This paper reports the effect of the axial position of the slot casing treatment on the performance of transonic compressor NASA Rotor 67 by unsteady numerical simulation. The interaction of the recirculation in the slots and flow near the blade tip is analyzed to understand the flow mechanisms. The relative importance of the mechanisms of stall margin improvement due to the slot casing treatment is evaluated with the relative weight method. The results show that the bleeding and injecting effect caused by the recirculation is the most important factor that affects the blockage in the blade tip region, which determines the stall margin improvement. When the slots cover the initial position of the tip leakage vortex (TLV) and the boundary layer separation zone downstream the shock, the recirculation is stronger due to the larger pressure difference between the front and rear part of the slots. Consequently, the slot casing treatment can reduce the blockage near the casing more effectively, which results in a larger stall margin improvement. Shifting the slots upstream or downstream will reduce the driving force for the recirculation and the effect extent on the low energy fluid of the slots, which decreases the stall margin improvement.

Conclusions

The effect of the axial skewed slot casing treatments with four different axial positions on the performance of NASA Rotor 67 was investigated by unsteady numerical simulations. The conclusions are summarized as follows:

(1) When the front part of the slots covers the initial position of the tip leakage vortex and the rear part of the slots covers the boundary layer separation zone downstream the shock, the recirculating mass flow in the slots is stronger due to the larger driving force. Thus, the slots can reduce the blockage due to the tip leakage vortex and the boundary layer separation more effectively, which causes a larger stall margin improvement. When the slots are shifted upstream compared to the optimal configuration, the recirculation is weakened and the blade suction boundary separation cannot be suppressed effectively. When the slots are shifted downstream compared to the optimal configuration, the recirculation is also weakened and the tip leakage vortex cannot be suppressed effectively. As a result, the stall margin improvement is reduced with the slots shifted upstream or downstream.

(2) The analysis of the interaction mechanisms between the slots and the flow near the blade tip indicate that the intensity of the recirculation in the slots, the blade loading and the inlet axial velocity have effect on the blockage, the shock position and the trajectory of the tip leakage vortex near the blade tip, which determines the stall margin improvement due to the slot casing treatments. Based on this, the relative importance of the factors that affect the effectiveness of the slots on the stall margin improvement is obtained with the relative weight method. The results show that the sequence of relative importance of the factors in the first level is the intensity of the recirculation in the slots, the blade loading, and the inlet axial velocity near the blade tip. And the sequence of relative importance of the factors in the second level is the blockage, the shock position, the inlet axial velocity near the blade tip.

The results have been published on Applied Thermal Engineering 126 (2017) 53–69.

Fig.1. Grid topology of the blade passage and the slot casing treatment.