Page 235 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

206 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

1.0 0.8
(Tw–Tb)/(Tw–Tb)max 0.6 P/D=1.1 (Tw–Tb)/(Tw–Tb)max (–) 1.0 P/D =1.1
0.8
0.6
0.4
0.4
P/D =1.2
P/D=1.2
0.2
0.0
0.0 P/D=1.3 0.2 P/D =1.3
0 5 10 15 20 25 30 0 5 10 15 20 25 30 35 40 45
(A) Angle (°) (B) Angle (°)
Fig. 5.13 Circumferential distribution of rod surface temperature. (A) Hexagonal fuel
assembly; (B) square fuel assembly.

diameter ratio of 1.3, the nonuniformity of the local heat-transfer coefficient is about
15%. The nonuniformity is much stronger in a square lattice than that in a triangular
lattice. At a pitch-to-diameter ratio of 1.3, the nonuniformity in a square lattice is still
as large as 40%.
In fuel assembly design, engineering correlations are usually applied, which give
only the overall average heat-transfer coefficient over the entire rod surface, and do
not take into account the nonuniformity of the local heat transfer. This average
approach will result in a large discrepancy for fuel assemblies with tight lattice.
The average approach can be well applied to cases with a large pitch-to-diameter ratio,
for example, P/D >1.3 for triangular lattices or P/D >1.6 for square lattices, where
the nonuniformity is <10%. Thus, for tight or semitight fuel assemblies, as in liquid-
metal-cooled reactors, the nonuniform distribution of the circumferential heat transfer
needs to be considered in the SCTH codes.
Yang et al. (2013) carried out theoretical and numerical investigation on the non-
uniform heat-transfer distribution. Starting from the conservation equations and intro-
ducing simplified assumptions, analytic solution of the conservation equations is
obtained, which show that the local heat-transfer coefficient depends on seven dimen-
sionless numbers, that is,

α P R f λ f λ f
¼ f θ, Re, Pr, , , , (5.60)
α D R cd λ cd λ c

After then, CFD analysis was carried out on three typical kinds of subchannels, that is,
interior subchannel, edge subchannel, and corner subchannel in regular rectangular
and hexagonal fuel assemblies. The CFD results show that the most important param-
eters affecting the local heat transfer are the angle θ, pitch-to-diameter ratio P/D,
Prandtl number, and thermal conductivity ratio of cladding to coolant. Based on
the theoretical analysis and CFD simulation, the following correlations are proposed
for the rectangular rod bundles: