Low noise axial flow blade with double stage serrations

Abstract

This invention relates to a low-noise axial flow blade with dual-stage serrations. It pertains to the field of axial flow fan technology. The blade trailing edge is divided into a lower-middle region (I) and an upper region near the blade tip (II) along the blade height. Both regions have serrations on their trailing edges, forming a dual-stage noise reduction structure. The trailing edge of region I is fitted using a Mie-squared function to match gradually sized serrations; the trailing edge of region II is fitted using a linear square root mixed function to match equidistant serrations. This invention can simultaneously and specifically suppress two core noise sources: trailing edge shedding vortices and tip clearance leakage vortices, significantly reducing fan aerodynamic noise while maintaining blade aerodynamic efficiency. It can be directly adapted to existing conventional fan installation structures, has a mature molding process, and is suitable for low-pressure, high-volume applications such as HVAC and industrial ventilation.

Description

Technical Field

[0001] This invention relates to the field of axial flow fan technology, specifically to a low-noise axial flow blade with double-stage serrations. Background Technology

[0002] With the rapid development of the HVAC industry and industrial ventilation, and the increasing demands for indoor environmental comfort in residential and commercial settings, axial fans, as core air delivery equipment, have seen their performance requirements gradually upgrade from traditional high air volume and high aerodynamic efficiency to comprehensive performance with high efficiency, low noise, and high reliability. Among these, fan operating noise has become a key indicator for measuring the core competitiveness of a product, and relevant industry standards and user demands are increasingly stringent in setting limits for fan noise.

[0003] During the operation of axial flow fans, noise sources mainly fall into two categories: mechanical vibration noise and aerodynamic noise. Aerodynamic noise is the core noise source of axial flow fans, accounting for over 80% of the total noise. Aerodynamic noise is further divided into two categories: one is low-frequency narrowband discrete noise, primarily caused by the interaction between leakage vortices in the blade tip gap and the trailing edge of subsequent blades during blade rotation, as well as periodic shedding vortices generated at the convergence of airflow on the pressure and suction surfaces of the blades at the trailing edge; the other is broadband turbulent noise, mainly caused by the interaction between boundary layer turbulence on the blade surface, wake turbulence, and the blade structure. Among these, trailing edge shedding vortices and leakage vortices in the blade tip gap are the most significant noise sources for axial flow fans and represent the core optimization directions for existing noise reduction technologies.

[0004] Currently, the mainstream noise reduction methods for axial flow blades in the industry mainly include three categories: reducing tip clearance, adding small winglets to the blade tip, and setting a single-stage serrated structure on the trailing edge of the blade. Among them, the solution of reducing tip clearance can reduce the intensity of leakage flow at the tip of the blade to a certain extent, but it requires extremely high machining and assembly precision of the impeller and the fan casing, which significantly increases the product manufacturing cost. At the same time, the vibration of the impeller and thermal expansion and contraction during the operation of the fan can easily cause fluctuations in tip clearance, and even pose a risk of blade tip rubbing against the fan casing, which greatly reduces the reliability of the equipment. Moreover, this solution cannot solve the noise problem caused by trailing edge shedding vortices, and the noise reduction effect has obvious limitations.

[0005] The solution of adding winglets to the blade tip can suppress the circumferential leakage flow at the blade tip and thus weaken the vortex at the blade tip gap. However, the winglet structure will significantly increase the aerodynamic drag of the blade, resulting in a decrease in the aerodynamic efficiency of the wind turbine and an increase in operating energy consumption. At the same time, the winglets themselves will generate new airflow disturbances and vortices, producing additional aerodynamic noise. The noise reduction gain is limited. Moreover, this structure will greatly increase the complexity of the blade forming mold, increase the production and manufacturing cost, and make it difficult to achieve large-scale promotion.

