In industrial production and scientific research, heat transfer is a basic and critical link. With the rapid development of electronic technology, aerospace engineering and new energy industry, the heat flux density of equipment continues to increase, and the requirements for thermal management are becoming more and more stringent. Traditional heat transfer methods (such as heat conduction, natural convection) have the disadvantages of low heat transfer efficiency, limited heat transfer distance and large temperature difference, which are difficult to meet the thermal management needs of high-power, miniaturized and high-precision equipment.

Heat pipe technology, invented by George Grover in 1963, has broken through the limitations of traditional heat transfer methods. As a passive heat transfer device, it relies on the phase change of the internal working fluid and the capillary force of the wick to realize ultra-high efficiency heat transfer, with a thermal conductivity hundreds or even thousands of times higher than that of pure copper. Compared with active heat transfer devices (such as fans, pumps), heat pipes have the advantages of no moving parts, low noise, long service life, compact structure and no external power input; compared with traditional passive heat transfer components (such as heat sinks), they have the characteristics of faster heat transfer speed, longer heat transfer distance and more uniform temperature distribution.

 

At present, heat pipes have been widely used in aerospace, electronic equipment cooling, solar energy utilization, waste heat recovery, industrial waste heat treatment and other fields. With the continuous innovation of material technology and structural design, heat pipe technology is developing towards miniaturization, high power, high temperature resistance and corrosion resistance. For practitioners, a systematic understanding of the working principles, structural characteristics, application scenarios and performance influencing factors of heat pipes is the basis for rational selection, design and application. This paper focuses on the core technology of heat pipes, systematically sorts out the working principles, classification, application scenarios and technical development trends, and combines industry standards and practical engineering experience to provide a comprehensive technical analysis, helping practitioners avoid technical mistakes and improve the efficiency and reliability of thermal management systems.

 

 

 

Heat pipes are typical passive heat transfer devices, whose core is to realize efficient heat transfer through the phase change of working fluid and the capillary action of wick structures. Their structural composition is relatively simple, mainly including three core components: shell, wick and working fluid. The passive heat transfer process is completed through the cyclic phase change (evaporation-condensation) of the working fluid and the capillary-driven circulation, without any external power input.

 

 

The typical structure of a heat pipe consists of three core components and auxiliary components, each of which plays a key role in ensuring the heat transfer performance and reliability of the heat pipe. The specific composition is as follows:

 

- Shell: The outer wall of the heat pipe, which is a closed container used to accommodate the working fluid and wick, and also plays the role of supporting and isolating the external environment. The shell is usually made of high thermal conductivity, high pressure-bearing capacity and corrosion-resistant materials, such as copper, aluminum, stainless steel, titanium alloy and nickel-based alloy. The selection of shell material is determined by the working temperature, working pressure and service environment of the heat pipe. For example, copper shells are widely used in low-temperature and medium-temperature heat pipes (below 200℃) due to their high thermal conductivity; stainless steel or titanium alloy shells are used in high-temperature, high-pressure or corrosive environments.

 

- Wick Structure: The core component that realizes capillary-driven circulation of the working fluid, attached to the inner wall of the shell. The wick is a porous structure with high capillary force, which can absorb the condensed working fluid and transport it back to the evaporation section through capillary action. Common wick structures include screen wick, grooved wick, sintered wick and composite wick. The screen wick has the advantages of simple structure and low cost, suitable for low-heat-flux scenarios; the grooved wick has high capillary force and good heat transfer performance, suitable for medium-heat-flux scenarios; the sintered wick has ultra-high capillary force and excellent heat transfer capacity, suitable for high-heat-flux and miniaturized heat pipes; the composite wick combines the advantages of multiple wick structures, further improving the heat transfer performance and reliability.

