NASA Technical Reports Server (NTRS) 19960014640: A Study of Large Droplet Ice Accretions in the NASA-Lewis IRT at Near-Freezing Conditions

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by  NASA Technical Reports Server (NTRS)

NASA/TM—1996-107142/REV1 ARL–MR–294
AIAA–96–0934
U.S. ARMY
RESEARCH LABORATORY
A Study of Large Droplet Ice Accretions in the
NASA Glenn IRT at Near-Freezing Conditions
Dean R. Miller and Harold E. Addy, Jr.
Glenn Research Center, Cleveland, Ohio
Robert F. Ide
U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio
June 2005
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NASA/TM—1996-107142/REV1 ARL–MR–294
AIAA–96–0934
U.S. ARMY
RESEARCH LABORATORY
A Study of Large Droplet Ice Accretions in the
NASA Glenn IRT at Near-Freezing Conditions
Dean R. Miller and Harold E. Addy, Jr.
Glenn Research Center, Cleveland, Ohio
Robert F. Ide
U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio
Prepared for the
34th Aerospace Sciences Meeting and Exhibit
sponsored by the American Society of Aeronautics and Astronautics
Reno, Nevada, January 15–18, 1996
National Aeronautics and
Space Administration
Glenn Research Center
June 2005
Document Change History
This printing, numbered as NASA /TM—1996–107142/REV1, June 2005, replaces the previous version, NASA Technical
Memorandum 107142, January 1996, in its entirety. It contains the following change:
Figure 1 has been modified to make the document compliant with “NASA Internet Publishing Content Guidelines,” NASA
Information Technology Requirement NITR–2810–3.
Note that at the time of writing, the NASA Lewis Research Center
was undergoing a name change to the
NASA John H. Glenn Research Center at Lewis Field.
Both names may appear in this report.
Available from
NASA Center for Aerospace Information National Technical Information Service
7121 Standard Drive 5285 Port Royal Road
Hanover, MD 21076 Springfield, VA 22100
Available electronically at http://gltrs.grc.nasa.gov
A STUDY OF LARGE DROPLET ICE ACCRETIONS
IN THE NASA GLENN IRT
AT NEAR-FREEZING CONDITIONS
Dean Miller and Harold E. Addy, Jr.
National Aeronautics and Space Administration
Glenn Research Center
Cleveland, Ohio
Robert F. Ide
U.S. Army Research Laboratory
National Aeronautics and Space Administration
Glenn Research Center
Cleveland, Ohio
NASA/TM—1996-107142/REV1 1
second test entry, only the results from the Twin Otter
At the present time the icing database concerning wing were available at the time of this writing. It is
large droplets at near-freezing temperatures is vet)' anticipated that the results from the second test entry
limited. This is a result of several :factors. First, these with the NACA-23012 wing will be presented in a
conditions occur infrequently when considered on a later report.
statistical basis. Secondly, iflarge droplet icing clouds
are defined to have droplet distributions with Median FACILITY AND MODEL DESCRIPTION
Volumetric Diameter (MVD) ranging from 40 to
400~ then this condition falls outside the FAR-part Icing Research Tunnel
25 icing certification envelope. Typically, the The NASA IRT is a closed-loop refrigerated wind
emphasis in icing tunnels and icing flight programs tunnel. The test section is 6 ft high and 9 ft wide. and
has been on aircraft certification within the FAR-part contains a turntable assembly which allows for model
25 envelope, where icing cloud MVD's are less than angle-of- attack changes. A 5000 hp fan provides
401Jlll. Therefore, the icing tunnel or flight databases airspeeds up to 400 mph (empty test section). The
with this condition are very limited. refrigeration heat exchanger can control the air
temperature from +40 OF to -40 Of'. The water spray
Given the fact that this combination of large droplet system is capable of simulating icing clouds with
with near-freezing temperatures is an important issue MVD of 14 to 40~m, and Liquid Water Content
for the icing community to deal with. the Icing (LWC) of 0.3 - 3.0 glm3• Figure I shows a schematic
Technology Branch at NASA-Lewis Research Center view of the IRT.
has attempted to study the large droplet phenomena.
