259333 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.