Steven T. Sommer

Low Solidity Cascade Diffuser and Scroll Optimization for Centrifugal Compressors

One of the major concerns of the centrifugal compressor designer is the trade-off between the compressor’s efficiency and its range. The compressors equipped with vaneless diffusers benefit from a broader operational range than conventional vaned diffusers, but with decreased compression efficiency. One solution found in recent years is the Low Solidity Diffuser, which displays a range greater than conventional vaned diffusers, by preventing rotating stall, and efficiency higher than vaneless diffusers. The study focuses on the impact of geometry elements on the flow field through the vanes’ passages, and on the change of performance with the location of the vanes relative to the scroll tongue. This paper presents results of experimental and Computational Fluid Dynamics (CFD) investigations — for centrifugal compressors with low solidity cascade diffusers. The vanes are based on a NACA airfoil profile and have been designed using commercial software from ConceptsNREC: Compal and AxCent. The analysis part was performed using CFX software from Ansys, Inc. The performance parameters recorded are pressure ratios, isentropic efficiency, flow angles and velocity field profiles.Copyright

Numerical Analysis of Blade Geometry Generation Techniques for Centrifugal Compressors

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 designers 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.Copyright