Experiments on the whirling of pitched blade impellers in baffled mixing vessels

Abstract

The whirling characteristics of a 45° pitched-blade impeller (PBI) operating in a baffled mixing vessel were studied experimentally. 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 tank-diameter ratios of D/DT=12, 914, 57, and one six-blade PBI with D/DT=57. The smallest impeller spans half of the tank diameter as is standard in most industries. The larger diameter impellers being specific to the polymer production industry. Experiments were conducted at speeds up to 1.2 times the natural bending frequency of the rotor shaft assembly. Impeller orbits together with torsional vibrations were measured directly using high speed imaging and strain gauges mounted at the root of the shaft. The results indicated that the impeller motions were random and could be modelled as an axisymmetric normal distribution. The mean square response of the impeller׳s deflection was dependent on the rotational speed and the rotational direction of the impeller.