Integrated microbubble generator

Abstract

The invention discloses an integrated micro-bubble generator which comprises a shell, cover plates are arranged at the upper end and the lower end of the shell, a gas phase inlet is formed in the side wall of the shell, a plurality of micro-bubble generating structures are arranged in the shell, and each micro-bubble generating structure comprises a shell. A liquid inlet area inner part, a gas suction area inner part, a mixing area inner part and a crushing area nozzle are sequentially arranged in the shell in the liquid flowing direction. The air suction area inner piece comprises a center cylinder, a rotational flow plate is arranged on the outer wall of the lower end of the center cylinder, and the shell wall, corresponding to the center cylinder, of the shell is integrally arranged to be a micropore pipe. The mixing area inner piece comprises a conical center assembly, a spiral plate is arranged on the center assembly, and the wall face, corresponding to the mixing area inner piece, of the shell has a certain gradient and is in an uneven shape. The micro-nano bubble generator is simple in structure and convenient to maintain, micro-nano bubbles can be stably generated, the gas-liquid mixing mass transfer efficiency is improved, and the problems that in the prior art, energy consumption is high, and bubbles are large and unstable are solved.

Description

Technical Field

[0001] The invention relates to the field of microbubble generators, and in particular to an integrated microbubble generator. Background Art

[0002] In the chemical industry, mass transfer and reaction processes play a vital role in improving production efficiency, optimizing product quality, and reducing energy consumption and environmental pollution. As an emerging physical method, microbubble generator technology has shown great potential in enhancing mass transfer and reaction. It can produce bubbles with a diameter of micrometers. These tiny bubbles have a larger specific surface area and a longer residence time in the liquid, which can significantly improve mass transfer efficiency and reaction rate.

[0003] Traditional mass transfer and reaction technologies usually use stirring, spraying and other methods to achieve gas-liquid mixing and reaction. However, these methods often have problems such as small gas-liquid contact area, low mixing efficiency and high energy consumption. The venturi tube microbubble generator has the problem of insufficient gas flux, and the traditional microporous tube has the problem of bubble aggregation and uneven gas phase dispersion due to insufficient turbulent kinetic energy.

[0004] Chinese patent CN 221471476 U discloses a coupled micro-bubble generator, which uses swirl flow to feed liquid and a venturi throat to feed air. This structure extends the throat length and increases the size of the device. Otherwise, the gas phase inlet area is too small, and it is also easy to cause bubble aggregation.

[0005] Chinese patent CN 114849508 A discloses a venturi tube type micro bubble generator, which is a hollow swirl assembly coaxially installed upstream of the traditional venturi flow channel, so that part of the water flow is sucked and mixed with the gas phase injected by the gas injection pipe when passing through the central cavity of the hollow swirl assembly, and the other part of the water flow forms a swirl under the action of the swirl blades; the device only relies on negative pressure to inhale air, and the air intake is too low, so the bubble diameter is small, but the gas volume is difficult to support industrial applications. At the same time, the liquid is divided and mixed, because one of the streams is axial flow and the other is swirl flow, the fluctuation is large during mixing, and the bubbles are easy to gather, and the overall pressure loss of the equipment is too large. Summary of the invention

[0006] In order to solve the problems of small gas flux, easy aggregation of bubbles, poor mixing effect, etc. of the existing microbubble generator, the present invention provides an integrated microbubble generator, which realizes bubble generation under the conditions of strong shearing and turbulence of liquid swirl and negative pressure suction of gas. The addition of the mixing zone and the breaking zone in the generator of the present invention realizes the spiral strong mixing effect and the necking and foam breaking effect of the gas-liquid two phases. The shell and gas phase flow channel design of the microbubble generator also meet the application conditions of the large-flux gas chemical industry, and has the advantages of large gas flux, small bubble size, strong gas-liquid mixing effect, and easy disassembly and maintenance.

[0007] The present invention is achieved by adopting the following technical solutions.

