Study of Particulate Matter Formation and Evolution in
Near-Field Aircraft Plumes using a One-Dimensional
Microphysical Model
by
Jianye Zhang
B.S.E in Aerospace Engineering (2004)
The University of Michigan, Ann Arbor, Michigan
Submitted to the Department of Aeronautics and Astronautics
in Partial Fulfillment of the Requirements for the Degree of
Master of Science in Aeronautics and Astronautics
at the
Massachusetts Institute of Technology
May 2007
L§Ye L
02007 Massachusetts Institute of Technology. All Rights Reserved
Signature of A uthor...............................
............
....................................................
Department of Aeronautics and Astronautics
May 25, 2007
C ertified by...............................
Ian Waitz
Jerome C. Hunsaker Professor of Aeronautics and Astronautics
Thesis Supervisor
Accepted by...........................
..............
Jaime Peraire
Professor of Aeronautics and Astronautics
Chair, Committee on Graduate Studies
MASSACHUSETTS INSTFUTE
OF TECHNOLOY
JUL 1 1 2007
L
L__
R
Mo~j
Abstract
Environmental concerns have led to a growing effort to investigate and characterize the
particulate matter (PM) emissions from aircraft engines. This thesis presents a onedimensional microphysics and chemical kinetics model that is used to study the formation
and evolution of particulate matter in near field aircraft plumes at ground level. The initial
exhaust properties were obtained from engine data and plume mixing profiles were generated
using FLUENT*. The initial gas species concentrations were estimated based on a two-step
constrained equilibrium process. Parametric studies of the effects of ambient temperature,
ambient relative humidity, engine operating conditions and engine design parameters on the
aerosol formation were investigated using the one dimensional model following a centerline
trajectory up to lkm downstream of the engine exit plane. PM formation and evolution
characteristics along trajectories from several radial locations were also investigated and
compared to the results from the centerline study. The results from this study show that
binary homogeneous H2 SO 4 -H 20 nucleation strongly depends on ambient conditions such as
ambient temperature and relative humidity whereas the condensational growth of soot
particles is most dependent on engine power settings. For example, the nucleation mode
sulfur emissions index varies from 0.01 mg/kg-fuel at 299 K and 80% relative humidity to 9
mg/kg-fuel for the same level of relative humidity at 288K. The nucleation mode sulfur Elm
increased from 0.01 mg/kg-fuel at 10% relative humidity and 7% engine power setting to
about 9 mg/kg-fuel at 80% relative humidity for the same power setting and ambient
temperature. The amount of SvI condensed on soot particles changes from around 2 mg/kgfuel to around 8 mg/kg-fuel as the engine power increases from 7% to 100% for the same
10% relative humidity. Engine design parameters such as engine bypass ratio as well as core
and bypass total temperature also play important roles in both the homogenous nucleation
and heterogeneous condensation processes in the plume. It was found that at 80% relative
humidity and 100% power setting, the sulfur emission index increased from 1.8 mg/kg-fuel
at the high BPR of 9 to a value of 2.5 mg/kg-fuel for the limiting case of zero BPR. The
sulfur emission index for the nucleation mode increased from around 0.7 mg/kg-fuel at the
higher core and bypass total temperature (15 K above the CFM56-2C1 values) to 3.5 mg/kgii
fuel at the lower temperature (15 K below the CFM56-2C1 engine values).The study sheds
light on the PM formation mechanisms in near field aircraft plumes and provides guidance on
the improvement of future experimental measurement techniques.
iii
Acknowledgements
First of all, I would like to thank Prof. Ian Waitz for all his support and guidance throughout
my study at MIT. I was deeply motivated by his constant encouragement and strong belief in
my abilities. His sheer energy, continuous strives for excellence and absolutely positive
attitude towards life and work have all been great inspirations to me. He has been a great
mentor and a role model to me and I enjoyed working with him immensely.
I would also like to express my gratitude to Dr. Richard Miake-Lye, Dr. Hsi-Wu Wong and
Dr. Paul Yelvington at Aerodyne Research Inc. for their support and advice in my research.
They are all brilliant researchers who gamer my total respect for their knowledge and passion
in their field. They are also great individuals who are caring and friendly and it has just been
a wonderful experience working with them. It is also important to mention the help I received
from Ms. Jennie Leith of PARTNER as well as Ms. Lori Martinez and Ms. Holly Anderson
from GTL. They are all wonderful people to work with. I would especially like to thank Dr.
Yifang Gong for his assistance in setting up the computer cluster for my research as well as
his help in answering my various technical questions.
On a personal note, I would like to thank my parents for their love and faith in me which is
the single most import source of strength to me. None of my achievement has been
conceivable without their love and support.
Lastly, I would like to thank my officemates for their company during my two years stay at
MIT. They are all wonderful individuals sparkling with talents. I especially want to thank
Anuja Mahashabde for her advice and help in my study and research. I enjoyed our
conversations a great deal.
This work was supported by Transport Canada and the Office of Environment and Energy,
U.S. Federal Aviation Administration, under FAA Cooperative Agreement No. 03-C-NEMIT, Amendment Nos. 017 and 024. The effort was managed by Carl Ma. Any opinions,
findings, and conclusions or recommendations expressed in this material are those of the
author and do not necessarily reflect the views of the FAA or Transport Canada.
iv
Table of Contents
A b stract ............................................................................................................................................
ii
Acknowledgem ents ..........................................................................................................................
iv
Table of Contents ..............................................................................................................................
v
List of Figures ..................................................................................................................................
vi
List of Tables .................................................................................................................................
viii
Nom enclature...................................................................................................................................ix
Chapter 1 Introduction.......................................................................................................................I
1. 1 B ack gro un d .............................................................................................................................
1
1.2 Thesis Organization .................................................................................................................
3
Chapter 2 M ethodology .....................................................................................................................
5
2.1 Initial Exhaust Properties at Engine Exit Plane....................................................................
5
2.2 Plume M ixing Profiles .......................................................................................................
6
2.3 One Dimensional M icrophysics M odel ...............................................................................
Chapter 3 Results and Discussions...............................................................................................
10
19
3.1 Representative Results from the Simulation ......................................................................
20
3.2 Effects of Engine Power Setting and Relative Humidity .....................................................
24
3.3 Effects of Ambient Temperature ........................................................................................
33
3.4 Effects of Engine Design Parameters .................................................................................
39
3.5 Study of Trajectories from Different Radial Locations ...........................................................
43
Chapter 4 Conclusions.....................................................................................................................49
4.1 Summ ary of major findings...............................................................................................
49
4.2 Recom mendations for future work ......................................................................................
52
Appendix A Tables..........................................................................................................................53
Bibliography ...................................................................................................................................
v
54
List of Figures
Figure 1 GAMBIT* grid showing detailed geometry of the exit section of the CFM56-2C1
Engine
Figure 2 Simulation results from FLUENT®: a) axial velocity contour from FLUENT*
simulation up to 30m downstream of the engine exit; b) Centerline axial velocity profile up
to lkm downstream of engine exit; c) Centerline static temperature profile up to 1km
downstream of engine exit; d) Centerline static pressure profile up to lkm downstream of
engine exit.
Figure 3 Representative simulation results from the one dimensional microphysics model: a)
Evolution of liquid droplets size distribution as a function of downstream distance; b)
Evolution of embryo size distribution as a function of time; c) Soot particle size distributions
at different downstream locations showing condensational growth; d) Comparison of final
size distribution of soot and liquid droplets at lkm downstream; e) Evolution of S1 mass
fraction in vapor phase, soot coatings and volatile droplets.
Figure 4 Sulfur mass emission index atlkm downstream as a function of engine power
settings and ambient relative humidity for an ambient temperature of 288 K and a fuel sulfur
content of 383 ppm: a) 10% RH, b) 40% RH, c) 60% RH, d) 80% RH.
