Similar to the presentation of predicted fw, Fig. 5 shows that the
Nusselt (Nu) number distributions for the FLiNaK salt along the
axial direction under various values of Rein number. Near the inlet
region (x=D < w10), a significant decrease in the Nu number
occurs, which is essentially caused by the development of
temperature profile from the uniform inlet profile. Similar to the
definition of Lh, the thermal entrance length (Lth) can be defined as
the minimum pipe length required to meet the criterion of
jðNu NuNÞ=NuNj < 1%. Based on the predicted results shown in
Fig. 5, Lth is about 7D. It is worth noting that Lth is predicted to be
independent of the Rein number, which is similar to the Sparrow’s
analytical result [30] for Pr ¼ 10 and 100. As the salt passes
downstream, the thermal condition becomes to be fully developed.
Fig. 5 also indicates that the fully-developed Nu (NuN) number
increases with the increasing Rein number.
Because of their high Pr number, the thermal behavior of molten
salts has been examined experimentally in a number of previous
studies [8e17]. However, it is difficult and expensive to directly
perform such experiments for molten salts, making CFD simulations
particularly important for predicting their thermal characteristics.
Fig. 6 shows a comparison of NuN number predicted by the
present CFD predictions with the experimental data reported by