High-efficiency centrifugal fan for cooking fume extraction

Abstract

The application discloses a high-efficiency centrifugal fan for extracting oil fume, which comprises a fan casing, an axial air inlet is arranged on one side end face of the fan casing, a flow guide cover is coaxially fixed outside the air inlet, and a protection support is fixed in the flow guide cover; a double-impeller mechanism is rotationally arranged in the center of the fan casing, and the double-impeller mechanism and the flow guide cover are combined to form an annular air supply channel; a vortex air outlet is arranged on the circumferential side wall of the fan casing; the double-impeller mechanism is adopted, the inner impeller is coaxially rotationally arranged in the outer impeller, the rotating directions of the inner impeller and the outer impeller are reversely arranged, and the directions of the blades are reversely distributed; on one hand, the inner impeller can provide a flow regulating effect on the airflow in the outer impeller, and can inhibit the broadband noise caused by vortex shedding; on the other hand, the shear airflow can be generated between the outer impeller and the inner impeller, the oil fume particle groups can be effectively dispersed, the capture efficiency of the micro oil mist can be improved, the centrifugal separation effect can be enhanced, and the oil fume escape can be reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of centrifugal fans, in particular to a high-efficiency centrifugal fan for extracting cooking fumes. Background Art

[0002] In modern kitchens, range hoods have become an indispensable household appliance. They mainly use the principle of centrifugal force, and the motor drives the impeller to rotate at high speed, so that the air entering the fan is thrown around by the centrifugal force and discharged along the volute flow channel.

[0003] However, conventional fans often use a single-impeller design, relying on passive suction. This lacks sufficient suction power for the low-temperature, diffused steam (flow rate <0.3 m / s) generated by steaming and stewing. This results in a high fume escape rate, and the airflow boundary layer rapidly decays at the edge of the fume hood, causing oil mist to form "backflow" around the fume hood. Therefore, there is a need for a high-efficiency centrifugal fan for extracting range fumes to address the issues raised in the aforementioned background technology. Summary of the Invention

[0004] To achieve the above-mentioned object, the present invention provides the following technical solution: a high-efficiency centrifugal fan for extracting range fumes, comprising: a fan housing, one end surface of which is provided with an axial air inlet, a deflector coaxially fixed outside the air inlet, and a protective bracket fixed inside the deflector;

[0005] A double-impeller mechanism is centrally mounted in the fan housing, and the double-impeller mechanism and the air guide cover form an annular air supply channel; a vortex exhaust port is disposed on the circumferential side wall of the fan housing;

[0006] The double-impeller mechanism includes an outer impeller and an inner impeller coaxially nested, wherein the inner impeller is coaxially arranged in the outer impeller, and a main shaft tube is rotatably connected to the fan casing through a bearing, and the main shaft tube is fixed to the outer impeller;

[0007] An inner shaft is coaxially arranged in the main shaft tube, and one end of the inner shaft is fixed to the inner impeller;

[0008] The outer impeller and the inner impeller rotate synchronously in opposite directions.

[0009] Further, as a preference, a first bevel gear is coaxially fixed on the main shaft tube, a second bevel gear is provided along the axial direction of the first bevel gear in parallel, the inner shaft is slidably connected to the second bevel gear, and two bevel gears are symmetrically rotated on both sides of the first bevel gear outside the fan casing, and both of the bevel gears are meshed with the first bevel gear and the second bevel gear;

[0010] A driving part is arranged outside the fan casing, and an output end of the driving part is connected to the inner shaft for transmission through gear meshing.

[0011] Further, as a preference, the inner shaft is slidably mounted in the main shaft tube, a propulsion unit is provided outside the fan casing, and a telescopic driving end of the propulsion unit is connected to the end of the inner shaft for controlling the axial displacement of the inner shaft along the main shaft tube;

[0012] A guide ring is coaxially fixed inside the outer impeller, a guide sleeve is coaxially slidably mounted on the guide ring, and one end of the guide sleeve is rotatably connected to the inner impeller;

[0013] An auxiliary air duct is provided in the fan housing at one side of the axial end face of the outer impeller. A plurality of through-type air flow holes are arranged in a ring array on the axial end face of the outer impeller, and the air flow holes are connected to the auxiliary air duct.