[0008] An integrated micro-bubble generator comprises a shell, wherein cover plates are arranged at the upper and lower ends of the shell, a gas phase inlet is arranged on the side wall of the shell, and a plurality of micro-bubble generating structures are arranged in the shell, wherein the bubble generating structures comprise an outer shell, and liquid inlet zone internals, air suction zone internals, mixing zone internals and crushing zone nozzles are arranged in sequence in the outer shell along the liquid flow direction; the air suction zone internals comprise a central cylinder, a swirl plate is arranged on the outer wall of the lower end of the central cylinder, and the outer shell wall corresponding to the central cylinder is integrally configured as a microporous tube; the mixing zone internals comprise a conical central component, a spiral plate is arranged on the central component, and the outer shell wall surface corresponding to the mixing zone internals has a certain slope and is uneven.

[0009] By adopting the above technical solution, the gas phase enters the air intake area of ​​the microbubble generating device after passing through the shell, and the liquid is accelerated in the liquid inlet area and then swirls and shears the gas in the air intake area to generate microbubbles. The internal parts in the air intake area also have the function of increasing the flow rate to provide negative pressure air intake. The gas phase inlet at the air intake area adopts a microporous structure and has the function of generating microbubbles. Then the two-phase flow enters the mixing area to disperse the fluid, enhance turbulence and strengthen mixing, and finally passes through the crushing area. The center of the crushing area has multiple nozzle structures for secondary bubble crushing.

[0010] Furthermore, a plurality of limiting cylinder segments are symmetrically arranged on the upper and lower end covers, and the number of the micro-bubble generating structures is equal to that of the limiting cylinder segments and they are assembled and fixed concentrically with the limiting cylinder segments.

[0011] Furthermore, the central component of the liquid inlet area internals adopts a hollow or solid conical or truncated cone structure; the central component of the liquid inlet area internals adopts a truncated cone structure, and the diameter of the small end of the truncated cone is 1 / 10-1 / 5 of the diameter of the microbubble generating structure.

[0012] Furthermore, the central cylinder is a solid or hollow structure; the central cylinder is a hollow structure, and the outer wall of the central cylinder is a microporous tube wall.

[0013] Furthermore, the swirl plates are radially arranged on the same circumferential surface with the central cylinder as the center, and the swirl plates are straight plates or arc-shaped plates.

[0014] Furthermore, an air suction zone annular flow channel is formed between the shell and the central cylinder, and the width of the air suction zone annular flow channel is 1 / 4-1 / 5 times the inlet radius of the liquid inlet zone.

[0015] Furthermore, the central component of the mixing zone internals adopts a hollow or solid conical or truncated cone structure with a cone angle range of 6°-15°; the central component of the mixing zone internals adopts a truncated cone structure with a diameter of the small end of the cone being 1 / 10-1 / 5 of the diameter of the microbubble generating structure.

[0016] Furthermore, the spiral plate is a variable pitch and variable diameter spiral with a pitch between 1mm-8mm and a diameter-changing slope that is smaller than the cone angle of the central component of the mixing zone interior. The spiral plate is symmetrically arranged in two groups, and the outer diameter of the bottom end of the spiral plate is the same as the bottom diameter of the central component of the mixing zone interior.

[0017] Furthermore, the nozzle in the crushing zone adopts a structure of equal diameter followed by expansion or gradually reducing diameter followed by expansion.

[0018] Furthermore, the crushing zone nozzle adopts an equal diameter and then expanding structure, the diameter of the nozzle at the minimum diameter is 2-3 times the width of the annular flow channel in the suction zone, and the gradual expansion angle is 15-30°; the crushing zone nozzle adopts a gradually narrowing diameter and then expanding diameter structure, the length of the gradually narrowing diameter is 1 / 4-1 / 6 of the expansion length, and the angle of the gradually narrowing diameter is the same as the angle of the gradually expanding neck, which is 6°-15°.

[0019] This application has the following beneficial effects.

[0020] (1) The flow channel in the liquid inlet area of ​​the present invention gradually shrinks, the flow channel in the air intake area is extremely narrow, and the flow channel in the mixing area gradually expands, simulating the negative pressure air intake and throat turbulent cavitation effects of the Venturi tube. However, the available opening area in the air intake area is much larger than the diameter of the traditional Venturi tube, retaining its effect while increasing the gas flux. The air intake area gas phase inlet uses a microporous tube for the initial generation of bubbles, and the air intake area inlet is set to a swirl plate to enhance the shear and turbulence effects and refine the generated bubbles;

[0021] (2) The present invention introduces a mixing zone, which effectively avoids the pipeline gas wall flow phenomenon and cyclone phase separation phenomenon under the action of the spiral internal parts, and strengthens the gas-liquid two-phase mixing effect;

