Independent Study
Atmospheric Effects on Aircraft Gas Turbine Life
Revised June 27, 2011
Kevin Roberg
Table of Contents
Table of Figures ............................................................................................................................................. 4
Table of Tables .............................................................................................................................................. 5
1.
Abstract ............................................................................................................................................. 6
2.
Introduction ...................................................................................................................................... 6
3.
Atmospheric Model .......................................................................................................................... 6
3.1.
Introduction to Atmospheric Model ............................................................................................. 6
3.2.
Basic Atmosphere ......................................................................................................................... 7
3.2.1.
Atmospheric Composition .................................................................................................... 7
3.2.2.
Atmospheric Structure .......................................................................................................... 8
3.2.3.
Standard Model of the Atmosphere ..................................................................................... 8
3.3.
Moisture in the Atmosphere....................................................................................................... 13
3.3.1.
Measures of Water Content ............................................................................................... 14
3.3.2.
Humidity/Altitude Relationship .......................................................................................... 16
3.3.3.
Conclusion ........................................................................................................................... 19
3.4.
Atmospheric Boundary Layer...................................................................................................... 19
3.4.1.
Introduction ........................................................................................................................ 19
3.4.2.
ABL Formation and Structure.............................................................................................. 19
3.4.3.
Consequences of Mixing ..................................................................................................... 20
3.5.
Inversions .................................................................................................................................... 21
3.5.1.
Importance of Inversions .................................................................................................... 21
3.5.2.
Synoptic Weather................................................................................................................ 21
3.5.3.
Boundary Layer Thickness Measurements ......................................................................... 22
3.6.
Atmospheric Particulates ............................................................................................................ 23
3.6.1.
Introduction ........................................................................................................................ 23
3.6.2.
Particulate Size Distributions .............................................................................................. 24
3.6.3.
Sources ................................................................................................................................ 26
3.6.4.
Concentration ..................................................................................................................... 27
3.6.5.
Transport in Free Troposphere ........................................................................................... 28
3.6.6.
Global Distribution .............................................................................................................. 29
4.
4.1.
Impacts on Mechanical Systems ..................................................................................................... 30
Wear............................................................................................................................................ 30
4.1.1.
Basic Wear Process ............................................................................................................. 30
4.1.2.
Areas Most Impacted by Wear ........................................................................................... 31
4.1.3.
Impact of Particle Size on Wear .......................................................................................... 31
4.1.4.
Impact of Particulate Concentration on Wear .................................................................... 32
4.1.5.
Impact of Humidity on Particulate Wear ............................................................................ 32
4.2.
Fouling......................................................................................................................................... 33
4.2.1.
Fouling Introduction and Impacts ....................................................................................... 33
4.2.2.
Parameters Impacting Fouling ............................................................................................ 33
4.2.3.
Impact of Humidity on Fouling............................................................................................ 34
4.3.
Conclusion ................................................................................................................................... 35
Works Cited ................................................................................................................................................. 36
Table of Figures
Figure 1: Atmospheric Pressure by Altitude ............................................................................................... 11
Figure 2 Atmospheric Pressure Adjusted to Sea Level Actual .................................................................... 12
Figure 3 Temperature and Humidity over Chung-Li Taiwan December 2003 and May 2004 .................... 16
Figure 4 Averaged Relative Humidity Readings Taken at Terra Nova Bay Antarctica ................................ 17
Figure 5 Calculated Mixing Ratio Profile ..................................................................................................... 18
Figure 6: Annual Mean Mixing Ratio in Middle Latitudes (Dutton, p. 87) .................................................. 18
Figure 7: Stull 4.9 ........................................................................................................................................ 20
Figure 8: Temperature profile with altitude showing inversions ............................................................... 22
Figure 9 Average planetary boundary layer height retrieved using Giovanni ............................................ 23
Figure 10: Particle Size Distribution ............................................................................................................ 24
Figure 11: Average particle diameter by height ......................................................................................... 25
Figure 12 Particulate Source Regions ......................................................................................................... 26
Figure 13 Particulate Density by Altitude ................................................................................................... 28
Figure 14 Particle contribution from transport by year.............................................................................. 29
Figure 15 Global mean particle density ...................................................................................................... 30
Figure 16: Eroded High Stage Compressor Blades (left) and High Pressure Turbine Blades (Right) (Dunn,
p. 341) ......................................................................................................................................................... 31
Figure 17: Wear Rate vs. Partial Pressure of Water Vapor After Fang Figure 7 ......................................... 32
Figure 18: Fang Figure 7 .............................................................................................................................. 33
Figure 19 % Power Loss vs. Absolute Humidity .......................................................................................... 35
Table of Tables
Table 1: Composition of the Atmosphere (Stull, p. 8) .................................................................................. 7
Table 2 Cold Temperature Error Table (Federal Aviation Administration, pp. 7-2-4) ................................ 13
Table 1: Standard Particle Size Distribution................................................................................................ 25
1. Abstract
2. Introduction
This study examines the impacts of the atmosphere on the life of aircraft gas turbines. Since gas
turbines operate from ground level up to approximately 40,000 ft, and throughout the world, it is
necessary to understand a large portion of the global atmosphere. This paper begins by developing a
model to describe the average atmosphere, with the ability to account for sea level temperature
and pressure. The paper then considers the impact of local variations in the atmospheric boundary
layer. Once the general structure of the atmosphere is understood particulate distribution and
transport within the atmosphere is studied.
