Patent Application
20060162906
The present invention relates generally to a heat pipe as a heat transfer device, and more particularly to a heat pipe with a screen mesh wick structure.
As electronic industry continues to advance, electronic components such as central processing units (CPUs), are made to provide faster operation speeds and greater functional capabilities. When a CPU operates at a high speed, its temperature frequently increases greatly. It is desirable to dissipate the heat generated by the CPU quickly.
To solve this problem of heat generated by the CPU, a cooling device is often used to be mounted on top of the CPU to dissipate heat generated thereby. It is well known that heat absorbed by fluid having a phase change is ten times more than that the fluid does not have a phase change; thus, the heat transfer efficiency by phase change of fluid is better than other mechanisms, such as heat conduction or heat convection. Thus a heat pipe has been developed.
The heat pipe has a hollow pipe body receiving a working fluid therein and a wick structure disposed on an inner wall of the pipe body. During operation of the heat pipe, the working fluid absorbs the heat generated by the CPU or other electronic device and evaporates. Then the vapor moves to the condensing section to release the heat thereof. The vapor cools and condenses at the condensing section. The condensed working fluid returns to the evaporating section and evaporates into vapor again, whereby the heat is continuously transferred from the evaporating section to the condensing section.
In general, movement of the working fluid depends on capillary pressure of the wick structure. Usually the wick structure has following four configurations: sintered power, grooved, fiber and screen mesh. For the thickness and pore size of the screen mesh can be easily changed, the screen mesh is widely used in the heat pipe.
It is well recognized that the capillary pressure of a screen mesh increases due to a decrease in pore size of the screen mesh. In order to obtain a relatively large capillary pressure for a screen mesh, a mesh screen having a small-sized pores is usually adopted. However, it is not always the best way to choose a screen mesh having small-sized pores, because the flow resistance to the condensed working fluid also increases due to the decrease in pore size of the screen mesh. The increased flow resistance reduces the speed of the condensed working fluid in returning back to the evaporating section and therefore limits the heat transfer performance of the heat pipe. As a result, a heat pipe with a screen mesh that has too large or too small pore size often suffers dry-out problem at the evaporating section as the condensed fluid cannot be timely return back to the evaporating section of the heat pipe.
Therefore, there is a need for a heat pipe with a screen mesh which can provide simultaneously a relatively large capillary pressure and a relatively low flow resistance so as to effectively and timely bring the condensed fluid back from its condensing section to its evaporating section and thereby to avoid the undesirable dry-out problem at the evaporating section. There is also a need for a heat pipe with a screen mesh which has a range of pore sizes so that the heat pipe can operate under different conditions without the undesirable dry-out problem at the evaporating section.
A heat pipe in accordance with a preferred embodiment of the present invention comprises a pipe body having an inner wall and a screen mesh disposed on the inner wall of the pipe body. The screen mesh is in the form of a multi-layer structure with at least one layer thereof has an average pore size different from that of the other layers. The layer with large-sized pores is capable of reducing the flow resistance to the condensed fluid to flow back, whereas the layer with small-sized pores is still capable of providing a relatively large capillary pressure for the condensed fluid in the heat pipe.
Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which:
The pipe body 20 is typically made of high thermally conductive materials such as copper or copper alloys. The screen mesh 30 is saturated with a working fluid (not shown), which acts as a heat carrier for carry thermal energy from the evaporating section 70 toward the condensing section 90 when undergoing phase change from a fluid state to a vaporous state. The working fluid may be water, alcohol or other material having a low boiling point and the heat pipe 10 is vacuumed; thus, the working fluid can easily evaporate to vapor during operation.
Along a longitudinal direction of the pipe body 20 from the evaporating section 70 to the condensing section 90, the screen mesh 30 has a multi-layer structure, which includes in sequence a first layer 40, a second layer 50 and a third layer 60. In this embodiment, the first, second and third layer 40, 50, 60 correspond to the evaporating, adiabatic and condensing section 70, 80, 90 of the heat pipe 10, respectively. Each layer of the screen mesh 30 has an average pore size different from that of the other layers. The first layer 40 has the smallest average pore size, whereas the third layer 60 has the largest average pore size. That is, the three layers 40, 50, 60 are arranged side by side in such a manner that the average pore sizes thereof gradually increase along the longitudinal direction from the evaporating section 70 toward the condensing section 90. According to the general rule, the capillary pressure of the screen mesh 30 and its flow resistance to the condensed fluid increase due to a decrease in pore size of the screen mesh 30; the multi-layer construction of the screen mesh 30 is thus capable of providing a capillary pressure gradually increasing from the condensing section 90 toward the evaporating section 70, and a flow resistance gradually decreasing from the evaporating section 70 toward the condensing section 90.
According to the above-mentioned general rule, the capillary pressure of a wick and its flow resistance to the condensed fluid increase due to a decrease in pore size of the wick; the inner layer 460 and the intermediate layer 450 have a relatively larger average pore size and therefore are capable of providing a relatively low resistance to the condensed working fluid to flow back. The outer layer 440, however, has a relatively smaller average pore size and therefore is capable of having a relatively high capillary pressure for drawing the condensed working fluid back to the evaporating section. Thus, the multi-layer construction of the screen mesh 430 is capable of providing between these layers, along the radial direction of the pipe body 20 a gradient of capillary pressure gradually increasing from the central axis X-X of the pipe body 20 toward the inner surface of the pipe body 20, and a gradient of flow resistance gradually decreasing from the inner surface of the pipe body 20 toward a central axis X-X of the pipe body 20. Furthermore, the outer layer 440 with small-sized pores is also capable of maintaining an increased contact surface area with the inner surface of the pipe body 20, as well as a large contact surface with the working fluid saturated in the screen mesh 430, to thereby facilitate heat transfer between the working fluid in the heat pipe 410 and a heat source outside the heat pipe 410 that needs to be cooled.
Also the layers can be arranged variably according to the heat flux of the heat source. As shown in
Each layer or section of the screen mesh as shown above has the same length or thickness along the longitudinal or the radial direction of the heat pipe 10. It is to be understood that the thickness or length of each layer or section can be changed and different from the others. As shown in
It is understood that the invention may be embodied in other forms without departing from the spirit thereof. Thus, the present example and embodiment is to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 094101782 | Jan 2005 | TW | national |
| 094108396 | Mar 2005 | TW | national |