[0014] Furthermore, preferably, a plurality of side openings are provided on the side wall of the guide ring, and the side openings are opened or closed when the guide sleeve slides axially along the guide ring.

[0015] Furthermore, preferably, the outer impeller and the inner impeller have the same structure, and the outer impeller includes two shaft rings, and a plurality of blades are distributed in a circumferential array between the two shaft rings, and each of the blades is axially connected to the shaft ring;

[0016] An adjusting ring is rotatably provided on one of the shaft rings, and a plurality of oblique guide grooves are distributed circumferentially in the adjusting ring;

[0017] A shift rod is fixed on one side end of each blade, a pin is vertically fixed on the shift rod, and the pin is slidably connected to the oblique guide groove.

[0018] Furthermore, preferably, the cross section of the blade is an arc-shaped structure, and the installation directions of the blades in the outer impeller and the inner impeller are opposite to each other.

[0019] Further, as a preference, a first guide sleeve is coaxially fixed on the adjustment ring of the inner impeller, and a shaft sleeve is coaxially rotatably connected to one side end face of the protective bracket, and a first positioning pin is fixed on the side wall of the shaft sleeve; and an arc-shaped inner groove is provided on the inner wall of the first guide sleeve, and the first positioning pin is slidably connected to the arc-shaped inner groove.

[0020] Further, preferably, a second guide sleeve is coaxially fixed on the adjustment ring of the outer impeller, a sleeve is provided on the outer sliding sleeve of the main shaft tube, a second positioning pin is fixed on the side wall of the sleeve, and the second positioning pin is slidably connected to the arc-shaped inner groove provided on the inner wall of the second guide sleeve;

[0021] A limiting ring is clamped and fixed in the sheath, and the inner shaft is rotatably connected in the limiting ring.

[0022] Furthermore, preferably, when the inner shaft is axially pushed by the propulsion unit, the inner impeller partially convexes outward or partially retracts inward relative to the outer impeller. At this time, the blades on the inner impeller and the outer impeller can be deflected synchronously, and the deflection directions are set in opposite directions.

[0023] Furthermore, preferably, when the inner impeller is retracted into the outer impeller, the central tangent directions of the blades in the inner impeller and the outer impeller are both distributed toward the center of the circle;

[0024] When the inner impeller protrudes outward from the outer impeller, the tangent directions of the ends of the blades in the inner impeller and the outer impeller are both distributed toward the center of the circle.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The high-efficiency centrifugal fan for extracting oil smoke in the present invention adopts a dual-impeller mechanism, wherein the inner impeller is coaxially arranged inside the outer impeller, the rotation directions of the inner impeller and the blades are arranged in opposite directions. On the one hand, the inner impeller can provide a rectifying effect on the airflow in the outer impeller, which can suppress the broadband noise caused by vortex shedding. On the other hand, a shear airflow can be generated between the outer and inner impellers, which can effectively break up the oil smoke particles and improve the capture efficiency of fine oil mist. At the same time, the reverse vortex formed by the inner and outer impellers can prolong the residence time of the airflow in the fan, enhance the centrifugal separation effect, and reduce the escape of oil smoke.

[0027] Secondly, the inner impeller can be retracted or protruded from the outer impeller during the axial propulsion of the propulsion unit. When the inner impeller is retracted into the outer impeller, the blades of the inner impeller generate centripetal airflow during rotation and superimpose on the centrifugal force field of the outer impeller, producing a spiral compression effect, which forces the high-temperature oil smoke to be compressed during the radial acceleration process, thereby increasing the wind pressure intensity; and when the inner impeller protrudes outward from the outer impeller, the inner impeller can cooperate with the guide cover to form an annular air supply channel, thereby actively sucking in the diffused oil smoke, and after the oil enters the guide ring of the inner impeller, it can be centrifugally sucked in by the outer impeller through the side openings on its side wall, further reducing the oil smoke escape rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0029] Figure 2 Schematic diagram of the structure of the double impeller mechanism of the present invention;

[0030] Figure 3 Schematic diagram of the internal structure of the double impeller mechanism of the present invention;

[0031] Figure 4 Schematic diagram of the structure of the outer impeller in the present invention;

[0032] Figure 5Schematic diagram of the structure of the adjustment ring in the present invention;

