The effect of turbulence on the minimum thickness of a liquid film flowing down a vertical tube

Abstract

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 100⁰C water flowing down a 6mm 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 90⁰, the minimum film thickness increased by 22% above the laminar value and the corresponding minimum flowrate increased by 75%. The model presented in this paper can be used to calculate the thermal performance of falling film evaporators over the whole range of flowrate.