Heat transfer and power consumption of non-Newtonian fluids in agitated vessels

Abstract

A new shear rate model is proposed for determining the apparent viscosity of non-Newtonian fluids: ▪ where f is defined as the flow regime coefficient. This model is based on the assumption that the torque offered by the rotating impeller is proportional to the average shear stress exerted on the wall surface of the agitated vessel. Under investigation were MIG impellers, flat-blade disc turbines, Pfaudler impellers, plate paddles, anchors and a semi-elliptical board impeller. The agitated vessel was equipped with three types of cooling tubes: inner helical coil, external helical coil and vertical tubes, the vertical tubes acting as a heat-exchanging surface as well. Heat transfer coefficients of non-Newtonian fluids in agitated vessels were correlated in the present model by using a generalized Reynolds number Re * and the dimensionless group εD 4/v 3 a , where ε is the power per unit mass. These heat transfer correlations for both the jacket and the colling tube can be used in the design and scale-up of industrial agitated vessels. Computation results indicated that the present shear rate model permits a good correlation of film coefficients for both the jacket and the cooling tube under different conditions and with different geometrical configurations for different types of impellers and cooling tubes, baffled or unbaffled, including the transition and turbulent regimes. The precision of the present model was compared with that of previous models.