PENALTY-FREE NOISE REDUCTION IN AXIAL-FLOW INDUSTRIAL FANS: EMBEDDING MERIDIONAL-FLOW-ALIGNED RIBLETS IN THE PRELIMINARY DESIGN

Abstract Ribbed blades are an effective means of attenuating fan noise emissions. However, their acoustic effectiveness also causes a quantifiable penalisation of the aerodynamic performance, especially in stable operation. In the literature, such noise-reduction features are invariably added after the aerodynamic design, so that the penalty is accepted rather than accounted for. This paper investigates whether blade riblets can instead be incorporated within the preliminary design procedure itself, so that their penalty is compensated by the design method rather than suffered afterwards. Recent CFD investigations by the authors showed that rotor-axis-aligned riblets moderately decrease fan pressure coefficient and efficiency in non-free-vortex designs, whereas this penalisation becomes negligible when the blade loading induces only a reduced radial migration of the meridional flow. Riblets therefore penalise the least when aligned with the meridional flow within the blade channel. A reduced version of a well-known design method builds the blade with aerofoil sections lying on conical surfaces approximating the meridional stream-surfaces. Riblets are thus applied on sections already following the flow, their cost offset by the conical positioning benefit over cylindrical repositioning. The hypothesis is tested on a benchmark constant-swirl fan with 0.5 hub-to-tip ratio, the effect quantified by a low-order CFD model. The penalty practically equals the conical-positioning benefit, so the ribbed fan matches the conventional design: a noise-reduction feature at no aerodynamic cost. A dedicated campaign validates the model and confirms measurable noise reduction in stable operation.