The strong adverse pressure gradient in axial compressor induces the flow to separate at the rear part of the blade, and the tip leakage flow even makes the compressor flow field much more complicated. Many researches concern its steady performance while ignoring its transient characteristics. In realistic case, however, the flow field is always fluctuating, especially at off-design conditions, which are usually accompanied by large scale flow separations. In light of insufficient ability of URANS (unsteady Reynold-average Navier-Stokes) in precision and in predicting detailed vortex, the tip leakage flows as well as its interaction with inlet boundary layer in a compressor cascade with stationary casing at off-design condition are investigated using Large Eddy Simulation (LES) in the present investigation. The incidence of the inlet flow is 8 degrees at an inlet Mach number of 0.12, and inlet boundary layer is also considered (test profile of total pressure is used in numerical boundary conditions).The structured grids consist of 64 million nodes with average *y^+^* value of the first grid layer near wall to be about 0.4, and Δ*x^+^* and Δ*z^+^* are within 30 and 20, respectively, according to the recommendation of others’ works. The LES results are compared with RANS solution first, and compared with RANS, LES could capture more detailed vortex structures inside the compressor and the interaction between tip leakage vortex and passage vortex is observed clearly. Besides, the induce vortex and tip separation vortex are also captured. Also, the transition process could be observed on the blade surface away from the casing. Afterwards, compressor cascade with upstream bleeding slot is simulated using LES, so as to investigate the effect of slot type bleeding, which is a typical method in actual application for axial compressor bleeding, on the evolution of tip vortex and its interaction with passage vortex, as well as loss mechanisms. As far as the authors know, LES hasn’t been used in the simulation of bleeding in compressor. Therefore, this part of investigation emphasis on understanding the role the flow control method, which is bleeding slot in present work, plays in a compressor cascade with tip clearance. The bleeding mass flow rate is 2.8% of the flow rate at inlet. The vortex structure is identified by Q-criterion in the current research. According to the iso-surface of Q, it could be observed that the tip leakage flow is jetted from the tip clearance and forms two counter-rotating vortices (tip leakage vortex and induce vortex), due to the different influences of blade tip and casing. In case without bleeding slot, passage vortex interacts with tip leakage vortex at about 30% blade chord downstream the leading edge, while no obvious passage vortex could be observed in case with bleeding slot. In the rear case, boundary layer of the casing doesn’t roll up into vortex and just moves toward the suction surface of blade and mix up with tip leakage vortex. The size of tip leakage vortex and induce vortex are larger at the front part of blade in case with bleeding slot than that without bleed slot. After tip leakage vortex interacting with passage vortex in case without bleeding slot, however, the size of vortex increases rapidly and the loss increases accordingly.


Vortex Structures Analysis and Loss Mechanisms in Axial Compressor Cascade With and Without Bleeding Slot Using Large Eddy Simulation

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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