Aggregation In Homogeneous, Isotropic Turbulence

Aggregation in homogeneous, isotropic turbulence

Aggregation of solid particles suspended in a carrier fluid is an important particle growth mechanism. In many cases it is an unwanted phenomenon as it interferes negatively with processes designed for generating particle size distributions based on nucleation and growth. In other cases aggregation is desired as it for instance improves the filterability of a slurry. Aggregation and flow dynamics are tightly connected. For aggregation events to occur, particles need to collide and stick together. For the bigger particles we are interested in, collisions are mainly brought about by velocity gradients in the flow field the particles are suspended in (orthokinetic aggregation); not so much by Brownian motion. The same velocity gradients that bring together particles also exert mechanical forces on aggregates that could break them again. We study this dynamic process of the generation of aggregate size distributions (ASD's) by means of numerical simulations that fully resolve the fluid and particle dynamics. Turbulence with well-defined properties is created by means of linear forcing in a fully periodic, three dimensional domain. In the domain we release uniformly sized spherical primary particles that we make sticky by letting them interact through a square-well potential. We study the ASD and its dynamics as a function of the strength of the turbulence, and as a function of the strength of the square-well potential. The figure shows some impressions of the simulations. In the left panel all particles in the domain are shown; they are colored by the size of the aggregate they are part of (red: small; blue: big). The right panel shows the four biggest aggregate at the same moment in time (each aggregate has a different color; the periodic conditions show in the sense that the red aggregate is connected through the side boundaries).