 

- Working Fluid: The medium that realizes heat transfer through phase change, filled in the closed shell after vacuum treatment. The working fluid must have good thermal conductivity, low boiling point (matching the working temperature), high latent heat of vaporization, good compatibility with the shell and wick (no chemical reaction, no corrosion), and low viscosity. Common working fluids include water, methanol, ethanol, acetone, liquid metal (such as sodium, potassium, lithium) and refrigerant. Water is the most widely used working fluid for low-temperature and medium-temperature heat pipes (50-200℃) due to its high latent heat of vaporization and low cost; liquid metal is used for high-temperature heat pipes (above 500℃), such as aerospace and high-temperature waste heat recovery scenarios.

 

- Auxiliary Components: Including end caps, vacuum seals and filling ports. The end caps are used to seal the shell and ensure the airtightness of the heat pipe; the vacuum seals are used to prevent air leakage and ensure the vacuum environment inside the heat pipe (the vacuum degree is usually 10^-3 - 10^-5 Pa); the filling port is used to fill the working fluid and evacuate the air inside the shell, which is sealed after the filling is completed.

 

 

The working principle of heat pipes is based on two core physical phenomena: phase change heat transfer (evaporation and condensation) and capillary action. The entire heat transfer process is a closed cycle without external power input, which is divided into four stages: heat absorption and evaporation, vapor flow, heat release and condensation, and capillary-driven liquid return. The four stages are carried out continuously, realizing efficient and passive heat transfer.

 

 

The heat pipe is divided into three sections according to the function: evaporation section (evaporator), adiabatic section (insulator) and condensation section (condenser). When the evaporation section is in contact with the heat source, the heat from the heat source is transferred to the working fluid in the wick through the shell and the wick. When the temperature of the working fluid reaches its boiling point (or saturation temperature under the internal vacuum pressure), the working fluid absorbs a large amount of latent heat and evaporates rapidly, forming high-temperature and high-pressure vapor. The latent heat of vaporization of the working fluid is much larger than the sensible heat, so the evaporation process can absorb a large amount of heat in a short time, realizing efficient heat absorption.

 

 

After the working fluid evaporates into vapor, a pressure difference is formed between the evaporation section and the condensation section (the vapor pressure in the evaporation section is higher than that in the condensation section). Driven by this pressure difference, the high-temperature and high-pressure vapor flows rapidly from the evaporation section to the condensation section through the central channel of the heat pipe. The adiabatic section is used to isolate the external environment, reduce heat loss during the vapor flow process, ensuring that most of the heat is transferred to the condensation section.

 

 

When the high-temperature vapor reaches the condensation section, it contacts the inner wall of the condensation section (which is in contact with the heat sink or cold source). The vapor releases a large amount of latent heat and condenses into liquid working fluid. The released latent heat is transferred to the heat sink or cold source through the shell of the condensation section, realizing heat release. The condensation process is the reverse of the evaporation process, and the latent heat released is equal to the latent heat absorbed during evaporation, ensuring the continuity of the heat transfer cycle.

 

 

The condensed liquid working fluid adheres to the wick in the condensation section. Under the action of the capillary force generated by the porous structure of the wick, the liquid working fluid is absorbed and transported back to the evaporation section against gravity (even in the vertical direction or inverted state). The capillary force is the core driving force for the liquid return, which ensures that the working fluid can circulate continuously between the evaporation section and the condensation section, forming a closed passive heat transfer cycle.

 

 

The performance of heat pipes is evaluated by a variety of indicators, which directly determine their application scope and heat transfer effect. The key performance indicators are as follows:

 

- Thermal Conductivity: The most important performance indicator of heat pipes, which reflects the efficiency of heat transfer. The thermal conductivity of heat pipes is usually 100-1000 times that of pure copper (the thermal conductivity of pure copper is about 401 W/(m·K)), and the thermal conductivity of high-performance heat pipes can reach more than 10,000 W/(m·K).

 

- Heat Transfer Capacity: Also known as the heat load, it refers to the maximum heat that the heat pipe can transfer continuously without failure. It is affected by the working fluid, wick structure, shell material and working temperature, and is usually expressed in watts (W).

