CFD MODELING OF CFB: FROM KINETIC THEORY TO TURBULENCE, HEAT TRANSFER, AND POLY-DISPERSED SYSTEMS

摘要

In the last three decades, a number of two-and three-dimensional multiphase CFD codes were developed.They solve Navier-Stokes equations for each phase, with coupling through the drag.Flow regimes for bubbling beds and risers have been successfully computed.Granular flow kinetic theory gives theoretical equations of state for each phase and transport properties, such as particulate viscosities that have been measured experimentally.The kinetic theory explains the existence of the undesirable core-annular flow in small and large FCC risers.In some situations, the standard drag model needs to be modified to account for the formation of clusters.The energy minimization multi-scale (EMMS) approach, which minimizes the energy, is being actively pursued by a number of research groups.Experiments show that, for dense bubbling flow, and probably for dense riser flow, the turbulent granular temperatures exceed the laminar granular temperatures predicted by kinetic theory.However, the CFD codes are able to compute the turbulent properties, such as the turbulent viscosities and turbulent thermal conductivities.In view of this, it is now possible to obtain innovative designs for multiphase reactors, such as those needed to capture CO2 from flue gases.To account for distribution and variation in particulate phase properties caused by particulate processes such as agglomeration, breakage, and chemical reactions, the population balance equation (PBE) coupled with CFD should be used.The coupling algorithm and numerical method of moments will be discussed along with a CO2 sorption case in a riser section of a circulating fluidized bed (CFB).