The paper aims at discovering the relationship between the spanwise gradient of blade
circulation and the total efficiency gain due to forward sweep, in the case of low-speed
axial flow fans, at the design point. For this purpose, an extensive set of literature
data have been processed and evaluated by statistical means. A trend function has
been established for quantifying the aforementioned relationship. By such means, it
has been pointed out that the efficiency gain due to forward sweep tends to increase
with the spanwise gradient of blade circulation. This means that the purposeful incorporation
of forward sweep in the design process offers an increased potential for efficiency
improvement in the case of rotors of controlled vortex design. The maximum efficiency
gain has been observed to be 2% to 3%, due to an in-house developed design method.
It has been noted that the efficiency improvement due to forward sweep tends to be
minor for free-vortex rotors. Furthermore, the efficiency tends to deteriorate if
spanwise decreasing blade circulation occurs in forward-swept rotors. Remarks have
been made on the means of simultaneous reduction of loss and noise related to low-speed
axial fans, for a prescribed aerodynamic performance.