Analysis of the radial, axial and tangential porosity variations of packed beds of spheres

Abstract

The porosity of a packed bed of spherically shaped particles influences different thermo-hydraulic phenomena in such packings. An understanding of the porous structures in such packings is therefore important for the design, analysis, operation, and future improvement of pebble bed reactors. The objective of this study is to address the analysis of the radial, axial and tangential variations of the porosity of packed pebble beds which contain spherical particles. In this dissertation, an evaluation of the distribution of porosity of a model of the HTR-10 packed bed reactor was done by generating a Discrete Element Method (DEM) model and calculating the porosity through semi-analytical methodologies. The validity of the model was evaluated by altering selected aspects of the DEM approach and comparing these to relevant results. Through this, an ideal DEM approach was determined from which the overall porosity, as well as porosity in three different sections of the packed bed, were determined. Correlations of the axial variation in porosity in the packed bed sections were also derived. Dividing the packed bed into 34 separate regions, allowed for a piece wise constant description of the porosity distribution in the packed bed to be provided along with the average porosity in each of these regions. A vertical line-based methodology was developed to calculate the porosity of packed beds containing spherical particles. This allowed for the determination of the overall porosity, angular variation in porosity, and the tangentially averaged radial variation in porosity. The results obtained were validated through comparison with existing methodologies. This vertical line-based methodology was shown to have the capability of providing a detailed analysis of a packed bed with spherical particles. By dividing the packed bed into sections of axial heights, radial ranges and angular ranges a 3D variation in porosity throughout a packed bed can be obtained.