round casing 12 containing a working fluid therein, a main wick structure 14 and an auxiliary wick structure 18.
The casing 12 is made of a highly thermally conductive material such as copper or aluminum. The casing 12 includes an evaporator section 121, an opposing condenser section 122, and an adiabatic section 123 disposed between the evaporator section 121 and the condenser section 122.
The main wick structure 14 is tube-shaped in profile, which is evenly distributed around and attached to an inner surface of the casing 12. The main wick structure 14 defines a receiving space therein. The main wick structure 14 extends along a longitudinal direction of the casing 12. The main wick structure 14 is usually selected from a porous structure such as fine grooves, sintered powder, screen mesh, or bundles of fiber, and provides a capillary force to drive condensed working fluid at the condenser section 122 to flow towards the evaporator section 121 of the heat pipe 10.
The auxiliary wick structure 18 is a longitudinal hollow tube, which is received in the receiving space of the main wick structure 14 and extends along the longitudinal direction of the casing 12. The auxiliary wick structure 18 has a ring-like transverse cross section. The auxiliary wick structure 18 longitudinally defines a liquid channel 172 therein. An outer diameter of the auxiliary wick structure 18 is much smaller than a bore diameter of the main wick structure 14.
The auxiliary wick structure 18 is a multi-layered structure, which is outwardly and radially formed by a plurality of round layers such that each successive layer is attached to a previous layer. In the embodiment, the auxiliary wick structure 18 includes a first layer 181 at an inner side and a second layer 182 at an outer side and attached immediately around the first layer 181. An inner peripheral surface of the first layer 181 longitudinally defines the liquid channel 172 therein. An inner peripheral surface of the main wick structure 14 and an outer peripheral surface of the second layer 182 of the auxiliary wick structure 18 cooperatively define a vapor channel 171 in the casing 12. The outer peripheral surface of the auxiliary wick structure 18 has a bottom side 164 contacting with the inner peripheral surface of the main wick structure 14, and a top side 165 spaced from the inner peripheral surface of the main wick structure 14.
The first and the second layers 181, 182 are formed by weaving a plurality of metal wires, such as copper wires. A plurality of pores is formed in the first and the second layer 181, 182, which provides a capillary action to the working fluid. The metal wires of the first layer 181 has a greater wire diameter than that of the metal wires of the second layer 182, whereby the metal wires of the first layer 181 have a greater mechanical strength to support the whole auxiliary wick structure 18, which prevents the auxiliary wick structure 18 from collapsing down to thereby maintain the intended shape of the pores and the liquid channel 172. Moreover, since the metal wires of the second layer 182 have a smaller wire diameter than the metal wires of the first layer 181, the second layer 182 has a smaller pore size than the first layer 181, whereby the second layer 182 has a greater capillary action to absorb more working fluid.
The working fluid is saturated in the main and the auxiliary wick structures 14, 18 and is usually selected from a liquid such as water, methanol, or alcohol, which has a low boiling point and is compatible with the main and the auxiliary wick structures 14, 18. Thus, the working fluid can easily evaporate to vapor when it receives heat at the evaporator section 121 of the heat pipe 10.
In operation, the evaporator section 121 of the heat pipe 10 is placed in thermal contact with a heat source, for example, a central processing unit (CPU) of a computer, which needs to be cooled. The working fluid contained in the evaporator section 121 of the heat pipe 10 is vaporized into vapor upon receiving the heat generated by the heat source. Then, the generated vapor moves via the vapor channel 171 towards the condenser section 122 of the heat pipe 10. After the vapor releases the heat carried thereby and is condensed into the condensate in the condenser section 122, the condensate is brought back by the main wick structure 14 and the auxiliary wick structure 18 to the evaporator section 121 of the heat pipe 10 for being available again for evaporation.
Referring to FIG. 3, a method for manufacturing the heat pipe 10 includes the following steps: providing an elongated pole, and weaving a plurality of first metal wires on an outer peripheral surface of the pole to form the first layer 181; weaving a plurality of second metal wires on an outer peripheral surface of the first layer 181 to form a second layer 182; removing the pole from the first layer 181 to form the auxiliary wick structure 18, wherein the first layer 181 defines the liquid channel 172 therein; providing a casing 12 having a main wick structure 14 attached to an inner peripheral surface thereof, inserting the auxiliary wick structure 18 into the casing 12; vacuuming an interior of the casing 12 and filling the working fluid into the casing 12; and sealing the casing 12.
Table 1 below shows an average of maximum heat transfer rates (Qmax) and an average of heat resistances (Rth) of forty-five conventional round grooved heat pipes and forty-five round heat pipes 10 formed in accordance with the present disclosure. Qmax represents the maximum heat transfer rate of the heat pipe at an operational temperature of 50° C. Rth is obtained by dividing the margin between an average temperature of the evaporator section 121 and an average temperature of the condenser section 122 of the heat pipe 10 by Qmax. A diameter of the transverse cross section and a longitudinal length of each of the conventional grooved heat pipes are 6 mm and 160 mm, which are respectively equal to the transverse diameter and the longitudinal length of each of the present heat pipes 10. Table 1 shows that the heat resistance of the present round heat pipe 10 is significantly less than that of the conventional round grooved heat pipe, whilst the Qmax of the round heat pipe 10 in accordance with the present disclosure is significantly more than that of the conventional round grooved heat pipe.