[0022] (3) The present invention introduces a crushing zone and uses a nozzle structure to effectively perform secondary crushing of bubbles, prevent the bubbles from growing, and ensure the bubble generation effect;

[0023] (4) The present invention adopts a structural form in which a shell is connected to multiple microbubble generating structures, which is conducive to industrial application, installation, disassembly and maintenance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the structure of the present invention;

[0025] Figure 2 It is a structural schematic diagram of the microbubble generating structure of the present invention;

[0026] Figure 3 is a schematic diagram of the structure of the swirl plate in Example 1 of the present invention;

[0027] Figure 4 is a schematic diagram of the structure of the swirl plate in Example 2 of the present invention;

[0028] Figure 5 It is a schematic diagram of the structure of the nozzle in the crushing zone in Example 2 of the present invention;

[0029] Figure 6 It is a schematic diagram of the structure of the liquid inlet zone internals, the air intake zone internals and the mixing zone internals in Example 2 of the present invention.

[0030] Among them, 1. limit cylinder section; 2. shell; 3. gas phase inlet; 4. micro bubble generating structure; 5. cover plate; 6. liquid inlet area internals; 7. suction area internals; 71. central cylinder; 72. swirl plate; 8. microporous tube; 9. mixing area internals; 91. central cone; 92. spiral plate; 10. crushing area nozzle; 11. shell. DETAILED DESCRIPTION

[0031] The present patent application is further described below in conjunction with embodiments.

[0032] Example 1

[0033] like Figure 1 As shown, an integrated micro-bubble generator includes a limiting cylinder segment 1, a shell 2, a gas phase inlet 3, a cover plate 5 and a micro-bubble generating structure 4. Two cover plates 5 are respectively connected to the upper and lower ends of the shell 2, and a plurality of micro-bubble generating structures 4 are arranged in the shell 2 according to a certain rule, and supported by the cover plates 5 at the upper and lower ends. Specifically, a plurality of limiting cylinder segments 1 are connected to the cover plates 5 at the upper and lower ends, and the number of micro-bubble generating structures 4 is equal to that of the limiting cylinder segment 1 and they are concentrically assembled with the limiting cylinder segment 1 and fixed by threaded connection to form a seal, and the two end portions of the micro-bubble generating structure 4 extend out of the limiting cylinder segment 1. More specifically, the shell 2 is cylindrical, the cover plate 5 is welded at both ends of the shell 2, the limit tube section 1 is symmetrically welded on the cover plates 5 at the upper and lower ends, the limit tube section 1 has an internal thread, and the connection between the microbubble generating structure 4 and the limit tube section 1 has an external thread, that is, the liquid inlet area thread of the microbubble generating structure 4 corresponds to the limit tube section thread on the lower end cover plate 5, and the crushing area thread of the microbubble generating structure 4 corresponds to the limit tube section thread on the upper end cover plate 5. A gas phase inlet 3 is provided on the side wall of the shell 2, and 1-4 gas phase inlets 3 are arranged, evenly arranged in the middle of the shell 2, and welded to the shell 2 as a whole. In this embodiment, one gas phase inlet 3 is provided, and the gas phase inlet 3, the shell 2, and the cover plates 5 at the upper and lower ends are combined into a closed cylindrical air cavity, which connects the external environment with the micropores in the microbubble generating structure 4 to ensure gas entry.

[0034] like Figure 2 As shown, the microbubble generating structure 4 is cylindrical, and plays the role of liquid intake and gas-liquid mixing. It is divided into a liquid intake area, a gas intake area, a mixing area, and a crushing area along the axial direction.

[0035] A liquid inlet area inner member 6 is arranged at the center of the liquid inlet area. In this embodiment, the liquid inlet area inner member 6 is a solid conical structure.

[0036] The air suction zone is provided with a microporous tube 8 and an air suction zone internal part 7. The outer wall of the air suction zone of the microbubble generating structure 4 is set as a microporous tube 8 as a whole. The gas phase is sucked into the liquid phase space in the tube by the microporous tube 8, and microbubbles are generated under the shearing action of the micropores and the liquid. The air suction zone internal part 7 includes a central cylinder 71 and a swirl plate 72. The swirl plate 72 is welded on the central cylinder 71 at the entrance of the air suction zone. The swirl plates 72 are radially arranged on the same circumferential surface with the central cylinder 71 as the center. In this embodiment, the central cylinder 71 is a solid structure, and the swirl plate 72 is an arc plate.

