Impact Mechanism Of Single Circumferential Grooves At Various Locations On The Stability Of Axial Compressor

In order to understand the impact of the circumferential groove location on the stall margin and the stall mechanism of the compressor, a tip-sensitive low-speed axial single rotor compressor was tested and numerically simulated. Both the measured and the calculated data showed that the single groove near the mid-chord of blade tip generates the maximum stall margin improvement (SMI), and the location for the groove with the minimal SMI is around 20%～30% axial tip chord (Cax) aft of blade leading edge.The evolution of the axial locations of the interface between tip leakage flow and main flow (TLF/MF interface) in the throttling process, which was identified by the axial shear stress lines and entropy on the casing, were analyzed. It was found that the movement of the TLF/MF interface exhibited two routes, and the single circumferential grooves were thus classified into quasi-modal- and spike-type grooves.For the spike-type grooves, the interface between tip leakage flow and main flow (TLF/MF interface) moves forward and reaches the blade leading edge in the near stall condition. The spike-type disturbance is formed when the TLF/MF interface spills at the rotor leading edge. For the quasi-modal-type grooves, the TLF/MF interface moves forward when the compressor operates in the large flow rate. The interface location does not shift anymore since the interface location crosses with the groove, and it is still located in the blade passage far from the rotor leading edge plane.To understand the two different evolutions of the TLF/MF interface, the quasi-modal-type single groove ("the front groove") located at 20% Cax aft of the rotor leading edge and the spike-type single groove ("the middle groove") located in the middle of the blade tip chord were selected to make a comparison of the flow structure in the tip clearance region. The results show that for the front groove, the interaction of the tip leakage flow with the groove is the strongest at the near-stall point. The injections into and out of the groove bring two impacts on the tip clearance flow. First, the inject flow into the groove near the pressure side accelerate the local incoming main flow, which inhibits the tip leakage flow from reaching the pressure side of the neighboring blade. This is the reason for the observed phenomenon that the TLF/MF interface stays at the location of the front groove at the near-stall point. Second, the groove flow driven by the axial pressure gradient forms into a counter-clockwise vortex along the circumference in the groove, which induces the clockwise vortex in the tip gap. For the middle groove, the interaction of the tip leakage flow with the groove as well as the injection in and out of the groove is much weaker. Tip leakage vortex is the main flow structure in the tip gap like the smooth casing. Therefore, TLF/MF interface achieve at the rotor leading edge plane at the near-stall point.The further analysis of the axial momentum balance and blockage in the blade passage indicates that the impact on the axial momentum distribution of tip clearance flow is the underlying mechanism for the various SMI generated by the spike-type grooves. The middle groove reduces the negative axial momentum of the tip leakage flow and thus pushes the TLF/MF interface downstream most. This is probably the reason for its largest SMI among all single groove investigated. For the quasi-modal-type front groove, the blockage in the blade passage is the largest at the same flow rate near stall, which possibly results from the clockwise vortex along the circumference in the tip gap. The highest blockage is believed to be responsible for the earlier stall of the compressor with the front groove.