Similarities between single and multilayer particle deposition and resuspension experiments in turbulent flows

This report is a comparative review of several particle transport experiments, which were performed within the framework of the two large scale European projects namely THINS (Thermal Hydraulics of Innovative Nuclear Systems) and ARCHER (Advanced Reactor for Cogeneration of Heat & Electricity R&d). Background motivation is the carbonaceous dust issue of the High Temperature Reactor (HTR). Radio-contaminated dust was found deposited all over the inner surfaces in the primary circuit of different HTR research reactors after a certain operational time. In case of an accidental scenario such as the depressurisation of the primary circuit the dust may be remobilised by the turbulent flow and becomes a considerable source term when escaping the system boundaries. The safety assessment requires a fundamental understanding of the interaction between the wall-deposited particle multilayer and the turbulent flow field. Thus, we performed a set of experiments to explore the particle deposition and resuspension characteristics in various geometries by means of high-end measurement instrumentation.
Two air-driven small-scale test facilities were designed to generate a particle laden turbulent flow field under well-defined conditions. The flow was downscaled using Reynolds similarity and the aerosol particles were described by its aerodynamic mobility. The first facility was designed for the THINS project and is called Gas Particle Loop. Here, the particle deposition and resuspension was studied in a horizontal turbulent duct flow. The flow field was captured by means of a stereoscopic Particle Image Velocimetry system. The airborne particles were classified using Aerodynamic Particle Sizer spectrometry and the wall deposited particles were captured by means of optical microscopy and laser distance sensors. It was found that the deposition of micron-sized particles mainly depends on the aerodynamic particle size and the fluid friction velocity. Particle resuspension was found to take place in intermittent events once a critical friction velocity was exceeded. The second facility was constructed for the ARCHER project and is called Pebble Bed Loop. Here, a particle laden turbulent flow in a pebble bed was generated. The aerosol particles were radio-labelled with the isotope fluorine 18 and the spatiotemporal distribution of the particle deposits in the pebble bed was recorded by means of positron emission tomography (PET) during various flow regimes. Assuming a homogeneous distribution of the initial activity over the particles, the activity recorded by the PET scanner directly correlates to the amount of deposited particles. The geometry of the pebble bed was captured by means of a gamma ray computed tomography (CT) scan. The result of this campaign is a time resolved 3D PET-CET overlay. Basic characteristics of the particle deposition and resuspension behaviour in the GPLoop were relocated in the results of the experiments in the PBLoop. The available data set provides a new insight into the particle transport behaviour in turbulent flows and may be used for the development of numerical methods for the prediction of the dust behaviour during accidental scenarios of HTRs.