Reinforced heat rod for protecting permafrost

Abstract

The application discloses an enhanced heat rod for protecting permafrost, and belongs to the technical field of permafrost protection, which comprises a gravity heat rod unit, a loop heat pipe cluster unit and a radiation refrigeration unit. The gravity heat rod unit is a heat rod filled with heat transfer working medium, and is sequentially divided into a condensation section, an adiabatic section and an evaporation section from top to bottom, and is respectively located above the ground, the active layer and the permafrost layer. The loop heat pipe cluster unit comprises a plurality of loop heat pipes radially arranged around the heat rod, and is filled with nanofluid working medium in sequence from an evaporator, a gas pipeline, a condenser and a liquid pipeline. The evaporator and the liquid pipeline are arranged in the heat rod, and the condenser and the gas pipeline are arranged outside the heat rod. The radiation refrigeration unit is coated on the condensation section and the outer part of the condenser corresponding to the condensation section. The combination of the gravity heat rod, the loop heat pipe technology and the radiation refrigeration technology can reduce the internal thermal resistance of the heat rod, improve the external cold energy capturing capacity, and realize the cooling protection of the permafrost.

Description

Technical Field

[0001] This invention belongs to the field of permafrost protection technology, and specifically relates to an enhanced heat pipe for protecting permafrost. Background Technology

[0002] Under conditions of global warming, permafrost is showing a trend of warming and degradation, and its melting, settlement, and deformation pose a significant threat to infrastructure such as buildings, power tower foundations, oil pipelines, and transportation structures. Therefore, how to cool and protect permafrost and maintain its thermal stability is a major challenge facing engineering construction and operation in permafrost regions. Especially when foundation damage extends to infrastructure, efficient and rapid cooling protection is required. Currently, heat pipes are the primary cooling measure for permafrost. Essentially, they are enlarged coreless gravity heat pipes filled with liquid ammonia. The evaporation section is located below the permafrost layer, and the condensation section is located above the air environment. When the air temperature is lower than the permafrost temperature, the liquid working fluid in the evaporation section absorbs heat, vaporizes, and rises to the condensation section. After dissipating heat and liquefying in the condensation section, it flows back to the evaporation section by gravity, thus cyclically absorbing heat from the permafrost foundation. It is evident that the circulation power of gravity heat pipes mainly relies on the buoyancy of the gas and the gravity of the liquid, and its advantage is that it is passively driven and consumes zero energy.

[0003] Years of application have shown that heat pipes have the following main defects: (1) Carrying limit. In order to meet the unidirectional heat dissipation requirements of permafrost, the first modification of heat pipes compared with conventional heat pipes is to eliminate the capillary liquid core, see "Heat Pipes and Heat Pipe Supports" (GB / T 27880). This results in the heat pipes not being able to be arranged horizontally, but must be vertical or inclined, and the liquid working fluid relies on gravity to return to maintain unidirectional heat conduction, and the evaporation heat absorption effect is concentrated in the middle and lower part of the evaporation section; on the other hand, the gaseous and liquid working fluids are in contact, which means that when the gaseous working fluid inside the heat pipe rises at a sufficiently high speed, the shear force at the liquid-gas interface will slow down the return speed of the liquid working fluid, or even carry it into the airflow, thereby reducing the return flow of the working fluid. When the entrainment is severe, it will cause the evaporation section to dry out, that is, form the carrying heat transfer limit, resulting in defects such as low working fluid circulation efficiency and poor temperature uniformity of the evaporation section. (2) Start-up limit. To meet the heat dissipation efficiency and cooling requirements of permafrost, the second modification of the permafrost heat pipe compared to conventional heat pipes is to enlarge the structural size, increase the amount of working fluid, and increase the transport pressure drop accordingly. Due to the low phase change activity of the working fluid and the small pressure difference between the condensation section and the evaporation section inside the heat pipe, the working fluid can only start phase change heat transfer when the temperature difference between the atmosphere and the permafrost is higher than a certain threshold (generally ≈5℃). The heat pipe is usually only in a fluctuating state from mid-October to mid-March of the following year, resulting in defects such as large start-up temperature difference and short effective operating time. (3) Condensation limit. The outer wall of the heat pipe uses conventional metal spiral blades, and the heat dissipation efficiency is limited by air temperature and wind speed. The ability to capture natural cold energy is insufficient, and the maximum heat transfer capacity is limited by the condensation capacity, which makes it impossible for the heat pipe to maintain a stable heat transfer power. Moreover, in the warm season, the metal pipe is prone to heat accumulation and temperature rise under solar radiation and is transferred to the permafrost layer. The aforementioned defects result in a small cooling range and limited impact area for heat pipes, typically with an effective cooling radius of only about 1.5 m, making them prone to differential deformation. Especially on the Qinghai-Tibet Highway, the insufficient cooling efficiency of the heat pipes in winter, coupled with the heat accumulation effect during the warmer season, easily triggers a thermal runaway chain reaction, leading to severe longitudinal cracking of the highway. Given the ongoing expansion of infrastructure construction and the upgrading of existing facilities on the Qinghai-Tibet Plateau, the optimization and improvement of heat pipes is urgently needed. Summary of the Invention

[0004] The purpose of this invention is to provide an enhanced heat pipe for protecting permafrost, aiming to solve the technical problems of existing permafrost heat pipes in terms of carrying capacity, start-up capacity, and condensation capacity.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] An enhanced heat pipe for protecting permafrost includes a gravity heat pipe unit, a loop heat pipe bundle unit, and a radiative cooling unit. The gravity heat pipe unit is a cylindrical heat pipe filled with a heat transfer medium. The heat pipe is divided into a condensation section, an insulation section, and an evaporation section from top to bottom. The insulation section and the evaporation section are inserted into the permafrost. The condensation section extends above the ground. The length of the insulation section is consistent with the thickness of the active layer of the foundation. The length of the evaporation section is consistent with the temperature rise and degradation depth within the permafrost layer.