Figure 5 Contour plots of sulfur mass emission index at lkm downstream as a function of
engine power and relative humidity at an ambient temperature of 288 k and a fuel sulfur
content of 383 ppm: a) Soot mode; b) Nucleation mode
Figure 6 Sulfur mass emission index at 80s plume age as a function of engine power settings
and ambient relative humidity for an ambient temperature of 288 k and a fuel sulfur content
of 383 ppm: a)10% RH b) 40% RH c) 60% RH d) 80% RH
Figure 7 Contour plots of sulfur mass emission index at 80s plume age as a function of
engine power and relative humidity at an ambient temperature of 288 k and a fuel sulfur
content of 383 ppm: a) Soot mode; b) Nucleation mode
vi
Figure 8 Sulfur mass emission index at 1 km downstream as a function of engine power
settings and ambient relative humidity for an ambient temperature of 299 k and a fuel sulfur
content of 383 ppm. a) 10% RH b) 40% RH c) 60% RH d) 80% RH
Figure 9 Contour plots of sulfur mass emission index at 1km downstream as a function of
engine power and relative humidity at an ambient temperature of 299 k and a fuel sulfur
content of 383 ppm: a) Soot mode; b) Nucleation mode
Figure 10 Sulfur mass emission index at 80s plume age as a function of engine power
settings and ambient relative humidity for an ambient temperature of 299 k and a fuel sulfur
content of 383 ppm. 10% RH b) 40% RH c) 60% RH d) 80% RH
Figure 11 Contour plots of sulfur mass emission index at 80s plume age as a function of
engine power and relative humidity at an ambient temperature of 299 k and a fuel sulfur
content of 383 ppm: a) Soot mode; b) Nucleation mode
Figure 12 Effects of engine bypass ratio on aerosol formation mechanism: a) Nucleation
mode b) Soot Mode
Figure 13 Effects of engine core and bypass exhaust total temperature: a) Nucleation mode
b) Soot Mode
Figure 14 Results of uncertainty study of the trajectories from radial locations at 80% relative
humidity a) Soot mode b)Nucleation Mode
Figure 15 Results of uncertainty study of the trajectories from radial locations at 60% relative
humidity a) Soot mode b)Nucleation Mode
vii
List of Tables
Table 1. The parameters used for log-normally distributed soot particles under different
power settings
Table 2. Engine design parameter variations
viii
Nomenclature
APEX
ARI
Aircraft Particle Emission Experiment series
=
Aerodyne Research Inc.
BHN
Binary Homogeneous Nucleation
BPR
Engine Bypass Ratio
Elm
=
Mass Emission Index
Number Emission Index
EIn
FAA
=
Federal Aviation Administration
FAR
=
Fuel to Air Ratio
ICAO
International Civil Aviation Organization
KQUN
Kinetic Quasi-Unary Nucleation
PM
Particulate Matter
Bij
coagulation kernel between soot particles in size bin i and droplets in size binj
b(x)
radius where the local properties are Ile of the centerline values at x
Ca
concentration of H2 SO4
Cjk
=
constant
Cs
=
concentration of SO3
c
mean particle thermal speed
ci, cj
speeds of particles i andj
D
average diffusivity
ix
Di, Dj
diffusivities of particles i and]
Dv
diffusivity of water vapor
d
average diameter
di, d
diameters of volatile particles i andj
dp,i
diameter of the soot particles in size bin i
JfL
plume centerline exhaust gas fraction at x
(x)
Gi
collision factor between aerosols and soot particles in size bin i
Ic, Im, Iqc
integral constants
Kg
Brownian coagulation kernel between particles i andj
k
spread constant
MeO
initial excess momentum per unit density
Mw,a
molecular mass of H2SO4
Mw,i
molecular weight of species i
Mw,s
molecular mass of SO 3
Ni
number density of droplets in size bin i
NO
=
Avogadro's number
mi
time-dependent condensed mass on the soot particles in size bin i
ni
concentration of the embryo with size i
Paj
partial pressure of aerosol vapor with sizej
P 0
saturation vapor pressure of aerosol with sizej
a, j
Pv
P00
V
=
partial pressure of water vapor
saturation pressure of water vapor
x
Ta
ambient total temperature
T.. (x)
plume centerline temperature at x
710
=
initial exhaust total temperature
t
time
ua
coflowing velocity
U(CL
plume centerline velocity at x
(x)
initial jet velocity
u=
Xamb,i
=
mass fraction of gas-phase species i or concentration of particulate species i in
the ambient
Xi
=
mass fraction of a gaseous species i, concentration of a aerosol species i, or
condensed mass on a soot particle in size bin i
x
axial downstream distance
ad
mass accommodation coefficient of SO 3 or H 2 SO 4 on inactivated (uncoated)
soot surface
atw
=mass accommodation coefficient of aerosols on activated (coated) soot
surfaces
fu
=
condensation rate of the embryos with size i
evaporation rate of the embryos with size i
y=
3
=
mean free distance between the particles
C
=
fuel sulfur conversion factor
91
=
activated (coated) surface fraction of the soot particles in size bin i
xi
K
=
Kelvin factor
molecular mean free path in air
pa
=
00
cO
density of the plume
average number of sites (molecules) per unit area of soot surface
=
molar production rate of species i
xii
Chapter 1
Introduction
1.1 Background
Environmental impacts of aviation have become increasingly important due to the rapid
growth of the global aviation industry. Because of the surge in demand for air transportation,
emissions of some pollutants from aviation are increasing against a background of emission
reductions from many other sources. Millions of people are adversely affected by these side
effects of aviation [1].
It is known that besides major chemical species such as N 2 , 02, CO 2 and H20, aircraft
engines also emit other species in much lower concentration that may have significant
environmental impacts. Particulate matter (PM) emissions composed of solid particles,
volatile aerosols, and solid particles coated with volatile liquid, belong to this category.
In order to understand the characteristics of particulate matter emissions from aircraft
engines, both experimental measurements and numerical simulation tools have been
developed and employed. Field measurements such as the Aircraft Particle Emission
Experiment series (APEX-1, JETS/APEX-2, APEX-3 [2,3] ) have been conducted to
investigate the particle and trace gas emissions from commercial aircraft engines as a
function of fuel composition, engine power, plume age and local ambient conditions.
Through these experimental studies, the impact of fuel sulfur and aromatic content on soot
and secondary particle formation has been examined. In addition, the evolution of particle
characteristics and chemical composition within the engine exhaust plume has been studied
I
2
as the plume cooled and mixed with ambient air [4]. Although these experiments provide
insight and important data for use in understanding the impact of aircraft emissions,
repeatable and accurate measurements from the experiments are hard to achieve because
some important factors affecting particle characteristics and chemical composition such as
ambient temperature and relative humidity are outside of experimental control. Furthermore,
these experiments are too costly to cover the whole range of important parameters affecting
aircraft emissions. Thus, detailed numerical models have been developed to complement
these experimental studies to gain a more complete understanding of the impacts of
important factors such as ambient conditions, engine design parameters and engine operating
conditions on the PM formation from aircraft engines.
Some previous numerical models have been developed to investigate the particulate matter
formation mechanism for aircrafts cruising at high altitudes [5-15]. There have also been
many studies on PM emissions from automobile diesel engines [16-21]. However, studies on
PM emissions from aircraft engines at ground level where APEX experiments were
conducted are not available. Thus, modeling tools are needed to understand ground level PM
formation mechanisms for aircraft engines. The results from the ground level model can be
used to interpret the APEX measurement data and to provide guidance on improved sampling
methods and probe design in future aircraft emission measurements. In this regard, a detailed
one-dimensional microphysics and chemical kinetics model has been developed by ARI and
MIT to study the PM formation mechanism for ground level aircraft emissions. A description
of the model is presented in this thesis. Parametric studies employing this model are
presented to estimate the effects of ambient conditions, engine operating conditions and
2
3
engine design parameters on the PM formation and chemical composition in the near-field
aircraft plumes.
1.2 Thesis Organization
In this thesis, a detailed one-dimensional microphysical model is used to study the aerosol
formation microphysics in near-field aircraft plumes at ground level. Parametric studies of
ambient conditions and engine operating parameters are presented, and the effects of these
parameters on the formation of aircraft-emitted aerosols are discussed.
Chapter 2 provides a detailed description of the methodology used in the study of the
ground level particulate matter formation in near field aircraft plumes. To perform the
modeling tasks, initial exhaust properties at the engine exit plane were first calculated based
on engine thermodynamic cycle data and a two-step equilibrium process [22]. Plume mixing
profiles were then determined using the fluid dynamics software FLUENT®. Dilution
profiles from several different radial locations were generated. Lastly, the main tool used in
the analysis of particulate matter formation, the one-dimensional microphysical model is
presented. Since binary homogeneous nucleation (BHN) of H 2SO 4 -H 20 forms the basis of
particle growth in near-field aircraft emissions, the focus of the work is the microphysical
processes related to H 2 SO 4-H 20 aerosols and the model thus does not explicitly consider the
effects of ion-induced [35, 36] or organics-enhanced
nucleation processes [37]. These
processes will be studied in future work with the model.
Chapter 3 shows the results and discussions of the parametric study of the effects of
ambient conditions, engine operating conditions and design parameters on particulate matter
3
4
formation in the near field plume. Simulations were carried out at two different ambient
temperatures (288K and 299K), four different ambient relative humilities (10%, 40%, 60%
and 80%), and three different engine power settings (7%, 65% and 100% engine thrust) for a
CFM56-2C1 engine, which was one of the aircraft engines investigated during the APEX
campaigns. Engine design parameters such as BPR and core/bypass exhaust temperatures
were also studied to analyze their effects on PM formation in the near field plume. In
addition, PM formation and evolution characteristics along trajectories from several radial
locations were investigated and compared to the results from the centerline study. The
parameter space explored provides a comprehensive investigation of the sensitivity of the
formation of near field particulate matter at ground level to these factors.
