Effects of Nonuniform Tip Clearance on Fan Performance and Flow Field

Nonuniform tip clearance, caused by the deformation of the casing (casing ovalization), exists in all fans to some extent. However, description of the flow behavior with this kind of nonuniform clearance is seldom given in the open literature. In this paper, both steady and unsteady calculations are performed to study the influence of the nonuniform clearance on fan performance and flow field. The investigation method is based on three dimensional RANS equations, carried out by the ANSYS CFX fluid solver. A new factor τ, which is in the range of 0 to 1, is defined to represent the nonuniformity of the tip clearance. A series of numerical models with different tip clearance nonuniformities τ are investigated. In addition, the original, uniform tip clearance is also performed serving as a reference. The results indicate that the fan performance is mainly affected by two factors: the clearance nonuniformity τ and the average clearance level. There is a clear decreasing trend of the fan performance with increasing nonuniformity and average tip clearance level. The maximum clearance can contribute to the reduction of stall margin. Furthermore, the effects of nonuniform clearance on tip leakage vortex and aerodynamic force are also discussed in detail. The deformation of the casing can affect the pressure fluctuation and thus increase the amplitude of the aerodynamic force.