[0037] An annular flow channel in the air intake zone is formed between the microporous tube 8 in the air intake zone of the present application and the central cylinder 71. The width of the annular flow channel in the air intake zone is 1 / 4-1 / 5 times the inlet radius of the liquid inlet zone. The design of the annular flow channel in the air intake zone is the key to ensuring that the gas effectively enters and generates bubbles that meet the requirements. Preferably, the width of the annular flow channel is 1 / 5 of the inlet radius.

[0038] The microporous tube 8 in the air intake area is made of metal, ceramic or silicon carbide, and the micropore diameter ranges from 0.01μm to 20μm. For the air intake area that needs to withstand high temperature or high pressure, metal materials can be selected, and for the air intake area that needs corrosion resistance, ceramic or silicon carbide materials can be selected. The micropore diameter is a key factor affecting gas permeability and filtering effect. For the air intake area of ​​ultrafine bubbles, a smaller micropore diameter can be selected, such as between 0.01μm and 1μm; for the air intake area that requires higher gas permeability, a larger micropore diameter can be selected, such as between 5μm and 20μm. In this embodiment, the micropore diameter of the microporous tube 8 in the air intake area is 5 microns.

[0039] The mixing zone internal part 9 is arranged at the center of the mixing zone. In the present embodiment, the mixing zone internal part 9 includes a solid central cone 91 and a variable pitch spiral plate 92. The cone angle range of the central cone 91 is 6°-15°, and preferably, the cone angle is 8°; the spiral plate 92 is a variable pitch and variable diameter spiral, and the pitch is selected in a combination between 1mm-8mm, and the variable diameter slope is less than the cone angle; the spiral plate 92 is symmetrically arranged in two groups, and the outer diameter of the bottom end of the spiral plate 92 is the same as the bottom diameter of the central cone 91. The pitch of the spiral plate 92 is a key factor affecting the fluid flow and mixing effect. One or more pitch values ​​can be selected. Preferably, the pitch of the present embodiment increases from 1mm to 4mm in a step-like manner from bottom to top. The wall surface of the outer shell 11 of the mixing zone of the present application has a certain slope and is uneven, and the inner wall surface of the tail end is flush with the outer side of the inner wall surface of the inlet of the nozzle 10 in the crushing zone. The flow channel of the mixing zone of the present application shows a trend of gradual expansion.

[0040] A plurality of crushing zone nozzles 10 are provided in the crushing zone, and the diameter at the minimum diameter of the nozzle is 2-3 times the width of the annular flow channel in the air suction zone, and the gradual expansion angle is 15-30°. In this embodiment, there are 4 crushing zone nozzles 10, and the crushing zone nozzles 10 adopt an equal diameter and then expansion structure, and the diameter at the minimum diameter is 2 times the width of the annular flow channel in the air suction zone, and the gradual expansion angle is 10°.

[0041] In this embodiment, the swirl plate 72 and the central cylinder 71 of the suction zone internal part 7, the spiral plate 92 and the central cone 91 of the mixing zone internal part 9 are all cast in one piece, and the equal diameter surfaces of the liquid inlet zone internal part 6, the suction zone internal part 7 and the mixing zone internal part 9 are connected to each other, and are connected by threads. More specifically, the conical bottom surfaces of the liquid inlet zone internal part 6 and the mixing zone internal part 9 are provided with extended studs, and the two end surfaces of the cylinder of the suction zone internal part 7 are provided with threaded holes, and the three are connected in sequence.

[0042] The diameter of the microbubble generating structure 4 is determined by the volume processing capacity of the device and the liquid flow rate in the microbubble generating structure 4. The liquid flow rate range is 3-7m / s. Preferably, the volume processing capacity is 50m 3 / h, the liquid flow rate in the microbubble generating structure 4 is 3m / s, and the diameter of the microbubble generating structure 4 is 20mm.