Chapter 4 presents a summary of the major findings from this study as well as some
recommendations for future work.
4
5
Chapter 2
Methodology
2.1 Initial Exhaust Properties at Engine Exit Plane
In order to perform the parametric studies using the one dimensional microphysics model, the
initial exhaust properties at the engine exit plane have to be determined first. These
properties include the thermodynamics properties of the exhaust gas at the engine exit plane,
initial core flow and bypass flow velocities and initial concentrations of the gaseous species.
The engine type examined in this study is the CFM International's CFM56-2C1. This
engine was used in the APEX-1 measurement campaign. Cycle performance data for the
engine at different power settings (7%, 65%, and 100%) were used to establish the
thermodynamic properties such as temperature and pressure at different engine stations.
Initial exhaust temperature, pressure and velocities at the core and bypass flow at the engine
exit plane were also obtained.
The initial gaseous species concentrations were determined using a two-step equilibrium
process [22]. The first step was carried out in the engine combustor. The initial gaseous
species concentrations in the combustor were estimated based on combustor fuel-to-air ratio
(FAR), where the jet fuel was modeled as butene (C 4 H8 ).
The gaseous species in the
combustor were assumed to achieve an equilibrium state in the combustor and final gas
species concentrations at the combustor exit were determined based on this equilibrium
calculation. In the second step, which occurred in the turbine section of the engine, the final
gas species concentrations from the combustor were used as initial conditions for the turbine
5
6
calculation. Again, the gaseous species were assumed to achieve an equilibrium state in the
gas turbine. The final gas species concentrations were then obtained based on the equilibrium
calculation in the turbine. However, it is known that not all gas species are in equilibrium in
the turbine. Those that are not in equilibrium such as CO and NOx were fixed at the levels
reported in the ICAO data base [23]. Other species such as SOx were estimated based on the
fuel composition.
The final gaseous species concentrations from this two-step equilibrium
process were used as the initial exhaust gas concentration at the engine exit plane.
2.2 Plume Mixing Profiles
In addition to the initial exhaust properties discussed in the previous section, mixing profiles
of the near-field aircraft plume were also needed to model the particulate matter formation in
aircraft emissions. These mixing profiles represent the dilution history of the exhaust gas in
the aircraft plume. To simulate the complex turbulent mixing in the near-field engine plume,
the computational fluid dynamics program FLUENT® was employed. Two-dimensional
computational grids were first constructed with detailed engine exit geometries in GAMBIT®
for the engine type under investigationl. Figure 1 shows the GAMBIT® grid used for
FLUENT® calculations. Initial exhaust properties at the engine exit such as temperature,
pressure and velocities for both core flow and bypass flow were obtained from the engine
cycle data. Together with ambient temperature and pressure specified, these data were
applied as initial conditions for the calculations of mixing profiles in FLUENT®. Initial
exhaust data for each engine type were obtained for three different power settings (7%, 65%
6
-
- ----- -----
7
and 100%). Axisymmetric calculations of the mixing history downstream of the engine exit
plane were carried out.
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---
---
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----
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May 07, 2007
FLUENT 6.1 (axi, dp, coupled imp, ske)
Figure
1 GAMBIT* grid showing detailed geometry of the exit section of the CFM56-2C1 Engine
Figures 2a and 2b show the velocity contours up to 30m downstream of the engine exit plane,
and velocity mixing profile up to 1km downstream generated with FLUENT* simulation
using representative engine exit conditions. Figure 2c and 2d show the static temperature and
static pressure mixing profiles. Static temperature, static pressure, velocity and concentration
profiles as a function of distance downstream of the engine exit plane were extracted along
one-dimensional trajectories from the plume mixing history. Mixing profiles along the
7
8
exhaust plume centerline, as well as trajectories starting from certain radial locations at the
engine exit plane were calculated. Each set of mixing profiles for different power settings
and different ambient conditions includes a static temperature profile, a static pressure profile,
a velocity profile and a concentration profile. These sets of mixing profiles, together with
initial exhaust properties at different power settings and ambient conditions, were used as
inputs to the one dimensional microphysics model to investigate the formation of particulate
matter under the corresponding power setting and ambient conditions.
3.47e+02
3.29e+02
3.11 e+02
2.94e+02
2.76e+02
2.58e+02
2.41 e+02
2.23e+02
2.05e+02
1 .87e+02
1.70e+02
1
.52e+02
1 .34e+02
1.1 7e+02
9.89e+01
8.1 2e+01
6.36e+01
4.59e+01
2.82e+01
1 .05e+01
-7.22e+00
May 07. 2007
FLUENT 6.1 (axi, dp, coupled imp, ske)
Contours of Axial Velocity (m/s)
Figure 2a Axial velocity contour from FLUENT* simulation up to 30m downstream of the engine exit
8
I-
pnmmmmwwwll
;im
- -- ---
.
-
-
-
- --
-
-,- -- - -
900
le+03 1.1e+03
9
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ay
ron
I
4.00e+02
3.50e+02
3.00e02
2.50e+02
2.00e+02
Axial
Velocity
(mIS)
1.50o+02
1.00e.02
5.00e+01
0.00e00
-5.00t+01
-1.000+02
0
100
200
300
400
500
600
700
800
Position (m)
May 07, 2007
Axial Velocity
FLUENT 6.1 (xi, dp, coupled exp, lke)
Figure 2b Centerline axial velocity profile up to 1km downstream of engine exit
1S0nntria
7.50+02
-
7.00e+02
6.50o+02
-
6.00e+02
-
5.50t+02 -
Static
Temperature
(k)
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-
4.50 .02
4.00e+02
-
3.50t+02
-
3.00e+02
-
2.30e+02
0
100
200
300
400
500
600
700
800
900
lU+03 1.1l+03
Position (m)
"taticTemperxture
UENT 6.1 (oxi,
Mly 07, 2007
Ace)
dp, coupled exp.
Figure 2c Centerline static temperature profile up to 1km downstream of engine exit
9
10
10
ntrnP
_
1
1.04e+05
1.02e+05
1.00c+05
Static
Pressure
(pascal)
9-80e+04
9.60e+04
9.40e+04
9.20e+04
I
0
100
200
300
400
500
600
700
800
900
l+03 1.1t+03
Position (m)
Stitic Pretsurc
FLUENT 6.1 (axi,
Figure 2d Centerline static pressure profile up to
dp,
May 07, 2007
coupled exp, -ce)
1km downstream of engine exit
2.3 One Dimensional Microphysics Model
A one-dimensional microphysics and chemical kinetics model has been developed by ARI
and MIT with Dr Hsi-Wu Wong from ARI being the main developer of the numerical model.
The remainder of this chapter presents a detailed description of the model which is extracted
from a paper jointly authored by Dr Hsi-Wu Wong, Dr Paul E. Yelvington, Dr Richard C.
Miake-Lye from ARI and Mr. Jianye Zhang and Prof Ian A. Waitz from MIT[24].
In the one-dimensional microphysics model presented here, the time evolution of any
gaseous or particulate species in a jet engine exhaust downstream of the engine-exit plane
can be described as [11]:
10
11
dX
-- = dX
dt
dt
+ dX
chemis,dt
+ dX
dt
mixing
(1)
microphysics
In this model, both liquid aerosols and coated black carbon soot particles are assumed to
be perfectly spherical. The three terms on the right hand side (RHS) of Equation (1)
correspond to the rates of production or disappearance from chemical reactions, wake
dilution and mixing, and microphysical processes, respectively.
To evaluate the contribution of chemical reactions, the first term on the RHS of Equation
(2) can be expressed as:
= Oi -M i -- 1(2)
dt
PA
chemistry
Equation (2) applies to the gaseous species only, since particulate species do not
participate in any of the chemical reactions in this model. In this work, a gas phase reaction
mechanism containing 35 chemical species and 181 chemical reactions derived from the NOx
and SOx combustion mechanism by Mueller et al. [25] was used, and the molar production
rates of the species were evaluated by CHEMKIN subroutines [26]. Sulfuric acid (H2 SO 4 )
was formed from reacting water with SO 3, and the rate constants for the reaction were taken
from the previous study [6].
