Experimental and numerical comparisons among various skewed blades of low speed axial fan rotors

Literature indicates that there is lack of study related to the effects of axially skewed blades on the aerodynamic performance of axial flow fans. In this study rotors of various blade skews, namely, axial, circumferential and axial-circumferential types, are studied and their aerodynamic performances are compared to those of the baseline radial rotors of two types, namely, constant chord and varying chord. The skew in a blade can be considered as combination of sweep and dihedral. The skew change from the circumferential to axial type causes reversal of dihedral induced radial blade force and hence a reversal of meridional streamline curvature which in turn reverses the modulation of the end-wall loads. The inherent forward sweep in the forward skewed blades also modulates hub and tip loads. Thus, in a forward axial skew blade the forward sweep and positive dihedral combine favorably to unload the tip and to upload the hub. In the forward circumferentially skewed blades, forward sweep and the negative dihedral have mutually countering effects on end-wall load modulation. The highlight of this study is that it has demonstrated experimentally the unloading of the tip in the forward axially skewed blades which resulted in reduced losses and better performance including increased stall margin. Reduced tip loading was also verified from the casing pressure sensor measurements which showed weakening of tip vortex. The relative influence of sweep and dihedral on performance was additionally demonstrated in the axially-circumferentially skewed rotor of the constant chord type, with improvements attributable to higher sweep.