JP2005240681A - Axial flow fan - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the performance and the efficiency by stabilizing and optimizing an eddy current generated in back of an inner cylinder in an axial flow fan. <P>SOLUTION: This axial flow fan is constructed so that an outer cylinder 12 is disposed concentrically on the outer periphery of the inner cylinder 11 through a ventilation passage 13, an impeller 16 having a plurality of moving blades 17 is rotatably supported in the inner cylinder 11, the outer cylinder 12 is provided with a diffuser 19 for recovering the dynamic pressure of fluid increased in pressure to the static pressure. The total length of the inner cylinder 11 is set to 1/2 times to 2/3 times as long as the total length of the diffuser 19. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an axial fan used for ventilation equipment and the like, and particularly relates to the shape of an inner cylinder extending to the rear diffuser side.

  FIG. 6 is a schematic view of an axial fan having an ideal inner cylinder, and FIGS. 7 and 8 are schematic views of a conventional axial fan.

  In a general axial fan, as shown in FIG. 6, an outer cylinder 002 is concentrically disposed on the outer periphery of an inner cylinder 001, and an annular ring is formed between the inner cylinder 001 and the outer cylinder 002. A ventilation path 003 is provided. A hub 004 is fixed to the front end portion of the inner cylinder 001, and a plurality of stationary blades 005 are installed between the outer cylinder 002 and the hub 004 at equal intervals in the circumferential direction. An impeller 006 is rotatably mounted on the front end portion of the inner cylinder 001 via a hub 004, and a plurality of moving blades 007 are attached to the outer peripheral portion of the impeller 006 so as to protrude into the ventilation path 003. ing. On the other hand, the outer cylinder 002 is provided with a diffuser 008 and a duct 009 for recovering the dynamic pressure of the fluid whose pressure has increased to a static pressure, and this diffuser 008 has an inner diameter from the front end portion to the rear end portion of the inner cylinder 001. It has an enlarged shape.

  Accordingly, the fluid introduced into the ventilation path 003 between the inner cylinder 001 and the outer cylinder 002 is accelerated by the rotating moving blade 007 and reaches the diffuser 008 via the stationary blade 005. The dynamic pressure (velocity energy) is converted into static pressure (pressure energy) by the diffuser 008 and discharged into the duct 009.

  In such an axial fan, since about half of the pressure increase immediately after the stationary blade 005 is discharged as a dynamic pressure, a diffuser 008 that recovers the dynamic pressure to a static pressure is required. When the duct 009 is provided on the downstream side of the diffuser 008, the pressure distribution in the duct 009 affects the performance of the diffuser 008, and the flow velocity distribution at the outlet of the diffuser 008 becomes the pressure distribution in the duct 009. There is a possibility that the fluid flow is disturbed and the performance is deteriorated due to the mutual influence. It is considered that this is because when the flow of the fluid is decelerated in the ventilation path 003, the flow causes separation or asymmetric distortion in the diffuser 008. In the axial fan, energy is imparted by turning the fluid into a swirl flow by the moving blade 007, static pressure is increased by eliminating the swirl flow velocity of the fluid by the stationary blade 005, and the fluid is decelerated by the downstream diffuser 008. It is a mechanism for increasing pressure. For this reason, when the rotor blade 007 imparts a turning force to the fluid, the fluid flow is biased toward the outer peripheral side, which has a fundamental problem that the flow velocity on the inner peripheral side is reduced.

  Therefore, various structures for the inner cylinder 001 have been proposed in order to solve the problems caused by such an axial fan. For example, as shown in FIG. 6, the inner cylinder 001 is streamlined with the outer diameter of its rear portion gradually reduced to prevent flow separation at the diffuser 008. However, it is difficult to form the inner cylinder 001 in a streamlined manner, and there is a problem that the manufacturing cost increases. Therefore, conventionally, as shown in FIG. 7, the inner cylinder 011 has a cylindrical shape, and the rear end portion is extended to the outlet of the diffuser 008, or the inner cylinder itself is eliminated as shown in FIG. ing.

  In addition, there exist the following patent documents 1 and 2 as such a conventional axial flow fan, for example.

