Analysis of the pressure drop through structured packed beds

Abstract

Accurately analysing the pressure drop over structured packed beds is of great importance to certain reactor designs, such as the Pebble Bed Modular Reactor (PBMR). In previous studies, the High Pressure Test Unit (HPTU) was used to conduct pressure drop experiments for three structured packed beds with separate porosities, designated as the Pressure Drop Test Section (PDTS) experiments, to aid in the design of the PBMR. However, conducting physical experiments to analyse the pressure drop over packed beds is often too expensive and time-consuming. Computational Fluid Dynamics (CFD) offers the possibility to accurately simulate the flow and analyse the pressure drop over structured packed beds. CFD was used to validate the PDTS experiments. Previous attempts at using Computational Fluid Dynamics to simulate the flow and analyse the pressure drop over structured packed beds have not produced methods which can be universally applied. Therefore, the successful simulation of the experiments in this study will require unique methods. The structured packed beds of the HPTU were replicated using a Computer Aided Design program, and the flow over the beds were simulated using CFD. SolidWorks (2016) was the CAD-program used to generate the packed beds, and Star-CCM+ was used to simulate the flow explicitly. The work done in this study is an extension of preliminary work completed by previous studies. A residual analysis was conducted to establish favourable simulation settings that will reduce the residuals of the momentum, energy and continuity equations to obtain converged solutions. A mesh dependence study was conducted to determine the influence of the mesh fineness on the pressure drop over the bed. Furthermore, the influence of prism layers and the thickness of the prism layers on the pressure drop was also investigated. These investigations formed part of a methodology development. The validity of the methodology was determined by comparing the results of the simulations with experimental results. It was found that it was not possible to accurately simulate the flow over structured packed beds for all required flow conditions, but accurate results were generated for laminar and slightly turbulent flow conditions. The methodology which was developed during this study could aid in future attempts to simulate the flow of a working fluid over a structured packed bed. The thickness and number of the prism layers was also shown to have an influence on the pressure drop. It was also proven that the HPTU experiments conducted on the Pressure Drop Test Sections can be successfully modelled using CFD.