Welded heat exchange tube and manufacturing method thereof

Abstract

The invention relates to the technical field of heat exchange tubes, in particular to a welded heat exchange tube and a manufacturing method thereof. The welded heat exchange tube comprises a plurality of channels formed on the inner wall surface of the heat exchange tube; a rib formed between the plurality of channels; wherein the surfaces of the grooves and the ribs are rough surfaces formed through sand blasting treatment, the rough surfaces are provided with a plurality of tiny pits, and a selection method for the size of sand blasting particles is provided according to the optimal opening radius of the pits.

Description

Technical Field

[0001] The present application relates to the technical field of heat exchange tubes, and particularly relates to a welded heat exchange tube and a manufacturing method thereof. Background Art

[0002] Dry evaporators are widely used in fields such as refrigeration air-conditioning and chemical industry. Its working principle is that in a shell-and-tube heat exchanger, the fluid on the shell side is a high-temperature heating fluid, and the fluid on the tube side is an evaporating fluid; under the heating of the high-temperature fluid on the shell side, the liquid enters one end of the heat exchange tube and becomes vapor / superheated vapor when flowing out of the other end of the heat exchange tube, and a phase change occurs during the flow process to generate vapor. The evaporator with this heat exchange method is called a "dry evaporator", and its key component is the evaporation heat exchange tube, commonly known as the "dry steam tube".

[0003] The performance of the heat exchange tube determines the performance and price of the heat exchanger. In order to improve the heat transfer performance of the "dry steam tube", the related technology is to process spiral grooves on the inner wall surface of the heat exchange tube, commonly known as the "internally threaded tube". The arrangement of multiple spiral grooves on its inner wall surface not only increases the heat exchange area compared with the smooth wall surface, but also changes the flow pattern of the fluid, thereby improving the heat transfer coefficient. The conventional processing method of the internally threaded tube is to make it by placing a lining core mold made of alloy material into a seamless copper tube or stainless steel tube and then rolling it outside the tube. Currently, the internally threaded tubes produced by this processing method are the mainstream products in the market.

[0004] In order to improve the heat exchange capacity of the internally threaded tube, people have tried to change the geometric structure of the grooves and ribs on the inner wall surface to increase the heat transfer coefficient. For example, a kind of internally threaded tube disclosed in the Chinese patent document with the publication number CN2539948Y is characterized in that cut-off ridge type grooves with regular / irregular intervals are arranged on the tooth top ridges to increase the heat exchange area; a kind of internally threaded tube disclosed in the Chinese patent document with the publication number CN202947520U is characterized in that main and secondary teeth with different heights are arranged on the surface and are staggered, and this technology increases the fluid turbulence. However, limited by the aforementioned processing method, the effect of these technologies on increasing the heat transfer coefficient is not significant. Summary of the Invention

[0005] The purpose of the present application is to solve at least one of the above technical problems, and to provide a welded heat exchange tube and a manufacturing method thereof.

[0006] In a first aspect, the present application provides a welded heat exchange tube, comprising:

[0007] Multiple grooves formed on the inner wall surface of the heat exchange tube;

[0008] Ribs formed between the multiple grooves;

[0009] Wherein, the surfaces of the groove and the rib are both rough surfaces formed by sandblasting, and the rough surfaces have a plurality of micro-pits.

[0010] In some possible embodiments, the channel extends spirally.

[0011] In some possible embodiments, the plurality of channels includes a first channel and a second channel, and the first channel and the second channel are arranged to cross each other in a manner with opposite spiral directions.

[0012] In some possible embodiments, the spiral angle of the channel is 15 - 60°, the depth of the channel is 0.1 - 0.3 mm, and the pitch of the channel is 0.3 - 1.0 mm.

[0013] In a second aspect, the present application provides a method for manufacturing a welded heat exchange tube, including:

[0014] Providing a metal strip, the metal strip having a first surface and a second surface arranged opposite to each other;

[0015] Rolling the first surface with a roller having grooves and protrusions on its surface, thereby forming a plurality of channels and ribs located between the plurality of channels on the first surface;

[0016] Performing sandblasting on the first surface with sand grains to make the surfaces of the channels and ribs become rough surfaces having a plurality of micro-pits;

[0017] Winding the metal strip into a tube with a butt joint seam and welding the butt joint seam; wherein, the first surface constitutes the inner surface of the tube, and the second surface constitutes the outer surface of the tube.

