Aerodynamic Workings of Blade Tip End-Plates Designed for Low-Noise Operation in Axial Flow Fans

The use of improved blade tip geometries is addressed as an effective design concept for passive noise control in industrial fans. These concepts, based on geometrical implementations of datum blade by means of end-plates at the tip, are shown to influence effectively the fan rotor-only aero acoustic signature because of the modifications of tip leakage flow behaviour. The aerodynamic merits of the proposed blade tip concepts are investigated by experimental and computational studies in fully-ducted configuration. The nature of the flow mechanisms in the blade tip region is correlated to the specific end-plate design features, and their role in creation of overall stage acoustic emissions clarified. By means of such tools, the tip leakage flow structures of the fans are analysed in terms of vortical structure detection, chordwise leakage flow evaluation, and loading distribution. Rotor losses are also investigated within the passage and invoking classical tip loss model. It was found that the tip geometrical modification markedly affects the multiple vortex behaviour of leakage flow, by altering the near-wall fluid flow paths on both blade surfaces. The improvement of rotor efficiency curves were assessed and correlated to the control of tip leakage flows exploited by the tip end-plates.