Basic knowledge of heatpipe design
Considerations in heat pipe design
Heat pipes are widely used in current thermal dissipation design, including our common laptops and mobile phones. The following factors need to be considered in the design of heat pipe:
heatpipe Qmax or heat source .
working temperature。
copper material.
working fluid .
Wick Structure.
Length and diameter of heat pipe.
heat contact area.
condenser contact area.
gravity direction.
Influence of heat pipe bending and flatness.
What materials can be used to construct heat pipes?
Heat pipe is mostly metal seamless steel pipe, and different materials can be used according to different needs,such as copper, aluminum, carbon steel, stainless steel, alloy steel, etc. The pipe can be standard round or special-shaped.such as oval, square, rectangular, flat, corrugated pipe, etc. The pipe diameter range from 2mm to 200mm or even larger. The length can range from a few millimeters to more than 100 meters. Copper and aluminum are mostly used as raw materials in most design solutions. Non ferrous metals are used as pipes mainly to meet the requirements of compatibility with working fluid.
What is wick structure? How does it affect the performance of heat pipes?
Groove structure:The capillary limit is the lowest, but the effect is the best when the condenser is located above the evaporator.
Mesh structure: It has the most uniform cotton core,and its working principle is evaporator is located above the condenser.
Sintered structure:The performance is best in the direction of gravity. Because the sintered powder metal core is bonded to the pipe wall through metal, its heat conduction from the pipe wall to the core or vice versa is the best of the four common cores.
How do heat pipe length and diameter affect the performance ?
The steam pressure difference between condenser and evaporator determines the rate of steam propagation between condenser and evaporator. In addition, the diameter and length of heat pipe will affect the steam transmission speed, so it must be considered in the design of heat pipe.
How does the orientation affect the heat pipe performance ?
The structure with high capillary limit can overcome gravity and transfer more working fluid from the condenser to the evaporator. However, as mentioned earlier, the sintered powder metal core heat absorber with the highest capillary limit works best under gravity assisted conditions (the evaporator is above the condenser), see below pictures about the gravity orientation to heatpipe performance.
How does heat pipe bending affect performance?
If the heat pipe is bent too tight, the wick may crack (powder metal sintering) or collapse and be clamped (wire mesh). Therefore, the bending of the heat pipe may reduce the heat that can be transmitted. The experimental results show that if the bending radius is equal to or greater than 3 times of the heat pipe diameter , the bending will not affect the performance obviously.
How does flatteningaffect the performance of heat pipe?
If the heat pipe is flattened, the thickness of the heat pipe will be reduced. Therefore, excessive flattening of the heat pipe will reduce the heat that can be transmitted and even completely block the passage of steam. The experimental results show that proper flattening will not affect the performance, but excessive flattening will affect the performance. If the thickness of the steam channel after flattening is greater than 2mm, the performance will not be reduced compared with the circular pipe.
How does the heat pipe working temperature affect the performance ?
The working temperature of heat pipe will affect the performance of heat pipe. The higher the temperature, the better the performance to a certain extent. This is due to the lower viscosity of the working fluid at higher temperatures, which allows more working fluid to flow from the evaporator to the oil core through the condenser. At higher temperatures, the working fluid can also become more volatile into a gaseous state.
Does heat pipe reliable ?
The heat pipe has no moving parts and has very high reliability. However, must be careful in the design and manufacture of heat pipes. Two manufacturing factors will reduce the reliability of heat pipe: tightness and cleanliness. Any leakage in the heat pipe will eventually cause the heat pipe to fail. Some external factors may also shorten the life of heat pipes, such as drop, vibration, force impact, thermal shock and corrosive environment.
