**Modal Simulation of Axial Flow Fan Blades and Its Influence on Aerodynamic Noise**
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**Introduction**
Axial flow fans are widely used in various industrial applications due to their efficiency and compact design. However, these fans can experience blade oscillation caused by centrifugal forces and unstable airflow, leading to fatigue damage, reduced performance, and increased aerodynamic noise. To understand and mitigate these issues, modal simulation is essential.
The blade surface of an axial flow fan is complex and irregular, making it difficult to analyze using classical methods. Therefore, finite element analysis (FEA) is employed. ANSYS is a powerful tool for performing modal analysis, while software like Unigraphics (UG) helps in creating accurate CAD models of the impeller. These models are then imported into ANSYS for detailed simulation.
**Impeller CAD Model and Data Import**
The axial flow fan under study has four blades made of ABS plastic. It operates at 860 RPM with a hub diameter of 0.147 meters and an outer impeller diameter of 0.42 meters. Using a three-coordinate measuring instrument, the external dimensions of the blade were captured, and the surfaces were reconstructed using UG's surface stitching function.
To speed up the analysis, the cyclic symmetry feature of ANSYS was used. A single 90° sector of the impeller was modeled, and the rest were replicated using symmetry. The UG model was imported into ANSYS via a direct interface, ensuring accurate geometry representation.
**Preprocessing and Solution**
Physical properties of the impeller material—ABS plastic—were input, including density, Young’s modulus, and Poisson’s ratio. For meshing, 10-node tetrahedral elements (SOLID92) were used for the 3D model, while 2D triangular elements (MESH200) were applied to the surface. The mesh was generated and copied across the symmetric sectors.
Boundary conditions were applied to the hub, restricting movement in the X and Z directions. Cyclic symmetry constraints were set using the CYCGEN macro, followed by solving the modal frequencies using the BlockLanczos method between 20 Hz and 200 Hz.
**Experimental Validation**
An experimental hammering test was conducted to compare simulation results with real-world data. The measured frequencies were close to the simulated ones, confirming the accuracy of the model. The first six modes showed significant influence on aerodynamic noise, as they affected the blade's angle of attack and lift distribution.
**Prestressing and Rotational Softening**
When the impeller rotates, centrifugal forces cause prestress, which increases stiffness and raises modal frequencies. However, rotational softening, due to large displacements, can lower the frequency. This balance must be considered for accurate predictions.
**Vibration Modes and Noise Analysis**
Several vibration modes were analyzed. The first few modes involved torsional motion, while higher-order modes exhibited more complex patterns such as wave-like deformations. These modes directly affect airflow, causing pressure fluctuations and increasing noise levels.
Understanding these modes is crucial for designing quieter fans. Future work includes analyzing the dynamic response of the blade under operational loads and predicting noise using aerodynamic acoustic formulas.
**Conclusion**
This study demonstrates how modal simulation helps identify critical vibration modes that contribute to aerodynamic noise. By considering both stress stiffening and rotational softening, engineers can optimize fan designs for better performance and reduced noise. Further research will focus on improving noise prediction and control techniques.
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