With the continuous improvement of laptop performance, the CPU frequency in computers is also rising, and the corresponding heat generated is increasing. However, the narrow space of laptops cannot quickly dissipate the heat generated by the CPU, and continuous high temperatures will seriously affect the stability of computer operation. Micro heat pipes are an emerging technology developed with the development of electronic components. Since Cotter proposed the concept of heat pipes in 1984, the structure of micro heat pipes has evolved from gravity heat pipes, pulsating heat pipes, micro-grooved heat pipes to loop heat pipes that use capillary force generated by the wick in the evaporator to drive the loop. This paper will focus on the new grooved flat micro heat pipes.
1. Research Overview of Heat Pipes in Laptop Cooling
Most of the heat in laptops comes from computer microprocessors. At present, due to the continuous improvement of the integration of electronic components, the number of circuits in computer chips is increasing day by day. Microprocessors not only have requirements on the maximum temperature, but also have higher requirements on the temperature uniformity during computer operation. Therefore, the performance of electronic component heat sinks directly affects the working performance of computers. Initially, the heat generated by microprocessors in laptops was dissipated by small fans, but this method was extremely inefficient and energy-consuming. Therefore, heat dissipation has always been one of the technical bottlenecks in the development of laptops.
Among many heat dissipation technologies, heat pipe technology is an effective heat dissipation technology proposed by IBM. Heat pipes transfer heat through the vapor-liquid phase change of the internal working fluid, with excellent thermal conductivity and isothermal performance. Moreover, heat pipes have low energy consumption and convenient installation, which solves the noise problem caused by traditional air cooling technology. Therefore, it has broad development prospects. However, due to the complexity of the internal heat transfer mechanism of heat pipes, the research on the internal heat transfer limit of heat pipes at home and abroad is still in a black box state. Only in-depth research on its internal mechanism can we better exert its good heat transfer characteristics and better solve the heat dissipation problem of laptops and other electronic devices.
2. Working Principle and Characteristics of Heat Pipes
Traditional heat pipes generally consist of a tube shell, a working fluid for transferring heat, and a vapor channel. According to the internal working characteristics of heat pipes, they can be divided into three parts: the heat absorption section (heating section), the heat dissipation section (cooling section), and the adiabatic section between the evaporation section and the condensation section. When one end of the heat pipe is heated, the liquid temperature rises and evaporates at the liquid surface until the saturated vapor pressure is reached. At this time, heat is transferred to the vapor in the form of latent heat. The vapor flows to the other end under a small pressure difference to release heat and condense into liquid. The liquid then flows back to the evaporation section along the porous material by capillary force. This cycle continues, and heat is transferred from one end of the heat pipe to the other. Heat pipes transfer heat through the phase change of the internal liquid working fluid, and the heat transferred is much more than that transferred through radiation, convection or conduction. That is, its thermal conductivity is much higher than that of common metal materials.
Characteristics of Heat Pipes：
a. Excellent thermal conductivity: The internal thermal resistance of heat pipes is very small, so they have excellent thermal conductivity. Of course, high thermal conductivity is relative. Due to the existence of temperature difference, it is impossible to violate the second law of thermodynamics, and the heat transfer capacity of heat pipes is limited by various factors, and there is a heat transfer limit.
b. Outstanding isothermal performance: The vapor in the inner cavity of the heat pipe is in a saturated state. The pressure of the saturated vapor depends on the saturated temperature. The temperature drop of the saturated vapor when flowing from the evaporation section to the condensation section is very small, so the heat pipe has excellent isothermal performance.
c. Wide temperature adaptation range and adjustable working temperature: When manufacturing heat pipes, by selecting different working liquids and tube shell materials, heat pipes can be adapted to work in the temperature range of -150～1500°C.
3. Heat Transfer Limit of Heat Pipes and Proposal of New Flat Micro Heat Pipes
Although the heat transfer performance of heat pipes is very high, they cannot transfer heat indefinitely. In actual use, there are many factors restricting the working capacity of heat pipes, such as capillary force, sound velocity, vapor viscosity, liquid boiling, etc. The type of heat transfer capacity of a heat pipe depends on the minimum value of each heat transfer limit at a certain working temperature.
With the continuous improvement of laptop CPU frequency, it is impossible to ensure the stable operation of the CPU without adopting special heat dissipation methods. People have conducted a lot of research on CPU heat sinks and developed a variety of CPU heat sink products. In recent years, CPU heat sinks have made great progress in design, structure, use effect and rigor. A new type of flat micro heat pipe proposed in this paper uses groove design instead of flat heat dissipation to increase the heat exchange area and improve heat exchange efficiency. Copper wires are used as the wick structure to increase capillary force. This new type of heat pipe has the following advantages: the sharp corners of the channels formed by copper wires can provide large capillary force; the flow channels for gas and liquid working fluids are separated, which can completely avoid friction at the gas-liquid interface; the copper wires arranged on the substrate can form more channels.
At present, the application of heat pipes in CPUs is not very popular. In China, except for energy institutes and research institutes that conduct research on mesh micro-tube technology, most other manufacturers are engaged in the manufacture of mesh or grooved micro heat pipes, which have the advantages of low cost and easy manufacturing. However, the use of these forms of capillary structures cannot provide large capillary pressure difference, resulting in the inability to improve the performance of micro heat pipes. The use of copper wires to form capillary structures and grooved heat dissipation surfaces can effectively improve the heat transfer and heat dissipation performance of micro heat pipes.
In the future, the development of this product and technology will not only help the upgrading of medical, instrument and other products, but also the related heat pipe products derived from micro heat pipe technology, such as heat pipe heat sinks and heat exchangers, can be widely used in various industries that require precise temperature control and heat sinks.
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Written by
CoolingThermal Engineering TeamCoolingThermal is an automation equipment manufacturer based in Kunshan, China, specializing in heat pipe and vapor chamber production equipment since 2017. Our engineering team designs, builds, and commissions complete production lines covering forming, degassing, welding, testing, and assembly processes. The technical content on this blog is written by the same team that develops the equipment — based on real production experience, not secondary research.
