Heat pipe is a kind of heat transfer element which depends on the phase change of working liquid in itself to realize heat transfer. It has the following basic characteristics.
1. Very high thermal conductivity
The heat pipe mainly relies on the vapor-liquid phase change heat transfer of the working liquid, with small thermal resistance, so it has a high thermal conductivity. Compared with silver, copper, aluminum and other metals, the heat pipe of unit weight can transfer several orders of magnitude more heat. Of course, the high thermal conductivity is also relatively speaking, the temperature difference always exists, which can not violate some laws of thermodynamics, and the heat transfer capacity of the heat pipe is limited by various factors, there are some heat transfer limits; the axial thermal conductivity of the heat pipe is very strong, and the radial heat pipe has not been greatly improved (except the radial heat pipe).
2. Excellent isothermal property
The steam in the inner cavity of the heat pipe is in the saturated state. The pressure of the saturated steam is determined by the saturated temperature. The pressure drop generated by the flow of the saturated steam from the evaporation section to the condensation section is very small. According to the equations in thermodynamics, the temperature drop is also very small, so the heat pipe has excellent isothermal property.
3. Variability of heat flux
The heat pipe can independently change the heating area of the evaporation section or the cooling section, that is to say, it can input heat with a smaller heating area, and output heat with a larger cooling area, or it can input heat with a larger heat transfer area, and output heat with a smaller cooling area, so as to change the heat flow density and solve some heat transfer problems that are difficult to be solved by other methods.
4. Reversibility of heat flow direction
A horizontal heat pipe with a core can be used as an evaporation section when heated at any end and a condensation section when the other end is cooled outwards because the internal circulation force is capillary force. This feature can be used for space temperature flattening of spacecraft and artificial satellite, as well as for chemical reactors and other devices that release heat first and then absorb heat.
5. Thermal diode and thermal switch performance
The heat pipe can be made into a thermal diode or a thermal switch, the so-called thermal diode is to allow the heat flow to only one direction, but not the opposite direction; the thermal switch is when the temperature of the heat source is higher than a certain temperature, the heat pipe starts to work, when the temperature of the heat source is lower than this temperature, the heat pipe will not conduct heat.
6. Constant temperature characteristics (controllable heat pipe)
The thermal resistance of each part of the common heat pipe basically does not change with the change of the heating capacity, so when the heating capacity changes, the temperature of the spare part of the heat pipe also changes. But people have developed another kind of heat pipe, the variable heat pipe, which makes the thermal resistance of the condensation section decrease with the increase of the heating capacity and increase with the decrease of the heating capacity. In this way, when the heating capacity of the heat pipe changes greatly, the temperature of the steam changes very little and the temperature is controlled, which is the constant temperature characteristic of the heat pipe.
7. Adaptability of environment
The shape of the heat pipe can change with the conditions of heat source and cold source. The heat pipe can be made into the rotating shaft of the motor, the blade of the gas turbine, drill, scalpel, etc. the heat pipe can also be made into the separated type to adapt to the heat exchange under the condition of long distance or the hot fluid can not be mixed; the heat pipe can be used for both the ground (gravity field) and the space (no gravity field).