It has been shown in many cases that a notable aerodynamic stability enhancement can be achieved using casing treatments (CTs) on transonic compressors. This advantage, however, often involves degradation in efficiency at design point conditions. In order to analyze the correlations between efficiency, surge margin and other flow quantities on the one hand and the geometric parameters related to axial slots on the other, an automated multi objective geometry optimization of axial slots is performed. This involves the usage of time accurate URANS simulations for each new CT design the optimization tool proposes. The axial slots are generated using a parametric design, which can produce slots of different size, shape and position. Three operating points are simulated. One at design point (ADP) conditions, a second at reduced speed working line conditions and a third at reduced speed close to the stability limit. Based on the results of the CFD simulations two objective values are calculated. These are, first, an increased efficiency at working line conditions and, second, an increased surge margin at reduced speed. The test case used for the study is the first stage of DLR’s transonic research compressor Rig250. The rig is representative for the front stages of a heavy duty gasturbine compressor. The computational domain includes the IGV as well as the first rotor and stator. The rotor of the configuration is tip-critical for the studied part speed condition. The result of the optimization is a Pareto front with all optimal geometries regarding surge margin and efficiency. It is found that efficiency at design point can be exchanged against surge margin at reduced speed. The working principles and flow phenomena of the Pareto-optimal axial slots are analyzed in detail to obtain a better understanding of the mechanisms leading to the extension in surge margin.

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