The contribution of wake dilution and mixing in Equation (1) can be written as:
dX
dt
d'b
('X
X
=(x -Amb
mixing
11
I)-
(3)
f(t)
12
wheref(t) is the exhaust gas fraction determined from the semi-empirical approach described
in Section 2.2. Note that
dX
'
= 0 for condensed mass on soot.
dt mixing
The third term on the RHS of Equation (1) describes the contribution of microphysical
processes, and can be further divided into three elements:
dX,
dt
dX,
microphysics
dt
+ dX
nucleation
dt
(4)
+ dX,
coagulation
dt soot
where the first term on the RHS of Equation (4) describes the binary homogeneous
nucleation of sulfuric acid (H 2 SO 4 ) and water (H 2 0), the second term describes the
coagulation of liquid embryos and droplets, and the third term describes the microphysical
processes taking place on the soot particle surface.
Nucleation rates of H 2SO 4-H 20 embryos were evaluated using kinetic quasi-unary
nucleation (KQUN) theory [27, 28] as a more accurate alternative to classical nucleation
theory in this work. KQUN theory is based on kinetic rates, and assumes that water reaches
equilibrium with embryos of different sizes immediately. This permits binary nucleation of
H 2SO 4-H 20 to be treated as unary nucleation of sulfuric acid. The embryo formation rates
due to homogeneous binary nucleation are derived explicitly from the condensation and
evaporation rates:
dn,
1
=2 -$,nj
dt
2
-
12
72n2 -
2n2+73n3
(i=2)
(5-2)
13
dn,
+-1
-
d= 272n2_+
dti=I
(i)
- ZI3n,
(5-3)
In Equation (5), m is a user-specified value representing the size of the largest embryo
tracked in the model, which was set at 40 in our work. Compared with classical binary
homogeneous nucleation theory of H2SO 4-H 2 0 cluster formation, this kinetic based theory
has the advantage of capturing particle distributions in a rapidly diluting engine exhaust
where dramatic changes in temperature, relatively humidity and sulfuric acid concentration
take place [22]. We believe that the quasi-unary nucleation theory is a more robust and
appropriate approach to study aircraft-emitted aerosols where engine exhaust mixes rapidly
with ambient air.
The second term in the RHS of Equation (4) corresponds to the coagulation of different
liquid particles. This term includes the condensational growth of sulfuric acid monomer on
H2 SO 4-H 2 0 embryos or droplets, which can be expressed as [29]:
S
d
ZKI
i+j=k
(6)
n n -Kknin
=
where Kj can be evaluated as [30]:
Ku(dd)=
8dD(7)
d/(d +o)+4D/(d+c)
1 2
, and 5
in which d = (d,+d)/2, D=(D,+D 1 )/2, c = (c,2+c2+
13
(,2+
2)1
14
When Equation (6) applies to the embryos included in Equation (5), i, j, k and / denote
the size of the embryos and are no bigger than m, which is the size of the largest embryo
included in Equation (5). Because tracking the exact composition of large particles is
impractical, the fully stationary sectional bin approach [31] was used to track droplets that
are beyond the largest embryo specified. In this case, 1 denotes the total number of sectional
bin tracked, which is a user-specified value, and ni and nj are the concentrations of the
particle droplets in the i-th and j-th bins. Note that mass conservation has to be taken into
account across the boundary between the embryos and the binned particles.
The first and second terms on the RHS of Equation (6) correspond to the formation and
disappearance
explicitly
of aerosol
considers
droplets,
sulfuric
respectively.
Since
quasi-unary
acid condensing on or evaporating
nucleation
theory
from embryos, the
condensational growth in Equation (6) does not apply to the H 2SO 4 -H 2 0 embryos governed
by Equations (5-1)-(5-3). The amount of water change from coagulation is also calculated
based on the equilibrium compositions of different particle sizes.
The third term on the RHS of Equation (4) describes the microphysical interactions
between nucleated H 2 SO 4-H 2 0 aerosols and the soot particles. The size evolution of soot
particles can be expressed as the combination of soot activation and condensational growth
as [11]:
dm d
mdm
dt
dt
14
+d
activation
dt
'di
condensation
(8)
15
where mi is the time-dependent condensed mass on the soot particles in the i-th size bin. The
soot activation can be further divided into adsorption and scavenging:
dm
dt
dm9d
+
'
dt adsorp. dt scav
di
d
actaion
(9)
where adsorption can be achieved by H2SO4 and its precursor S03 as:
dmn,
d
12
=-a
d
dt adsorp.
, i2N .(1-O)-(CsMw
d
4
+CAM
a)
(10)
in which ad was set to be 0.018 in accordance with recent laboratory findings [32].
The scavenging of H2 SO 4-H 20 embryos and droplets can be written as:
dm
dt
1
=-
Ej-B,N
'Ad
-dp'I-
scav=4
2
(I1-
,)M
(
As shown in Equations (10) and (11), the activation and scavenging rates are linearly
dependent on the inactivated (uncoated) fraction of the soot particle surfaces. Therefore, 61
can be expressed as:
d-= dO'
+ d',
dt
dt adsorp
dt scav
d6i
dt
where aO is set at 5x 1018/
N
4
0
adsorp
d~i
dt
=I
-
(12)
N- (1 -0) -(C + Ca)
c-
B, N
p
1-0,
(13)
(14)
p
scav4A
[11].
The growth on soot particles can be broken into contributions arising from condensation
of both sulfur-containing aerosols and water vapor:
15
16
dmi
dt
dm
condensatin
dt
(15)
+ dmi
dt
sulfur
water
in which:
dm
d
=
2
Mj
Ld
dm
= 2GNo D d,,
P
dt water
and
Paj - 1CPa
2JtG)V
Z D,,au, " R T
1
jJ-Mj]
(16)
P, -wPP
4Pv
(17)
-jM
KRT)WV
where Gi can be expressed as [33]:
G =_
+
1+ 2A
(18)
3awd, i
d, i_
where aw is set at unity for both water vapor and sulfur-containing aerosol condensation on
coated soot particles.
In Equation (16), the sulfur-containing liquid embryos and droplets of all sizes contribute
to the condensational growth of soot particles. For sulfuric acid monomers, the pressure
difference term Pa,j - P" is present, since sulfuric acid monomers have noticeable vapor
pressure under the conditions of interest and can evaporate from coated soot surfaces. For
other larger embryos or droplets, Pa.
is assumed to be zero and the pressure difference
16
17
terms are reduced to partial pressure of the condensing aerosol species since these aerosols
have very low saturation vapor pressure under our conditions of interest.
17
18
18
19
Chapter 3
Results and Discussions
This chapter presents the results of the parametric study of the effects of ambient conditions,
engine operating conditions and engine design parameters on particulate matter formation in
the near field plume. Simulations were carried out at two different ambient temperatures
(288K and 300K), four different ambient relative humilities (10%, 40%, 60% and 80%),
three different engine power settings (7%, 65% and 100% engine thrust) for the CFM56-2C 1
engine investigated during the APEX campaigns. Engine design parameters such as BPR and
core/bypass exhaust temperatures were also studied to analyze their effects on PM formation
in the near field plume. The PM formation mechanisms in near-field aircraft plumes
following centerline trajectories up to lkm downstream of the engine exit plane were
simulated within the parameter space described above. In addition, a radial location
sensitivity study was also carried out. PM formation and evolution characteristics along
trajectories from several radial locations were investigated and compared to the results from
the centerline study.
In this parametric study, the following assumptions were made. The fuel sulfur conversion
ratio to Sv1 [34] was assumed to be 1%, i.e. Sv1 mass was assumed to bel% of the total sulfur
mass at the engine exit plane. The initial soot particle size distribution at the engine exit
plane was assumed to follow a log-normal distribution with 30 bins from 3 nm to 250 nm in
diameter. The mean and standard deviation of the distribution were obtained based on
APEX-i experimental data and these distribution parameters vary with engine power settings
19
20
as shown in Table 1. To investigate the evolution of H 2SO 4 -H 2 0 volatile particles in the near
field aircraft plume, these volatile particles were divided into two categories according to
their size: embryos and droplets. Embryos are H 2SO 4-H 2 0 oligomers, which are particles
smaller than or equal to a user-specified size set at 40 acid molecules. Droplets are particles
beyond the largest embryos tracked.
It should also be noted that the modeling tool used in this study does not cover all the
microphysics processes
in near field aircraft plumes. For example, both ion-induced
nucleation and organics-enhanced nucleation were absent from this model since the current
work focused on binary homogeneous nucleation (BHN) of H 2 SO 4-H 20 which forms the
basis of particle growth in near-field aircraft emissions. Although having its limitation and
space for improvement, this numerical model provides a valuable tool with which parametric
studies on factors affecting PM formation in near field aircraft plumes can be carried out. The
results from these parametric studies are presented in the following sections.