The important modes of turbine deterioration related to these atmospheric parameters are
discussed. The impact of various values of the atmospheric parameters on the rates of degradation
are developed.
Identification of Relative Severity
3. Atmospheric Model
3.1.Introduction to Atmospheric Model
It is said in New England “Don’t like the weather? Wait a minute, it will change.” This is in fact true in
the majority of locations on earth. It is also true that change can be found in any direction, including
up.
This study examines the effect of atmospheric conditions on the life of aircraft gas turbines. It is
impractical, if not impossible, to model the instantaneous structure of the atmosphere during every
flight in the life of an engine. Fortunately, it is possible to describe an average atmosphere which,
over the course of many flights and a long period of time, closely resembles the environment
experienced in operation.
The relationships developed in this study describe average conditions. They will rarely, if ever,
correctly describe instantaneous conditions exactly. Where there structure of the atmosphere can
consistently differ from the average, such as a ground level inversion present each morning or
evening, these effects are described.
3.2.Basic Atmosphere
3.2.1. Atmospheric Composition
The atmosphere is composed of a mixture of gasses. For the purposes of analyzing the
atmosphere using the ideal gas law, those gases must be known in order to assign an accurate
gas constant. Dry air, that is the atmosphere excluding water vapor is:
Table 1: Composition of the Atmosphere (Stull, p. 8)
Symbol Name
N2
Nitrogen
Fractional
Molecular Volume
Molecular
Weight
Fraction Weight
28.01
78.08 21.870208
32
20.95
6.704
O2
Oxygen
Ar
Ne
He
Argon
Neon
Helium
H2
Hydrogen
Xe
Xenon
Carbon
dioxide
131.3 0.000009
44.01
0.035
CH4
Methane
16.04
0.00017
N2O
Nitrous
Oxide
44.01
0.00003
CO
Carbon
Monoxide
28.01
0.0035
SO2
Sulfur
Dioxide
64.06 0.000014
O3
Ozone
NO2
Nitrogen
Dioxide
CO2
39.95
20.18
4
2.02
0.93 0.371535
0.0018 0.0003632
0.0005
0.00002
0.00005
1.01E-06
48 0.000012
Totals
46.01 0.000005
100.00
1.182E-05
0.0154035
2.727E-05
1.32E-05
0.0009804
8.968E-06
5.76E-06
2.301E-06
28.96
Dividing the Universal Gas Constant by the Molecular weight of dry air:
8.314472 𝑘𝑃𝑎 ∙ 𝐾 −1 ∙ 𝑚3 ∙ 𝑘𝑚𝑜𝑙
𝑘𝑔
28.96𝑘𝑚𝑜𝑙
yields the gas constant for dry air:
ℛ𝑑 = 0.287053 𝑘𝑃𝑎 ∙ 𝐾 −1 ∙ 𝑚3 ∙ 𝑘𝑔−1 (Stull, p. 8)
3.2.2. Atmospheric Structure
The earth’s atmosphere is divided into distinct layers delineated by temperature extremes.
Beginning at the ground where temperatures are warmest, with energy being derived from
absorption of visible light. Temperature decreases with distance from the ground until the
stratosphere is reached. Temperature increases through the statopause until the level where
most ultraviolet light is absorbed, the stratopause. After passing the stratopause temperature
again declines through the mesosphere, until entering the troposphere where most other
radiation is absorbed. (Stull, p. 13)
The troposphere is the portion of the atmosphere nearest the ground in which nearly all clouds
and weather occur (Stull, p. 13). The temperature of the troposphere decreases linearly with
altitude. The top portion of the troposphere is the tropopause. Within the tropopause
temperature is constant. The transition from troposphere nominally occurs at a height of 11
km. The tropopause continues to a height of 20 km. Since aircraft activity using gas turbines is
generally confined to altitudes well below 20 km only the tropopause and troposphere will be
considered in this study.
3.2.3. Standard Model of the Atmosphere
A standard model of the atmosphere exists for the purpose of modeling atmospheric behavior.
The 1976 U.S. Standard Atmosphere is widely used for modeling the relationships between
temperature, pressure, and height.
3.2.3.1.
Geopotential Height
In modeling the atmosphere meteorologists apply adjustments to account for the variation
in acceleration due to gravity (Stull, p. 12) and centrifugal force (Dutton, p. 65) due to
differences in distance from the center of the earth. The adjustment for geopotential
height is given by:
𝑍=
980.6160[1−2.64×10−3 𝑐𝑜𝑠2𝜙+5.9×10−6 𝑐𝑜𝑠2 2𝜙]𝑐𝑚/𝑠2 ∙𝑎
980𝑐𝑚/𝑠2
𝑧
(𝑎+𝑧) (Dutton, p. 65)
Where 𝑎 is equal to the average radius of the earth, 6356.766 km, 𝜙 represents latitude,
and 𝑧 is altitude above sea level. Using the above equation shows the difference between
0 and 90 degrees latitude is 0.53%. Similarly, the difference between true height and
geopotential height at 15 km is 0.24%.
3.2.3.2.
