Hydrodynamical Analysis of Nanometric Aluminum/Teflon Deflagrations
BY
SHAWN C. STACY, B.S.M.E.
A THESIS
IN
MECHANICAL ENGINEERING
Submitted to the Graduate Faculty
of Texas Tech University in
Partial Fulfillment of
the Requirements for
the Degree of
MASTER OF SCIENCE
IN
MECHANICAL ENGINEERING
Approved
Michelle Pantoya
Chairperson of the Committee
Valery Levitas
Brandon Weeks
Accepted
Fred Hartmeister
Dean of the Graduate School
May 2008
Texas Tech University, Shawn C. Stacy, May 2008
Acknowledgments
I am exceptionally grateful to everyone that has helped me accomplish so much
while at Texas Tech University. Specifically, I’m thankful for the opportunities and
guidance of Dr. Michelle Pantoya has given me over the last two years. With her help, I
have grown much as a researcher and I am more prepared for any challenges ahead of
me. I would also like to thank Dr. Mark Grimson at the Texas Tech Imaging Center for
helping me with the finer points of electron microscopy. Also, I would like to
acknowledge Idaho National Laboratory for technical and financial assistance that was
critical to this work.
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Texas Tech University, Shawn C. Stacy, May 2008
Table of Contents
ACKNOWLEDGMENTS ................................................................................... II
TABLE OF CONTENTS ................................................................................... III
ABSTRACT .......................................................................................................... V
LIST OF TABLES .............................................................................................. VI
LIST OF FIGURES .......................................................................................... VII
I. INTRODUCTION AND BACKGROUND ..................................................... 1
1.1 OVERVIEW ................................................................................................. 1
1.2 ALUMINUM COMPOSITE REACTIVE MATERIALS ............................. 1
1.3 NANO-SCALE COMPOSITES.................................................................... 3
1.4 UNDERWATER TESTING ......................................................................... 5
1.4.1 SEQUENCE FOR UNDERWATER REACTIONS ................................. 5
1.4.2 UNDERWATER TESTING..................................................................... 8
II. EXPERIMENTAL SETUP AND PROCEDURE ....................................... 11
2.1 SAMPLE PREPARATION......................................................................... 11
2.2 EXPERIMENTAL SETUP ......................................................................... 14
2.3 DATA ACQUISITION ............................................................................... 16
2.4 REAL CODE CALCULATIONS ............................................................... 20
III. RESULTS...................................................................................................... 23
3.1 GAS GENERATION .................................................................................. 23
3.2 PRESSURE ................................................................................................. 26
3.3 OBSERVATIONS ...................................................................................... 27
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3.4 TMD RESULTS .......................................................................................... 29
IV. DISCUSSION................................................................................................ 33
4.1 UNDERWATER TESTING OF DEFLAGRATIONS ............................... 33
4.2 HYDROPHOBIC TEFLON ........................................................................ 35
4.3 GAS GENERATION .................................................................................. 37
V. CONCLUSIONS ............................................................................................ 43
VI. FUTURE WORK.......................................................................................... 44
REFERENCES .................................................................................................... 55
APPENDIX .......................................................................................................... 58
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Abstract
The hydrodynamics of deflagrations from reactive materials (RM) submerged
underwater can be studied using a modified aquarium test. Normally loose powder RM
will disperse after being submerged in water. Introducing hydrophobic materials such as
Teflon into the reactant matrix, enables a barrier against permeation of water into the
reactants. Also, ignition via resistance heating can be difficult underwater because
significant energy is lost by convection off the wire into the water. Nano-Al particles
require significantly less energy for ignition than their micron scale counterparts such that
underwater ignition via resistance heating can be achieved. The objective of this study is
to examine the reaction hydrodynamics from a submerged nano Al-Teflon mixture as a
function of mixture composition and bulk density.
Submerged Aluminum/Teflon mixtures were ignited and the ensuing reaction was
recorded with a high speed camera and a pressure transducer. The resulting bubble shape,
size, and pressure histories along with the burn time and rate allow the analysis and
comparison of different fuel/oxidizer compositions and powder packing densities. Results
show that as the density of the powder decreases the reaction transitions from a slow jet
of multiple bubbles to quick single bubble. One observation is that as the percentage of
aluminum increases the bubble radius also increases even though there is less of the gas
producing Teflon in the mixture. This could imply that the excess aluminum is reacting
with water.
