Research On Compressors' Aerodynamic Rotating Instability Using Circumferential Mode Analysis Method

The unsteady flow in compressors results in performance degradation,noise problem and even operation accidents,and the relevant research will contribute to extend stable operation range of compressors.Rotating instability(RI)is a local aerodynamic instability that occurs at off-design conditions and a large tip clearance design,and in some situations it can cause blade vibrations and a significant increase in tip clearance noise.Using circumferential mode analysis method,the present thesis studies about the RI pressure fluctuation at tip region of an axial compressor setup.The main content of the thesis includes:Experiments measuring circumferential mode characteristics of compressors usually have a high demand for the number of sensors,and aliasing of circumferential wave number happens when not enough sensors can be provided.A calibration and prediction of higher circumferential mode numbers were proposed using two sets of measuring points(doubleuniform sampling points,DUSP),which can improve the maximum mode number by five times.Unsteady pressure fluctuations were studied,and high circumferential mode characteristics were captured on both the blade passing frequency(BPF)and the rotational instability frequency(RIF)under different flow rate conditions based on this novel method.The characteristic RI spectrum with a broadband hump was present in a large range of flow conditions.Both the frequency range and the dominant circumferential mode number decreased as the flow rate decreased;based on the calibrated mode characteristics,a rotating aerodynamic source model was used to explain the side-by-side peak of RIF spectrum and rotating characteristics of RI,where the source frequency decreases with flow rate.A compressed sensing(CS)based signal processing technique was applied using both DUSP and randomly selected points,which can effectively increase the spatial resolution of the signals and provide the measurements with robustness.Strength of the pressure fluctuations varied significantly in time domain with the help of CS method.Furthermore,a proper orthogonal decomposition(POD)method was applied to obtain the mode component of tip flow and to give a comparison with the traditional Fourier modes.A discussion on influence of spatial resolution with POD performance was obtained.It turns out that the two mode descriptions matched each other well.