Axial Flow Fan Design Principle

In view of the contradiction between the low air pressure in the performance of air conditioner fans and the requirement of large air volume and low noise in use, this study selected an axial flow fan prototype with excellent performance for air conditioning as a comparative research model for curved swept design. The basic structure of the prototype: impeller outer diameter 409mm, impeller diameter 120mm, impeller ratio 0.29, number of blades 4, flow rate 2220m3/h, static pressure 20Pa, speed 880r/min. The design of the new fan is based on the premise of ensuring the flow rate, focusing on improving the internal flow distribution of the fan to reduce noise.
The design of the new fan is based on previous fluid dynamics research and existing quasi-3D design methods, using leading edge bending technology and CFD/CAD coupling. The structural characteristics of the typical prototype wind wheel are estimated by CAD, and the commercial software FLUENT is used to perform full 3D CFD calculations to examine its external and internal flow characteristics, and compare and analyze with experimental results; the conventional method is used to make a preliminary quasi-3D impeller design, and CFD calculations are performed to examine its external and internal flow characteristics; the CAD/CFD results of the prototype fan and the new fan are compared and analyzed, and the relevant parameters of the initial design are adjusted to achieve a better aerodynamic layout plan; CFD is used to predict the performance of the fan after adjustment, and the relevant parameters are further adjusted according to the calculation results to further optimize the fan.
In addition, parameters such as blade chord length, blade installation angle, blade camber angle, and airfoil mid-arc line need to be considered in the design. These parameters are optimized and reasonably matched with each other to improve the aerodynamic-acoustic performance of the fan.
Taking the above factors into consideration, the basic structure of the new fan is designed: the fan outer diameter is 408 mm, the impeller diameter is 100 mm, the impeller ratio is 0.245, the number of blades is 4, the blade tip/blade root chord length is 276/80 mm, the forward bend angle is 44°, and the forward sweep angle is 20°.
In the same outdoor unit, the prototype fan and the new fan are numerically simulated respectively. The actual machine is appropriately simplified in the calculation, and the influence of the heat exchanger, outlet grille, motor and its bracket is not considered, but the side inlet is considered.
The finite volume method is used to discretize the control equations, and the three-dimensional steady flow field of the entire outdoor unit is solved implicitly. The Spalart-Alm arcs turbulence model is used for the calculation, the convection term adopts the second-order upwind difference format, and the pressure-degree coupling is solved by the standard SIM PLE algorithm.
Due to the complexity of the fan system, the entire calculation domain uses unstructured grids, and the total number of grids is 110 x 104. The area where the impeller is located is defined as the rotating area, which has a large number of grids. The pressure boundary conditions are used for both the inlet and the outlet. The inlet is atmospheric conditions, and the outlet is given different back pressures. When the residual value of each calculation is less than 1 x 10-3, the calculation is considered to be converged.