[0007] The loop heat pipe bundle unit includes several loop heat pipes, each loop heat pipe being sequentially connected by an evaporator, a gas pipe, a condenser, and a liquid pipe. The interior of each loop heat pipe is a fluid loop filled with a nanofluid working fluid. Several loop heat pipes are radially arranged around the central axis of the heat pipe at its periphery. The evaporator and liquid pipe are located inside the heat pipe, while the condenser and gas pipe are located outside the heat pipe.

[0008] The radiative cooling unit is coated on the outside of the condensing section of the heat pipe and on the outside of the condenser corresponding to the condensing section on the loop heat pipe.

[0009] Preferably, the heat pipe is a sealed hollow tube, the interior of the heat pipe is filled with a heat transfer medium, and its cavity is in a vacuum state; the outside of the loop heat pipe is provided with an insulation sleeve and a phase change energy storage sleeve, the insulation sleeve corresponds to the insulation section of the heat pipe; the phase change energy storage sleeve corresponds to the evaporation section of the heat pipe, and the interior of the phase change energy storage sleeve is filled with a phase change energy storage material.

[0010] Furthermore, the heat transfer medium filled inside the heat rod is liquid ammonia; the phase change critical temperature of the phase change energy storage material is between the heat transfer medium vaporization endothermic temperature and the temperature threshold corresponding to the stable state of permafrost.

[0011] Furthermore, a spiral baffle is installed on the outer wall of the condensing section, and the condenser of the loop heat pipe passes through the spiral baffle from top to bottom.

[0012] Furthermore, the spiral spoiler is a long strip of aluminum alloy plate that is spirally wound around the outside of the condensation section, and the aluminum alloy plate is covered with round holes.

[0013] Furthermore, the upper and lower ends of the loop heat pipe are U-shaped bend I and U-shaped bend II, respectively. The two ends of U-shaped bend I are connected to the condenser and the liquid pipeline, respectively, forming a condensing end U-shaped pipe; the two ends of U-shaped bend II are connected to the evaporator and the gas pipeline, respectively, forming an evaporating end U-shaped pipe.

[0014] The U-shaped elbow I has a radius of curvature of 50mm, a pipe diameter of 20mm, a pipe wall thickness of 1mm, and an inner diameter of 18mm; the U-shaped elbow II has a radius of curvature of 50mm, a pipe diameter of 30mm, a pipe wall thickness of 1mm, and an inner diameter of 28mm.

[0015] Preferably, the evaporator is divided into two sections: the lower section consists of multiple parallel capillary bundles, and the upper section is a straight tube. A capillary wick is installed on the inner wall of the capillary bundle. The capillary wick is made of a mixture of metal fibers and ceramic whiskers.

[0016] Furthermore, the evaporators and condensers of several loop heat pipes operate in parallel, and the evaporators and liquid lines of these loop heat pipes are all connected through a liquid receiver. The outlet of the liquid line is connected to the top of the liquid receiver, and the inlet of the evaporator is connected to the bottom of the liquid receiver. There are six loop heat pipes, that is, six loop heat pipes are radially evenly distributed on the cylindrical liquid receiver at a 60° angle.

[0017] Furthermore, the working fluid of the nanofluid is selected as a nanomagnetic fluid, and the nanoparticles inside the nanomagnetic fluid are nanoscale metal or metal oxide particles.

[0018] The heat pipe is equipped with a float valve inside, and a permanent magnet is installed on the top of the float valve. The permanent magnet and the float valve are located at the lower part of the capillary bundle of the evaporator and can change with the rise and fall of the liquid ammonia level at the bottom of the heat pipe.

[0019] Furthermore, the radiation cooling unit is a dielectric / polymer composite radiation cooling film with a 3D porous structure.

[0020] The technological advancements achieved by this invention compared to existing technologies are as follows:

[0021] This invention combines gravity heat pipes with loop heat pipe technology and radiative cooling technology. By separating the gas and liquid pipelines of the loop heat pipe, it reduces the flow and migration resistance of the heat transfer medium inside the heat pipe while maintaining zero energy consumption. This improves the phase change efficiency of the internal heat transfer medium and enhances the heat pipe's ability to capture external cold energy, thereby achieving cooling and protection of permafrost. This invention significantly reduces thermal resistance, using gravity, capillary force, and magnetism to drive the heat transfer cycle and permafrost cooling. It overcomes the technical limitations of conventional gravity heat pipes, such as carrying capacity, start-up limits, and condensation limits. It broadens the climate adaptability to the cold season's atmospheric temperature range, improves the adaptability to different permafrost protection scenarios for various structures, and achieves more real-time and effective maintenance of the thermal stability of permafrost engineering projects. Attached Figure Description

[0022] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0023] In the attached diagram:

[0024] Figure 1 An external view of an enhanced heat pipe for protecting permafrost, provided as an embodiment of the present invention;

[0025] Figure 2 for Figure 1 A schematic diagram of a scenario where a medium-reinforced heat pipe is applied to a railway subgrade.

[0026] Figure 3 for Figure 2 A schematic diagram of the cross-section of the railway subgrade;

[0027] Figure 4 This is a schematic diagram of the structure of the heat pipe in an embodiment of the present invention;

[0028] Figure 5 This is a schematic diagram of the loop heat pipe structure in an embodiment of the present invention;

[0029] Figure 6 This is a schematic diagram of the arrangement of several loop heat pipes in an embodiment of the present invention;

[0030] Figure 7 This is a schematic diagram of the structure after the heat pipe and the loop heat pipe are combined in an embodiment of the present invention;

[0031] Figure 8 for Figure 1 Schematic diagram of the fit between the upper spiral baffle of the medium-enhanced heat pipe and the condenser;

[0032] Figure 9 This is a schematic diagram of the lower part of the enhanced heat pipe in an embodiment of the present invention;

[0033] Figure 10 for Figure 9 A schematic diagram showing the connection between the central liquid reservoir and the loop heat pipe;

[0034] Figure 11 for Figure 9 A schematic diagram showing the connection between the permanent magnet, the float valve, and the loop heat pipe;

[0035] In the picture:

[0036] 100-Heat rod, 101-Condensation section, 102-Insulation section, 103-Evaporation section, 104-Liquid ammonia; 105-Threaded end cap, 106-Spiral baffle, 107-Insulation sleeve, 108-Phase change energy storage sleeve.