3.1 Representative Results from the Simulation
A set of representative simulation results from the one dimensional microphysics model
are shown in Figure 3. The PM formation in the exhaust plume of the CFM56-2C1 engine
was simulated. The simulation was carried out along the centerline of the exhaust plume up
to 1km downstream of the engine exit plane at an ambient temperature of 288K and a relative
humidity of 80%. The sulfur content of the fuel was assumed to be 383 ppm, which is the
sulfur content of one type of jet fuel used in the APEX measurements. The engine power
setting was 7%.
20
21
5
10
I
0
101
100
200
600
400
500
Distance (m)
300
800
700
900
1000
--
5
Log(dN/dlog(Dp)
Figure 3a Evolution of liquid droplets size distribution as a function of downstream distance
40
5
35
30
0
25
0
20
15
-5
10
5
-10
10,
101
10
Log(N)
Time (s)
Figure 3b Evolution of embryo size distribution as a function of time
7
x 101
6
-
/7 \\\
\i \\\
5
- -
4
0)
\ \\
3
-
-
Initial Distribution
100m Distribution
200m Distribution
400m Distribution
800m Distribution
Final Distribution
2
1
-
n
0
10
20
40
30
50
60
70
80
Particle Diameter (nm)
Figure 3c Soot particle size distributions at different downstream locations showing condensational growth
21
22
x 1015
10
Nucleation Mode
-- - S oot Mode
8
6
C"
*0
4
2
n
102
101
Particle Diameter (nm)
Figure 3d Comparison of final size distribution of soot and liquid droplets at
100%
90%
* Vapor Phase
80%
o Soot Coatings
m Volatile Droplets
70%
0
U
1km downstream
60%
50%
40%
30%
20%
10%
0%
0
446
631
Distance (m)
773
894
1000
Figure 3e Evolution of SvI mass fraction in vapor phase, soot coatings and volatile droplets.
Figure 3 depicts the evolution of size distributions of both the volatile H 2 SO 4 -H 2 0
particles and soot particles. The evolution of volatile H2SO 4-H 20 particles is shown in
Figure 3a and 3b. Figure 3a shows the size distribution of liquid droplets as a function of
downstream distance. As shown in the figure, the size distribution of liquid droplets reaches
equilibrium at around 800m downstream of the engine exit plane. The geometric mean
22
23
diameter grows to about 18 nm at the equilibrium distance. Further downstream from this
equilibrium distance, the concentration of the liquid droplets declines due to increased
mixing with ambient air as well as condensation of liquid on soot particles. Figure 3b shows
the evolution of size distribution of the embryos tracked. These H2 SO 4-H 2 0 oligomers form
the basis for the liquid droplet growth. Figure 3c shows the soot particle growth downstream
of the engine exit plane. The geometric mean diameter increases from the initial 16.0 nm to
about 22 nm at lkm downstream. Figure 3d presents a comparison of the final soot size
distribution with that of the volatile particle size distribution. It can be seen in the figure that
soot particles reache a larger geometric mean diameter relative to the liquid droplets,
although the overall particle concentration of the liquid droplets is much greater than that of
the soot particles. This can be explained by the high ambient relative humidity and low
engine power setting which will be discussed in detail in the following section. To see this
difference in growth of sulfur containing aerosols more clearly, a sulfur (Sv) mass budget
plot is shown in Figure 3e.
Svi mass in the sulfur containing aerosols can be classified into three categories: (1) S mass
in the vapor phase which includes H2 SO 4-(H 20)n monomers and their gas phase precursors
S03
and H2 SO 4, i.e. vapor mode; (2) SvI mass in soot coatings formed from the
condensational growth of soot particles, i.e. soot mode; (3) Sv' mass in volatile droplets and
embryos formed through homogeneous binary H2 SO 4-H 20 nucleation, i.e. nucleation mode.
In Figure 3e, Sv1 mass in both volatile droplets and soot coatings increases as a function of
the downstream distance. The Svi mass growth in both modes can be explained by the
condensational growth in the case of soot particles and homogeneous binary nucleation in the
23
24
case of aerosol droplets. The mass fraction of SvI in aerosol droplets i.e. the nucleation mode
dominates at 1km downstream with about 88% of overall SvI. On the other hand, the amount
of SvI in soot coating is much lower with about 7% of the total Svl mass. The remaining 5%
of the SvI mass is in the vapor phase. This dominance of the nucleation mode and relative
insignificance of the soot mode can be explained by the combined effects of high ambient
relative humidity and low engine power settings. Investigation of both parameters will be
detailed in the section that follows. It is anticipated that condensational growth of soot
particles will continue downstream since homogeneous nucleation has already reached
equilibrium. Gas phase monomers and their precursors, as well as embryos and liquid
droplets from nucleation mode, will continue to condense on the soot particles while
depleting SvI mass from both vapor phase and volatile particles. Further observation from
Figure 3e indicates the possibility of a final equilibrium achieved between all three modes.
Mixing with ambient air further downstream will cause sulfuric acid to be diluted enough to
reach its saturation vapor pressure. H2 S0 4 condensation onto and evaporation from the soot
coatings will reach a final equilibrium. At this stage, the SvI mass will remain constant in the
vapor phase.
3.2 Effects of Engine Power Setting and Relative Humidity
The effects of engine power setting and relative humidity on PM formation and evolution in
near field aircraft plumes are investigated in this section. Plots of sulfur mass emission
indices were generated as a function of both engine power setting and ambient relative
humidity. The parametric study was carried out at an ambient temperature of 288K and a fuel
24
25
sulfur content of 383 ppm. Sulfur mass emission indices (sulfur mass contained per unit mass
of fuel, Elm) were converted from SV1 mass fraction found in the previous mass budget plots.
Sulfur mass emission indices (Elm) of both the soot mode (mass of SvI in soot coatings) and
the nucleation mode (mass of SV1 in volatile droplets) were calculated. Three different engine
power settings, i.e. 7%, 65%, 100%, representing three ICAO points (idle, climb-off and
take-off respectively) and four different ambient relative humidities, i.e. 10%, 40%, 60% and
80% were studied to see their effects on PM formation in the near field plume.
Simulation results at 1km downstream of the engine exit plane were presented in Figure 4
and Figure 5. It can be seen from Figure 4 that sulfur Elm of the soot mode is strongly
dependent on the engine power settings. As the engine power increases from 7% to 100%,
the amount of Svi condensed on soot particles changes from around 2 mg/kg-fuel to around 8
mg/kg-fuel. This 4 fold increase of sulfur Elm can be explained by the larger soot surface
area available for condensation at higher power settings. As shown in Table 1, different
engine power settings have different initial soot particle size distributions. Soot particles at
higher engine power setting tend to have larger surface areas which make condensation of
vapor phase monomers and volatile droplets onto soot particles much easier. It should be
noted that the increase of soot mode sulfur Elm from 65% to 100% is more drastic compared
to the increase from 7% to 65%. This difference can be explained by another factor
influencing condensational growth of soot particles in the plume, i.e. residence time, which is
the amount of time soot particles have spent in the plume. Longer residence time leads to
extended period of condensation of gas phase molecules and liquid droplets onto soot. At
lower engine power setting, the time to reach 1km (the plume age at 1km) is much longer
25
26
compared to that at higher engine power setting (about 900 seconds for 7%, about 120s for
65% and roughly 80s for 100% power setting). The longer residence time at lower engine
power settings leads to more condensational growth of soot particles. This compensates for
the decrease in sulfur Elm due to smaller surface area available for condensation at low
power settings.
a)
10% RH, 383 PPMM,1km
(
_
m Nucleation
7 -
Soot Mode
Mode
6543
W II
0
65%
7%
100%
Power Setting %
b)
40% RH, 383PPMM, 1km
10 8
o>
6
E
4-
m Nucleation Mode
*Soot Mode
E
L]
2-
0
7%
65%
Power Setting %
26
100%
27
c)
60% RH, 383PPMM, 1km
10
E
E
M
9
8
7
6
5
4
3
2
1
0
7%
65%
100%
Power Setting
d)
80%RH, 383PPMM,lkm
10
9
8
8>
. Nucleation Mode
Soot Mode
)6
lie5
E 4
E 3
LU
2
1
0
7%
65%
100%
Power Setting %
Figure 4 Sulfur mass emission index atlkm downstream as a function of engine power settings and ambient
relative humidity for an ambient temperature of 288 K and a fuel sulfur content of 383 ppm.
a) 10% RH b) 40% RH c) 60% RH d) 80% RH
It is also observed in Figure 4 that soot mode Elm was not significantly influenced by
variations of ambient relative humidity. The changes of soot Elm among different relative
humidities were less than 1 mg/kg-fuel. However, the sulfur Elm of the nucleation mode is
very sensitive to ambient relative humidity changes. At low relative humidities such as 10%
and 40%, nucleation mode sulfur Elm is insignificant compared to that of soot mode. The
27
28
highest nucleation mode sulfur Elm at low relative humidities is less than 0.01 mg/kg-fuel
compared to around 8 mg/kg-fuel for the soot mode at the same relative humidity. The
amount of nucleation mode sulfur Elm increases considerably at 60% and 80% relative
humidities. The nucleation mode sulfur Elm at 80% relative humidity and 7% engine power
reaches around 9 mg/kg-fuel, which is the highest among all these conditions. The strong
dependence of nucleation mode sulfur Elm on ambient relative humidity is due to the fact
that water molecule plays a critical part during the binary homogeneous nucleation process.
