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test cells intended for aero-engines. There
is still work to do an improving the icing
tests and one of the most important
elements is the simulation of a defined
cloud. Some icing clouds consist of a
mixture of supercooled water droplets and
particles of dry ice. These can be produced
at Pyestock based an different techniques
for the production of both components: ice
particles are produced by milling chips of
prepared ice blocks and introduced into the
main flow upstream of the liquid water
injection. There is a problem in measuring
the uniformity of the droplet size
distribution in a 2.6 m diameter cell.
Therefore at RAE they calibrate individual
nozzles in a controlled environment in a
low speed wind tunnel especially adapted
for this purpose. The two cells have large
test chambers of 6 m and 7.6 m diameters,
the latter can accomodate a helicopter
without rotors. A majority of the aeroengine
testing is done in connect mode, but
it is more realistic to do it in free jet
mode. The icing cloud is produced by a
number of airblast water spray nozzles on
rakes . A new rake for testing of large
engines requires 310 nozzles for a 2.64 m
duct. Five channels of closed circuit TV
are available for remote viewing: one
channel is a strobed view of rotating parts
whilst highspeed cine up to 1200 frames per
second records the ice shedding and
particle trajectories. Measuring the water
content and droplets’ diameter in the real
test cells close to the engine is the
preferred solution but it is very difficult
to achieve in large scale engineering
environment with delicate instruments. The
alternative solution at Pyestock is an open
circuit wind tunnel with a 0.31 m2 working
section and 80 m/s wind speed. Spray
nozzles characteristics (figure 24) show
the nozzles’ operational limits. Another
problem is the cooling down of the droplets
in the airflow. The water is injected at
20‘C to prevent freezing on the nozzle; are
the droplets supercooled at the time they
impact on the target? The state of the
droplets determines the ice accretion
forms. A 10 micron droplet comes to thermal
equilibrium in a -5-C airflow in under a
meter, but a 30 micron droplet might
require 5 meters. Thus the distance between
the spray rake and the target needs to
exceed 5 meters.
Hr Creismeas of C.E.Pr reported on a
numerical code developed at C.E.Pr. and
ONERA (30). As said in above paper the
distance between spray rake and test item
is in excess of 5 m and there can be a
significant modification in the droplets’
mean diameter. The test requires a given
mean droplet diameter and it is useful to
have a method to predict the evolution. The
theory is based on the thermal exchanges
between the liquid water and the gasflow,
taking into account the hygrometry. This
numerical code is named H.A.GI.C
(Modelisation et Analyse du GIvrage en
Caisson). The equations are based on the
continuity, momentum and heat equation. The
drag force of the droplet in airflow is
based on the value of Cx which is taken as:
(24/Re). (1+0.15
3-5
Theoretical results were compared with
experimental results. The starting section
is 0.25 m where mass distribution in
diameter intervals are given in table 2 for
4 Cases. The diameters in a section 1.95 m
from the injector were measured and
calculated and figure 25 gives an example
of the comparison.
5. Ice relevant cloud physical varameters.
Since 1983 the icing of aircraft is
investigated in the DLR - Institute for
Atmospheric Physics using an icing research
aircraft (33). The aim is to get
information about the dependance of the
icing relevant cloudphysical parameters on
cloud type and the height above cloud base.
The total water content (fluid and solid
particles), the median volume diameter and
the temperature T were collected during
vertical and horizontal flight through
clouds. A result is shown in figure 26. The
TWC of clouds of a high pressure area
increases about linearly with height above
cloud base. Its maximum is located just
below the cloud top and has values between
0.40 and 0.50 g/m3. Temperature is
decreasing with height and the median
volume diameters (MVD) are rather small and
are situated between 11 and 23 pm. The
phase of the particles is always fluid. The
MVD of clouds in the range of a Warm front
fluctuates strongly between small and large
values and maximum values are between 100
and 460 pm. The phase of the particles in
clouds in the range of a warm front would
vary between fluid and solid.
6. Low temperature and fuel vroblems.
The low temperature operations could
impact aircraft missions as the fuel can
form solid wax precipitates which may cause
plugging of filters or blockage of fuel
transfer lines. The Naval Air propulsion
center of Trenton was concerned with the
problem of availability of the F-44 (JP5)
fuel. This fuel with very low freezing
point accounts for only a small fraction,
less than 1% of total refinery production.
The relaxation of the freezing point
specification to that of the commercial
Jet-A specification would lead to a higher
fraction of the crude (25).
This study requires the determination
of the fuel temperature in the fuel tanks
which is very expensive if it has to be
done experimentally. NAPC looked for a CFD-
code that could be modified to handle the
calculations. They chose the PHOENICS 84 as
the base code performing fluid-flow, heat
transfer and chemical reaction calculations
simultaneously. The major developments are:
optimum grid selection, turbulence
modelling, phase change modelling and
expert system. The user-friendly menu
driven format allows the user to operate in
a menu driven step-wise format, depicted in
figure 27, where he has to input the
dimensions of the tank and the amount of
fuel desired. The boundary conditions are
either the experimental skin temperature of
the tank, or the calculated skin
temperature based on air temperature and
air speed. Figure 28 compares test and
calculated data. The model predicts
accurately, in both two and three