Heat pipe

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Heat pipes transfer heat through the phase change latent heat of the working medium inside a sealed vacuum tube shell, with their thermal transfer performance resembling that of superconductors in electrical conductivity. They are characterized by high thermal transfer capacity and efficiency.

The gravity heat pipe is first evacuated and filled with a small amount of working fluid inside the sealed tube. At the lower end of the heat pipe, the working fluid absorbs heat and vaporizes into steam. Under a slight pressure difference, the steam rises to the upper end of the heat pipe, releasing heat to the surroundings and condensing into a liquid. The condensed liquid, under the force of gravity, returns to the heated section along the inner wall of the heat pipe, where it is reheated and vaporizes again. This cycle repeats continuously, transferring heat from one end to the other without interruption.

The working principle of heat pipes

In the evaporation section of the heated tube, the working fluid inside the tube core is heated and evaporates, carrying away heat. This heat is the latent heat of vaporization of the working fluid. The steam flows from the central channel to the condensation section of the heat pipe, condensing into a liquid while releasing latent heat. Under the action of capillary forces, the liquid returns to the evaporation section. Thus, a closed loop is completed, transferring a large amount of heat from the heating section to the cooling section.

When the heating section is below and the cooling section is above, with the heat pipe vertically positioned, the return of the working fluid is sufficiently provided by gravity, eliminating the need for a capillary structure in the core. Such a heat pipe without a porous core is called a thermosyphon. The thermosyphon structure is simple and widely used in engineering applications.

Basic Function of Heat Pipes

Typical heat pipes are composed of a shell, a wick, and end caps, and the interior of the tube is evacuated.1.3×(10^-1 - 10^-4) Pa vacuum is applied, then filled with an appropriate working fluid. After the capillary porous material in contact with the inner wall of the tube is fully filled with fluid, it is sealed. One end of the tube is the evaporation section (heating section), and the other end is the condensation section (cooling section). An insulation section can be placed between the two sections as needed for the application. When one end of the heat pipe is heated, the fluid in the wick evaporates and vaporizes, flowing to the other end under a small pressure difference to release heat and condense into a liquid. The liquid then flows back to the evaporation section along the porous material due to capillary force.

Such cycles continue, with heat being transferred from one end of the heat pipe to the other.—End. In the process of achieving this heat transfer, the heat pipe involves the following six interrelated main processes:

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

（2) The liquid evaporates at the (liquid-vapor) interface within the evaporation section.

（3) Steam within the steam chamber flows from the evaporation section to the condensation section.

（4) Steam condenses at the vapor-liquid interface within the condensation section:

（5) Heat is transferred from the (vapor-liquid) interface to the cold source through the absorbent core, liquid, and tube wall.

（6) Due to capillary action, the condensed working fluid returns to the evaporation section within the liquid core.

Heat Pipe Classification

Due to the wide range of applications, types, and designs of heat pipes, along with their differences in structure, materials, and working fluids, there are many ways to classify heat pipes. Common classification methods include the following.

1. Temperature Variations

Heat pipes can be categorized according to the working temperature inside the tube, including low-temperature heat pipes (-273℃ - 0°C, room temperature heat pipes (0-250°C), medium temperature heat pipes (250-450°C), high temperature heat pipes (450-1000°C), etc.

2. Recirculation Power Classification

Heat pipes can be classified according to the motive power of the working fluid's return, including core heat pipes, two-phase closed-loop thermosyphon pipes (also known as gravity heat pipes), gravity-assisted heat pipes, rotating heat pipes, electrohydrodynamic heat pipes, magnetohydrodynamic heat pipes, osmotic heat pipes, and more.

3. Method of Combination

By dividing according to the combination of the casing and the working fluid, (which is a conventional method of classification), it can be categorized into copper—Water-heated pipes, carbon steel—Water-heated pipes, copper steel composite—Water-heated pipes, aluminum—Acetone heat pipes, carbon steel·Rong heat pipes, stainless steel·Sodium heat pipes, etc.

4. Structural Form Variations

By structural form, they can be divided into regular heat pipes, split heat pipes, capillary pump loop heat pipes, micro heat pipes, flat heat pipes, radial heat pipes, and more.

5. Function Classification

Heat pipes are categorized by their functions, including heat transfer heat pipes, thermal diodes, thermal switches, thermal control heat pipes, simulation heat pipes, refrigeration heat pipes, and so on.

Basic Characteristics of Heat Pipes

Thermal conductivity

The heat pipe primarily relies on the phase change of the working fluid between vapor and liquid to transfer heat, resulting in a very low thermal resistance and high thermal conductivity. Compared to metals like silver, copper, and aluminum, a heat pipe of the same weight can transfer several orders of magnitude more heat. Of course, high thermal conductivity is relative, and temperature differences always exist; it is impossible to violate thermodynamic laws. The heat transfer capability of the heat pipe is limited by various factors, and there are certain heat transfer limits. The axial thermal conductivity of the heat pipe is strong, but there is no significant improvement in radial conductivity (except for radial heat pipes). [1]

Isothermal properties

The steam inside the heat pipe is in a saturated state. The pressure of the saturated steam depends on the saturation temperature. The pressure drop generated as the saturated steam flows from the evaporation section to the condensation section is very small. According to the equations in thermodynamics, the temperature drop is also very small, thus the heat pipe possesses excellent isothermal properties.

3. Variability

Heat pipes can independently alter the heating area of the evaporation section or the cooling section, meaning they can input heat with a smaller heating area and output heat with a larger cooling area, or vice versa, with a larger heat transfer area inputting heat and a smaller cooling area outputting heat. This allows for the adjustment of heat flux density and can address heat transfer challenges that are difficult to solve with other methods.

4. Reversibility

A horizontally placed heat pipe with a core, as its internal circulation is driven by capillary force, either end can act as the evaporation section when heated, while the other end dissipates heat outward to become the condensation section. This feature is applicable for temperature equalization in spacecraft and artificial satellites in space, as well as for heat exchangers and other devices that first release heat and then absorb it. [1]

5. Switching Performance

Thermosiphons can be made into thermal diodes or thermal switches. A thermal diode allows heat to flow in only one direction, not the opposite; a thermal switch starts working when the heat source temperature exceeds a certain threshold, and stops conducting heat when the temperature falls below that threshold. [1]

6. Constant Temperature Characteristics

The thermal resistance of the various parts of a standard heat pipe generally does not change with the amount of heat applied, so when the heat amount varies, the temperatures of the different parts of the heat pipe also change accordingly. However, people have developed another type of heat pipe...——Variable thermal conductivity tube, which reduces the thermal resistance of the condensation section as the heating amount increases and increases it as the heating amount decreases. This allows the steam temperature to change minimally under conditions of significant heating amount fluctuations, achieving temperature control. This is the isothermal characteristic of the heat pipe. [1]

7. Environmental Adaptability

The shape of heat pipes can vary according to the conditions of the heat source and the cold source. They can be made into motor shafts, gas turbine blades, drill bits, surgical knives, and more. Heat pipes can also be designed as separate units to accommodate long-distance or situations where hot fluids cannot be mixed for heat exchange. They can be used both on the ground (within the gravitational field) and in space (in the absence of gravity field).