A Free Streamline Model for Fully Separated Turbomachine Flows

Abstract

This paper describes the use of the free streamline approach to determine flows in fully separated cascades, and to thereby provide a rational basis for estimating the performance of axial turbomachinery at severely off-design conditions. On the assumption of two-dimensional, incompressible flow, the free streamlines that bound the separated region are determined using the hodograph method, in which the methods of complex analysis are used to define a tractable numerical problem. The resulting jet-wake flow profile at cascade exit is then presumed to undergo mixing at constant area, from which performance parameters for the cascade can be determined. The proposed method is verified by comparison against flat plate cascade solutions and by checks against momentum conservation. Validation of the free streamline approach is carried out by comparison against several cascade configurations for which experimental data and simulation results are available. In general, the predictions based upon the present theoretical model are found to be good, especially for negative incidences such as those that occur in the compressor section of an engine operating at sub-idle conditions. This suggests that the present approach can be used in the estimation of turbomachinery performance maps far from the design point, especially during early design phases.