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List of Tables
1. POWDER SPECIFICATIONS.......................................................................... 12
2. ACTUAL MASS PERCENTS OF AL AND TEFLON FOR MIXTURES ... 13
3. REAL CODE RESULTS FOR INCREASING PRESSURE .......................... 14
4. EXPERIMENTAL RESULTS........................................................................... 23
5. PERCENT TMD AND BUBBLE SHAPE RESULTS..................................... 30
6. REAL CODE RESULTS FOR TEFLON AND 80% PURE ALUMINUM .. 50
7. REAL CODE VOLUME PRODUCT RESULTS FOR AL + TEFLON WITH
A MASS PERCENTAGE OF WATER ............................................................ 50
8. REAL CODE ADIABATIC FLAME TEMPERATURE RESULTS FOR AL
+ TEFLON WITH A MASS PERCENTAGE OF WATER ........................... 61
9. EXPERIMENTAL RESULTS FOR DISPLACEMENT ENERGY, BUBBLE
GROWTH RATE, AND PRESSURIZATION RATE .................................... 62
10. PERCENT OF WATER REQUIRED FOR COMPARISON IN REAL
CODE RESULTS ................................................................................................ 63
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List of Figures
1. HEAT OF COMBUSTION COMPARISON OF AL/TEFLON, TNT, AND
THERMITE MIXTURES CALCULATED USING REAL CODE ................. 2
2. UNDERWATER EXPLOSIVE BEHAVIOR .................................................... 8
3. SEM MICROGRAPH OF ALUMINUM AND TEFLON AT 50% TMD. ... 12
4. TANK SETUP INCLUDING THE SAMPLE BLOCK (A), IGNITION
WIRE LEADS (B), AND UNDERWATER BLAST PRESSURE SENSOR
(C). ........................................................................................................................ 16
5. OVERALL TEST SETUP WITH THE HIGH SPEED CAMERA, TANK,
AND FIBER OPTIC LIGHT GUIDE ............................................................... 17
6. EXAMPLE OF A SINGLE BUBBLE REACTION WITH 14
MILLISECONDS BETWEEN EACH FRAME .............................................. 17
7. LIGHTED CONDITIONS WITH THE HIGH SPEED CAMERA (A),
TANK SETUP (B), FIBER OPTIC LIGHT GUIDE (C), AND VARIAC
TRANSFORMER (D) ......................................................................................... 18
8. PHANTOM SOFTWARE FOR RADIUS ANALYSIS. .................................. 19
9. THE RADIUS IS OBTAINED FROM TWO POINTS PER FRAME ON
THE SURFACE OF THE BUBBLE. THE POINTS ARE AVERAGED FOR
THE FINAL RADIUS CURVE. ........................................................................ 20
10. MAXIMUM RADIUS DATA FROM HIGH SPEED VIDEOS SHOWING
THE MAXIMUM RADIUS INCREASES WITH POWDER
EQUIVALENCE RATIO................................................................................... 25
11. MAXIMUM BUBBLE VOLUME ..................................................................... 26
12. A REPRESENTATIVE PRESSURE PROFILE FOR ONE SECOND ........ 27
13. EXPERIMENTAL PRESSURE RESULTS AT MAXIMUM RADIUS ....... 27
14. EXAMPLES OF UNBURNED MICRON ALUMINUM AND TEFLON
REACTIONS ....................................................................................................... 28
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15. ILLUSTRATION SHOWING THE INFLUENCE OF CARBON ON
BUBBLE TRANSPARENCY ............................................................................ 29
16. AIR BUBBLE BEFORE REACTION .............................................................. 29
17. %TMD AND BURN TIME ANALYSIS .......................................................... 30
18. FROM LEFT TO RIGHT: THE JET OF BUBBLES, THE BALL OF
BUBBLES, THE MUSHROOM SHAPED BUBBLE, AND FINALLY THE
SINGLE BUBBLE THAT IS USED FOR THE RADIUS ANALYSIS. ........ 31
19. RADIUS OSCILLATIONS FOR BLOCK RIGHT1....................................... 32
20. DETAILED RADIUS OSCILLATIONS FOR TIME .02 TO .03 SECONDS
FOR BLOCK RIGHT1 ...................................................................................... 32
21. PENDULUM ANALOGY .................................................................................. 35
22. REAL CODE CALCULATION FOR THE AL/TEFLON WITHOUT AND
WITH 50% WATER. THE EQUIVALENCE RATIO IS ONLY
CALCULATED FOR THE AL/TEFLON POWDER..................................... 38
23. REAL CODE CALCULATION OF ADIABATIC FLAME
TEMPERATURE FOR DIFFERENT MASS PERCENTS OF WATER ..... 40
24. RESULTS SHOWING GAS PRODUCTION INCREASES WITH FUEL
RICH MIXTURES AND MOST OF THE GAS PRODUCTION INCREASE
IS FROM HYDROGEN GAS (AOCHI, 2000)................................................. 41
25. HEAT OF COMBUSTION (KJ/MOLE) .......................................................... 58
26. HEAT OF COMBUSTION (KJ/M3) ................................................................. 59
27. HEAT OF COMBUSTION (KJ/KG) ................................................................ 60
28. DATA FROM TABLE 4 PLOTTED TO SHOW THE EFFECT OF
SAMPLE MASS .................................................................................................. 64
29. FIRST ATTEMPT NORMALIZING RADIUS DATA .................................. 54
30. SECOND ATTEMPT NORMALIZING RADIUS DATA ............................. 66
31. SECOND ATTEMPT NORMALIZING RADIUS DATA IN TERMS OF
VOLUME PER MOLES .................................................................................... 67
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Chapter I
Introduction and Background
1.1 Overview
Reactive materials (RM) are of interest in the combustion community. These
materials have the adaptability that allows maximum performance and still have the
safety required for various applications. Furthermore, nanometer-scale RM offer
enhanced performance over their micron-scale counterparts. To better understand how
nanometer-scale RM or RM in general behave, hydrodynamic studies can be done.
