where Dp is the predicted pressure difference and Dx is the corresponding
mesh distance. It is clearly revealed in this figure that fw
for the FLiNaK salt is predicted to decrease from the tube entrance
and approaches a fully-developed value that is dependent on the
Rein number. The decay trend of fw in the front region (x=D < 10) is
essentially resulted from the developing velocity profile of the salt
from its uniform value at the inlet. A detailed examination of Fig. 2
also shows that a transition region (10 < x=D < w40) for thefw
distribution exists between the developing and the fully-developed
flow regions. As the salt flows downstream, the fully-developed
condition is reached, causing fw to be invariant along the pipe.
The hydraulic entrance length (Lh) needed for the FLiNaK salt to
reach the fully-developed flow condition can also be obtained from
the fw distributions shown in Fig. 2.
Fig. 3 compares the entrance length obtained from the present
CFD predictions (dots) with the calculations from the correlation
given byx=D ¼ 4:4Re0:6 [27] (black line). In the present simulations,
the entrance length is determined as the location at which
the value of jðfw fw;NÞ=fw;Nj is less than 1%. Compared with the
correlation, the CFD model clearly overestimates the entrance