Multilayer deposition and resuspension experiments of graphite particles between periodic steps in turbulent channel flows

This report describes an experimental study about graphite particle multilayer deposition and resuspension between periodic steps in turbulent channel flows. Background motivation is the carbonaceous dust safety issue in the High Temperature Reactor. In the present study the particle deposition and resuspension scenarios were fluid dynamically downscaled and were reproduced under simplified conditions in an air-driven small-scale test facility. Periodic steps were placed in a square duct test section to induce a complex flow field characterized by flow separations, recirculation and reattachment zones.
The turbulent flow field was recorded by means of a stereo Particle Image Velocimetry system. Polydisperse graphite particles matching the size range of HTR dust were dispersed into the flow field during a pure deposition regime. The multilayer build up between the periodic steps was measured using a laser distance sensor coupled with a linear stage. The particle layer build up showed a linear tendency against time and the deposition velocities are in good agreement with similar investigations. After the deposition scenario the aerosol generator was switched off and the fan speed of the test facility was stepwise increased to initiate particle resuspension. It was found that the resuspension started to begin in the reattachment area between the steps. The entire multilayer was already removed by the flow at friction velocities at which single particle resuspension starts to begin. Furthermore, particle bed porosity determinations were performed by gravimetrical and x-ray methods to characterize the packing properties. The present results indicate similarities between the single and the multilayer particle deposition and resuspension process.