In the context of global energy conservation and carbon reduction, and the rapid development of high-power, miniaturized equipment (e.g., electronic chips, aerospace engines), the demand for high-efficiency, passive heat transfer technology is increasingly urgent. Traditional heat transfer methods (e.g., conduction, convection, radiation) have limitations such as low heat transfer efficiency, high energy consumption, and poor temperature uniformity, which are difficult to meet the heat dissipation and heat recovery requirements of modern equipment and industrial processes.
 

 

Heat pipe technology, invented by R.S. Gaugler in 1942 and further developed by George Grover in 1963, is a revolutionary passive heat transfer technology that leverages the phase change of working fluids to achieve efficient heat transfer. A typical heat pipe consists of a sealed tube, a wick structure, and a working fluid, which operates without external power, relying only on the capillary force of the wick and the phase change of the fluid to complete the cyclic heat transfer process. The thermal conductivity of heat pipes can reach 10³ to 10⁴ W/(m·K), which is 10 to 100 times that of copper (401 W/(m·K)) and aluminum (237 W/(m·K)), making it an ideal choice for high-efficiency heat transfer scenarios.

 

The core advantages of heat pipe technology include: passive operation (no external power consumption), high heat transfer efficiency, excellent temperature uniformity, compact structure, light weight, and strong adaptability to extreme environments (e.g., high temperature, low temperature, vacuum). These advantages enable heat pipe technology to be widely used in aerospace, electronics, energy, metallurgy, and other fields, playing an irreplaceable role in thermal control, waste heat recovery, and energy utilization.

 

However, the practical application of heat pipe technology still faces many challenges: the performance of heat pipes is highly dependent on the working fluid, wick structure, and operating conditions; the manufacturing process of high-performance heat pipes (e.g., micro heat pipes, high-temperature heat pipes) is complex and costly; the reliability and service life of heat pipes in harsh environments (e.g., high corrosion, high vibration) need to be further improved. To solve these problems, it is necessary to deeply understand the passive heat transfer principles of heat pipes, clarify the key factors affecting their performance, and explore targeted application solutions.

 

This paper focuses on the core of heat pipe technology, systematically elaborates on its structure, classification, and passive heat transfer principles, analyzes the key factors affecting heat transfer performance, explores typical applications in various fields, and discusses future development trends, aiming to provide professional technical support for the research, development, and application of heat pipe technology.

 

 

 

The structure of a heat pipe is relatively simple but highly integrated, and its performance is closely related to the design of each component. According to the structural characteristics, working fluid, and application scenarios, heat pipes can be classified into different types, each with distinct characteristics and applicable fields.

 

 

A typical heat pipe consists of three core components: a sealed container (tube shell), a wick structure, and a working fluid. These components work together to complete the passive heat transfer cycle, and each component plays a crucial role in the heat transfer performance of the heat pipe:

 

- Sealed Container (Tube Shell): The shell is the outer structure of the heat pipe, which is usually made of materials with good thermal conductivity, pressure-bearing capacity, and compatibility with the working fluid, such as copper, aluminum, stainless steel, nickel-based alloys, and ceramics. The shell must be hermetically sealed to maintain the vacuum environment inside the heat pipe and prevent the leakage of the working fluid. The shape of the shell can be cylindrical, flat, rectangular, or micro-scale, depending on the application scenario (e.g., cylindrical heat pipes for industrial waste heat recovery, flat heat pipes for electronic device cooling).

 

- Wick Structure: The wick is a porous structure attached to the inner wall of the shell, which is the core component that provides capillary force to drive the working fluid circulation. The wick must have good capillary performance, high thermal conductivity, and large porosity to ensure the smooth flow of the condensed working fluid. Common wick structures include screen wicks (wire mesh), grooved wicks (axial grooves, spiral grooves), sintered wicks (sintered metal powder), and composite wicks (combination of grooved and sintered structures). Each wick structure has its own advantages: screen wicks are easy to manufacture and low-cost; grooved wicks have high capillary force and low flow resistance; sintered wicks have high porosity and excellent heat transfer performance.

