Two-Phase Closed Thermosyphon with Internal Adiabatic Section for Efficient Ultra-Long-Distance Heat Exchange

This application claims the benefit of and priority to Chinese Patent Application No. 202410424226.8, filed Apr. 10, 2024, and Chinese Patent Application No. 202410588787.1, filed May 13, 2024, each of which is hereby incorporated herein by reference herein in its entirety.

The present disclosure relates to the technical field of heat transfer devices, and in particular to a two-phase closed thermosyphon with an internal adiabatic section for efficient ultra-long-distance heat exchange.

Two-Phase Closed Thermosyphons (TPCT), which operate without requiring external energy, are highly efficient heat transfer devices with simple structures and have been widely applied in the fields such as heat exchangers, aeronautics and astronautics, energy storage systems and engineering construction in cold regions.

Compared with conventional heat pipes, TPCT do not contain capillary wicks. Specifically, when a TPCT begins operation, vapor is formed after endothermic evaporation of a working medium in the evaporation section, rises to the condensation section under the action of a pressure difference and condenses into liquid in the condensation section, and the condensate flows back to the evaporation section along a pipe wall under the action of gravity. After undergoing the circulating evaporation and condensation of the working medium, the heat energy from the evaporation section is consistently transferred to the condensation section.

The adiabatic section in the middle of the conventional TPCT uses an external thermal insulation to partially reduce heat exchange between the working medium inside the pipe and environment. In the early engineering applications of the TPCT, the adiabatic sections were occasionally used. The thermal insulation performance of the external thermal insulation type is not satisfactory due to limitations in the adiabatic material's performance, thickness and susceptibility to construction disturbances. In projects using TPCT, such as frozen earth subgrade protection along Qinghai-Tibet Railway and geothermal exploitation, a TPCT design solution in which an adiabatic section is arranged in the middle is generally not used.

During the operation of the conventional TPCT, vaporous-liquid two-phase working mediums circularly flow in the same closed cavity, and the heat exchange between a rising high-temperature vaporous and a falling low-temperature liquid significantly reduce the heat transfer efficiency of the TPCT. Additionally, due to the viscosity and surface tension of working medium, under conditions of a higher axial heat flux density and fast circulating flow of the working medium, a large shear force arises by the upward flowing vapor and the downward flowing liquid, the liquid is entrained back to the condensation section again by the vapor flow, causing an entrainment limit phenomenon, and significantly reduces the heat transfer capacity of the TPCT. Currently, several solutions exist for the entrainment limit, such as using internal pipe-assisted flow guide or steel cable-assisted flow guide (see Chinese Patents with Publication No. CN 1731064 A and Publication No. CN 111551058 A). The above measures solve the entrainment limit problem to some extent, but do not fundamentally overcome the shortcomings of intense heat exchange between the high-temperature vapor and the low-temperature liquid in the middle section of the TPCT, which significantly restricts the application of TPCT in ultra-long-distance heat exchange scenarios.

During the operation of the TPCT, the radial heat flux gradually increases with increasing in a temperature difference between the evaporation section and the ambient environment, intensifying the boiling phenomenon. Once the critical heat flux is reached, resulting bubbles continuously cover the inner surface of the evaporation section, causing a boiling limit and also greatly reducing the maximum heat transfer capacity of the TPCT.

During the operation of the TPCT, as the heat flux continually increases, the condensate flowing back along the pipe wall is reheated when it has not returned to the evaporation section, forming a blank region between a liquid bath and a liquid film at this moment. This blank region prevents the back flow of the condensate to the evaporation section, further limiting the maximum heat transfer capacity of the TPCT.

Whether there is an inefficient external adiabatic section, a small entrainment limit, a boiling limit under a large temperature difference or other factors, the adverse effect on TPCTs with short distances, low temperature differences and low power are always negligible, and they can meet the usage requirements under conventional conditions. In certain specific application of TPCT, such as deep geothermal energy exploitation, cooling deep frozen soil at pile ends, or cooling the middle of a wide-range foundation by horizontal pipes, minimizing energy loss in middle sections of the TPCT during transmission is essential. Therefore, in projects requiring efficient ultra-long-distance heat exchange, such as in geothermal exploitation and deep refrigeration, it is necessary to incorporate an efficient long-service-life adiabatic section in the middle of the TPCT, to significantly reduce the heat exchange between the high-temperature vapor and the low-temperature liquid while remarkably reducing the internal-external heat exchange in the middle section of the TPCT.

