Coaxial double-pipe heat exchanger for medium-deep layer geothermal exploitation and machining method of coaxial double-pipe heat exchanger

Abstract

The invention provides a coaxial double-pipe heat exchanger for medium-deep geothermal exploitation and a processing method of the coaxial double-pipe heat exchanger. The coaxial double-pipe heat exchanger comprises an enhanced heat exchange outer cavity pipe for absorbing heat from a soil warming layer, a coaxial heat preservation inner cavity pipe for conveying heat exchange medium circulating water and a water return port for communicating the inner cavity pipe with the outer cavity pipe. A plurality of heat exchange strengthening protrusion sets are arranged on the inner wall of the heat exchange strengthening outer cavity pipe, a protrusion structure body is hemispherical and is integrally formed, and the roots of the protrusion structure body are smoothly connected in an arc mode so as to reduce stress concentration. The layout of the protrusion sets is matched with the temperature difference distribution rule of medium-deep layer terrestrial heat, different axial distribution densities are adopted in a low-temperature layer, a medium-temperature layer and a high-temperature layer, and the high-temperature layer is distributed most densely so as to strengthen heat exchange. The convex structure is used as a vortex generator, so that the laminar flow boundary layer of the pipe wall is effectively damaged, the boundary thermal resistance is reduced, the heat exchange performance is remarkably improved, the corrosion and fracture risks of a traditional welding process are avoided, the on-way resistance and the operation cost are reduced, and the reliability and the economical efficiency of a geothermal system are improved.

Description

Technical Field

[0001] This invention relates to the field of heat exchanger technology, specifically to a coaxial sleeve heat exchanger for medium-deep geothermal extraction and its processing method, and more particularly to an internally convex coaxial sleeve heat exchanger suitable for medium-deep geothermal extraction. Background Technology

[0002] With the development of the energy industry, my country's energy structure has undergone tremendous changes compared to the traditional energy structure. Medium-deep geothermal resources, due to their large reserves and high stability, have become one of the core directions for clean energy development and are gradually being applied in northern my country. Coaxial tube heat exchangers are key equipment for medium-deep geothermal development; their outer tubes, as the core heat transfer carrier between "rock and soil heat" and "working fluid inside the tube," directly determine the overall energy efficiency of the geothermal utilization system.

[0003] Currently, the design and application of coaxial heat exchange casings for medium-deep geothermal wells still face the following technical challenges: First, traditional coaxial heat exchange casings for geothermal wells use a smooth outer tube design, which easily generates a thick laminar boundary layer inside the tube. This results in low convective heat exchange efficiency between the fluid inside the tube and the wall, a prominent heat exchange bottleneck in the high-temperature layer, low utilization of geothermal resources, and serious energy waste. Second, the commonly used heat exchange enhancement schemes for medium-deep geothermal wells mostly involve adding fins, flow dividers, and other technical solutions to the tube wall, which are complex in structure and have high production costs.

[0004] Patent document CN112923592A discloses a heat exchange enhancement scheme for a shell-and-tube heat exchanger with internal and external fins in medium-deep geothermal wells. While this scheme can effectively improve the heat exchange performance of the outer tube and increase the final outlet water temperature, it is difficult to adapt to the high-temperature and high-pressure environment of medium-deep geothermal wells. The welded heat exchange fins and the roots of the heat exchange plates are prone to fracture due to stress concentration, and the working fluid contains impurities such as chloride ions, calcium and magnesium ions, which easily cause the fins to corrode and fail. At the same time, the welding process not only increases the processing cost by 40%-50%, but may also create micro-gaps that lead to working fluid leakage and pollute groundwater resources.

[0005] Patent document CN218781451U discloses a design scheme for a casing heat exchanger for medium-deep geothermal wells using a diverter pipe and spiral fins. While this scheme can improve the uniformity of water flow temperature and heat exchange performance at the outlet of the outer pipe, the added structures such as fins and diverter plates significantly increase the friction resistance, requiring a more powerful circulating pump to drive the geothermal system, thus reducing the system's energy efficiency ratio. Furthermore, the added fins are mostly arranged throughout the entire pipe, and this "over-enhancement" of non-high-temperature sections further increases the friction resistance without providing any substantial heat exchange benefit, reducing the overall economic efficiency of the system.