 

- Working Fluid: The working fluid is the medium that completes heat transfer through phase change (evaporation and condensation), and its selection is closely related to the operating temperature range of the heat pipe. The working fluid must have appropriate boiling point, high latent heat of vaporization, good thermal conductivity, low viscosity, and compatibility with the shell and wick materials. Common working fluids include water (for medium temperature: 20℃ to 150℃), ethanol (for low temperature: -50℃ to 80℃), methanol (for low temperature: -60℃ to 100℃), mercury (for high temperature: 200℃ to 600℃), and molten salts (for ultra-high temperature: 600℃ to 1200℃).

 

In addition to the three core components, some heat pipes are equipped with additional structures to improve performance, such as heat fins (to expand the heat transfer area), insulation layers (to reduce heat loss), and non-condensable gas (NCG) traps (to remove non-condensable gases generated during operation, which can affect heat transfer efficiency).

 

 

Heat pipes can be classified into different types according to various criteria, including operating temperature, wick structure, working fluid, and application scenarios. The main classification methods are as follows:

 

- Classification by Operating Temperature: According to the operating temperature range, heat pipes are divided into low-temperature heat pipes (-273℃ to 0℃), medium-temperature heat pipes (0℃ to 300℃), high-temperature heat pipes (300℃ to 1000℃), and ultra-high-temperature heat pipes (>1000℃). Low-temperature heat pipes are mainly used for cryogenic equipment (e.g., liquid nitrogen storage, space cryogenic thermal control) with working fluids such as liquid helium and liquid nitrogen; medium-temperature heat pipes are the most widely used, with working fluids such as water, ethanol, and methanol, suitable for electronic cooling, waste heat recovery, and building energy conservation; high-temperature heat pipes use working fluids such as mercury, sodium, and potassium, suitable for aerospace engines, industrial high-temperature waste heat recovery; ultra-high-temperature heat pipes use molten salts or refractory metals as working fluids, suitable for nuclear reactors and high-temperature furnaces.

 

- Classification by Wick Structure: Based on the wick structure, heat pipes are divided into screen-wick heat pipes, grooved heat pipes, sintered-wick heat pipes, composite-wick heat pipes, and capillary-pumped loop (CPL) heat pipes. Sintered-wick heat pipes have the best heat transfer performance and are widely used in high-power electronic cooling and aerospace thermal control; grooved heat pipes have low flow resistance and are suitable for large-scale industrial waste heat recovery; CPL heat pipes have long-distance heat transfer capacity and are used for space station thermal control.

 

- Classification by Working Fluid: Heat pipes can be divided into water heat pipes, organic heat pipes (ethanol, methanol), metal heat pipes (mercury, sodium), and molten salt heat pipes. Water heat pipes are the most common, with high latent heat of vaporization and low cost, suitable for medium-temperature scenarios; metal heat pipes are used for high-temperature scenarios due to their high boiling points; organic heat pipes are suitable for low-temperature scenarios.

 

- Classification by Application Scenarios: According to the application fields, heat pipes are divided into aerospace heat pipes (e.g., satellite thermal control, aerospace engine cooling), electronic heat pipes (e.g., CPU cooling, power module cooling), industrial heat pipes (e.g., waste heat recovery, boiler heat exchange), solar heat pipes (e.g., solar water heaters, solar power generation), and building heat pipes (e.g., floor heating, wall heat transfer).

 

 

 

The core advantage of heat pipe technology lies in its high-efficiency passive heat transfer mechanism, which combines phase change heat transfer and capillary action to achieve efficient heat transfer without external power. The entire heat transfer process of a heat pipe consists of four consecutive stages: heat absorption and evaporation, vapor flow, heat release and condensation, and liquid reflux. These stages form a closed cycle, ensuring continuous and efficient heat transfer.

 

 

The passive heat transfer cycle of a heat pipe can be divided into four key stages, which are closely linked and form a closed loop:

 

1. Heat Absorption and Evaporation (Evaporator Section): The evaporator section (heating section) of the heat pipe is in contact with the heat source. When heat is transferred from the heat source to the evaporator section, the working fluid in the wick absorbs heat and evaporates into vapor. The latent heat of vaporization of the working fluid is large, so a small amount of fluid evaporation can absorb a large amount of heat, achieving efficient heat absorption.