Temperature
The 1976 Standard Atmosphere provides equations defining temperature curves for
standard atmospheric temperature at altitudes up to 51 km. The vast majority of aircraft
activity takes place below 15 km, so two of these equations are of interest. First, for
altitudes less than 11 km, i.e. within the troposphere:
𝐾
𝑇 = 288.15𝐾 − (6.5 𝑘𝑚
)𝐻 (Stull, p. 13)
Note that 288.15K is equal to 15°C, which is standard sea level temperature. (Stull, p. 7).
𝐾
The value 6.5 𝑘𝑚
is the temperature lapse rate, which is fairly constant regardless of
location and season (Dutton, p. 83). For altitudes between 11 km and 20 km in the
standard atmosphere, and area of atmosphere known as the tropopause, temperature is
constant:
𝑇 = 216.65𝐾 (Stull, p. 13)
The tropopause begins when the temperature reaches 216.65K, so it is evident considering
the constant lapse rate that the tropopause will begin lower than 11 km when sea level
temperatures are below 15°C, and higher than 11 km when sea level temperatures are
above 15°C (Dutton, p. 83). Thus the equations for standard atmosphere can be easily
adapted to any atmosphere by adjusting the ground level temperature:
𝐾
𝑇 = 𝑇𝑠𝑙 − (6.5𝑘𝑚) ∙ 𝐻
The maximum altitude for which the above is valid is:
𝐻=
216.65𝐾
𝐾
𝑇𝑠𝑙 − (6.5𝑘𝑚
)
Beyond this altitude the remainder of the atmosphere of interest to this paper will be at
216.65K.
These equations are applicable for most day and night conditions but at night, in cases
where the surface is cooler than that atmosphere, such as winter or marine conditions
temperature may increase with altitude briefly before resuming the normal stratospheric
pattern. This condition is called an inversion and should be accounted for if examining
winter conditions or low level marine areas. (Dutton, p. 67)
3.2.3.3.
Pressure
The pressure of the Standard Atmosphere is described by equations dependent on the
temperatures calculated above. The equation for pressure in the troposphere is:
288.15𝐾 −5.255877
)
𝑇
P=(101.325𝑘𝑃𝑎) ∙ (
(Stull, p. 13)
And the pressure in the tropopause is given by:
P=(22.632𝑘𝑃𝑎) ∙ 𝑒 −0.1577∙(𝐻−11𝑘𝑚)
Again, it is desirable to generalize these equations for an atmosphere with arbitrary sea
level temperature. This has already been accomplished for the stratospheric pressure
equation by generalizing its temperature equation. The pressure equation for the
tropopause must be modified by replacing the constant 11 km with the maximum
troposphereic altitude calculated above.
−0.1577∙(𝐻−
P=(22.632𝑘𝑃𝑎) ∙ 𝑒
3.2.3.4.
216.65𝐾
𝐾 )
𝑇𝑠𝑙 −(6.5
)
𝑘𝑚
Pressure Altitude
Ambient pressure is used to determine altitude in aircraft. Different methods are used
depending on intended altitude. In all jurisdictions there exists a pressure altitude above
which standard atmospheric pressure is used as a baseline for determining altitude. In the
United States that altitude is 18,000 ft above mean sea level (MSL) Below 18,000 ft MSL
altitude is adjusted to actual pressure at a station within 100 nautical miles of aircraft
position. (Federal Aviation Administration, pp. 7-2-1)
Since this paper is primarily concerned with the local atmosphere an aircraft experiences,
altitude will be normalized to local conditions.
The formula used for calibrating altimeters is:
𝑝 0.1903
518.4°𝑅
ℎ𝑝 = 0.00357°𝑅/𝑓𝑡 [1 − (𝑝 )
0
] (Leishman, p. 213)
This is simply the standard atmosphere equation from above re-arranged and using English
units with the exception that true height, not geopotential height, is used. Given this, and
the minimal differences between geopotential and true height within altitudes of interest,
true height will be used in this paper.
It should be noted that the above equation does not compensate for temperature. If the
temperature is above standard for the indicated altitude an aircraft is higher than the
altimeter indicates. Alternately an aircraft is lower than indicated if the outside
temperature is lower than standard. (Federal Aviation Administration, pp. 7-2-3) This
effect is illustrated in Figure 1 below.
Atmospheric Pressure by Altitude
140
Pressure (kPa)
120
100
80
0°C
60
15°C
40
30°C
20
0
0
5
10
15
20
Altitude (km)
Figure 1: Atmospheric Pressure by Altitude
The discussion thus far has not considered the effect of density. Though in any column of
air pressure will decrease with temperature as altitude increases, a low temperature
column of air will tend to have a higher than standard sea level pressure as a result of
higher air density. The converse is true of warm air. As a result Figure 1 shows impossibly
small atmospheric pressures at sea level. To achieve a more accurate result actual, rather
than calculated, sea level pressure must be used as a starting point. This is done by
replacing 101.325 kPa in the pressure equation with an appropriate value to yield the
actual sea level atmospheric pressure. For example, to achieve a more realistic sea level
pressure of 101.327 kPa at 0°C the equation becomes:
288.15𝐾 −5.255877
)
𝑇
P=(134.2𝑘𝑃𝑎) ∙ (
The resulting curve is shown against the standard curve in Figure 2.
