Design of a tip appendage for the control of tip leakage vortices in axial flow fans

This paper reports on numerical investigations of passive control techniques used for the performance enhancement of a large diameter axial flow cooling fan through modification of the blade tip geometry. Using open-source software, a novel meshing strategy is developed to carry out both steady and unsteady numerical computations of a periodic section of an axial flow fan. Analysing the flow near the blade tip with a reduction in tip clearance, two predominant flow phenomena are identified. These two flow phenomena are further investigated with the aim of controlling them through implementation of a tip appendage design. Both introduced end-plate designs indicate effective control of each relevant flow phenomena. The constant thickness (CT) end-plate design is found to increase all fan performance characteristics at lower than design point (DP) flow rates while increasing the fan's peak efficiency plateau towards the rotor stall margin. However, none of the CT end-plate designs are able to improve the fan's performance characteristics at its DP. The introduction of a novel trailing edge (TE) end-plate design is found to increase all fan performance characteristics across the entire evaluated stable operating range, with an indicated increase of 37.3 percent in total-to-static pressure rise and a 2.9 percentage point increase in total-to-static efficiency at the fan's DP flow rate. The aerodynamic performance results attest to the associated benefits of the investigated passive control techniques.