One goal of this effort is to increase the size of the Icing Cloud Spray System
existing database on large droplet ice accretions at The spray system that generates the icing cloud in the
near-freezing temperatures. To achieve this goal, the IRT is composed. of eight spraybars containing a total
NASA-Lewis Icing Research Tunnel has been utilized of 95 spray nozzles. The spraybars are located in the
to conduct experimental large droplet icing settling chamber upstream of the contraction section as
investigations. shown in Figure 1. The spray nozzles used are air
assist type atomizers which are not commercially
The first experimental large droplet investigations at available but are specially built for NASA. A cross
near-freezing temperatures were conducted in the sectional view of the nozzle is shown in Figure 2.
NASA-Lewis Icing Research Tunnel (IRT) in
December 1994, and also during May 1995. As a Two types of nozzles are currently utilized in the IRT
follow on to these investigations, another more spray system and are designated Standard and Mod-I.
comprehensive experimental investigation was The Mod-l nozzles were used for the large droplet
conducted in the IRT during the last quarter of 1995. tests described in this report. This is because they
we
The primary objectives of this latter investigation produce L levels closer to those expected for large
were: (I) To document the existing capability of the droplets in natural icing clouds, than do the Standard
IRT to produce icing clouds of large droplets at near nozzles. The Mod-l nozrles are normally used to
freezing temperatures, and (2) To conduct a produce icing clouds with MVD of 14 to 40~ using
parametric study of large droplets at near-freezing air pressures from 10 to 80 psig.
temperatures on a commuter-class aircraft wing
section, by varying temperature, droplet size, airspeed, Model
angle-of-attack, flap-setting, and de-icer boot cycling The test article was a 77.25 inch chord Twin Otter
time. wing section with a 30"/0 chord flap. The leading edge
of the model was outfitted with a full-span pnewnatic
This experimental investigation of large droplet ice de-icer boot extending to 10% chord on the suction
accretions was comprised of two tests, each one surface, and to II % chord on the pressure surface. A
utilizing a different commuter- dass aircraft wing picture of the test article is shown in Figure 3.
section. The first test entry utilized a full-scale 77 .25
inch chord Twin Otter wing section with a 30% chord The model was also instrumented with 16 Type-T
flap. The second entry utilized a full-scale wing thermocouples at a spanwise location of 42 inches
section with a NACA-23012 aitfoil section, and a above the tunnel floor. The thennocouples were
nominal chord length of 68 inches at half-span. Due located at various chordwise positions. both on the
to time constraints in reducing the data from the
NASA/TM—1996-107142/REV1 2
boot and on the metal skin of the airfoil as shown in Tlrfermjnation of Droplet Size
Figure 4. A very thin patch ofboot material (.01 inch) Droplet size distributions were measured in the center of
was used to cover the boot thermocouples, providing the tunnel test section using two laser-based instruments.
physical protection and also providing a good boot A furward scattering specttomerer probe, model FSSP-
surface temperature measurement. 100, was used to measure droplets with diameters between
2 and 47"", and ao opIical amy probe, model 0AP230X,
LARGE DROPLET SPRAY CALmRATION was used to measure droplets with diameters from 47 to
450~ Both of these instruments are manufactured by
The calibration of the IRT's icing cloud was extended Particle Measuring Systems Inc. in Boulder, Colorado.