 

- Temperature Uniformity: The temperature difference between the evaporation section and the condensation section of the heat pipe is very small (usually less than 1-5℃), which can realize uniform temperature distribution of the heat source, avoid local overheating of the equipment.

 

- Capillary Limit: The maximum heat transfer capacity limited by the capillary force of the wick. When the heat load exceeds the capillary limit, the wick cannot transport the condensed liquid back to the evaporation section in time, resulting in dry-out of the evaporation section and failure of the heat pipe.

 

- Working Temperature Range: The temperature range in which the heat pipe can work normally, which is determined by the boiling point and freezing point of the working fluid. For example, water-based heat pipes work in the range of 50-200℃, and liquid metal heat pipes work in the range of 500-1500℃.

 

 

Heat pipes can be classified into different types according to structural form, working fluid type, working temperature range and application scenario. Each type has its own unique structural characteristics and applicable scope. The detailed classification is as follows:

 

- Classification by Structural Form:

        

 

  - Sintered Heat Pipe: The wick is made of metal powder (such as copper powder) sintered on the inner wall of the shell, with high capillary force and excellent heat transfer performance, suitable for high-heat-flux and miniaturized scenarios (such as CPU cooling, high-power LED cooling).

 

  - Grooved Heat Pipe: The inner wall of the shell is processed with axial or spiral grooves, which form the wick structure. It has the advantages of simple processing, low cost and high heat transfer efficiency, suitable for medium-heat-flux scenarios (such as aerospace equipment, industrial heat exchangers).

 

  - Screen Heat Pipe: The wick is made of metal screen (such as copper screen) wrapped on the inner wall of the shell, with simple structure and low cost, suitable for low-heat-flux and large-size scenarios (such as solar water heaters, waste heat recovery systems).

 

  - Micro Heat Pipe: The inner diameter is less than 1 mm, with compact structure, suitable for miniaturized electronic devices (such as mobile phones, tablets, microprocessors).

 

  - Loop Heat Pipe (LHP): A special type of heat pipe with a separate evaporator and condenser, connected by a vapor line and a liquid line, with long heat transfer distance and high heat transfer capacity, suitable for large-scale thermal management systems (such as satellite thermal control, large-scale data centers).

 

- Classification by Working Fluid Type:

        

 

  - Water-Based Heat Pipe: Working fluid is water, suitable for low-temperature and medium-temperature scenarios (50-200℃), widely used in electronic equipment, solar energy utilization and industrial waste heat recovery.

 

  - Organic Working Fluid Heat Pipe: Working fluid is methanol, ethanol, acetone, etc., suitable for low-temperature scenarios (-50-100℃), such as refrigeration equipment, low-temperature electronic devices.

 

  - Liquid Metal Heat Pipe: Working fluid is liquid metal (sodium, potassium, lithium, etc.), suitable for high-temperature scenarios (500-1500℃), such as aerospace engines, high-temperature furnaces, nuclear reactors.

 

- Classification by Working Temperature Range:

 

  - Low-Temperature Heat Pipe: Working temperature below 100℃, suitable for electronic equipment cooling, refrigeration systems, etc.

 

  - Medium-Temperature Heat Pipe: Working temperature 100-500℃, suitable for solar energy utilization, industrial waste heat recovery, etc.

 

  - High-Temperature Heat Pipe: Working temperature above 500℃, suitable for aerospace, nuclear energy, high-temperature industrial processes, etc.

 

 

 

Heat pipes, with their advantages of ultra-high thermal conductivity, passive heat transfer, compact structure and reliable operation, are widely used in various fields that require efficient thermal management. Their application fields cover aerospace, electronic equipment, solar energy utilization, industrial manufacturing, nuclear energy and other industries. The specific application scenarios are as follows:

 

 

The aerospace field has extremely strict requirements for thermal management, as the equipment operates in extreme environments (high vacuum, large temperature difference, strong radiation), and the weight and volume of the equipment are strictly limited. Heat pipes, as passive heat transfer devices with high efficiency, light weight and no moving parts, have become the core components of aerospace thermal control systems.

