Design optimization of axial slot casing treatment in a highly-loaded mixed-flow compressor

In this paper, a novel axial slot casing treatment design is introduced via optimization to improve the stall margin of the compressor with minimal efficiency loss. Besides the conventional design parameters including the height, length, position, inclination angle and open area ratio of the slots, a series of new parameters are put forward to increase the freedom of the slot meridional profile. An in-house design optimization platform is constructed based on NSGA-II and Kriging surrogate model to perform the optimization. A Pareto Front is obtained through optimization, from which the optimal design is selected by comprehensively considering both stability and efficiency performances of the compressor, i.e. the stall margin is improved by 11.23% in conjunction with the peak efficiency reduced by 0.21%. The database is analyzed to investigate how different parameters affect the performance of the slots. Three non-dominated representative designs are selected from the Pareto Front and their influencing mechanisms on stability and efficiency are further studied. Results show that the application of slots would generate a circulated flow, which exerts suction and reinjection effects at the rotor tip region and alleviate the blockage in the blade passages. Entropy analysis indicates that the viscous shearing interaction between the reinjection flow and incoming main flow causes additional entropy generation, thus slightly reducing the peak efficiency of the compressor.