TABLE 1

	average of Qmax	average of Rth
Types of heat pipes	(unit: w)	(unit: ° C./w)

Conventional grooved	65	0.025
heat pipes
present heat pipes	95.5	0.024

As shown in FIG. 4, a flat heat pipe 50 in accordance with a second embodiment of the present invention is obtained by flattening the heat pipe 10 of FIGS. 1 and 2. The heat pipe 50 has the same structure as the heat pipe 10 except that the heat pipe 50 is a flat one. After the flattening operation, the auxiliary wick structure 18 is kept intact, and the auxiliary wick structure 18 spaces a gap from a top wall 52 of the heat pipe 50. The heat transfer capability of the flat heat pipe 50 is not decreased due to the flattening operation. The heat transfer capability of the flat heat pipe 50 is better than a conventional flat heat pipe whose wick structure is damaged in the flattening operation.
Table 2 below shows an average of maximum heat transfer rates (Qmax) and an average of heat resistances (Rth) of ten conventional flat grooved heat pipes and ten present heat pipes 50, which are flattened to have a height of 3.5 mm. Before these heat pipes are flattened, they have the same transverse diameter and longitudinal length as the heat pipes mentioned in Table 1. Qmax and Rth in Table 2 have the same meaning as the Qmax and Rth in Table 1. Table 2 shows that the heat resistance of the present flat heat pipe 50 is significantly less than that of the conventional flat grooved heat pipes, whilst the Qmax of the flat present heat pipes 50 is significantly more than that of the conventional flat grooved heat pipes.

TABLE 2

	average of Qmax	average of Rth
Types of heat pipes	(unit: w)	(unit: ° C./w)

Conventional grooved	32	0.055
heat pipes
Present heat pipes	64	0.033

It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.

Claims

1. A heat pipe comprising:

a casing;

a main wick structure received in the casing and attached to an inner surface of the casing;

a multi-layered auxiliary wick structure received in the main wick structure, an inner peripheral surface of the main wick structure and an outer peripheral surface of the auxiliary wick structure cooperatively defining a vapor channel therebetween, the auxiliary wick structure extending along a longitudinal direction of the casing and defining a liquid channel therein; and

a working fluid contained in the casing and saturated in the main wick structure and the auxiliary wick structure;

wherein the auxiliary wick structure is formed by a plurality of layers radially disposed on each other in which each outer layer is attached immediately around an adjacent inner layer.

2. The heat pipe as claimed in claim 1, wherein the auxiliary wick structure comprises a first inner layer and a second outer layer attached around the first layer, an inner surface of the first inner layer defining the liquid channel, the inner surface of the main wick structure and an outer peripheral surface of the auxiliary wick structure defining the vapor channel.

3. The heat pipe as claimed in claim 1, wherein each layer of the auxiliary wick structure is formed by weaving a plurality of wires.

4. The heat pipe as claimed in claim 3, wherein the wires of an inner layer have a greater wire diameter than that of the wires of an adjacent outer layer of the auxiliary wick structure.

5. The heat pipe as claimed in claim 1, wherein an outer diameter of the auxiliary wick structure is smaller than a bore diameter of the main wick structure.

6. The heat pipe as claimed in claim 1, wherein the casing is round.

7. The heat pipe as claimed in claim 1, wherein the casing is flattened.

8. A method for manufacturing a heat pipe comprising the steps of:

providing an elongated pole, weaving a plurality of first wires on an outer peripheral surface of the pole to form a first layer;

weaving a plurality of second wires on an outer peripheral surface of the first layer to form a second layer;

removing the pole from the first layer to form an auxiliary wick structure, the auxiliary wick structure defining a liquid channel therein;

providing a casing having a main wick structure, the main wick structure attached to an inner surface of the casing, inserting the auxiliary wick structure into the casing;

vacuuming the casing and filling a working fluid into the casing; and

sealing the casing.

9. The method as claimed in claim 8, wherein the plurality of first wires of the first layer has a greater wire diameter than that of the plurality of second wires of the second layer.