CN109723569B - Shear type rectangular nozzle device for researching plasma enhanced jet mixing - Google Patents

Abstract

The invention relates to a shearing type rectangular nozzle device, which is applied to the experimental research of plasma-enhanced jet mixing, and belongs to the technical field of plasma flow control. The nozzle arrangement includes an inner nozzle, an outer nozzle and a plenum. The gas collection chamber is assembled from a front end cover, a rear end cover and an intermediate isolation plate. The inner nozzle includes an inner nozzle bracket and two inner nozzle wall plates, the inner nozzle bracket is fixed on the middle partition plate, and the two inner nozzle wall plates are both fixed on the inner nozzle bracket. The outer nozzle is composed of an upper part of the outer nozzle and a lower part of the outer nozzle, and the outer nozzle is fixed on the front end cover. The invention can realize the laying and discharge of the plasma exciter, carry out the research on the plasma-enhanced jet mixing, and has various parameter adjustment functions. At the same time, the device of the present application has compact structure, simple parts processing, convenient disassembly and assembly, and high reliability.

Description

Shearing Rectangular Nozzle Setup for Studying Plasma-Enhanced Jet Blending

technical field

The invention relates to a shearing type rectangular nozzle device, which is applied to the experimental research of plasma-enhanced jet mixing, and belongs to the technical field of plasma flow control.

Background technique

Jets have important applications in aerospace vehicle combustion chambers and exhaust ports, chemical reactors and cooling systems, and it is of great significance to improve the blending, development characteristics and noise characteristics of jets. The plasma flow control technology based on surface dielectric barrier discharge is an effective means to improve the degree of jet mixing. As shown in Figure 1, the exciter consists of two parts, an exposed electrode and an implanted electrode, which are separated by an insulating medium layer and located on the upper side and the lower side of the insulating medium layer, respectively. High-frequency high-voltage excitation is applied between the two electrodes, which can generate a sufficiently large electric field gradient above the implanted electrode dielectric layer to ionize the air to generate plasma, and the charged particles move directionally under the action of the electric field, inducing the air to form a directional jet.

At present, experimental researches on plasma-enhanced jet mixing have been carried out at home and abroad, but most of them are common nozzle configurations with single components and relatively simple structure, and no nozzles specially used for plasma discharge have been developed. In addition, due to the fixed structure, the location for laying the plasma exciter is limited, and the layout is single, so it is difficult to carry out in-depth and diversified research.

SUMMARY OF THE INVENTION

The purpose of this application is to solve the problems existing in the above-mentioned existing nozzle experimental devices, and to provide a shear-type two-dimensional rectangular nozzle device for studying plasma-enhanced jet blending. Surface dielectric barrier discharge, pasting copper foil electrodes (ie plasma exciters) at different positions of its structure and applying high voltage can form near-wall plasma jets and realize the pneumatic intervention of plasma on shear jets. At the same time, it can realize the installation of single or multiple plasma exciters at different positions (inner nozzle inner wall, outer wall surface and outer nozzle inner wall surface, outer wall surface) and layout at any angle, as well as various characteristic parameters of the nozzle (inner nozzle slit width, The adjustment of the outer nozzle slit width and premixing length) can investigate different influencing factors and carry out rich research.

In order to solve the above-mentioned technical problems, the technical scheme adopted in this application is:

A shear-type rectangular nozzle device for studying plasma-enhanced jet blending, comprising an inner nozzle, an outer nozzle, and a plenum, the inner and outer nozzles both projecting from the interior of the plenum, and the an inner nozzle extends from inside said outer nozzle; wherein:

A front end cover and a rear end cover are respectively provided on the front side and the rear side of the gas collection chamber, and a middle isolation plate is arranged at the connection of the front end cover and the rear end cover, and the central area of the middle isolation plate is a rectangular hollow area; the front end cover is provided with a front end cover air intake hole, and the rear end cover is provided with a rear end cover air intake hole; the air collection chamber uses the middle partition plate to form two separate air collection spaces, the front end cover, the rear end cover The air inlet hole of the end cover is connected with an external parallel air supply pipeline, and supplies air to the outer nozzle and the inner nozzle respectively.