 

2. Vapor Flow (Adiabatic Section): The vapor generated in the evaporator section has a higher pressure than the condenser section (due to the temperature difference), so it flows from the evaporator section to the condenser section through the adiabatic section (insulated section). The adiabatic section is designed to reduce heat loss during vapor flow, ensuring that most of the heat is transferred to the condenser section.

 

3. Heat Release and Condensation (Condenser Section): The condenser section (cooling section) of the heat pipe is in contact with the cold source. When the high-temperature vapor flows into the condenser section, it releases heat to the cold source and condenses into liquid. The latent heat of condensation released by the vapor is transferred to the cold source, completing the heat transfer process.

 

4. Liquid Reflux (Wick Capillary Action): The condensed liquid in the condenser section is driven by the capillary force of the wick structure to flow back to the evaporator section, completing the cycle. The capillary force is generated by the surface tension of the liquid and the porous structure of the wick, which is the core driving force for the passive operation of the heat pipe, eliminating the need for external pumps or fans.

 

The entire cycle is continuous and passive, with no moving parts, ensuring high reliability and long service life of the heat pipe. The heat transfer efficiency of the heat pipe is mainly determined by the phase change heat transfer efficiency, capillary force, and fluid flow resistance.

 

 

The high-efficiency passive heat transfer of heat pipes relies on two core mechanisms: phase change heat transfer and capillary action. These two mechanisms work together to ensure the efficient operation of the heat pipe:

 

- Phase Change Heat Transfer Mechanism: Phase change (evaporation and condensation) is the core of heat pipe heat transfer, as it can transfer a large amount of heat with a small temperature difference. The latent heat of vaporization of the working fluid is much larger than the sensible heat (heat absorbed or released by temperature change), so the phase change process can achieve higher heat transfer density. For example, the latent heat of vaporization of water is 2260 kJ/kg, which means that 1 kg of water can absorb 2260 kJ of heat when evaporating at 100℃, which is equivalent to the heat absorbed by 5.4 kg of copper when its temperature increases by 100℃. The phase change heat transfer efficiency is affected by the working fluid, the surface condition of the wick (e.g., wettability), and the operating temperature.

 

- Capillary Action Mechanism: Capillary action is the driving force for the liquid reflux of the heat pipe, which is generated by the surface tension of the working fluid and the contact angle between the fluid and the wick material. The wick structure is a porous medium, and the surface tension of the liquid causes the liquid to rise in the porous channels (capillary rise). The capillary force can be quantified by the Young-Laplace equation: $$P_c = \frac{2\sigma \cos\theta}{r}$$, where $$P_c$$ is the capillary pressure,$$\sigma$$ is the surface tension of the working fluid,$$\theta$$ is the contact angle between the fluid and the wick, and $$r$$ is the radius of the capillary channel. A smaller capillary radius, larger surface tension, and smaller contact angle (good wettability) can generate larger capillary force, ensuring smooth liquid reflux.

 

 

The heat transfer performance of heat pipes is affected by many factors, including the working fluid, wick structure, operating temperature, heat pipe size, and non-condensable gases. Understanding these factors is crucial for optimizing the design and application of heat pipes:

 

- Working Fluid Selection: The working fluid directly determines the operating temperature range and heat transfer efficiency of the heat pipe. The ideal working fluid should have high latent heat of vaporization, good thermal conductivity, low viscosity, appropriate boiling point, and compatibility with the shell and wick materials. For example, water is suitable for medium-temperature scenarios but cannot be used below 0℃ (freezing point), while ethanol is suitable for low-temperature scenarios due to its low freezing point (-114℃).

 

- Wick Structure Design: The wick structure affects the capillary force and liquid flow resistance. A good wick structure should have high porosity, small capillary radius, good wettability, and high thermal conductivity. Sintered wicks have better performance than screen or grooved wicks but are more expensive to manufacture. The thickness and pore size of the wick also affect the heat transfer performance: a thicker wick increases capillary force but increases flow resistance, while a smaller pore size increases capillary force but reduces flow rate.

