Influence of Boundary Conditions on CFD Simulation of Gas- particle Hydrodynamics in a Conical Fluidized Bed Unit

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The hydrodynamics of gas-particle conical fluidized bed containing dried TiO2 micro-size particles were studied experimentally and computationally. The pressure drop was studied by pressure measurements and mean solid velocity in the different axial and radial positions and their experimental values were measured by a parallel 3-fiber optical probe. The Eulerian-Eulerian multiphase model, including kinetic theory of granular flow using Gidaspow (1994) drag function was used in the computational fluid dynamics (CFD) simulation. The frictional viscosity of solids is predicted from the equation proposed by Schaeffer (1987), whereas semi-empirical boundary equations developed by Johnson and Jackson (1987) were applied for the tangential velocity and granular temperature of the solids at the wall. Three different types of boundary conditions (BC), consisting in free-slip/no-friction, no-slip/friction and high-slip/small-friction, were used in this work to compare the results of the model with the experimental data. The numerical predictions using free-slip/no-friction BC agreed reasonably well with the experimental pressure drop measurements, especially at superficial gas velocities higher than the minimum fluidization velocity, Umf. The results for simulated mean axial solid velocity show that the free-slip BC was in better agreement with the experimental data compared with the other boundary conditions. The model also provides reasonable predictions for particle circulation and formation of fountain zone, gulf-effect and air velocity.