[0037] 400-Loop heat pipe, 401-Straight pipe, 402-Capillary bundle, 403-Gas line, 404-Condenser, 405-Liquid line, 406-U-bend I, 407-U-bend II;

[0038] 5-Liquid reservoir; 6-Nano fluid working medium; 7-Permanent magnet; 8-Float valve; 9-Active foundation layer; 10-Permafrost layer; 11-Roadbed. Detailed Implementation

[0039] The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments. The embodiments of the present invention will now be described with reference to the accompanying drawings.

[0040] To address the existing limitations of permafrost heat pipes, such as carrying capacity, startup limits, and condensation limits, current optimization methods primarily focus on external structural aspects like filling rate and tilt angle. However, since heat pipe operation relies on heat transfer through the phase change of the working fluid, optimization efforts have limited effectiveness. Therefore, in recent years, the industry has introduced active cooling technology to improve heat pipe performance. For example, patent application CN202311599322.8 discloses an all-season heat pipe system that adds a power-driven semiconductor cooling chip to the condensation section of the heat pipe. This eliminates the operational downtime during the warm season, but requires additional external power supply equipment, resulting in high energy consumption, high investment, and unreliable durability. It is mainly suitable for scenarios with severe thermal melting defects requiring emergency response and is difficult to promote on a large scale.

[0041] Overall, heat pipes remain the mainstay of permafrost protection in the future due to their technological advantages of zero-energy utilization of natural cold energy and high phase-change heat carrying efficiency. Given the technological limitations of actively driven refrigeration components, further improving the heat transfer performance of the heat pipe itself is key to enhancing the level of permafrost protection technology.

[0042] like Figure 1 , Figure 2 and Figure 3 As shown in the figure, an enhanced heat pipe for protecting permafrost provided by an embodiment of the present invention includes a gravity heat pipe unit, a loop heat pipe bundle unit, and a radiative cooling unit. The gravity heat pipe unit is a cylindrical heat pipe 100 filled with a heat transfer medium. The heat pipe 100 is divided into a condensation section 101, an insulation section 102, and an evaporation section 103 from top to bottom. The insulation section 102 and the evaporation section 103 are inserted into the permafrost. The condensation section 101 extends above the ground. The length of the insulation section 102 is consistent with the thickness of the active layer 9 of the foundation. The length of the evaporation section 103 is consistent with the temperature rise and degradation depth within the permafrost layer 10. The heat transfer medium filled inside the heat pipe is liquid ammonia 104. The boiling point of liquid ammonia at normal pressure is -33.5℃, which is much lower than the temperature of the permafrost layer. First, liquid ammonia absorbs heat and evaporates into gas in the evaporation section, thus cooling and protecting the permafrost around the pipe. Then, the ammonia gas flows upward into the condensation section under a small pressure difference and condenses, releasing heat. Finally, the condensed liquid flows back to the evaporation section by gravity, completing the entire cycle.

[0043] The loop heat pipe bundle unit includes several loop heat pipes 400. Each loop heat pipe 400 is sequentially connected by an evaporator (composed of a straight pipe 401 and a capillary bundle 402), a gas pipe 403, a condenser 404, and a liquid pipe 405. The interior of each loop heat pipe 400 is a fluid loop filled with nanofluid working fluid 6. Several loop heat pipes 400 are radially arranged around the central axis of the heat rod 100. The evaporator and liquid pipe 405 are located inside the heat rod 100, while the condenser 404 and gas pipe 403 are located outside the heat rod 100.

[0044] The radiative cooling unit is coated on the exterior of the condensing section 101 of the heat rod 100 and the exterior of the condenser 404 corresponding to the height range of the condensing section 101.

[0045] The working principle of the aforementioned enhanced heat pipe is as follows: using a gravity heat pipe unit as the main body, and optimizing and improving the cooling and protection effect of permafrost through loop heat pipes and radiative cooling units. Specifically, several loop heat pipes are radially arranged around the heat pipe, with the evaporator and liquid pipeline of the loop heat pipes located inside the heat pipe, and the condenser and gas pipeline located outside the heat pipe. The gas pipeline and liquid pipeline of the loop heat pipes are designed separately to focus on improving the carrying capacity and start-up limits of the gravity heat pipe unit; the radiative cooling unit is applied to the condensation section of the heat pipe and the exterior of the condenser corresponding to the height range of the condensation section to focus on improving the condensation limit of the gravity heat pipe unit.

[0046] As a preferred structure, such as Figure 4 As shown, the heat pipe 100 is a sealed hollow tube with threaded end caps 105 at both the top and bottom. The heat pipe 100 is filled with liquid ammonia 104, and its cavity is in a vacuum state. The loop heat pipe 400 is externally equipped with an insulation sleeve 107 and a phase change energy storage sleeve 108. The insulation sleeve 107 corresponds to the insulation section 102 of the heat pipe 100. The phase change energy storage sleeve 108 corresponds to the evaporation section 103 of the heat pipe 100, and its interior is filled with phase change energy storage material. The phase change critical temperature of the phase change energy storage material should be between the vaporization endothermic temperature of liquid ammonia 104 and the temperature threshold corresponding to the stable state of permafrost.

[0047] In its specific fabrication, the heat pipe 100 is manufactured according to the requirements of "Heat Pipes and Heat Pipe Supports" (GB / T 27880), including material selection, size design, and fabrication. Specifically, the heat pipe 100 is made of carbon steel with an outer diameter of 89mm, a wall thickness of 2mm, and an inner diameter of 85mm; the height of the condensing section 101 is set to 2.0~4.0m. Generally, the heights of the condensing section 101, the adiabatic section 102, and the evaporating section 103 of the heat pipe 100 are set to 3.0m, 2.0m, and 4.0m, respectively; the filling mass of liquid ammonia 104 is 2500g.

