The mean residence time is determined by the mean film velocity and the tube length. Once the hydrodynamic velocity profile is fully developed the velocity profile in gravity controlled regimes is constant. The profile is determined by the balance of wall shear stress and gravity.
A typical residence time distribution can be seen in the picture below, which shows the normalised conductivity response by salt injected in the film flow as a function of time. The black curve shows the initial impulse at the top of the unit. The red curve reflects the response at the bottom this means tube exit.
Residence time distribution in turbulent film flow
Under turbulent film flow conditions the response signal reflects very much the initial impulse, indicating the the residence time of fluid particles in the film is very even. This is an importand fact when processing residence time sensitive products such as in food or polymer processing. A second fact is the meassured very short residence time of the liquid, proving that exposure to hot surfaces is short.
Residence time distribution in laminar / wavy film flow
In comparison to the turbulent case the response indicates hold up of liquid near to the tube wall. It is evidence that such liquid is exposed for a longer period of time to higher wall temperatures. Aim is to prevent such cases.