The inner nozzle includes an inner nozzle bracket and two inner nozzle wall plates, the inner nozzle bracket is fixed on the middle partition plate, the inner nozzle bracket is provided with support ribs, and the two inner nozzle wall plates are fixed The inner nozzle is assembled on the support rib; the inner nozzle wall plate first passes through the inner nozzle gasket, and then protrudes from the rectangular hollow area of the middle partition plate; the inner nozzle is formed by the rear The air inlet hole of the end cover realizes the air supply;

The outer nozzle is composed of an upper part of the outer nozzle and a lower part of the outer nozzle, the outer nozzle is fixed on the front end cover, and air is supplied by the air inlet hole of the front end cover.

Further, the front end cover, the rear end cover and the middle isolation plate are provided with bolt holes in corresponding positions, and the hexagon socket bolts pass through the bolt holes on the front end cover, the middle isolation plate and the rear end cover in turn and connect the front end cover, the rear end cover, and the rear end cover. The end caps and the intermediate spacer plate are assembled as a plenum.

Further, the inner nozzle bracket includes a bracket wall plate and a support rib, the support rib is arranged perpendicular to the bracket wall plate, the bracket wall plate is arranged parallel to the middle partition plate, and the two inner nozzle wall plates are perpendicular to each other. The inner nozzle gasket is arranged between the bracket wall plate and the intermediate isolation plate; the bracket wall plate, the inner nozzle gasket and the intermediate isolation plate are in corresponding There are waist-shaped holes at all positions, and the inner hexagon bolts pass through the support wall plate, the inner nozzle gasket and the waist-shaped holes of the intermediate isolation plate in sequence to fix the inner nozzle bracket on the intermediate isolation plate.

Further, the upper part of the outer nozzle and the lower part of the outer nozzle both include a base and an outer nozzle wall plate, the outer nozzle wall plate is arranged perpendicular to the base, the base is arranged parallel to the front end cover, and the two bases Corresponding positions with the front end cover are provided with waist-shaped holes, and the inner hexagon bolts pass through the two bases and the waist-shaped holes of the front end cover in turn to fix the outer nozzle on the front end cover.

Further, the ventilation holes of the inner nozzle and the outer nozzle are all rectangular, and the length-to-width ratio of the ventilation holes is greater than 50; both sides of the inner nozzle wall plate and both sides of the outer nozzle wall plate can be laid with plasma exciters. Or directly used as the dielectric barrier layer of the plasma exciter; the ends of the two outer nozzle wall plates are also provided with positioning holes.

Further, sealing rubber rings are provided on both side assembly surfaces of the inner nozzle gasket, the middle isolation plate and the outer nozzle base close to the front end cover.

further,

The geometric parameters of the two inner nozzle wall plates are the same, and they are aligned in the length and width directions respectively;

The outer nozzle wall plates all include a flat wall and side walls located on both sides of the flat wall, in a "C" shape enclosing structure; the thickness of the flat walls of the two outer nozzle wall plates is the same, and the thickness of the side walls of the two outer nozzle wall plates is the same. It is also the same; the height of the side wall of each outer nozzle wall is uniformly long and one short, the height of the long side wall is equal to the sum of the height of the short side wall and the thickness of the wall, and the short side wall of the outer nozzle wall is the same as the length of the other outer nozzle wall. The side walls are arranged correspondingly, wherein a slit is left on the outer side of the short side wall, and the width of the slit is the same as the thickness of the side wall;

The central area of the outer nozzle base adjacent to the front end cap is hollow for passing through another outer nozzle wall; the sum of the length of the outer nozzle wall adjacent to the front end cap and the thickness of its base is equal to the length of the other outer nozzle wall.

Further, the inner nozzle and the outer nozzle are made of ceramics and polytetrafluoroethylene; the gas collection chamber is made of stainless steel.

Further, the straight sides of the waist-shaped holes are arranged along the vertical direction.

Further, the front end cover is provided with 5 front end cover air intake holes, the rear end cover is provided with 4 rear end cover air intake holes, the front end cover air intake holes and the rear end cover air intake holes are both. They are arranged horizontally and equally spaced, and all are connected to the external air supply pipeline.