2.5
100
2
Pressure (kPa)
120
80
1.5
60
1
40
0.5
20
0
0
-1
Divergence (kPa)
Atmospheric Pressure by Altitude Adjusted to
Sea Level Actual
0°C
15°C
Divergence
-0.5
1
3
5
7
9
11
13
15
Altitude (km)
Figure 2 Atmospheric Pressure Adjusted to Sea Level Actual
This adjustment, equivalent to setting an altimeter to an airfield barometer measurement
produces a slow deviation from the correct value as altitude increases which is in line with
cold temperature error data supplied to pilots as shown in Table 2.
Table 2 Cold Temperature Error Table (Federal Aviation Administration, pp. 7-2-4)
3.2.3.5.
Density Altitude
Though density altitude will not be used extensively in this paper it is an important value in
aviation and is helpful to clarify the discussion above. Warm air is generally less dense than
cold air, consequently warm air usually has a higher pressure than cold air. Confusion can
arise since this is the opposite of the altitude relationship. It must be recalled that the
temperature altitude relationships described above only hold within a column of air with
constant sea level pressure. Altitude calculations must be continually corrected to sea level
pressure.
Density altitude refers to an alternate method of computing altitude using air density
rather than pressure. This method is generally used on the ground for the purpose of
calculating required take off distance. As temperature rises, density altitude increases.
Density is important to flight dynamics, but is less important to the concerns of this paper.
3.3.Moisture in the Atmosphere
Water vapor can make up from 0 to 4% of air by volume. The amount of water in the air affects the
atmosphere in diverse ways. As a result there are diverse measures of the quantity used to examine
various effects. For the purposes of this paper certain measures are of more utility than others. In
this section the measures will be examined and their respective advantages discussed.
3.3.1. Measures of Water Content
3.3.1.1.
Relative Humidity
Perhaps the most commonly heard measure of water content in the atmosphere is
Relative Humidity. Relative humidity is a comparison of the amount of water in the air to
the amount that can remain in vapor form under present conditions. That is:
𝑅𝐻
𝑒
𝜌
𝑟
= = ≈
100% 𝑒𝑠 𝜌𝑠 𝑟𝑠
Where e represents partial pressure, 𝜌 represents absolute humidity, and r represents
mixing ratio. Each of these measures will be discussed in more detail. The subscript s
represents saturation. This is the maximum amount of water that can remain in vapor
form the present temperature. So as temperature varies, the same absolute amount of
water vapor will result in different relative humidities. Relative humidity provides a
measure of how easily water, or perspiration will evaporate so it is a useful measure of
comfort to include in weather reports. However, if the actual amount of water in the
atmosphere is of primary concern, relative humidity is a cumbersome measurement
because temperature must also be specified to determine the absolute quantity of water.
3.3.1.2.
Dew Point
If relative humidity measures the amount of water vapor relative to the maximum amount
that can exist at the current temperature, then there must be a temperature at which that
same amount of water can no longer be held as vapor. This temperature is the dew point.
The dew point is 100% relative humidity for a given quantity of air. Dew point is easily
measured by chilling a mirror until dew forms, and provides a value that can be converted
into the amount of water in the atmosphere without having to know temperature. (Stull,
p. 102) If current temperature is known, dew point provides a measure of relative
humidity either own its own or by calculating saturation for that temperature.
While dew point is a useful measurement technique, it can be converted into values that
are more useful for computation.
3.3.1.3.
Partial Pressure
Partial pressure is the portion of atmospheric pressure contributed by water vapor. It is
independent of the other gasses in the air. The maximum partial pressure, saturation
pressure, is determined by:
𝑒 = 0.611𝑘𝑃𝑎 ∙ 𝑒𝑥𝑝 [
0.611𝑘𝑃𝑎∙(𝑇−273.16𝐾)
] (Stull,
𝑇−35.86𝐾
p. 98)
The dew point temperature can be entered to determine the partial pressure of water
vapor in the atmosphere. If current temperature is entered the formula yields the
saturation partial pressure. These values can be used to determine relative humidity.
Partial pressure is a convenient measure because it includes both water vapor quantity
information and temperature information. It can also be used to calculate several other
values.
3.3.1.4.
Absolute Humidity
Absolute humidity is a measure of the quantity of water vapor in a given volume. It can be
calculated from partial pressure as follows:
𝜌𝑣 =
𝑒
ℜ𝑣 ∙𝑇
𝑒
𝑃
= 𝜀 ∙ 𝜌𝑑 (Stull, p. 99)
Where ℜ𝑣 is the water vapor gas constant, 0.4615 𝑘𝑃𝑎 ∙ 𝐾 −1 ∙ 𝑚3 ∙ 𝑘𝑔−1 (Stull, p. 446)
3.3.1.5.
Water Vapor Mixing Ratio
Water vapor mixing ratio is the ratio of the mass of water vapor to the mass of dry air. This
makes it useful for performing calculations within the ideal gas equation. The water vapor
mixing ratio is given by:
𝜀∙𝑒
𝑟 = 𝑃−𝑒 (Stull, p. 99)
Where:
𝜀=
3.3.1.6.