Aquarium tests offer a unique way to hydrodynamically analyze RM and characterize
performance. Combustion characteristics that are difficult to obtain under normal burn
testing conditions can easily be obtained with underwater burns. It is possible to
determine an estimate of the gas generation and kinetic energy released. In addition,
aluminum-based RM have a special interest because of the aluminum/water reaction that
can produce hydrogen along with large amounts of energy. Hydrogen production is of
significant interest for its use in alternative fuels.
1.2 Aluminum Composite Reactive Materials
Aluminum composite RM have a high energy density and the combustion
performance that allows them to be used in various applications from new types of
propellants to underwater cutting torches. Aluminum reacts very readily and acts as a
metal reducing agent in reduction/oxidation reactions (metal combustion). On exposure
to air, aluminum forms a passive layer of aluminum oxide that is a barrier preventing the
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remaining aluminum from reacting. The aluminum oxide shell allows the aluminum RM
to be handled safely and also could change the reaction mechanism. Aluminum can be
combined with metal oxides (known as Thermites), other metals (known as
intermetallics), and other compounds (such as Teflon). Aluminum-based reactions are
extremely exothermic (Fischer & Grubelich, 1998). Also, based on the REAL code
(Belov, 2004) calculations shown in Figure 1, aluminum/water and aluminum/Teflon
reactions have a higher energy density than a conventional explosive (TNT) and common
Thermite reactions. Heat of Combustion results in kJ/mole, kJ/m3, and kJ/kg are
presented in the appendix.
cal/cc
C7H5N3O6 (TNT)
cal/g
2Al+Fe2O3
2Al+MoO3
2Al+3H2O
4Al+3C2F4
0
1000
2000
3000
4000
5000
6000
7000
Figure 1. Heat of Combustion comparison of Al/Teflon, TNT, and Thermite mixtures. The values
were calculated using REAL code
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1.3 Nano-scale composites
Advances in powder manufacture have allowed particles of aluminum to be
commercially available in the nanometer range. Nano-scale particles have one dimension
less than 100 nanometers (Son, 2004). RM made with the smaller particles can have
improved combustion performance. Combustion performance can be characterized by
ignition dynamics and energy propagation. Specifically, parameters such as flame speed,
energy generated, energy required for ignition, temperature change, pressure change, and
completeness combustion can be used to characterize performance. For example, Nanoscaled reactive materials have a faster flame speed (Bockmon et al., 2005) and lower
apparent activation energy (Granier & Pantoya, 2004) than their micron-scaled
counterparts.
The reaction speed is greatly affected by the smaller particle size. Smaller particle
size allows for a larger surface area to volume ratio, decreasing the distance the fuel and
oxidizer have to travel to react and increasing the likelihood of contact between the
different reactants.
Another reason, although not completely based on size of the
particles, is that for loosely packed Thermites, the reaction is convection controlled (Asay
et al., 2004). Energy is quickly transferred to the RM in front of the flame faster, allowing
the flame to propagate faster with conduction than convection. The change in the mode of
heat transfer is only valid for mixtures that have porosity to allow gas to disperse between
particles. The mode of heat transfer and the surface area to volume both are influences to
increase the nano-scaled RM reaction speed.
