| The scientific analysis of mixing, essential for process scaleup and design, has not received much attention. In this research work we attempt to understand the mixing process in a model mixer, the Brabender Farinograph, which is widely used in mechanical dough development and dough mixing studies. It is well known that in the dough being mixed, the rate of shear differs across the mixer and that the rate of shear varies with time. The shearing action is extremely non-homogeneous and mean shear rates ranging from 10 s{dollar}sp{lcub}-1{rcub}{dollar} to 10{dollar}rmsp4 ssp{lcub}-1{rcub}{dollar} have been reported. Attempts have been made to use the "coaxially rotating concentric cylinders" geometry as a model for the Brabender Farinograph (Goodrich and Porter, 1967). However, this approach does not take into account the complex geometry of the blades.; Using the technique of Laser-Doppler Anemometry, three dimensional velocity values and velocity gradients were determined at various locations in the Farinograph, with three model fluids having different rheological properties and the local shear rate was calculated. The shear rate distribution in the Farinograph as a function of motion of the blades and fluid rheology was determined.; Using experimentally obtained shear rate data for three model fluids (Newtonian corn syrup, non-Newtonian CMC (2%) and a fluid with high zero shear viscosity, Carbopol-940 (0.11%)), an equation to predict shear rate as a function of location, fluid rheology and speed of rotation of the blades was set up. An empirical shear rate distribution function was defined, providing the distribution of shear rates across the mixer. These distributions depend upon fluid rheology. A method to predict power requirements in mixing was also elucidated.; On the microscopic scale, mixing is the stretching, folding, redistribution/dispersion of material elements. An attempt was made to quantify distributive mixing in terms of lineal stretch ratio and dispersive mixing in terms of the relative rates of material element deformation and rotation. The nature and local efficiency of mixing were found to be dependent on fluid rheology. |
