Fluid Force Testing Machine for Centrifugal Impellers in Precession Motion

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The vibration motion of an overhung centrifugal impeller can be considered as the superposition of whirling and precession motion. The present study is to develop a testing machine, by which the fluid force and moment acting on a centrifugal impeller in the precession motion can be measured.In the first year (1989), the design and construction of the testing machine was conducted. Measuring devices including 4-element rotating force sensor and data acquisition and processing system were also planned.In the second year (1990), the testing machine was finally adjusted along with its subsystems and set into experiment. A centrifugal impeller (model of a boiler feed pump) with 350 mm diameter was chosen as test impeller. The fluid force and moment on this impeller were measured during precession motion with various precession/rotational speed ratio at five given flow rate. The impeller was installed in vaneless diffuser and in vaned diffuser.The testing machine functioned as expected. The test result revealed that :1) The moment on the impeller exerts destabilizing effect on the shaft vibration when the rotational speed is higher than twice the critical speed of the shaft.2) The force plays less important role for the shaft vibration, since the force associated with precession motion is canceled by that associated with whirling motion.