The beneficial effects of the present invention are:

1. The above-mentioned gas collecting chamber adopts an intermediate isolation plate, which is assembled with the inner nozzle bracket and the inner nozzle gasket. The gas collecting chamber is divided into two separate gas collecting spaces, which are respectively connected with the outer nozzle and the inner nozzle, so that the two The nozzle has the function of independent adjustment of the air supply.

2. The air intake holes of the rear end cover and the front end cover of the gas collection chamber are distributed at equal distances horizontally, and the pipelines are connected in parallel. This layout method can ensure that the flow rate at different positions of the nozzle is basically the same. In addition, the length-width ratio of the nozzle is required to be greater than 50. Combining the above two points makes the jet have good two-dimensional flow characteristics, that is, the jet flow field is consistent in the transverse direction.

3. The above-mentioned inner nozzle and outer nozzle are made of non-metallic materials such as ceramics and polytetrafluoroethylene with good insulation, which can meet the insulation requirements of the plasma exciter discharge on the carrier, and prevent the high voltage applied by the exciter and metal gas gathering. Conduction occurs between the chamber and the outside, and the wall of the nozzle can directly act as a dielectric barrier for the plasma exciter due to its insulating properties.

4. The dimensions of the above-mentioned inner nozzle bracket and inner nozzle gasket can be serialized, and the nozzle wall material can be serialized. According to the experimental research to investigate the influence of different materials and size parameters, the corresponding parts can be selected for assembly.

5. The above-mentioned inner nozzle and outer nozzle adopt the method of separate processing and reassembly. The plasma exciter can be laid before assembly, which can realize the convenient and fast laying of the exciter in any position, and has a strong performance in the experiment. operability.

Description of drawings

Fig. 1 is the structural schematic diagram of SDBD exciter;

Figure 2 is an isometric view of the sheared rectangular nozzle device of the present invention;

3 is a front view of the shearing rectangular nozzle device according to the present invention;

4 is a side sectional view of the shearing rectangular nozzle device according to the present invention;

5 is an exploded view of the assembly of the shearing rectangular nozzle device according to the present invention;

6 is a schematic diagram of the inner nozzle support of the shearing rectangular nozzle device according to the present invention;

7 is a schematic diagram of the upper part of the outer nozzle of the shearing rectangular nozzle device according to the present invention;

8 is a schematic diagram of the lower part of the outer nozzle of the shearing rectangular nozzle device according to the present invention;

Among them, 11. Gas collection chamber; 120. Front end cover; 121. Hexagon socket head bolt; 122. Front end cover air inlet; 130. Middle partition plate; Inner nozzle gasket; 21. Inner nozzle; 220. Inner nozzle bracket; 221. Hexagon socket head bolt; 222. Bracket wall plate; 223. Support rib; 224. Waist hole; 230. Inner nozzle wall plate; 320. Lower part of outer nozzle; 321. Base; 330. Upper part of outer nozzle; 331. Hexagon socket bolt; 332. Long side wall; 333. Short side wall; 334. Waist hole; flat wall.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, but not to limit the present invention.

A shearing rectangular nozzle device for studying plasma-enhanced jet blending, with reference to Figures 2 to 5, includes an inner nozzle 21, an outer nozzle 31 and a plenum 11, the inner nozzle 21 and the outer nozzle 31 are The gas collection chamber 11 protrudes from the inside, and the inner nozzle 21 protrudes from the outer nozzle 31 .

A front end cover 120 and a rear end cover 140 are respectively provided on the front side and the rear side of the gas collection chamber 11 , and an intermediate isolation plate 130 is provided at the connection between the front end cover 120 and the rear end cover 140 . The intermediate isolation plate The central area of 130 is a rectangular hollow area for passing through the inner nozzle 21 . Specifically, the front end cover 120 , the rear end cover 140 and the middle isolation plate 130 are provided with bolt holes at corresponding positions, and the hexagon socket head bolts 121 pass through the front end cover 120 , the middle isolation plate 130 and the bolts on the rear end cover 140 in sequence. hole and assemble the front end cover 120 , the rear end cover 140 and the middle partition plate 130 as the plenum 11 . The air collecting chamber is divided into two separate air collecting spaces, which are respectively connected with the outer nozzle and the inner nozzle, so that the outer nozzle and the inner nozzle have the function of independent adjustment of air supply.

