Experimental Study of Pitched-Blade Impeller Vibration in Baffled Mixing Vessels

The vibrational characteristics of a 45° pitched-blade impeller (PBI) operating in a baffled mixing vessel were studied. Test conditions include different blade configurations, impeller sizes and rotational speeds. Experiments were conducted in three mixtures of water and glycerol, with and without baffles, operating in both clockwise and counterclockwise directions. Four different impellers were used: three, four-blade PBIs with diameter to tank diameter ratios of DDT = ½, DDT=91 4,DDT =57 and one six-blade PBI with DDT=57 . Experiments were conducted at speeds up to 1.2 times the natural frequency of the rotor. Impeller orbits were measured directly using high speed imaging together with torsional vibrations, using strain gauges mounted at the root of the shaft. The results indicated that the impeller deflections were random and could be modelled as a normal distribution. The mean square response of the impeller's deflections was dependent on the rotational speed and the direction of the impeller's axial thrust force. The results indicated that the baffles also had a strong effect on the vibration characteristics for small tip clearances.;A linear model for the steady-state orbit of a standard 45° PBI in a baffled mixing vessel that accounts for both structural and hydrodynamic forces which are sensitive to geometric parameters, fluid properties and rotational speed. Model results for the expected mean square vibration response of the four-blade PBI with a DDT=57 were compared with the experimental data. The model correctly predicts the mean square deflection and the asymmetry of the shaft's vibration amplitude with rotation direction.