Loop heat pipes (LHPs) are passive heat transfer devices that play an important role in spacecraft thermal control systems, as well as the cooling of electronic equipment in aviation and submarine applications. They mainly consist of an evaporator, a reservoir, a vapor line, a liquid line, and a condenser. Unlike conventional heat pipes, LHPs do not have an integral distributed capillary structure inside; instead, their operation relies solely on the capillary structure within the evaporator to maintain the loop system, with gas-liquid two-phase fluid circulating in the loop. After years of development, LHPs have evolved into various types such as capillary pumped loop heat pipes, flat evaporator type loop heat pipes, and thin-type loop heat pipes.
1. Conventional Loop Heat Pipes
In a conventional LHP, when a heat load is applied to the evaporator, the liquid working fluid evaporates on the outer surface of the fully wetted capillary core in the evaporator. The generated vapor flows out from the vapor channels into the vapor line, and then enters the condenser where it condenses into a liquid and becomes subcooled. The reflux liquid replenishes the capillary core of the evaporator through the liquid line, forming a continuous cycle. The circulation of the working fluid is driven by the capillary pressure generated by the capillary core of the evaporator without the need for external power.
2. Capillary Pumped Loop Heat Pipes
Similar to LHPs, there is also a capillary pumped loop (CPL) which was first proposed by Stenger working at the NASA Lewis Research Center. Although CPLs and LHPs operate on the same principle, the structural difference of the separated compensation chamber and evaporator results in different operating characteristics. The operating temperature of the LHP evaporator is not determined by the set temperature of the reservoir as in the CPL, but is maintained in balance through complex energy and mass transfer between the evaporator and the liquid compensator. Compared with LHPs, the main advantage of CPLs is that they can better regulate the operating temperature of the device over a wider heat load range to achieve precise temperature control.
3. Flat Evaporator Type Loop Heat Pipes
With the miniaturization of electronic equipment, a type of LHP with a flat evaporator has emerged. Compared with traditional LHPs, a flat evaporator type LHP of the same size can increase the contact area with heat-generating devices, and the capillary core is heated more uniformly, enabling it to better exert the heat transfer capacity of the LHP. The angle between the temperature gradient and the velocity gradient of the working fluid flow in the flat evaporator is small. From the perspective of the field synergy principle, flat-type LHPs have more advantages than traditional LHPs, which gives them greater potential in the field of heat dissipation for high heat flux electronic devices.
4. Thin-Type Loop Heat Pipes
Studies have found that materials with high thermal conductivity (such as composite metals and graphite sheets) only provide good heat dissipation performance in the planar direction. However, heat pipe and vapor chamber technologies offer much better heat transfer capabilities than solid thermal conductive materials. Loop heat pipes (LHPs) are two-phase flow heat transfer systems that can achieve long-distance heat transfer without auxiliary power. LHPs have been researched and developed for applications in high heat-generating components such as CPUs, but they have not been widely used in consumer electronic products. The high heat generation problem of consumer electronics under 5G communication systems may provide an application opportunity for ultra-thin loop heat pipes.
It is reported that researchers from Fujitsu Laboratories Ltd and the Department of Mechanical System Engineering at Nagoya University published their research results on the application of submillimeter-thin loop heat pipes made of two copper plates for heat dissipation in 5G smart electronic devices in the journal Applied Thermal Engineering.
The loop heat pipe (LHP) developed by Japanese researchers for heat dissipation in ultra-thin and compact electronic devices has the following characteristics:
- A new type of loop heat pipe with a submillimeter thickness manufactured using two copper plates;
- Groove structures formed on thin copper plates to generate capillary force;
- Non-woven fabric made of copper fibers used as the wick to maintain the liquid phase;
- The thermal resistance of the thin loop heat pipe with a thickness of 0.4 mm is 0.21 K/W;
- The ultra-thin loop heat pipe with a thickness of 0.4 mm has a thermal resistance of 0.21 K/W at a thermal power of 7.5 W.
Structure of the New Submillimeter Thin-Type Loop Heat Pipe
- The new thin-type LHP is manufactured using only two thin copper plates, which can simplify the manufacturing process and reduce costs, and the thickness of the LHP can be further reduced from the current 0.6 mm;
- In the thin-type LHP constructed with two thin copper plates, capillary force is generated through fine groove structures formed using a "semi-etching" manufacturing process. Capillary grooves for the working fluid to flow in parallel/vertical directions are constructed on the two thin copper plates respectively through an etching process, and the two copper foils are bonded by diffusion welding to form a capillary wick structure with orthogonal grooved cores;
- The working fluid of the thin-type LHP is water, because the combination of water and copper is common in conventional heat pipes. The selection of water ensures the reliability of long-term application in electronic equipment. In addition, the surface tension of water is greater than that of other working fluids, thus increasing the capillary force;
- Prototype research shows that the optimal working fluid filling ratio for effective heat transfer in the thin-type LHP is 40±2 vol%.
Orthogonal Grooved Cores Formed by Etching and Diffusion Bonding in Thin-Type LHPs
Flow Path Layout of the Thin-Type LHP Constructed with Two Copper Plates and the Capillary Groove Structure of Type-A Prototype
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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.
