A study has been carried on to determine the effect of the sweep angle on the noise generated by axial flow fans. To do so, two fans with radial and forward sweep angles, having the same aerodynamic overall performances, were designed and tested at the LEMFI test rig.
 |
 |
Forward fan |
Radial Fan |
Simultaneously, numerical simulations were performed using ANSYS CFX 10.0 software. To reproduce the experimental setup, the geometry of the test rig was created using DesignModeler in ANSYS Workbench 10.0 and meshed with ANSYS CFX-Mesh 10.0. ANSYS TurboGrid 10.0 was used to mesh the fans.
 |
 |
Numerical test rig meshed with ANSYS CFX-Mesh and including the fan |
Mesh the for radial fan created using ANSYS TurboGrid |
The flow through the fan exhibited a transient behavior. The true transient prediction was achieved by simultaneously solving all blade passages of the entire fan. An adequate time step was used to capture the main mixing process due to the rotation. Transient simulation convergence is difficult to judge, because of the large number of different interaction frequencies and the respective continuous oscillation of all data. The residual levels were monitored, but more importantly the time-averaged signals of important integral flow quantities were monitored. The simulations were run until the time-averaged signals became constant. The predicted pressure rise and overall efficiency to the experimental data at design point were compared. The predicted values are very close to the experimental ones.
 |
 |
Pressure rise versus mass flow rate |
Efficiency versus mass flow rate |
Instantaneous velocity components, calculated downstream of the fans, were compared to the measured data. For example, the instantaneous axial component for the radial fan. From the transient simulations, ANSYS CFX 10.0 automatically computes the unsteady loading (acoustic rotating dipole) exerted on the surface of the blade considered as main source of noise. These forces were used as input for the noise level prediction. The Ffowcs, Williams and Hawkings (FW-H) model was used to predict the noise levels generated by the two fans. The chart at the bottom of this page gives a comparison of the tonal noise generated by the two fans and results from SPLNoise are also reported in this figure. It was found that the fan with the forward sweep angle has lower noise levels. The measurements confirmed this result and the overall noise was reduced, for the forward fan, by 4 dB(A) relative to the one generated by the radial fan.
Instantaneous axial component, Radial fan
Comparison of predicted Noise levels for the two fans
[1] Belamri, T., Kouidri, S., Fedala, D. and Rey, R. "Comparative study of the aeroacoustic behavior of two axial flow fans with different sweep angles", Paper FEDSM2005-77242, Proceedings of ASME FEDSM'05, 2005 ASME Fluid Engineering Summer Conference Houston, TX, USA, June 16-23, 2005.
[2] Belamri, T. and Wang, G., " Aeroacoustic Study of Tonal Noise generated by Axial Flow Fans", HT-FED2004-56908, , Proceedings of ASME HT-FED'04, 2004 ASME Heat Transfer/Fluid Engineering Summer Conference Charlotte, NC, USA, July 11-15, 2004.
Content