[0006] Patent document CN113959103A discloses a coaxial casing heat exchanger for drilling in medium-deep formations, comprising an outer tube and an insulated inner tube. The bottom of the inner side of the outer tube has an arc-shaped protrusion, and the inner wall is provided with threaded blades. Circulating water flows in from the inlet at the upper casing head of the outer tube, exchanges heat with the formation through the annular space, flows in from the radial opening at the bottom of the inner tube, and then flows out from the outlet at the top through the inner tube channel. The drawback of this design is that the threaded blades significantly increase the friction resistance along the inner wall of the outer tube, requiring a high-power circulating pump and reducing the system's energy efficiency ratio.

[0007] In view of the above-mentioned shortcomings of the existing technology, there is an urgent need to provide a heat exchange enhancement coaxial sleeve that can effectively improve the heat exchange efficiency of the outer tube, minimize the pressure difference along the pipe, and conform to the temperature difference and heat flow distribution law of deep geothermal heat. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the purpose of this invention is to provide a coaxial tube heat exchanger for medium-deep geothermal extraction and its processing method.

[0009] According to the present invention, a coaxial tube heat exchanger for medium-deep geothermal extraction includes an outer tube, an inner tube coaxially sleeved inside the outer tube, multiple sets of heat exchange enhancement protrusions disposed on the inner wall of the outer tube, and a return water port disposed on the surface of the inner tube and connecting the outer tube and the inner tube.

[0010] The heat exchange enhancement protrusions are used to form vortices when the heat exchange medium flows through the outer cavity tube, so as to achieve efficient heat exchange between the outer cavity tube and the heat exchange medium. The inner cavity tube is used to transport the heat exchange medium. After the heat exchange medium absorbs heat through the outer cavity tube, it flows into the inner cavity tube through the return water port and is transported to the ground.

[0011] Preferably, the protrusion structure of the heat exchange enhancement protrusion group is hemispherical.

[0012] Preferably, the diameter of the protrusion structure does not exceed 15% of the diameter of the outer cavity tube, and the root of the protrusion structure is smoothly connected to the inner wall of the outer cavity tube.

[0013] Preferably, the protrusions in the heat exchange enhancement protrusion group are symmetrically distributed circumferentially along the inner wall of the outer cavity tube, and the number of protrusions in the same group is not less than 6. The multiple sets of heat exchange enhancement protrusions are distributed along the central axis of the outer cavity tube, and the protrusion structures in two adjacent sets of heat exchange enhancement protrusions are staggered in the circumferential direction of the outer cavity tube, with a staggered angle of not less than 10°. The diameter of the inner circle at the top of the protrusion of the heat exchange enhancement protrusion group is larger than the outer diameter of the inner cavity tube.

[0014] Preferably, the outer cavity tube and the heat exchange strengthening protrusion assembly are integrally formed from metal materials, including one or more of aluminum alloy, carbon steel, or alloy steel.

[0015] Preferably, the distribution density of the heat exchange enhancement protrusion group along the central axis of the outer cavity tube increases with the depth of the stratum. As the depth gradually increases, the geothermal layer is divided into a low-temperature layer, a medium-temperature layer, and a high-temperature layer.

[0016] Preferably, the number of heat exchange enhancement protrusions on the inner wall of the outer cavity tube in the low temperature layer does not exceed 100 per 100m; the number of heat exchange enhancement protrusions on the inner wall of the outer cavity tube in the medium temperature layer does not exceed 250 per 100m; and the number of heat exchange enhancement protrusions on the inner wall of the outer cavity tube in the high temperature layer is not less than 400 per 100m.

[0017] Preferably, the return water outlet is located at the high-temperature end of the inner cavity tube, which extends into the high-temperature layer during use.

[0018] According to the present invention, a processing method for a coaxial tube heat exchanger for medium-deep geothermal extraction is provided, wherein the heat exchange strengthening protrusion group is processed by a cold working integral forming process, specifically by using a hydraulic bulging process or an external rolling process to locally plastically deform the outer tube wall material, forming an inner protrusion structure by inward concavity.

