Effects of Backward-Forward Blade Skew on the Performance of an Axial Flow Fan Designed for Automotive Applications

Abstract Axial fans are conventionally designed assuming cylindrical stream tubes since secondary radial flows inside the machine are usually neglected in the preliminary design phase. This assumption is acceptable in the case of a rotor designed using a free vortex law, FVD, where minimal radial flows are present. However, in the case of a fan designed according to a controlled vortex approach, CVD, the assumption of two-dimensional flows over cylindrical surfaces is not suitable due to the occurrence of non-negligible radial fluid migration. According to the literature, a non-radial stacking, NRS, could be employed to restore the flow to cylindrical flow surfaces, as approximately occurs when an FVD is used. However, when applied correctly, the NRS is not only employed to mitigate radial flow migration, but also to enhance fan efficiency, increase stall resistance, and reduce noise emission, both in terms of overall level and tonal peaks. The most appropriate stacking law depends on many variables, such as vortex law, rotational speed, flow rate, hub-to-tip ratio, tubed or un-tubed fan, and shrouded or un-shrouded fan. For this reason, no general rules can be found in the literature for applying the stacking law to control the secondary flows and thus improve the performance of a CVD rotor. The current work shows a sensitivity analysis of the geometric parameters of a line-arc stacking law on the performance of a CVD axial flow fan. Different circumferential blade skew configurations are simulated using a single-blade channel CFD approach. The results obtained are analyzed using a statistical approach to identify the geometric parameters of the law that most influence the machine’s performance.