Higher ambient relative humidity enhances the nucleation process thus leading to a greater
contribution to the total sulfate mass by volatile H2 SO 4 -H 2 0 droplets. The decrease in
nucleation mode sulfur Elm towards higher engine power at 80% relative humid is the result
of increased competition from soot mode since condensational growth of the soot particles is
encouraged at high engine power settings.
a)
80
8
70
7
60
5
-50
4
9 40
3
30
2
20
10
0
20
40
60
Engine Power (%)
28
80
0
100 Elm (mg/kg-fuel)
29
b)
80
8
70
60
6
a
_
-50
-5
4
S40
3
30
2
20
10
1
0
20
40
60
Engine Power (%)
80
10 0
0
EIm (mg/kg-fuel)
Figure 5 Contour plots of sulfur mass emission index at 1km downstream as a function of engine power and
relative humidity at an ambient temperature of 288 k and a fuel sulfur content of 383 ppm:
a) Soot mode; b) Nucleation mode
Figure 5 presents a summary of the effects of engine power settings and ambient relative
humidities on aerosol formation mechanism in a contour plot. The strong dependence of soot
mode on engine power settings and significant influence of relative humidity on the
nucleation mode is depicted in the contour plot. Figure 6 and 7 shows the results of the
parametric study at a plume age of 80s to eliminate the effects of residence time on soot
particle condensational growth. In Figure 6, the soot mode sulfur Elm dropped sharply from
the value of about 2 mg/kg-fuel at 65% engine power to a value of about 0.2 mg/kg-fuel at
7% power compared to the previous moderate decrease for the 1km results shown in Figure 4.
The sulfur Elm of the nucleation mode at this low power setting and short resident time was
29
30
also diminished compared to the value at lkm. The reason for this decrease in nucleation
mode is due to the fact that within this short residence time, the binary homogeneous
nucleation process has not reached equilibrium. The volatile H2 SO 4 -H 2 0 particles are still
growing for this young plume age of 80s. Figure 7 presents a summary of the effects of
engine power settings and ambient relative humidity for the case of a 80s plume age.
a)
10% RH 383 PPMM, 80s
98
Nucleation Mode
7 -
Soot Mode
E4
0
65%
Engine Power %
7%
100%
b)
40% RH, 383PPMM, 80s
10 m Nucleation Mode
8 -
Soot Mode
c6
E
E
i
4 -
2 0
65%
7%
Power Setting %
30
100%
7-
31
c)
60% RH, 383PPMM, 80s
10
9
8
7
0)
6
0)
5
E 4
E 3
ijj 2
1
0
[ U
UNucleation
Mode
Soot Mode
65%
7%
100%
Power Setting
d)
80%RH, 383PPMM, 80s
10 9
8
E Nucleation Mode
N Soot Mode
7
6 5
E
E
mw
4
3
2
10
7%
65%
100%
Power Setting %
Figure 6 Sulfur mass emission index at 80s plume age as a function of engine power settings and ambient
relative humidity for an ambient temperature of 288 K and a fuel sulfur content of 383 ppm.
a)
10% RH b) 40% RH c) 60% RH d) 80% RH
31
32
a)
80
70
60
50
E
40
30
20
10
0
20
40
60
80
100
EIm (mg/kg-fuel)
Enaine Power (%)
b)
80
70
60
L5
50
E
4
40
3
30
2
20
1
-
10
0
20
40
60
Enaine Power (%)
80
10 0
-O
Elm (mg/kg-fuel)
Figure 7 Contour plots of sulfur mass emission index at 80s plume age as a function of engine power and
relative humidity at an ambient temperature of 288 K and a fuel sulfur content of 383 ppm:
a) Soot mode; b) Nucleation mode
32
33
3.3 Effects of Ambient Temperature
The effect of ambient temperature on the aerosol formation mechanism is described in this
section. The ambient temperature was changed from the previous 288 K to a higher
temperature at 299 K. The sulfur content of the fuel was kept at 383 ppm and the sulfur
conversion ratio was also constant at 1% of the total sulfur mass. Simulations at this ambient
temperature were carried out for the same set of engine power settings (7%, 65% and 100%)
and ambient relative humidity (10%, 40%, 60% and 80%). Figure 8 and Figure 9 show the
results of the simulations in terms of sulfur mass emission index for both the nucleation
mode and the soot mode at 1km downstream of the engine exit plane.
Comparing Figure 8 to Figure 4, the sensitivity of nucleation mode sulfur emission to
ambient temperature variation is seen. At a higher ambient temperature of 299 K, the sulfur
emission index observed for the nucleation mode is insignificant even at high ambient
relative humidity compared to the previous restuls at a lower ambient temperature of 288K.
The nucleation mode sulfur emission index found at 299 K and 80% relative humidity is 0.01
mg/kg-fuel compared to 9 mg/kg-fuel for the same level of relative humidity at 288K.
Although the values of sulfur Elm at this higher ambient temperature are rather insignificant,
the effects of relative humidity on the nucleation mode sulfur emission index are still
observed. The sulfur Elm of the nucleation mode increases from 0.0002 mg/kg-fuel at 10%
relative humidity and 65% power to 0.01 mg/kg-fuel at 80% relative humidity for the same
engine power setting.
33
34
a)
10%RH 383PPMM FSC 299.26k
9
8
7
0 Nucleation Mode
E Soot Mode
6
5
4
E
E 3
w~
2
1
0
7%
65%
Power Setting %
100%
b)
40%RH 383PPMM FSC 299.26k
M
Nucleation Mode
M Soot Mode
E
W
7%
65%
Power Setting %
100%
c)
60%RH 383PPMM FSC 299.26k
* Nucleation Mode
U Soot Mode
W
E
65%
Power Setting %
7%
100%
d)
80%RH 383PPMM FSC 299.26k
10
9
8
7
6
5
4
3
2
1
0
N Nucleation Mode
E Soot Mode
7%
65%
Power Setting %
100%
Figure 8 Sulfur mass emission index at 1km downstream as a function of engine power settings and ambient
relative humidity for an ambient temperature of 299 K and a fuel sulfur content of 383 ppm.
a)
10% RH b) 40% RH c) 60% RH d) 80% RH
34
35
a)
80
8
70
7
60
V
6
5
50
E
I
03
4
40
w
3
30
2
20
100
20
60
40
Engine Power (%)
80
100
Elm (mg/kg-fuel)
b)
x 10-
.10
80
9
70
8
60
-0
7
6
50
E
(D
7
5
40
4
3
30
2
20
1
10
0
20
60
40
Engine Power (%)
80
100Elm (mg/kg-fuel)
Figure 9 Contour plots of sulfur mass emission index at 1km downstream as a function of engine power and
relative humidity at an ambient temperature of 299 K and a fuel sulfur content of 383 ppm:
a) Soot mode; b) Nucleation mode
35
36
On the other hand, the soot mode sulfur mass emission index was not significantly affected
by the ambient temperature variation and is a still mainly a function of engine power settings.
The soot mode emission index changes from a maximum value of 8.7 mg/kg-fuel at 100%
engine power setting, 60% relative humidity for the 288k ambient temperature case to a
maximum value of 8.8mg/kg-fuel for the case at 100% power setting, 80% relative humidity
and an ambient temperature of 299K. The slight increase across all power settings from an
ambient temperature of 288K to 299K is due to lessened competition from the nucleation
mode which is suppressed at a higher ambient temperature. The reduction in soot mode
sulfur emission index at high relative humidity for the low ambient temperature case is not
observed in the case of a higher ambient temperature of 299K. This is also due to the fact that
the nucleation mode is suppressed at this higher ambient temperature setting and no
competition is experienced by the soot mode even at higher relative humidity. The soot mode,
meanwhile, is still strongly affected by the change in engine power settings. The sulfur
emission index for soot mode increases from 2.4 mg/kg-fuel at 7% engine power and 10%
relative humidity to 8.4 mg/kg-fuel at 100% engine power and 10% relative humidity.
However at the same time, there is also a slight drop in soot mode sulfur emission index from
7% engine power to 65% power. The reason for this phenomenon was the competing effects
of soot particle surface area and residence time in the plume, both of which were explained in
section 3.2.