[0018] In some possible embodiments, before winding the metal strip into a tube with a butt joint seam, the method further includes: determining the particle size of the sand grains for sandblasting according to the target application conditions of the heat exchange tube.

[0019] In some possible embodiments, the method for determining the particle size of the sand grains according to the target application conditions of the heat exchange tube includes:

[0020] 1) Providing a second metal strip, the second metal strip having a front surface and a back surface arranged opposite to each other, and the second metal strip having the same thickness as the metal strip;

[0021] 2) Rolling the front surface with the roller, thereby forming a plurality of third channels and third ribs located between the plurality of third channels on the front surface;

[0022] 3) maintaining the same process conditions as those used in the sandblasting process of the first surface, sandblasting the front surface with second sand particles of different particle sizes, so that the surfaces of the third grooves and the third ribs on the surface thereof become a second roughened surface having a plurality of second micro-pits;

[0023] 4) Using an electron microscope, the opening radii and corresponding occurrence frequencies of the second micro-pits in each region were measured, and a distribution graph of the opening radii and occurrence frequencies of the second micro-pits for each particle size was generated. This database was then established for the second sand particles of different particle sizes, the second micro-pit opening radii, and the corresponding occurrence frequencies.

[0024] 5) According to the following formula (1), the optimal opening radius r of the small pits on the rough surface is calculated, that is, r c <r<2r c ,

[0025]

[0026] Among them, r c is the critical opening radius; Re is the Reynolds number of the fluid in the heat exchange tube under the target application conditions; σ is the surface tension of the fluid in the heat exchange tube under the target application conditions; T s is the saturation temperature of the fluid in the heat exchange tube under the target application conditions; ρ v The gas density in the heat exchange tube under the target application conditions; h fg is the latent heat of vaporization of the fluid in the heat exchange tube under the target application conditions; ΔT is the wall superheat under the target application conditions;

[0027] Then, according to the database, the particle size of the second sand particles corresponding to the area where the second micro-pits appear the most times within the value range of the optimal opening radius r is determined, that is, the target particle size;

[0028] Therefore, the target particle size is determined as the particle size of the sand particles.

[0029] In some possible implementations, before winding the metal strip into a tube with a butt seam, the method further includes:

[0030] Determine whether the number of target pits in the plurality of micro pits reaches a set ratio, wherein the opening radius of the target pit is greater than r c and less than 2r c .

[0031] In some possible implementations, before winding the metal strip into a tube with a butt seam, the method further includes:

[0032] Determine that the number of the target pits per unit area reaches a set number.

[0033] The welded heat exchange tube provided by the present application includes: a plurality of channels formed on the inner wall surface of the heat exchange tube; ribs formed between the plurality of channels; wherein, the surfaces of the grooves and the ribs are both rough surfaces formed by sandblasting, and the rough surfaces have a plurality of tiny pits. In this way, the heat exchange performance of the heat exchange tube can be further improved. Description of the Drawings

[0034] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings of the embodiments will be briefly introduced below. Obviously, the drawings described below only relate to some embodiments of the present application and do not limit the present application.

[0035] Figure 1 It is a schematic structural view of the heat exchange tube provided by an embodiment of the present application after being partially cut open.

[0036] Figure 2 is Figure 1 A schematic structural view of the heat exchange tube shown during the manufacturing process.

[0037] Figure 3 is Figure 2 The A-A sectional view of.

[0038] Figure 4 It is a schematic view of the inner wall surface of the heat exchange tube provided by an embodiment of the present application under an electron microscope.

[0039] Figure 5 It is a flowchart of the manufacturing method of the heat exchange tube provided by an embodiment of the present application.

[0040] Figure 6 It is a flowchart of the manufacturing method of the heat exchange tube provided by an embodiment of the present application.

[0041] Figure 7 It is a distribution diagram of the equivalent radius of the pit outlet per unit area and the number of occurrences in an embodiment of the present application.

[0042] Figure 8 It is a comparison of the experimental values of the heat transfer coefficients of the heat exchange tube and the traditional internal thread tube under different mass flow rate conditions in an embodiment of the present application.