[0048] Preferably, the critical temperature value of the phase change energy storage material is determined according to the relevant standards or specifications corresponding to the application scenario. For example, when applied to permafrost protection in highway engineering, according to the "Technical Specification for Highway Design and Construction in Permafrost Areas" (JTG / T 3331-04), the temperature threshold corresponding to the stable state of permafrost is -3.0℃. Therefore, the critical temperature value of the phase change energy storage material should be lower than -3℃. The purpose is to overcome the shortcomings of permafrost, such as low specific heat capacity and thermal conductivity, and slow heat transfer rate, to increase the heat exchange during the start-up of the heat pipe in low-temperature and frigid weather. This prevents cold accumulation and energy loss due to excessively low outer wall temperature of the evaporation section, while also storing energy and maintaining an appropriate temperature gradient, which is beneficial for improving the continuity of cold energy diffusion and effective cooling capacity. Simultaneously, the outer casing of the phase change energy storage material is made of stainless steel to protect the internal materials from foundation deformation and damage.

[0049] In the specific manufacturing process, the phase change energy storage material uses ammonium nitrate (NH4NO3) solution in hydrated crystal salts. Its phase change temperature is controlled between -5 and -10℃, which is between the liquid ammonia evaporation temperature inside the heat pipe and the low temperature stability threshold of permafrost. Its beneficial effect is to improve the mismatch between the short operating time of the heat pipe and the poor thermal conductivity of permafrost. Through its high thermal conductivity and large specific heat capacity, it can achieve continuous heat absorption on permafrost and rapid heat release on the heat pipe, that is, to achieve cascaded cold storage and optimize the cooling effect of the heat pipe on permafrost.

[0050] Meanwhile, the insulation sleeve 107 uses a rubber-plastic insulation pipe that can adapt to low temperature and water-rich conditions. The height of the insulation section corresponds to the thickness of the active layer of the permafrost foundation. This is to prevent the backflow liquid in the condensation section from absorbing heat and vaporizing in the insulation section, thus avoiding affecting the vaporization rate of the evaporation section and the protection effect of the permafrost. Simultaneously, the rubber-plastic insulation pipe is encased in a stainless steel pipe to protect the internal materials from foundation deformation and damage. During manufacturing, both the insulation sleeve and the phase change energy storage sleeve are 40mm thick. This is to facilitate drilling and implantation using standard-sized drill bits (175mm diameter) from conventional drilling rigs, ensuring a tight fit between the pipe and the stratum, guaranteeing heat transfer efficiency between the interfaces, and preventing surface water seepage.

[0051] Further optimize the above structure, such as Figure 4As shown, a spiral baffle 106 is installed on the outer wall of the condensing section 101, and the condenser 404 of the loop heat pipe 400 passes through the spiral baffle 106 from top to bottom. The spiral baffle 106 is a corrugated aluminum alloy plate with a width of 40mm and a spiral spacing of 10cm. 3mm diameter holes are drilled into the aluminum alloy plate. Its function is to increase the heat exchange area and enhance the induction and turbulence of the airflow, creating turbulence on the outer wall of the condensing section. The beneficial effect is that, compared with the spiral heat dissipation blades on the outer wall of a traditional heat pipe, the turbulence allows for a longer contact time between the air and the condensing section, enabling the capture of more cooling capacity under the same air temperature and flow rate conditions.

[0052] In specific embodiments of the present invention, such as Figure 5 , 6 As shown, the upper and lower ends of the loop heat pipe 400 are U-shaped bends I 406 and II 407, respectively. The two ends of U-shaped bend I 406 are connected to the condenser 404 and the liquid pipeline 405, respectively, forming a condensing end U-shaped pipe. The two ends of U-shaped bend II 407 are connected to the evaporator and the gas pipeline 403, respectively, forming an evaporating end U-shaped pipe. The evaporator and condenser respectively function as heat absorbers and heat dissipators, absorbing heat from the permafrost to cool it down and dissipating heat to the outside. The condenser 403 is connected to the spiral baffle 106 on the outer wall of the condensing section 101 of the gravity heat pipe unit, which helps to increase the heat exchange area between the condenser 403 and the surrounding atmosphere, improving heat exchange efficiency.

[0053] As a preferred structure, such as Figure 5 As shown, the evaporator is divided into two sections: the lower section consists of multiple parallel capillary bundles 402, and the upper section is a straight pipe 401. Capillary cores are installed on the inner wall of the capillary bundles 402, providing capillary force for the migration of the nanofluid working medium 6. The capillary cores are made of a mixture of metal fibers and ceramic whiskers, with the following performance specifications: average pore size <1μm (0.8~1.2μm percentage >95%), open porosity >70%, equivalent thermal conductivity <3W / (m·K), and compressive strength >50MPa. The beneficial effect of arranging capillary cores 405 inside the evaporator 401 is that the high-performance capillary cores provide the capillary driving force for the migration of the liquid nanofluid working medium 6, allowing it to flow uniformly within the height range of the capillary bundles 402 and evaporate, generating an endothermic cooling effect. The resulting beneficial effects are as follows:

[0054] First, it improves the carrying limit problem of the evaporation section 103 of the heat pipe 100, and avoids the liquid ammonia 104 in the condensation section 101 being affected by the superheated airflow of the liquid ammonia 104 in the adiabatic section 102 and the upper part of the evaporation section 103 during the gravity backflow process after condensation. Second, when the heat absorption effect of the vaporization of liquid ammonia 104 inside the heat pipe 100 is limited to the bottom of the evaporation section 103, or when it cannot transfer heat due to the start-up limit, the loop heat pipe 400 can still work normally with the assistance of the capillary wick 405, thus improving the start-up limit problem and the uniformity of heat absorption and cooling of the heat pipe 100. Third, it enhances the anti-gravity capability, overcomes the limitation that the traditional heat pipe 100 can only be arranged vertically and at a large angle, and enables the loop heat pipe to operate stably in any installation position.

[0055] In the specific design, the dimensions of the straight tube 401 are: diameter 30mm, wall thickness 1mm, inner diameter 28mm, and height 2.0m, corresponding to the adiabatic section 101 of the gravity heat pipe unit. The purpose is to reduce the flow resistance and vaporization heat absorption rate of the nanofluid working fluid. The dimensions of the capillary bundle 402 are: diameter 6mm, wall thickness 1mm, inner diameter 4mm, and height 4.0m, corresponding to the evaporation section 103 of the gravity heat pipe unit. The purpose is to increase the vaporization heat absorption rate of the nanofluid working fluid.

