Stall Inception of Axial Fan with Foam Metal Casing Treatment

The effect of the foam metal casing treatment on the stall inception of an axial flow fan is experimentally and numerically investigated in this paper. The foam metal casing treatment is a wide casing groove filled with a foam metal ring (GFM). It is found there is a link between the axial location of the GFM and the prestall behavior patterns. When the GFM is placed at locations near the leading edge of the rotor blade, the fan stalls via the shortlength-scale disturbance (spikes). However, at locations near the midchord of blade, the fan experiences a longlength-scale disturbance before the rotating stall. Particularly at location 35 mm downstream of the blade leading edge (represented by CT8), there appears two long-length-scale disturbances in the prestall process. The stall inception mechanism of the fan in CT8 is presented with focus on the behavior of the tip leakage behavior during the throttling. Detailed observation shows that the second long-length-scale disturbance corresponds to the unsteady forward spillage of the partial tip leakage flow, while the first long-length-scale disturbance is related to the classical modal wave. The transition of the stall inception pattern and the trigger mechanism of the second long-length-scale disturbance are also discussed.