JP 2003-278893 A JP 2000-170688 A

  However, even with the above-described conventional axial fan, a conventional problem cannot be solved, and the performance deteriorates due to separation of the fluid flow or asymmetric distortion. That is, as shown in FIG. 7, when the inner cylinder 011 is formed in a cylindrical shape and the rear end portion is extended, the boundary layer along the wall surface of the inner cylinder is expanded, and separation of the flow occurs. In addition, since the inner cylinder 011 has a flat rear end, the flow velocity varies due to fluctuations in the pressure distribution in the peripheral direction of the inner cylinder 011 and a drift occurs, an irregular vortex is generated, and the flow becomes asymmetric. Unable to achieve pressure recovery. On the other hand, as shown in FIG. 8, when the inner cylinder itself is eliminated, the flow channel area suddenly expands immediately after the stationary blade 005, resulting in instability and a vortex, resulting in a large pressure loss.

  SUMMARY OF THE INVENTION The present invention solves such problems, and an object thereof is to provide an axial fan that improves performance and efficiency by stabilizing and optimizing vortices generated behind an inner cylinder. To do.

  In order to achieve the above object, an axial fan according to the invention of claim 1 includes an inner cylinder housing a rotating shaft, an outer cylinder disposed concentrically on the outer periphery of the inner cylinder via a ventilation path, An impeller having a moving blade that is rotatably supported by the rotating shaft on the inner cylinder and protrudes into the ventilation path, and a dynamic pressure of a fluid that is provided on the outer cylinder and has increased in pressure by the moving blade is made static. In the axial fan having the recovering diffuser, the total length of the inner cylinder is set to be 1/2 to 2/3 times the total length of the diffuser.

  In the axial fan according to a second aspect of the present invention, the position of the rear end surface of the inner cylinder is provided at a position where the flow passage area at the front end portion of the diffuser expands from two to four times.

  An axial fan according to a third aspect of the present invention includes an inner cylinder housing a rotating shaft, an outer cylinder disposed concentrically on the outer periphery of the inner cylinder via an air passage, and the inner cylinder by the rotating shaft. A shaft comprising a rotor wheel having a moving blade that is rotatably supported and protrudes from the ventilation path, and a diffuser that is provided on the outer cylinder and recovers the dynamic pressure of the fluid whose pressure has been increased by the moving blade to a static pressure. In the flow fan, a stabilizing plate extending backward is provided at a rear end portion of the inner cylinder.

  In the axial fan according to a fourth aspect of the present invention, the stabilizer is provided along the radial direction, and the total length is set to be 1 to 2 times the outer diameter of the rear end portion of the inner cylinder. It is a feature.

  An axial fan according to a fifth aspect of the present invention includes an inner cylinder that accommodates a rotation shaft, an outer cylinder that is disposed concentrically on the outer periphery of the inner cylinder via a ventilation path, and the inner cylinder that is provided with the rotation shaft. A shaft comprising a rotor wheel having a moving blade that is rotatably supported and protrudes from the ventilation path, and a diffuser that is provided on the outer cylinder and recovers the dynamic pressure of the fluid whose pressure has been increased by the moving blade to a static pressure. In the flow fan, a concave portion is provided at a rear end portion of the inner cylinder.

  In an axial fan according to a sixth aspect of the present invention, the overall length of the recess is set to be 1/5 to 1/2 times the outer diameter of the rear end portion of the inner cylinder.

  An axial fan according to a seventh aspect of the present invention includes an inner cylinder that accommodates a rotation shaft, an outer cylinder that is disposed concentrically on the outer periphery of the inner cylinder via an air passage, and the inner cylinder that is provided with the rotation shaft. A shaft comprising a rotor wheel having a moving blade that is rotatably supported and protrudes from the ventilation path, and a diffuser that is provided on the outer cylinder and recovers the dynamic pressure of the fluid whose pressure has been increased by the moving blade to a static pressure. In the flow fan, a fluid intake port is provided at a front end portion of the inner cylinder, while a fluid discharge port is provided at a rear portion of the inner cylinder, and a distance from the fluid intake port to the fluid discharge port is determined by the diffuser. It is characterized in that the length is set to 1/2 to 2/3 times the total length.

  In an axial fan according to an eighth aspect of the present invention, the outer diameter of the inner cylinder is set so that the rear side gradually becomes a smaller diameter.

  According to the axial fan of the first aspect of the invention, the outer cylinder is disposed concentrically on the outer periphery of the inner cylinder via the ventilation path, and the impeller having the moving blades is rotatably supported on the inner cylinder. Since the diffuser that restores the dynamic pressure of the fluid whose pressure has increased to the static pressure is provided, and the total length of the inner cylinder is set to 1/2 to 2/3 times the total length of the diffuser, the total length of the inner cylinder By making it shorter than the diffuser, the wall surface area is reduced, it is possible to ensure the sufficient pressure recovery by suppressing the generation of the boundary layer where the fluid separates, and stabilize the vortex generated downstream to suppress the pressure loss As a result, the performance and efficiency can be improved.

