Influence of geometric scaling on the stability and range of a turbocharger centrifugal compressor

In this work the stability behavior of small-scale centrifugal compressor is evaluated in detail and the influence of design features typical for small-scale applications is shown. The impeller used in this study represents the design features of machines typically used in small turbochargers like a low blade count, hi h blade loading and a large relative tip gap. The work is evaluating data obtained in an enlarged research facility and in an actual scale turbocharger application. Both facilities are using a geometrical similar impeller and all nondimensional parameter are maintained. The Reynolds number is adjusted by changing the inlet pressure and thus the density of the air. This setup allows measurements with high accuracy on the enlarged research stage and simple parameter studies on the small-scale model. Comparing the operating characteristics of both scales shows the validity of this approach. For the range of Reynolds numbers present, the stability of the compressor is not affected by the geometric scaling. As the user of the compressor system wishes to operate at a wide range and under varying load demands but always in stable condition the knowledge of the stability margin and the kind of instabilities is vital. An analysis of the instable phenomena limiting the range of the centrifugal compressors is shown. The analyses are interpreting the pressure fluctuations gained with high response pressure transducer located in the diffuser for the characterization of the system stability. A similar overall compressor characteristic and stability range is obtained for both scales investigated. The flow structure within the diffuser is shown in a timeresolved manner using a 3D Laser Doppler Anemometer. It is shown how the flow structure is affected by the leakage flow through the tip gap. It is forming a strong jet-wake pattern resulting in a non-uniform flow and sheared velocity triangles.