[0043] The shell 2 of the present application mainly plays the role of support and gas flow channel. The external gas enters the shell 2 from the gas phase inlet 3, and enters the liquid flow channel through the microporous tube 8 of the microbubble generating structure 4. The microbubble generating structure 4 is a Venturi nozzle-like structure as a whole. The liquid enters from the liquid inlet and contracts and accelerates through the liquid inlet area. The swirl effect is generated by the internal part 7 in the suction area. The negative pressure working condition is generated under the action of the high-speed swirl liquid, and the gas in the gas flow channel is sucked into the suction area. Tiny bubbles are formed under the action of shear force and turbulent kinetic energy. The gas-liquid mixed flow enters the mixing area. Under the disturbance of the internal part 9 in the mixing area, the gas-liquid mixing is enhanced, and the traditional microporous tube wall flow phenomenon and swirl phase separation phenomenon are destroyed. Finally, it enters the crushing area. The nozzle has the effect of re-crushing large bubbles and weakening the aggregation effect of bubbles. Therefore, the generator of the present application can stably generate micro-nano bubbles.

[0044] Using the structural form and parameters of Example 1, water is used as the liquid medium and air is used as the gas medium. A microscope is used to observe the bubbles derived from the microbubble generating structure 4. The gas-liquid ratio is 0.1-0.8. The change in the size of the microbubbles is shown in Table 1. As the gas-liquid ratio increases, the average diameter of the bubbles increases. It can be seen that the microbubble generating structure 4 used in Example 1 of the present application has a good microbubble generating effect.

[0045] Table 1 Example 1 Microbubble generation structure Bubble average diameter

[0046] Gas-Liquid Ratio 0.1 0.3 0.5 0.8 Average bubble diameter / μm 83 108 159 188

[0047] Example 2

[0048] See also Figure 3-5 The difference between this embodiment and embodiment 1 is that:

[0049] In this embodiment, the swirl plate 72 of the inner part 7 of the air suction zone adopts a straight plate structure, and the structure of the nozzle 10 in the crushing zone is a structure with a gradually narrowing diameter and then an expanding diameter.

[0050] Specifically, the swirl plates 72 are straight plates, and a plurality of swirl plates 72 are distributed on the central cylinder 71 at an angle of 360°.

[0051] The necking section of the crushing zone nozzle 10 is a gradually narrowing diameter, the length of the gradually narrowing diameter is 1 / 4-1 / 6 of the expanded diameter length, the angle of the gradually narrowing diameter is the same as the angle of the gradually expanding neck, the angle is adjustable from 6° to 15°, and the inlet plane of the nozzle is completely perpendicular to the fluid flow direction.

[0052] See also Figure 6 In this embodiment, the central components of the liquid inlet area internal component 6 and the mixing area internal component 9 adopt a truncated cone structure, and the two truncated cone structures and the central cylinder 71 of the suction area internal component 7 are both hollow wall forms, and the bottom end of the liquid inlet area internal component 6 is connected to the gas phase space. Specifically, the diameter of the truncated cone small end of the liquid inlet area internal component 6 and the mixing area internal component 9 is 1 / 10-1 / 5 of the diameter of the micro bubble generating structure 4, and the thickness of the hollow wall is not more than 5 mm.

[0053] Furthermore, the central cylinder 71 of the suction zone inner part 7 is a microporous tube wall surface, and the gas can also enter from the center of the hollow cone of the liquid inlet zone inner part 6 and enter the liquid phase of the suction zone flow channel through the cylindrical microporous tube wall surface of the suction zone inner part 7.

[0054] The structural form and parameters of Example 2 are adopted, the length of the gradually narrowing diameter of the nozzle 10 in the crushing zone is 1 / 4 of the length of the expanding diameter, and the angle of the gradually narrowing diameter is the same as the angle of the gradually expanding neck, which is 6°; the diameter of the truncated cone of the central component of the internal component 6 in the liquid inlet zone and the internal component 9 in the mixing zone is 1 / 10 of the diameter of the microbubble generating structure 4, and the other parameters are the same as those in Example 1. Using water as the liquid medium and air as the gas medium, a microscope is used to observe the bubbles derived from the microbubble generating structure 4, with a gas-liquid ratio of 0.1-0.8. The change in the size of the microbubbles is shown in Table 2. As the gas-liquid ratio increases, the average diameter of the bubbles increases. Compared with Example 1, this embodiment is more applicable to high gas-liquid ratios and has a better microbubble generating effect.

