Three-dimensional flow measurement of a sphere-packed pipe by a digital hologram and refractive index-matching method

Molten-salt used as a coolant in fusion reactors plays a significant role in the design of advanced reactors. Investigation of thermal behavior is necessary in an actual environment of a facility where heat transfer enhancement takes place under a high Pr number fluid flow such as in case of FLiBe. For the development of a facility, it is necessary to be able to monitor fluid motion of a basic heat transfer promoter such as a sphere-packed pipe (SPP). In the present study, to discern the complex flow structures in SPP, digital holographic PTV visualization is carried out by a refractive index-matching method using a sodium iodide (NaI) solution employed as a working fluid. Hologram fringe images of particles behind the spheres can be observed, and the particles’ positions can be reconstructed by a digital hologram. Consequently, 3-D velocity-fields around the spheres are obtained by the reconstructed particles’ positions. The velocity between pebbles is found to be faster than that in other regions.