Non-Uniform Turbulence Modeling in Isolated Unsteady Diffuser Computational Models for a Vaned Centrifugal Compressor

Recent advancements in computational fluid dynamics (CFD) enable new and more complex analysis methods to be developed for early design stages. One such method is the isolated unsteady diffuser model, which seeks to reduce the computational cost of unsteady CFD when modeling diffusion systems in centrifugal compressors with vaned diffusers by isolating the diffuser from the computational domain and prescribing an unsteady and periodic inlet boundary condition. An initial iteration of this computational methodology was developed and validated for the Centrifugal Stage for Aerodynamic Research (CSTAR) at the High-Speed Compressor Laboratory at Purdue University. However, that work showed discrepancies in flow structure predictions between full-stage and isolated unsteady CFD models, and it also presented a narrow scope of only a single loading condition. Thus, this work addresses the need for improvement in the modeling fidelity. The original methodology was expanded by including a more accurate, non-uniform definition of turbulence at the diffuser inlet and modeling several loading conditions ranging from choke to surge. Results from isolated unsteady diffuser models with non-uniform turbulence modeling were compared with uniform turbulence isolated unsteady diffuser models and full-stage unsteady models at four loading conditions along a speedline. Flow structure predictions by the three methodologies were compared using 1D parameters and outlet total pressure and midspan velocity contours. The comparisons indicate a significant improvement in 1D parameter and flow structure predictions by the isolated unsteady diffuser models at all four loading conditions when including more accurate non-uniform turbulence, without a corresponding increase in computational cost. Additionally, both isolated diffuser methodologies accurately track trends in 3D flow structures along the speedline.