  When a liquid flows down a vertical tube at a low flowrate, it may not be
able to completely cover the surface as a film, and the flow will comprise a
number of rivulets separated by unwetted areas. Predictions of the minimum film
thickness and corresponding minimum flowrate below which films cannot be
sustained are needed in many industrial heat and mass transfer operations. In
this paper a model is developed using the minimum energy method to calculate
the minimum film thicknesses that can be maintained for flows down a vertical
tube. It is concluded that inclusion of the effect of turbulence in the model
can significantly increase the values of the calculated minimum film
thicknesses. In these calculations (for 100 deg C water flowing down a 6 mm
inside diameter tube) the effect of turbulence increased as the tube to liquid
contact angle increased. For a flow at the highest contact angle of 90 degrees,
the minimum film thickness increased by 22 % above the laminar value and the
corresponding minimum flowrate increased by 75 %. The results of the
calculations are compared to experimental data obtained from a falling film
evaporator.
