Comparison of The Wake in Free Stream and The Wake in Confined Open Channel of Nuclear Reactor Fuel Assembly

We empirically compare the wake of a streamwise-aligned cylinder-a short cylinder facing the flow (i.e. cylinder axis parallel to the flow direction) - placed in both: the free stream of a wind tunnel and in the confined channel between fuel rods of nuclear fuel assembly for pressurized water reactors VVER (i.e. with triangular symmetry of the rod arrangement). This kind of channel is special as the walls are concave (in classical pipe-flow, the walls are convex), and the channel is open to the neighboring channels. Stereo Particle Image Velocimetry (PIV) is applied to a plane perpendicular to the flow in three positions (from 2.3D to 11.3D,D is the cylinder diameter) downstream of the gird holding the obstacle alone, or the fuel rods as well. As expected, the wake is completely different due to the different topology of the flow domain. When we compare the confined wake (with rods) to the unconfined wake (no rods), then the following observations are made: at early stage, the velocity deficit is deeper; later (since z= 4.5D ) it is shallower. The area directly influenced is smaller at all stages. Fluctuations display larger values at all distances. The skewness of stream-wise velocity distribution shows a ring of negative skewness, but this ring is much weaker and it disappears completely at last explored distance (z = 11.3D ). This is the most pronounced difference, which seems to be of principal importance. Future investigations (experiments & data analysis) are needed.