[0019] Preferably, the external rolling process employs multi-point rolling forming, comprising the following steps: S1: Fix the prefabricated smooth metal pipe onto the rotating fixture; S2: Using hard alloy rollers evenly distributed on the outside of the steel pipe, radial pressure is applied to the pipe wall under the control of CNC program, causing the pipe wall material to undergo local plastic deformation under pressure, bulging inward to form a bulging structure of heat exchange strengthening bulge group; S3: Drive the steel pipe to rotate and repeat step S2 to process protrusions at different circumferential positions of the steel pipe, ultimately forming a heat exchange strengthening protrusion group.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention uses a heat-enhancing protrusion structure as a vortex generator to form a good vortex, which can fully break the laminar boundary layer on the inner wall of the outer tube and induce the fluid to generate a secondary flow perpendicular to the mainstream direction. Compared with traditional smooth-walled geothermal pipes, it forms a more thorough three-dimensional mixing effect, thereby effectively reducing boundary thermal resistance, significantly improving the heat exchange performance of the outer tube, and improving the utilization rate of medium and deep geothermal resources.

[0021] 2. By adopting a hemispherical protrusion structure, this invention occupies a small cross-sectional area of ​​the main channel, thus avoiding large pressure differentials and severe pressure drops along the flow path. Compared with finned heat exchange enhanced geothermal pipes, a circulation pump with lower power can be selected, reducing the operating cost of the geothermal system and improving its economic efficiency.

[0022] 3. This invention directly stamps or rolls hemispherical protrusions onto the pipe to form an integral shape. Compared with finned heat exchange enhanced geothermal pipes, this eliminates the risk of weld corrosion and thermal stress fracture. At the same time, it simplifies the processing technology, eliminates the need for additional materials, improves the reliability of the geothermal system, and further reduces costs.

[0023] 4. This invention adopts a segmented protrusion group discrete design scheme, which is adapted to the temperature difference and heat flow distribution law of medium and deep geothermal layers. Compared with the existing "over-enhanced" scheme of fin arrangement throughout the pipe, it can accurately enhance heat exchange for different temperature zones, thereby making more efficient use of geothermal resources, optimizing the layout of heat exchange areas, and improving the overall economic efficiency of the system. Attached Figure Description

[0024] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram illustrating the structure of the internally convex coaxial sleeve heat exchanger, which is the main feature of this invention. Figure 2 This is a schematic diagram illustrating the heat exchange process of the internally convex coaxial sleeve heat exchanger in soil, which is the main feature of this invention. Figure 3a This is a schematic diagram illustrating the eddy current generation and heat exchange effects of a smooth-walled coaxial sleeve, which is the main feature of this invention. Figure 3b This is a schematic diagram illustrating the eddy current generation effect and heat exchange effect of the internally convex coaxial sleeve heat exchanger, which is the main feature of this invention.

[0025] Figure label: 1. Outer cavity tube, 2. Inner cavity tube, 3. Heat exchange enhancement protrusion group, 4. Return water port. Detailed Implementation

[0026] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0027] The invention will be further described in detail below with reference to the accompanying drawings. This invention discloses a coaxial tube heat exchanger for medium-deep geothermal extraction and its processing method, and proposes a layout principle adapted to deep geothermal temperature differences and heat flow, thereby solving the problems of low heat exchange performance and serious energy waste in existing geothermal systems, and improving the heat exchange efficiency of heat exchange tubes more efficiently and cost-effectively. Figure 1 As shown, the internally convex coaxial sleeve heat exchanger mentioned in this invention includes an outer cavity tube 1, an inner cavity tube 2, multiple sets of heat exchange enhancement protrusions 3, and a return water port 4. The inner cavity tube 2 is coaxially sleeved inside the outer cavity tube 1 and is used to transport the heat exchange medium. It is continuously lowered into the geothermal well from inside the outer cavity tube 1. The high-temperature end of the inner cavity tube 2 faces downwards and extends to the high-temperature layer during use. The return water port 4 is located at the high-temperature end of the inner cavity tube 2. The heat exchange enhancement protrusions 3 are used to form vortices when the heat exchange medium flows through the outer cavity tube 1, achieving efficient heat exchange between the outer cavity tube 1 and the heat exchange medium. The heat exchange enhancement protrusions 3 are disposed on the inner wall of the outer cavity tube 1, and multiple sets of heat exchange enhancement protrusions 3 are distributed along the central axis of the outer cavity tube 1, with their distribution distance determined by a discrete layout formula. The return water inlet 4 is located on the wall of the inner tube 2, connecting the outer tube 1 and the inner tube 2. After absorbing heat through the outer tube 1, the heat exchange medium flows into the inner tube 2 through the return water inlet 4 and is transported to the ground. The ends of the inner tube 2 and the outer tube 1 that extend underground are closed structures, preventing the liquid inside the tubes from leaking into the surrounding underground environment.