[0006] The single-stage serrated structure at the trailing edge of the blade is currently the most widely used aerodynamic noise reduction solution. It breaks down large-scale vortices at the trailing edge into smaller-scale vortices through the serrated structure, reducing the mutual interference between the vortices and the mainstream, thereby achieving noise reduction. However, existing single-stage serrated structures all adopt uniform serrated parameters and trailing edge profile design throughout the entire blade height. In contrast, the rotational velocity of axial flow blades increases linearly from the blade root to the blade tip along the blade height direction. The airflow angle of attack, pressure difference between the pressure and suction surfaces, airflow characteristics, and vortex generation mechanisms differ significantly at different blade height positions: the core noise source in the lower and middle regions of the blade is the trailing edge shedding vortex, while the core noise source in the upper region near the blade tip is the tip clearance leakage vortex and the tip separation vortex. Existing single-stage sawtooth structures with uniform parameters cannot simultaneously adapt to two completely different vortex characteristics. They can only achieve limited noise reduction for a single noise source and cannot achieve high-range noise suppression across the entire blade. In some cases, noise reduction may occur in one area while aerodynamic disturbances and noise increases in another area. Furthermore, the trailing edge profiles of existing single-stage sawtooth structures mostly adopt simple straight lines and arcs without fine-tuning the profile fitting optimization for different vortex mechanisms. This results in unstable noise reduction effects and easily damages the original high-efficiency aerodynamic profile of the blades, leading to a decrease in aerodynamic performance such as fan airflow and air pressure. It is impossible to achieve a balance between high efficiency and low noise. Summary of the Invention

[0007] The purpose of this invention is to provide a low-noise axial flow blade with double-stage serrations to solve the problems mentioned in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a low-noise axial flow blade with double-stage serrations, comprising an impeller, the impeller being fixedly mounted on the output shaft of a motor, the motor being fixed to the middle of a mesh cover, a wind tunnel being fixed to the port of the mesh cover, and the wind tunnel surrounding the outside of the impeller; the impeller comprising at least five sets of blade units, the blade root of each blade unit being fixed to a central bushing of the impeller, each blade unit having an arc-shaped leading edge on its front side and a trailing edge on its rear side, and a blade tip at the outer end of each blade unit, the connection between the blade tip and the leading edge being the blade tip, the windward side of the blade unit being the blade suction surface when rotating, and the leeward side being the blade pressure surface; characterized in that the trailing edge of the blade is divided into two sections, region I and region II, along the blade height direction, region I being the trailing edge section in the lower middle part of the blade, and region II being the upper trailing edge section near the blade tip, both the trailing edges of region I and region II having serrated structures, forming a double-stage serrated noise reduction structure.

[0009] Furthermore, the radius of the blade root of the blade unit is... The radius of the leaf tip is The radial range of region I is to The radial range of region II is to ;in From leaf height coefficient definition, , The value ranges from 0.35 to 0.45; From leaf height coefficient definition, , The value ranges from 0.85 to 0.95.

[0010] Furthermore, the trailing edge profile of region I is fitted using a "Mi-type" function, and its expression in Cartesian coordinates is: ,in A=33.26, B=9.68.

[0011] Furthermore, the sawtooth spacing of region I The relationship between the sawtooth height H1 and the following formula is satisfied: ,in The value of K is 0.6-0.8; when the sawtooth of region I extends along the tail edge profile in the negative X-axis direction, the spacing and height of the sawtooth are provided with an amplification factor K, the value of K is 0.85-0.95.

[0012] Furthermore, the trailing edge profile of region II is fitted using a linear square root mixture function, and its expression in Cartesian coordinates is: ,in C=0.8925, D=3.375.

[0013] Furthermore, the serrated spacing of region II With the height of the saw teeth Satisfying the relation: ,in The value is 0.4-0.6; the sawtooth of region II is a sawtooth with equal height and equal spacing, and the magnification factor K=1.

[0014] Furthermore, the extended trailing edge profile of region I, the radius of the leading edge of the blade, and the radius of the blade tip... The length of the chord intercepted by the arc segment is The actual chord length at the tip of the blade unit is , The value is greater than The value.