  According to the axial flow fan of the second aspect of the invention, the position of the rear end surface of the inner cylinder is set to a position where the flow passage area at the front end of the diffuser is expanded from two to four times. By setting the optimum length to ensure a sufficient pressure recovery, the vortex generated at the rear end of the inner cylinder can be reliably stabilized.

  According to the axial fan of the invention of claim 3, the outer cylinder is concentrically disposed on the outer periphery of the inner cylinder via the ventilation path, and the impeller having the moving blade is rotatably supported on the inner cylinder. The cylinder is provided with a diffuser that restores the dynamic pressure of the fluid whose pressure has increased to static pressure, and a stabilizer plate extending backward is provided at the rear end of the inner cylinder. Vortices can be generated, and performance and efficiency can be improved by stabilizing and optimizing the generated vortices.

  According to the axial fan of the invention of claim 4, the stabilizer is provided along the radial direction, and the total length is set to be 1 to 2 times the outer diameter of the rear end portion of the inner cylinder. By generating them in an orderly manner, pressure loss can be suppressed.

  According to the axial fan of the invention of claim 5, the outer cylinder is disposed concentrically on the outer periphery of the inner cylinder via the ventilation path, and the impeller having the moving blade is rotatably supported on the inner cylinder. Since the diffuser that restores the dynamic pressure of the fluid whose pressure has increased to the static pressure is provided and a recess is provided at the rear end of the inner cylinder, a vortex is generated in the recess at the rear end of the inner cylinder. The vortex does not expand and can be stabilized, and as a result, the performance and efficiency can be improved.

  According to the axial fan of the sixth aspect of the invention, since the overall length of the recess is set to 1/5 to 1/2 times the outer diameter of the rear end of the inner cylinder, A stable vortex can be generated.

  According to the axial fan of the seventh aspect of the invention, the outer cylinder is disposed concentrically on the outer periphery of the inner cylinder via the ventilation path, and the impeller having the moving blade is rotatably supported on the inner cylinder. The cylinder is provided with a diffuser that restores the dynamic pressure of the fluid whose pressure has risen to static pressure, and a fluid intake port is provided at the front end of the inner cylinder, while a fluid discharge port is provided at the rear of the inner cylinder, and fluid is supplied from the fluid intake port. Since the distance to the discharge port is set to 1/2 to 2/3 times the total length of the diffuser, by forming a fluid flow path from the fluid intake port to the fluid discharge port inside the inner cylinder The boundary layer where the fluid peels off at the outer peripheral surface of the inner cylinder can be accelerated, and this acceleration can suppress the generation of the boundary layer and ensure sufficient pressure recovery, as well as the vortex generated downstream. Pressure loss can be suppressed, and as a result, performance and efficiency can be improved. That.

  According to the axial fan of the eighth aspect of the invention, since the outer diameter of the inner cylinder is set so that the rear side gradually becomes smaller, the boundary layer is reliably accelerated by the fluid from the fluid discharge port, and the boundary layer is generated. Can be suppressed.

  Embodiments of an axial fan according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

  FIG. 1 is a schematic diagram of an axial fan according to Embodiment 1 of the present invention.

  In the axial fan of the first embodiment, as shown in FIG. 1, an outer cylinder 12 is concentrically disposed on the outer periphery of the inner cylinder 11, and an annular shape is provided between the inner cylinder 11 and the outer cylinder 12. A ventilation path 13 is provided. A hub 14 having the same diameter as that of the inner cylinder 11 is fixed to the front end portion of the inner cylinder 11, and a plurality of stationary blades 15 are arranged at equal intervals in the circumferential direction between the outer cylinder 12 and the hub 14. It is erected. Further, at the front end portion of the inner cylinder 11, an impeller 16 having a spherical front end and the same diameter as the inner cylinder 11 is rotatably mounted via a hub 14. Protrudes into the ventilation path 13 and a plurality of rotor blades 17 are attached at equal intervals in the circumferential direction.

  On the other hand, the outer cylinder 12 includes a front duct 18, a diffuser 19, and a rear duct 20. The diffuser 19 converts (recovers) the dynamic pressure (kinetic energy) of the fluid whose pressure has increased into static pressure (pressure energy), and has a shape in which the inner diameter increases from the front end portion to the rear end portion of the inner cylinder 11. It has become.