 

- Operating Temperature: The operating temperature affects the physical properties of the working fluid (e.g., surface tension, viscosity, latent heat of vaporization) and the thermal conductivity of the heat pipe. The heat transfer efficiency of the heat pipe is the highest near the boiling point of the working fluid; when the temperature is too high or too low, the performance of the working fluid degrades, leading to a decrease in heat transfer efficiency.

 

- Non-Condensable Gases (NCGs): NCGs (e.g., air, carbon dioxide) generated during the manufacturing process or operation of the heat pipe will accumulate in the condenser section, blocking the condensation of vapor and reducing the heat transfer area, thereby significantly reducing the heat transfer efficiency of the heat pipe. Therefore, heat pipes must be manufactured in a vacuum environment, and NCG traps are often installed to remove accumulated NCGs.

 

- Heat Pipe Size and Shape: The length, diameter, and shape of the heat pipe affect the vapor flow resistance and liquid reflux efficiency. Longer heat pipes have larger vapor flow resistance, which may lead to dry-out (insufficient liquid reflux) in the evaporator section; larger diameter heat pipes can increase the heat transfer capacity but are heavier and bulkier. Flat heat pipes are suitable for narrow spaces (e.g., electronic devices), while cylindrical heat pipes are suitable for large-scale heat transfer scenarios.

 

 

 

With its high-efficiency passive heat transfer advantages, heat pipe technology has been widely applied in various fields, from aerospace to daily life, covering thermal control, waste heat recovery, energy utilization, and other aspects. The application of heat pipe technology can significantly improve heat transfer efficiency, reduce energy consumption, and enhance the reliability of equipment.

 

 

The aerospace field has strict requirements for thermal control systems, such as light weight, compact structure, passive operation, and strong adaptability to extreme environments (vacuum, high temperature, low temperature). Heat pipe technology is an ideal choice for aerospace thermal control, and its main applications include:

 

- Satellite and Space Station Thermal Control: Satellites and space stations operate in a vacuum environment, with large temperature differences between the sunlit side and the shadow side (up to 200℃). Heat pipes are used to transfer heat from the high-temperature area (sunlit side) to the low-temperature area (shadow side) or to the radiator, ensuring that the temperature of the satellite payload (e.g., sensors, electronic equipment) is within the normal operating range. For example, the International Space Station (ISS) uses a large number of heat pipes and heat pipe radiators to control the temperature of the cabin and equipment, ensuring long-term stable operation.

 

- Aerospace Engine Cooling: Aerospace engines operate at extremely high temperatures (up to 1500℃), and the cooling of key components (e.g., turbine blades, combustion chambers) is crucial for engine safety. High-temperature heat pipes (using sodium, potassium as working fluids) are embedded in the turbine blades to transfer heat from the high-temperature surface to the cooling channel, reducing the temperature of the blades and extending their service life. For example, some advanced aircraft engines use heat pipe cooling technology to improve the turbine inlet temperature and engine efficiency.

 

 

With the miniaturization and high-power density of electronic devices (e.g., CPUs, GPUs, power modules), the heat dissipation problem has become a key bottleneck restricting their performance and service life. Traditional cooling methods (e.g., heat sinks, fans) have limited heat dissipation capacity, while heat pipe technology can achieve efficient passive cooling with compact structure and low noise. Main applications include:

 

- Computer and Server Cooling: Heat pipes are widely used in CPU and GPU cooling of desktop computers, laptops, and servers. A typical CPU cooler consists of heat pipes, a heat sink, and a fan: the heat pipes absorb heat from the CPU, transfer it to the heat sink, and the fan blows air to dissipate the heat. Compared with traditional heat sinks, heat pipe coolers have higher heat dissipation efficiency and lower noise, which can effectively reduce the temperature of the CPU and improve the stability of the computer.

 

- Power Electronic Device Cooling: High-power electronic devices (e.g., IGBT modules, rectifiers) generate a large amount of heat during operation, requiring efficient cooling to ensure their reliability. Heat pipes are used to transfer heat from the electronic devices to the radiator, achieving passive or semi-passive cooling. For example, in new energy vehicles, heat pipes are used to cool the power battery and motor controller, improving the safety and service life of the vehicle.

