This paper presents a study of stall inception mechanisms in a low-spe
ed axial compressor. Previous work has identified two common flow brea
kdown sequences, the first associated with a short length-scale distur
bance known as a ''spike,'' and the second with a longer length-scale
disturbance known as a ''modal oscillation.'' In this paper the physic
al differences between these two mechanisms are illustrated with detai
led measurements. Experimental results are also presented that relate
the occurrence of the two stalling mechanisms to the operating conditi
ons of the compressor. It is shown that the stability criteria for the
two disturbances are different: Long length-scale disturbances are re
lated to a two-dimensional instability of the whole compression system
, while short length scale disturbances indicate a three-dimensional b
reakdown of the flow-field associated with high rotor incidence angles
. Based on the experimental measurements, a simple model is proposed t
hat explains the type of stall inception pattern observed in a particu
lar compressor. Measurements from a single-stage low-speed compressor
and from a multistage high-speed compressor are presented in support o
f the model.