Page 233 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

204 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

τ
f M ¼ (5.58)
β G ij u i u j

where τ is deduced from CFD simulation:

∂u
0 0
τ ¼ ρ u v ¼ ρ ε M (5.59)
∂y
Flow scattering
Flow scattering is caused by the structure inserts in FAs, for example, spacer grids,
which disturb the flow structure and usually enlarge the turbulence strength of the
flow. Therefore, it is often treated in the same way as the turbulent mixing and taken
into consideration by modifying the turbulent mixing coefficient. The effect of flow
scattering depends on the structure of the channel insert itself, structure of the sub-
channels, and the thermal-hydraulic conditions. For each design condition, it should
be usually measured experimentally. Therefore, in SCTH codes, the user has to define
the effect of the flow scattering via the turbulent mixing coefficient.

Large scale oscillation
In LMRs, especially SFR, fuel assemblies have tight lattice arrangement. Experimen-
tal studies show that the transversal exchange in rod bundles with tight lattice has
completely different characteristics than that in a rod bundle with wide lattice. The
so-called flow pulsation or flow oscillation phenomenon was responsible for high
mixing in a tight lattice rod bundle, as presented in Fig. 5.11. Vortices are transported
in the longitudinal direction quasiperiodically with this oscillating flow. The interac-
tion between the transported vortices results in a gain in the momentum transfer and
increases in the mixing. The flow oscillation depends highly on the configuration of
the subchannels and was also reproduced both by experimental studies (Rehme, 1992)
and by numerical simulations using CFD (e.g., Yu and Cheng, 2012). Unfortunately,
the phenomenon of the large-scale oscillation is not considered in most SCTH analysis

Fig. 5.11 Flow oscillation in the gap.