Response Tests on the Unsteady Fluid Forces for the Oscillating Centrifugal Impeller

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The first year was devoted to the estimation of the fluid force moment on the shroud of precessing impeller, from unsteady pressure measurements. It was shown that the destabilizing region increases as the increase is leakage flow rate, caused by the increase in the mean tangential velocity of the leakage flow. It was also found the increase of resistance at the outerradius causes the increase in the force measurements. A numerical analysis based on the bulk flow model was made. It was found that analysis can simulate most of important aspects experiments. It was also made clear that the leakage flow can be basically treated as a 2-D inviscid unsteady flow within variable clearance.In the 2nd year, direct force measurements on a whirling open-type compressor impeller which was designed and manufactured in the preceding year. To identify the phase relationship of the impeller rotation and the whirling motion, two triggering signals from the main shaft and the whirling sleeve was put in an AND circuit and a triggering signal was put out when the rotational and whirl signal were both on. The triggering signal was used to start accumulating the force signal in a FFT.An interesting result which suggests the possibility of both forward and backward whirl instability was obtained. However, a detailed inspection of the fluctuating force suggests the existence of unexpected shaft vibration. Hence we should continue the experiments to obtain more reliable data.