  In this embodiment, the inner cylinder 11 has a cylindrical shape, and the rear end portion is a flat surface along the radial direction. The total length of the inner cylinder 11 is 1/2 to 2/3 times the total length of the diffuser 19, that is, the total length of the inner cylinder 11 is 1 / 2L to 2/2 of the total length L of the diffuser 19. It is set to 3L.

  Further, it is known that the amount of pressure that can be effectively recovered by the diffuser 19 depends on the position of the rear end portion of the inner cylinder 11. In this embodiment, the position of the rear end surface of the inner cylinder 11 is changed. The flow path area at the front end portion of the diffuser 19 is set to a position where it is enlarged from 2 times to 4 times (preferably from 2.5 times to 3.0 times). In other words, the position of the rear end surface of the inner cylinder 11 at a position where the ratio of the rear flow area A2 to the flow area A1 at the front end of the diffuser 19 is 2 to 4 (preferably 2.5 to 3.0). I try to let them.

  Therefore, when a fluid is introduced from the front duct 18 into the ventilation path 13, the fluid is accelerated by the rotating moving blade 17 and reaches the diffuser 19 through the stationary blade 15. The diffuser 19 converts dynamic pressure (velocity energy) into static pressure (pressure energy) and discharges it to the rear duct 20.

  At this time, since the wall surface area of the inner cylinder 11 is reduced, the boundary layer itself peeling from the wall surface does not occur. The length of the boundary layer is proportional to the length of the inner cylinder 11, but the pressure recovery in the diffuser 19 is proportional to the square of the length of the inner cylinder 11. Therefore, if the length of the inner cylinder 11 is halved, the boundary layer length is also halved and the pressure recovery is increased to 3/4. Therefore, the pressure loss due to the boundary layer is halved. In addition, although a vortex is generated at the rear end of the inner cylinder 11, this vortex is a stable low-speed flow and can reduce the dead water area, and the pressure loss due to this is a pressure of (1-3 / 4). Therefore, it is possible to prevent the performance from being deteriorated as a result of the subtraction.

  As described above, in the axial fan of the first embodiment, the outer cylinder 12 is disposed concentrically on the outer periphery of the inner cylinder 11 via the ventilation path 13, and the impeller 16 having the moving blade 17 on the inner cylinder 11. Is provided with a diffuser 19 that recovers the dynamic pressure of the fluid whose pressure has increased to a static pressure, and the total length of the inner cylinder 11 is reduced from 1/2 times the total length of the diffuser 19 to 2 / The length is set to 3 times.

  Accordingly, the wall surface area is reduced by making the total length of the inner cylinder 11 shorter than that of the diffuser 19, and it is possible to suppress the generation of the boundary layer from which the fluid peels and to ensure sufficient pressure recovery, and to occur downstream. The pressure loss can be suppressed by stabilizing the vortex, and as a result, the performance and efficiency can be improved.

  Further, by increasing the distance between the rear end portion of the inner cylinder 11 and the rear duct 20, the distortion of the downstream flow due to the expansion of the boundary layer of the inner cylinder 11 can be reduced, and the influence on the rear duct 20. And the influence of the pressure distribution in the rear duct 20 on the performance of the diffuser 19 can be eliminated. Further, it is only necessary to shorten the overall length by making the inner cylinder 11 into a cylindrical shape, which can contribute to simplification of the structure and cost reduction.

  2 is a schematic diagram of an axial fan according to a second embodiment of the present invention, FIG. 3A is a rear view of an inner cylinder of the axial fan according to the second embodiment, and FIGS. 3-2 and 3-3 are It is a rear view of the inner cylinder showing the modification of the stabilizer in the axial fan of Example 2.

  In the axial fan of the second embodiment, as shown in FIGS. 2 to 3-1, the inner cylinder 21 has a cylindrical shape, and a lid plate 22 having a flat surface perpendicular to the longitudinal direction is fixed to the rear end portion. Yes. And this inner cylinder 21 is provided with the recessed part 24 by fixing the guide ring 23 which has the same outer diameter as the inner cylinder 21 to the rear-end part. The overall length (depth) of the recess 24 is 1/5 to 1/2 times the outer diameter of the rear end of the inner cylinder 21, that is, the outer diameter D of the rear end of the inner cylinder 21. On the other hand, the overall length of the recess 24 is set to 1 / 5D to 1 / 2D.