The present disclosure aims to address the technical problem by providing a two-phase closed thermosyphon (TPCT) with an internal adiabatic section for efficient ultra-long-distance heat exchange, featuring a simple structure, suitability for ultra-long distances, and high heat transfer efficiency.

To address the above problem, the TPCT with an internal adiabatic section for efficient ultra-long-distance heat exchange, includes a condensation section, an adiabatic section and an evaporation section. Inside, a vapor guiding core pipe and a liquid guiding core pipe both pass through the adiabatic section. A top sealing partition plate and a bottom sealing partition plate are arranged at both ends of the adiabatic section to fix a wall of TPCT, the vapor guiding core pipe and the liquid guiding core pipe. A sealed cavity is formed by the top and the bottom sealing partition plates, along with the wall of TPCT, the vapor guiding core pipe and the liquid guiding core pipe. The cavity remains sealed from the outside, and the top sealing partition plate and the bottom sealing partition plate remain sealed against the vapor and the liquid guiding core pipes. The vapor guiding core pipe has a top in communication with a top of the condensation section, and a bottom fixed to the bottom sealing partition plate and in communication with the evaporation section. The liquid guiding core pipe has a top fixed to the top sealing partition plate and in communication with the condensation section, and a bottom in communication with a bottom of the evaporation section.

The vapor guiding core pipe and the liquid guiding core pipe are open-ended pipes with port sections of arbitrary shapes.

A distance from a top opening of the vapor guiding core pipe to the top of the condensation section is less than or equal to a length of the condensation section, with a minimum value ensuring a maximum outlet flow rate of a vaporous working medium in the vapor guiding core pipe. Similarly, a distance from a bottom opening of the liquid guiding core pipe to the bottom of the evaporation section is less than or equal to a length of the evaporation section, and with a minimum value ensuring a maximum outlet flow rate of a liquid working medium in the liquid guiding core pipe.

The vaporous working medium flows through the vapor guiding core pipe, while the liquid working medium flows through the liquid guiding core pipe.

The surfaces of the top sealing partition plate and the bottom sealing partition plate have arbitrary shapes.

The cavity is either filled with adiabatic material or vacuumed.

Compared with a conventional technology, the present disclosure has the following advantages:

In the figures:—vapor guiding core pipe,—liquid guiding core pipe,—wall of TPCT,—top sealing partition plate,′—bottom sealing partition plate,—cavity,—vaporous working medium, and—liquid working medium.

As shown in, a two-phase closed thermosyphon (TPCT) an internal adiabatic section for efficient ultra-long-distance heat exchange includes a condensation section, an adiabatic section and an evaporation section. The TPCT is internally equipped with a vapor guiding core pipeand a liquid guiding core pipe. Both the vapor guiding core pipeand the liquid guiding core pipepass through the adiabatic section, with a top sealing partition plateand a bottom sealing partition plate′ arranged at two ends of the adiabatic section respectively to fix the wall of TPCT, the vapor guiding core pipeand the liquid guiding core pipe. A cavityis enclosed by the top sealing partition plateand the bottom sealing partition plate′ along with the wallof TPCT, the vapor guiding core pipeand the liquid guiding core pipe. This cavityremains sealed from the outside, and the top sealing partition plateand the bottom sealing partition plate′ remain sealed from the vapor guiding core pipeand the liquid guiding core pipe; a top of the vapor guiding core pipeis in communication with a top of the condensation section, and a bottom of the vapor guiding core pipeis fixed to the bottom sealing partition plate′ of the adiabatic section and is in communication with the evaporation section; and a top of the liquid guiding core pipeis fixed to the top sealing partition plateof the adiabatic section and is in communication with the condensation section, and a bottom of the liquid guiding core pipeis in communication with a bottom of the evaporation section.

In the pipes, the vapor guiding core pipeand the liquid guiding core pipeare open-ended pipes, with sections of pipe ports in various shapes, including a horizontal cut shape and the like. The ports of the vapor guiding core pipeand the liquid guiding core pipemay also be designed, as required, with beveled cuts to facilitate the high-speed circulation of the working medium.

The distance from the top opening of the vapor guiding core pipeto the top of the condensation section is less than or equal to the length of the condensation section, and has a minimum value meeting a requirement for the maximum outlet flow rate of a vaporous working mediumin the vapor guiding core pipe. Similarly, the distance from a bottom opening of the liquid guiding core pipeto the bottom of the evaporation section is less than or equal to the length of the evaporation section, and has a minimum value meeting a requirement for the maximum outlet flow rate of a liquid working mediumin the liquid guiding core pipe. The specific lengths of the vapor guiding core pipeand the liquid guiding core pipeshould be determined according to an actual operating environment.

