Comparative Study of Unsteady Flows in a Transonic Centrifugal Compressor with Vaneless and Vaned Diffusers

To reduce vibration and noise level, the impeller anddiffuser blade numbers inside an industrial compressor aretypically chosen without common divisors. The shapes of volutes orcollectors in these compressors are also not axis-symmetric. Whenimpeller blades pass these asymmetric structures, the flow fieldin the compressor is time-dependent and three-dimensional. Toobtain a fundamental physical understanding of thesethree-dimensional unsteady flow fields and assess their impact onthe compressor performance, the flow field inside the compressorsneeds to be studied as a whole to include asymmetric and unsteadyinteraction between the compressor components. In the currentstudy, a unified three-dimensional numerical model was built for atransonic centrifugal compressor including impeller, diffusers,and volute. HFC 134a was used as the working fluid. Thethermodynamic and transport properties of the refrigerant gas weremodeled by the Martin-Hou equation of state and power laws,respectively. The three-dimensional unsteady flow field wassimulated with a Navier-Stokes solver using the k−εturbulent model. The overall performance parameters are obtainedby integrating the field quantities. Both the unsteady flow fieldand the overall performance are analyzed comparatively for eachcomponent. The compressor was tested in a water chiller systeminstrumented to obtain both the overall performance data and localflow-field quantities. The experimental and numerical resultsagree well. The correlation between the overall compressorperformance and local flow-field quantities is defined. Themethodology developed and data obtained in these studies can beapplied to the centrifugal compressor design and optimization