The inner nozzle 21 includes an inner nozzle bracket 220 and two inner nozzle wall plates 230. The inner nozzle bracket 220 is fixed on the middle partition plate 130, and the inner nozzle bracket 220 is provided with support ribs 223. Each inner nozzle wall plate 230 is fixed on the support rib 223 and combined to form the inner nozzle 21 ; The hollow area protrudes; the inner nozzle 21 is supplied with air by the air inlet hole 141 of the rear end cover.

Referring to FIGS. 2-6 , the inner nozzle bracket 220 includes a bracket wall 222 and a support rib 223 , the support rib 223 is arranged perpendicular to the bracket wall 222 , and the bracket wall 222 is arranged parallel to the middle partition plate 130 , the two inner nozzle wall plates 230 are arranged perpendicular to the bracket wall plates 222 and fixed on the support ribs 223 . The inner nozzle gasket 150 is disposed between the bracket wall plate 222 and the intermediate spacer plate 130 . The bracket wall plate 222 , the inner nozzle gasket 150 and the middle partition plate 130 are all provided with waist-shaped holes at corresponding positions, and the inner hexagon bolts 221 pass through the bracket wall plate 222 , the inner nozzle gasket 150 and the middle partition plate 130 in sequence. The waist-shaped hole 224 fixes the inner nozzle bracket 220 on the middle partition plate 130 .

5, 7-8, the outer nozzle 31 is composed of an outer nozzle upper part 330 and an outer nozzle lower part 320, the outer nozzle 31 is fixed on the front end cover 120, and is realized by the front end cover air intake hole 122 gas supply.

The outer nozzle upper part 330 and the outer nozzle lower part 320 both include a base 321 and an outer nozzle wall, the outer nozzle wall is arranged perpendicular to the base 321, and the base 321 is arranged parallel to the front end cover 120, Corresponding positions of the two bases 321 and the front end cover 120 are provided with waist-shaped holes 334, and the inner hexagon bolts 331 pass through the waist-shaped holes of the two bases 321 and the front end cover 120 in turn to fix the outer nozzle 31 on the front end cover. 120 on.

The front end cover 120 is provided with five front end cover air intake holes 122, the rear end cover 140 is provided with four rear end cover air intake holes 141, the front end cover air intake holes 122 and the rear end cover air intake The holes 141 are arranged horizontally and equally spaced, and are connected to the external air supply pipelines, that is, the air supply pipelines are connected in parallel, and this arrangement can ensure that the flow rates at different positions of the nozzles are basically the same. In addition, the ventilation holes of the inner nozzle 21 and the outer nozzle 31 are both rectangular, and the aspect ratios are greater than 50. Combining the above two points makes the jet have good two-dimensional flow characteristics, that is, the jet flow field has uniformity in the transverse direction. .

Sealing rubber rings are provided on both sides of the inner nozzle gasket 150 , the middle isolation plate 130 and the outer nozzle base 321 close to the front end cover 120 .

The two inner nozzle wall plates have the same geometric parameters and are aligned in the length and width directions respectively.

The outer nozzle wall plates all include a flat wall 336 and side walls located on both sides of the flat wall 336, in a "C" shape surrounding structure; the flat walls 336 of the two outer nozzle wall plates have the same thickness, and the two outer nozzle wall plates have the same thickness. The thickness of the side wall is also the same; the height of the side wall of each outer nozzle wall plate is uniformly long and one short, the height of the long side wall 332 is equal to the sum of the height of the short side wall 333 and the thickness of the flat wall 336, and the short side wall 333 of the outer nozzle wall plate is the same as the other. The long side walls 332 of an outer nozzle wall plate are arranged correspondingly, wherein a slit is left outside the short side wall 333, and the width of the slit is the same as the thickness of the side wall;

The central area of the outer nozzle base 321 close to the front end cover 120 is hollow for passing through another outer nozzle wall; the sum of the length of the outer nozzle wall adjacent to the front end cover 120 and the thickness of its base 321 is equal to the length of the other outer nozzle wall.

