An experimental and kinetic study of syngas-air combustion at elevated temper...
DMS_vargas
1. Design and Development of a Thermophoretic
Soot Sampling System for High-Pressure
Laminar Diffusion Flames
Alex M. Vargas
Supervisor: Professor ¨Omer L. G¨ulder
Departmental Masters Seminar
University of Toronto Institute for Aerospace Studies
March 31, 2016
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2. Presentation Outline
Introduction
Motivation
Research Objectives
Experimental Apparatus
High-Pressure Combustion Chamber and Burner
Thermophoretic Soot Sampling System
Results and Discussion
Flame Disturbance Caused by the Sampling Probe
TEM Image Analysis
Sources of Error
Conclusion and Recommendations
Conclusion
Recommendations and Future Work
References
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3. What is combustion?
A chemical reaction between fuel and oxidizer [1].
Involves chemical kinetics, thermodynamics, transport phenomena
(heat and mass transfer), and fluid mechanics.
Main mechanism for energy production around the world [2].
Conversion of energy stored in chemical bonds of hydrocarbon fuels
into a form that is practical (e.g., electricity, transportation and heat).
However, some of the underlying chemical mechanisms, such as soot
formation, are still not well understood [3].
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4. Motivation
The presence of soot in combustion systems could be both beneficial
and problematic.
Beneficial
Soot increases the heat transfer rate from the combustion gases to the
circulating fluids [1].
Problematic
Medical and environmental research indicate overwhelmingly that soot
particles in the atmosphere are understood to be one of the main causes
of lung-disease and one of the leading factors contributing to global
warming [4–8].
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5. What is Soot?
Soot is a carbonaceous particle that forms from gas-phase
combustion processes [9].
Figure: Aggregate soot particle from a methane-air laminar diffusion flame collected at
5.4 atm.
Soot particle characteristics:
General atomic formula is C8H.
Consists of a number of roughly spherical primary particles that are
linked together.
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6. Flame Types
Flow: Laminar or turbulent.
Fuel and oxidizer mixing:
Pre-mixed.
Diffusion.
Laminar diffusion flames were used in this work.
Methane diffusion flames at high pressures.
Figure: Depiction of methane laminar diffusion flames.
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7. Flame Types: Pre-mixed vs. Diffusion
Most jet propulsion and diesel engines operate on diffusion
combustion [1].
Studies using premixed combustion:
Due to oxidation, premixed flames cannot simulate the rich pyrolysis
need to form soot [9].
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8. Flame Types: Turbulent vs. Laminar
Most practical combustion systems operate on high-pressure
turbulent combustion.
Difficult to spatially and temporally track soot formation in
turbulent flames.
Shorter residence times.
High levels of intermittency.
Laminar flames are more tractable at elevated pressures.
Easier to obtain spatially and temporally resolved data.
Approximations to apply laminar results to turbulent systems.
Flamelet Hypothesis: proposes that turbulent flames are a collection
of deformed laminar flames [18, 26, 27].
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10. Soot Morphology Parameters
The architecture of soot particles is known as soot morphology.
The soot morphology parameters are:
dp− Soot particle size.
Df− Soot fractal dimension.
kf− Soot fractal pre-factor.
Rg− Radius of gyration.
P = 4.0 atm
E9_4
E9_6 E9_17
F6_21
F6_28
Figure: Aggregate soot particle collected at 4 atm from a laminar diffusion flame.
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11. Studies of Soot Morphology
Numerical studies are able to simulate soot particle formation
[10–12].
Experimental studies are able to measure soot particle sizes and
concentrations using non-intrusive and intrusive techniques.
Non-intrusive techniques use light scattering and extinction methods
to measure soot particle sizes and concentrations in flames [19–25].
Intrusive Techniques use thermophoretic sampling and transmission
electron microscopy to analyze soot particle sizes.
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12. Research Objectives
Objectives
1. To design and build a thermophoretic sampling system to
integrate with the existing high-pressure combustion chamber
at the University of Toronto Institute for Aerospace Studies
(UTIAS)
2. To carry out proof-of-principle experiments on laminar
diffusion flames to show the capabilities of the designed
system at elevated pressures.
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13. High-Pressure Combustion Chamber (HPCC)
The experiments were performed
in the HPCC at UTIAS which
has been used in many previous
studies [31, 34–38, 49–51].
Weighs 680 kg.
Manufactured from A36
stainless steel for protection
against corrosion [52].
The cylindrical chamber is
made from a 25.4 cm (10
inch) diameter pipe.
Wall thickness of 18.24
mm (0.718 inch).
