3D Unsteady CFD Simulation of the Unsteady Flow in a Centrifugal Fan

B3 - Unsteady Aerodynamics and Flow Simulations

The purpose of this paper is to study the pressure fluctuation in a high performance centrifugal fan. Using Computational Fluid Dynamics (CFD), the three – dimensional unsteady flow field in the whole domain of impeller and volute casing is solved. Since these fluctuation are causing severe dynamic loads their assessment is crucial during the design process in view of life time prognoses. The Scale Adaptive Simulation (SAS) is used to model the turbulence in Navier – Stokes equation. The unsteadiness of the flow is induced by the interaction of impeller – volute and the complex geometry of the impeller. This paper focuses on the pressure fluctuation depending on these two phenomena. The calculations of the impeller with traditional spiral volute and combination of vaneless diffuser and collection volute are carried out at design point. Because of the rotating motion the volute and impeller domains are analyzed separately. The results show a periodical unsteadiness of the pressure in both cases. In the pressure field, the blade passing frequency and its multiples dominate. Due to the interaction between impeller and volute, the pressure fluctuation is strong at the impeller outlet and in the vicinity of the tongue considering the volute domain. With the application of the vaneless diffuser and the collection volute the distance between the impeller outlet and the tongue is increased. In addition, the diffuser effect is decreased because of the deceleration of the flow. Compared to the traditional spiral volute, the pressure fluctuation is considerably reduced through the application of vaneless diffuser and collection volute. The pressure distribution in the impeller is analyzed in rotational coordinates. In both cases the pressure fluctuation remains at the same level. As the flow changes its direction from axial to radial in the impeller, the pressure fluctuation reaches its maximum, which is almost three times higher than the one in the volute. The paper underlines the importance of numerical flow simulation and accurate turbulence models in understanding the origin of flow instabilities and qualifying their impact on the compounding dynamic loads.