Environmental Engineering Reference
In-Depth Information
accompany the evaporation of a liquid film. Such a film can flow on a vertical wall [3, 4], or
on an inclined wall [5, 6, 7]. The wall itself can present a porous surface [8, 9, 10]. Most
works deal with evaporation in a free medium [15, 16] or a semi-confined medium like a
canal [11, 12]. However the works dealing the evaporation of a thin film in a closed cavity are
rare. In the latter case, the yield of distillation depends both on evaporation and condensation.
To improve the yield of this process, we need a better understanding of competition between
the transfer phenomena that accompany distillation in a cavity and of the manner according to
which the energy is distributed during the operation of the cell.
The complexity of the studied process and of the occurring phenomena needs the use of a
combination between experience and modeling. In a previous work [14], a model describing
the behavior of the distillation cell where the parameters influencing its yield are incorporated
has been produced. In the present work, an analysis of the experimental results from the
energetic point of view has been performed. Particular care is set on the distribution of the
energy to better explain the yield of the process.
Position of the Problem
The cavity is a parallelepiped of small width b , formed by an adiabatic of height h, of
length l , closed on the two main sides by two vertical plates, distant of b . The plates are
square, identical and of dimensions: 0.5m x 0.5m. On the internal face of the plate heated
from the outside by a constant heat flux density q f , a very thin film of water flows, of
thickness δ (y) varying with the height y , entering with a mass flow rate m feed and exiting
with a mass flow rate m out . These mass flow rates refer to the surface area of the heated wall.
The opposite plate is maintained at a constant temperature T c allowing the condensation on its
internal face.
Experimental Setup and Measurement Device
An experimental setup including a distillation cell, separate devices for heating, cooling
for the condensation plate, and water feeding has been performed. The distillation cell is a
parallelepiped cavity of form factor H = h/b =10, of dimensions: l = 50 cm, h = 50 cm and b
= 5 cm. The active walls of the cell, i.e. the vertical walls between which the heat and mass
exchange take place, are two square stainless steel plates of thickness 1 cm, of side 50 cm.
These walls are distant of 5 cm and are separated by a thick perspex frame to reduce the side
heat losses. The wall supporting the film is provided with the heating system. A resistance cut
in a carbon sheet allows us to distribute the flux imposed at the wall. A water cooling system
has been machined in the opposite wall to control its temperature and ensure its cooling. To
realize a thin falling film, a very thin fabric with meshes of very low dimensions has been
spread on the heated wall. When wetted, this fabric adheres to the wall by capillarity.
Furthermore, this fabric presents the advantage to reduce the thermal resistance at the contact
of the wall surface. The water feeding is provided by a tube perforated along a generating line
and linked to a constant level tank. The feeding tube placed at the top of the cell is surrounded
by the fabric in order to form the liquid film.
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