Experimental Investigations On The Circumferential Propagation Characteristics Of Tip Leakage Flow Unsteadiness In Axial Compressor And Its Influence Factors

Rotating stall is an unstable phenomenon which restricts the safety operation of compressors, which makes the early warning and control of stall become the research focus of researchers at home and abroad for many years. Stall precursors, typically including modal and spike types, usually can be detected during stall inception. The types of stall precursors can be discriminated by the incidence at the tip of the rotor blade and the track of tip leakage flow. Further studies show that the tip leakage flow shows remarkable periodic unsteady fluctuation before classical stall precursors at near stall working conditions when with relative large tip clearance size. The fluctuation of tip leakage flow and the rotating pressure wave induced by it provide new ideas and perspectives for the generation mechanism of different kinds of stall precursors and the development process of rotating stall.Experiments have been conducted to explore the transformation trends of the frequency and propagation characteristics of unsteady tip leakage flow during the throttling process under different tip clearance sizes, shaft speeds and inlet distortions. The similarities and differences of the results under different conditions are compared and concluded. The main work and results are shown as below:1. A single rotor axial compressor is taken as research object. On the casing two lines of sensors circumferentially arranged at the leading edge and 20% blade chord were used to collect the dynamic data as well as the sensors arranged chordwise from 60% blade chord to 20% blade chord upstream blade leading edge in three rows and sensors respectively at lcm upstream blade leading edge and lcm downstream of blade trailing edge. Power spectrum density, pressure contour and space Fourier transform are used to analyze the frequency and propagation characteristics of unsteady tip leakage flow at every working condition. Several characteristic curves are collected before dynamic data collection to ensure the repeatability in every experiment.2. The influence of tip clearance size on unsteady tip leakage flow is studied by changing the casings. The results show that unsteady tip leakage flow shows up at a certain flow point during the throttle process, which gets earlier when the tip clearance increases and will always exist if the tip clearance size is large enough. When tip clearance increases with a certain shaft speed and at same flow rate, the amplitude of unsteady tip leakage flow increases, the dominant frequency decreases, the propagation speed doesn’t change much and mode orders decrease. During the throttling process, if the shaft speed and the tip clearance keep constant, the most remarkable position of unsteady tip leakage flow moves upstream in blade passages and at 20% blade chord at near stall conditions. Furthermore, during the throttling process, the peak amplitude of unsteady tip leakage flow increases, and the corresponding frequency first increases and then decreases after a certain point. Still the propagation speed increases during the throttle process, and the mode orders start as blade number and decreases afterward. When the tip clearance is large enough, the mode orders at large flow coefficient are lower than blade numbers. The propagation characteristics at blade leading edge are the same as those at 20% blade chord where the signals are clearer.3. The influence of shaft speed on unsteady tip leakage flow is studied by controlling the direct current motor. When the shaft speed increases while the tip clearance stays the same, the dominant frequency of unsteady tip leakage flow increases and the relative frequency nondimensionalized by blade passing frequency increases too. The propagation speed increases, but the relative speed nondimensionalized by shaft speed doesn’t change much, as well as the mode orders.4. The effect of inlet hub distortion on unsteady tip leakage flow is studied by putting a distortion piece at compressor inlet. With hub distortion, the characteristic curves change a lot, but the development trends of the frequency and propagation characteristics of unsteady tip leakage flow and also the effects of tip clearance and shaft speed are similar to those with uniform inlet. But the flow field is disturbed and the disturbance is more remarkable. The effect level of tip clearance size and shaft speed is changed.5. As the tip clearance size, shaft speed and inlet condition change, most of the frequency and propagation characteristics of unsteady tip leakage flow will change. But some parameters have the same features before stall. For example, the axial position where the unsteady tip leakage flow has the biggest amplitude is at 20% blade chord, and the propagation speed will be between 0.45Ns-0.5Ns.So the frequency and propagation characteristics of unsteady tip leakage flow can be used as stall warning signals, which has guiding meanings to the detection and control of rotating stall.