Increase the Efficiency of Rotor-Only Axial Fans with Controlled Vortex Design Blading

B2 - Theoretical & Numerical Methods for Axial Fans

Controlled Vortex Design (CVD) by the second half of the last century has shown to be attractive for the performance increase of axial-flow rotors originally designed according to the free vortex criterion. During the last decade, the advancements in the knowledge of the effect due to non-radial staking line of the blade have allowed to improve performance and to reduce noise of tube-axial industrial fans. However, many highly loaded rotor-only axial fans of current production reach an efficiency level which hardly complies to the restrictions imposed by the newer European directives.
The paper starts with a conceptual discussion of the efficiency losses involved with the adoption of CVD bladings in rotor-only tube-axial fans. Then, a design criterion aimed at increasing the efficiency without reducing total pressure of this type of fan is introduced. Finally, the criterion is applied to a production axial fan model featuring a value of 0.44 for the hub-to-tip ratio, an almost constant swirl velocity distribution at the rotor outlet. and a quite low blade Reynolds number. The criterion is exploited performing experimental tests on each rotor defined by the successive steps suggested in the design guidelines. The experimental data show that the design criterion allowed an increase of peak efficiency up to 5% at blade positioning angles higher than design condition and an extension of the high efficiency operation keeping similar values of the total pressure.