Modelling the build-up of a thixotropic fluid under viscosimetric and mixing conditions

Abstract

The rheological behaviour of a thixotropic fluid was studied during build-up under viscosimetric and mixing conditions. The rheological characterisation was performed with high-shear low-shear tests and the behaviour was modelled with the stretched exponential model and the three parameters were functions of the applied low shears. The mixing conditions were performed on a small-scale equipment with an anchor impeller and the K p and K s factors were identified with three shear thinning fluids exhibiting a plateau viscosity at low shear rates. High-speed low-speed tests were carried out with the thixotropic fluid and the torque increased with time as under viscosimetric conditions. We applied the Metzner and Otto approach to predict the power consumption of our device with that of the previously identified rheological model. The results show that this approach gave good predictions of the initial and final viscosity values, yet demonstrated that the build-up kinetic was lower for mixing conditions. The viscosimetric data suggested that our thixotropic fluid exhibited a yield stress during build-up. We applied the method used for the mixing problem of a Bingham fluids with an anchor. This method consists of keeping the K s factor constant and determining an experimental correlation for the K p factor based on the fluid yield stress and the operating conditions. This method enhanced our modelling, leaving the experimental correlation for the K p factor to be verified with pilot scale equipment and other types of impellers.