[0056] The condenser 404 is a copper tube with a fixed diameter of 20mm, a wall thickness of 1mm, an inner diameter of 18mm, and a height of 5.0m. The purpose of the condenser tube diameter (20mm) being lower than the evaporator tube diameter (30mm) is to create a difference in the flow velocity of the nanofluid working medium 6 inside the condenser and evaporator with the same circulation flow rate, thereby increasing the pressure difference and meeting the circulation requirements of the nanofluid working medium, which corresponds to the low-pressure environment for evaporation heat absorption and the high-pressure environment for condensation heat dissipation. This reduces the start-up temperature difference required for the phase change cycle of the nanofluid working medium, thus overcoming the start-up limit of the existing heat pipe.

[0057] Gas line 403 is a copper tube with a gradually decreasing diameter, from 30mm to 20mm, with a wall thickness of 1mm and a height of 4.0m. The diameter gradually decreases along the gas flow direction to increase the transport velocity of the gaseous nanofluid working medium, overcome the influence of gravity, and ensure that the gaseous nanofluid working medium can spontaneously migrate upwards into the condenser based on its density.

[0058] The liquid pipeline 405 is a copper tube with a gradually increasing diameter, from 20mm to 30mm, with a wall thickness of 1mm and a height of 3.0m. The diameter gradually increases along the direction of liquid flow to improve the reflux efficiency of the liquid nanofluid working medium, allowing the droplets to overcome the flow resistance of the pipe wall after collection and flow or drip rapidly downwards. This ensures that the nanofluid working medium can quickly reflux or drip directly back to the evaporator after condensation, promoting the circulation of the nanofluid working medium.

[0059] The loop heat pipe is fabricated as follows: First, a U-shaped bend I with a radius of curvature of 50 mm, a diameter of 20 mm, a wall thickness of 1 mm, and an inner diameter of 18 mm is used to connect the condenser 404 to the liquid line 405 to form a condensing end U-shaped tube. Then, a U-shaped bend II with a radius of curvature of 50 mm, a diameter of 30 mm, a wall thickness of 1 mm, and an inner diameter of 28 mm is used to connect the capillary bundle 402 of the evaporator to the gas line 403 to form an evaporating end U-shaped tube. Finally, the evaporating end U-shaped tube and the condensing end U-shaped tube are connected to form a closed loop, wherein the inlet of the evaporator's straight pipe 401 is connected to the outlet of the liquid line 405, and the outlet of the gas line 403 is connected to the inlet of the condenser 404, that is, the two U-shaped tubes are connected vertically to form a closed loop.

[0060] The working principle of the loop heat pipe is as follows: First, when liquid nanofluid working fluid 6 flows into the capillary wick inside the capillary bundle 402 of the evaporator, the nanofluid working fluid vaporizes due to its boiling point being lower than the ambient temperature. The phase change process absorbs a large amount of heat (latent heat), generating a cooling effect. Second, due to the low density of the gaseous nanofluid working fluid 6, it moves upward in a vacuum environment and is transported to the condenser 404 through the gas pipe 403. Then, after releasing heat in the condenser 404, the gaseous nanofluid working fluid 6 condenses into a gas-liquid mixture or liquid state and enters the liquid pipe 405. Under the action of gravity, it moves downward through the liquid pipe 405 and returns to the straight pipe 401 of the evaporator. Finally, the liquid nanofluid working fluid 6 re-enters the capillary wick inside the capillary bundle 402 through the straight pipe 401, and under the action of capillary force, it covers the entire inner wall of the capillary bundle 402, continuing to absorb heat and evaporate. This cycle repeats continuously, utilizing the phase change process of evaporation and condensation of the nanofluid working fluid 6 to achieve efficient heat transfer. It is evident that, compared to the gravity heat pipe 100, the loop heat pipe 400 drives the circulation of the nanofluid working fluid through a combination of capillary force and gravity.

[0061] Therefore, compared to the straight-through structure of the traditional heat pipe 100, the closed-loop structure of the loop heat pipe 400 differs from the structure where the evaporation section 101 and the condensation section 103 are directly connected. The gas pipe 403 and the liquid pipe 405 of the loop heat pipe 400 are relatively separated, avoiding the entrainment effect caused by the contact between the rising airflow and the descending liquid flow during the circulation of the nanofluid working medium 6. At the same time, combined with the capillary core of the evaporator capillary bundle 402 and the specially designed gas pipe 403 and liquid pipe 405 with gradually changing pipe diameters, the carrying limit defects are comprehensively improved.

[0062] In a specific embodiment of the present invention, the structure of the loop heat pipe bundle unit is as follows: the evaporators (composed of straight pipes 401 and capillary bundles 402) and condensers 404 of several loop heat pipes 400 operate in parallel; the outlets of the liquid pipes 405 of all loop heat pipes 400 are connected to the top surface of the liquid reservoir 5, and the inlets of the evaporators of all loop heat pipes 400 are connected to the bottom surface of the liquid reservoir 5. The loop heat pipes 400 are evenly distributed on the cylindrical liquid reservoir 5, forming an overall annular columnar structure. The beneficial effects of using the above-mentioned loop heat pipe bundle unit are: each loop heat pipe operates independently and does not affect each other, sharing the cooling load of permafrost; the circulation volume of the nanofluid working fluid 6 is automatically and dynamically distributed according to the impedance of each branch, reducing the overall impedance; it has high fault tolerance and is easy to expand.

[0063] In specific manufacturing, the liquid storage tank 5 is a cylindrical liquid storage tank with an inner diameter of 50mm and a height of 100m; the number of loop heat pipes 400 is 6, that is, the 6 loop heat pipes 400 are radially evenly distributed on the cylindrical liquid storage tank at an angle of 60°.

