Carbon fiber transport was studied using mathematical models established for other pollution problems. It was demonstrated that resuspension is not a major factor contributing to the risk. Filtration and fragmentation tests revealed that fiber fragmentation shifts the fiber spectrum to shorter mean lengths in high velocity air handling systems

Elber, W.

English

NASA Technical Reports Server

DISSEMINATION, RESUSPENSION, AND FILTRATION 
OF CARBON FIBERS 
Wolf Elber 
NASA Langley Research Center 
INTRODUCTION ---- 
The carbon fiber study has been structured like other atmospheric pollution 
problems. The source produces the material, in this case carbon fibers; the 
atmosphere transports and distributes them; and at the end of the chain the 
fibers produce effects, in this case the disruption of electrical devices. 
Both the source and the effects elements of this study had to be generated 
specifically for this problem; whereas the transportation element could be 
mostly adapted from other pollution studies. 
For the fire plume aspect of transportation, the smoke stack models were used 
for the determination of the height at which the plume becomes neutrally 
buoyant. However, the differences between an uncontained open fire plume 
and an industrial smoke stack are large enough to have required some model 
verification testing. This work was performed at the White Sands Missile 
Range. 
Many models exist for the cloud transport phase of the problem. These models 
have a large empirical data base from pollution work with gases such as sulphur 
dioxide from industrial sources, with liquid droplets from aeria7 spraying, and 
with solid particles such as fly ash from smoke stacks. Those models are only 
sensitive to the still air fall velocity of the individual particles, and have 
been adopted into the risk assessment studies because they have proven 
successful in work for the Environmental Protection Agency. 
Resuspension of particles was found to be a less developed much more complex 
science which had developed around the need to understand soil erosion in some 
states and long term radioactive pollution in other states. However, what 
appeared applicable to round soil particles did not appear to be valid for our 
high aspect ratio fibers. A special series of tests was therefore conducted to 
monitor the resuspension of fibers from a desert land patch with a known 
deposition of fibers. The results of those tests led to the conclusion that 
resuspension was a minor problem and that the risk assessment should be based 
on the first-time source. 
Filtration of fibers and the their entry into buildings is a subject for which 
models existed and only specific filter tests had to be performed. The transfer 
function models are essentially micro-dissemination models assuming perfect 
mixing of the atmosphere within the buildings. 
Many organizations‘were involved in assembling the methodology which prescribes 
the path for the fibers from the source to the vulnerable equipment. This 
presentation contains a brief outline of the methodology used and the data 
developed for the four main topic areas: Fire Plume, Dissemination, 
Resuspension , and Filtration. 
59 
https://ntrs.nasa.gov/search.jsp?R=19800010919 2020-03-21T02:25:22+00:00Z
MEASURES OF FIBER POLLUTION 
Typically a vulnerable e7ectric instrument may blow cooling air over the 
electric circuits, and blow the fibers across contact pairs. The rate at 
which fibers might strike a contact pair and cause a failure is therefore 
proportional to the concentration of fibers in the air. In this presentation 
the symb 
9 
1 C is used for concentration. Its units are fibers/unit volume of 
air (f/m ). The total risk of failure of an instrument is proportional to the 
exposure E, which is defined as the time-integral of the fiber concentration, 
E= JCdt f-s/m3 
or if the concentration C is constant for a time t, the exposure is the 
product of time and concentration 
E=Cxt f-s/m3 
For most instruments the risk of failure is only a function of the exposure E, 
and it has therefore become the main measure of carbon fiber pollution. 
Most detectors of fibers deposit the fibers on a surface, and the deposition 
density D is the number of fibers per unit area (f/m2). 
The deposition density is related to the exposure through the deposition 
velocity 
D=ExV f/m2 
where v is the flow velocity of the air for filter type collectors or the 
fibers' free fall velocity V, for flat surfaces. 
METEOROLOGY FOR DISSEMINATION 
The strength of the sun's heating of the ground has the greatest influence on 
the turbulence of the atmosphere and on the dispersion of particles. 
The sketches in Figure 1 show a smoke stack in three weather conditions. In 
sunny weather with low winds the radiation from the sun first heats the ground. 