 

 

Industrial waste heat is a massive underutilized secondary energy source, and heat pipe technology is an efficient way to recover low-grade and medium-grade waste heat (50℃ to 300℃) due to its high heat transfer efficiency and passive operation. Main applications include:

 

- Flue Gas Waste Heat Recovery: Heat pipe heat exchangers are used to recover the waste heat of low-temperature flue gas (100℃ to 200℃) from industrial boilers, furnaces, and kilns. The heat pipe heat exchanger has high heat transfer efficiency, small volume, and low resistance, which can recover the waste heat of flue gas to heat combustion air, feed water, or domestic hot water, reducing fuel consumption and carbon emissions. For example, a steel plant uses heat pipe heat exchangers to recover the waste heat of blast furnace flue gas (200℃ to 300℃) for heating combustion air, reducing fuel consumption by 10% to 15%.

 

- Waste Water Waste Heat Recovery: Heat pipes are used to recover the waste heat of high-temperature waste water (50℃ to 150℃) from petrochemical, food processing, and metallurgical industries. The waste heat recovered can be used for process heating, domestic hot water supply, or heating, improving energy utilization efficiency. For example, a food processing plant uses heat pipe heat exchangers to recover the waste heat of processing waste water (60℃ to 80℃) for heating production workshops, reducing energy consumption by 20%.

 

 

Solar energy is a clean and renewable energy source, but its utilization is limited by intermittency and low energy density. Heat pipe technology can improve the efficiency of solar energy utilization by enhancing heat transfer. Main applications include:

 

- Solar Water Heaters: Heat pipe solar water heaters use heat pipes to absorb solar energy and transfer it to the water tank. The heat pipe has high heat transfer efficiency and can quickly absorb solar energy even in low light conditions, improving the heating efficiency of the water heater. Compared with traditional solar water heaters, heat pipe solar water heaters have faster heating speed, higher thermal efficiency, and better frost resistance.

 

- Solar Power Generation: In concentrated solar power (CSP) systems, heat pipes are used to transfer the concentrated solar energy to the working fluid (e.g., molten salt), generating high-temperature steam to drive the turbine to generate electricity. Heat pipes can improve the heat transfer efficiency between the solar collector and the working fluid, reducing heat loss and improving the overall efficiency of the CSP system.

 

 

In addition to the above fields, heat pipe technology is also widely used in building energy conservation, medical equipment, and cryogenic engineering:

 

- Building Energy Conservation: Heat pipes are used in building wall systems, floor heating, and ventilation systems to improve heat transfer efficiency and reduce energy consumption. For example, heat pipe wall systems can transfer heat from the outside to the inside in winter and from the inside to the outside in summer, reducing the energy consumption of air conditioning and heating.

 

- Medical Equipment: Heat pipes are used in medical equipment (e.g., MRI machines, laser equipment) to cool high-power components, ensuring the stability and accuracy of the equipment. For example, MRI machines generate a large amount of heat during operation, and heat pipes are used to dissipate the heat, ensuring the normal operation of the machine.

 

- Cryogenic Engineering: Low-temperature heat pipes are used in cryogenic equipment (e.g., liquid nitrogen storage tanks, cryogenic refrigerators) to transfer heat and maintain the low-temperature environment. The working fluid of low-temperature heat pipes (e.g., liquid helium, liquid nitrogen) has low boiling points, which can achieve efficient heat transfer in cryogenic environments.

 

 

 

 

Although heat pipe technology has significant advantages and wide applications, it still faces many challenges in practical application, which restrict its further development and large-scale promotion:

 

- High Manufacturing Cost: The manufacturing process of high-performance heat pipes (e.g., sintered-wick heat pipes, high-temperature heat pipes) is complex, requiring strict vacuum sealing, wick sintering, and working fluid filling processes, resulting in high manufacturing costs. This limits the application of heat pipe technology in low-cost scenarios.

 

- Performance Limitations in Extreme Environments: In harsh environments (e.g., high corrosion, high vibration, ultra-high temperature), the reliability and service life of heat pipes are affected. For example, in corrosive environments, the shell and wick materials are easily corroded, leading to fluid leakage; in high vibration environments, the wick structure may be damaged, affecting capillary action.