[0028] The raised structures of the heat exchange enhancement protrusion group 3 are hemispherical, with a diameter not exceeding 15% of the diameter of the outer cavity tube 1. The center of the protrusion group is in contact with the inner wall surface, and the root edge of the protrusion structure forms a smooth arc-shaped connection with the inner wall of the outer cavity tube 1. The raised structures in the same heat exchange enhancement protrusion group 3 are symmetrically distributed circumferentially along the inner wall of the outer cavity tube 1, and the number of protrusions in the same group is not less than 6. The raised structures in two adjacent heat exchange enhancement protrusion groups 3 are staggered in the circumferential direction of the outer cavity tube 1, with a staggered angle of not less than 10°. The diameter of the inscribed circle at the top of the raised structure in the same heat exchange enhancement protrusion group 3 is larger than the outer diameter of the inner cavity tube 2. The outer cavity tube 1 and the heat exchange enhancement protrusion group 3 are integrally formed from metal materials, including one or more of aluminum alloy, carbon steel, or alloy steel, without the need for additional welding and splicing processes.

[0029] The hemispherical protrusion structure designed in this invention can be directly stamped / rolled from the pipe material, forming an integrated structure from the deformation of the base material. There are no welding points, thus eliminating the risks of weld corrosion and thermal stress fracture compared to common finned heat exchange enhanced geothermal pipes. The manufacturing process is simpler and requires no additional materials, improving the reliability and economy of the geothermal system. Simultaneously, the hemispherical protrusion structure occupies a very small cross-sectional area of ​​the main flow channel, avoiding large pressure differentials along the flow path and thus preventing drastic pressure drops. Therefore, compared to common finned heat exchange enhanced geothermal pipes, a lower-power circulation pump can be selected, resulting in lower operating costs and superior economic efficiency for the entire geothermal system.

[0030] The following will describe the specific design concept of the present invention through specific embodiments and in conjunction with the accompanying drawings, making the technical content of the present invention clearer and easier to understand. The present invention can be embodied in many different embodiments, and the scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0031] refer to Figure 1 After a conventional geothermal well is drilled, the soil layers are roughly divided into shallow, medium-deep, and deep geothermal layers based on local geological conditions. Shallow geothermal layers typically extend 0–200 m below the surface, medium-deep layers extend 200–3000 m, and deep geothermal layers extend 3000 m and deeper. For ease of discussion, this invention roughly divides the working area into low-temperature, medium-temperature, and high-temperature layers to highlight their temperature differences. The specific depth range represented by each layer depends on the actual geothermal distribution characteristics at the well's location. From the low-temperature layer to the medium-temperature layer and then to the high-temperature layer, the depth represented by each layer gradually increases.

[0032] The internally convex coaxial tube heat exchanger mentioned in this invention patent has different layout patterns in different geothermal layers. The distribution pattern of the heat exchange enhancement protrusion group 3 along the pipeline adapts to the temperature difference distribution pattern of medium-deep geothermal layers. The distribution density along the central axis of the outer cavity tube 1 increases with the depth of the formation, thereby improving heat exchange performance and system economy. The segmented protrusion group discrete design scheme proposed in this invention is more in line with the distribution pattern of temperature difference and heat flow in medium-deep geothermal layers, and more fully and effectively utilizes the distribution pattern of internal and external temperature differences for heat exchange enhancement. Compared with the "excessive heat exchange enhancement" of low-temperature sections in common geothermal pipe heat exchange enhancement methods, it is more efficient and targeted in utilizing geothermal resources and optimizing the heat exchange area. In the low-temperature layer, the number of heat exchange enhancement protrusion groups 3 on the inner wall of the outer cavity tube 1 does not exceed 100 groups per 100m; in the medium-temperature layer, the number of heat exchange enhancement protrusion groups 3 on the inner wall of the outer cavity tube 1 does not exceed 250 groups per 100m; in the high-temperature layer, the number of heat exchange enhancement protrusion groups 3 on the inner wall of the outer cavity tube 1 is not less than 400 groups per 100m.

