Using sweep to extend the stall-free operational range in axial fan rotors

The paper discusses the use of sweep as a remedial strategy to control the aerodynamic limits in low-speed axial fan rotors. In this respect, the present work contributes to the understanding of the potential effect of blade lean on the shifting of the rotor stall margin. Numerical investigations have been undertaken on highly loaded fans of non-free vortex design, with the ideal total head rise coefficient typical of the industrial application range. Two rotors with identical nominal design parameters and, respectively, with 35° forward swept blades and unswept blades have been studied. The investigation has been carried out using an accurate in-house developed multilevel parallel finite element RANS solver, with the adoption of a non-isotropic two-equation turbulence closure. The pay-off derived from the sweep technology has been assessed with respect to the operating range improvement. To this end, the flow structure developing through the blade passages and downstream of the rotors, as well as loss distributions, have been analysed at design and near-peak pressure operating conditions. The analyses of three-dimensional flow structures showed that, sweeping forward the blade, the non-free vortex spanwise secondary flows are attenuated, and a control on the onset of stall is recovered. Moreover, the swept rotor features a reduced sensitivity to leakage flow effects. Consequently, it operates more efficiently approaching the throttling limit.