A simplified theory to justify forward sweep in low hub-to-tip ratio axial fan

Many modern axial flow compressor and fan rotors feature forward swept blades. However, there was no general consensus on advantages due to blade forward sweep until some years ago, also because of the difficulties in isolating the effect of blade sweep alone from the global aerodynamic blading behaviour. Thus, while some researchers try to collect in a “sweep package” all modifications connected to the introduction of blade sweep, there is no attempt in the recent literature to formalise a theory on the aerodynamic effect of sweep angle on axial flow fan blades. This paper first recalls the swept wing concept suggested by Adolf Busemann, then the original swept wing meaning is re- interpreted to present a simplified theory of blade forward sweep. This theory allows to explain the not yet completely understood changes in total pressure and efficiency due to the inclusion of sweep in the design of high circulation gradient controlled vortex design rotors. A methodology to select the degree of forward sweep to be chosen in the preliminary design of constant-swirl bladings for rotor-only axial flow fan featuring low hub-to-tip ratio is suggested as well. A specifically designed unswept fan and its modified configurations according to the suggested theory has been verified by using CFD. The forward swept design increases the unswept fan total pressure in the whole operation range from about 1% to 4% and expands the stall margin at throttling by more than 10% of the design flow rate leaving efficiency and duty unchanged.