[0015] Furthermore, the motor is an EC external rotor motor, the impeller is fixed on the external rotor of the EC external rotor motor, the air duct is a guide ring structure, and a preset blade tip gap is provided between the inner wall of the air duct and the blade tip.

[0016] Furthermore, the blade unit is a high-efficiency three-dimensional flow blade, and the blade suction surface and blade pressure surface are spatial curved surface structures based on three-dimensional flow theory.

[0017] Furthermore, the impeller has 5-9 sets of blade units, with multiple sets of blade units evenly distributed along the circumference of the impeller; the serrations in Region I and Region II are both oblique triangular, and the tooth tips and roots of the serrations are provided with arc transition sections.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] Based on the design of high-efficiency three-dimensional flow blades, this scheme designs the trailing edge profile in segments. The upper trailing edge profile is normalized using a linear square root mixed function, while the middle and lower trailing edge profiles are processed using a "Mi-type" function. Different design methods are used on the trailing edge profile to design double-stage sawtooths for different vortex generation mechanisms.

[0020] The blade trailing edge is the region where the airflow from the pressure and suction surfaces of the blade converges, and its profile design plays a significant role in weakening the blade wake vortex. In axial blades, the airflow velocity varies at different radii along the blade height. This design utilizes segmented design of the blade trailing edge and employs different functions to refine the description of different regions, thereby effectively reducing pressure pulsation at the blade trailing edge, weakening the interference of trailing edge shedding vortices on the mainstream, and effectively reducing the amplitude of low-frequency narrowband noise.

[0021] Axial blades have a pressure difference between their pressure and suction surfaces. During operation, the airflow at the blade tip on the pressure surface bypasses the tip clearance, forming vortices. These leakage vortices interact with the trailing edges of subsequent blades, increasing blade operating noise. This design employs a two-stage sawtooth segmentation to individually weaken the tip vortices, reducing turbulence at the blade trailing edge and lowering aerodynamic noise. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the EC external rotor fan with axial flow blades installed according to the present invention.

[0023] Figure 2 This is a front view of the axial flow impeller of the present invention;

[0024] Figure 3 This is a rear view of the axial flow impeller of the present invention;

[0025] Figure 4 This is the mapping relationship of the trailing edge of the blade in the Cartesian coordinate system according to the present invention;

[0026] Figure 5 This is a magnified view of a portion of the sawtooth pattern in region I of the present invention;

[0027] Figure 6This is a magnified view of a portion of the sawtooth pattern in region II of the present invention;

[0028] Figure 7 This is a front view of a single blade of the present invention;

[0029] Figure 8 This is a front view of a single blade of the present invention.

[0030] In the diagram: 1. Net cover; 2. Air duct; 3. Motor; 4. Impeller; 5. Blade root; 6. Blade leading edge; 7. Blade tip; 8. Blade apex; 9. Blade suction surface; 10. Blade trailing edge; 11. Blade pressure surface. Detailed Implementation

[0031] Please see Figure 1 —8, a low-noise axial flow blade with double-stage serrations, including an impeller 4, the impeller 4 is fixedly installed on the output shaft of a motor 3, the motor 3 is fixed in the middle of a mesh cover 1, and a wind tunnel 2 is fixed at the port of the mesh cover 1, the wind tunnel 2 surrounds the outside of the impeller 4; the impeller 4 includes at least five sets of blade units, the blade root 5 of the blade unit is fixed to the middle shaft sleeve of the impeller 4, the front side of the blade unit is provided with an arc-shaped blade leading edge 6, the rear side is provided with a blade trailing edge 10, the outer end of the blade unit is provided with a blade tip 8, the connection between the blade tip 8 and the blade leading edge 6 is the blade tip 7, the windward side of the blade unit when rotating is the blade suction surface 9, and the leeward side is the blade pressure surface 11; the blade trailing edge 10 is divided into two sections, region I and region II, along the blade height direction, region I is the trailing edge section in the middle and lower part of the blade, and region II is the upper trailing edge section of the blade near the blade tip 8, both the trailing edges of region I and region II are provided with serration structures, forming a double-stage serration noise reduction structure.

