Study of Aerodynamic Noise Generated from a Forward Curved Fan due to Rotating Stall Cell

C1 - Noise of Centrifugal Fans

In order to clarify the influence of rotating stall cell on the properties of a forward curved fan, the aerodynamic characteristic and noise of the fans have been estimated experimentally. Moreover, the influence of unsteady internal flow on the aerodynamic noise in the low frequency domain has been analyzed by the numerical simulation. A two-dimensional forward curved blade was employed for the impeller. The diameter of the impeller is 125 mm; the chord length is 9 mm; the span length is 50 mm; the number of blades is 40. The forward curved fan with a shroud is named as MF9S, and the fan without shroud is called as MF9. The two examined impellers are totally same dimensions except for the existence of the shroud. The CFD code of SCRYU/Tetra produced by Software Cradle Co., Ltd. was used for the numerical simulations. In the numerical simulation, the representative length of the impeller and the diffuser is modeled equivalently to the actual system. Approximately 4.5 million grid elements were employed to solve the entire flow field of the fan. The spatial distribution of the sound source was visualized by using the right-hand side of Powell-Howe’s equation. The static pressure of MF9S became high than that of the MF9. The maximum efficiency of MF9S was increased 1.6% than MF9 due to the improvement of the static pressure. On the other hand, the fan noise of MF9S at the maximum efficiency point became approximately 1 dB larger than MF9 due to the influence on the broadband noise in the vicinity of the 200 Hz. The rotating stall cell was formed at the front side of the impeller of MF9. The cell was rotated around the impeller with 40% rotation speed of the impeller. The cell formed seven pseudo blades around the impeller. We clarified experimentally that the broadband noise in the low frequency domain is generated at the vicinity of the frequency which is rotational speed of the cell.