(131d) Multiscale Models for Use in Wet Gas-Solid Fluidized Beds

Authors: 
Girardi, M., Princeton University
Radl, S., Graz University of Technology
Sundaresan, S., Princeton University

Simulations provide a convenient approach for the study of multiphase fluidized beds due to their ability to probe flow details, such as individual particle velocities and solid volume fraction profiles, that are difficult to measure in experiments.  Yet, one shortcoming of simulation studies is their ability to model flow behavior at the large length scales necessary to study industrial systems.  To advance beyond this limitation, filtering methods may be employed that bridge the gap between particle scale and industrial scale simulations.  While multiscale modeling and filtering methods have been developed for fluidized beds of monodisperse and non-cohesive particles (Agrawal 2001, Igci 2008, Van der Hoef 2008, Milioli 2013), progress is limited in beds with wet particles, which are important in pharmaceutical and energy industries.

Our primary objective is to develop an effective drag force model for use in large-scale simulations of wet gas-fluidized beds.  We have performed well-resolved Euler-Lagrange simulations (commonly referred to as CFD-DEM) of gas-particle fluidization in small periodic domains.  The particles are assumed to be coated by a thin liquid film, and so, when they collide, a pendular liquid bridge forms that provides a cohesive force.  In this way, we permit for agglomeration formation as well as breakup.  The primary means by which particles (and agglomerates of particles) are suspended in the bed is through the drag force exerted by the gas.  By systematically filtering the results from such simulations, we determine the filtered drag coefficient and develop a model for it as a function of filter size, solid volume fraction, and liquid properties (specifically, we consider a Bond number and a dimensionless liquid loading level).  Extrapolation allows us to estimate the asymptotic limit of the filtered drag coefficient for large filter sizes.  We then perform CFD-DEM simulations with coarse grids using the filtered drag force models and compare the results with the coarse features of well-resolved CFD-DEM simulations.  The details of this comparison will be described in this presentation.

Agrawal, K., Loezos, P. N., Syamlal, M., & Sundaresan, S. (2001). The role of meso-scale structures in rapid gas–solid flows. Journal of Fluid Mechanics, 445, 151–185.

Igci, Y., Andrews, A. T., Sundaresan, S., & O’Brien, T. (2008). Filtered Two-Fluid Models for Fluidized Gas-Particle Suspensions. AIChE Journal, 54(6), 1431–1448.

Milioli, C. C., Milioli, F. E., Holloway, W., Agrawal, K., & Sundaresan, S. (2013). Filtered Two-Fluid Models of Fluidized Gas-Particle Flows : New Constitutive Relations. AIChE Journal, 59(9), 3265–3275.

Van der Hoef, M. a., Van Sint Annaland, M., Deen, N. G., & Kuipers, J. a. M. (2008). Numerical Simulation of Dense Gas-Solid Fluidized Beds: A Multiscale Modeling Strategy. Annual Review of Fluid Mechanics, 40(1), 47–70.