Effect of Blade Loading on the Structure of Tip Leakage Flow in a Forward-Swept Axial-Flow Fan

An experimental analysis using three-dimensional laser Doppler velocimetry (LDV) measurement and computational analysis using the Reynolds stress model of the commercial flow solver, FLUENT, have been conducted to give a clear understanding of the effect of blade loading on the structure of tip leakage flow in a forward-swept axial-flow fan operating at the maximum efficiency condition (⊘ = 0.25) and two off-design conditions (⊘ = 0.21 and 0.30). As the blade loading increased, the onset position of the rolling-up of tip leakage flow moved upstream and the trajectory of tip leakage vortex (TLV) center was more inclined toward the circumferential direction. Because the casing boundary layer became thicker and the mixing between the through-flow and the leakage jet with the different flow direction was enforced, the streamwise vorticity decayed more rapidly with the blade loading increasing. A distinct TLV was observed downstream of the blade trailing edge at ⊘ = 0.30, but it was not found at ⊘ = 0.21 and 0.25. In comparison with LDV measurement data, the computed results predicted the complex viscous flow patterns inside the tip region in a reliable level. Influence of the relative motion of the casing wall on the structure of tip leakage flow was also investigated. Since it enhanced the strength of TLV by dragging the fluid through the tip clearance region, the magnitudes of the streamwise vorticity for the stationary casing wall were considerably larger than those for the rotating casing wall. Therefore, for the rotating casing wall, the high momentum region related with the blockage effect of TLV was reduced in comparison with that of the stationary casing wall.