The flow visualization in the complicated flow geometry of the pebble bed of the high temperature gas-cooled reactor is investigated to identify the stagnation points at which internal hot spots are expected. A particle image velocimetry method was employed to visualize flow for the pebble bed in the structure of the face centered cubic. The wind tunnel was designed to provide the same Reynolds number of $2.1614×104$ as the pebble bed nuclear reactor. Scaling law determined the diameter of the pebble as 120 mm, which is two times bigger than the reference when we use air as a coolant rather than helium. The present scaled up design reduces the load of high speed imaged acquisition and the flow field measured by 4000 frames/s. It was found that the present method identified flow field successfully, including the stagnation points suspected to produce hot spots on the surface of the pebble bed. The present data are useful in evaluating the three-dimensional computational fluid dynamics analysis.

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