(361f) Solids Suspension Simulations At Multiple Scales

Solids suspension simulations at multiple scales

Solids suspension in mixing tanks is a multi-scale process
with the tank size as the macro-scale and the particle size (or the
interparticle spacing) as micro-scale. Resolving all scales in a single
simulation is not doable given the usually large width of the length scale
spectrum. Simulation approaches to the suspension process recognize this and
parameterize part of the length-scales range: In Euler-Euler approaches
individual particles are not considered and the focus is on the global (turbulent)
flow characteristics and the distribution of the phases in the tank. Euler-Lagrange
simulations do consider individual solid particles but not the details of the
flow around them. Simulations that do resolve the flow around particles (?particle-resolved
simulations') are feasible but have severe limits in terms of the number of
particles and the sizes of the systems to be simulated (certainly much smaller sizes
than even a lab-scale mixing tank). In this paper we suggest strategies how to relate
simulations at different levels to enhance our understanding of the solids
suspension process. The emphasis will be on what happens near the bottom of a
tank in a not completely suspended situation. Here particles forming a granular
bed on the bottom get entrained by the flow generated by the impeller. As an
example, the figure shows a particle-resolved simulation of the start-up of
this entrainment process in a miniature mixing tank.

Figure. Start-up of the suspension process for monosized
spherical particles. Realizations after (from left to right) 2, 4 and 32
impeller revolutions.