Numerically Aided Design of a High-Efficient Casing Treatment for a Transonic Compressor

Abstract

The transonic NASA Rotor 37 is well known for the occurrence of rotating stall at operation points close to stall. Several experimental and numerical investigations have shown that the instabilities observed mainly origin near the blade tip. Due to this property this rotor is theoretically suitable for a successful application of casing treatments. The first issue of this paper is to numerically investigate the flow mechanisms leading to compressor stall at design and off-design conditions. The numerical simulations carried out demonstrate that at design speed the behaviour of tip leakage flow in combination with the angle of attack at the blade tips is the key factor which limits the flow stability. Whereas at off-design, a steep angle of attack near the blade tip causes vast flow separations at the blade’s suction side. The second issue of the paper is to design an adapted casing treatment for the NASA Rotor 37. This issue is very demanding since the design total pressure ratio is with 2.1 extremely high and therefore, high-loss blockage zones can be easily caused by a misdesigned casing treatment. Based on the observations made in the first part guidelines are suggested for designing a casing treatment which efficiently enhances the compressor’s flow stability at design and off-design conditions. Casing treatments geometries are presented which address these suggested guidelines. Three-dimensional time accurate CFD-simulations are carried out to verify these designed casing treatments.