Effectiveness of Blade Forward Sweep in a Small Industrial Tube-Axial Fan
3.6 Case Studies: Aerodynamics
It is well-known that introduction of forward sweep in the radial stacking line of rotor blades for low-speed axial fans allows for an appreciable gain in the stall margin at the expense of a reduction of the fan pressure at design point. In addition, several rotors with blade circulation that increases from the hub to tip show a few percentage gain in the maximum fan efficiency. Accordingly, the literature has suggested some methods that allow for including the sweep angle already in the preliminary design in order to limit the extensive Experimental and/or Computational Fluid Dynamics trial and error work required to find the distribution of sweep angle best suited to a specific fan design. The authors presented few years ago a technique to increase either the performance or the efficiency of an existing arbitrary vortex design tube-axial fan by introduction of a uniform radial distribution of blade forward sweep. According to CFD calculations, this technique allowed for an appreciable increase of the fan pressure at design point in a 560mm tube-axial fan without penalties in fan efficiency. Moreover, other calculations have predicted a relevant increase of the maximum fan pressure if 6 degrees of additional forward sweep at the blade tip are superimposed to the distribution of sweep suggested by the technique.
The paper presents the results of experimental tests performed on a 315mm tube-axial fan similar to the fan considered in the CFD analyses. The fan features a hub-to-tip ratio equal to 0.4 and a tip clearance value typical of industrial fans of equal size. To quantify the effects of blade forward sweep on the performance and efficiency figures of this small-size tube-axial fan three different stacking line of the rotor blades have been tested: unswept, forward swept and forward swept with additional sweep at the blade tip. The differences between these experimental results and the CFD predictions obtained for the similar fan of medium-size are presented as well.