8Stability and Range

A sufficiently large compressor range is a major design target for centrifugal compressors. In most cases it is limited by choking and surge. However, when operating at high pressure levels, the operating range may also be limited by flow induced vibrations resulting from rotating stall or other fluid dynamic excitations (Bonciani et al. 1980).

Most prediction methods are relatively accurate in terms of choking limit but often show large discrepancies in the prediction of the minimum mass flow. This is related to the complexity of the phenomena that are involved and the interaction between them.

Although surge and stall are very different phenomena, both names are commonly used for the same unsteady phenomena. One should therefore start with a clear definition.

The fluid in a decelerating flow experiences an adverse pressure gradient. As the kinetic energy is much smaller in the boundary layers than in the free stream, that part of the fluid may fail to overcome the pressure gradient, fixed by the free stream, and return flow or an inversion of the through flow velocity component occurs. From the macroscopic point of view, this reorientation of the flow may be steady (steady stall) or unsteady (rotating stall).

In steady stall, such as in the two‐zone model flow, zones of separated flow rotate at the same speed as the impeller and do not constitute a dynamic load on the impeller blades (Figure 3.40). Rotating stall is when the axi‐symmetric character of the ...

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