to include the large droplet sizes chosen for this
investigation (99~ and 160~). The LWC was Each probe was alternately placed in the center of the
fixed at a single value for each of the above droplet tunnel and the stabilized icing cloud was sampled for 100
sizes. No further calibration was required for the seconds at an airspeed of 195 mph. For these tests only
401Jlll droplets, since these droplets are mthin the evety other sprnybm was opernted to reduce the droplet
standard IRT cloud calibration envelope. numbe< densities to acceptable levels fur the FSSP to
make valid measurements. Reference I details the
Determination of Liquid Water Content comparison of using all spraybors and half-spraybars on
To detennine the liquid water content (LW C), nine 1.5 the FSSP results and the rationale for why half-spray
inch diameter vertical cylinders spaced at nine inches results are more acc_eptaeble.
were mounted in the test section of the tunnel The
cylinders were ro1ated at approximately 60 lpm They The standard PMS data correction algorithms were used
were exposed to the icing cloud for a predetennined including an "activity" correction for the FSSP and bin
length of time. The resulting ice accretion was depth of field and amy width cornx:tions fur the
detennined by measuring the __ of the iced OAP. The droplet distributions from each probe were
cylinders using a probe much tmversed each cylinder combined and the median volumetric __ (MVD)
from 6 inches above the tunnel floor to 10 inches from the was calculated The LWC was also computed from the
tunnel ceiling. The _ reconled was the iced __ combined droplet distribution as a check against the
measurement at vertical intervals of 1.25 inches for each rotating cylinder L WC.
cylinder.
As an alternate to the PMS data correction algorithm, the
The recorded data was used to plot the L WC uniformity NCAR (Baumgardner) methodology was used to
in the test section and to calculate the central vaIue of determine the effi:d it woold have 00 the calculated
LWC. The cenIIlIl LWC value was calculated using the MVD and LW C.' A comparison of the calculated values
equatioo: ofMVD and LWC resulting tiom the PMS and NCAR
methods is shown in Table I and the numbe< density and
LWC distributions are shown in Figure 5. For both
C' I!.r
LWC ~ -:-=::--:-: spray conditions, the NCAR method resulted in a decrease
Eb*V*tI2 of23 to 30 percent in theMVD and an increase of330 to
389 _ in the LWC. The LWC values resuIting
where: from the PMS correction algorithm were judged to be
Ar is the ice thickness on the cylinder at the more reasonable since they agreed much more closely
center location with the rotating cylinder values. As can be seen in Figure
Et, is the average collection efficiency 5, the droplet distributions genernted with the OAPIPMS
V is the airspeed correction algorithm are smoother and exhibits a better
t istime mau:h to the FSSP distributions than the OAPINCAR
C is a constant which includes the ice density correction algorithm. Therefore the MVD values
calculated with the PMS algorithm were used as the
The ice density is assumed to be 0.88. Using the units of representative values for these spray conditions.
Ar in inches, V in miles per hour and t in seconds, the
value ofC is 5.0 x104. The time is divided by 2 since one DROPLET SUPERCOOLING INVESTIGATION
half of the cylinder is always exposed to the cloud
impingement. When the IRT is used to produce icing clouds with
normal cloud droplet sizes of 14 to 40fllll, droplet
NASA/TM—1996-107142/REV1 3
supercooling is accelerated by the expansion and
subsequent cooling of the pressurized air used in the Figure 6 presents experimental ice shape tracings
nozzles to promote droplet breakup. The air and water resulting from 3 different initial spraybar air and water
are normally heated to approximately 175°F to temperatures and a 160J.Ull MVD icing cloud. A
preclude droplet freeze-out. Low atomizing air comparison of the three ice tracings reveals that the
pressures of 2 to 5 psig were used to generate large ice shapes are virtually identical for this range of
droplets, thereby significantly reducing expansion spraybar air and water temperatures. This
cooling. Since large droplets contain a larger amount experimental result confirms the AEDC code
of mass to cool, the loss of expansion cooling raised predictions and lends credibility to the code
concerns that the droplets might not reach ambient predictions that the droplets are supercooled (with a
temperature before impinging on the model. temperature close to that of the airstream).