 

- Limited Heat Transfer Capacity of Micro Heat Pipes: With the miniaturization of electronic devices, micro heat pipes (diameter < 1mm) are increasingly used, but their heat transfer capacity is limited due to the small size of the wick and the high flow resistance of the working fluid. Improving the heat transfer capacity of micro heat pipes is a key technical bottleneck.

 

- Compatibility of Working Fluid and Materials: The working fluid must be compatible with the shell and wick materials to avoid chemical reactions that generate non-condensable gases or corrode the components. However, for high-temperature and corrosive scenarios, it is difficult to find suitable working fluid and material combinations.

 

 

With the continuous advancement of material science, manufacturing technology, and energy conservation and carbon reduction goals, heat pipe technology will develop towards high efficiency, miniaturization, high temperature, and intelligence, and the following trends will become increasingly prominent:

 

- High-Efficiency and Low-Cost Manufacturing Technology: Develop new manufacturing processes (e.g., 3D printing, automatic sintering) to reduce the manufacturing cost of heat pipes and improve production efficiency. For example, 3D printing technology can fabricate complex wick structures and heat pipe shapes, improving heat transfer performance and reducing manufacturing costs.

 

- Miniaturization and Integration: Develop micro heat pipes and micro-channel heat pipes with higher heat transfer capacity to meet the cooling needs of miniaturized, high-power electronic devices. Integrate heat pipes with other components (e.g., heat sinks, chips) to form integrated thermal management systems, reducing volume and improving heat transfer efficiency.

 

- High-Temperature and Corrosion-Resistant Heat Pipes: Develop new materials (e.g., ceramic matrix composites, high-temperature alloys) and working fluids (e.g., molten salts, refractory metals) to improve the high-temperature resistance and corrosion resistance of heat pipes, expanding their application in harsh environments (e.g., nuclear reactors, high-temperature industrial furnaces).

 

- Intelligent Thermal Management Systems: Integrate heat pipe technology with sensors, Internet of Things (IoT), and artificial intelligence (AI) technologies to form intelligent thermal management systems. Real-time monitor the heat transfer performance of heat pipes, adjust the operating parameters according to the heat load changes, and predict potential failures, improving the reliability and efficiency of the system.

 

- Integration with Renewable Energy and Carbon Reduction Technologies: Combine heat pipe technology with renewable energy (e.g., solar energy, wind energy) and carbon capture technologies to improve energy utilization efficiency and reduce carbon emissions. For example, use heat pipes to recover waste heat from renewable energy power generation systems, or use heat pipes in carbon capture equipment to improve the efficiency of CO₂ capture.

 

 

 

Heat pipe technology, as a high-efficiency passive heat transfer technology, relies on the phase change of working fluids and the capillary action of wick structures to achieve efficient heat transfer without external power, with significant advantages such as high thermal conductivity, uniform temperature distribution, compact structure, and long service life. This paper systematically elaborates on the basic structure and classification of heat pipes, deeply analyzes the passive heat transfer principles (phase change heat transfer and capillary action), and explores the key factors affecting heat transfer performance.

 

Heat pipe technology has been widely applied in aerospace, electronic devices, industrial waste heat recovery, solar energy utilization, and other fields, playing an important role in thermal control, energy conservation, and carbon reduction. However, it still faces challenges such as high manufacturing cost, performance limitations in extreme environments, and limited heat transfer capacity of micro heat pipes. With the continuous development of material science and manufacturing technology, heat pipe technology will develop towards high efficiency, miniaturization, high temperature, and intelligence, and its application scope will be further expanded.

 

For relevant practitioners, it is necessary to deeply understand the heat transfer principles of heat pipes, optimize the design of heat pipe components (working fluid, wick structure, shell), and develop targeted application solutions according to different scenarios. At the same time, strengthen the research and development of new materials and manufacturing processes, reduce manufacturing costs, and improve the reliability and performance of heat pipes. In the future, heat pipe technology will continue to play an important role in promoting energy conservation and carbon reduction, and making important contributions to the sustainable development of global energy and the environment.