The effect of numerical diffusion and the influence of computational grid over gas-solid two-phase flow in a bubbling fluidized bed

The numerical diffusion effects appear due to the discretization process of the convective terms of the transport equations. This phenomenon takes place also in the numerical simulation of gas-solid two-phase flows in bubbling fluidized beds (BFB). In the present work a comparative analysis of the numerical results obtained using two interpolation schemes for convective terms, namely FOUP (First Order UPwind) and a high order scheme (Superbee) is presented. The equations are derived by considering the Eulerian-Eulerian gas-solid two-fluid model and the kinetic theory of granular flows (KTGF) for modeling solid phase constitutive equations. For that purpose the MFIX (Multiphase Flow with Interphase eXchanges) code developed at NETL (National Energy Technology Laboratory, US Department of Energy) is used. The numerical diffusion is analyzed by considering a single bubbling detachment and its hydrodynamic process in a two-dimensional BFB. The bubble shape is used as a metric for the description of the results. The influence of the computational grid is also analyzed. It is concluded that the Superbee scheme produces better results and this scheme is recommended for discretizations of the convective terms in coarse grids. The FOUP scheme can be used only in fine grids but it requires a high computational effort. In this study it is also verified that the analysis about estimating uncertainty in grid refinement can be applied in specific points of the grid when a monotonic convergence in time and space occurs.

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