Drag Reduction Mechanism of Bionic Riblets and Their Application on Centrifugal Fan Blades

Bio-inspired riblet technology has progressed in recent years in suppressing near-wall flow separation and reducing drag. However, significant benefits are obtained only when the riblet geometry matches the local flow conditions. Further study is still required before riblets can be reliably applied to low-speed fan blades, whose flow fields are inherently complex. In the present work, large eddy simulation (LES) is used to examine how riblets oriented perpendicular to the flow direction influence the aerodynamic performance and flow characteristics of an airfoil typical of low-speed turbomachinery. Riblets with various groove widths and heights are designed, and the effect of their placement along the airfoil suction surface (SS) is also investigated. The friction coefficient, pressure coefficient, and near-wall velocity distributions of the different configurations are compared. Based on the friction coefficient, the airfoil SS can be divided into three regions: a rapidly decreasing region (RDR) near the leading edge, a stable region (SR) in the middle, and a fluctuating region (FR) at the trailing edge. It is found that riblets of different sizes placed within the RDR can suppress vortex development and improve the near-wall flow. As a result, the RDR exhibits better adaptability to riblet parameters than the SR and FR. Motivated by these findings, riblets were applied to the blades of a backward centrifugal fan, specifically within the blade RDR, with a uniform width and height of 0.5 mm. Fan test results demonstrate that riblets covering 10-20% of the blade camber line increase the fan efficiency by 4.5% and the total pressure by 4.2%, respectively. The aerodynamic performance of the fan under low-flow conditions is markedly improved.