A Critical Analysis of the Differences Among Design Methods for Low-Speed Axial Fans

Some methodological differences exist among the design methods for axial-flow fans. These differences generate confusion in the mind of the unexperienced fan designer, who is unaware of which method ensures the achievement of the required pressure rise at highest efficiency. In this work three important differences that appear comparing the classic methods of fan blading design are highlighted and analyzed: i) the choice of the airfoil, ii) the choice of the solidity distribution, and iii) the computation of the stagger angle of the blade elements. A fourth aspect regards the selection of the rotor number of blades. This aspect is treated in relation to the dubious applicability of the drag annulus correlation by Howell to low hub-to-tip ratio fan design and analysis. CFD simulations are performed on three case-study rotor-only fans, comparing blades systematically designed varying the airfoils (British C4 vs American NACA-65), the solidity distribution (Diffusion Factor criterion against the arbitrary selection of the blade-element lift coefficient) and the computation of the stagger angle (with respect to the inlet flow velocity or the mean one). The accuracy of XFOIL-predicted data at low Reynolds number (e.g., 300000) in designing small-to-medium fans is discussed as well. For each of the previous design aspects, results suggest the best indication among those suggested in the classic literature to achieve fan requirements.