A computer code developed by the Arnold Figure 7 provides additional experimental evidence
Engineering Development Center (AEDC) was used to which appears to support the above conclusion. Figure
determine the effect of spraybar water temperature on 7 is a time-history plot of temperature obtained from a
the degree of droplet cooling.3 Table 2 shows the leading-edge thermocouple. Temperature
predicted final droplet temperature for initial water measurements are plotted for the first 7S seconds of a
temperatures of 50, 100, and 200 OF. For all code 160J.l.m MVD icing spray. The icing spray was
runs,the tunnel test section airspeed was 195 mph, the initiated at t=O seconds. Initially, the leading-edge was
tunnel total temperature was 28 OF, and the initial near the tunnel total temperature of 28 OF. Visual
relative humidity at the spray location was 83%. Td - observation of this leading-edge region indicated that
TR represents the difference between the droplet the icing cloud first impinged on the model at 9
temperature and the air temperature. seconds. Note that at this time the leading-edge
temperature began to dip. Over the next 5 seconds the
It can be seen from Table 2, that the initial water temperature continued to decrease to a minimum of
temperature had a negligible effect on the final droplet 1.5 OF below the initial value. At t=21 seconds
temperature. Vatying the initial water temperature complete :freezing of the leading edge area was
from 50 "F to 200 "F resulted in only a 0.1 OF change observed, which is where the temperature profile of
in droplet temperature. Also, the maximum Figure 7 began leveling at 32°F.
temperature difference (between the droplets and the
air) was predicted to occur for the largest droplet size The temperature profile shown in Figure 7 suggests
of 1601Jlll, where the droplets were predicted to be 1.8 that the impinging droplets were initially cooling the
OF warmer than the air. airfoil surface below the total temperature of the
tunnel (approximately 28.5 oF). To cool the leading
An experiment was conducted to examine the validity edge by 1.5 "P, the droplet temperature would have to
of the code results for icing spray clouds with MVO's have been no greater than 27 "P. It was likely that the
of 99~ and 160J.l.m. Experimental icing runs were droplet temperature was actually between the
conducted in the IRT at an airspeed of 195 mph and a airstream static temperature of 21.7 OF and 27 "F.
total temperature of28 OF. A temperature of28°F was Note that the AEDC code predicted that the 160J.Ull
suitable for this investigation because it was near droplets would be 1.9 OF above the airstream static
freezing, yet didn't exhibit appreciable water runback temperature (see Table 2); thus by this prediction, the
and random ice shedding. The result was a traceable droplet temperature would be approximately 23.6 "P.
ice shape. For each droplet size, the atomizing air and
water temperatures were varied between nominal Clearly, the AEDC analysis and experimental
values of 50 "P, 100 "P, and 200 of. Ice was accreted observations lead to the conclusion that the large
on the model, and ice shape tracings were obtained. droplets produced in the IRT are supercooled and
These ice shape tracings were then compared to approach the static temperature of the airstream.
determine how the initial spray and water temperature
affected the resultant ice accretion. It should be PARAMETRIC INVESTIGATION
mentioned that nominal spraybar temperature values
were not always achieved. An attempt was made to The objective of this portion of the test program was to
get as close as possible to the nominal values for those study how parameters such as temperature, droplet
cases. size, AOA, airspeed, flap setting, and de·icer boot
NASA/TM—1996-107142/REV1 4
cycle times affect large droplet ice accretions. was found that as the droplet MVD was increased, the
Additionally, it was desired to compile a database of LWC of the icing cloud became more nonuniform over
large droplet ice accretions which could be used to the area. of the test section. After review of the large
better characterize this phenomena. Table 3 lists the droplet spray calibration data, it was decided. that the
range of parameter values which were evaluated for 160}lJll MVD cloud: 1) provided an acceptable area of
their effect on large droplet ice accretions. A test unifonn Lw e at the center of the test section and 2)
matrix was developed using the parameter values provided the largest droplet size distn"bution which
defined in Table 3 and subsequently used to guide the could be confidently measured with the available
testing in the IRT. droplet size measuring instrumentation.
