Structure of Tip Leakage Flow in a Forward - Swept Axial - Flow Fan Operating at Different Loading Conditions

An experimental analysis using three-dimensional Laser Doppler Velocimetry (LDV) measurement and computational analysis using the Reynolds stress model in FLUENT are 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 (φ=0.25) and two off-design conditions (φ=0.21 and 0.30). As the blade loading increases, the onset position of the rolling-up of tip leakage flow moves upstream and the trajectory of tip leakage vortex center is more inclined toward the circumferential direction. Because the casing boundary layer becomes thicker and the mixing between the through-flow and the leakage jet with the different flow direction is enforced, the streamwise vorticity decays more fast with the blade loading increasing. A distinct tip leakage vortex is observed downstream of the blade trailing edge at φ=0.30, but it is not observed at φ=0.21 and 0.25.