The Role of Forward Sweep in Subsonic Axial Fan Rotor Aerodynamics at Design and Off-Design Operating Conditions

Abstract

The role that forward sweep plays in the aerodynamics of subsonic axial fan rotor is herein discussed, with emphasis on the combined effects of non-uniform three-dimensional work distribution and modified stacking lines. To study blade forward sweep effects numerical investigations have been undertaken on highly loaded fans of non-free vortex design, with ideal total head rise coefficient typical of industrial application range. The results of two rotors with identical overall design parameters and, respectively, with 35-deg forward swept blades and unswept blades have been compared. The investigation has been carried-out using an accurate in-house developed multi-level 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 three different operating conditions. The studies showed that the forward swept blade operates more efficiently in particular at low volume flows, with a delayed onset of stall. The analyses of three-dimensional flow structures showed that, sweeping forward the blade, the flow centrifugation on blade suction surface is reduced and non-free vortex spanwise secondary flows is attenuated. Moreover, reduced fluid mechanical losses have been also pointed out in rotor with swept blades.