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

Abstract

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.

End-plate for noise-by-flow control in axial fans

“…When the coefficient of acoustic energy dissipation results for the datum AC90/6 fan are studied, Fig. 8a, it is apparent that the tip leakage vortex development results in an inviscid swirling core, an observation that is self-consistent with the conclusions of other scholars [8,54]. When the coefficient of acoustic energy dissipation results for the fan AC90/6/TF, fan AC90/6/TFvte and fan AC90/6/TFmvb are studied, Fig.…”

“…The grid refinement towards the solid surfaces controlled the dimensionless distance value to approximately 1 on the first nodes row. Standard boundary condition settings were adopted [33,54]. The Dirichlet conditions for the relative velocity components were imposed at the inflow section, half a mid-span chord upstream of the leading edge.…”

Aerodynamic Design of Low-Speed Axial-Flow Fans: A Historical Overview

“…Another strategy of sound reduction involves the installation of endplates at the blade tip (see Figure 8) to modify the behavior of the leakage flow and the related vortical structure [52]. Apart from the acoustic interests, the effects of forward-swept designs on the performance and efficiency of fans with non-free-vortex blades have then been investigated by Corsini and Rispoli [53] and Vad [54,55]. These authors respectively highlighted the advantages of swept blades into providing an extension of the stall margin and to obtain an efficiency gain in case of rotors designed with NFV techniques featuring positive gradients of circulation along the radius.…”

Experimental Characterisation of the Far-Field Noise in Axial Fans Fitted with Shaped Tip End-Plates

“…Corsini and coworkers [17,19] designed the TF end-plates in accordance with the theory behind vortex control in aircraft wings; that is, they determined end-plate dimensions in proportion to the radial dimension of the leakage vortex that they wished to control, estimated in this case as 0.1-0.2 blade spans in tip-limited low-speed rotors of axial compressors [20] and fans [21].…”