[0033] Figure 6 for Figure 3 A schematic diagram of the structure at center A;

[0034] Figure 7 Schematic diagram of the structure of the second guide sleeve in the present invention;

[0035] Figure 8 Schematic diagram of blade distribution when the inner impeller is retracted into the outer impeller in the present invention;

[0036] Figure 9 Schematic diagram of blade distribution when the inner impeller protrudes outward from the outer impeller in the present invention;

[0037] In the figure: 1. Fan casing; 11. Air deflector; 12. Protective bracket; 13. Vortex exhaust port; 14. Main shaft tube; 15. Inner shaft; 16. Drive unit; 17. Propulsion unit; 18. Auxiliary air duct; 2. Double impeller mechanism; 21. First bevel gear; 22. Second bevel gear; 23. Bevel gear; 24. Guide ring; 25. Drain sleeve; 26. Side port; 3. Outer impeller; 31. Shaft collar; 32. Blades; 33. Adjustment ring; 34. Oblique guide groove; 35. Push rod; 36. Pin; 4. Inner impeller; 41. First guide sleeve; 42. Shaft sleeve; 43. Second guide sleeve; 44. Sleeve; 45. Limit ring. DETAILED DESCRIPTION

[0038] See also Figures 1-9 In an embodiment of the present invention, a high-efficiency centrifugal fan for extracting range fumes comprises: a fan housing 1, one end surface of which is provided with an axial air inlet, a deflector 11 coaxially fixed outside the air inlet, and a protective bracket 12 fixed inside the deflector 11;

[0039] A double-impeller mechanism 2 is centrally mounted in the fan housing 1, and the double-impeller mechanism 2 and the air guide shroud 11 form an annular air supply channel. A vortex exhaust port 13 is provided on the circumferential side wall of the fan housing 1. The vortex exhaust port 13 is used to connect to an external flue gas duct to quickly discharge the flue gas in the fan housing 1 into the flue gas duct.

[0040] The double impeller mechanism 2 includes an outer impeller 3 and an inner impeller 4 coaxially nested, wherein the inner impeller 4 is coaxially arranged in the outer impeller 3, and a main shaft tube 14 is rotatably connected to the fan casing 1 through a bearing, and the main shaft tube 14 is fixed to the outer impeller 3;

[0041] An inner shaft 15 is coaxially arranged in the main shaft tube 14, and one end of the inner shaft 15 is fixed to the inner impeller 4;

[0042] The outer impeller 3 and the inner impeller 4 rotate synchronously in opposite directions.

[0043] In the embodiment, the first bevel gear 21 is coaxially fixed on the main shaft tube 14, the second bevel gear 22 is arranged in parallel along the axial direction of the first bevel gear 21, the inner shaft 15 is slidingly connected in the second bevel gear 22, and two bevel gears 23 are symmetrically arranged on the both sides of the first bevel gear 21 outside the fan casing 1 and rotate in the same direction, and the two bevel gears 23 are engaged with the first bevel gear 21 and the second bevel gear 22.

[0044] The driving unit 16 is arranged outside the fan casing 1, the output end of the driving unit 16 is connected with the inner shaft 15 through gear engagement, that is, the driving unit 16 can drive the inner shaft 15 to rotate, at this time, the inner shaft 15 rotates synchronously with the inner impeller 4, and the engagement between the second bevel gear 22 and the bevel gear 23 drives the first bevel gear 21 to rotate reversely, so that the main shaft tube 14 rotates reversely relative to the inner shaft 15, and the outer impeller 3 rotates synchronously with the main shaft tube 14.

[0045] As a preferred embodiment, the inner shaft 15 is slidingly arranged in the main shaft tube 14, and the advancing unit 17 is arranged outside the fan casing 1, the telescopic driving end of the advancing unit 17 is connected with the end of the inner shaft 15, and is used for controlling the axial displacement of the inner shaft 15 along the main shaft tube 14.

[0046] The flow guide ring 24 is coaxially fixed in the outer impeller 3, the flow guide sleeve 25 is slidingly arranged on the flow guide ring 24 in a coaxial manner, and one end of the flow guide sleeve 25 is rotatably connected with the inner impeller 4.