ℜ𝑑
ℜ𝑣
= 0.622 (Stull, p. 99)
Virtual Temperature
Virtual temperature is a modification to the ideal gas law used by meteorologists to
account for reduced density due to water vapor in air. (Dutton, p. 258) Virtual
temperature is defined by:
𝑇𝑣 = (1 + 0.61 ∙ 𝑟)𝑇 (Stull, p. 8)
The total mass of water in a column of air can be represented in the same was as the mass
of a column of air was treated in developing the Standard Model of the Atmosphere
(Dutton, p. 258). As such, water vapor pressure i.e. partial pressure, will vary according to
the same rules as air pressure, and it is appropriate to substitute 𝑇𝑣 into the generalized
tropospheric temperature equations developed above.
𝐾
𝑇𝑣 = 𝑇𝑣_𝑠𝑙 − (6.5𝑘𝑚) ∙ 𝐻
By using this temperature a slightly different pressure altitude relationship is obtained.
Humidity is not taken into account in altimeter calibration, this is a minor source of error.
Care must be taken to remember that virtual temperature is not actual temperature. It is a
construct to be used in conjunction with the ideal gas law to adjust for water in the gas
mixture.
3.3.2. Humidity/Altitude Relationship
Published data shows that the humidity altitude relationship depends on weather conditions
and does not follow a general equation. Such data is shown in Figure 3 (Chiang & Subrata
Kumar Das, 2009).
Figure 3 Temperature and Humidity over Chung-Li Taiwan December 2003 and May 2004
However, is data is averaged over a longer period of time relative humidity begins to appear
more constant from the surface up to the tropopause. Figure 4 shows the average of data
collected over a 12 year period (Tomasi et al, 2004) .
Figure 4 Averaged Relative Humidity Readings Taken at Terra Nova Bay Antarctica
For the purpose of an average analysis, if we assume relative humidity remains constant as
altitude increases then:
𝑒 = 𝑅𝐻 ∙ 𝑒𝑠
The resulting mixing ratio profile appears as shown in Figure 5.
Mixing Ratio Profile Assuming
Constant RH
Altitude (km)
16
14
12
10
8
6
4
2
0
0.001
0.01
0.1
10% RH
50% RH
100% RH
1
10
100
Mixing Ratio (g/kg)
Figure 5 Calculated Mixing Ratio Profile
Comparison with the averaged measured profile shown in Figure 4 demonstrates reasonable
correspondence in overall shape and magnitude allowing for some smoothing across the tropopause.
Figure 6: Annual Mean Mixing Ratio in Middle Latitudes (Dutton, p. 87)
If the humidity profile for the atmosphere at a specific time is desired then actual
atmospheric soundings are required. If averaged are to be used assuming a constant
relative humidity up to the tropopause appears to be a reasonable assumption.
In a still atmosphere with no precipitation the amount of water, measured either by
absolute humidity or mixing ratio, will remain stable throughout daily temperature
variations. If local temperature drops below the dew point, dew will form. In cases when
inversions occur due to a relatively cool surface, low lying fog may form though
moisture remains in vapor form at higher elevations. As discussed above, when
analyzing winter conditions or low lying marine areas it must be recalled that such a
situation can occur.
3.3.3. Conclusion
Thus far a model has been developed that can be adjusted for average local temperature, pressure and
humidity. At this point in our study we are able to replicate with reasonable accuracy ambient
temperature, pressure, and water vapor content from sea level to 15 km.
3.4.Atmospheric Boundary Layer
3.4.1. Introduction
The atmospheric boundary layer (ABL) is the portion of the atmosphere that is affected by the
turbulence generated by heating of the earth’s surface by solar radiation (Stull, p. 65).
Understanding of the ABL is important because it strongly influences the distribution of
moisture in the lower troposphere and the ground level concentration of atmospheric
particulates.
3.4.2. ABL Formation and Structure
The ABL forms within the context of the standard atmosphere (Stull 66). Recalling the
discussion of density above, consider a parcel of air near the surface which is warmed by the
sun. Warming the air decreases its density which causes it to buoyantly rise. This tendency of
the air to move from its original altitude is referred to as instability (Stull, p. 66). As the air rises,
it will encounter an altitude at which it is stable. Surface air will continue to be heated by solar
radiation, resulting in a continuous circulation from the surface to this stable level. This
constant churning results in homogeneous atmospheric properties in the first one to four
kilometers of the troposphere (Stull, p. 66). That is, if air from various altitudes is normalized to
the same pressure it will have the same virtual temperature. Above this mixed layer the
troposphere has the same temperature, on average, as predicted by the standard atmosphere
model (Stull, p. 67).
Clearly, this will result in a sharp temperature shift at the top of the mixed layer. This creates a
stable layer which is often referred to as the capping layer. The area above is called the free
troposphere (Stull, p. 67). The capping layer prevents air exchange from the lower troposphere
to the free troposphere.
The extent to which the boundary layer follows this pattern with vary with the degree of solar
heating. In the case that the surface temperature is similar to air temperature, often associated
with overcast daytime conditions, there will be minimal circulation, and a weak capping layer.
Under these conditions the atmosphere will closely resemble the standard atmosphere with
little evidence of a boundary layer.
Finally, the surface may be cooler than the atmosphere. A thin, 20-500m (Stull, p. 66), layer of
stable air will form. As circulation ceases, the air between the stable layer and capping
inversion will become stable. This area becomes known as the residual layer (Stull, p. 69). It
continues to contains humidity and pollutant contents similar to the boundary layer that
existed previously (Stull, p. 69).
