This paper presents both experimental and unsteady RANS investigations of a slot-type casing treatment at a transonic axial flow compressor rotor. Experimental results show that at 60% and 98% of rotor design wheel speeds, approximately 100% and 200% extra extensions of the rotor operation ranges are achieved, respectively. On the other hand, there are about 3.6% and 2.0% drops of efficiencies at 60% and 98% speeds respectively if comparisons are made at the same peak-efficiency mass flow rates of the solid casing case. If comparing the respective peak efficiencies for the solid casing case with those for the treated casing case, there are still about 3.4% and 0.7% drops at 60% and 98% speeds, respectively. As for the unsteady RANS study, an in-house unsteady RANS code has been used to study the casing treatment flow at several operating points, i.e., the peak efficiency and the near stall with regard to the solid casing case at 60% speed and 98% speed, respectively. It is shown that the interactions between the blade passage flow and the casing treatment flow exhibit different manner at two rotating speeds. The flow condition in which the rotor operates, i.e., either the subsonic condition at the 60% speed or the transonic condition with passage shock presented at the 98% speed, is one of the determinate factors that are responsible for the manner the casing treatment works. The loss production due to casing treatment is also particularly discussed.

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