Casing Treatment: Its Potential and Limitations

Casing treatment is an advanced design feature intended to improve the stability of a compressor. Various investigations have been conducted based on both experimental and numerical studies at least over the last 50 years. In general, it has been demonstrated that a careful design of a casing treatment can substantially improve the stall margin of a tip-limited compressor although this generally comes with an aerodynamic efficiency penalty. This review paper revisits important past studies on casing treatments to clarify findings on the sometimes-conflicting results and understand what lessons are generally applicable to a wider group of machines. Circumferential grooves over a rotor tip have been in use for some time. Much recent work has focused on axial slots over a rotor tip, because of the significant stall margin improvement and the better efficiency potential of this configuration. This paper includes both configurations, with more emphasis on the latter. The paper concludes that casing treatment offers powerful benefits to a compressor and deserves continuing research and development. Good benefits result from axial slots which start ahead of the rotor leading edge and cover the front part of the rotor tip and include skew (tangential lean in the direction of rotation). Contributing physical mechanisms include increasing the axial momentum of the casing flow casing and reducing the aerodynamic blockage to improve the casing boundary layer and the tip leakage flow. The role of impacting unsteadiness is yet to be further clarified. Areas of further research are recommended, both experimental and analytical, including further work to understand and control the loss mechanisms, studying and controlling stage interaction effects in multistage compressors, and the simultaneous design optimization of the rotor blade with the casing treatment. Multi-disciplinary issues such as manufacturing cost, weight, length, durability, aeromechanics effects, and tolerance to dust and ice also deserve further attention.