[0043] Description of the Reference Numerals:

[0044] 100 - heat exchange tube;

[0045] 1 - channel, 2 - rib, 3 - weld seam, 4 - tiny pit. Detailed Embodiments

[0046] To make the objectives, technical solutions, and advantages of this application clearer, the following will clearly and completely describe the technical solutions of the embodiments of this application in conjunction with the accompanying drawings of the embodiments of this application. Obviously, the described embodiments are some, but not all, of the embodiments of this application. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of this application without creative efforts fall within the scope of protection of this application. It can be understood that, without conflict, some technical means described in various embodiments herein can be replaced or combined with each other.

[0047] In the description of this application, if there are terms such as "first", "second", etc., they are only used to distinguish the described objects and do not have any sequential or technical meaning. Thus, the objects defined with "first", "second", etc. may explicitly or implicitly include one or more of such objects. Moreover, for example, the term "first element" itself does not imply the existence of a "second element", and the term "second element" itself does not imply the existence of a "first element". In addition, similar terms such as "one" or "a" do not indicate a quantity limitation but rather indicate the existence of at least one, and "multiple" means not less than two.

[0048] In the description of this application, the terms "comprising" and "having" indicate the presence of the described features, numbers, operations, elements, and / or combinations thereof, but do not exclude the presence or addition of one or more other features, numbers, operations, elements, and / or combinations thereof.

[0049] In the description of this application, referring to "one embodiment" or "some embodiments", etc. means that in one or more embodiments of this application, the specific features, structures, or characteristics described in connection with that embodiment are included. Thus, statements such as "in one embodiment", "in some embodiments", "in other some embodiments", "in still other embodiments", etc. that appear in different places in this specification are not necessarily all referring to the same embodiment, but rather mean "one or more but not all embodiments", unless otherwise specifically emphasized in a different way.

[0050] Figures 1 to 4 Fig. 13 shows a welded heat exchange tube 100 provided by an embodiment of this application, which is formed by curling and welding a steel strip, has a steel pipe structure, and is applied as a dry steam pipe in an evaporative heat exchanger.

[0051] The heat exchange tube 100 is an internally threaded heat exchange tube 100, and a plurality of spirally extending channels 1 are formed on its inner wall surface, and ribs 2 that also extend spirally are formed between the channels 1. Among them, the surfaces of the channels 1 and the ribs 2 are both rough surfaces formed by sandblasting, and the rough surfaces have a plurality of micro-pits 4.

[0052] The depth and diameter of the aforementioned micro-pits 4 can be at the micron level, for example, 2 - 10 um. The spiral angle of the channel 1 can be 15 - 60°, the depth can be 0.1 - 0.3 mm, and the pitch can be 0.3 - 1.0 mm.

[0053] According to the heat exchange tube 100 provided by the embodiment of the present application, the surfaces of the channel 1 and the rib 2 are configured as rough surfaces formed by sandblasting, which can change the flow state of the heat exchange working medium inside the tube, thereby helping to improve the heat transfer coefficient of the heat exchange tube 100.

[0054] In this embodiment, the spiral directions of each channel 1 are the same and parallel to each other without crossing.

[0055] In some other embodiments, the multiple channels 1 include multiple first channels and multiple second channels, and the first channels and the second channels are arranged to cross each other in a way that the spiral directions are opposite. Thus, the inner wall surface of the heat exchange tube 100 is formed into a grid-like wall surface.

[0056] The embodiment of the present application also provides a manufacturing method for the welded heat exchange tube 100 with the above configuration. The method includes:

[0057] S501, providing a metal strip, which has a first surface and a second surface arranged opposite to each other.

[0058] The metal strip can be a steel strip, such as a stainless steel or ordinary carbon steel strip, or can also be a copper strip or an aluminum strip.

[0059] S502, using a roller with grooves and protrusions on its surface to roll the first surface, thereby forming multiple channels and ribs located between the multiple channels on the first surface.

[0060] In order to obtain spiral channels and ribs on the inner surface of the finally produced heat exchange tube, the roller should roll the first surface in a direction inclined with respect to the width of the metal strip. It can be understood that when the roller rolls the first surface of the metal strip, the protrusions on the roller squeeze the material on the first surface inward to form inclined channels, and the grooves on the roller provide a space for the material on the first surface to form inclined ribs.

[0061] In some other embodiments, the roller rolls the first surface of the metal strip successively in two mutually crossing inclined directions, thereby forming grid-like crossed channels on the first surface.

[0062] S503, using sand grains to perform sandblasting on the first surface, so that the surfaces of the channels and ribs obtained in step S502 become rough surfaces with multiple micro-pits.

