Effectiveness of Blade Forward Sweep in a Small Industrial Tube-Axial Fan

Case Studies: Aerodynamics

It is well-known that forward sweep in the radial stacking line of rotor blades for low-speed axial fans generates 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 increasing blade circulation from hub to tip show a small percentage gain in the maximum fan efficiency. Some methods were suggested in the literature to include 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 the CFD calculations performed for a 560 mm tube-axial fan, this technique was able to increase the fan pressure at design point without decreasing the efficiency. The superimposition of 6 degrees of forward sweep at the blade tip, in addition to the distribution suggested by this technique, resulted in a significant increase of the maximum fan pressure.
The paper presents the results of experimental tests performed on a 315 mm 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 typical for industrial fans of this size. To quantify the effects of blade forward sweep on the performance and efficiency figures of this small 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. Experimental data prove the effectiveness of the design method for these small fans.