Development of a geometric model for the study of propagating stall inception based on flow visualization in a linear cascade

Abstract

Flow visualization movies of flow through a cascade of compressor blades showed propagating stall at stagger angles of 36.5 and 45 degrees for angles of attack of 20 degrees and higher. At a stagger angle of 25 degrees, the development of a steady, separated boundary layer occurred with no propagation. The observed propagating stall process was the development of a vortex in the boundary layer and its subsequent shedding. The shedding mechanism was observed to be the interference by the reverse flow from the previously stalled passage with the vortex flow in the stalled passage. This dissipated the vortex in the blade passage and the incoming flow then flushed the stagnated vortex out of the passage.

Measurements of propagation speeds showed that the propagation speed is related to the blockage of the passage, that stagger angle has an insignificant effect on propagation speed, and that propagation speed is proportional to the relative velocity.

Based on the observations, a geometric model was developed to predict the onset of propagating stall. This model showed that increased solidity, decreased stagger angles, and operation at low angles of attack make a cascade more resistant to propagating stall inception. The model shows the relation of the operating point of a compressor to the stall inception point. When expanded to include all significant aspects of blade geometry, the model may provide a basis for controlling propagating, and hence, rotating, stall inception based on the blade row·geometry.