Key considerations for heat pipe design

    The heat pipe is a kind of heat transfer element, which makes full use of the heat conduction principle and the fast heat transfer property of the cooling medium. The heat of the hot object is quickly transferred to the outside of the heat source through the heat pipe, and its thermal conductivity has far exceeded that of any known metal.

    Heat pipes are often used in current heat dissipation design, including our common notebook computers, mobile phones, etc. The following factors should be considered in the design of heat pipe: heat load or heat to be transferred; Operating temperature; Pipe; Working fluid; Capillary structure; Length and diameter of heat pipe; Contact length of evaporation zone; Contact length of compensation area; Direction; The effect of heat pipe bending and flattening, etc.

The following factors shall be considered in the design of heat pipe：

1、 Selection of working fluid

① The working fluid shall adapt to the working temperature zone of the heat pipe and have appropriate saturated vapor pressure;

② The working fluid shall be compatible with the shell and the wick material, and shall have good thermal stability;

③ The working fluid shall have good comprehensive thermophysical properties;

2、 Structure of liquid suction core

The selection of the wick is a complex problem. From the point of view of providing the maximum heat transfer rate, the wick is required to have a very small effective capillary radius r., To provide the maximum capillary pressure, the permeability K value should be large to reduce the pressure loss of the return liquid, and the thermal conductivity resistance should be small to reduce the radial thermal conductivity resistance. It is difficult to make the wick with the same structure meet all the above requirements, so there are composite wick structures and trunk wick, but the manufacturing difficulty and cost are increased. Therefore, when selecting the liquid suction core, attention should be paid to selecting the simplest structure on the basis of meeting the heat transfer requirements. For heat pipes used on the ground, gravity reflux shall be used as much as possible, and thermosyphons without liquid absorption cores shall be used.

3、 Operating temperature

Under the specified design conditions, the temperature of the cold source and heat source is known, and the heat transfer conditions are also clear, so the operating temperature range of the heat pipe itself can be calculated by a general heat transfer formula. The working temperature here generally refers to the steam temperature of the working liquid in the heat pipe during operation. When a good heat pipe works, the working fluid must be in a vapor liquid two-phase state. Because the melting point of the selected working fluid should be lower than the working temperature of the heat pipe, the heat pipe can work normally. Figure 3-59 lists the temperature range of melting point, boiling point and critical point (vertical short line on the line segment) that can be used as the working liquid of heat pipe. It can be seen from the figure that these liquids overlap in some temperature regions, that is, several working liquids can be selected in some temperature regions. It is necessary to consider factors such as saturation pressure, price, thermal stability, nontoxicity, etc. in turn, and compare them to make a choice.

4.   There are  four common capillary Heat pipes structures, including grooves, wire mesh, sintered powder, metal and fiber. The capillary structure is lined on the inner wall of the heat pipe container and allows the liquid to flow from one end of the heat pipe to the other through capillary action. Each capillary structure has its advantages and disadvantages. There is no perfect capillary structure. Each capillary structure has its own limit.

  The heat pipe has no moving parts and has high reliability. However, care must be taken 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. If the internal chamber is not thoroughly cleaned, when the heat pipe is heated, the residue will produce non condensable gas and reduce the performance of the pipe.