Separated Flow Topology in Compressors

Abstract

When a multi-stage high-speed compressor is operated away from its design point extreme incidence is caused in some blade rows. This results in large, localised separations which are three-dimensional in nature. In this paper topological reasoning is used to describe the behaviour of these three-dimensional separations. It is shown that two classes of separation exist: One in which the flow progresses from attached to separated in a smooth way; and another where there is a discontinuity in the response of the flow topology. It is shown that the global structure of the flow depends upon the type of topological response that occurs. When the response is discontinuous, non-axisymmetric cells of separated blades are formed. When the response is smooth, the resultant separated flow is axisymmetric. The paper is split into two broad sections: The first section presents examples of the two different classes of topological response that can occur in a single blade row, it also shows how an engineer can achieve a different response by altering the blade design. The second section covers the analysis of a multi-stage high-speed compressor. The compressor initially presents discontinuous behaviour with rotating cells of separations. It is then redesigned to reduce the severity of the cell behaviour or remove it entirely