The objective of this paper is the study and the analysis of the complex phenomena related to the internal flow in a centrifugal fan, using Computational Fluid Dynamics (CFD) tools, completed with experimental investigation in order to validate the used numerical models. The CFD analysis concerns 2D and 3D unsteady flow. The studied phenomena are the interactions and unsteadiness induced by the motion of the rotating blades relatively to the volute and their impact on the aeroacoustic behavior of the fan. Thus, 3D and 2D unsteady calculations using Unsteady Reynolds Averaged Navier Stokes (URANS) approach has been applied on a hybrid mesh grid whose refinement has been studied and adapted to the flow morphology. Turbulence has been modeled with the k-ω-Shear Stress Model (SST) model. The computational domain has been divided into two zones, a rotating zone including the impeller and stationary zone including the volute. A sliding mesh technique has been applied to the interfaces in order to allow the unsteady interactions between the two zones. The overall performances predicted by the computations have been validated at different flow rate. For each geometry modeling (2D and 3D), the unsteady part of the study is illustrated by analyzing the pressure fluctuations on different points from the lateral surface of the volute. The analysis of the wake generated by the rotation of the blower shows that the volute tongue is the main zone of unsteadiness and flow perturbations. In order to predict the acoustic pressures, the unsteady flow field variables provided by the CFD calculations have been used as inputs in the Ffowks Williams-Hawkings equations.

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