What Is an Axial Fan?

When an axial fan works, the blades push the air in the same direction as the shaft, so it is called an axial fan. Axial fans are divided into large axial fans, medium axial fans, and small axial fans. Choose the appropriate axial fan according to the place of use. Generally speaking, large-scale axial flow fans are mainly suitable for exhausting in places such as dust and gravel yards; medium-sized axial flow fans are mainly suitable for indoor ventilation and heat dissipation, such as granaries; small axial flow fans are mainly applicable Ventilation and heat dissipation with mechanical equipment, such as electrical cabinets.

Axial fans with shrouds only near the trailing edge of the impeller are widely used in outdoor units of split room air conditioners. In these systems, in addition to requiring compact structures and large air volumes, low noise is a key index. However, compared with low-pressure fans for power, this low-pressure fan has many different characteristics in terms of structural configuration, design method, and internal flow phenomenon, and it is not easy to grasp the fan's low-noise design and grasp of the internal flow mechanism. At home and abroad, the noise reduction research of this type of fan is mostly carried out by experimental methods or some specific methods after the blade is formed. However, there are few reports on the improvement of the performance of the air-conditioning fan moving blade design.

Based on the conventional quasi-three-dimensional design method of the axial flow impeller, the leading edge bending technology is adopted, and the CFD / CAD method is used to design the axial flow fan of the air conditioner to improve the internal flow of the fan at the design point. Noise provides the basis. The three-dimensional Navier-Stokes equation and the Spalart-Allmaras turbulence model were used to calculate and analyze the internal flow of the entire outdoor unit, and to estimate the performance of the fan to reduce the corresponding experimental workload ^[1]

There are two types of fans: axial and radial. They are divided by the direction of air flow during fan operation. When an axial fan works, the blade pushes the air to flow in the same direction as the shaft, so it is called an axial flow fan. When a radial fan works, the blade pushes the air to flow in a direction perpendicular to the shaft (that is, radial). Called a runoff fan.

Axial fans are divided into large axial fans, medium axial fans, and small axial fans. Choose the appropriate axial fan according to the place of use.

Generally speaking, large-scale axial flow fans are mainly suitable for exhausting in places such as dust and gravel yards; medium-sized axial flow fans are mainly suitable for indoor ventilation and heat dissipation, such as granaries; small axial flow fans are mainly applicable Ventilation and heat dissipation from mechanical equipment, such as:

Aiming at the contradiction of the low performance of the air conditioner fan due to the low wind pressure, the use of a large amount of air volume and low noise is required, this research selects a good performance axial flow fan prototype as a comparative research model for bending design. Basic structure of the prototype: the outer diameter of the impeller is 409mm , Luyi diameter 120 mm, Luyi ratio 0. 29, number of blades 4, flow 2220 m ³ / h, static pressure 20 Pa, speed 880 r / min. The design of the new fan is based on the premise of ensuring the flow, and focuses on improving the internal flow distribution of the fan to reduce noise.

The design of the new fan is based on the previous fluid dynamics research and its existing quasi-three-dimensional design method, using leading edge bending technology, and applying the CFD / CAD combination method. CAD estimation of the structural characteristics of a typical prototype wind wheel , Use commercial software FLUENT to perform full 3D CFD calculations to investigate its external and internal flow characteristics, and compare and analyze with experimental results; use conventional methods for preliminary quasi-three-dimensional impeller design, and perform CFD calculations to investigate its external and internal flow characteristics ; Compare and analyze the CAD / CFD results of the prototype fan and the new fan, adjust the relevant parameters of the initial design to achieve a better aerodynamic layout plan; use CFD to predict the performance of the adjusted fan, and further adjust the correlation based on the calculation results Parameters to further optimize the fan.

In addition, the design needs to consider the blade chord length, blade installation angle, blade camber angle, airfoil mid-arc and other parameters. These parameters are optimally selected and reasonably matched with each other to improve the aero-acoustic performance of the fan.

Taking into account the above factors, the basic structure of the new fan is designed: the outer diameter of the fan is 408 mm, the diameter of Luyi is 100 mm, the ratio of Luyi is 0.245, the number of blades is 4, the blade top / root chord length is 276/80 mm, and the forward bending angle 44 °, swept forward 20 °.

In the same outdoor unit, numerical simulations were performed on the prototype fan and the new fan. The actual machine was appropriately simplified in the calculation without considering the effects of the heat exchanger, the outlet grille, the motor and its bracket, but the side Into the wind.

The discrete volume control method of the finite volume method is used to solve the 3D stationary flow field of the outdoor unit. The calculation uses the Spalart-A llm arcs turbulence model, the convection term adopts the second-order upwind difference scheme, and the pressure and date are calculated using the standard SIM PLE algorithm.

Due to the complexity of the fan system, the entire computing domain uses unstructured grids, with a total of 110 x 10 ⁴ grids. The area where the impeller is located is defined as the rotation area and enjoys a larger number of grids. Both the import and export use pressure boundary conditions, the inlet is atmospheric, and the outlets are given different back pressures. When the residual value of each calculation is less than 1 x 10 ^-3 , the calculation is considered to be convergent ^[1]

a. The aerodynamic performance of the new axial flow fan remains basically unchanged. At the same flow rate, the measured noise of the new axial flow fan is lower than the prototype wind wheel by 0.5-1.3dB (A level sound pressure level);

b. The new axial fan reduces the lateral migration capacity of the suction surface and the vortex area near Luyi, enhances its radial migration capacity, reduces the area of secondary vortex in the blade channel, increases the mainstream area, and slows down the mainstream , Noise reduction;

c. The new axial fan basically eliminated the leading edge separation phenomenon of the prototype fan in the middle of the wingspan ^[3] .