Numerical Analysis of Blade Geometry Generation Techniques for Centrifugal Compressors Available to Purchase

It is a known fact that machined impellers result in improved compressor performance compared to cast impellers of the same design. The performance improvements can be attributed to better surface finish, more accurate geometric definition (tighter dimensional tolerances), well defined edges, and the lack of blade tip fillet on shrouded impellers. In addition, it has been observed through experimental investigations that the construction method of the impellers has an impact on performance. For flank-milled machined impellers, a hub and shroud blade profile is connected by pre-determined straight-line-elements (SLE) - which would correspond to a tool path - to generate the blade surface according to the design intent of the compressor engineer. For cast impellers, the method of connecting hub and shroud blade profile points leads to an arbitrary surface definition and is dependent upon a designer's interpretation of blade profile data and/or the solid model, as well as the CAD software. Although the shape of the hub and shroud profiles are preserved, the resulting blade surface defined by connecting these two profiles may not correspond to the design intent of the compressor engineer. Because the blade surface deviates from the design intent, the compressor performance can deteriorate. Foundries rely on a full 3D design model to create tooling for cast impellers, as opposed to hub and shroud profiles typically required of a 5-axis machining program. Therefore, these construction differences become significant for cast impellers. This paper presents computational fluid dynamic investigations of two types of impellers - with blade surfaces generated using SLE and using CAD arbitrary definitions. Because there are many different mathematical definitions that CAD tools employ for curves, the resulting arbitrary blade surface is not unique. The numerical results will help understand the causes of the performance difference as well as the effects of SLE blades to the flow through the impeller. Input conditions for computational dynamic simulations are based on experimental results. All references to experimental data in the present paper are for cast impellers. Therefore the differences in performance are attributed to blade definition (SLE vs. other) and not to differences resulting from manufacturing methods.