Flow characteristics in vertical film flow




Types of evaporators

Working Principles


Residence time

Heat Transfer

Liquid distribution


The liquid film can be observed in different hydrodynamic conditions. This conditions are characterised by the so called film Reynolds number, which is determined as follows:

 Re(film) = 4 G / h

G = mass flow per circumference 

h = fluid viscosity 

When comparing data in literature care has to be taken what kind of Re(film) definition is used


  • pure laminar
    This flow condition can hardly ever  be encountered in technical processes. In general pure laminar film flow is present in case the Film Reynolds number is below 30. Only in very viscous flows this flow condition can be encountered. But even than  in Literature it is mentioned that wavy behaviour was observed!.

Falling Film laminar flow
Filmflow ati ReFilm= 10. Already wavy laminar behaviour

Under this flow conditions the Nusselt law for film thickness and subsequent heat transfer can be applied. This means Heat transfer increased with thinner liquid films and therefor smaller Film Reynolds numbers

  • laminar wavy
    From a certain mass flow first superficial waves can be observed. They are characterised by a faster velocity compared to the average velocity of the sub layer. The thickness of a wavy laminar fluid film is reduced compared to a pure laminar film. Smaller average film thickness and increased partial turbulence yield a higher heat transfer compared to pure laminar flow conditions.

Falling Film wavy laminar
Film flow at ReFilm= 500

  • turbulent
    Apart from the near to the wall laminar sub layer the flow is fully turbulent. In this region heat transfer increases with increased turbulence which means with increased Reynolds number

Falling Film turbulent
Filmflow at ReFilm= 5000, fully turbulent