[0033] like Figure 2 As shown, the heat exchange enhancement protrusion group 3 is more densely distributed in the middle and deep layers, where the soil temperature is higher and the geothermal resources are more abundant. The dense heat exchange enhancement protrusion group 3 can efficiently conduct the heat flow of the soil and rock layer to the heat-conducting fluid flowing in the outer cavity tube 1. At the same time, the heat exchange enhancement protrusion group 3 breaks the laminar boundary layer formed by the fluid in the original outer cavity tube 1 on the smooth wall, effectively reducing the thermal resistance between the solid and liquid interfaces, making the heat exchange of the solution more uniform. After the heat exchange is completed, the solution flows back to the inside of the inner cavity tube 2 through the return water port 4 opened at the bottom of the inner cavity tube 2, and finally the heat-exchanged solution is discharged through the inner cavity tube 2.

[0034] This paper takes a common coaxial geothermal casing in northern my country as an example for in-depth analysis. In this example, the outer diameter of the outer tube 1 is 0.089 m, the inner diameter is 0.08 m, and the outer diameter of the inner tube 2 is 0.055 m. A 30 cm section is selected as the heat exchange enhancement type outer tube 1 of the coaxial casing. The heat exchange enhancement effect of a single group of heat exchange enhancement type protrusions 3 is numerically calculated and analyzed. In this example, the heat exchange enhancement type protrusions are hemispheres with a diameter of 1.28 cm, uniformly distributed along the inner wall of the heat exchange enhancement type outer tube 1 at a circumferential angle of 60°. Considering the acceleration due to gravity, the circulating heat exchange medium in this example is liquid water, with an inlet velocity of 6 m / s and an initial temperature of 20 °C. To better simulate the thermal environment under the high-temperature layer, this embodiment sets the outer surface of the pipe wall as a variable temperature surface with a temperature gradient of 2.5℃ / 100m, which changes uniformly in the vertical direction. The material of the outer cavity pipe 1 is steel, and the maximum number of iterations in the numerical calculation is set to 500.

[0035] Calculations show that, under the action of a single set of protrusions, the heat transfer coefficient of the protrusion set increases from 2.17 × 10⁻⁶. 4 W / (m²•K) increased to 4.44×10 4 The heat transfer efficiency (W / (m²•K)) is increased by 104.61% compared to a smooth wall surface. The average surface heat transfer coefficient also increases from 2.02 × 10⁻⁶. 4 W / (m²•K) increased to 2.59×10 4 W / (m²•K) is increased by 31.18% compared to a smooth wall surface, and the velocity vector contour plot and trace plot clearly show that good vortices are formed at the protrusions. In summary, the heat transfer enhancement effect of a single set of protrusions is 104.61%, and the enhancement effect on the entire cavity section is 31.18%, which can form good vortices to break the laminar boundary layer formed on the inner wall of the outer cavity tube. Based on this, the vortex enhancement effect and heat transfer enhancement effect caused by adjacent sets of protrusions are simulated and calculated.

[0036] Calculations in this embodiment show that, under the interaction of adjacent protrusion groups, the average heat transfer coefficient of the inner wall of the outer cavity tube 1 increases from 2.59 × 10⁻⁶. 4 W / (m²•K) increased to 2.63×10 4 The heat transfer efficiency (W / (m²•K)) is approximately 30.2% higher than that of a smooth wall. Furthermore, the velocity vector cloud diagram and trace diagram clearly show that after passing through the protrusion group, the eddy currents are significantly enhanced, resulting in stronger fluid disturbance and effectively disrupting the laminar boundary layer, thus greatly reducing boundary thermal resistance. Based on this, it can be reasonably estimated that the introduction of the protrusion group can improve the heat transfer enhancement effect of the coaxial geothermal heat exchanger tube at the protrusions by approximately 105%, and the overall average heat transfer coefficient is increased by approximately 30.5% compared to a smooth wall. The enhanced eddy currents formed by adjacent protrusion groups effectively break the laminar boundary layer, further reducing fluid thermal resistance and improving heat transfer efficiency.