Some of the heat is conducted to the lower few meters of the air. This stack 
is unstable, and the heat is distributed vertically by convection, until the 
atmosphere is layered at the dry adiabatic lapse rate to the height of the 
inversion. Typically during the heating portion of the day the temperature 
distribution changes from State I to State II. The convection patterns are 
responsible for very rapid mixing of the smoke plume in the atmosphere. 
At the other end of the scale, with no solar heating the earth radiates out 
heat and cools. When the atmosphere is stably layered, the bottom layers will 
loose heat to the ground by conduction forming a stronger inversion. This 
atmosphere sustains no turbulence and smoke plumes mix extremely slowly. These 
conditions lead to the highest pollutant exposures downwind. 
60 
Between the two extremes there is the neutrally buoyant atmosphere with little 
or no heat flux. These conditions usually occur after the passage of fronts 
in strong winds under overcast skies. The turbulent mixing under such condi- 
tions is due to the turbulence accompanying the wind. The mixing is faster 
than in stable weather, but slower than in unstable weather. 
For dissemination analysis stability indices have been developed. The 
Pasquill-Gifford stability classes for the three main weather conditions are 
shown in Figure 1. Cloud spread angles have been empirically determined for 
each of these classes. 
PLUME MODELLING 
Figure 2 shows a schematic view of a smoke cloud rising from the site of a 
large pool fire. When the gases are hot and buoyant the fire plume is 
extremely turbulant and mixes rapidly with ambient air until it reaches 
neutral buoyancy. The thermal buoyancy in that phase dominates the spread 
and rise of the c7ouds, and special fire plume models have been developed to 
predict the height and the spread of the fire plume to the point at which it 
becomes neutrally buoyant, that is the "stabilization point". 
Beyond the stabilization point the meteorology determines the spread of the 
drifting smoke cloud. The Gaussian dissemination models have been developed 
to predict the exposure patterns downwind. The fire plume models and the 
dissemination models are matched together at the stabilization point. 
FIRE PLUME MODELS ------_ 
The fire plume models are based on material contained in a paper by 
G. A. Briggs (1970) presented at the Second International Clean Air 
Congress. 
Although the model was developed for smoke plumes from industrial exhaust 
stacks, it provides good estimates of the stabilization parameters for the 
concentrated liquid-fuel pool fires of interest here. 
The model is sensitive to the stability of the atmosphere and the logic 
changes between stable and unstable atmospheres. 
In stable temperatures for instance the stabilization height is given as 
Where F is the total heat flux, u is the wind velocity and s is the 
potential temperature gradient. 
The size of the cloud is determined by the entrainment of air and in most 
cases the diameter grows linearly with height so that 
D= 0.6 Ii 
61 
DISSEMINATION MODELLING 
The most appropriate dissemination models for a risk assessment are the 
Gaussian Dissemination Models. 
In these models the material is given the Gaussian bell-shaped distribution 
shown in Figure 3. As the cloud drifts downwind it grows in diameter 
depending on the instability of the atmosphere. The growth angles have been 
determined empirically from many observations of smoke plumes. 
As the cloud grows and drifts downwind it will begin to intercept both the 
ground and the inversion as shown in Figure 4. Inasmuch as the growth is 
dependent on the turbulence in the layer between the ground and the inversion, 
the models are refined not to allow the cloud to grow through the inversion, 
but to reflect back the pollutant into the layer between ground and inversion. 
At the same time particles will deposit on the ground at a rate dependent on 
particle concentration and their fall velocity. Empirical data show that 
reflection of 70% of the particles from the ground provides exposure patterns 
consistent with the fall velocity of the particles. 
Figure 5 shows two typical exposure patterns. The lines of constant exposure 
are termed "isopleths". In the stable atmosphere the spread of the cloud 
is narrow, but high exposure levels may persist for up to 700 km. In the 
unstable atmosphere the spread of the cloud is wide, but does not persist for 
the same distances. 
The deposition rate anywhere in the affected area is dependant on the concen- 
tration and the fall velocity 
The total deposition at one point is 
D = vs~C dt = vsx E 
and the total deposition over the entire area is 
or 
s D d/l =u,EdO s 
N = vc EdA r 
where N is the total number of particles in the pollutant cloud. This 
equation indicates that the fire and meteorological conditions only affect the 
distribution of the exposures, the total area coverage however is determined 
by the amount of material in the source. 
