Large Eddy Simulation Of Unsteady Flow Field In A Low-Speed Axial Compressor Rotor

The internal flow in compressor is essentially unsteady. The rational use of the unsteady effect has great potential in improving compressor performance. In this paper, Large-eddy simulations (LES) were conducted to study the unsteady flow field in two dimension moving compressor cascades and low speed axial compressor rotor. Here profiles of the cascades are three typical blade elements of this compressor rotor. The evolutions of vortex structures and oscillating frequency during compressor throttling process are focused on, especially the unsteady oscillating features induced by flow separation on suction surface. By imposing a series of monitor points near suction surface along blade chord, the signals of unsteady static pressure are collected. The unsteady oscillating features in time domain and frequency domain can be captured and analyzed. Thus the structures and frequency distribution of unsteady oscillation can be obtained.Flow around two dimension circular and flow through two dimension moving compressor cascade are used to study the effects of different sub-grid scale models and time steps. Vortex structures and oscillation frequency and amplitude are compared. Thus the suitable sub-grid scale model and time step can be selected to do the following simulations in this paper.Blade elements at5%,50%and95%spans are used to study unsteady flow field in two dimension moving compressor cascades. First, single-passage model is adopted to investigate the evolutions of flow structure and oscillation frequency during throttling process for each blade profile, and the fluid mechanism is explained. The similar evolution trends are obtained for all the three profiles. Unsteady oscillations resulted from flow around leading edge and separation on blade suction surface coexist. Both frequencies almost keep constant. Flow structures of separation on suction surface are of small scale. Then, the effects of simulation models with different passage numbers are compared for profile at50%blade span. The results show that both single-passage and multi-passage models can capture unsteady oscillating caused by flow around leading edge and the suction surface separation, the flow structures are similar. The only difference is the absolute value of unsteady oscillating frequencies. Further, the multi-passage model and valve-throttle model are used to study the evolution of unsteady flow field during throttling process. At working conditions near stall limit, large scale separation occur on suction surface. The oscillating frequency decreases dramatically and is lower than blade passing frequency.For the compressor rotor with no tip clearance, single-passage model is used to study the evolution of unsteady flow field with compressor throttling. The vortex structures and frequency features within whole blade span are analyzed. The results agree well with those of two dimension cascade simulation, which proves the guiding significance of the two dimension simulation results.