To eliminate effects of soot particle residence time in the plume, simulation results at a
uniform plume age of 80s were generated and compared to the results at 1 km downstream of
the engine exit plane. Figure 10 and Figure 11 show the results for a plume age of 80s. This
36
37
time the sulfur mass emission index increases consistently as the engine power increases
from 7% to 100% due to the sole effect of increased soot particle surface area for
condensational growth.
a)
10%RH, 383PPMM, 299.26k, 80s
10
" Nucleation Mode
9
" Soot Mode
7
6
5
75
E
E
W
4
3
2
1-
0 .m
65%
7%
100%
Power Setting %
b)
10%RH, 383PPMM, 299.26k, 80s
9
8
" Nucleation Mode
" Soot Mode
7
6
5
E 4
E 3
w~ 2
1
0
65%
7%
100%
Power Setting %
c)
10%RH, 383PPMM, 299.26k, 80s
10
" Nucleation Mode
9
8
E
E
E
iii
" Soot Mode
7
6
5
4
3
2
1-
0 -0
65%
7%
Power Setting %
37
100%
38
d)
80%RH, 383PPMM, 299.26k, 80s
10
N Nucleation Mode
M Soot Mode
9
7-
06
5S
E
W
43
2
0
7%
100%
65%
Power Setting %
Figure 10 Sulfur mass emission index at 80s plume age as a function of engine power settings and ambient
relative humidity for an ambient temperature of 299 k and a fuel sulfur content of 383 ppm.
a)
10% RH b) 40% RH c) 60% RH d) 80% RH
a)
x 10-
80
7
70
6
60
5
V
50
E
a)
4
40
3
a)
30
2
20
10
1
0
20
60
40
Engine Power (%)
38
80
100
n
EIm (mg/kg-fuel)
39
b)
80
8
70
7
60
1
6
-5
50
4
3
30
2
20
10
Figure
0
20
40
60
Engine Power (%)
80
100
0
EIm (mg/kg-fuel)
11 Contour plots of sulfur mass emission index at 80s plume age as a function of engine power and
relative humidity at an ambient temperature of 299 k and a fuel sulfur content of 383 ppm:
a) Soot mode; b) Nucleation mode
3.4 Effects of Engine Design Parameters
The influence of engine design parameters on the aerosol formation mechanisms is
investigated in this section. Key design parameters such as engine bypass ratio, exhaust core
total temperature and exhaust bypass total temperature were examined. The effect of each
design parameter on PM formation was investigated independently. The CFM56-2C 1 engine
examined in previous sections served as a standard reference point for comparisons with
engines having different design parameters. In this study, engine bypass ratio (BPR) was
varied independent of other design parameters between values of 9, 6 and 0 with 6 being the
BPR of the CFM56-2CI engine simulated in the previous sections. A BPR value of 9
39
~~1
__________
40
represents a high bypass turbofan engine such as GE90 while a BPR value of 0 represents the
configuration of a turbojet engine. In addition, the core and bypass exhaust total temperature
were varied together from the values of the CFM56-2C1 engine. A case with higher core and
bypass exhaust total temperature with a 15 K increment from the original values of the
CFM56-2C 1 engine and a case with lower core and bypass total temperature with 15 K
decrease from the values of the CFM56-2C 1 engine were investigated. Corresponding
GAMBIT' grids and FLUENT® simulations were generated and plume mixing profiles were
obtained for these different engine design parameters. Table 2 shows variations of the engine
design parameters under investigation. The simulations were carried out at an ambient
temperature of 288K. The sulfur content of the fuel was kept at 383ppm and a sulfur
conversion ratio of 1% was used. The study was done for the case of 100% engine power at
four different ambient relative humidities.
a)
10 *BPR
8 BPR
0 BPR
9 87
26q5--4
jJ
2
2
I
0
10%
60%
40%
Relative Humidity (%)
40
80%
9
6
0
41
b)
10
_
9
-
BPR = 9
*BPR = 6
fBPR = 0
8
76
5
3
2
1 0
60%
40%
Relative Humidity (%)
10%
80%
Figure 12 Effects of engine bypass ratio on aerosol formation mechanism:
a)
Nucleation mode b) Soot Mode
Figure 12 shows the results of the study on engine bypass ratio. From Figure 12a, the
strong dependence of nucleation mode on ambient relative humidity is again observed. It is
also clear that the nucleation mode was suppressed at high bypass ratio, though the effects
were not very significant. At 80% relative humidity and 100% power setting, the sulfur
emission index increased from 1.8 mg/kg-fuel at the high BPR of 9 to a value of 2.5 mg/kgfuel for the limiting case of zero BPR. The suppression of the nucleation mode at high BPR
can be explained by a hotter centerline temperature profile obtained for a high BPR
configuration. With other things equal, a larger bypass exhaust coflow tends to preserve the
temperature of the hot core flow and slow its cooling along the centerline. This higher
temperature profile along the centerline trajectory discourages the nucleation mode similar to
the effects for a higher ambient temperature studied in the previous section. On the other
41
42
hand, the soot mode at high BPR was enhanced due to reduced competition from the
nucleation mode. Figure 12b shows the effects of different BPR on soot mode.
a)
E dT = -15k
4
NdT =
Ok
EdT = +15k
3.5
3
8
2.5
<2
1. 5
1
0. 5
I
I
0
10%
60%
40%
Relative Humidity (%)
80%
b)
10
9
K
8
7
6
5
S
4
S
3
2
1
'
0
10%
'
60%
40%
Relative Humidity (%)
80%
Figure 13 Effects of engine core and bypass exhaust total temperature:
a) Nucleation mode b) Soot Mode
42
E dT = -15k
E dT = Ok
EdT = +15k
43
The influence of different core and bypass exhaust total temperature is depicted in Figure
13. A 15 K difference above and below the original core and bypass total temperature of the
CFM56-2C I engine was employed for the investigation. Figure 13a shows the effects of core
and bypass exhaust temperature variation on the nucleation mode for 100% engine power
setting at four different ambient relative humidities. As shown in the figure, a lower core and
bypass total temperature enhances the binary nucleation process leading to a higher value of
nucleation mode sulfur Elm. The sulfur Elm for the nucleation mode increased from around
0.7 mg/kg-fuel at the higher core and bypass total temperature (15 K above the CFM56-2C 1
value) to 3.5 mg/kg-fuel at the lower temperature ( 15 K below the CFM56-2C I engine data).
The reason is again the temperature effect on binary homogenous nucleation as described
before. Due to the competition between the soot mode and the nucleation mode at high
relative humidity and high engine power setting, the sulfur Elm of the soot mode decreased
from a value of 8.6 mg/kg-fuel at higher core and bypass total temperature to about 6.0
mg/kg-fuel for the lower temperature case as shown in Figure 13b.
3.5 Study of Trajectories from Different Radial Locations
In addition to the study of various effects on the aerosol formation along the centerline
trajectory, PM formation and evolution characteristics along trajectories from several radial
locations were investigated and compared to the results from the centerline study. Particle
pathlines released from 4 equally-spaced radial locations were generated in FLUENT® with
the outermost radial location being on the very edge of the engine core exhaust flow. Plume
mixing profiles were obtained for these radial locations and were used as input to the
43
44
microphysics model. The ambient temperature at which the study was performed was 288 K
and the sulfur content is 388 ppm. In order to observe the effects of radial trajectories on the
nucleation mode of the aerosol formation mechanism, two high ambient relative humidities
of 60% and 80% were selected to provide considerable nucleation mode sulfur Elm. Figure
14 shows the results for 80% relative humidity and Figure 15 shows the results for 60%
relative humidity.
In Figure 14, lower sulfur mass Elm was observed near the core edge for the soot mode.
The soot mode Elm decreased from a centerline value of 7.5 mg/kg-fuel to about 4.2 mg/kgfuel at the core flow edge (0.3012m from the centerline) at 80% relative humidity and 100%
engine power setting. This decline in soot mode Elm from the centerline to the core edge is
mainly due to a lower soot particle concentration near the core flow edge due to enhanced
mixing with surrounding air. As a result, there is less overall surface area available for
condensation onto soot particles leading to a decrease in soot mode Elm. On the other hand,
the nucleation mode Elm was enhanced near the core flow edge where the temperature near
the edge was lower compared to that along the centerline. The nucleation mode sulfur Elm
increased from 1.8 mg/kg-fuel at centerline to 5.4 mg/kg-fuel at the core edge. Besides the
temperature effects, reduced competition from the soot mode is another reason for this
increase in nucleation mode Elm near the core edge. The variations from the centerline result
for both nucleation and soot mode were most pronounced at 100% power setting where the
competition between the soot mode and the nucleation mode was most intense and the
temperature drop from the centerline to the core flow edge was the greatest.