The test matrix was developed with the assumption The lower limit airspeed of 125 mph represents a
that both droplet size and near·freezing temperatures typical approach condition for the Twin Otter aircraft,
are key parameters affecting large droplet ice while the upper airspeed limit of 195 mph was chosen
accretions. Therefore, a temperature "sweep" was to provide enough of an airspeed variation that its
incorporated into the matrix to investigate large effect on the large droplet ice accretions could be
droplet ice accretions at temperatures within +/- 4 cP studied.
of the freezing point. Previous large droplet studies
conducted in the IRT suggested that runback and ice The four angles of attack and the three flap settings
sliding may be important features of ice accretions at were considered to be typical of nominal Twin Otter
near-freezing temperatures, as these characteristics flight conditions. Also, because this was a rather large
had been observed at temperatures at or above 28 OF. chord model (77.25"), it was decided to limit the
Therefore, 28 OF was chosen as the lower temperature angle--of-attack to minimize tunnel blockage effects.
bound for the temperature sweep.
Four different de-icer boot cycling times were tested.
It was not possible to test all of the conditions reflected which ranged from no activation of the boot to cycling
in Table 3 within the tunnel time alotted for this test the boot in 6 minute intervals. A 3 minute boot cycle
program. Therefore, as the temperature sweep was time was the nonnal cycling interval, while the .7 min
conducted, one temperature was selected as an cycling time represented the fast cycling interval for
"anchor·point". This temperature remained fixed. the Twin Otter.
while the other parameters of interest were varied
(hence the name "anchor·poinf'). The selection of the Ice Shape Repeatability
"anchor point" was subjective and based on Since the effect of large droplet ice accretion on the
consultation with NASA colleagues who had aerodynamic peIformance of an aircraft is due to the
conducted previous large droplet investigations in the peculiar way it accumulates, the ice shape that forms is
IRT . Ice accretions obtained at the anchor point of key importance in studying these effects. Therefore,
temperature were believed to be most representative of a significant part of this study was to record large
the large droplet icing condition. The "anchor·points" droplet ice accretions under various conditions such
selected for this investigation were T = 30 OF at as: temperature, airspeed, droplet size, model angle-of·
t
airspeeds of 125 mph and 163 mph, and T = 32 OF at attack, model flap setting, and de·icer boot cycling
t
an airspeed of 195 mph. time. Underlying this type of study is the question of
bow well an ice shape is reproduced under a given set
In addition to the temperature sweep, large droplet ice of these parameters.
accretions were studied at two other temperatures: 5 cP
and 0 OF. The 5 OF condition was chosen based on There are two aspects to the question of repeatability
meteorological evidence suggesting that the large in this study. The first concerns the ability to closely
droplet condition may extend as low as 5 CP, while the reproduce icing conditions in the IRT. The second
o OF condition was chosen to obtain impingement centers arOWld how well an ice shape reproduces itself
infonnation with short duration icing sprays.4 in a given set of conditions. This latter aspect may at
first appear to be totally dependent upon the first.
However, in these tests, the ice protection system did
The droplet sizes selected for this investigation ranged
not completely remove the accwnulated ice and the
from the upper end of the standard IRT icing cloud
envelope to near the limit of what is currently shape of the residual ice was always somewhat
attajnable with the existing IRT spray bar system. It random. Furthennore, at total temperatures VCJy near
NASA/TM—1996-107142/REV1 5
freezing (32 +/-2 oF), the accumulated ice would often accretion began to form, there was a distinct ridge
be shed off the model independent of the ice protection noted aft of the active portion of the de-ieer boot on
system activation. This "self-shedding" occurred in a both the suction and pressure surfaces.
random manner, leaving behind a variable ice shape.