 

- Satellite Thermal Control: Satellites operate in space with large temperature differences (the temperature of the sun-facing surface can reach 100℃ above zero, and the temperature of the backlit surface can reach 100℃ below zero). Heat pipes are used to transfer the heat from the sun-facing surface to the backlit surface, or transfer the heat generated by the satellite's internal electronic equipment to the radiator, ensuring that the satellite's internal temperature is within the normal working range. For example, the thermal control system of the International Space Station (ISS) uses a large number of heat pipes to realize temperature control of the cabin and equipment.

 

- Aerospace Engine Cooling: Aerospace engines generate a lot of heat during operation, and the temperature of key components (such as turbine blades, combustion chambers) can reach thousands of degrees Celsius. High-temperature heat pipes (liquid metal heat pipes) are used to transfer the heat of key components to the radiator, reducing the temperature of the components and ensuring their safe operation.

 

- Spacecraft Cabin Temperature Control: Heat pipes are used to transfer the heat generated by the internal equipment of the spacecraft cabin to the external radiator, maintaining a stable temperature inside the cabin, providing a comfortable working environment for astronauts and ensuring the normal operation of electronic equipment.

 

 

With the development of electronic technology, electronic devices (such as CPUs, GPUs, high-power LEDs, 5G base stations) tend to be miniaturized, high-power and integrated, resulting in a sharp increase in heat flux density. The traditional heat sink cooling method can no longer meet the thermal management needs, and heat pipes have become the mainstream cooling solution for high-power electronic devices.

 

- Computer and Server Cooling: The CPU and GPU of computers and servers generate a lot of heat during operation, and the heat flux density can reach 100-500 W/cm². Heat pipes are used to transfer the heat from the CPU/GPU to the heat sink, and then dissipate the heat to the air through the fan, ensuring the stable operation of the computer and server. For example, high-end gaming laptops and data center servers all adopt heat pipe cooling systems.

 

- High-Power LED Cooling: High-power LEDs have high luminous efficiency but low heat conversion efficiency, and 70-80% of the input power is converted into heat. If the heat cannot be dissipated in time, the luminous efficiency and service life of LEDs will be greatly reduced. Heat pipes are used to transfer the heat from the LED chip to the heat sink, realizing efficient heat dissipation, extending the service life of LEDs. This technology is widely used in LED street lights, LED projectors and LED displays.

 

- 5G Base Station Cooling: 5G base stations have high power consumption and generate a lot of heat, and the equipment is usually installed outdoors, which is affected by the external environment. Heat pipes are used to transfer the heat from the internal electronic components of the base station to the external radiator, ensuring that the base station operates at a stable temperature, improving the reliability and service life of the equipment.

 

 

In the field of energy utilization, heat pipes are mainly used in solar energy utilization, waste heat recovery and geothermal energy development, which can improve energy utilization efficiency and reduce energy waste, in line with the concept of energy conservation and emission reduction.

 

- Solar Energy Utilization: Heat pipes are used in solar water heaters, solar collectors and solar power generation systems. In solar water heaters, heat pipes transfer the heat absorbed by the solar collector to the water tank, heating the water; in solar power generation systems, heat pipes transfer the heat from the solar collector to the working fluid of the power generation system, driving the turbine to generate electricity. Heat pipes can improve the heat collection efficiency of solar energy, reduce heat loss, and adapt to different sunlight conditions.

 

- Waste Heat Recovery: A large amount of waste heat is generated in industrial production (such as steel, cement, chemical industry), and the temperature of the waste heat is usually 100-500℃. Heat pipes are used to recover the waste heat, transfer it to the working fluid, and use the waste heat for heating, power generation or other industrial processes, improving energy utilization efficiency and reducing environmental pollution. For example, in steel plants, heat pipes are used to recover the waste heat of flue gas, heating the combustion air, reducing fuel consumption.