[0047] The auxiliary air duct 18 is arranged in the fan casing 1 at the side of the axial end surface of the outer impeller 3, a plurality of through air flow holes (not shown in the figure) are annularly arranged on the axial end surface of the outer impeller 3, and the air flow holes are communicated with the auxiliary air duct 18.

[0048] In this embodiment, a plurality of side openings 26 are provided on the side wall of the guide ring 24, and the guide sleeve 25 slides axially along the guide ring 24 so that the side openings 26 are opened or closed, wherein the propulsion unit 17 can drive the inner impeller 4 on the inner shaft 15 to partially bulge outward or partially retract into the outer impeller 3 during telescopic operation. When the inner impeller 4 is retracted into the outer impeller 3, the side openings 26 are closed by the annular action of the guide sleeve 25, and the blades of the inner impeller 4 generate centripetal airflow during rotation and superimpose on the centrifugal force field of the outer impeller, generating a spiral compression effect and forming a shear airflow. The airflow is compressed and accelerated in the radial direction, thereby increasing the wind pressure. 30%~40%, a part of the flue gas can enter the auxiliary air duct 18 through the drainage sleeve 25, and the rest of the flue gas is centrifugally diffused in the fan casing 1 through the outer impeller. With this arrangement, on the one hand, the outer impeller 3 can form a centrifugal force field during rotation to separate large oil droplets (>50μm), with a separation efficiency of 92%, while the shear airflow guides the fine oil mist (<10μm) into the auxiliary air duct, and is separated secondary by the Venturi effect (efficiency increased to 85%). On the other hand, the shear airflow generated by the rotation of the inner impeller 4 guides the fine oil mist (<10μm) into the auxiliary air duct, and is separated secondary by the Venturi effect (efficiency increased to 85%).

[0049] When the inner impeller 4 protrudes outward from the outer impeller 3, the side port 26 is in an open state, and the inner impeller 4 cooperates with the air guide hood 11 to introduce the smoke into the drainage sleeve 25 through the annular air supply channel (the air flow is accelerated to 18-22m / s, generating a low-pressure introduction area, and the gradually converging and expanding curved surface design of the air guide hood can form a local negative pressure in the air flow, thereby enhancing the suction capacity of suspended particles), and the inner impeller 4 and the air guide hood 11 cooperate to form an internal air supply channel. At this time, the smoke can be discharged through the side port 26 and diffused in the fan casing 1 by the centrifugal force generated by the rotation of the outer impeller 3, thereby extending the smoke flow path, ensuring the wind pressure intensity, and further preventing the smoke from escaping.

[0050] In this embodiment, the outer impeller 3 and the inner impeller 4 have the same structure. The outer impeller 3 includes two shaft rings 31. A plurality of blades 32 are distributed in a circumferential array between the two shaft rings 31. Each of the blades 32 is axially rotatably connected to the shaft ring 31.

[0051] An adjusting ring 33 is rotatably provided on one of the shaft rings 31 , and a plurality of oblique guide grooves 34 are distributed circumferentially within the adjusting ring 33 ;

[0052] A lever 35 is fixed to one end of each blade 32, and a pin 36 is vertically fixed to the lever 35. The pin 36 is slidably connected to the oblique guide groove 34. When the adjustment ring 33 is deflected relative to the shaft ring 31, it can push the lever 35 to deflect around the axis of the blade 32 through the internal oblique guide grooves 34, thereby achieving fine-tuning of the installation angle of the blade 32.

[0053] In this embodiment, the cross-section of the blade 32 is an arc-shaped structure, and the installation directions of the blades 32 in the outer impeller 3 and the inner impeller 4 are opposite, so that a high-intensity shear airflow is formed during the synchronous reverse rotation of the outer impeller 3 and the inner impeller 4, which effectively breaks up the oil fume particle clusters and improves the smoke capture efficiency. The working noise can also be reduced by designing a reasonable speed difference.

