Effects of Pressure Ratios of Axial Flow Compressors on the Deep Surge Frequencies Part II : Influences of Relative Locations of the Compressor

Macroscopic behaviors of the frequencies of deep-surges in systems of multi-stage compressors and flowpaths could be evaluated very roughly by a constant value of an effective reduced surge frequency fRsmeff, as described in Part I ofthe present study. However, some deviations from the rough average value could often appear, depending on the situations of the concerned systems. In the present study, the causes of the deviations are made clear by examinations on the numerical results. The deviations are concerned with the following three conditions; (1) a multi-stage compressor having a sufficiently high design pressure-ratio, (2) changes in the compressor speeds through near the design speed, and (3) a sufficiently short delivery plenum. In the situations affected by combined effects of the above three, the compressor could be located in a zone of the surge flow mode where the amplitude could vary noticeably in the axial direction. When the stall-responsible stages relocate from the front stages to the rear ones within the zone during the speed increase through near the design speed, the small difference in the relative location of the stall-responsible stages in the surge mode could sensitively affect the surge behaviors and lower the surge frequency. In the situations, incomplete stall recoveries, and sometimes subharmonic surges, tend to occur additionally, elongating the deep-surge periods and lowering the surge frequencies. It should be emphasized from the aspects of numerical-experimental observations that very local events as such could happen to have significant effects on the surge phenomena.