Impact of the Blade Fillet Radius on the Aerodynamic Performance of an Unshrouded Centrifugal Compressor

Fillets are often neglected in the flow analysis of centrifugal compressors due to the assumption of having little influence on the flow field and the difficulties related to the grid generation. Existing studies have shown an impact on efficiency, pressure ratio and choke margin by including fillets in the flow simulation. The goal of the present work is to investigate the impact of the fillet radius on the aerodynamic performance in an unshrouded centrifugal compressor. A parametric study on the blade hub fillet radius is conducted using the "RadiVer" test geometry and steady state CFD analysis. By using common methods for grid generation of turbomachinery blade passages, which are stacking the surface grids for each blade section, 3D features, such as fillets, cannot be meshed properly. Therefore, a new topology for 3D multi-block structured grid generation of radial impeller blades including hub fillets is proposed. Transfinite interpolation is used to create the initial grid and elliptic smoothing is applied afterwards for removing grid folding and improving grid quality. From the results of the flow simulations an improvement of the compressor performance with larger fillet radius is observed. Modifying the fillet radius mainly influences the flow downstream of the blade trailing edge. Even though the impeller efficiency decreases as the fillet size gets larger, due to the increased tip leakage flow, the diffuser performance improves, leading to an overall increase of the compressor efficiency. The improvement of the diffuser performance is attributed to the lower static pressure at the trailing edge as the fillet radius increases. The lower static pressure allows the flow to get sucked out from the backwards facing step and to distribute over the whole span of the diffuser. On the one hand, this leads to increased mixing losses. On the other hand, the entrapment of the flow at the backwards facing step can be avoided in that way, which overall leads to a reduced total pressure loss in the diffuser.