 

- Geothermal Energy Development: Heat pipes are used to extract geothermal energy from the ground, transfer the geothermal heat to the surface, and use it for heating, power generation and other purposes. Heat pipes can work in low-temperature geothermal fields (below 100℃), with high heat extraction efficiency and low cost.

 

 

In the industrial manufacturing field, heat pipes are used in industrial heat exchangers, high-temperature furnaces, metal processing and other processes, which can improve the heat transfer efficiency of equipment, reduce energy consumption and improve product quality.

 

- Industrial Heat Exchangers: Heat pipes are used in heat exchangers to transfer heat between two fluids, with high heat transfer efficiency, compact structure and small pressure drop. They are widely used in chemical industry, petroleum industry, power industry and other fields. For example, in the chemical industry, heat pipe heat exchangers are used to recover the heat of process fluid, reduce energy consumption.

 

- High-Temperature Furnaces: High-temperature furnaces (such as ceramic sintering furnaces, metal heat treatment furnaces) require uniform temperature distribution inside the furnace. Heat pipes are used to transfer heat inside the furnace, ensuring uniform temperature distribution, improving the quality of processed products.

 

- Metal Processing: In metal processing processes (such as welding, casting, forging), a large amount of heat is generated, and heat pipes are used to transfer the heat, reducing the temperature of the processing area, avoiding deformation of the workpiece, and improving processing precision.

 

 

In addition to the above fields, heat pipes are also widely used in nuclear energy, medical equipment, automotive industry and other fields:

 

- Nuclear Energy Field: Heat pipes are used in nuclear reactors to transfer the heat generated by nuclear fission to the cooling system, ensuring the safe operation of the reactor. High-temperature liquid metal heat pipes are used in fast neutron reactors, with high heat transfer capacity and strong radiation resistance.

 

- Medical Equipment: Heat pipes are used in medical equipment (such as MRI machines, laser medical equipment) to cool the internal electronic components, ensuring the stable operation of the equipment and improving the accuracy of medical diagnosis and treatment.

 

- Automotive Industry: Heat pipes are used in automotive engines, battery packs (new energy vehicles) to transfer heat, reducing the temperature of the engine and battery pack, improving the reliability and service life of the vehicle. For example, in new energy vehicles, heat pipes are used to cool the battery pack, ensuring that the battery operates at a stable temperature, improving battery performance and safety.

 

 

 

The performance of heat pipes is affected by many factors, including structural design, material selection, working fluid properties and service environment. Understanding these factors is crucial for optimizing the design of heat pipes and improving their performance. At the same time, with the continuous development of related technologies, heat pipe technology is showing new development trends.

 

 

- Wick Structure: The capillary force, porosity and thermal conductivity of the wick directly affect the liquid return capacity and heat transfer efficiency of the heat pipe. Sintered wicks have higher capillary force and better heat transfer performance than screen wicks and grooved wicks, but the processing cost is higher.

 

- Working Fluid Properties: The latent heat of vaporization, boiling point, viscosity and compatibility of the working fluid affect the heat transfer capacity and working temperature range of the heat pipe. Working fluids with high latent heat of vaporization and low viscosity can improve the heat transfer efficiency of heat pipes.

 

- Shell Material: The thermal conductivity, pressure-bearing capacity and corrosion resistance of the shell affect the heat transfer efficiency and service life of the heat pipe. Shell materials with high thermal conductivity (such as copper) can improve the heat transfer efficiency between the heat source and the working fluid.

 

- Vacuum Degree: The vacuum degree inside the heat pipe affects the boiling point of the working fluid and the resistance of vapor flow. A higher vacuum degree can reduce the boiling point of the working fluid, improve the evaporation efficiency, and reduce the vapor flow resistance.

