(354b) Modeling of Particle-Particle and Particle-Wall Contact Points in the 3D CFD Simulation of Transport in Fixed Beds of Spheres

The use of computational fluid dynamics (CFD) as a tool for obtaining detailed flow and heat and mass transport information in packed beds is growing, despite the apparent intractability of the geometry of the packing. In particular, the simulation by CFD of fixed beds at higher particle Reynolds number (Re > 1000) and low tube-to-particle diameter ratio (2 < N < 8) is of interest as highly-exothermic partial oxidation reactors and highly-endothermic steam reforming reactors operate under these conditions. Recently, studies that presented simulations of beds of a large number of particles (usually between 100 – 1000 spheres) have appeared in the literature, for flow and heat transfer simulations.

            In the development of meshes for CFD simulations of transport in fixed beds of spheres, particle-particle and wall-particle contact points often present difficulties. This paper presents a comparison between four methods of dealing with contact points in CFD simulations of both fluid flow and heat transfer in fixed beds of spheres, at commercially-relevant flow rates. These methods represent unselective (global) and selective (local) geometric manipulation of the spherical particles through either reducing or enlarging them. We give results for drag coefficient (C_D) and heat flow (Q) for flow past sphere-sphere and wall-sphere contact points, focusing on higher flow rates typical of industrial steam reformers (500 < Re < 10,000). Global methods, in which all particles in a bed are either shrunk or enlarged uniformly, change bed voidage giving erroneous results for C_D. Local methods, in which bridges are inserted or spherical caps are removed only at the points of contact, give much better results for C_D. The bridges approach is preferable for heat transfer, as fluid gaps reduce heat transfer too much, and particle overlaps increase it. A set of graphs is presented to allow estimation of the error introduced by the various methods of dealing with contact points. Examples are presented of the application of these methods to fixed bed transport simulations.