UWE Bristol Engineering showcase 2015

Florian Raabe
MSc Aerospace Design Engineering
Project Supervisor
Dr. Chris Toomer
Analysis of the Condensation within the double skin of a commercial
airliner
Introduction
A common problem commercial airliners are
dealing with is the condensation within the double
skin during cruise at an altitude of 37,000ft. The
condensation occurs due to the relatively cold
temperatures of about -56.5°C on the outside
which lowers the temperature within this gap and
causes the moisture within the air to condense on
contact with the inner surface of the outer skin..
As the water now adds up to the total weight of
the aircraft as it sticks to the insulation blankets or
accumulates in certain parts within the double
skin, it consequently increases the fuel
consumption. To cope with this problem the
Swedish company CTT Systems finally came up
with a system called Zonal Drying System (ZDS),
which will be described in detail in the paper but
basically functions by removing moisture from the
air and blowing dried air into the double skin and
thus reduces the amount of condensation.
Due to the complexity of the geometry and the
relating difficulty to set the according simulations
up the evaluation of the condensation will be
approached step-by-step. In addition to this the
step-by-step approach will start off with a very
basic and simplified fuselage compartment in
order to verify the setup from Part A.
Subsequently the geometry will be refined as well
as investigating and integrating proper
condensation into the model. Finally it will also be
described what needs to be considered for setting
a run up for a reference flight. This is to not miss
out important things and to accurately capture the
variations in condensation due to a varying
temperature gradient across the gap of the double
skin. On top of that a few possible simplifications
that can be done to address computational issues
will be outlined.
Condensation
Due to the intention to monitor the physical
condensation going on within a geometry, there
have been different approaches :
1. Following a steam jet tutorial from ANSYS (not
suitable)
2. Applying heat and mass transfer models as
well as drag models upon three fluid pairs (not
suitable)
3. Wall condensation example supplied by ANSYS
support
The initial example provided by ANSYS is shown in
the picture below. It turned out that this example
is working and enables determination of the actual
water at the bottom wall but needed to be
modified to better match the conditions of the
problem analysed in this investigation.
Conclusion
Summarising this paper, the most important point
shown, is the fact that condensation is very
complex and there are numerous ways of
approaching this specific thermodynamic topic.
The final wall condensation example successfully
simulated condensation that can be analysed and
monitored by the mass of H2O at the bottom wall
due to the sink function. According to the
approaches to condensation conducted before, it
was shown that these cases have not been
completely wrong but simply do not represent the
given problem properly. Moreover, it is concluded
that one needs to be very careful when setting a
condensation case up, as there are numerous ways
to approach condensation.
Also reviewing the initial simulations with the aid
of approaching the condensation case contribute
to a better understanding of the processes going
on, even though, the used a homogeneous flow
field. Finally it can be said that the investigation
done in this paper form a good basis to further
analyse the condensation within the double skin of
an aircraft. It was shown how a homogeneous flow
field will behave within a fuselage’s geometry as
well as how to monitor the actual mass of
condensed water. The actual mass of water then
can be analysed in specific
parts of the geometry by specifying individual
surfaces and thus can solve the question of the
hot spots of condensation.
Project summary
The aim of this study was to broaden the
knowledge gained in Part A of this Thesis and
to further investigate the condensation that
takes place within the double skin of a
commercial airliner. Accordingly the
condensation has been approached stepbystep
and was finally realised with the aid of a
wall condensation example for a simple box.
The step-by-step approach now includes a
refinement process of the geometry as well as
the integration of condensation and using
inhomogeneous flow fields.
Project Objectives
It is intended to demonstrate the differences
of e.g. the water volume fraction or flow
direction and the corresponding velocity due
to different temperature gradients. Also it is
intended to finally show the condensation or
condensation rates for a reference flight and
to outline the differences.
Project Conclusion
Finally it can be concluded that integrating
the condensation can be quite complex as
there are different ways of condensation
which require different setups.