[0032] In practical assembly applications, the central bushing of impeller 4 and the output end of motor 3 are doubly fixed through interference fit, circumferential key connection, or fastening fasteners. This ensures that there is no relative sliding between the bushing and the motor output end during blade rotation, avoiding vibration and additional noise caused by transmission clearance, while also facilitating impeller disassembly, maintenance, and replacement. The mesh cover 1 adopts a grid-type protective structure, which can be formed by stamping metal sheet or injection molding engineering plastic. The grid of mesh cover 1 is set with an appropriate guide angle along the air inlet direction, which can not only provide physical protection for impeller 4 and motor 3, preventing foreign objects from entering the fan and causing damage to the components, but also perform preliminary rectification of the inlet airflow, reducing the inlet turbulence, and working synergistically with the trailing edge noise reduction structure to further optimize the low-noise operation of the fan. The air duct 2 and the mesh cover 1 are fixedly connected by bolts, snap-fit, or welding. The inner wall of the air duct 2 is a smooth and continuous arc surface. Its curved surface profile matches the rotation profile of the blade tip 8 of the impeller 4, ensuring that the blade tip gap between the blade tip 8 and the inner wall of the air duct 2 is evenly distributed in the circumferential direction. This avoids the aggravation of local leakage flow caused by uneven gaps and reduces the generation of blade tip gap vortices from the source.

[0033] The radius of the leaf root of the blade unit is The radius of the leaf tip is The radial range of region I is to The radial range of region II is to ;in From leaf height coefficient definition, , The value ranges from 0.35 to 0.45; From leaf height coefficient definition, , The value is 0.85-0.95. By dividing the radial range, the blade is divided into two independent noise reduction zones along the blade height direction, corresponding to different noise sources and vortex characteristics, so as to achieve targeted noise reduction design and solve the defect that the existing single-stage sawtooth structure cannot adapt to the airflow characteristics of the entire blade.

[0034] The trailing edge profile of region I is fitted using a Mi-type function, and its expression in Cartesian coordinates is as follows: ,in A=33.26, B=9.68. This region corresponds to the lower part of the blade. In this region, the blade rotational velocity is relatively low, and the airflow angle of attack changes gently along the blade height. The core source of aerodynamic noise is the wake vortex generated by the convergence of airflow on the pressure and suction surfaces of the blade. By fitting the trailing edge profile using a Mie function, a gradient-like smooth convergence of airflow on the pressure and suction surfaces can be achieved at the trailing edge, avoiding pressure pulsations caused by sudden changes in flow velocity and fundamentally reducing the intensity of wake vortex generation.

[0035] Sawtooth spacing in region I The relationship between the sawtooth height H1 and the following formula is satisfied: ,in The value of K is 0.6-0.8; when the serrations in region I extend along the trailing edge profile in the negative X-axis direction, the spacing and height of the serrations are magnified by a factor K, with a value of 0.85-0.95. This variable-size serration structure is adapted to the "Mi-type" trailing edge profile, which can break the large-scale wake vortex generated in the middle and lower part of the blade into multiple small-scale micro-vortices, greatly reducing the mutual interference between the vortices and the mainstream airflow. At the same time, the gradually changing serration size can accurately match the airflow parameters at different blade height positions, ensuring the vortex breaking effect in the entire region and effectively reducing the amplitude of low-frequency narrowband noise.

[0036] The trailing edge profile of Region II is fitted using a linear square root mixture function, and its expression in Cartesian coordinates is: ,in C=0.8925, D=3.375. This region corresponds to the upper part of the blade near the tip, where the blade rotational velocity is high and the influence of tip clearance leakage flow is most significant. The core source of aerodynamic noise is the tip clearance leakage vortex and the tip separation vortex. The trailing edge profile fitted by the linear square root mixing function can guide and correct the airflow direction in the tip region, avoid the positive impact of leakage vortex on the blade trailing edge, reduce the circumferential propagation intensity of leakage vortex, and prevent it from coupling resonance with the trailing edges of adjacent blades.

