TIP-LEAKAGE VORTEX MINIMIZATION IN DUCTED AXIAL FANS USING NOVEL PRESSURE SIDE TIP PLATFORM EXTENSIONS

Efficiency of an axial fan unit used in ducted fan based propulsion systems is closely related to its tip leakage mass flow rate and the level of tip/casing interactions. The present experimental study uses a stereoscopic Particle Image Velocimeter to quantify the three dimensional mean flow observed at just downstream of a ducted fan unit. After a comprehensive description of the tip leakage influenced fan exit flow, a number of tip treatments based on pressure side extensions are introduced. Various tip leakage mitigation schemes are introduced by varying the chordwise location and the width of the extension in the circumferential direction. The current study shows that a proper selection of the pressure side bump location and width are the two critical parameters influencing the success of tip leakage mitigation. Significant gains in axial mean velocity component are observed when a proper pressure side tip extension is used. It is also observed that a proper tip leakage mitigation scheme significantly reduces the tangential velocity component near the tip of the axial fan blade. Reduced tip clearance interactions are essential in improving the energy efficiency of ducted fan systems. Aeroacoustic gains are also possible. A reduction or elimination of the momentum deficit in tip vortex structures are also expected to reduce the adverse performance effects originating from the unsteady and highly turbulent tip leakage flows rotating against a stationary casing