2011 Volume 65 Issue 1 Pages 544-551
In general, concentrated suspension shows non-Newtonian behavior under shearing due to flocculation of suspended powder particles. Thus it is of importance to predict the thixotropic behavior of concentrated suspension to transporting and handling of them. However, the thixotropic behavior is strongly influenced by the formation of particle aggregate due to the interparticle forces and the fragmentation of the aggregate due to shear stress. In this study, we propose the thixotropy model for concentrated and flocculated suspension based on fractal concept to predict the time-dependent shear viscosity. We adopt the viscosity model proposed by Mills based on the assumptions of mean-field theory. According to the model, the viscosity of flocculated suspension is expressed by an effective volume fraction of fractal aggregates instead of solid volume fraction. In order to determine the size distribution in fractal aggregates, a population balance model proposed by Barthelmes, et al. is introduced to account the coagulation and fragmentation of the aggregates under shear flow. From the evolution of the aggregate size distribution, the mean effective volume fraction can be calculated at arbitrary time and then it is applied to the viscosity model. Further the fractal dimension accounting for the aggregate structure is determined by the rheological approach proposed by Chougnet, et al. The model prediction is in good agreement with experimental data.
