Axial Dispersion and Mixing Phenomena of the Gas Phase in a Packed Pebble-Bed Reactor


The knowledge and proper analyses of axial dispersion and the mixing phenomena of the coolant gas flow in the dynamic core of pebble-bed nuclear reactors are useful for safe design and efficient operation of these reactors. These processes can be characterized in terms of the residence time distribution and quantified in terms of the axial dispersion coefficient. Therefore, in this work, the axial dispersion coefficients of the gas phase and their residence time distributions were measured experimentally in a separate effect pilot-plant scale and cold-flow experimental setup of 0.3 m in diameter, using a sophisticated and advanced gaseous tracer technique. The non-ideal flow of the gas phase in the pebble bed was described using the axial dispersion model (ADM). The effect of the gas velocity on the axial dispersion was investigated using a range of velocities from 0.01 to 2 m/s, covering both the laminar and turbulent flow regimes. The effect of the bed structure (pebble size) on the axial dispersion coefficient was investigated, and the results indicate that the pebble size strongly affects axial dispersion and mixing in the packed pebble-bed reactor. The results show that the flow pattern of the gas phase does not deviate much from the idealized plug-flow condition at high flow rate, which depends on the gas flow rate and the bed structure of the pebble-bed. The present work provides insight on the extent of mixing and dispersion in the gas phase in the studied bed using an advanced gas dynamics technique and methodology that properly accounts for the external dispersion.


Chemical and Biochemical Engineering

Research Center/Lab(s)

Center for High Performance Computing Research

Keywords and Phrases

Boltzmann Equation; Dispersions; Flow of Gases; Flow Patterns; Gas Dynamics; Gases; Mixing; Molten Materials; Nuclear Reactors; Pebble Bed Reactors; Pilot Plants; Pipe Flow; Rotating Disks; Axial Dispersion Coefficients; Axial Dispersion Model; Axial Dispersions; Gas Tracers; Laminar and Turbulent Flow; Mixing and Dispersions; Mixing Process; Pebble Beds; Residence Time Distribution

International Standard Serial Number (ISSN)


Document Type

Article - Journal

Document Version


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© 2016 Elsevier Ltd, All rights reserved.

Publication Date

01 Feb 2016