Aerodynamic Optimization of a Transonic Centrifugal Compressor by Using Arbitrary Blade Surfaces

A transonic centrifugal compressor was aerodynamically optimized by means of a numerical optimization process. The objectives were to increase the isentropic efficiency and to reduce the acoustic signature by decreasing the amplitude of pre shock pressure waves at the inlet of the compressor. The optimization was performed at three operating points on the 100% speed line in order to maintain choke mass flow and surge margin. At the design point, the specific work input was kept equal. The baseline impeller was designed by using ruled surfaces due to requirements for flank milling. To investigate the benefits of arbitrary blade surfaces, the restrictions of ruled surfaces were abolished and fully 3D blade profiles allowed. In total therefore, 45 parameters were varied during the optimization. The combined geometric and aerodynamic analysis reveals that a forward swept leading edge and a concave suction side at the tip of the leading edge are effective design features for reducing the shock strength. Beyond that the blade shape of the optimized compressor creates a favorable impeller outlet flow, which is the main reason why the performance of the vaneless diffuser improves. In total a gain of 1.4%-points in isentropic total-to-static efficiency, evaluated by CFD at the exit plane of the vaneless diffuser, is achieved.