Its distribution along the curvilinear coordinate is reported in Figs. 4 and 5 for both the smooth wall and the corrugated wall
pipes for the two case d = 0.031 and d = 0.056 respectively. As the fluid enters the coiled section, it starts to be affected by the centrifugal force due to the curvature of the tube and therefore the fluid is colder towards the outside wall where the axial velocity is expected to be maximum.