What Are The Types of Capillary Structures of Heat Pipes?

Heat pipe is a heat conduction element that relies on the working liquid phase change of its own internal filling to achieve heat transfer, and has the following basic characteristics:

1. High thermal conductivity;

2. Excellent temperature equalization;

3. Reversible heat flow direction;

4. Thermal diodes and thermal switching;

5. Environmental adaptation.

Thermal switching means that the heat pipe can be made into A one-way heat conduction type, such as when the temperature of section A is higher than that of section B, the heat pipe works; When the temperature in section A is lower than that at end B, the heat pipe does not work.

The working principle of the pipeline and the frozen soil buried heat pipe is to make use of the switching property of the heat pipe. In winter, the ground temperature is higher than the air temperature, and the heat pipe works to absorb and distribute the temperature of the frozen soil layer into the air, making the temperature of the frozen soil layer lower and more stable. In summer, the air temperature is high and the heat pipe does not work.

This is also known as gravity heat pipe, its inner wall is smooth without capillary structure. The reflux of the liquid can only rely on the gravity of the working medium and has a unidirectional effect.

However, in the application scenarios of most electronic devices, the use of heat pipes has no fixed direction, so it is necessary to adapt to the working fluid reflux in all directions. At this point, the capillary structure of the inner tube wall is very important, and the capillary structure needs to transport the working fluid in the case of anti-gravity, ensuring that anywhere as a condensing section, the liquid can quickly return to the evaporation end.

According to the needs of the use scenario and considering the processing cost, there are currently three main types of capillary structure.

1. Powder sintering structure (Powder)

Look at the following physical disassembly diagram, from the diagram, you may think that it is very soft, but in fact, this capillary core is neither soft nor loose, but very strong. Because it is a substance of copper powder heated by high temperature, so as they cool, they restore the original hard texture of the metal.

In fact, the process of making such a heat pipe is relatively complex, first of all, a round rod mold is placed in the center of the copper pipe, and then the loose copper powder is injected, and the copper powder reaches a certain density after vibration; Then the product is placed in a reducible atmosphere furnace and heated to a certain temperature. Before the copper powder is completely melted, the edge of the copper powder particles will first melt, adhere to the surrounding copper powder, and stick to the copper pipe wall, and finally form the hollow structure you see now.

The sintering temperature and holding time need to be reasonably matched to ensure the bonding strength, but the copper powder particles cannot be melted. Of course, in actual production, this process has been very mature, the cost has been greatly reduced, and it is the most used form of capillary structure.

Copper powder sintered capillary core

The main production methods of copper powder are electrolysis, water atomization and REDOX method. The copper powder produced by electrolysis method has the characteristics of dendritic microscopic shape, higher purity and better formability than the surface, but the production of point-hydrolysis copper powder has high energy consumption, high cost and serious environmental pollution. At present, copper powder produced by water atomization is more commonly used. At present, the copper powder produced by the water atomization + REDOX method (now AOR method) commonly used in the world is modified on the surface, and its microscopic shape is not planned coral, which is developed than the surface and has good formability, and it has a low loose density of electrolytic copper powder (generally 1.5 ~ 3.0g/cm3). It also has good fluidity of water atomized copper powder (general fluidity is less than 35s/50g), and the process performance is stable, so it can replace most electrolytic copper powder.

2. Groove Structure

The application of trench structure has a long history. From the figure, we can see that the wall of the pipe is different from the smooth surface of the sintered copper pipe, but there is a circle of fine grooves along the direction of the copper pipe drawing.

It is these grooves that act as capillary forces in heat pipes. The returned liquid is rapidly conducted through these channels in the heat pipe.

Obviously, the finer the grooving, the better the capillary force should be, but the material production cost will also increase.

From the point of view of heat pipe production cost, the process of this heat pipe is relatively simple, and many processes are produced without filling powder, and the manufacturing cost is relatively low.

Grooved capillary core structure

3, metal Mesh (Mesh)

This structure is also very common now.

Like sintered powder, wire mesh is also designed to provide capillary force. This braided mesh, with cross-lap between the wires, forms pores, as well as capillary forces, which can overcome gravity and achieve rapid transport of working fluid.

Of course, its capillary properties are not as good as sintered powder capillary core.

Wire mesh capillary core structure

Used as a capillary structure of copper wire mesh, generally use the number of mesh as a nominal specification, commonly used 100 mesh, 200 mesh, 300 mesh and so on.

4. Complex Structure

In view of the advantages and disadvantages of the above structures, there are also composite pipe structures for comprehensive use.

Such as copper powder + copper mesh, groove + copper mesh and so on. For long-distance working medium transport, one or more metal braided wires can also be sintered on the surface of the powder or mesh to increase the return speed and increase the return flow.