[0063] S504. Wind the metal strip into a tube with a butt joint seam and weld the butt joint seam; wherein, the first surface forms the inner surface of the tube, and the second surface forms the outer surface of the tube.

[0064] S505. Anneal the welded tube.

[0065] To enable readers to better understand the technology of the embodiments of this application, the phase change process and heat transfer mechanism completed inside the dry steam tube are introduced as follows:

[0066] The phase change process completed inside the dry steam tube is a process in which a liquid forms vapor in a flowing state. In heat transfer, it is defined as a flow boiling process. According to classical heat transfer theory, there are two heat transfer mechanisms for flow boiling: forced convection heat transfer and nucleate boiling heat transfer, and the contributions of the two are superimposed on each other, as expressed by the following formula:

[0067] h = Sh 沸 + Fh 对 (2)

[0068] where, h is the heat transfer coefficient on the inner surface of the tube, h 沸 is the heat transfer coefficient component caused by nucleate boiling, h 对 is the heat transfer coefficient component caused by forced convection, and S and F are the corresponding coefficients.

[0069] As described above, measures should be taken in both forced convection heat transfer and nucleate boiling to strengthen the flow boiling inside the tube. Setting spiral channels and ribs on the inner wall surface not only increases the heat transfer area, but also the protruding ribs and concave grooves destroy the velocity and temperature distributions at the bottom layer of the boundary layer, strengthening the convective heat transfer. On the other hand, an important means to strengthen nucleate boiling heat transfer is to increase the vaporization nuclei and the number of bubbles. The formation of vaporization nuclei is closely related to the geometric state of the surface. The pits and cavities on the heating surface are the direct sources of vaporization nuclei. Undoubtedly, increasing the pits (or cavities) on the heating surface as much as possible is an effective method to increase the boiling heat transfer coefficient. According to classical boiling theory, if the pits on the heating surface become active vaporization nuclei and develop into bubbles, the outlet radius of the pits must be greater than the critical radius. As expressed by the following formula (3),

[0070]

[0071] where, r' is the critical opening radius of the pit, σ is the surface tension of the fluid, T s is the saturation temperature of the fluid, ρ v is the gas density, h fg is the latent heat of vaporization, and ΔT is the wall superheat.

[0072] When the outlet radius of the pit is greater than the critical radius r', it can develop into an activated vaporization core and evolve into a bubble; conversely, it is impossible to develop into an activated vaporization core or a bubble. Although the classical formula (3) is simple, it reveals the conditions that must be met for the generation of bubble cores, that is, the functional relationship between the size of the opening of the pit and the wall superheat. Under a certain superheat condition, the opening size of the pit must reach a certain value to generate a vaporization core. However, the theoretical derivation of the famous classical formula (3) is based on the analysis of a single pit in pool boiling. Obviously, the flow boiling phenomenon in the dry evaporation tube is a different heat transfer process from the pool boiling phenomenon, and the heat transfer mechanism is different. First, the boiling in the flow process in the dry evaporation tube is affected by the fluid flow, and the role of its flow boundary layer cannot be ignored. The boiling process shows that during the process of the bubble core developing into a bubble, on the one hand, the thin gas film existing at the bottom of the bubble continuously transports vapor to the bubble to promote its growth, but on the other hand, a part of the area at the top of the spherical bubble is exposed to the mainstream and is cooled by the mainstream fluid at a lower temperature, thus hindering the growth of the bubble. There is a temperature boundary layer with a higher temperature near the heating wall, which is helpful for the growth of the bubble. When the Reynolds number Re is small and the boundary layer is thick, larger-diameter spherical bubbles are allowed to grow, that is, the opening radius of the pit can be larger; conversely, when the Reynolds number Re is large and the boundary layer is thin, the outlet radius of the pit should be smaller to be conducive to the growth of the bubble.

[0073] It can be seen that, different from pool boiling, the classical formula (3) mentioned above should not be directly used to determine the critical opening radius of the pit in the dry evaporation tube of flow boiling, but the influence of the Reynolds number Re on the critical opening radius should be considered. Through a large number of studies and experiments, the inventor found that for the dry evaporation tube of flow boiling, it is appropriate to use the formula to determine the critical opening radius of the pit on the inner wall of the dry evaporation tube, where Re is the Reynolds number of the fluid in the heat exchange tube (dry evaporation tube) in the target application scenario.