As the temperature was decreased still further, less
As has been found in previous studies, the ability to and less runback was observed aft of the boot probably
reproduce icing conditions in the IRT is very good and because more of the water was freezing earlier during
continues to improve as enhancements are contirrua1ly the runback process. Eventually, a temperature was
being made to the tunneLS Moreover, results found reached where the ice ridge on the boot (aft of the
during the droplet cooling portion of this study also . active portion) appeared to reach a relative maximwn.
indicated that the IRT reproduces icing conditions This temperature was 30 OF for V :;: 195 mph, while
very well. at 125 mph this temperature was 28 OF.
Variation in ice shapes formed under nearly identical The pictures in Figure 10 visually depict what a strong
conditions can be seen in Figures 8 and 9. Figure 8 effect the total temperature has on the large droplet ice
shows ice shape tracings from the spanwise centerline accretion. The lines on the model indicate 5, 10, and
of the airfoil for three separate test runs at the same set 15% chord locations. Figure lOa shows the ice
of conditions. The active portion of the ice protection accretion on the suction surface for a total temperature
system extends to approximately 7.5% chord on the of 32 "P. Runback extended from just aft of the deicer
suction surface. It can be seen that the ridge of boot at 10010 chord to beyond 15% chord. Note the
residual ice forms in the general vicinity of the edge of lack of a defined ridge just aft of the active portion of
the active portion of the boot. However, the ridge the boot at 7.5% chord. Any ice ridge that formed
varies considerably in height and shape as well as in tended to be very "slushy" and subsequently was
location from 7% to 9.5% chord. Similar results can blown off the ailfoil at this temperature.
be seen in Figure 9, which shows three nearly identical
cloud runs at a total temperature of 31"F, rather than Dropping the total temperature to 30 "F resulted in the
at 32°F. accretion shown in Figure lOb. There was much less
runback aft of the boot, and a thin layer of glaze ice
This random variation in ridge location, height and was visible between 7.5% to 10% chord. However, the
shape was found throughout this entire series of tests. ice was ''wet'' and lacked adhesive strength. because
This characteristic of near-freezing temperature large the upper portion of the ridge was shed. Also, we see
droplet ice accretion could well be a cause of the the beginning of ice protuberances or nodules just aft
unpredictability ofit's effect on aircraft pelformance. of 10% chord. These nodules grew nonnal to the
surface and were consistently found just aft of an ice
Total Temperature Effect ridge.
The effect of total temperature on large droplet ice
accretions was investigated by varying the total When the total temperature was further decreased to
temperature from 36 OF down to 5 "F, and then 28 OF, we saw a marked change in the ice accretion.
comparing the features of the resultant ice accretions. Now the ridge appeared to have more structural
This was done at the following nominal condition: strength and, in this case, had resisted shedding. Also
MVD = 160.,un, ADA= 0°, Flap = 0 0, Boot cycle = 3 note the lack of runback and the very well defined
min, Spray = 18 minutes, and V = 125 and 195 mph. nodule growth aft of the de-icer boot. It is very
Generally speaking, the trends noted at 195 mph were evident from this sequence of pictures in Figure 10,
similar to those observed at 125 mph. The only that small changes in total temperature can have a
difference was in the temperature at which trends were very significant effect on the resultant large droplet ice
observed to begin or end. accretion when the total temperature is near 32 OF.
At T = 36 OF, all the impinging water ran back to the It was also noted that a liquid region existed at the
t
trailing-edge and was blown off the model leading-edge of the ailfoil. The width of this zone was
Decreasing the temperature to 34 OF, less runback was observed to directly correlate with total temperature.
noted and an ice accretion formed at V :;: 195 mph. The warmer the temperature, the wider this zone was.
The temperature had to be decreased to 32 OF before Boot thermocouple temperature data verified this
an accretion was observed at 125 mph. When the ice observation. Table 4 compares leading-edge boot
NASA/TM—1996-107142/REV1 6