Calculations of inlet distortion induced compressor flow field instability

Abstract

Calculations of the onset of flow instability are presented for low speed multistage axial compressors operating with asymmetric inlet flow. The most important feature of the calculation procedure is the modelling of the fluid dynamic interaction between the spoiled and unspoiled sectors of the compressor. The calculations show that annulus averaged slope of the compressor pressure rise characteristic equal to zero is a useful approximate stability criterion for situations where the dynamics of the compressor flow field do not couple strongly to the compression system or the structure of the imposed distortion is not similar to that of the eigenmodes of the flow in the compressor annulus. This approximate criterion is used to investigate the relationship between the present model and the “parallel compressor” model. Calculations are also performed for cases of interest when compressor and compression system are closely coupled, as well as situations in which the compressor is subjected to a rotating distortion. (This would occur, for example, in a two-spool engine with the low compressor in rotating stall). These first-of-a-kind computations, and the accompanying description of the physical mechanisms, show that the stability of the flow in the compressor can be adversely affected if the temporal or spatial structure of the distortion is such that resonant type responses can be evoked either from the compressor or from compressor/compression system interactions.