3.4.3. Consequences of Mixing
The mixing within the ABL results in a uniform mixing ratio up to the capping inversion.
Stull Figure 4.9
Figure 7: Stull 4.9
The impact of ABL mixing on particulate concentration will be considered later in this study.
3.5.Inversions
3.5.1. Importance of Inversions
Though it is desirable to understand the thermal and humidity profiles resulting from the ABL,
especially for aircraft that operate primarily below 2km, the most important aspect of the ABL
is its capping inversion.
Strong inversions prevent the exchange of air and its contents, such as particulates and
humidity, between the boundary layer and the free atmosphere. This can result in high
concentrations of particulates at ground level. Thus it is important to understand when
inversions will be present, and at what altitudes.
3.5.2. Synoptic Weather
Inversion strength is influenced by both the conditions in the ABL, and the interactions of air
masses at frontal boundaries, synoptic weather (Stull, p. 68). Strong inversions will occur at
boundaries between warm and cold air masses (Stull, p. 263). These tend to be low altitude
inversions which result in shallow mixing regions (Zhang, p. 5529).
The nature of the air mass also affects the strength of the inversion. Within a high pressure
mass the inversion is strong (Stull, p. 68) as is the tendency for mixing resulting in highly
homogeneous properties up to a relatively high altitude inversion (Zhang, 5530). Within a low
pressure system there will be a weak, if any, capping inversion (Stull, 68). Under these
conditions there will be constant gradients from the surface up to the tropopause, as in the
standard atmosphere.
Figure 8: Temperature profile with altitude showing inversions
3.5.3. Boundary Layer Thickness Measurements
NASA GEOS data is widely used as source data for Boundary Layer Thickness in atmospheric
modeling (Lin, p. 1726). GEOS data is available from NASA through the MERRA data set.
Planetary Boundary Layer Thickness is contained in the 2 dimensional turbulence data set:
tavg1_2d_flx_Nx. The Giovanni tool is available for plotting desired parameters over any time
scale from 1974 through approximately 2 months preceding the current date. An example of
Giovanni output is shown below.
Figure 9 Average planetary boundary layer height retrieved using Giovanni
This information will be used to obtain the crucial boundary layer height parameter when
modeling atmospheric structure.
3.6.Atmospheric Particulates
3.6.1. Introduction
Particulates of a variety of sizes are suspended in the atmosphere. These particles come from a
variety of natural and anthropogenic sources. These particulates are of importance because
they can result in fouling of aircraft gas turbine aerodynamic surfaces (Stalder, p. 364), and can
cause wear to those same surfaces (Warren, 2008, p. B1). This study will examine the sources
of contamination with the aim of understanding variation by region. Vertical concentration
profiles will also be examined.
This study will not include events which result in extreme conditions typically avoided by
aircraft, such as volcanic eruption and catastrophic forest fires. Conditions resulting from
operations on unimproved surfaces, e.g. helicopter landings on loose soil, will also not be
considered.
3.6.2. Particulate Size Distributions
For air quality measurements particle sizes are usually reported in terms of PM-10 and PM-2.5.
PM-10 refers to the quantity of particles 10 micrometers in diameter or less. Likewise, PM-2.5
refers the particles 2.5 micrometers or less. Thus, PM-2.5 is a subset of PM-10. PM-2.5 is often
referred to a fine particles, while PM-10 may be referred to as a coarse particle measurement
(Environmental Protection Agency, 2010, p. 20)
PM-10 was the dominant measurement until the mid 1990’s when evidence increasingly
showed that PM-2.5 was a more important measure of the human health impact of particles
(Environmental Protection Agency, 1995, p. 26). In recent literature PM-2.5 is more commonly
reported.
Neither PM-2.5 nor PM-10 provides a complete profile of particles in the air. Average
atmospheric particulate profiles have been reported for the purposes of aircraft contamination
exposure estimation (SAE, 2007, p. 5).
Figure 10: Particle Size Distribution
This figure agrees well with the recommended particulate distribution given in SAE ARP986
(SAE, 2008) which is shown below. This particle distribution will be used for scaling reported
PM-2.5 and PM-10 particulate to obtain a complete ground level particle distribution.
Table 3: Standard Particle Size Distribution
The mean particle size, and thus the particle distribution, remains remarkably constant with
respect to altitude (Zhang, 5530), so the same distribution will be used for all altitudes.
Figure 11: Average particle diameter by height
3.6.3. Sources
3.6.3.1.
Natural
Naturally occurring atmospheric particulates originate from regions with loose soil and
sufficient winds to lift those particulates from the surface (Engelstaedter, p. 76). The
largest emitter is Saharan Africa, which accounts for the majority of global dust emissions
(Engelstaedter, 75). The below figure shows measurement of dust concentration in terms
of an aerosol index (Engelstaedter, 73) and highlights the importance of Saharan Africa as
an emitter of natural particulates.
Figure 12 Particulate Source Regions
It should be noted that the data source used tends to exclude results less than 1 km from
the surface which means anthropogenic high concentration events will not be shown
(Englestaedter, 77). This is because, as discussed above, high concentration anthropogenic
events are associated with a shallow boundary layer, usually much less than 1 km.