  Further, the inner cylinder 21 is fixed with a horizontal stabilizer 25 extending rearward at the rear end, and the total length of the stabilizer 25 is 1 to 2 times the outer diameter of the rear end of the inner cylinder 21. , That is, the total length of the stabilizing plate 25 is set to a length of 2 to 2D with respect to the outer diameter D of the rear end portion of the inner cylinder 21.

  Accordingly, when the fluid is converted from dynamic pressure (velocity energy) to static pressure (pressure energy) by the diffuser, a large number of vortices smaller than the outer diameter of the inner cylinder 21 are generated in the concave portion 24 and bulge outward. It does not interfere with the pressure rise. In addition, the generated vortex is divided up and down symmetrically by the stabilizing plate 25, and no drift is generated around the vortex.

  As described above, in the axial fan according to the second embodiment, the guide ring 23 is fixed to the rear end portion of the inner cylinder 21 to provide the recess 24, and the stability extends to the rear end portion of the inner cylinder 21 backward. A plate 25 is provided. Therefore, by generating a plurality of vortices in the recess 24 of the inner cylinder 21, the vortices can be stabilized without expanding, and the stabilizer 25 can make the vortices symmetrical. As a result, it is possible to improve performance and efficiency by stabilizing and optimizing the vortex.

  In addition, in this Example 2, the horizontal stabilizer 25 was fixed to the rear-end part of the inner cylinder 21, However, The shape and attachment direction of this stabilizer are not limited to this structure. For example, as shown in FIG. 3B, a cross-shaped stabilizing plate 26 is fixed to the rear end portion of the inner cylinder 21, or as shown in FIG. You may fix the stabilizer 26 which makes a shape. In this case, the number of stabilizing plates is not limited to these, and may be set as appropriate. In order to maintain the axial symmetry of the downstream flow, it is appropriate to arrange the stabilizing plates at equal intervals in the circumferential direction. Further, the shape of the inner cylinder 21 may be a cylindrical shape or a conical shape, and the length thereof may be the same as that of the diffuser or may be as short as in the first embodiment.

  FIG. 4 is a schematic diagram of an axial fan according to the third embodiment of the present invention, and FIG. In addition, the same code | symbol is attached | subjected to the member which has the same function as what was demonstrated in the Example mentioned above, and the overlapping description is abbreviate | omitted.

  In the axial fan of the present embodiment, as shown in FIG. 4, the inner cylinder 31 has a conical shape such that the outer diameter gradually decreases toward the rear side, and the first cylinder portion 32 having a large diameter and a small diameter are formed. The second cylindrical portion 33 is configured. The front end portion of the first cylindrical portion 32 opens slightly larger than the outer diameter of the hub 14, and a fluid intake port 34 is provided at a connection portion with the hub 14. The second cylindrical portion 33 has a front end opening slightly smaller than the outer diameter of the rear end portion of the first cylindrical portion 32, and a fluid discharge port 35 is provided at a connection portion with the first cylindrical portion 32, and the rear end The part is closed. The distance from the fluid intake port 34 to the fluid discharge port 35 is set to be 1/2 to 2/3 times the entire length of the diffuser 19.

  Accordingly, the fluid is accelerated by the moving blade 17 and reaches the diffuser 19 via the stationary blade 15 where the dynamic pressure (velocity energy) is converted into static pressure (pressure energy). At this time, the fluid that has passed through the static pressure 15 is guided to the diffuser 19, but a part of the fluid enters the first cylindrical portion 32 of the inner cylinder 31 from the fluid intake port 34, and again from the fluid discharge port 35. Is erupted. Therefore, the fluid flowing on the outer peripheral surface of the first cylindrical portion 32 is accelerated by the fluid (jet) ejected from the fluid discharge port 35, and the flow velocity distribution here can be made constant. As shown in FIG. 5, since a part of the fluid branches to the fluid intake port 34, a slight pressure loss occurs in the main flow, but the static pressure is sufficiently recovered by the jet from the fluid discharge port 35. It will be.

  As described above, in the axial fan according to the third embodiment, the inner cylinder 31 has a conical shape, the fluid intake port 34 is provided at the front end portion, the fluid discharge port 35 is provided at the intermediate portion, and the fluid intake port 34 The distance to the fluid discharge port 35 is set to 1/2 to 2/3 times the total length of the diffuser 19. Therefore, by forming a fluid flow path from the fluid intake port 34 to the fluid discharge port 35 inside the inner cylinder 31, the boundary layer where the fluid peels off at the outer peripheral surface of the inner cylinder 31 can be accelerated. This acceleration suppresses the generation of the boundary layer to ensure sufficient pressure recovery and stabilizes the vortex generated downstream to suppress pressure loss, resulting in improved performance and efficiency. Can be achieved.

