Numerical investigation on a bimodal mixing system of solid–liquid mixture in an industrial mixing cooker

Abstract

The powder mixing for foods is regarded to be unique because tons of products are manufactured through powder mixing processes in food industries; hence there is no room for doubt about the importance of a better understanding of the mixing mechanism. On the other hand, the powder mixing for foods has not been established so far. In this study, a series of simulations of the powder mixing in a liquid are performed using the discrete element method–computational fluid dynamics method. A monodispersed and bimodal mixing systems are computed to evaluate the effect of the particle density and the mixing shaft obliquity in a particular mixing cooker. The monodispersed system was computed purely for a purpose of reference to compare with the bimodal system. By performing a simulation on a monodispersed and bimodal mixing systems with very small density differences between solids as well as solid and liquid, this study provides significant evidence and novel insights into understanding the mixing mechanism of food products. In the monodispersed mixing system, the particles with the density close to water are revealed to be affected by the diffusive mixing which is due to buoyancy and the drag force. Furthermore, an increase in mixing shaft obliquity is shown to improve mixing performance in the monodispersed mixing system. In the bimodal mixing system the effect of the gravitational force is shown to be extremely important for the efficient powder mixing, even if the density difference is very small as 5%. The heavier particles initially positioned in the upper layer are drawn down into the layer of the lighter particles by gravitational force, facilitating mixing and enhancing efficiency. These are new findings which showed unique particle behaviour in mixing systems. This study provides new insights into the specific powder mixing mechanism in a liquid which are frequently encountered in the food industries.