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Another factor for RM materials is activation energy. Activation energy is the
energy required for an energetic material to ignite. Lower activation energy will allow a
reaction to occur more easily. For instance, with low activation energy a simple wooden
match would be able to ignite the reaction while for higher activation energies a powerful
laser is needed. The energy imparted on the sample with the match is much less than the
energy from the laser. Particle size plays a large role in the apparent activation energy for
aluminum reactions. For example, a composition made from Molybdenum trioxide,
MoO3, and nanometric aluminum particles will ignite very readily compared to micron
scale aluminum particles (Granier & Pantoya, 2004). The lower apparent activation
could allow for more complete burns with smaller aluminum particles. The lower
apparent activation energy would allow the flame to propagate to areas of inhomogeneity
(excess fuel or oxidizer, inconsistent density, impurities) when they normally would not.
Finally, the alumina shell plays a key role in changing how nano-scaled aluminum
particles react with an oxidizer. The alumina shell is an oxide layer that forms on the
surface of aluminum from exposure to oxygen. The alumina shell might act much like a
pressure vessel, containing the molten aluminum under compression until the shell fails.
When the shell fails the shell could be violently ejected to the surroundings allowing the
molten aluminum core to be completely exposed. The influence of the alumina shell to
allow the molten aluminum inside the particles to react unimpeded is called melt
dispersion (Levitas et al., 2006). Melt dispersion helps account for the increased reaction
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speed that is not explained by the diffusion mechanism and the surface area to volume
ratio increase alone.
1.4 Underwater Testing
Underwater explosive testing is an important type of research for reactive
materials. Water provides a more dense environment enabling observation of the gasliquid interface. Testing RM underwater allows for monitoring the pressure of the
shockwave emanating from the reaction and more importantly for this work, tracking the
interface between the gas products and water. The interface can easily be seen
underwater, unlike in gas environments were the intermixing of gas from the reaction and
the environment is too turbulent and the gas from the reaction diffuses too readily. The
interface makes it possible to determine the amount of kinetic energy released and the
volume of gas products produced from a reaction. Underwater explosive testing allows
for the characterization of RM in a controlled and repeatable setup.
1.4.1 Sequence for underwater reactions
To understand underwater testing of RM, underwater explosion/reaction behavior
must be understood. Underwater explosions have a sequence of distinct phases starting
from the pre-reaction initial conditions to oscillations and collapse (Cole, 1948).
Knowing these phases is important because different data can be obtained from each
phase.
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The initial conditions and setup, particularly depth of the reaction, are important
for analysis. The reaction will displace less water at greater depths and also be more
unlikely to break the surface of the water. Another important factor is the reflection of
acoustic waves based on structures surrounding the explosive sample. Reflected waves
can cancel out or provide interference for the initial acoustic wave resulting in erroneous
results.
The next step in the sequence is ignition and reaction. The activation energy
barrier of the sample must be overcome in order for the reactive material to ignite. Once
ignited, the reactive material will begin to propagate if the energy generated is greater
than the activation energy. The reactants will rapidly transition into products and energy
in the form of light, heat and physical expansion. In many cases the intermediate products
and final products are gases causing large increases of pressure on the order of tens of
thousands of atmospheres (Cole, 1948).
After the initial reaction the pressure concentrated at the reactive zone starts to
expand spherically. In the water surrounding the gas bubble, there will be an outgoing
acoustic pressure wave. The wave will have a sharp, near vertical rise that then will
decay from the peak to the center of the reaction. The profile will also drop in amplitude
the peak moves away from the center of the reaction. If the velocity of the peak is faster
then the speed of sound in water the wave can be called a shockwave, otherwise it is
know as an acoustic wave. The compressibility of water has much larger role in
mathematical equations for shockwave than acoustic waves. The first oscillation is not
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used in calculations due to these effects. In contrast, the pressure inside the gas bubble
will be isotropic and homogenous but will change with the diameter of the bubble. The
pressure inside the bubble will decrease to reach equilibrium with the pressure of the
surrounding water forcing the bubble to grow. Once the bubble reaches maximum radius
it will start to oscillate (Bocksteiner, 1996; Menon & Mihir, 1997).
The gas bubble will oscillate for a reason very similar to why a pendulum will
swing back and forth. The pressure is the driving potential in the bubble similar to the
height driving the movement in a pendulum. In both cases kinetic and potential energy
are exchanged with each other until the energy is completely dissipated and the
oscillations stop. For the bubble oscillations, the highest pressure will be when the bubble
is at the minimum radius. When the bubble is at its maximum radius it will have the least
amount of pressure. These two extremes will be the points of highest potential energy,
while the highest kinetic energy will be when the pressure is equal to the hydrostatic
pressure outside the bubble. The bubble will oscillate between the maximum and
minimum radii until pressure equilibrium is reached with the surrounding water and all
the energy is lost from viscous dissipation. These oscillations are illustrated in Figure 2.