[0064] In a specific embodiment of the present invention, the nanofluid working medium 6 is selected as a nanomagnetic fluid, and the nanoparticles inside the nanomagnetic fluid are nanoscale metal or metal oxide particles. The nanofluid working medium is a novel phase-change heat carrier, a ternary suspension composed of nanoparticles, a base liquid, and a dispersant. Liquid ammonia is selected as the base liquid, and anhydrous methanol is used as the dispersant. The concentration of liquid ammonia should be controlled between 23% and 27% to ensure that the evaporation temperature of the ammonia water is lower than the lowest temperature of permafrost. Compared to liquid ammonia 104, the nanofluid working medium 6 has a higher thermal conductivity and can increase active nucleation sites through the particle effect of nanoparticle deposition on the wall surface, thus enhancing heat transfer. Simultaneously, the mass fraction of metal or metal oxide particles is 3.0% to prevent the high concentration of nanofluid from increasing the viscosity of the base liquid and avoiding excessive flow resistance.

[0065] The nanoscale metal or metal oxide particles in the nanomagnetic fluid of this invention are magnetic, which has the advantage that their migration dynamics can be adjusted by applying a magnetic field. In this embodiment, the nanoparticles are preferably iron(III) oxide (Fe3O4).

[0066] The aforementioned magnetic field is applied using a permanent magnet 7, which is a cylinder with a diameter of 40 mm and a height of 20 mm. The magnetic field of the permanent magnet 7 serves two purposes: First, before heat transfer begins inside the loop heat pipe 400, the nanoparticles exhibit high transport activity in the magnetic field, reducing the startup power required for the nanofluid working medium 6 to transition from static to dynamic states. This provides a barrier during the startup phase, enhancing the rates of nucleation boiling, bubble generation, and growth, thereby improving startup performance and heat transfer efficiency. Second, once the nanofluid working medium 6 inside the loop heat pipe begins to undergo phase change cycling, the nanoparticles are drawn to the evaporation end under the influence of the magnetic field, creating more active nucleation sites.

[0067] As a preferred embodiment, a float valve 8 is installed inside the heat pipe 100, and a permanent magnet 7 is installed on top of the float valve 8. The permanent magnet 7 and the float valve 8 are located at the lower part of the capillary bundle 402 of the evaporator and can change with the rise and fall of the liquid ammonia 104 at the bottom of the heat pipe 100. The variation law of the magnetic field environment of the loop heat pipe 400 is as follows: the farther away the permanent magnet 7 is, the better the circumferentiality of the magnetic field, the worse the unidirectionality, and the weaker the magnetic force. The effect of the magnetic field on the nano-magnetic fluid is mainly to reduce the starting resistance and stimulate the phase change activity. Conversely, the closer the permanent magnet 7 is, the worse the circumferentiality of the magnetic field, the better the unidirectionality, and the stronger the magnetic force. The effect of the magnetic field on the nano-magnetic fluid is mainly to reduce the operating resistance and improve the circulation efficiency. When the ambient air temperature is high, the liquid ammonia 104 inside the gravity heat pipe mainly accumulates in the upper condensation section 101 in a gaseous state (ammonia) and is difficult to liquefy and flow back. That is, when the heat pipe 100 stops running, the liquid level of the liquid ammonia 104 at the bottom of the evaporation section 103 is low, and the average distance between the permanent magnet 7 and the loop heat pipe is far. At this time, the magnetic field has a beneficial effect on the loop heat pipe 400 to stimulate phase change activity and start the cycle heat transfer. When the ambient air temperature is low, the ammonia gas liquefaction and reflux rate in the condensation section 101 inside the heat pipe 100 is fast. That is, when the heat pipe 100 is operating normally, the liquid ammonia 104 at the bottom of the evaporation section 101 is at a high level, and the average distance between the permanent magnet and the loop heat pipe is close. At this time, the magnetic field has the beneficial effect of reducing the operating resistance and accelerating the circulation heat transfer of the loop heat pipe 400. In conjunction with the capillary wick, it drives the liquid nanofluid working medium 6 to move downward, thereby improving the distribution coverage and heat absorption and cooling uniformity of the liquid nanofluid working medium 6 in the evaporation section 103 and improving the cooling effect of permafrost.

[0068] Therefore, the beneficial effects of the combination of nano-magnetic fluid and permanent magnet in the loop heat pipe are as follows: On the one hand, the high thermal conductivity and large heat transfer area of ​​nanoscale metal or metal oxide particles are utilized to enhance heat transfer, especially by increasing active nucleation sites through nanoparticles, thereby improving the heat exchange and phase change rate of the working fluid; on the other hand, the phase change activity and circulation efficiency of the nano-magnetic fluid working fluid 6 are further enhanced through the cooperation of the nano-magnetic fluid and the magnetic field of the permanent magnet. Combined with the aforementioned design of the pipe diameter difference between the evaporator and condenser 404, the starting limit defects are comprehensively improved, the requirement for the low-temperature value of the atmospheric temperature corresponding to the starting temperature in the cold season is reduced, and the effective operating time is increased.

[0069] This invention employs a combination of a gravity heat pipe unit and a loop heat pipe cluster unit: the gravity heat pipe unit serves as the framework, and the loop heat pipe cluster unit is attached and fixed to the gravity heat pipe unit, such as... Figure 7-11 As shown. The specific production process is as follows:

[0070] First, the liquid receiver 5 is installed at the center of the bottom end inside the adiabatic section 102 of the heat pipe 100, and is fixed by welding with a cross bracket. Second, the U-shaped bend I 406 of the loop heat pipe passes through the side wall of the threaded end cap 105 at the top of the condensing section 101 of the heat pipe 100, and the U-shaped bend II 407 of the loop heat pipe 400 passes through the side wall of the threaded end cap 105 at the bottom of the evaporating section 103 of the heat pipe 100. The passage is sealed by welding to prevent leakage of the heat transfer medium 104 inside the heat pipe 100. Then, the evaporator and liquid pipeline 405 of each loop heat pipe 400 are located inside the heat pipe 100, and the condenser 404 and gas pipeline 403 are located outside the heat pipe 100. The gas pipeline 403 is located in the phase change energy storage sleeve 108 on the outer wall of the evaporating section 103 of the heat pipe 100. The condensers 404 of several loop heat pipes 400 are evenly arranged on the outer wall of the condensing section 101 of the external heat pipe 100, maintaining a distance of 1.0 cm from the outer wall, and are fixed on the spiral baffle 106. They cooperate with the spiral baffle 106 to form an air vortex near the condensing section 101 of the heat pipe 100, thereby improving the heat exchange efficiency.