If the material could be uniformly distributed at exposure levels E over 
an area A , then we have the simole relation 
N ’ = vs EA 
62 
Figure 6 is a parametric plot of the area covered to an exposure E as a 
function of the mass of single fibers in the source. As an example, the 
worst case analyzed in the risk assessment represented a source of 375 kg of 
single fibers. The plot show5 that this amount of fibers could cover a whole 
city to an exposure of 5 X 10 , or five city b7ocks to an exposure of 708. 
The damage potential from such a release will be discussed in the next 
presentation. 
RESUSPENSION OF CARBON FIBERS 
Carbon fibers deposited on a surface may be resuspended by a disturbance such 
as wind or traffic. This phenomenon was investigated to determine if 
resuspension could represent a significant contribution to the carbon fiber 
risk. 
Figure 7 shows the logic diagram for the type of surface on which fibers are 
first deposited. Fibers deposited on water will sink and cannot be 
resuspended. Fibers deposited in vegetated surfaces will fall so deep that 
the winds required to resuspend the particles cannot reach the particles. 
But fibers deposited on flat hard surfaces could be resuspended. One test 
series was conducted to monitor the resuspension of fibers from a desert 
surface. From an original source of 23 kg scattered over an area of 60 X 80 m 
The daily downwind fiber flux is plotted in Figure 8. The data was collected 
for three years. The vertical and horizontal distributions were not defined, 
but on the assumption that the flux was uniform over a downwind area of 
1000 m2, we can calculate a total flux of 0.1 kg of 4 mm fibers or less than 
0.4% of the available total source. At the same time the average length of the 
captured fibers changed from an initial mean length of 7 mm to a final mean 
length of 1 mm, while the source material left on the ground clearly retains 
the initial lengths of 7 mm. The fragmentation indicates that the fibers 
released are broken from the clumps of source fibers, most probably by the 
saltation of'sand particles. 
Because the fraction of fibers resuspended is small, because only special areas 
are suitable for resuspension, 
process of resuspension, 
and because fibers appear to be fragmented in the 
the phenomenon of resuspension was not considered 
further in the risk assessment. 
TRANSFER FUNCTIONS 
The transfer function of a building or instrument enclosure is the ratio of 
internal fiber exposure to external fiber exposure. 
Filtration, airflow, and settlement velocity are the parameters controlling 
the transfer function. 
Filter tests have been carried out at the Ballistics Research Laboratories to 
define the filter transmission factors as a function of fiber length for many 
63 
comon filter media. Figure 9 shows the results of such tests on a typical 
window-screen and a furnace filter. Both filters are more effective against 
long fibers than against short fibers. As a result the mean fiber length as 
we71 as the number of fibers are reduced by the filter. 
Figure 9 shows an initial exponential fiber length distribution, and the 
distribution after filtration. 
to 1" 
The mean fiber length has changed from L"= 2 mm 
= 0.9 mm, and the transfer function for fibers longer than 7 in is 0.73. 
Such analytical refinements have not been introduced into the risk assessment. 
Instead, the contractors have used the fi7ter factors appropriate for the 2 mm 
mean spectrum length and have assumed that the transmitted spectrum remains 
exponential. 
The Bionetics Corporation has tested filtration equipment from commercial 
aircraft. Both water separaters and air cleaners have very low transmission 
factors, but also cause severe fiber fragmentation. In a separate test the 
fragmentation of fibers was evaluated by passing 3 mm fibers through a curved 
duct. Figure 70 shows a schematic of the test apparatus together with the 
test results. At low flow speeds most fibers travel through the curved tube 
intact, but at fiber speeds of 17 m/s and higher virtually all fibers were 
fragmented into lengths less than 7 mm. 
Aircraft ventilation air drawn from the compressor stages of the turbines 
typically would undergo many turns and much higher speeds. We have therefore 
assumed that fibers longer than 1 mm would be fragmented into lengths smaller 
than 7 mm in aircraft engines. 