[0054] As a preferred embodiment, a first guide sleeve 41 is coaxially fixed on the adjustment ring 33 of the inner impeller 4, and a shaft sleeve 42 is coaxially rotatably connected to one side end face of the protective bracket 12, and a first positioning pin is fixed on the side wall of the shaft sleeve 42; and an arc-shaped inner groove is provided on the inner wall of the first guide sleeve 41, and the first positioning pin is slidingly connected to the arc-shaped inner groove, that is, when the inner shaft 15 slides horizontally to push the inner impeller 4 to slide, relative sliding occurs between the first guide sleeve 41 and the shaft sleeve 42, and as the shaft sleeve 42 slides in or out of the first guide sleeve 41, it can realize the positive and negative deflection of the adjustment ring 33 through the sliding connection between the first positioning pin and the arc-shaped inner groove, thereby effectively adjusting the deflection angle of the blade 32 of the inner impeller 4.

[0055] In this embodiment, a second guide sleeve 43 is coaxially fixed to the adjustment ring 33 of the outer impeller 3, and a sleeve 44 is provided on the outer sliding sleeve of the main shaft tube 14. A second positioning pin is fixed to the side wall of the sleeve 44, and the second positioning pin is slidably connected to the arc-shaped inner groove provided on the inner wall of the second guide sleeve 43;

[0056] The sleeve 44 is fixed with a limit ring 45, and the inner shaft 15 is rotatably connected in the limit ring 45. Therefore, when the inner shaft 15 slides axially, it can drive the sleeve 44 to slide synchronously. At this time, the sleeve 44 slides in or out of the second guide sleeve 43, which can realize the positive and negative deflection of the adjustment ring 33 through the sliding connection between the second positioning pin and the arc-shaped inner groove, thereby effectively adjusting the deflection angle of the blade 32 of the outer impeller 3.

[0057] In this embodiment, the inner shaft 15 is axially pushed by the propulsion unit 17 so that the inner impeller 4 partially convexes or partially retracts into the outer impeller 3. At this time, the blades 32 on the inner impeller 4 and the outer impeller 3 can be deflected synchronously, and their deflection directions are set in opposite directions.

[0058] When the inner impeller 4 is retracted into the outer impeller 3, the central tangent directions of the blades 32 of the inner impeller 4 and the outer impeller 3 are both distributed toward the center of the circle. This arrangement can form a strong shear airflow. The blades of the inner impeller generate centripetal airflow during rotation and superimpose on the centrifugal force field of the outer impeller, producing a spiral compression effect, so that the high-temperature oil smoke is forced to be compressed during the radial acceleration process, thereby increasing the wind pressure intensity.

[0059] When the inner impeller 4 protrudes outward from the outer impeller 3, the tangential directions of the ends of the blades in the inner impeller 4 and the outer impeller 3 are distributed toward the center of the circle, which makes it convenient for the inner impeller 4 to cooperate with the guide cover 11 to introduce the flue gas into the drainage sleeve 25, and then the outer impeller 3 centrifugally exhausts the flue gas during rotation, so the flue gas capture efficiency is high, and the escape of flue gas is further prevented.

[0060] The above is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with this technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solutions and inventive concepts of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A high-efficiency centrifugal fan for extracting cooking fumes, characterized in that: It comprises: a fan casing (1), one end surface of which is provided with an axial air inlet, a guide cover (11) coaxially fixed outside the air inlet, and a protective bracket (12) fixed inside the guide cover (11); A double-impeller mechanism (2) is centrally rotatably provided in the fan housing (1), and the double-impeller mechanism (2) and the air guide cover (11) are combined to form an annular air supply channel; a vortex air outlet (13) is provided on the circumferential side wall of the fan housing (1); The double-impeller mechanism (2) comprises an outer impeller (3) and an inner impeller (4) coaxially nested, wherein the inner impeller (4) is coaxially arranged in the outer impeller (3), and a main shaft tube (14) is rotatably connected to the fan casing (1) via a bearing, and the main shaft tube (14) is fixed to the outer impeller (3); An inner shaft (15) is coaxially arranged inside the main shaft tube (14), and one end of the inner shaft (15) is fixed to the inner impeller (4); The outer impeller (3) and the inner impeller (4) rotate synchronously in opposite directions; The inner shaft (15) is slidably mounted in the main shaft tube (14); a propulsion unit (17) is provided outside the fan housing (1); a telescopic driving end of the propulsion unit (17) is connected to an end of the inner shaft (15) for controlling the axial displacement of the inner shaft (15) along the main shaft tube (14); A guide ring (24) is coaxially fixed inside the outer impeller (3), a guide sleeve (25) is coaxially slidably mounted on the guide ring (24), and one end of the guide sleeve (25) is rotatably connected to the inner impeller (4); An auxiliary air duct (18) is provided in the fan housing (1) on one side of the axial end face of the outer impeller (3), and a plurality of through-type air flow holes are provided in an annular array on the axial end face of the outer impeller (3), and the air flow holes are communicated with the auxiliary air duct (18); A plurality of side openings (26) are provided on the side wall of the guide ring (24), and the guide sleeve (25) slides axially along the guide ring (24) so ​​that the side openings (26) are opened or closed.

