Page 225 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

196 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

a
f b ¼ n (5.17)
Re
The coefficient a and the exponent n are given by the code users.
For calculating the friction pressure drop in rod bundles with wire wraps, the cor-
relation of Rehme (1973a) is widely accepted and is expressed as below:

Δp f P b 1 ρ u 2 e
¼ f a (5.18)
Δz P t D h 2
Here, P b stands for the perimeter of the rod bundle without box, P t for the total perim-
eter including the box, and u e the effective velocity, which takes the sweeping effect of
the wire wraps into consideration and is expressed by

( ) 1=2
2:16
1=2 D W 2
ð
u e ¼ u ð P=DÞ +7:6 P=DÞ (5.19)
H
The modified friction factor f a is determined by
64 0:0816
f a ¼ + 0:133 (5.20)
Re
Re m
m
with

ρ u e D h
Re m ¼ (5.21)
μ
For rod bundles with spacer grids, which are considered as local hydraulic resistance,
the local pressure-drop coefficient is calculated by the correlation of Rehme (1973b):

ρ u 2
2
△p sg ¼ k s ε (5.22)
2
Here, ε stands for the blockage ratio. The coefficient k s depends slightly on Reynolds
number and has the value in the range 6–7.

5.2.2.2 Heat transfer
Heat transfer in liquid metals with low molecular Prandtl number is characterized by
the high contribution of the molecular conduction, compared with the conventional
fluids with large Prandtl number like water. In the open literature, a large number
of correlations are available (Cheng et al., 2006). Most of them take the expression as.

Nu ¼ a + b Pe n (5.23)