It is noted that, geometric characteristics of corrugations are the
same for all corrugated tubes. However, depending on tubes arrangements
and corrugations type (convex or concave) each
arrangement (see Fig. 4) can create a particular streamlines and
pathlines along the annular space between the two tubes. Obviously,
these different streamlines cause various amounts of Nusselt
number and other thermal or frictional characteristics. This phenomenon
is shown schematically in Fig. 6. An example view of the
water streamlines in the annulus is shown in Fig. 6 for two different
arrangements of corrugated tubes. Indeed, in Fig. 6(a), both of the
point A and B are convex, but in Fig. 6(b) point A is convex and point
B is concave.