Experimental investigation of micro hotspots in pebble bed reactors using unobstructed flow visualization technique
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A typical packed bed is a column where a fluid is flowing through packed grain-like material. One important and particular application involves energy transfer as in the Advanced Gas Cooled Pebble Bed Reactors for nuclear power generation. In this case, a "fuel compact" is in the form of spherical coated particles and Helium is used as the coolant fluid. The actual models that describe fluid flow parameters inside the reactor are based on a macro-scale approach that does not accurately describe the flow behavior at every location in the reactor. Flow visualization and full velocity field measurements at the micro-scale level would help in the development of models able to describe fluid flow inside packed beds at pore level. Experimental techniques to date have not been able to give detailed local fluid flow and heat transfer properties through spherical pebble reactors. One of the difficulties using optical techniques such as Particle Image Velocimetry (PIV) in porous media is the need for optical access of the medium. In order to overcome this complexity, the solid particles used as packing material and the fluid flowing through the column can be chosen in such a way that their refractive indices are matched. In this way, the optical accessibility required by the flow visualization technique can be achieved. In this work, the measurements of velocity fields using Particle Tracking Velocimetry (PTV) in the midplane of a packed bed are presented. The use of Matching Refractive Index (MRI) methods allowed PTV measurements to be performed at the center of the packed bed square channel. The results in this investigation showed the velocity distribution at the pore scale level. Vortices were identified inside the pores between beads. Vorticity maps were calculated from the obtained velocity fields at the pore level. Higher vorticity zones were identified near the boundary of the packing material under certain pore configurations. Identification of vortices and their length scales can help in the understanding of heat transfer parameters, specifically, within the pore, where information extracted can be used to help understand the complex heat transfer phenomena at pore-scale and micro-hotspots due to local poor heat transfer.
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