[0037] Sawtooth spacing in region II With the height of the saw teeth Satisfying the relation: ,in The value ranges from 0.4 to 0.6; the sawtooth in region II is a uniformly spaced sawtooth with an amplification factor K=1. This uniformly spaced sawtooth structure can continuously and uniformly cut and dissipate leakage vortices and separation vortices in the blade tip region, further weakening vortex energy and reducing turbulence intensity at the blade trailing edge. This reduces broadband aerodynamic noise in the blade tip region from the source, forming a two-stage synergistic high-noise reduction system for the entire blade with the sawtooth structure in region I.

[0038] The extended trailing edge profile of region I, the radius of the leading edge 6 of the blade, and the blade tip. The length of the chord intercepted by the arc segment is The actual chord length at the tip of the blade unit is , The value is greater than The numerical value. This structural design allows the blade trailing edge to form a suitable contraction trend along the blade height direction, matching the airflow pressure distribution on the blade surface, further optimizing the outflow state of the trailing edge airflow, reducing airflow separation at the trailing edge, and ensuring the aerodynamic efficiency of the blade while reducing noise.

[0039] Motor 3 is an EC external rotor motor, and impeller 4 is fixed on the external rotor of the EC external rotor motor. The air duct 2 is a guide ring structure, and a preset blade tip gap is provided between the inner wall of the air duct 2 and the blade tip 8. This blade structure can be directly adapted to the installation interface of existing conventional EC external rotor fans without requiring significant modifications to the fan's grille, air duct, motor, and other supporting components. It has good versatility and can be directly applied to the upgrade and optimization of existing fan products, reducing product modification costs.

[0040] The blade unit is a high-efficiency three-dimensional flow blade, with the suction surface 9 and pressure surface 11 being spatial curved surface structures based on three-dimensional flow theory. The blade unit is integrally injection molded or die-cast from aluminum alloy, with no splicing welds in the overall structure. This ensures the structural strength and fatigue resistance of the blade, meeting the requirements of long-term high-speed operation of the wind turbine, while also guaranteeing the forming accuracy of the blade's aerodynamic profile, avoiding adverse effects of forming errors on aerodynamic performance and noise reduction. The leading edge 6 of the blade adopts a smooth arc transition structure, with the curvature of the leading edge gradually changing along the blade height direction. This reduces the impact of the leading edge on the airflow during blade rotation, reduces the generation of leading edge separation vortices, and forms a full-blade-height vortex suppression system with the double-stage sawtooth structure of the trailing edge, achieving a wide-band noise reduction effect.

[0041] The impeller 4 has 5-9 sets of blade units, which are evenly distributed around the circumference of the impeller 4. The serrations in regions I and II are both oblique triangular, and the tooth tips and roots of the serrations are provided with arc transition sections. The arc transition sections can avoid stress concentration in the serration structure, improve the structural stability of the blades, and reduce airflow disturbance at the edge of the serrations, further optimizing the noise reduction effect.

[0042] like Figure 1 As shown: The mesh cover, motor, and impeller are secured together with fasteners. During operation, the motor powers the blades to rotate clockwise around the motor shaft. The blades perform work on the air, which enters through the inlet and exits through the outlet. Electrical energy is converted into mechanical energy and then into kinetic energy, thus transporting the air. During airflow transport, the double-stage serrated structure at the blade trailing edge simultaneously suppresses and breaks up eddies in the airflow, achieving low-noise operation without affecting the fan's airflow, air pressure, or other aerodynamic performance.

