What is the Conduit Theory?

The heat pipe is generally a hollow cylindrical pipe. When a temperature difference occurs between the two ends of the heat pipe, the liquid at the evaporation end will quickly vaporize, bringing the heat to the condensation end, and the speed is very fast.

Chinese name: Heat pipe

Foreign name: heat pipe
Function: Thermal conductivity

History of Heat Pipe Development

The heat pipe technology is a heat transfer element called "heat pipe" invented by GMGrover of LosAlamos National Laboratory in the United States in 1963. It makes full use of the principle of heat conduction and the fast heat transfer properties of the cooling medium. The heat of the heating object is transmitted through the heat pipe. Quickly transferred outside the heat source, its thermal conductivity exceeds the thermal conductivity of any known metal. Heat pipe technology has been widely used in aerospace, military and other industries before. Since its introduction into the radiator manufacturing industry, people have changed the design of traditional radiators and got rid of the single cooling mode that relies solely on high air volume motors to obtain better heat dissipation. The use of heat pipe technology enables the radiator to achieve satisfactory results even with a low-speed, low-air-volume motor, which solves the problem of air-cooled noise, which opens up a new world of heat dissipation industry. ^[1]

Heat pipe heat pipe

(Or heat pipe) is a special material with fast temperature uniformity. The hollow metal tube body makes it lightweight, and its fast temperature uniformity makes it excellent in thermal superconductivity. ; The application range of heat pipes is quite wide. The earliest application was in the aerospace field. It has been widely used in various types of heat exchangers, coolers, natural geothermal references, etc., and plays the role of rapid heat conduction. It is also a heat sink for electronic products today. The most common and efficient thermal (non-thermal) component in the world. ^[2]

How heat pipes work

The heat pipe is basically a closed cavity containing a working fluid. The liquid-vapor two-phase change of the continuous circulation of the working fluid through the working fluid in the cavity, and the vapor & liquid fluid returns to the liquid back and forth between the heat absorbing and exothermic ends. Convection, so that the surface of the cavity shows the characteristics of rapid and uniform temperature and achieves the purpose of heat transfer;

Its action mechanism is that the liquid-phase working fluid evaporates into a vapor phase at the heat-absorbing end. At this moment, a local high pressure is generated in the cavity, which drives the vapor-phase working fluid to flow to the heat-releasing end at high speed. After the end is condensed into a liquid phase, it is returned to the endothermic end by gravity / capillary force / centrifugal force ... It can be seen that when the heat pipe is activated, the airflow is driven by the pressure difference of the air pressure, and the liquid flow must adopt or design a suitable backflow driving force according to the operating state during use.

When the heat pipe is ideally operated, the working fluid is in a state where the liquid and vapor coexist, and there is no temperature difference between the two phases, that is, the entire cavity is at a uniform temperature. At this time, although there is thermal energy in and out of this cavity system, However, the endothermic and exothermic ends are isothermal, forming a thermal superconducting phenomenon of isothermal heat transfer.

Heat pipe body structure

The heat pipe must form a closed cavity by the structure of the pipe body. The pipe body must not only have the structural function of bearing the internal and external pressure difference, but also be a medium material for heat transfer into and out of the cavity. Therefore, except for the demonstration heat pipe, glass will be used. The material is used to show the internal action phenomenon, and other practical heat pipes are made of metal.

The small-sized heat pipes used in the electronic heat dissipation industry are mostly made of copper, and aluminum or titanium pipes are also used due to weight considerations.

Non-condensing gas

If there are impurity gases (such as air) in the heat pipe other than the working fluid, these impurity gases are called non-condensable gas because they do not participate in the evaporation-condensation cycle. In addition to the increase in startup temperature, When the heat pipe is actuated, it will be compressed by the vapor-phase actuating fluid to the condensing end, occupying a certain cavity space, resulting in a pipe body that should be uniform temperature. There is a significant temperature difference between the effective actuation section and the non-condensing gas section, and serious Affects its thermal conductivity; these non-condensable gases may come from:

Incomplete vacuum during heat pipe process

Air leaks into the body gap

The cavity is not clean, and it reacts with the working fluid or the tube wall.

Classification of heat pipes

There are different classifications of heat pipes, usually:

Liquid phase reflux method: thermosiphon, capillary ...

According to working temperature:

Operating temperature	Main working fluid
Extremely low temperature (-273 ~ -70 )	Helium, Argon, Krypton, Nitrogen, Methane
Low temperature (-70 ~ 200 )	Freon, ammonia, acetone, methanol, ethanol, heptane, water
Medium temperature (200 ~ 500 )	Naphthalene, Dowtherm, thermox, sulfur, mercury
High temperature (500 ~ 1000 )	Cesium, rubidium, potassium, sodium
Extremely high temperature (> 1000 )	Lithium, calcium, lead, indium, silver

Why do heat pipes have such a good thermal conductivity?

From the perspective of thermodynamics, the endothermic and exothermic objects are relative. Whenever there is a temperature difference, the phenomenon of heat transfer from high temperature to low temperature is bound to occur. From the perspective of three methods of heat transfer: radiation, convection, and conduction, heat conduction is the fastest. The heat pipe uses evaporative cooling to make the temperature difference between the two ends of the heat pipe so that the heat is quickly transmitted. The general heat pipe is composed of a shell, a wick and an end cap. The inside of the heat pipe is pumped into a negative pressure state and filled with an appropriate liquid. This liquid has a low boiling point and is easily volatile. The tube wall has a liquid wick, which is composed of a capillary porous material. One end of the heat pipe is an evaporation end, and the other end is a condensing end. When one end of the heat pipe is heated, the liquid in the capillary tube rapidly evaporates, and the vapor flows to the other end under a slight pressure difference, and releases heat, re-condenses into a liquid, and the liquid is again porous The material flows back to the evaporation end by the action of capillary force, so the cycle is not stopped, and the heat is transferred from one end of the heat pipe to the other end. This cycle is fast and heat can be conducted continuously. ^[1]