3.6.3.2.
Anthropogenic
Anthropogenic sources of atmospheric particulates are primarily related to combustion.
These include burning of fossil fuels and biomass for energy and transportation, as well as
burning of biomass. Human activities can also exacerbate the natural emissions discussed
above through improper land use.
Anthropogenic sources can account for nearly 75% of measureable particulates in urban
areas (Baek, p. 211), and about 65% in suburban areas (Baek, 210) . The anthropogenic
portion of measure particulates represents an increase over the natural background
quantity of particulate (Jones, p. 4467). The amount of particulate in the atmosphere due
to anthropogenic sources is sensitive to the level of human activity, even varying
noticeably between weekday and weekend automotive traffic levels (Jones, 4467). Given
the variability and significance of anthropogenic sources it is clear that this study will
require local average particulate levels to develop an understanding of particulate impacts
at any given location.
3.6.4. Concentration
Particulate concentration at ground level is heavily dependent upon boundary layer thickness
(Zhang, 5529). Conditions within the boundary layer also strongly affect the variation in
particulate distribution with respect to altitude due to the variations in mixing discussed above.
This variation will strongly impact the dust conditions at any given moment, however, average
conditions will give a reasonable representation of long term operation within a specific area.
Average concentrations will be considered in two regions: the boundary layer and the free
troposphere. Within an average boundary layer the air may be considered to be well mixed.
Likewise, the residual layer under night time conditions will remain well mixed. For this reason,
concentration within the boundary layer will be considered constant up to the top of the
boundary layer.
Since the boundary layer is topped by a capping inversion particulates have difficulty ascending
above the boundary layer, and therefore generally have low concentrations above that height.
This atmospheric structure is clearly shown for several inversion heights (Type 1~ 3 km, Type 2~
2 km, Type 3~ 1 km) in the figure below. The different colors in the plot represent different
dates and slightly different inversion heights.
Figure 13 Particulate Density by Altitude
3.6.5. Transport in Free Troposphere
Atmospheric particulates are known to be transported between regions. Dust from Asian
sources contributes to 41% of major dust events in the Western United States (Fairlie, p. 1251).
Dust originating in Saharan Africa can be transported across the Atlantic Ocean to North and
South America, north to Europe, and east to the Middle East Region and Asia (Engelstaedter,
80).
While seasonably variable, these particulate transport paths follow a predictable annual cycle.
Years with El Nino events show increased dust transport, and there may be an increasing in the
trend due to climate change. The cycle is sufficiently consistent, however, that the effect of
transport can be considered to be accounted for in local averages (Engelstaedter, 91).
Figure 14 Particle contribution from transport by year
It must be considered whether aircraft operating within the particle transport paths will be
exposed to higher concentrations of particles. The particles involved in transport are at the
level of the top of the boundary layer. Concentrations closely resemble the concentration in the
originating region (Colette, p. 393). Therefore the model discussed above will be appropriate
for an aircraft encountering a transport layer on departure. Air aircraft at cruise will be
operating well above the top of the boundary layer so this contamination is not of concern.
Therefore the model developed above is adequate.
3.6.6. Global Distribution
The global distribution of particulate contaminants is available from satellite measurements
(van Donkelaar, 2010). The figure below shows global averages for 2001 through 2006. These
reference values will be used in combination with the model developed above to estimate
particulate profiles for cities of interest. Combined with the boundary layer thickness
measurements discussed above, this information will be used to determine the amount of
exposure experienced by aircraft operating in the region.
Figure 15 Global mean particle density
4. Impacts on Mechanical Systems
Aircraft gas turbine engines are designed to accommodate variation of the parameters
discussed in the atmospheric model: pressure, temperature, humidity. While the variation of
these parameters affects the thermodynamics of the engine, possibly demanding operation at
modified power settings in certain environmental conditions, such impacts would be highly
specific to individual designs and will not be considered in this study.
Particulate exposure affects all turbine engines. As has been introduced above, atmospheric
conditions can greatly influence the quantity of particulate exposure an engine experiences.
This section will discuss how atmospheric conditions also affect the impact of particulate
exposure.
4.1.Wear
4.1.1. Basic Wear Process
The primary factors that impact particulate wear are (Finne, p. 81):
1)
2)
3)
4)
5)
6)
Angle of Impingement
Particle Rotation at Impingement
Particle Velocity Impingement
Particle Size
Surface Properties
Shape of the Surface
7)
8)
9)
10)
Stress Level at the Surface
Particle Shape and Strength
Particle Concentration in the Fluid Stream
Nature of the Carrier Gas and its Temperature
Clearly, many of these factors are a function of the particular area, not the environment in
which it operates. Of this list, Particle Concentration is the sole factor which may be expected
to vary with environment based on what has been learned above.
4.1.2. Areas Most Impacted by Wear
Foreign Object Debris (or Damage), without denying that local atmospheric conditions may
affect the local concentration of birds and rain-hoods, is outside the scope of this study.
Particles discussed will be limited to the size range outlined in Table 3. Within a gas turbine
engine the severity of particulate wear increases with material temperature (Tabakoff, p. 543) ,
that is, wear is most severe on either side of the combustor (Ogaji, p. 28) (Sunderarajan, p.
346). So while all areas of the engine will eventually succumb to particulate wear, the impacts
will first be seen in the high compressor, high turbine or combustor.
