Evolution of multiple stall cells with short length-scale in an axial compressor rotor was investigated experimentally. In a low-speed research compressor rotor tested, a short length-scale stall cell appeared at first, but did not grow rapidly in size unlike a so-called "spike-type stall inception" observed in many multi-stage compressors. Alternatively, the number of cells increased to a certain stable state (a mild stall state) under a fixed throttle condition. In the mild stall state the multiple stall cells, size of which was on the same order of the inception cell (of a few blade spacings), were rotating at 72% of rotor speed and at intervals of 4.8 blade spacings. With further throttling, a long length-scale wave appeared overlapping the multiple short length-scale waves, then developed to a deep stall state with a big cell. The ensemble averages of the axial velocity components upstream and downstream of the rotor, which were obtained phase-locked to the cell rotation, showed that the stall cells were of a part-span type.