[0043] like Figure 2 and 3 The diagram illustrates the main range of the two-stage sawtooth pattern. During blade rotation, the airflow from the pressure and suction surfaces converges at the blade trailing edge, forming a wake vortex. The sawtooth pattern in region I, combined with the Mie-style trailing edge function, disperses this wake vortex, reducing its interference with the main flow. Simultaneously, the airflow at the blade tip on the pressure surface bypasses the tip clearance, forming a vortex. This leakage vortex interacts with the trailing edge of subsequent blades. Region II, through a linear square root mixing function, regulates the trailing edge, effectively avoiding the interaction between the airflow and the blade, thus reducing airflow disturbance in this region. The noise reduction structures in the two regions operate independently yet collaboratively, simultaneously addressing the two core noise sources: wake shedding vortices and tip leakage vortices. Compared to existing single-stage sawtooth structures, this provides a wider noise reduction coverage and greater targeting.

[0044] like Figure 4As shown: The trailing edge function of the blade is given as a piecewise function. Region I in the first quadrant is a "Mi-type" function, and region II in the third quadrant is a linear square root mixed function. The specific expression is as follows:

[0045]

[0046] The unknown values ​​for the function in this design scheme are: This piecewise function allows for precise control of the blade trailing edge profile, ensuring that the trailing edge profile is perfectly matched with the airflow characteristics of different blade height regions, thus guaranteeing the design accuracy and actual effectiveness of the noise reduction structure.

[0047] like Figure 5 As shown: The sawtooth spacing and height are defined by the following relationship: Furthermore, as the sawtooth moves closer to the negative X-axis along its trailing edge, there is an amplification factor, which is the ratio of the spacing and height of subsequent sawtooths to the corresponding preceding sawtooth. .

[0048] like Figure 6 As shown: The sawtooth spacing and height are defined by the following relationship: Sawtooth magnification factor .

[0049] like Figure 7 As shown: The radius of the wheel hub. The radius is the area at the tip of the blade. The region containing the serrated section (I) experiences an increase in linear velocity on the blade surface as the blade height increases during axial flow impeller operation. The wake vortices of the blades contribute significantly to blade noise, necessitating the addition of a serrated structure to the blade trailing edge to balance the velocity difference between the pressure and suction surfaces and reduce interference with the mainstream flow. , From leaf height coefficient definition. ,in addition .

[0050] like Figure 8 As shown: The chord length of the arc segment is taken from the extended line of the trailing edge profile where the serration of segment I is located, the radius of the leading edge of the blade, and the blade tip. This is the chord length at the leaf tip of a real leaf. This is the area where the serrations of section II are located. As the leaf height continues to increase, In this region, the blade tip separation vortex and the tip gap vortex are the main contributors. By designing the trailing edge profile in this region using function fitting and adding equal-height serrations, the interaction between this vortex and the blade trailing edge is weakened, thereby reducing the blade noise.

[0051] In another optional embodiment of the present invention, the sawtooth tooth shape of region I and region II can be replaced by trapezoidal teeth or circular arc teeth, as long as the sawtooth structure can achieve the breaking and dissipation of the trailing edge vortex, all of which are within the protection scope of the present invention; in another optional embodiment of the present invention, the number of blade units can be adapted and adjusted according to the air volume and air pressure design requirements of the fan, as long as multiple sets of blade units are evenly distributed along the circumference of the impeller to meet the dynamic balance performance requirements of the impeller operation.

[0052] The low-noise axial flow blade with dual-stage serrations provided in this embodiment utilizes a segmented design of the blade trailing edge. For different blade height regions, corresponding trailing edge profile fitting functions and serration structures are matched to address the vortex generation mechanisms and core noise sources, forming a dual-stage synergistic noise reduction system. Compared to existing conventional single-stage serrated blades, this system effectively suppresses both trailing edge detachment vortices and tip leakage vortices, significantly reducing low-frequency narrowband and wideband aerodynamic noise during axial flow fan operation. Simultaneously, it ensures no reduction in blade aerodynamic efficiency, perfectly meeting the low-noise, high-efficiency requirements of low-pressure, high-volume scenarios such as air conditioning system cooling and industrial building ventilation. Furthermore, the blade structure's molding process is mature and can be directly adapted to existing conventional fan installation structures without significant modifications to supporting components, demonstrating excellent industrial application prospects and facilitating mass production and market promotion.