The vaporous working mediumcirculates in the vapor guiding core pipe, while the liquid working mediumflows through the liquid guiding core pipe.

The surfaces of the top sealing partition plateand the bottom sealing partition plate′ are in arbitrary shapes, such as concave shapes, convex shapes and funnel shapes. The top sealing partition platemay be designed with a surface to facilitate liquid collection, while the surface of the bottom partition′ may be designed to facilitate gas flow guide according to practical requirements.

The cavityis filled with adiabatic material (i.e. filled with various high-thermal-resistance mediums having poor heat transfer properties), and is alternatively formed as a vacuum cavity through vacuumization. The cavityis illustrated in the shape of a circular cross section, but may also be in any other shape.

The outer contour of the cross section of the TPCT may take various shapes, such as cylinder etc., and an outer contour of an axial longitudinal section of the TPCT may be in an | or L shape, or other irregular-shaped TPCTs. A straight cylindrical TPCT facilitates the flow of the vaporous working mediumand the liquid working mediuminside pipe, reducing the flow resistance of the vaporous working medium. The L-shaped curved TPCT increase larger heat exchange area between the liquid working mediumand the wallof TPCT in the evaporation section. After the surface area of a liquid bath is increased, a movement distance of generated bubbles to reach an interface of the liquid bath is shortened, and more bubbles are evaporated in the surface area of the liquid bath in a short time, leading to more rapid evaporation and higher heat and mass transfer rates. The vertical condensation section ensures the backflow of the liquid working medium, allowing it return to the bottom of the evaporation section through the liquid guiding core pipe. The specific structure of the TPCT can be designed according to an actual engineering purpose and site constraints.

The adiabatic section formed by the top sealing partition plate, the bottom sealing partition plate′ and the cavitymay vary as needed.

The vapor guiding core pipeand the liquid guiding core pipemay be either parallel or non-parallel to the axial direction of the TPCT.

The vapor guiding core pipeand the liquid guiding core pipedo not intersect within the adiabatic section, and outer walls may not be arranged adjacently to each other without a gap. In the present disclosure, the liquid working mediumis heated and vaporized into the vaporous working mediumin the evaporation section, and the vaporous working medium flows to the condensation section along the vapor guiding core pipeunder the action of pressure difference and is liquefied into the liquid working mediumin the condensation section. The liquid working mediumthen flows back to the evaporation section through the liquid guiding core pipeunder the action of gravity.

Compared with conventional TPCTs, the present disclosure, by arranging the vapor guiding core pipe and the liquid guiding core pipe side by side inside the adiabatic section, effectively isolating heat exchange between the adiabatic section of the conventional TPCT and environment. This configuration ensures unidirectional flow of the working medium, overcomes entrainment limits, and significantly reduces heat exchange between the vaporous and liquid working mediums in the adiabatic section. During operation, the coldest liquid working mediumdirectly returns to the bottom of the evaporation section, where it flows upward without being significantly affected by boiling and dry-up limit. The vaporous working mediumbegins to be cooled from the top of the condensation section. Even if the vaporous working medium is condensed into the liquid working medium in the upper part, the vaporous working medium can still further be cooled in the process of downward flowing along the pipe wall, and the condensation and cooling effects of the condensation section can be fully played.

In the closed cavity of the TPCT, the pressure drop of the upward flowing vapor flow is smaller than that of the condensate flowing back under the action of gravity, enabling separated, directional circulation of the vaporous and liquid working mediums due to these pressure differences. With the special structural design of the adiabatic section in the present disclosure, the heat exchange between the inside and outside of the pipe and between the vaporous working medium and the liquid working medium can be greatly reduced. Moreover, the length and position of the cavityformed by the top sealing partition plateand the bottom sealing partition′ can be adjusted substantially according to actual operating requirements, ensuring more heat exchange occurs between the evaporation section and the condensation section, thus achieving efficient, high-power heat transfer of the ultra-long TPCT.

As shown in, a TPCT with an internal adiabatic section for efficient ultra-long-distance heat exchange is made of carbon steel, has an inner diameter of 80 mm, an outer diameter of 89 mm and the total length of 2.1 m, wherein an evaporation section is 1.25 m, a condensation section is 0.6 m, and a adiabatic section is 0.25 m. Ammonia is selected as a working medium of the TPCT, the filling rate is 30% (defined as a ratio of the volume of the working medium to the volume of a cavity of the TPCT, excluding the vacuum partition layer in the adiabatic section when calculating the cavity volume).

