|In general Falling Film evaporators are designed to operated
under conditions were no nucleated boiling takes place. Heat from the tube
wall is transfered via convection or conduction through a liquid film to
the vapour-liquid interface were evaporation takes place. In the majority
of applications evaporation takes place on the tube side, with vapour
liquid separation at the bottom of the tube. The film heat transfer
coefficient depend on the hydrodynamic of the annular liquid film. The film
can be divided in pure laminar, laminar-wavy and full turbulent flow. (see
also flow characteristics) The
transition is characterised by the Film Reynolds number and the Kapitza
In laminar film flow, the heat transfer mechanism is one of conduction through the film. Indicting that with thinner films at lower circulations the heat transfer increased.
In this scenario the Nusselt solution for Film flows can be used:
Whereby the film Reynolds number is defined as follows:
G = mass flow per circumference
h = fluid viscosity
With increasing Reynolds number due to lower viscosity or higher mass flow the liquid film becomes wavy laminar, here according to Chun Seban the heat transfer can be calculated as follows:
Nu (film, wavy laminar) = 0.82 Re(film) (-0.22)
The waviness of the film adds turbulents to the flow and therefore compared to the pure laminar the heat transfer increases.
Nu (film, wavy laminar) = 0.0097 Re(film) (0.29) . 0.0038 Pr(film) (0.63)
- Heat transfer can be enhance in case vapour shear at the liquid vapour interface leads to more agitated thiner liquid films.
(1) Chun, K. R. and Seban, R. A., Heat transfer to evapaporating liquid films.Journal of Heat Transfer, 197 I, 93, 391-396
(2) Falling film evaporation of single component liquids
Inr. J. Hear Mass Transfer. Vol. 41, No. 12, pp. 1623-1632, 1998