In the present study, to discern the complex flow structures in a sphere-packed pipe (SPP), digital holographic PTV visualization is carried out by a refractive index-matching method using a sodium iodide (Nal) 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, with the result that the velocity in the other region can contribute to the thermal enhancement in thin thermal layer such as in the case of high Pr number fluid flow.