[0071] The heat transfer mode of the combined gravity heat pipe unit and loop heat pipe cluster unit is as follows: the technical advantages of the gravity heat pipe are its large heat transfer capacity and high efficiency, while the technical advantages of the loop heat pipe are its low thermal resistance and good temperature uniformity. Therefore, on the one hand, the loop heat pipe compensates for the carrying capacity and start-up limitations of the gravity heat pipe, resulting in a significantly lower start-up temperature difference for the enhanced heat pipe compared to conventional gravity heat pipes. On the other hand, it fully utilizes the high-efficiency heat transfer performance of the gravity heat pipe after initiating a phase change cycle in a low-temperature atmospheric environment. This broadens the atmospheric temperature range suitable for the operation of the enhanced heat pipe and increases its effective operating time and cumulative heat exchange.

[0072] In a specific embodiment of the present invention, the radiative cooling unit is a dielectric / polymer composite radiative cooling film with a 3D porous structure. The film material is applied by directly brushing it onto the outer wall of the condensation section 101 of the heat rod 100, the spiral baffle 106, and the outer wall of the condenser 404 located outside the condensation section 101.

[0073] The aforementioned radiation-cooling thin film is a zero-energy, zero-pollution cooling technology. Its principle lies in the precise, broad-spectral selectivity control within the material to achieve passive cooling. The material utilizes randomly dispersed alumina particles and layered, disordered micro- and nano-pores to enhance its spectral performance (98.26% reflectivity in the solar radiation band and 97.56% emissivity in the atmospheric window band), thereby reducing the heat gain from solar radiation on metallic materials. The beneficial effects of using this radiation-cooling film are as follows:

[0074] On the one hand, increasing the reflectivity to solar radiation prevents the metal pipes from heating up rapidly under high temperatures and solar radiation, thus preventing natural heat from being transferred to the permafrost layer through the metal pipes when the heat pipes stop working during the warm season, mitigating the negative effects of natural heat erosion during the warm season. On the other hand, increasing the outward heat emission rate of the metal pipes creates a unidirectional heat transfer path, enhancing the efficiency of capturing natural cooling during the cold season. Combined with the specially designed spiral baffle 106 of the aforementioned heat pipe 100, the condensation limit defect is comprehensively improved, and the heat transfer efficiency is enhanced.

[0075] In summary, this invention, by fully utilizing the zero-energy consumption, high heat flux density, and high-capacity heat transfer capabilities of gravity heat pipes, has the following advantages:

[0076] First, the introduction of a loop heat pipe cluster, with separate gas and liquid piping designs, reduces flow resistance and leverages the low thermal resistance of the loop heat pipes for long-distance heat transfer and cooling of deeper areas of permafrost. Second, the evaporator incorporates a capillary wick; due to the capillary distribution effect, the evaporator provides reliable cooling and temperature equalization, and utilizes anti-gravity heat transfer capabilities, allowing for microgravity operation and free use at any angle between 0° and 90°. Third, the introduction of a radiative cooling film and airflow turbulence components allows for the capture of more natural cold energy under the same air temperature and wind speed conditions, effectively overcoming condensation limitations and creating a more effective "self-cooling" thermal control system. When applied to permafrost protection, on the one hand, it broadens the temperature range corresponding to effective heat absorption and cooling, overcomes the carrying limit problem of traditional gravity heat pipes when the heat exchange temperature difference is too large, and the start-up limit defect when the heat exchange temperature difference is too small, thus improving the adaptability to climate environment; on the other hand, it broadens the range of heat pipe arrangement angles, overcomes the deficiency that a single gravity heat pipe cannot be arranged horizontally, and improves the adaptability to permafrost protection scenarios.

[0077] The manufacturing process of the enhanced heat pipe provided by this invention is as follows:

[0078] Step 1: Prepare the tube body of the heat pipe. Prepare the tube body of the gravity heat pipe, as well as the threaded end caps at the top and bottom, according to the aforementioned material and size requirements.

[0079] Step 2: Fabricate the components of the loop heat pipe. Fabricate the evaporator, gas piping, condenser and liquid piping, liquid receiver, and U-bend I and U-bend II respectively.

[0080] Step 3: Weld the threaded end cap and the U-bend. Drill a 20mm diameter hole at a 60° angle on the side wall of the top threaded end cap, insert the U-bend I through it, and then weld the gap to seal. Drill a 30mm diameter hole at a 60° angle on the side wall of the bottom threaded end cap, insert the U-bend II through it, and then weld the gap to seal.

[0081] Step 4: Seal the gravity heating rod. First, connect the outlets of the six liquid lines to the top inlet of the receiver, and connect the inlets of the six evaporator straight pipes to the bottom outlet of the receiver, then insert them into the heating rod body. Second, connect the inlets of the six liquid lines to the outlet of U-bend I on the top threaded cap, and connect the outlets of the six evaporator capillary bundles to the inlet of U-bend II on the bottom threaded cap. Then, seal the top and bottom threaded caps to the gravity heating rod body.

[0082] Step 5: Seal the heat pipe loop. Connect the outlets of the six condensers to the inlets of U-bend I on the top threaded cap, connect the inlets of the six gas lines to the outlets of U-bend II on the bottom threaded cap, and connect the outlets of the six gas lines to the inlets of the six condensers one by one. Secure the external condensers together with spiral baffles.

[0083] Step 6: Injection of the Nano-Magnetic Fluid Working Material into the Loop Heat Pipe. First, the nano-magnetic fluid working material is prepared using a two-step method: first, nanoparticles are prepared, then mixed with a base liquid. A dispersant is added to disperse the nanofluid and ensure its stability. At room temperature (25℃), iron(III) oxide particles with a diameter of 10-20 nm are prepared using the sol-gel method. This technology is a well-known and mature process in the field and will not be elaborated further here. Next, the prepared iron(III) oxide particles are dispersed in liquid ammonia to form a suspension. Then, the suspension is mixed with anhydrous methanol and dispersed using a magnetic stirrer and an ultrasonic oscillation device for 30 minutes each, completing the preparation. Finally, 350g of the nano-magnetic fluid working material is injected into the loop heat pipe bundle using vacuum filling technology.