Figure 16: Eroded High Stage Compressor Blades (left) and High Pressure Turbine Blades (Right) (Dunn, p. 341)
4.1.3. Impact of Particle Size on Wear
Particle size has a limited impact upon wear. For particle sizes greater than 50-100µm erosion
rates do not vary with respect to particle size (Finne, p. 87) (Sunderarajan, p. 340). As discussed
above, the distribution of particles less than 100µm is suitably constant with respect to
elevation and region. Therefore, for global averages, it is sufficient to understand the average
particle distribution, but it is also critical to remember that if local pollution conditions skew the
particle size distribution towards 100µm wear rates can be severely affected.
4.1.4. Impact of Particulate Concentration on Wear
For the most part, wear rate is simply related to the number of particle impacts (Sunderarajan,
p. 340). Therefore, in an increased particulate concentration the number of impacts will
predictably increase, and the wear rate will increase commensurately. At extreme particle flux
rates, which are outside the scope of this study, particle interference may affect wear rates
(Sunderarajan, p. 342). For the purposes of this study wear rate will increase linearly with
particulate exposure.
4.1.5. Impact of Humidity on Particulate Wear
Humidity can have an effect on the wear rate of materials (Fang, p. 144) (Lancaster, p. 371). At
or near room temperature the effect can be pronounced as the wear rate varies approximately
in relation to the partial pressure of water vapor:
Wear Rate vs. Partial Pressure
15
25 Degrees
10
35 Degrees
5
45 Degrees
55 Degrees
0
0
5
10
15
20
Figure 17: Wear Rate vs. Partial Pressure of Water Vapor After Fang Figure 7
Much more clearly than the presentation in Fang (Fang, p. 149):
Figure 18: Fang Figure 7
This effect is likely important in external components and associated systems. However, since
the most crucial wear occurs in the hot section of the gas turbine where variations in water
vapor pressure are insignificant this effect has little bearing on this study.
4.2.Fouling
4.2.1. Fouling Introduction and Impacts
Fouling is the degradation in performance of a turbine engine due to the build-up of
contaminants on the various aerodynamic surfaces. Though fouling of the hot section can occur
as a result of dirty fuel (Stalder, p. 363), this study will focus solely on fouling due to
atmospheric contaminants. This type of fouling is primarily limited the compressor section
(Naeem, p. 248) (Stalder, p. 363). Compressor section fouling accounts for 70-85% of gas
turbine engine performance deterioration (Naeem, p. 248).
4.2.2. Parameters Impacting Fouling
Naeem lists several factors affecting fouling (Naeem, p. 248) :






compressor’s design,
compressor’s airfoil-loading,
aerofoil’s incidence,
aerofoil’s surface-smoothness and coating-material,
type and condition of airborne pollutant, and
operational environment (e.g. a high humidity increases the rate of fouling).
The engine design factors are mostly outside the scope of this paper. Obviously, exact rate
impacts for any engine will depend upon these parameters, but a general discussion of the
environmental influences upon fouling can be made.
Fouling is caused by particles smaller than 2-10µm (Kurz et al, p. 95). The quantity of those
particles in a local atmosphere will vary as discussed above. These local variations have a major
impact on the rate of fouling deterioration an engine experiences (Stalder, p. 365). Since
particles measured by PM2.5 are in the range most likely to cause fouling the particle
measurements this study will use for determining particulate severity are directly applicable to
fouling and local discrepancies from the average distribution will not have as severe an impact
as in the discussion of erosion.
4.2.3. Impact of Humidity on Fouling
Fouling results from particles adhering to water and oil, if present, on surfaces within the
engine (Stalder, p. 365) (Kurz et al, p. 95). Water is present due to the pressure drop
experienced at the inlet to the engine (Stalder, p. 365). The rate of fouling is related to the total
humidity, which as discussed above is directly related to the partial pressure of water vapor in
the surrounding atmosphere. Stalder presents data showing that the rate of power loss will
peak for a certain level of atmospheric humidity. The x-axis in the figure below shows tons of
water passing through the engine in 70 hours.
Figure 19 % Power Loss vs. Absolute Humidity
As humidity increases some amount of the water present will flow off the aerodynamic
surfaces rather than adhere, resulting in some particulates being washed away thus reducing
the fouling rate (Stalder, p. 366). The inlet pressure drop leading to droplet formation will vary
even between aircraft using the same engine so the rate of increase in fouling and peak
humidity for fouling cannot be generalized.
In the event that hydrocarbons are present, the water will have very little ability to wash
contaminants away and contamination will continue at a high rate until reaching an equilibrium
point (Stalder, p. 366). This is typically the case in aircraft jet engines (Naeem, p. 248).
4.3.Conclusion
This section has shown that the exact impact on a specific engine of operating in various levels
of particulates and humidity depends upon design factors specific to that engine so a generic
plot of increased erosion or fouling is not possible. It is possible, however, to state that
increased particle concentrations will resulted in increased erosion and fouling for any engine,
and the fouling will be more severe at higher levels of humidity. The next section will examine,
based on the model developed in the previous section, which regions of the world have the
worst combined particle concentrations, atmospheric boundary layer depth, and humidity
levels for aircraft gas turbine operation.
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