[0055] Table 2 Example 2 Microbubble generation structure bubble average diameter

[0056] Gas-Liquid Ratio 0.1 0.3 0.5 0.8 Average bubble diameter / μm 99 112 137 153

[0057] The embodiments of this specific implementation method are all preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention. Therefore, all equivalent changes made based on the structure, shape, and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. An integrated microbubble generator, comprising a housing (2), wherein cover plates (5) are provided at the upper and lower ends of the housing (2), and a gas phase inlet (3) is provided on the side wall of the housing (2), characterized in that: A plurality of microbubble generating structures (4) are arranged in the shell (2). The bubble generating structures (4) include a shell (11). A liquid inlet zone internal component (6), an air suction zone internal component (7), a mixing zone internal component (9) and a crushing zone nozzle (10) are arranged in sequence in the shell (11) along the liquid flow direction; the air suction zone internal component (7) includes a central cylinder (71), a swirl plate (72) is arranged on the outer wall of the lower end of the central cylinder (71), and the shell wall of the shell (11) corresponding to the central cylinder (71) is entirely arranged as a microporous tube (8); the mixing zone internal component (9) includes a conical central component, a spiral plate (92) is arranged on the central component, and the wall surface of the shell (11) corresponding to the mixing zone internal component (9) has a certain slope and is uneven.

2. An integrated microbubble generator according to claim 1, characterized in that: A plurality of limiting cylinder segments (1) are symmetrically arranged on the upper and lower end cover plates (5); the number of the micro bubble generating structures (4) is equal to that of the limiting cylinder segments (1) and they are assembled and fixed concentrically with the limiting cylinder segments (1).

3. The integrated microbubble generator according to claim 1, characterized in that: The central component of the liquid inlet area internal component (6) adopts a hollow or solid conical or truncated cone structure; the central component of the liquid inlet area internal component (6) adopts a truncated cone structure, and the diameter of the small end of the truncated cone is 1 / 10-1 / 5 of the diameter of the microbubble generating structure (4).

4. The integrated microbubble generator according to claim 1, characterized in that: The central cylinder (71) is a solid or hollow structure; the central cylinder (71) is a hollow structure, and the outer wall of the central cylinder (71) is a microporous tube wall.

5. The integrated microbubble generator according to claim 1, characterized in that: The swirl plates (72) are radially arranged on the same circumferential surface with the central cylinder (71) as the center. The swirl plates (72) are straight plates or arc-shaped plates.

6. The integrated microbubble generator according to claim 1, characterized in that: An air intake annular flow channel is formed between the outer shell (11) and the central cylinder (71), and the width of the air intake annular flow channel is 1 / 4 to 1 / 5 times the inlet radius of the liquid inlet zone.

7. The integrated microbubble generator according to claim 1, characterized in that: The central component of the mixing zone internal component (9) adopts a hollow or solid conical or truncated cone structure, and the cone angle range is 6°-15°; the central component of the mixing zone internal component (9) adopts a truncated cone structure, and the diameter of the small end of the truncated cone is 1 / 10-1 / 5 of the diameter of the microbubble generating structure (4).

8. The integrated microbubble generator according to claim 1, characterized in that: The spiral plate (92) is a variable pitch and variable diameter spiral with a pitch between 1 mm and 8 mm, and a diameter change slope that is smaller than the cone angle of the central component of the mixing zone internal component (9); the spiral plate (92) is symmetrically arranged in two groups, and the outer diameter of the bottom end of the spiral plate (92) is the same as the bottom surface diameter of the central component of the mixing zone internal component (9).

9. The integrated microbubble generator according to claim 1, characterized in that: The crushing zone nozzle (10) adopts a structure of equal diameter followed by expansion or gradually reducing diameter followed by expansion.

10. The integrated microbubble generator according to claim 9, characterized in that: The crushing zone nozzle (10) adopts a structure of equal diameter and then expanding diameter, the diameter at the minimum diameter of the nozzle is 2-3 times the width of the annular flow channel in the suction zone, and the gradual expansion angle is 15-30°; the crushing zone nozzle (10) adopts a structure of gradually reducing diameter and then expanding diameter, the length of the gradually reducing diameter is 1 / 4-1 / 6 of the expansion length, and the angle of the gradually reducing diameter is the same as the angle of the gradually expanding neck, which is 6°-15°.

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