[0074] Based on the foregoing description, in some embodiments, before step S504, the manufacturing method may further include:

[0075] Determine that the proportion of the number of target pits among multiple micro pits reaches a set ratio, where the opening radius of the target pit is not less than the critical opening radius r c .

[0076] Considering that the opening of the pit on the surface after sandblasting treatment is often in an irregular shape, and the outlet radius measured by the electron microscope shows a corresponding radius. Due to the influence of the opening shape, it is found in the experiment that within a certain range greater than the critical opening radius, the opening radius is the effective radius. Therefore, the opening radius of the target pit is set to be greater than the critical opening radius r c and less than twice the critical opening radius, that is, within the range of 2r c .

[0077] The aforementioned set ratio can be, for example, 50%. That is, to meet the requirement that the proportion of the number of target pits in the tiny pits formed after sandblasting reaches at least 50%.

[0078] In some embodiments, before step S504, the method further includes: determining that the number of target pits per unit area reaches a set number. For example, an electron microscope can be used to detect the number of target pits on 1 mm 2 only when the number of target pits on 1 mm 2 reaches 200, step S504 is carried out.

[0079] The opening radius of the pits formed by the sandblasting process is related to the particle size of the sand grains. Therefore, in some embodiments, before step S503, or even before step S501, the method further includes:

[0080] S5021, determining the particle size of the sand grains used for sandblasting according to the target application conditions of the heat exchange tube to be manufactured. Then, in step S503, the first surface is sandblasted with sand grains of this particle size.

[0081] Specifically, please refer to Figure 6 , in step S5021, determining the particle size of the sand grains according to the target application conditions of the heat exchange tube can specifically include:

[0082] S601, providing a second metal strip, the second metal strip having a front surface and a back surface arranged oppositely, and the second metal strip having the same thickness as the metal strip in step S501;

[0083] S602, rolling the front surface with a roller, thereby forming a plurality of third channels and third ribs located between the plurality of third channels on the front surface.

[0084] The roller used in this step S602 can be the same roller as the roller in step S502, so that the third channels formed have corresponding shapes and sizes to the channels formed in step S502, and the third ribs formed have corresponding shapes and sizes to the ribs formed in step S502.

[0085] S603, sandblasting different regions of the front surface with second sand grains of different particle sizes respectively, so that the surfaces of the third channels and third ribs in each region become a second rough surface with a plurality of second tiny pits.

[0086] In this step S603, the sandblasting process conditions should be consistent with the sandblasting process conditions for the first surface in step S503.

[0087] S604. Use an electron microscope to detect the different opening radii and corresponding occurrence frequencies of the second micro-pits per unit area in each region (as shown in Figure 7 ), make a distribution diagram of the different pit opening radii and occurrence frequencies, and thus establish a database of the second sand grains of different particle sizes, the outlet radii of the second micro-pits, and the corresponding occurrence frequencies;

[0088] S605. According to the above formula (1), calculate the value range of the optimal opening radius r of the micro-pits on the rough surface (i.e., the rough surface in S503), that is, r c < r < 2r c , and then, according to the distribution diagram of the pit opening radii and occurrence frequencies of the sandblasted surfaces of different particle sizes in the database set in S604 above, determine the particle size of the second sand grains corresponding to the region with the most occurrences of the second micro-pits within the value range of the optimal opening radius r (i.e., within the range of r c < r < 2r c

[0089] Exemplarily, the target application conditions (atmospheric pressure) of a heat exchange tube to be manufactured include: the fluid medium is R32, the saturation temperature T of the fluid medium s = 10 °C, the wall superheat of the fluid ΔT = 4 °C, the mass flow rate of the fluid is 250 kg / m 2 ·s, and the Reynolds number Re = 17642. Then, based on formula (4), the calculated critical radius r c is 3.8 um. Please refer to Figure 7 , after the corresponding region of the second metal strip is sandblasted with sand grains of 120 mesh (corresponding to the particle size of the sand grains) and the formed rough surface is detected by a microscope, there are 436 pits per unit area (1 mm 2 ), among which 274 pits have an opening radius of 3.8 - 7.6 um, accounting for 63%. In contrast, it is better than the rough surfaces formed by sand grains of other particle sizes. Thus, it is determined that in step S503, sand grains of 120 mesh are used to sandblast the first surface.

[0090] The experimental results show that under the above experimental conditions, the in-tube heat transfer coefficient of the welded internal thread tube after sandblasting is increased compared with that of the traditional internal thread tube, and the maximum increase can be more than 30%, as shown in Figure 8 .