[0037] The calculation results of the above embodiments are used to illustrate the working principle of the coaxial shell heat exchanger for medium-deep geothermal extraction mentioned in this invention. As shown in Figure 3, the inner convex coaxial shell heat exchanger mentioned in this invention, with the same pipe diameter, has heat exchange enhancement protrusions 3 arranged on its inner wall. These heat exchange enhancement protrusions can act as high-quality vortex generators, forming good vortices to fully break the laminar boundary layer formed on the inner wall of the coaxial geothermal pipe. They can also induce the fluid to generate secondary flow perpendicular to the mainstream direction. This three-dimensional mixing process is more efficient than simple two-dimensional fin heat exchange. At the same time, this staggered design can prevent the fluid from forming a stable wake channel behind the protrusions. When the fluid bypasses the first set of protrusions and separates, it will directly collide with the second set of protrusions located in the downstream gap, thereby forcing the fluid to continuously change its flow direction. This not only enhances radial mixing but also significantly increases the secondary flow intensity Se, making the heat exchange efficiency 10%-15% higher than the traditional in-line method. Therefore, compared with traditional geothermal pipes with smooth walls, the convex coaxial sleeve heat exchanger mentioned in this invention can effectively reduce boundary thermal resistance, thereby significantly improving the heat exchange performance of the outer cavity tube 1 and making full use of medium-deep geothermal resources.

[0038] Compared to currently used smooth-section outer tube heat exchangers, the internally convex coaxial sleeve heat exchanger mentioned in this invention, as shown in the figure, transforms the stable laminar flow of the circulating water into a high-quality vortex flow when flowing through the intermediate and high-temperature layers of the outer tube 1. This significantly increases the secondary flow intensity Se, disrupting the uniform laminar thermal boundary layer formed on the inner wall of the outer tube 1 and allowing for thorough mixing of the hot and cold fluids. Consequently, the convective heat transfer coefficient h on the inner wall surface is significantly improved, increasing the heat transfer per well and raising the outlet water temperature T of the inner tube 2.

[0039] The internally convex coaxial tube heat exchanger designed in this invention utilizes a hemispherical protrusion group and its layout to generate excellent vortices, effectively breaking the laminar boundary layer on the inner wall of the outer tube 1. This reduces fluid thermal resistance and increases the heat transfer coefficient of the outer tube 1 of the coaxial geothermal heat exchanger. The proposed segmented protrusion group discrete design scheme better conforms to the distribution patterns of temperature difference and heat flow in medium-deep geothermal systems, and more fully and effectively utilizes the distribution patterns of internal and external temperature differences for heat transfer enhancement. Compared to the "over-heat transfer enhancement" of low-temperature sections in common geothermal pipe heat transfer enhancement methods, this design is more efficient and targeted in utilizing geothermal resources and optimizing the heat transfer area. Simultaneously, this structure avoids the corrosion and fracture risks associated with traditional welding processes, reduces frictional resistance and operating costs, and improves the reliability and economy of the geothermal system.

[0040] Furthermore, the present invention also provides a preferred processing method for the aforementioned outer cavity tube 1. To ensure the mechanical strength of the protruding structure and eliminate welding thermal stress, the heat exchange strengthening protrusion group 3 is processed using a cold working integral forming process. Specifically, this involves locally plastically deforming the tube wall material of the outer cavity tube 1 through a hydraulic bulging process or an external rolling process, causing it to concave inward to form an inner protruding structure. Specifically, when using the external rolling process, multi-point rolling forming is employed, including the following steps: S1: Fixing a pre-fabricated smooth metal steel tube onto a rotating fixture; S2: Using carbide rollers evenly distributed on the outside of the steel tube, applying radial pressure to the tube wall under CNC program control, causing the tube wall material to locally plastically deform under pressure, protruding inward to form the protruding structure of the heat exchange strengthening protrusion group 3; S3: Driving the steel tube to rotate, repeating step S2, processing the protruding structure at different circumferential positions of the steel tube, ultimately forming the heat exchange strengthening protrusion group 3. This cold working process not only achieves seamless forming, but also produces a work hardening effect on the raised parts, increasing their surface hardness by 15%-20% compared to the base material. This significantly enhances the wear resistance of the raised structure under high-speed water flow and extends the service life of the geothermal pipe.

