A kind of unsteady casing treatment was used to enhance the stall margin of an axial flow fan

which did not result in any recognizable efficiency loss.So

it is quite important to understand the mechanism of stall margin improvement with such casing treatment.The present investigation firstly developed a technical method for measuring and analyzing the inception wave

and then made some detailed comparisons with the existing results.The experimental results show that the unsteady casing treatment will delay the non-liner development of stall process by suppressing the propagation of inception wave in comparison to the evolution of the inception wave without casing treatments

while the inception point will not be changed with the application of the unsteady casing treatment.