Can conduct experiments up
to 110 atm.
Figure: UTIAS Combustion Chamber
[50].
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14. Co-Flow Laminar Diffusion Flame Burner
3.00 mm
6.22mm
4.76 mm
7.5 °
B
DETAIL: B
25.40 mm
Sintered
Metal Foam
Air
Fuel and Diluent Mixture
Figure: Schematic of the
co-flow diffusion flame burner
that was used in this work [53].
The exit diameter of the fuel
tube is 3.0 mm.
Exit diameter of the co-flow air
tube is 25.4 mm.
The burner has two features
that help improve the stability
of the resulting flame.
1. Fuel tube exit tip is tapered.
2. Metal porous inserts.
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15. Burner Enclosure
Modified with a horizontal slot to allow physical access to the flame.
HEIGHT
SLOT
63.5 mm
3.5 mm
STAINLESS
STEEL CASING
BURNER NOZZLE
SLOT
IGNITER
HOLDER
QUARTZ WINDOW
44.5 mm
Figure: Modified burner enclosure for the burner assembly in the combustion chamber at
UTIAS.
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16. Typical Thermophoretic Sampling Systems
Common intrusive technique used for
other nano-scale particle morphology
research in the biology and material
science fields [55].
In general, there are two types of
thermophoretic sampling:
Ordinary - samples taken along sampling
path.
Localized - samples taken at a desired
location.
Thermophoretic soot sampling uses 3 mm
diameter copper grids to collect the soot
particles.
TEM-GRID
WIDTH
FLAME
SAMPLING PATH
BURNER
THICKNESS
Figure: Ordinary thermophoretic
sampling scheme (Lee et al.
2008)
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17. Thermophorectic Sampling Works Based on
Thermophoresis
Figure: Particle motion induced by a temperature gradient [56]
Nano-scale particles inside a non-isothermal environment are driven
from the high to low temperature regions. This particle transport
down the temperature gradient is known as thermophoresis [56].
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18. Thermophoretic Sampling System (TSS)
The thermophoretic sampling has three main components:
Thermophoretic sampling disk (TSD).
Motor drive system (MDS).
Programmable control system (PCS).
Air intake port
Flame enclosure
Sampling probe
Igniter holder
Fuel tube
Burner tube
Quartz window
Gearbox
Stepper motor
Encoder
Sampling disk
Homing limit switch
Base plate
Slot in the
flame enclosure
Figure: Schematic of the thermophoretic sampling system developed in this work.
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19. Thermophoretic Sampling Disk (TSD)
Responsible for sweeping the sampling probes through the flame.
Technical Specifications:
Houses 10 probes.
Probe arms extend radially outwards 109.5 ±1.25 mm.
3 mm thick stainless steel.
86 mm
109.5 mm
23.75 mm
4 mm
R1.625 mm
TEM grid
Figure: Top view of the TSD and detailed view of the sampling probe arm.
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20. Motor Drive System (MDS)
Drives the rotation of
the sampling disk.
Technical
specifications:
Max angular
velocity – 80 rev
s .
Velocity
Repeatability
±0.02% of set rate.
Static Torque 1.09
Nm (155 oz-in).
Gearbox
Motor drive system
mounting bracket
Gearbox output shaft (flat-top)
Stepper motor
Homing switch
Homing switch mounting bracket
Figure: Motor Drive System.
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21. Programmable Control System (PCS)
The PCS allows the user to control the probe exposure time.
If it is too long, soot aggregates could stack, compromising the
clarity of the TEM images and burn the grid [22, 47].
if it is too short, there is a possibility that soot particles would not
be collected [22, 47].
Home position
Record button
Figure: Touch screen interface of the PCS.
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23. Transmission Electron Microscopy (TEM)
An electron microscope is used to take pictures of soot aggregates
at magnifications ranging from 80,000× – 400,000×.
Depict soot particles as small as 0.204 nm in size in ideal TEM
conditions.
TEM images are analyzed manually and with software to determine
the soot morphology characteristics.
Figure: TEM image of an aggregate soot particle from an methane-air laminar diffusion flame
at 10 atm.
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24. Experimental Test Conditions
Methane and air were used as the fuel and oxidizer for all the
experiments.
Methane flow rate was kept as 0.55 mg/s at all pressures.
Corresponds to a carbon mass flow rate of 0.41 mg/s.
Co-flow air mass flow was kept at 0.34 g/s.
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25. Results - Flame Shape Disturbance
The flame disturbance was analyzed by capturing high frame-rate
image series of the sampling process in a similar manner to the work
presented in [48].