[0041] In the description of this application, it should be understood that the terms "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0042] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A coaxial sleeve heat exchanger for medium-deep geothermal extraction, characterized in that, It includes an outer cavity tube (1), an inner cavity tube (2) coaxially sleeved inside the outer cavity tube (1), multiple sets of heat exchange enhancement protrusions (3) disposed on the inner wall of the outer cavity tube (1), and a return water port (4) disposed on the wall of the inner cavity tube (2) and connecting the outer cavity tube (1) and the inner cavity tube (2). The heat exchange enhancement protrusion group (3) is used to form vortices when the heat exchange medium flows through the outer cavity tube (1), so as to realize efficient heat exchange between the outer cavity tube (1) and the heat exchange medium. The inner cavity tube (2) is used to transport the heat exchange medium. After the heat exchange medium absorbs heat through the outer cavity tube (1), it flows into the inner cavity tube (2) through the return water port (4) and is transported to the ground.

2. The coaxial casing heat exchanger for medium-deep geothermal extraction as described in claim 1, characterized in that, The protrusion structure of the heat exchange enhancement protrusion group (3) is hemispherical.

3. The coaxial casing heat exchanger for medium-deep geothermal extraction as described in claim 2, characterized in that, The diameter of the protrusion structure does not exceed 15% of the diameter of the outer cavity tube (1), and the root of the protrusion structure forms a smooth arc-shaped connection with the inner wall of the outer cavity tube (1).

4. The coaxial casing heat exchanger for medium-deep geothermal extraction as described in claim 1, characterized in that, The protrusions in the heat exchange enhancement protrusion group (3) are symmetrically distributed along the inner wall of the outer cavity tube (1) in the same group, and the number of protrusions in the same group is not less than 6. Multiple sets of heat exchange enhancement protrusions (3) are distributed along the central axis of the outer cavity tube (1). The protrusion structures in two adjacent sets of heat exchange enhancement protrusions (3) are staggered in the circumferential direction of the outer cavity tube (1) with a staggered angle of not less than 10°. The inner circle diameter at the top of the protrusion structure of the heat exchange enhancement protrusion group (3) is larger than the outer diameter of the inner cavity tube (2).

5. The coaxial casing heat exchanger for medium-deep geothermal extraction as described in claim 1, characterized in that, The outer cavity tube (1) and the heat exchange strengthening protrusion group (3) are integrally formed by metal materials, including one or more of aluminum alloy, carbon steel or alloy steel.

6. The coaxial casing heat exchanger for medium-deep geothermal extraction as described in claim 1, characterized in that, The distribution density of the heat exchange enhancement protrusion group (3) along the central axis of the outer cavity tube (1) increases with the depth of the stratum. As the depth below the surface gradually increases, the geothermal layer is divided into a low temperature layer, a medium temperature layer and a high temperature layer.

7. The coaxial casing heat exchanger for medium-deep geothermal extraction as described in claim 6, characterized in that, In the low-temperature layer, the number of heat exchange enhancement protrusions (3) on the inner wall of the outer cavity tube (1) does not exceed 100 per 100m; in the medium-temperature layer, the number of heat exchange enhancement protrusions (3) on the inner wall of the outer cavity tube (1) does not exceed 250 per 100m; in the high-temperature layer, the number of heat exchange enhancement protrusions (3) on the inner wall of the outer cavity tube (1) is not less than 400 per 100m.

8. The coaxial casing heat exchanger for medium-deep geothermal extraction as described in claim 7, characterized in that, The return water inlet (4) is located at the high-temperature end of the inner cavity tube (2), which extends into the high-temperature layer during use.

9. A method for processing a coaxial casing heat exchanger for medium-deep geothermal extraction as described in any one of claims 1 to 8, characterized in that, The heat exchange strengthening protrusion group (3) is manufactured by cold working integral forming process, specifically by using hydraulic expansion process or external rolling process to locally plastically deform the wall material of the outer cavity tube (1) and form an inner protrusion structure by indentation.

10. The processing method as described in claim 9, characterized in that, The external rolling process employs multi-point rolling molding and includes the following steps: S1: Fix the prefabricated smooth metal pipe onto the rotating fixture; S2: Using hard alloy rollers evenly distributed on the outside of the steel pipe, radial pressure is applied to the pipe wall under the control of CNC program, so that the pipe wall material undergoes local plastic deformation under pressure and bulges inward to form the bulging structure of heat exchange strengthening bulge group (3). S3: Drive the steel pipe to rotate and repeat step S2 to process protrusions at different circumferential positions of the steel pipe, and finally form a heat exchange strengthening protrusion group (3).

Images