Effect Of Tip Clearance On Stall And Circumferential Single Grooves Improving The Stability On Axial-flow Compressors

Under the support of China National Natural Science Fund, this dissertation focuses on the aerodynamic stability of compressors. As the first step, the isolate subsonic NPU-rotor was experimentally studied for its stall mechanism. A numerical study was then carried out to give a detailed discussion for the stability of rotor, the influence of tip clearance on stall and its coupled effect with circumferential grooves. The same problems were further studied on rotor 37 to find out the differences and similarities in a transonic compressor.The amplitude of blade passing frequency reaches its maximum value at peak-efficiency point, goes down as the throttling of compressor and then rises again after stall occurs. This feature is significant for detecting aerodynamic stall and of great value for engineering application. A threshold of blade passing frequency can be defined for the monitor of the compressor. When the compressor operates near stall, the blade passing frequency will fall off this threshold and thus give a warning for the happen of stall.The numerical study provided a more detailed observation, that both of leading edge spill flow and the trailing edge reverse flow develop remarkably in the tip region of rotor when it comes to the near stall operating point. The above phenomenon is actually induced by the effect of axial momentum. To give a quantitative evaluation for the interaction between main flow and leakage flow, a new parameter(Ba) was defined using the ratio between their area average momentum. For the subsonic rotor in study, the stall starts only when Ba=0.5. Another numerical study for Rotor 37 gave a similar conclusion. As the result of the interaction between main flow and leakage flow, the leakage vortex expands and breaks when it goes through shock. The induced low energy fluid thus leads to blockage for tip passage. The start of stall in transonic compressor also lies in the balance between axial and leakage momentum. However, the value of Ba for the near-stall operating point of transonic rotor is lower than 0.5.This study has confirmed the significant influence of tip-clearance on the stable operating range of compressor. Aerodynamic stall can be induced by two different mechanism, i.e. the boundary layer separation and the interaction between main flow and leakage flow. The variation of tip-clearance will induce different stall types. For both subsonic and transonic compressor, the increase of tip-clearance will lead to the upstream movement of the interface between main flow and leakage flow. When it comes to a certain large value, the leakage flow will thus spill out from leading edge near stall point. This explains why there was no leading edge spill flow and the trailing edge reverse flow found in some previously investigated compressor cases. The variation of tip-clearance also influence the value of Ra and Ba. As the decrease of tip-clearance, the growing trend of Ba becomes lower. Quantitatively, in a stable operating compressor, the value of Ba is always lower than 0.5. For a fixed stall type, compressors with lower Ba will generally have better stability. The stall type will change as the tip-clearance decreases to a certain value. In this circumstance, the value of Ba will not be able to give an accurate description for the best clearance.For the study of the coupling effect between circumferential groove and tip clearance, a special phenomenon is noted that the PS-SS pressure difference is locally reduced by single circumferential groove but rises to a larger value in its downstream region, i.e. the so-called “over-recovery”. For case with small tip clearance, the stall margin can be significantly improved by both of the leading-edge and mid-chord single groove without the cost of efficiency and total pressure ratio. While for large tip clearance case, groove will induce a drop for efficiency, pressure ration and stall margin. As shown by an afterward mechanism study, it is mainly because the small tip-clearance suppresses the over-recovery induced by the single groove. A further study for the depth of groove is carried for the small clearance case. The result shows that groove with too large depth induces strong internal swirling flow and thus large loss generation. A suitable control of groove depth will give consideration for both radial suction and loss reduction, and therefore is significant for its effectiveness in compressor.