[0091] ​It can be understood that, based on the methods of S601 to S605, the corresponding relationships between various sand grain sizes and the outlet radius and occurrence frequency distribution of micro-pits on the sandblasted surface can be established in advance and stored in a computer. When it is necessary to manufacture a heat exchange tube with a sandblasted inner wall surface of the required structure, only the optimal sand grain size needs to be determined according to the aforementioned pre-stored corresponding relationship, and then sand grains of this size are selected for sandblasting processing.

Claims

1. A welded heat exchange tube, characterized in that: include: A plurality of grooves are formed on the inner wall surface of the heat exchange tube; ribs formed between the plurality of channels; The surfaces of the grooves and the ribs are rough surfaces formed by sandblasting, and the rough surfaces have a plurality of tiny pits.

2. The welded heat exchange tube according to claim 1, characterized in that: The channel extends in a spiral shape.

3. The welded heat exchange tube according to claim 2, characterized in that: The plurality of channels include a first channel and a second channel, and the first channel and the second channel are arranged to cross each other in a manner that the spiral directions are opposite to each other.

4. The welded heat exchange tube according to claim 2 or 3, characterized in that: The helix angle of the groove is 15-60°, the depth of the groove is 0.1-0.3 mm, and the pitch of the groove is 0.3-1.0 mm.

5. A method for manufacturing a welded heat exchange tube, characterized in that: include: Providing a metal strip having a first surface and a second surface disposed opposite to each other; Rolling the first surface with a roller having grooves and protrusions on its surface, thereby forming a plurality of grooves and ribs between the plurality of grooves on the first surface; sandblasting the first surface with sand particles to make the surfaces of the grooves and the ribs rough surfaces with a plurality of tiny pits; The metal strip is wound into a tube with a butt joint, and the butt joint is welded; wherein the first surface constitutes the inner surface of the tube, and the second surface constitutes the outer surface of the tube.

6. The manufacturing method according to claim 5, characterized in that Before winding the metal strip into a tube with a butt joint, the method further includes: determining the particle size of the sand particles according to target application conditions of the heat exchange tube.

7. The manufacturing method according to claim 6, characterized in that The method for determining the particle size of the sand particles according to the target application conditions of the heat exchange tube includes: 1) providing a second metal strip, wherein the second metal strip has a front surface and a back surface opposite to each other, and the second metal strip has the same thickness as the first metal strip; 2) rolling the front surface with the roller to form a plurality of third grooves and third ribs between the plurality of third grooves on the front surface; 3) maintaining the same process conditions as those used in the sandblasting process of the first surface, sandblasting the front surface with second sand particles of different particle sizes, so that the surfaces of the third grooves and the third ribs on the surface thereof become a second roughened surface having a plurality of second micro-pits; 4) Using an electron microscope, the opening radii and corresponding occurrence frequencies of the second micro-pits in each region were measured, and a distribution graph of the opening radii and occurrence frequencies of the second micro-pits for each particle size was generated. This database was then established for the second sand particles of different particle sizes, the second micro-pit opening radii, and the corresponding occurrence frequencies. 5) Calculate the optimal opening radius r of the tiny pits on the rough surface according to the following formula: c <r<2r c , Among them, r c is the critical opening radius; Re is the Reynolds number of the fluid in the heat exchange tube under the target application conditions; σ is the surface tension of the fluid in the heat exchange tube under the target application conditions; T s is the saturation temperature of the fluid in the heat exchange tube under the target application conditions; ρ v The gas density in the heat exchange tube under the target application conditions; h fg is the latent heat of vaporization of the fluid in the heat exchange tube under the target application conditions; ΔT is the wall superheat under the target application conditions; Then, according to the database, the particle size of the second sand particles corresponding to the area where the second micro-pits appear the most times within the value range of the optimal opening radius r is determined, that is, the target particle size; The target particle size is determined as the particle size of the sand particles.

8. The manufacturing method according to claim 7, characterized in that Before winding the metal strip into a tubular shape with a butt seam, the method further comprises: Determine whether the number of target pits in the plurality of micro pits reaches a set ratio, wherein the opening radius of the target pit is greater than r c and less than 2r c .

9. The manufacturing method according to claim 8, characterized in that Before winding the metal strip into a tubular shape with a butt seam, the method further comprises: It is determined that the number of the target pits per unit area reaches a set number.

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