(112a) On the Effect of Frictional Stress Modeling in Dense Fluidized Beds for Minimum Fluidization Conditions and in Bubbling Regime

The Euler-Euler approach commonly used to simulate bubbling fluidized applies the kinetic theory of granular flow to model the stress of solid phase. This theory, understood as an extension of the kinetic theory of gases, assumed that particle-particle collisions were instantaneous and binary. However, in dense regimes such as encountered in bubbling fluidized bed, the frictional contacts between neighboring particles may have a strong influence. Johnson and Jackson [1] proposed to sum the stress derived by the kinetic theory to the stress provided by frictional stress model. Several practical models derived from the soils mechanics were proposed for modeling frictional stress [2-3].

In recent works, some authors [4-6] investigated the performance of frictional models [2-3] for Euler-Euler simulations of dense fluidized beds. They studied the bubbles size and shape, pressure drop, bed expansion and particles circulation. However, these models were found to be very sensitive to the chosen activation threshold.

In this paper, according to experimental data and numerical simulations of the transition from fluidized beds to fixed bed, a new method was proposed to determine the activation threshold of frictional models. The 3D numerical simulations by NEPTUNE_CFD code [7-8] were carried out on a large number of particle types to predict the minimum fluidization velocity and the bed expansion in the transition regime. It was demonstrated that the threshold value is not sensitive to the particle size but is strongly dependent on the shape and surface properties of the particles.

Then, hybrid approach frictional-kinetic was applied to simulate bubbling fluidized beds of particle Geldart group A powder known to be very sensitive to mesh refinement [9]. Very fine meshes of several million cells were used to demonstrate that a frictional model was needed to achieve a mesh convergence. Finally, the influence of the introduction of this frictional stress were evaluated on the macroscopic flow variables such as drop pressure and time averaged solid volume fraction, but also on the particle velocity and solid volume fraction time-averaged variances.

References

[1] P.C. Johnson, R. Jackson, Frictional-collisional constitutive relations for granular materials, with application to plane shearing, J. Fluid Mech. 176 (1987) 67–93.

[2] D.G. Schaeffer, Instability in the evolution equations describing incompressible granular flow, J. Differ. Equ. 66 (1987) 61–74.

[3] A. Srivastava, S. Sundaresan, Analysis of a frictional–kinetic model for gas–particle flow, Powder Technology. 129 (2003) 72–85.

[4] N. Reuge, L. Cadoret, C. Coufort-Saudejau, S. Pannala, M. Syamlal, B. Caussat, Multifluid Eulerian modeling of dense gas–solids fluidized bed hydrodynamics: Influence of the dissipation parameters, Chem. Eng. Sci. 63 (2008) 5540–5551.

[5] A. Passalacqua, L.A. Marmo, A critical comparison of frictional stress models applied to the simulation of bubbling fluidized beds, Chem. Eng. Sci. 64 (2009) 2795–2806.

[6] Farzaneh, M., Almstedt, A.-E., Johnsson, F., Pallarès, D., Sasic, S., The crucial role of frictional stress models for simulation of bubbling fluidized beds. Powder Technology 270, (2015) 68–82.

[7] Neau, H., Fede, P., Laviéville, J., Simonin, O.. High performance computing (HPC) for the fluidization of particle-laden reactive flows. In : The 14th International Conference on Fluidization, From Fundamentals to Products, Noordwijkerhout, Netherlands (2013).

[8] R. Ansart, P. Garcia Triñanes, B. Boissière, H. Benoit, J.P.K. Seville, O. Simonin. Dense gas article suspension upward flow used as heat transfer fluid in solar receiver : PEPT experiments and 3D numerical simulations. Powder Technology, Volume 307, (2017) 25-36.

[9] Agrawal, K., Loezos, P., Syamlal, M., Sundaresan, S., The role of mesoscales structures in rapid gas-solid flows. Journal of Fluid Mechanics 445, (2001) 151–185.