[0084] Step 7: Liquid ammonia filling of the gravity heat pipe. Using vacuum filling technology, a one-way valve is added to the threaded cap at the top to inject 2500g of liquid ammonia into the gravity heat pipe.

[0085] Step 8: Installation of the outer casing of the gravity heat pipe. Install and fix the insulation casing and the phase change energy storage casing to the outer walls of the insulation section and evaporation section of the gravity heat pipe, respectively.

[0086] In practical applications, the enhanced heat pipes will be arranged in the same planar configuration as ordinary heat pipes. For example... Figure 2 , 3 As shown, taking the application of heat pipes in railway subgrade as an example, the reinforced heat pipes are evenly and symmetrically arranged on both sides of the subgrade. According to the requirements of the standard "Heat Pipes and Heat Pipe Racks" (GB / T 27880), the longitudinal spacing of ordinary heat pipes is generally 3.0m to 5.0m; preferably, the longitudinal spacing of new heat pipes is increased to 5.0m to 8.0m. The evaporation section of the reinforced heat pipes can be arranged vertically, inclined, or horizontally.

[0087] Compared with existing heat pipe technologies, the enhanced heat pipe for permafrost protection provided by this invention has the following advantages: This invention combines gravity heat pipe with loop heat pipe technology and radiative cooling technology. While maintaining the technical characteristics of zero energy consumption, it separates the gas pipeline and liquid pipeline of the loop heat pipe, forming a new type of heat pipe device that can significantly reduce thermal resistance. The working fluid circulation is driven by gravity, capillary force, and magnetic force, overcoming the technical defects of conventional gravity heat pipes such as carrying limit, start-up limit, and condensation limit. It broadens the climate adaptability to the cold season atmospheric temperature range, improves the adaptability to the scene of permafrost protection for different structures, and achieves more real-time and effective maintenance of the thermal stability of permafrost engineering.

[0088] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A reinforced heat pipe for protecting permafrost, characterized in that: It includes a gravity heat pipe unit, a loop heat pipe bundle unit, and a radiative cooling unit. The gravity heat pipe unit is a cylindrical heat pipe filled with a heat transfer medium. The heat pipe is divided into a condensation section, an insulation section, and an evaporation section from top to bottom. The insulation section and the evaporation section are inserted into the permafrost. The condensation section extends above the ground. The length of the insulation section is consistent with the thickness of the active layer of the foundation. The length of the evaporation section is consistent with the temperature rise and degradation depth within the permafrost layer. The loop heat pipe bundle unit includes several loop heat pipes, each loop heat pipe being sequentially connected by an evaporator, a gas pipe, a condenser, and a liquid pipe. The interior of each loop heat pipe is a fluid loop filled with a nanofluid working fluid. Several loop heat pipes are radially arranged around the central axis of the heat pipe at its periphery. The evaporator and liquid pipe are located inside the heat pipe, while the condenser and gas pipe are located outside the heat pipe. The upper and lower ends of the loop heat pipe are U-shaped bend I and U-shaped bend II, respectively. The two ends of U-shaped bend I are connected to the condenser and the liquid pipeline, respectively, forming a condenser-end U-shaped pipe; the two ends of U-shaped bend II are connected to the evaporator and the gas pipeline, respectively, forming an evaporator-end U-shaped pipe. The evaporator is divided into two sections: the lower section consists of multiple parallel capillary bundles, and the upper section is a straight tube. Capillary wicks are installed on the inner wall of the capillary bundles. The straight tube corresponds to the adiabatic section of the gravity heat rod unit, and the capillary bundle corresponds to the evaporation section of the gravity heat rod unit. The radiative cooling unit is coated on the outside of the condensing section of the heat pipe and on the outside of the condenser corresponding to the condensing section on the loop heat pipe.

2. The reinforced heat pipe for protecting permafrost according to claim 1, characterized in that: The heat pipe is a sealed hollow tube, and its interior is filled with a heat transfer medium, with its cavity in a vacuum state. The loop heat pipe is equipped with an insulation sleeve and a phase change energy storage sleeve. The insulation sleeve corresponds to the insulation section of the heat pipe. The phase change energy storage sleeve corresponds to the evaporation section of the heat pipe, and its interior is filled with a phase change energy storage material.

3. A reinforced heat pipe for protecting permafrost according to claim 2, characterized in that: The heat transfer medium filled inside the heat rod is liquid ammonia; the phase change critical temperature of the phase change energy storage material is between the heat transfer medium vaporization endothermic temperature and the temperature threshold corresponding to the stable state of permafrost.

4. A reinforced heat pipe for protecting permafrost according to claim 1, characterized in that: A spiral baffle is installed on the outer wall of the condensing section, and the condenser of the loop heat pipe passes through the spiral baffle from top to bottom.

5. A reinforced heat pipe for protecting permafrost according to claim 4, characterized in that: The spiral baffle is a long strip of aluminum alloy plate that is spirally wrapped around the outside of the condensation section, and the aluminum alloy plate is covered with round holes.

6. A reinforced heat pipe for protecting permafrost according to claim 1, characterized in that: The evaporators and condensers of several loop heat pipes operate in parallel. The evaporators and liquid lines of several loop heat pipes are all connected through a liquid receiver. The outlet of the liquid line is connected to the top of the liquid receiver, and the inlet of the evaporator is connected to the bottom of the liquid receiver.

7. A reinforced heat pipe for protecting permafrost according to claim 1, characterized in that: The working fluid is a nanomagnetic fluid, and the nanoparticles inside the nanomagnetic fluid are nanoscale metal or metal oxide particles. The heat pipe is equipped with a float valve inside, and a permanent magnet is installed on the top of the float valve. The permanent magnet and the float valve are located at the lower part of the capillary bundle of the evaporator and can change with the rise and fall of the liquid ammonia level at the bottom of the heat pipe.

8. A reinforced heat pipe for protecting permafrost according to any one of claims 1-7, characterized in that: The radiation cooling unit is a dielectric / polymer composite radiation cooling film with a 3D porous structure.

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