The Potential of Casing Treatments for Transonic Compressors: Evaluation Based on Axial-Slot and Rotor Blade Optimization

Abstract

Casing treatments (CTs) have proven their potential to increase the working range of a compressor stage, sometimes even with little or no decrease in efficiency. However, a positive impact on efficiency is only possible if the additional CT-losses are compensated by a reduction of other losses, especially at rotor tip. This appears to be increasingly difficult to achieve for highly efficient modern rotors. In order to analyse how CTs can contribute to improve the overall compressor design, extensive optimization studies are performed, aiming at increasing the stability and efficiency of a transonic compressor stage. Axial-slot CTs and the rotor are optimized separately with a high number of geometric parameters. Selected Pareto-optimal geometries of the two optimizations are combined to study various CTs on different rotors. It is shown that a significant increase in stability can be achieved using axial-slot CTs, exceeding the values that can be reached optimizing the rotor without CTs. However, no combination of optimized rotors and CTs is found that dominates the other geometries in terms of efficiency. Hence, the question whether a CT can contribute to an improved compressor design very much depends on the desired stage design. CTs provide a benefit if a maximum stability range is necessary or if certain design choices lead to a demand for a stability enhancement, that otherwise cannot be achieved. In order to gain a maximum efficiency, a design without CTs appears to be more promising in the first place. Designs with comparably high losses in the rotor tip region, e.g. due to large tip clearances, might also benefit of CTs in terms of efficiency.