(406f) Experimental Investigation on the Power Input of Viscoelastic Fluids in an Unbaffled Tank

Stirring processes involving viscoelastic fluids are relevant in food industry, polymer industry and biotechnology, e.g., in xanthan fermentation, polystyrene production or fermentation of substrate in biogas processes. The DFG project ‘‘Mixing of fluids with viscoelastic flow behaviour in stirred tanks’’ aims to find basic principles for the efficient design of stirring processes involving viscoelastic fluids. The present research examines the power input of viscoelastic fluids in an unbaffled tank with various agitator geometries.

Material and methods

Power input experiments are conducted in two geometrically similar unbaffled flat-bottom cylindrical tanks (diameter T=480mm, T=290mm and T=90mm) equipped with either a three-stage cross-beam agitator (vertical of inclined blades), a blade or a grid agitator. The torque is recorded with a 20Nm and an 1Nm strain gauge-based torque sensor in the two larger scales. An Anton Paar RheolabQC fitted with stereolithographic miniature agitators is used for the scale-down experiments. Stereo particle image velocimetry (PIV) in the intermediate scale is selectively applied to help with the interpretation of the data obtained from power input measurements.

Shear-thinning viscoelastic aqueous polymer solutions of xanthan gum, carboxymethyl cellulose, polyacrylamide and polyethylene oxide at different concentrations are used to cover a wide range of rheological properties. Additionally, constant viscosity elastic fluids, so-called Boger fluids, based on glucose syrup are investigated to experimentally separate shear thinning effects from the influence of elasticity. All fluids are rheologically characterized in terms of flow curve, first normal stress differences, storage and loss module in an Anton Paar MCR302 rotary rheometer with a cone-plate geometry.

Results

Based on the results obtained with Boger fluids compared to the Newtonian reference, elasticity itself is found to have practically no influence on the power input in the laminar regime. Shear-thinning fluids with pronounced elastic however may show a significant increase in power input compared to weakly elastic fluids that is indirectly related to elasticity. The PIV measurements show that elastic forces weaken the secondary flow and thereby reduce the local shear rates near the agitator (figure 1). This results in an increase of the effective viscosity. The Metzner-Otto constant may reduce as much as two thirds when comparing to the other extreme case.

A sharp elasticity-induced increase of the power input is manifested in the transitional regime where Boger fluids deviate from the Newtonian reference and approach a constant power number that is concluded to form an elastic limit as indicated in figure 2. A similar increase in power input is found for shear-thinning elastic fluids with a steep dependence of the first normal stress difference on the shear rate. When inertia forces dominate the fluid flow a decrease in power input below the Newtonian reference can be observed which is contributed to the shear-thinning characteristic.