Claims

1. A low-noise axial flow blade with double-stage serrations, comprising an impeller (4), the impeller (4) being fixedly mounted on the output shaft of a motor (3), the motor (3) being fixed to the middle of a screen (1), a wind tunnel (2) being fixed to the port of the screen (1), the wind tunnel (2) being arranged around the outside of the impeller (4); the impeller (4) comprising at least five blade units, the blade root (5) of the blade unit being fixed to the middle bushing of the impeller (4), the front side of the blade unit having an arc-shaped blade leading edge (6), the rear side having a blade trailing edge (10), the outer end of the blade unit having a blade tip (8), the connection between the blade tip (8) and the blade leading edge (6) being a blade tip (7), the windward side of the blade unit when rotating being a blade suction surface (9), and the leeward side being a blade pressure surface (11); characterized in that, The trailing edge (10) of the blade is divided into two sections along the blade height direction: Region I and Region II. Region I is the trailing edge section in the middle and lower part of the blade, and Region II is the upper trailing edge section near the blade tip (8). Both the trailing edges of Region I and Region II are provided with serrated structures to form a double-level serrated noise reduction structure. The trailing edge function of the blade is given as a piecewise function. Region I in the first quadrant is a "Mi-type" function, and Region II in the third quadrant is a linear square root mixed function. The specific expression in the Cartesian coordinate system is as follows: ； The unknown value of the function in this scheme is: .

2. The low-noise axial flow blade with double-stage serrations according to claim 1, characterized in that, The radius of the leaf root of the blade unit is The radius of the leaf tip is ; The radial range of region I is to The radial range of region II is to ;in From leaf height coefficient definition, , The value ranges from 0.35 to 0.45; From leaf height coefficient definition, , The value ranges from 0.85 to 0.

95.

3. The low-noise axial flow blade with double-stage serrations according to claim 2, characterized in that, The sawtooth spacing of region I The relationship between the sawtooth height H1 and the following formula is satisfied: ,in The value of K is 0.6-0.8; when the sawtooth of region I extends along the tail edge profile in the negative X-axis direction, the spacing and height of the sawtooth are provided with an amplification factor K, the value of K is 0.85-0.

95.

4. The low-noise axial flow blade with double-stage serrations according to claim 3, characterized in that, The sawtooth spacing of region II With the height of the saw teeth Satisfying the relation: ,in The value is 0.4-0.6; the sawtooth of region II is a sawtooth with equal height and equal spacing, and the magnification factor K=1.

5. The low-noise axial flow blade with double-stage serrations according to claim 2, characterized in that, The extended line of the trailing edge profile of region I, the radius of the leading edge (6) of the blade and the blade tip. The length of the chord intercepted by the arc segment is The actual chord length at the tip of the blade unit is , The value is greater than The value.

6. The low-noise axial flow blade with double-stage serrations according to claim 1, characterized in that, The motor (3) is an EC external rotor motor, the impeller (4) is fixed on the external rotor of the EC external rotor motor, the air duct (2) is a wind guide ring structure, and a preset blade tip gap is provided between the inner wall of the air duct (2) and the blade tip (8).

7. The low-noise axial flow blade with double-stage serrations according to claim 1, characterized in that, The blade unit is a high-efficiency three-dimensional flow blade, and the blade suction surface (9) and blade pressure surface (11) are spatial curved surface structures based on the three-dimensional flow theory.

8. The low-noise axial flow blade with double-stage serrations according to claim 1, characterized in that, The impeller (4) has 5-9 sets of blade units, and multiple sets of blade units are evenly distributed along the circumference of the impeller (4); the serrations of region I and region II are both oblique triangles, and the tooth tip and tooth root of the serrations are provided with arc transition sections.

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