Large eddy simulation is applied to solve the unsteady three-dimensional viscous flow in the whole impeller-volute configuration of a centrifugal fan. The results of the simulation are used to predict the impeller-volute interaction and to obtain the unsteady pressure, velocity, and vorticity fluctuations in the impeller and volute casing. The simulation at the design point is carried out with the wall-adapting local eddy-viscosity subgrid-scale model and a sliding mesh technique is applied to consider the impeller-volute interaction. The results show that a strongly unsteady flow field occurs in the impeller and volute casing of the fan, and the flow is characterized with obvious pressure and vorticity fluctuations, especially at the tongue and at the blade wake region. The large pressure fluctuation at the tongue and the large fluctuation of the blade wake vorticity appear as the blade wake is passing the tongue. Acoustic analogy and vortex sound theory are used to compute the radiated dipole and quadrupole sound fields, which are in good agreement with the experiment. The sound results show that the vortex sound theory is convenient for the broadband noise computation, and the dipole sound is much higher than the quadrupole sound. The dipoles, distributed over the volute tongue surface, are the dominant sound source of the fan.

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A numerical study on the aerodynamic noise generation of an industrial centrifugal fan with forward swept blades is carried out. Three-dimensional numerical simulations of the complete unsteady flowfield in the whole impeller — volute configuration are performed to obtain the aerodynamic sound sources. Then, aerodynamic sound is calculated using the Lowson equation and compared with the measurements. Moreover, the fan is modified for noise reduction by increasing the distance between the impeller tip and the volute tongue and sloping the volute tongue. The sound levels of the modified fan are lower than those of the original one over almost the entire range of frequencies analysed. The blade passing frequency level of the modified fan is decreased by about 15 dB at the design point. The method described and applied in this work provides a good qualitative prediction of the noise generation when designing a new fan, thus facilitating the choice of the lowest noise fan from several feasible alternatives.

Variable inlet guide vanes (VIGVs) can regulate pressure ratio and mass flow at constant rotational speed in centrifugal compressors as a result of inducing a controlled prewhirl in front of impellers. Radial inlets and VIGVs are typical upstream components in front of the first-stage impellers in many industrial centrifugal compressors. However, previous investigations on VIGVs in centrifugal compressors were mostly conducted under the condition of axial inlets, and this study aims to focus on the effects of radial inlet on the VIGVs performance of a centrifugal compressor stage. The axial inlet stage model is compared with the radial inlet stage model with splitters using numerical flow simulation. The flow from the radial inlet was nonuniform in both circumferential and radial directions; thus, the VIGVs, the impeller, the vaneless diffuser, and the return vane channel are modelled with fully 360 passages. The three-dimensional (3D) flow field is numerically simulated at VIGVs setting angles ranging from À20 to 60 . The overall stage performance parameters are obtained by integrating the field quantities. Though the splitters are equipped in the radial inlet, the overall stage polytropic efficiency decreases by an average of 4 per cent and total pressure ratio decreases by an average of 3.3 per cent in comparison with the axial stage model. This can be attributed to the effect of both flow non-uniformity induced by radial inlet and flow loss in the radial inlet at different VIGV setting angles. The flow loss in the radial inlet with splitters is the main reason of the stage performance decrease compared with the flow non-uniformity. The simulation results show that the performance of VIGVs is degraded by its inlet flow distortions resulting from a radial inlet. The results in this study can be applied to centrifugal compressor design and optimization.

The uncontinuous volute profile (UVP) is widely used in the squirrel cage fan to meet the space limitation in the air conditioning system. However, it usually causes an obvious performance drop due to the unreasonable impeller–volute interaction. This paper employs two improved design methods, i.e., downsized volute profile (DVP) and partial flow volute profile (PVP), to enhance the aerodynamic performance of the fan installed in a limited space. Experimental results validate the positive effects of these two design methods on the aerodynamic performance. The results obtained by the flow simulation reveal that the pressure field inside the fan with the improved volute profiles is much more uniform than that with the UVP; thus, the improved volute profiles are beneficial for the increase of the fan pressure rise and the reduction of the flow loss. For the investigated fan, it is suggested that the design method of the DVP is more suitable for slight space limitation, while the design method of the PVP is more suitable for significant space limitation.