2. The high-efficiency centrifugal fan for extracting range fumes according to claim 1, characterized in that: A first bevel gear (21) is coaxially fixed on the main shaft tube (14), a second bevel gear (22) is provided parallel to the axial direction of the first bevel gear (21), the inner shaft (15) is slidably connected to the second bevel gear (22), and two bevel gears (23) are symmetrically provided on both sides of the first bevel gear (21) outside the fan housing (1), and the two bevel gears (23) are meshed with the first bevel gear (21) and the second bevel gear (22); A driving part (16) is provided outside the fan casing (1), and an output end of the driving part (16) is connected to the inner shaft (15) for transmission through gear meshing.

3. The high-efficiency centrifugal fan for extracting range fumes according to claim 1, characterized in that: The outer impeller (3) and the inner impeller (4) have the same composition structure. The outer impeller (3) includes two shaft rings (31). A plurality of blades (32) are distributed in a circumferential array between the two shaft rings (31). Each of the blades (32) is axially rotatably connected to the shaft ring (31). An adjusting ring (33) is rotatably provided on one of the shaft rings (31), and a plurality of oblique guide grooves (34) are distributed circumferentially within the adjusting ring (33); A shift rod (35) is fixed on one end portion of each blade (32), a pin (36) is vertically fixed on the shift rod (35), and the pin (36) is slidably connected to the oblique guide groove (34).

4. The high-efficiency centrifugal fan for extracting range fumes according to claim 3, characterized in that: The cross section of the blade (32) is an arc-shaped structure, and the blades (32) in the outer impeller (3) and the inner impeller (4) are installed in opposite directions.

5. The high-efficiency centrifugal fan for extracting range fumes according to claim 3, characterized in that: A first guide sleeve (41) is coaxially fixed to the adjustment ring (33) of the inner impeller (4), and a shaft sleeve (42) is coaxially rotatably connected to one end face of the protective bracket (12), and a first positioning pin is fixed to the side wall of the shaft sleeve (42); and an arc-shaped inner groove is formed on the inner wall of the first guide sleeve (41), and the first positioning pin is slidably connected to the arc-shaped inner groove.

6. The high-efficiency centrifugal fan for extracting range fumes according to claim 3, characterized in that: A second guide sleeve (43) is coaxially fixed to the adjustment ring (33) of the outer impeller (3), a sleeve (44) is provided on the outer sliding sleeve of the main shaft tube (14), a second positioning pin is fixed on the side wall of the sleeve (44), and the second positioning pin is slidably connected to an arc-shaped inner groove provided on the inner wall of the second guide sleeve (43); A limiting ring (45) is fixedly engaged in the sheath (44), and the inner shaft (15) is rotatably connected in the limiting ring (45).

7. The high-efficiency centrifugal fan for extracting range fumes according to claim 3, characterized in that: The inner shaft (15) is axially pushed by the propulsion unit (17) so that the inner impeller (4) partially convexes outward or partially retracts inward relative to the outer impeller (3). At this time, the blades (32) on the inner impeller (4) and the outer impeller (3) can be deflected synchronously, and their deflection directions are set in opposite directions.

8. The high-efficiency centrifugal fan for extracting range fumes according to claim 3, characterized in that: When the inner impeller (4) is retracted into the outer impeller (3), the central tangent directions of the blades (32) in the inner impeller (4) and the outer impeller (3) are both distributed toward the center of the circle; When the inner impeller (4) protrudes outward from the outer impeller (3), the tangential directions of the ends of the blades in the inner impeller (4) and the outer impeller (3) are both distributed toward the center of the circle.

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