  The axial fan according to the present invention is intended to stabilize vortices generated by improving the shape of the inner cylinder. A supercharger for a ship, a supercharger for an automobile, to which this axial fan is applied, It is useful for industrial compressors and small aviation gas turbines.

It is the schematic of the axial flow fan which concerns on Example 1 of this invention. It is the schematic of the axial flow fan which concerns on Example 2 of this invention. It is a rear view of the inner cylinder in the axial fan of Example 2. It is a rear view of the inner cylinder showing the modification of the stabilizer in the axial fan of Example 2. It is a rear view of the inner cylinder showing the modification of the stabilizer in the axial fan of Example 2. It is the schematic of the axial flow fan which concerns on Example 3 of this invention. It is a graph showing the fluid pressure change of the ventilation path in the axial fan of Example 3. It is the schematic of the axial flow fan which has an ideal inner cylinder. It is the schematic of the conventional axial fan. It is the schematic of the conventional axial fan.

Explanation of symbols

11, 21, 31 Inner cylinder 12 Outer cylinder 13 Ventilation path 15 Stator blade 16 Winger 17 Rotor blade 19 Diffuser 23 Guide ring 24 Recess 25, 26, 27 Stabilizing plate 34 Fluid intake port 35 Fluid discharge port

Claims (8)

  An inner cylinder accommodating a rotating shaft, an outer cylinder arranged concentrically on the outer periphery of the inner cylinder via a ventilation path, and supported by the inner cylinder so as to be rotatable by the rotating shaft and projecting into the ventilation path In the axial fan including the impeller having the moving blade and the diffuser provided in the outer cylinder and recovering the dynamic pressure of the fluid whose pressure has been increased by the moving blade to static pressure, the total length of the inner cylinder is An axial fan characterized in that the length is set to 1/2 to 2/3 times the total length of the diffuser.   2. The axial flow fan according to claim 1, wherein the position of the rear end surface of the inner cylinder is provided at a position where the flow passage area at the front end portion of the diffuser is expanded two to four times. fan.   An inner cylinder accommodating a rotating shaft, an outer cylinder arranged concentrically on the outer periphery of the inner cylinder via a ventilation path, and supported by the inner cylinder so as to be rotatable by the rotating shaft and projecting into the ventilation path A rear end portion of the inner cylinder, wherein the rear end portion of the inner cylinder is provided with an impeller having a moving blade and a diffuser provided in the outer cylinder and recovering a dynamic pressure of a fluid whose pressure is increased by the moving blade to a static pressure. An axial fan characterized in that a stabilizing plate extending rearward is provided.   4. The axial flow fan according to claim 3, wherein the stabilizing plate is provided along a radial direction, and a total length is set to be 1 to 2 times an outer diameter of a rear end portion of the inner cylinder. An axial fan.   An inner cylinder accommodating a rotating shaft, an outer cylinder arranged concentrically on the outer periphery of the inner cylinder via a ventilation path, and supported by the inner cylinder so as to be rotatable by the rotating shaft and projecting into the ventilation path A rear end portion of the inner cylinder, wherein the rear end portion of the inner cylinder is provided with an impeller having a moving blade and a diffuser provided in the outer cylinder and recovering a dynamic pressure of a fluid whose pressure is increased by the moving blade to a static pressure. An axial fan characterized in that a recess is provided in the fan.   6. The axial flow fan according to claim 5, wherein the overall length of the concave portion is set to a length that is 1/5 to 1/2 times the outer diameter of the rear end portion of the inner cylinder. fan.   An inner cylinder accommodating a rotating shaft, an outer cylinder arranged concentrically on the outer periphery of the inner cylinder via a ventilation path, and supported by the inner cylinder so as to be rotatable by the rotating shaft and projecting into the ventilation path In an axial fan comprising an impeller having a moving blade and a diffuser provided in the outer cylinder and recovering the dynamic pressure of the fluid whose pressure has been increased by the moving blade to a static pressure, the front end of the inner cylinder While a fluid intake port is provided, a fluid discharge port is provided at the rear of the inner cylinder, and the distance from the fluid intake port to the fluid discharge port is from 1/2 times the entire length of the diffuser to 2/3. An axial fan characterized by being set to double the length.   8. The axial fan according to claim 7, wherein the outer diameter of the inner cylinder is set so that the rear side gradually becomes a smaller diameter.

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