The vapor guiding core pipeand the liquid guiding core pipehave a distance of 10 mm from an inner wall of the pipe, and a distance between the centers of circles of the two core pipes is 35 mm.

Open ends of the vapor guiding core pipeand the liquid guiding core piperespectively have a distance of 100 mm from the top and the bottom of the TPCT.

An internal adiabatic section is formed by the vacuum cavitybetween the top sealing partition plateand the bottom sealing partition′. There partition plates are first machined to ensure the proper hole passage distribution of the vapor guiding core pipeand the liquid guiding core pipe, and are integrally welded to the pipe wall of the adiabatic section. The vapor guiding core pipeand the liquid guiding core pipeeach with a diameter of 20 mm, are welded into hole passages reserved in both the top and bottom sealing partition plate. Then, either vacuumization or the addition of adiabatic material is used to form the adiabatic partition layer.

The machined adiabatic section is first welded to the evaporation section and then welded to the condensation section, and finally the evaporation section, the adiabatic section and the condensation section are assembled to form a complete unit. During welding, it is required to ensure that the adiabatic partition layer is not damaged and brackets should be mounted on the inner wall surfaces of both the evaporation and condensation sections to fix the core pipes.

As shown in, the TPCT of the present disclosure features a straight cylindrical structure. The liquid working mediumin the evaporation section performs heat exchange with the wallof TPCT, causing the temperature to rise as it absorbs external heat, which leads to bubbles formation in a vaporization core region. As the temperature rises constantly, the pressure in the liquid bath increases, the generated bubbles grow under the action of constantly input heat and move toward the surface of the liquid bath, and the bubbles are evaporated after reaching the surface of the liquid bath and rise to the condensation section through the adiabatic section along the vapor guiding core pipeunder the action of a pressure difference. After being sprayed from the vapor guiding core pipe, the vaporous working mediumperforms heat exchange with the wallof TPCT of the condensation section and is condensed to form the liquid working medium, the liquid working medium is further cooled in the process of downward flowing along the pipe wall so that the condensation and cooling effects of the condensation section can be fully played, and the liquid working medium flows back to the evaporation section along the liquid guiding core pipeunder the action of gravity. The adiabatic section uses a vacuum thermal insulation mode to avoid the heat exchange between the high-temperature vaporous working mediumand low-temperature liquid working mediumin the middle of the pipe, so as to ensure that more heat exchange occurs in the evaporation section and the condensation section. Since the flow pressure drop of the vaporous working mediumis less than a flow pressure drop of the liquid working mediumflowing back, the directional circulating flow of the working medium is achieved.

According the present disclosure, the internally arranged adiabatic section formed by the top sealing partition plate, the bottom sealing partition′ and the cavitymay vary in position according to an application scenario and a purpose. Under certain special conditions, the length and position of the internally arranged adiabatic section in the middle may be adjusted or extended. The present disclosure solves the problem of incomplete separation between the vaporous and the liquid working mediums in the adiabatic section of the conventional TPCTs, effectively prevents the heat exchange between the vaporous working medium and the liquid working medium in the adiabatic section, significantly improves the heat transfer efficiency of ultra-long TPCT, has the advantages of simple structure, low cost, good applicability and high heat transfer efficiency, and can be applied to ultra-long-distance heat transfer scenarios such as deep frozen soil cooling, artificial freezing, foundation cooling, and geothermal resource exploitation.

The heat transfer efficiency of a TPCT with an internal adiabatic section for efficient ultra-long-distance heat exchange of the present disclosure and the heat transfer efficiency of the conventional TPCT were experimented with an inclination angle of 90° (vertical mounting). The 2 experimental TPCTs have a length ratio of 2, the ambient temperature of the evaporation section was controlled to 10° C. and kept constant during the experiment, and the ambient temperature of the condensation section was gradually decreased from 10° C. to 0° C., −5° C., −10° C., −15° C. and −20° C.

is a curve graph showing the variation of heat flux rates in a TPCT with an internal adiabatic section for efficient ultra-long-distance heat exchange and a conventional TPCT vary as ambient temperature differences between respective evaporation sections and corresponding condensation sections thereof according to the present disclosure. As observed in the experimental range, the heat flux rates of the both TPCTs increases with increasing temperature differences between the evaporation and the condensation sections. When the temperature difference is less than 15° C., the heat flow rates of both show only a small difference, whereas the difference becomes more significant as the temperature difference increases. At the temperature difference is 30° C., the heat flux rate of a TPCT with an internally adiabatic section increases by 22% compared with the conventional TPCT.