 

- Service Environment: Factors such as temperature, humidity, vibration and radiation in the service environment affect the performance and reliability of heat pipes. For example, in high-temperature environments, the working fluid may decompose, and the shell and wick may be oxidized; in vibration environments, the wick may be damaged, affecting the liquid return capacity.

 

 

With the continuous demand for high-efficiency thermal management in various fields, heat pipe technology is developing towards miniaturization, high power, high temperature resistance, corrosion resistance and intelligence. The main development trends are as follows:

 

- Miniaturization and Integration: With the miniaturization of electronic devices, micro heat pipes (inner diameter less than 1 mm) and integrated heat pipe arrays are developing rapidly. They can be integrated into the structure of electronic devices, realizing efficient heat dissipation without increasing the volume of the equipment.

 

- High-Power and High-Heat-Flux: The heat flux density of high-power electronic devices and aerospace equipment continues to increase, requiring heat pipes to have higher heat transfer capacity. The development of new wick structures (such as composite sintered wicks) and high-performance working fluids (such as high-temperature liquid metals) can improve the heat transfer capacity of heat pipes, meeting the needs of high-heat-flux scenarios.

 

- High-Temperature and Corrosion-Resistant: In high-temperature and corrosive environments (such as aerospace engines, nuclear reactors, chemical industry), the demand for high-temperature and corrosion-resistant heat pipes is increasing. The development of new shell materials (such as ceramic matrix composites, nickel-based alloys) and corrosion-resistant working fluids can improve the high-temperature resistance and corrosion resistance of heat pipes.

 

- Intelligent Thermal Management: Combining heat pipes with sensor technology, control technology and intelligent algorithms, developing intelligent heat pipes that can monitor and adjust heat transfer performance in real time. For example, adding temperature sensors and flow control valves to heat pipes, realizing real-time monitoring of temperature and dynamic adjustment of heat transfer capacity.

 

- New Material and New Structure Innovation: The development of new materials (such as carbon nanotubes, graphene) and new structures (such as loop heat pipes, pulsating heat pipes) has brought new opportunities for the development of heat pipe technology. Carbon nanotube wicks have ultra-high capillary force and thermal conductivity, which can further improve the heat transfer efficiency of heat pipes; pulsating heat pipes have no wick structure, simple structure and high heat transfer capacity, suitable for complex heat transfer scenarios.

 

 

 

Heat pipe technology, as a high-efficiency passive heat transfer technology, relies on the phase change of working fluid and the capillary action of wick structures to realize ultra-high efficiency heat transfer without external power input. It has the characteristics of ultra-high thermal conductivity, uniform temperature distribution, compact structure, low cost and reliable operation, and has become the core component of thermal management systems in various fields.

 

This paper systematically elaborates on the structural composition and core passive heat transfer principles of heat pipes, clarifies the role of each component and the four-stage heat transfer cycle; classifies heat pipes according to structural form, working fluid type and working temperature range, and analyzes their characteristics and applicable scenarios; deeply explores the application scenarios of heat pipes in aerospace, electronic equipment, energy utilization, industrial manufacturing and other fields, and combines practical cases to illustrate their application advantages; finally, summarizes the key factors affecting heat pipe performance and the latest technical development trends, providing comprehensive and professional technical guidance for relevant practitioners.

 

In the actual application process, it is necessary to select the appropriate type of heat pipe according to the working conditions (heat flux density, working temperature, service environment) and performance requirements, optimize the structural design and material selection, and ensure the efficient and reliable operation of heat pipes. In the future, with the continuous innovation of material technology, structural design and intelligent technology, heat pipe technology will develop towards higher efficiency, higher power, smaller size and more intelligence, expanding its application scope to more high-end and complex fields. For practitioners, it is necessary to continuously learn new technologies and new standards, accumulate practical experience, and improve the level of selection, design and application of heat pipes to meet the growing thermal management needs of various industries.