The gas collection chamber 11 is made of stainless steel. The inner nozzle 21 and the outer nozzle 31 are made of non-metallic materials such as ceramics and polytetrafluoroethylene with good insulation, which can meet the insulation requirements of the plasma exciter discharge on the carrier, and prevent the high-voltage electricity applied by the exciter from gathering with the metal. The gas chamber and the outside are connected, and the nozzle wall plate can directly act as the dielectric barrier layer of the plasma exciter due to its insulating properties. The ends of the two outer nozzle wall plates are also provided with positioning holes 335 , which are arranged correspondingly up and down. Both sides of the inner nozzle wall plate and both sides of the outer nozzle wall plate can be laid with a plasma exciter or directly as a dielectric barrier for the plasma exciter. The above-mentioned inner nozzle and outer nozzle are all processed and reassembled separately. Before assembling the nozzle device, the dielectric barrier discharge plasma exciter is firstly laid at the target position, which can realize the convenient and fast laying of the exciter in any position. It has strong operability in the experiment. The dimensions of the inner nozzle bracket and the inner nozzle gasket can be serialized, and the material of the inner nozzle wall plate can be serialized. According to the experimental research to investigate the influence of different materials and size parameters, the corresponding parts can be selected for assembly.

The straight sides of the waist-shaped holes are all arranged in the vertical direction. The upper part 330 of the outer nozzle and the lower part 320 of the outer nozzle can be adjusted upward and downward respectively, so as to realize the change of the width of the gap of the outer nozzle. At the same time, the inner nozzle 21 can be adjusted up and down, thereby changing the distance from the upper and lower walls of the outer nozzle.

To sum up, some parts of the above nozzle device adopt the method of split processing and reassembly, which can not only replace the same type of parts with different sizes and materials, but also fine-tune the parts along a specific direction, which can not only develop various characteristic geometric parameters of the nozzle. , Research on the change of dielectric barrier materials, and realize the convenient and rapid laying of single or multiple plasma exciters in any position and along any direction of the nozzle. The device has compact structure, simple parts processing, easy disassembly and assembly, and high reliability.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (10)