3 mm
10 mm
Burner nozzle tip
Probe height relative to burner nozzle tip
Flame height relative to burner nozzle tip
A B C
0 mstr = 2.10 ms 4.20 ms
Stable
flame
Figure: Sampling of the methane-air laminar diffusion flame at 10 atm with a grid residence
time of 4.20 ms. Pictures were taken at 5000 frames per second with a high frame-rate
camera (Photron, model: SA5) and lens (Tamron, 180 mm F/3.5).
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26. Results - Flame Oscillation Disturbance
Observing the figure below, it is seen that the flame height
decreases and increases until a height stability is reached at 109.2
ms after the sampling process started.
0 11.4 15.0 19.6 21.8 23.8 31.8 38.4 45.6 55.0 60.4 66.6 75.8 84.4 95.0 106.4 120.6
10
8
6
4
2
0
Stable
flame
Stable
flame
Flameheight(mm)
Time (ms)
Sampling
time 1st oscillation 2nd oscillation
3rd, 4th, …
oscillations
Figure: Pictures were taken with a high frame-rate camera (Photron, model: SA5) and lens
(Tamron, 180 mm F/3.5). The pictures were taken at a rate of 5000 fps.
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27. Results - Soot Particle Measurement Summary
Table: Breakdown of the number of size measurements by pressure and grid residence time
Pressure Grid residence time
atm 4.2 ms 5.4 ms 7.56 ms
2.3 110 100 110
4.0 100 140 100
5.4 100 100 100
7.1 100 100 140
10.0 100 130 150
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28. Particle Size Measurements
Figure: Image of a soot
aggregate sampled from
a methane-air laminar
diffusion flame at 4
atm.
Figure: Image of a soot
aggregate particle with
ten primary particle
chosen and numbered.
Figure: Particle
size
measurement of
a soot primary
particle from
the aggregate
in the above
figure.
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30. Discussion
A 36% decrease in mean primary particle diameters suggests that
higher pressures significantly decreases soot particle sizes.
Assuming that soot nuclei form via coalescence of smaller PAH
clusters and consequent surface growths.
Increasing pressure is relatively slowing down this process in order to
arrive to smaller soot primary particle sizes at elevated pressures.
Park et al. modeled the coalescence of particles in the transition
regime [61].
Implementing the harmonic mean coagulation kernel → Coagulation
rates significantly decrease for particles in the transition to
near-continuum regime.
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31. Discussion
Assuming that the combustion gases in this flame are around 1500 K
[32]:
At pressures between atmospheric and 2.3 atm:
The size of the primary soot particles is comparable to the mean free
path of the combustion gases in a laminar diffusion flame.
At a pressure of 2.3 atm:
The Knudsen (Kn) number ≈ 20 (free molecular regime close to the
transition regime).
At a pressure of 5.4 atm:
Kn ≈ 10 (transition regime).
At a pressure of 10 atm:
Kn ≈ 5 (transition regime but approaching the near-continuum
regime).
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32. Sources of Error
Errors from the diagnostic system:
Distortion of the flame induced by the probe vibrations [47].
Mitigated by probe aspect ratio (thickness/width).
Accuracy of the angular positioning of the sampling disk was about
±0.02%.
Errors from the measurement analysis:
The particle size measurement uncertainty was estimated at about
10% given the number of samples taken [46].
Errors from the combustion chamber and gas delivery system:
The uncertainty was less than 1%.
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33. Conclusion
A novel thermophoretic sampling system that uses a rotating
sampling disk with multiple probes was designed and built.
1. Up to 10 samples to be taken in one experimental run.
2. Rotating disk design allows for easy and precise control of the grid
residence time down to 2.65 ms.
The mean primary particle size was observed to decrease
significantly by 36 % as the pressure increased from 2.3 to 10 atm.
It was argued that lower coagulation rates as a result of decreasing
Knudsen number is the leading cause for the mean soot particle size
decrement with increasing pressure.
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34. Recommendations and Future Work
There are two major design recommendations that would improve
the performance of the TSS:
1. Improve the method for attaching TEM grids to the sampling probe
arms.
To improve the removal process of the grids from the probe arms to
transfer them to the TEM.
2. Invest in a stepper motor and gearbox that are designed to function
in pressures above 10 atm.
Further TEM image processing of the samples could provide
insightful data bout the effects of pressure on soot fractal
properties. The results from this analysis could aid researchers to
determine the refractive indices of soot[16].
Investigating soot particle sizes for various biofuels at high-pressures
could be carried out at UTIAS.
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