How heat pipes work

The heat pipe is a kind of heat transfer element, which makes full use of the principle of heat conduction and the fast heat transfer properties of the cooling medium. thermal conductivity.

 

 

In 1963, heat pipe technology was invented by George Grover of Los Alamos National Laboratory.

 

The heat pipe is a kind of heat transfer element, which makes full use of the principle of heat conduction and the fast heat transfer properties of the cooling medium. thermal conductivity.

 

Heat pipe technology has been used in aerospace, military and other industries before. Since it was introduced into the radiator manufacturing industry, people have changed the design thinking of traditional radiators and got rid of the traditional heat dissipation mode that relies solely on high-volume fans to obtain better heat dissipation.

 

Instead, it adopts a new cooling mode with low speed, low air volume fan and heat pipe technology.

 

Heat pipe technology has brought an opportunity to the quiet era of computers and has been widely used in other electronic fields.

 

How heat pipes work?

 

The working principle of the heat pipe is: whenever there is a temperature difference, the phenomenon of heat transfer from the high temperature to the low temperature will inevitably occur. The heat pipe uses evaporative cooling, so that the temperature difference between the two ends of the heat pipe is very large, so that the heat is conducted quickly. The heat of the external heat source increases the temperature of the liquid working medium through the heat conduction of the tube wall of the evaporation section and the liquid absorbent core filled with the working medium; the temperature of the liquid rises, and the liquid surface evaporates until it reaches the saturated vapor pressure. way to pass to the steam. The vapor flows to the other end under a small pressure difference, releases heat, and condenses into liquid again, and the liquid flows back to the evaporation section along the porous material by capillary force. This cycle is rapid, and heat can be continuously conducted away.

 

Heat pipe technical features

 

·High-speed heat conduction effect. Light weight and simple structure

 

·Even temperature distribution, can be used for uniform temperature or isothermal action.·Large heat transfer capacity. Long heat transfer distance.

 

·There are no active components, and it does not consume power itself.

 

·There is no restriction on the direction of heat transfer, the evaporating end and the condensing end can be interchanged. ·Easy to process to change the direction of heat transfer.

 

Durable, long life, reliable, easy to store and keep. Why does heat pipe technology have such high performance? We have to look at this problem from a thermodynamic point of view.

 

The heat absorption and heat release of objects are relative, and whenever there is a temperature difference, the phenomenon of heat transfer from high temperature to low temperature will inevitably occur.

 

There are three ways of heat transfer: radiation, convection, and conduction, among which heat conduction is the fastest.

 

The heat pipe uses evaporative cooling to make the temperature difference between the two ends of the heat pipe very large, so that the heat can be quickly conducted.

 

 

A typical heat pipe consists of a tube shell, a wick and an end cap.

 

The production method is to pump the inside of the tube to a negative pressure of 1.3×(10-1~10-4)Pa and then fill it with an appropriate amount of working liquid, so that the capillary porous material of the liquid absorption core close to the inner wall of the tube is filled with liquid and then sealed.

 

The boiling point of the liquid decreases under negative pressure, and it is easy to volatilize. The tube wall has a liquid-absorbing wick, which is composed of capillary porous materials.

 

Heat pipe material and common working fluid

 

One end of the heat pipe is the evaporating end and the other end is the condensing end.

 

When one section of the heat pipe is heated, the liquid in the capillary evaporates rapidly, and the vapor flows to the other end under a small pressure difference, releases heat, and condenses into liquid again.

 

The liquid flows back to the evaporation section along the porous material by capillary force, and the cycle is endless. The heat is transferred from one end of the heat pipe to the other end. This cycle is carried out rapidly, and the heat can be continuously conducted.

 

Six Associated Processes of Heat Transfer in Heat Pipes

 

1. Heat is transferred from the heat source to the (liquid-vapor) interface through the wall of the heat pipe and the wick filled with working liquid;

 

2. The liquid evaporates on the (liquid-vapor) interface in the evaporation section, and 3. The steam in the steam chamber flows from the evaporation section to the condensation section;

 

4. The steam condenses on the vapor-liquid interface in the condensation section;

 

5. The heat is transferred from the (vapor-liquid) interface to the cold source through the wick, the liquid and the tube wall;

 

6. In the wick, the condensed working liquid is returned to the evaporation section due to capillary action.

 

Internal structure of heat pipe

 

The porous layer on the inner wall of the heat pipe has many forms, the more common ones are: metal powder sintering, groove, metal mesh, etc.

 

1.Hot slag structure

 

 

Literally, the internal structure of this heat pipe is like charred briquettes or hot slag.

 

In the seemingly rough inner wall, there are all kinds of tiny holes, they are like capillaries on the human body, the liquid in the heat pipe will shuttle in these small holes, forming a strong siphon force.

 

In fact, the process of making such a heat pipe is relatively complicated. The copper powder is heated to a certain temperature. Before it is completely melted, the forehead edge of the copper powder particles will first melt and adhere to the surrounding copper powder, thus forming what you see now. to the hollow structure.

 

 

From the picture, you might think it is very soft, but in fact, this hot slag is neither soft nor loose, but very strong.

 

Because it is a substance heated by copper powder at high temperature, after they cool, they restore the original hard texture of the metal.

 

In addition, from the manufacturing point of view, the manufacturing cost of the heat pipe with this process and structure is relatively high.

 

2. Groove structure

 

 

The internal structure of this heat pipe is designed like parallel trenches.

 

It also acts like capillaries, and the returning liquid is quickly conducted in the heat pipe through these grooves.

 

However, according to the precision and fineness of the slot, according to the process level and the direction of the groove, etc., it will have a great impact on the heat dissipation of the heat pipe.

 

From the perspective of production cost, the manufacture of this heat pipe is relatively simple, easier to manufacture, and relatively inexpensive to manufacture.

 

However, the processing technology of the heat pipe groove is more demanding. Generally speaking, it is the best design to follow the direction of the liquid return, so theoretically speaking, the heat dissipation efficiency is not as high as the former.

 

3. Multiple metal meshes

 

More and more common heat pipe radiators use this multi-metal mesh design. From the picture, you can easily see that the flocculent stuff inside the heat pipe is like a broken straw hat.

 

- Generally, the inside of this heat pipe is a metal fabric made of copper wires. There are many gaps between the small copper wires, but the structure of the fabric will not allow the fabric to dislocate and block the heat pipe.

 

From the perspective of cost, the internal structure of this heat pipe is relatively simple, and it is also simpler to manufacture.

 

Only one ordinary copper tube is needed to fill these multi-metal mesh fabrics. In theory, the heat dissipation effect is not as good as the previous two.