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Effect of inclined-blade impeller design on mixing and aeration in stirred vessels

The flow field and gas-liquid mass transfer coefficient generated by inclined-blade mixing impellers with modified blades has been studied. The work was focused on four-blade designs with specific alterations - variable blade width and leading edge wings. Three versions of inclined-blade geometry were examined: a general type of a trapezium-shaped blade TB, a four-blade shielded version of TB, indicated as STB, and a conventional inclined blade impeller with a 45° pitch PBT. The up-pumping mode of operation in a single impeller configuration was followed. Measurements of the power consumption, mixing time θ m and k L a have been carried out in a stirred vessel of 0.4m diameter to study the effect of the blade design on the specific blending and aeration characteristics. Computational fluid dynamics (CFD) has been used to uncover the flow patterns. Correspondence between the impeller shape and the flow patterns has been sought. The version STB is found to enhance axial circulation and to double the circulation loops at relatively low power number. Both TB versions showed enhanced mass transfer coefficients compared to the conventional one. The observed effects are explained by reduced zones of extremely low backpressure and decreased separation flow at the blades' suction sides that lead to reduced rotational flow and increased circulation.