Performance and inter-blade flow of axial flow fans with different blade angles of attack

Abstract

The predictions of fan performance on the pressure and flow rate characteristics of the axial flow fans using the conventional computational flow dynamics (CFD) approaches generally show a large deviation of about 20–30% from the experimental results. This article modifies the conventional CFD approaches by engaging the flow resistance of the fan test bench on the downstream area of the computational domain. The downstream pressure and flow rate are adjusted iteratively during the computational process. The results show that the prediction deviations using this method can be reduced to at most 3%, which is a great improvement compared to the conventional method. The inter-blade flows of three axial flow fans with different blade angles of attack are subsequently studied using this method. The results show that the flow patterns are drastically sensitive to the variation of the blade angle of attack, and the fan performances are closely related to the inter-blade flow behaviors. In the pre-stall regime, inappropriately designed blade angle of attack would cause the inter-blade flows in the region near the blade tip and the flows in the tip clearance region to present larger lateral and smaller axial velocity components with recirculation bubbles near the blade trailing edges. These flow behaviors cause the degradation of the fan performance. In the stall regime, boundary layer separation occurs to the suction surface of fan blades. Large recirculation bubbles appear near the trailing edges of blades and cause blockage effect against the axial flows. For the fans with inappropriately designed blade angle of attack, reverse flows can even be observed in the inter-blade passages with huge recirculation bubbles attaching to the trailing edges of fan blades.