Numerical Investigation on Slot Casing Treatment in a Transonic Axial Compressor Stage: Part 2 — Unsteady Simulations and Analysis

Slot-type casing treatment generally has a great potential of enhancing the operating range for tip-critical compressor rotors, however, with remarkable efficiency drop. In the first part of this two-part words, several configurations of slot casing treatment were tested in a 1.5 transonic compressor stage by steady simulations. One kind of arc-curve skewed slot contributed to considerable stall margin improvement with minimum efficiency loss. However, interaction between main passage and casing treatment was inherently unsteady. Steady simulation was inadequate to provide accurate compressor performance prediction and precise flow field details. Thus, this part was aimed at clarifying the differences between steady and unsteady simulations. The unsteady interaction process between main passage flow and slots were also detailed interpreted. Unsteady simulation was conducted by applying sliding interface between rotor passage and arc-curved skewed slots. Firstly, differences of compressor performance were examined between steady and unsteady methods. Results showed that steady simulation underestimated stall margin improvement and efficiency drop by casing treatment. Then analysis on aerodynamic parameters and specific flow fields were carried out at smooth casing peak efficiency and casing treatment near stall conditions. Unsteady simulation provided more than 50% larger mass flow rate entering or exiting slots opening surfaces at both operating conditions. It revealed that in unsteady simulation, casing treatment contributed to stronger suction/injection process, which promoted tip flow fields more effectively than steady simulation. Axial velocity deficit at rotor outlet was refilled by slots more effectively in unsteady simulation. In steady result, a large low momentum blockage existed inside rotor passage near tip region and prevented flow from entering the passage at near stall condition. While in unsteady simulation at the same condition, incoming flow was still able to travel across rotor passage in a high velocity. Further, instantaneous flow fields near tip region and inside the slots were particularly examined during a rotor blade passing period to elaborate the unsteady flow interaction. The mid-pitch surface of a representative slot was selected to represent the re-circulation procedure inside slots. Unsteady flow fields and spectrum analysis manifested that tip flow field was dominated by slots passing, while re-circulation process inside slots was dominated by blade passing. Low pressure region inside the blade passage facilitated the injection process. Circulation inside slots lagged behind the pressure variations beneath slots. When the slot was striding over the blade tip, intense injection didn’t emerge immediately beneath slots’ front portion. Until the high pressure region moved away from the slot opening surface, fluids inside the slots started to inject into the main flow in high speed.