44
- __m.
-
-
__ -
-
- I -
-
__ __
_
_ ---
45
a)
Soot Mode
8
0O.3012m from
00.2259m from
00.1506m from
00.0753m from
*CL
7
6
CL-Core
CL-Near
CL -Mid
CL-Near
Edge
Core Edge
Section
CL
F
bD
bD
S3
2
1
I
0
65%
7%
100%
Power Setting
b)
Nucleation Mode
10
9
00.3012m from CL-Core Edge
N0.2259m from CL-Near Core Edge
00.1506m from CL-Mid Section
00.0753m from CL-Near CL
*CL
8
7
6
C)
5
4
3
2
1
I
0
65%
7%
100%
Power Setting
Figure 14 Results of uncertainty study of the trajectories from radial locations at 80% relative humidity
a) Soot mode b) Nucleation Mode
45
46
The results for the 60% relative humidity case are shown in Figure 15. Again the soot
mode Elm decreased from a value of 8.7 mg/kg-fuel at the centerline to 5.4 mg/kg-fuel at the
core edge, while the sulfur Elm of the nucleation mode increased from 0.1 mg/kg-fuel at
centerline to a value of 4 mg/kg-fuel at the core edge. The changes in the nucleation mode
are more drastic at this 60% relative humidity level compared to that at 80% relative
humidity for the same engine power setting of 100%. The proposed explanation for this
difference is that at lower relative humidity, the effect of temperature drop on the nucleation
process becomes more pronounced. For higher relative humidity, abundance of water
molecules enhances the binary nucleation so much that a slightly lower temperature profile
does not contribute to the nucleation mode sulfur Elm by much, whereas at a lower relative
humidity with fewer participating water molecules, any decrease in temperature profile can
lead to a very much encouraged nucleation process.
a)
Soot Mode - 60%RH
1.OOE+01 -
*0.3012m
from CL-Core Edge
9. OOE+00
M0.2259m from CL-Near Core Edge
8.OOE+00
00.1506m from CL-Mid Section
0 0.0753m from CL-Near CL
7.OOE+00
6. OOE+00
MCL
5. OOE+00
E 4.
OOE+00
3. OOE+00
2. OOE+00
1. OOE+00
0. OOE+00
65%
7%
Power Setting
46
100%
47
b)
Nucleation Mode - 60%RH
1. OOE+01
C)
H0.3012m from CL- Core Edge
9. OOE+00
S0. 2259m from CL-Near Core Edge
8.OOE+00
00.1506m
from CL-Mid Section
0. 0753m from CL-Near CL
7. OOE+00
* CL
6. OOE+00
an
E
E
5. OOE+00
4. OOE+00
3. OOE+00
2. OOE+00
1. OOE+00
0. OOE+00
IL
65%
Power Setting
7%
100%
Figure 15 Results of uncertainty study of the trajectories from radial locations at 60% relative humidity
a)Soot mode b)Nucleation Mode
47
48
48
49
Chapter 4
Conclusions
In this work, the particulate matter formation mechanism in the near field aircraft plume at
ground level was investigated. A detailed one dimensional microphysics and chemical
kinetics model was employed to study the influence of ambient temperatures(288 K and 299
K), ambient relative humidity (10%, 40%, 60% and 80%), engine power settings(7%, 65%
and 100%) and engine design parameters (engine BPR and core/bypass total temperatures)
on the aerosol formation mechanism. Initial estimates of input conditions to the model such
as engine exit properties were obtained from data thermodynamic cycle analysis. Other
inputs such as plume mixing profiles were generated using FLUENT* and initial exhaust gas
species concentrations were calculated based on a two-step constrained equilibrium process.
The simulation was carried out following a centerline trajectory up to 1km downstream of the
engine exit plane. In addition, PM formation and evolution characteristics along trajectories
from several radial locations were investigated and compared to the results from the
centerline study.
4.1 Summary of major findings
It was found in this study that condensational growth of soot particles in the near field
aircraft plume, i.e. the soot mode, is strongly dependent on the engine power settings. As the
engine power increases from 7% to 100%, the amount of SvI condensed on soot particles
changes from around 2 mg/kg-fuel to around 8 mg/kg-fuel. This 4 fold increase of sulfur Elm
can be explained by the larger soot surface area available for condensation of vapor phase
49
50
monomers and volatile droplets at higher power settings. Another factor influencing
condensational growth of soot particles in the plume is the residence time. At lower power
setting, the residence time of soot particles in the plume is longer compared to that at high
power setting, leading to extended period of condensational growth. This effect compensates
the decrease in sulfur Elm due to smaller surface area available for condensation at low
power settings.
On the other hand, formation of volatile droplets and embryos through homogeneous
binary H 2 SO 4 -H
2
0 nucleation, i.e. the nucleation mode, is very sensitive to ambient relative
humidity changes. The nucleation mode sulfur Elm increased from 0.01 mg/kg-fuel at 10%
relative humidity and 7% engine power setting to about 9 mg/kg-fuel at 80% relative
humidity for the same power setting. The sharp increase at higher relative humidity is due to
the critical role water molecules play during the binary H 2 SO 4-H 20 nucleation process. It
was also observed that at high relative humidity and high engine power settings, there is
intense competition between nucleation mode and soot mode for participating species in both
processes.
Furthermore,
the
nucleation
mode sulfur Elm is strongly
influenced by
ambient
temperature change. The nucleation mode sulfur emission index found at 299 K and 80%
relative humidity is 0.01 mg/kg-fuel compared to 9 mg/kg-fuel for the same level of relative
humidity at 288K. Meanwhile,
the soot mode sulfur mass emission index was not
significantly affected by the ambient temperature variation and is a still mainly a function of
engine power settings. The soot mode emission index changes from a maximum value of 8.7
mg/kg-fuel at 100% engine power setting, 60% relative humidity for the 288K ambient
50
51
temperature case to a maximum value of 8.8mg/kg-fuel for the case at 100% power setting,
80% relative humidity and an ambient temperature of 299K.
In the engine design parameter study, it is found that the nucleation mode was suppressed
at high bypass ratio, though the effects were not very significant. At 80% relative humidity
and 100% power setting, the sulfur emission index increased from 1.8 mg/kg-fuel at the high
BPR of 9 to a value of 2.5 mg/kg-fuel for the limiting case of zero BPR. On the other hand, a
lower core and bypass total temperature enhances the binary nucleation process leading to a
higher value of nucleation mode sulfur Elm. The sulfur Elm for the nucleation mode
increased from around 0.7 mg/kg-fuel at the higher core and bypass total temperature (15 K
above the CFM56-2C1 value) to 3.5 mg/kg-fuel at the lower temperature ( 15 K below the
CFM56-2C1 engine value).
Lastly in the study for trajectories from different radial locations, lower sulfur mass Elm was
observed near the core edge for the soot mode due to decreased soot particle concentration
near the core flow edge. The soot mode Elm decreased from a centerline value of 7.5 mg/kg-
fuel to about 4.2 mg/kg-fuel at the core flow edge (0.3012m from the centerline) at 80%
relative humidity and 100% engine power setting. Nucleation mode Elm, however, was
enhanced near the core flow edge where the temperature near the edge was lower and the
competition from soot mode was also less intense compared to that along the centerline. The
nucleation mode sulfur Elm increased from 1.8 mg/kg-fuel at centerline to 5.4 mg/kg-fuel at
the core edge.
51
52
4.2 Recommendations for future work
In this study, both ion-induced nucleation and organics-enhanced nucleation were absent
from the microphysics model. Addition of these microphysical processes into this model is
desired in order to capture all the details of the PM formation mechanism in the near field
aircraft plume.
In addition, comparison of simulation results from the one dimensional model with
experimental data will be very useful to confirm the findings and verify the theories behind
the findings from the model, whereas these modeling results can serve to guide and improve
future PM measurement techniques.
52
Appendix A Tables
Table 1. The parameters used for log-normally distributed soot particles under different
power settings
Power
Initial Concentration
Settings (%)
(1/cm 3)
Geometric Mean Standard Deviation Estimated Surface
Area
Diameter (nm)
3
(m 2/cm )a
100
3.0x107
35.0
1.6
1.795x 10-7
65
7.5x 106
25.0
1.6
2.291x 10-
30
6.0x 106
20.0
1.4
9.456x 10-9
7
8.5x106
16.0
1.3
7.845x 10-9
was based on log-normally distributed spherical particles into 30 bins ranging
from 3 nm to 250 nm
a Estimation
Table 2. Engine Design Parameter Variations
Bypass
Core/Bypass Total Temperature Variation' dT
(K)
Ratio
+15
9
0
6
-15
0
bVariation from the engine data of CFM56-2C
1.
53
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