Models for the analysis of transfer functions have been available from previous 
studies. The transfer function for an enclosure can be shown to be the ratio 
of fiber inflow rate to the fiber loss rate. 
The inflow rate is given by the airflow rate times 
The fiber loss rate is made up of outflow losses, 
filter, and losses due to fiber deposition. 
filter transmission factor. 
losses in the recirculation 
Figure It shows the transfer function calculations for a 200 m2 residence with 
open windows protected by wire screens. The calcu lations show that even with 
open windows transfer functions as low as 0.07 are to be expected. 
CONCLUSIONS 
The elements of the fiber transport chain have been studied. The mathematical 
models had been established for other pollution problems and were found to be 
appropriate for the carbon fiber problem. 
A particular study was made to establish the possibility of resuspension. The 
data showed that resuspension cannot be a major contributing factor to the risk. 
Filtration and fragmentation tests were run to provide the necessary data base 
for transfer function calculations. The data showed that filters are much 
64 
more effective than assumed in the preliminary study and that in high velocity 
air handling systems significant fiber fragmentation will change the fiber 
spectrum to shorter mean lengths. 
65 
UNSTABLE - A NEUTRAL-C STABLE - F 
SUNNY - LOW WINDS CLOUDY - STRONG WIND NIGHT - LOW WINDS 
HEIGHT 
18 
1= 
TEMP 
HEIGHT 
TEMP 
HEIGHT 
i 
TEMP 
Figure l.- Dissemination meteorology with Pasquill-Gifford stability 
classes A, C, and F. 
Figure 2.- Characteristic smoke plume development. 
Figure 3.- Gaussian distribution of pollutant 
in a drifting cloud. 
REFLECTION SOURCE 
FULL REFLECTION 
PARTIAL REFLECTION 
GROUND 
Figure 4.- Pollutant cloud growth with reflection from 
inversion and ground. 
67 
AREA 
EXPOSURE 
TYPICAL F 
HIGH EXPOSURES, MALL AREAS 
LOW EXPOSllRES 
TOTAL DEPOSITION EQUATION: CEi A; = N/V5 
Figure 5.- Typical exposure patterns and area coverage definition. 
108. 
AREA 
COVERED M* lo7 
106. 
EXPOSURE, F ?YM-~ 
Figure 6.- parametric plot of carbon fiber area coverage limit. 
CITY 
. SUBURB 
DEVELOPMENT 
BLOCK 
. FOOTBALL FIELD 
LOT 
APARTMENT 
3 
68 
o RESUSPENSION POSSIBLE FROM FEW AREAS, 
Figure 7.- Resuspension logic chart. 
8000 
FIBER 
FLUX 
F/M*-DAY 
6000 
4000 
2000 
0 
TOTAL FLUX INTEGRAL = 4 X lo5 F/M* 
TOTAL FIBER MASS = 23 KG 
TOTAL RESUSPENDED = 0,l KG AT 4 MM 
= 0.4% OF AVAILABLE TOTAL I 
ASSUMED FIBER 
FLUX WINDOW 1000 M* 
DEPOSITION AREA 60 X 80 M 
i i i 
TIME, YEARS 
Figure' 8.- Resuspension data. 
69 
FILTER 'lo- BRL FILTER DATA 
TRANSMISSION 
FACTOR 
FURNACE FILTER 
WIRE-SCREEN FILTERED 
MEAN = 0,95 MM 
5 10 15 5 10 15 
FIBER LENGTH, MM FIBER LENGTH, MM 
Figure 9.- Filter data. 
FIBER 
SOURCE 
3 MM 
12 MM TUBE 
I 
FIBER 
', .I 
SAMPLING 
, SCREEN 'lo 
FRACTION 
4.1 MM 
085 
RANGE OF 
PREDICTIONS 1 
0, 
0 200 400 
FIBER IMPACT SPEED (FPS) 
Figure lO.- Fragmentation data. 
70 
T:F. = 
FILTER FACTUR * + 
( +L f .AiE$Jqd+$) 
3 -1 
0.5 m s - IO00 cfm = O.OI 
e ONE WINDOW FAN 
Figure 11 .- Transfer of carbon fibers into buildings. 
71 
-_ 


Dissemination, resuspension, and filtration of carbon fibers

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server