Determination of a Critical Length Scale for Clustering Instability in Granular Systems

In sharp contrast to familiar molecular fluids, granular fluids composed of inelastic particles are unstable to the formation of macroscopic density inhomogeneities known as clusters. Theses instabilities are of particular engineering interest in fluidized beds, such as those found in coal and biomass gasifiers, where they decrease gas-solid contact area and, subsequently, impact conversion. Clusters are characterized by a critical length scale which defines the system size at which clusters will first begin to appear. In order to quantify a critical length scale for clustering, molecular dynamics simulations are used. These simulations treat the granular particles as hard spheres enclosed in a cube with periodic, adiabatic boundaries. Preliminary simulation results are in good agreement with theoretical results generated by linear stability analyses of the hydrodynamic equations for granular flow. The long term application for the research discussed in this poster is the improvement of the gasification process through an enhanced knowledge of the clustering phenomenon. Although not truly representative of a gasfier, the simulations discussed in this poster are useful in obtaining a “basic principles” understanding of cluster formation and the system length scales required for their formation.