The design of a tip appendage for the passive control of tip leakage vortices in axial flow fans is investigated for the purpose of fan performance enhancement. The merging and subsequent bursting of the tip leakage vortex further downstream of the blade is associated with having a detrimental effect on fan performance characteristics. Periodic three-dimensional numerical computations are conducted using the open-source CFD library OpenFOAM and a newly implemented open-source meshing strategy.

Both steady and unsteady numerical computations are used to determine the effect of tip clearance on fan performance characteristics. An increase of 15.6 percent in total-to-static pressure rise and a 5.1 percentage point increase in total-to-static efficiency is found with a 66.6 percent reduction in tip clearance at the fan's design point flow rate. Normalized helicity contours are used in the visualization and quantification of the flow near the blade tip. The findings show a reduction in the relative tip leakage flow velocity through the tip gap and a reduction in the tip leakage vortex exit trajectory angle along the blade's chord with a reduction in tip clearance. This paper reports on numerical investigations that were performed on these two flow characteristics with the aim to control them through the implementation of a tip appendage design.

An average reduction of up to 6.8 percent in the tip leakage flow velocity through the tip gap is achieved through the introduction of three constant thickness end-plate designs. Comparing the pressure side constant thickness end-plate design to the fan datum, an increase in the fan's total-to-static pressure rise at lower than design point flow rates is achieved with an increase in the fan's peak efficiency plateau towards the rotor stall margin. However, none of the constant thickness end-plate designs are able to improve the fan's performance characteristics at its design point flow rate.

A novel trailing edge end-plate concept is introduced through modification of the downstream section of the blade tip. The trailing edge end-plate design is found to exhibit control of the tip leakage vortex exit trajectory angle while shifting the blade's peak loading distribution further downstream towards the blade's trailing edge. The introduction of the trailing edge end-plate design is found to increase the datum fan's performance characteristics across the entire evaluated stable operating range, with an indicated increase of 9.7 percent in total-to-static pressure rise and a 1.3 percentage point increase in total-to-static efficiency at the fan's design point flow rate.


Design of a Tip Appendage for the Control of Tip Leakage Vortices in Axial Flow Fans

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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