Structure of tip leakage flow in a forward-swept axial-flow fan

An experimental analysis using three-dimensional laser Doppler velocimetery (LDV) measurements and computational analysis using the Reynolds stress model of the commercial code, FLUENT, were conducted to give a clear understanding on the structure of the tip leakage flow in a forward-swept axial-flow fan operating at the peak efficiency condition, and to emphasize the necessity of using an anisotropic turbulence model for the accurate prediction of the tip leakage vortex. The rolling-up of the tip leakage flow was initiated near the position of the maximum static pressure difference, which was located at approximately 12% axial tip chord downstream from the leading edge of the blade, and developed along the centerline of the pressure trough on the casing. A reverse flow between the blade tip and the casing due to the tip leakage vortex acted as a blockage on the through-flow. As a result, high momentum flux was observed below the tip leakage vortex. As the tip leakage vortex proceeded to the aft part of the blade passage, the strength of the tip leakage vortex decreased due to the strong interaction with the through-flow and the casing boundary layer, and the diffusion of the tip leakage vortex by high turbulence. Through the comparative study of turbulence models, it was clearly shown that an anisotropic turbulence model, e.g., Reynolds stress model, should be used to predict reasonably an anisotropic nature of the turbulent flow fields inside the tip leakage vortex. In comparison with LDV measurement data, the computed results predicted the complex viscous flow patterns inside the tip region in a reliable level.