CFD simulations of expanding/contracting homogeneous fluidized beds and their transition to bubbling

This work presents a new Eulerian–Eulerian fluid dynamic model for monodisperse suspensions of solid particles fluidized by Newtonian incompressible fluids. The closure relationships for the fluid–particle interaction force distinguish the proposed equations of motion. The force consists of buoyancy, local fluid acceleration, drag and elastic forces. The closures of Mazzei and Lettieri [2007. A drag force closure for uniformly-dispersed fluidized suspensions. Chemical Engineering Science 62, 6142] based on the Richardson and Zaki [1954. Sedimentation and fluidization: part I. Transactions of the Institution of Chemical Engineers 32, 35] empirical correlation, and Mazzei et al. [2006. A revised mono-dimensional particle bed model for fluidized beds. Chemical Engineering Science 61, 1958], generalized for multidimensional applications, model the drag and elastic forces, respectively. For the first time these relations are solved in a computational fluid dynamic code. To test the drag force closure, we evaluate the steady-state expansion profiles of liquid-fluidized systems computationally, and compare them with those obtained using the closures of Wen and Yu [1966. Mechanics of fluidization. Chemical Engineering Progress Symposium Series 62, 100] and Ergun [1952. Fluid flow through packed columns. Chemical Engineering Progress 48, 89] and with experimental data. We successively analyze the behavior of uniform systems subjected to sudden changes in fluid flux, and compare the computational results with the theoretical predictions of the mechanistic one-dimensional model of Gibilaro et al. [1984. A simple mechanistic description of the unsteady-state expansion of liquid-fluidized beds. Chemical Engineering Science 39, 607]. To conclude the work, we investigate the stability of homogeneous gas-fluidized suspensions and the transition from the particulate to the aggregative state. Analytical and computational results (obtained by linear stability analysis and by integration of the equations of motion, respectively) are compared with experimental findings. The dynamics predicted are described and discussed; considerations on the meaning of fluid dynamic stability are debated.