1. A shearing rectangular nozzle device for studying plasma-enhanced jet mixing, characterized in that it comprises an inner nozzle (21), an outer nozzle (31) and a gas collection chamber (11), the inner nozzle ( 21) and the outer nozzle (31) both protrude from the inside of the plenum (11), and the inner nozzle (21) protrudes from the inside of the outer nozzle (31); wherein: A front end cover (120) and a rear end cover (140) are respectively provided on the front side and the rear side of the gas collection chamber (11), and the connection between the front end cover (120) and the rear end cover (140) is provided with an intermediate isolation plate (130), the central area of the intermediate isolation plate (130) is a rectangular hollow area; the front end cover (120) is provided with a front end cover air inlet (122), and the rear end cover (140) There is an air intake hole (141) on the rear end cover; The inner nozzle (21) includes an inner nozzle bracket (220) and two inner nozzle wall plates (230), the inner nozzle bracket (220) is fixed on the middle partition plate (130), and the inner nozzle A support rib (223) is provided on the bracket (220), and two inner nozzle wall plates (230) are both fixed on the support rib (223) and combined to form the inner nozzle (21); the inner nozzle wall plate (230) first passes through the inner nozzle gasket (150), and then protrudes from the rectangular hollow area of the intermediate partition plate (130); the inner nozzle (21) is passed through the rear end cover air intake hole (141) realize gas supply; The outer nozzle (31) is composed of an outer nozzle upper part (330) and an outer nozzle lower part (320). The outer nozzle (31) is fixed on the front end cover (120) and is inserted into the front end cover. The air hole (122) realizes air supply. 2. The shearing rectangular nozzle device for studying plasma-enhanced jet blending according to claim 1, wherein the front end cover (120), the rear end cover (140) and the middle partition plate (130) ) are provided with bolt holes at the corresponding positions, and the hexagonal socket head bolts pass through the bolt holes on the front end cover (120), the middle partition plate (130) and the rear end cover (140) in turn and connect the front end cover (120), the rear end cover ( 140 ) and the intermediate isolation plate ( 130 ) are assembled as a plenum ( 11 ). 3. The shearing rectangular nozzle device for studying plasma-enhanced jet blending according to claim 1, wherein the inner nozzle holder (220) comprises a holder wall (222) and a support rib (223) ), the support ribs (223) are arranged perpendicular to the bracket wall plate (222), the bracket wall plate (222) is arranged parallel to the middle partition plate (130), and the two inner nozzle wall plates (230) The inner nozzle gasket (150) is arranged perpendicular to the support wall plate (222) and fixed on the support rib (223); the inner nozzle gasket (150) is arranged between the support wall plate (222) and the middle partition plate (130); The bracket wall plate (222), the inner nozzle gasket (150) and the middle partition plate (130) are all provided with waist-shaped holes at corresponding positions, and the hexagon socket bolts pass through the bracket wall plate (222) and the inner nozzle gasket in sequence. (150) and the waist-shaped holes of the intermediate partition plate (130) fix the inner nozzle bracket (220) on the intermediate partition plate (130). 4. The shearing rectangular nozzle device for studying plasma-enhanced jet blending according to claim 1, wherein the outer nozzle upper part (330) and the outer nozzle lower part (320) both comprise a base (321) and an outer nozzle wall plate, the outer nozzle wall plate is arranged perpendicular to the base (321), the base (321) is arranged parallel to the front end cover (120), the two bases (321) and the front end Corresponding positions of the cover (120) are provided with waist-shaped holes, and the inner hexagon bolts pass through the two bases (321) and the waist-shaped holes of the front end cover (120) in turn to fix the outer nozzle (31) on the front end cover (120) on. 5. The shearing rectangular nozzle device for studying plasma-enhanced jet mixing according to claim 4, characterized in that the ventilation holes of the inner nozzle (21) and the outer nozzle (31) are both rectangular, And the length-to-width ratio of the vent holes is greater than 50; both sides of the inner nozzle wall plate and both sides of the outer nozzle wall plate can be laid with a plasma exciter or a dielectric barrier layer directly used as a plasma exciter; two outer nozzle walls The ends of the plates are also provided with positioning holes (335). 6. The shearing rectangular nozzle device for studying plasma-enhanced jet blending according to claim 4, characterized in that the inner nozzle gasket (150), the middle partition plate (130) and the front end cap Both sides of the assembly surfaces of the outer nozzle base (321) of (120) are provided with sealing rubber rings. 7. The shearing rectangular nozzle device for studying plasma-enhanced jet blending according to claim 4, characterized in that: The geometric parameters of the two inner nozzle wall plates are the same, and they are aligned in the length and width directions respectively; The outer nozzle wall plates both include a flat wall (336) and side walls located on both sides of the flat wall (336), and are in a "C" shape enclosing structure; the flat walls (336) of the two outer nozzle wall plates have the same thickness, and the two The thickness of the side walls of the outer nozzle wall plates is also the same; the height of the side walls of each outer nozzle wall plate is uniformly long and short, and the height of the long side wall ( 332 ) is equal to the sum of the height of the short side wall ( 333 ) and the thickness of the flat wall ( 336 ) , the short side wall (333) of the outer nozzle wall plate is arranged corresponding to the long side wall (332) of the other outer nozzle wall plate, wherein a slit is left on the outside of the short side wall (333), and the width of the slit is the same as the thickness of the side wall ; The central area of the outer nozzle base (321) close to the front end cover (120) is hollow for passing through another outer nozzle wall plate; the sum of the length of the outer nozzle wall plate close to the front end cover (120) and the thickness of the base (321) is equal to the other One outer nozzle wall length. 8. The shearing rectangular nozzle device for studying plasma-enhanced jet blending according to claim 1, wherein the inner nozzle (21) is made of a material of ceramic or polytetrafluoroethylene, The outer nozzle (31) is also made of a material of ceramics or polytetrafluoroethylene, and the gas collection chamber (11) is made of stainless steel. 9 . The shearing-type rectangular nozzle device for studying plasma-enhanced jet blending according to claim 3 , wherein the straight sides of the girdle-shaped holes are arranged in a vertical direction. 10 . 10. The shearing rectangular nozzle device for studying plasma-enhanced jet mixing according to claim 1, characterized in that, the front end cover (120) is provided with 5 front end cover air inlet holes (122) , the rear end cover (140) is provided with four rear end cover air intake holes (141), and the front end cover air intake holes (122) and the rear end cover air intake holes (141) are arranged horizontally at equal intervals are arranged and connected to the external air supply pipeline.

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