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Figure 2. Underwater Explosive Behavior
Based on viscosity of water and speed of the bubble rise from buoyancy, the
bubble will collapse into smaller bubbles. Depending on the RM and depth of the
reaction this could be as soon as it reacts or until it reaches the surface. Once the bubble
collapses, radius and bubble pressure data will make it difficult to obtain any useable
results, because there are multiple radii to measure and the pressure sensor will not be
inside of a bubble.
1.4.2 Underwater Testing
Underwater explosive testing is more time consuming and costly than normal
reactive material testing but the benefits are well worth while. The main benefit is
visualizing the volumetric gas production of the reaction. Underwater testing methods are
diverse allowing trade offs based on the data required and resources available.
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Underwater explosive testing is often carried out in open areas of water to
minimize the effects of acoustic reflections and damage to surroundings while being able
to use full sized charges and munitions. These tests can be done destructively with a real
munitions and targets or non-destructively with explosives in open water. Open water
tests are useful because they allow for repeatable experiments that can be used to
compare the characteristics of one explosive to another. Both of these types of tests are
time consuming to prepare and difficult to observe underwater. Conversely, there are also
aquarium tests which allow for quick setup and comparable results to open water nondestructive tests.
Aquarium tests are underwater explosive experiments conducted in a sturdy tank
filled with water. Aquarium tests commonly use pressure transducers to measure the
pressure wave and/or the bubble pressure. The tank setup also allows for adaptability
while still maintaining the ability for positioning the sensors and samples in the same
location each experiment. Researchers commonly suspended the RM samples in the
center of the tank (Frankel et al., 1982; 1996; Menon & Mihir, 1997; Aochi et al., 1999),
but other methods were also used (Goldstein & Johnson, 1982). Suspending the sample
reduces the influence of the walls and surface of the water on the pressure waves in the
tank. Each experimenter used a different ignition, pressure sensor, and camera setup that
suited their samples. The most important advantage aquarium tests have are lighting and
camera conditions that are impossible in an open water setup are made trivial in aquarium
experiments. With the right lighting conditions, it possible to film the entire sequence of
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events of an underwater reaction with a high speed camera. Aquarium testing does have
its disadvantages. The wave reflections cause problems for data acquisition and for tank
durability. The wave reflections from the tank walls and the water surface will make
shockwave analysis difficult but will not strongly influence the bubble radius and
pressure (Menon & Mihir, 1997). Finally, the tank must be cleaned after each experiment
because of the solid product residue. The advantages of aquarium tests make them an
invaluable research tool besides the disadvantages.
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Chapter II
Experimental Setup and Procedure
2.1 Sample Preparation
Reactive material samples were obtained by mixing aluminum and Teflon (PTFE)
at various fuel/oxidizer balances. The aluminum powder was obtained from Novacentrix
(Austin, Tx). The powder is spherical, with an average diameter of 80 nanometers, and
covered with thin oxide (alumina) shell roughly 2 nanometers thick. The alumina shell
was determined to be 20% of the total particle mass. X-ray diffraction, TEM imaging,
and BET surface area analysis was used to find the particle size and purity. The Teflon
obtained from Dupont (Zonyl MP1150) particles have an average diameter of 200
nanometers and are also spherical. An SEM micrograph of the mixture is shown in Figure
3.
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Figure 3. SEM micrograph of Aluminum and Teflon at 50% TMD.
In addition to the nanoscale aluminum, micron-scale aluminum was also tested.
The micron aluminum (Atlantic Equipment Engineers) was 1-3 microns in diameter,
spherical and 99% pure. Molybdenum trioxide, MoO3, (Novacentrix) was also used as an
oxidizer for initial experiments. The Molybdenum trioxide particles were rectangular, 44
nanometers in the smallest dimension, and 99% pure. The powders are displayed in
Table 1.
Powder
Aluminum
Teflon-PTFE
Aluminum
MoO3
Particle Size
80nm
200nm
1-3µm
44nm
Table 1. Powder Specifications
Manufacture
Novacentrix
Dupont
Atlantic Equipment Engineers
Novacentrix
Purity
80%
100%
99%
99%
Form Factor
Spherical
Spherical
Spherical
Rectangular
The aluminum and Teflon powders were combined at mass percents shown in
Table 2. Mass percents were given by the chemical balance reaction in Equation (1)
(Asay et al., 2004) and the equivalence ratio, Φ, Equation (2). The mass percents took
into account the difference between active aluminum content the overall mass of the
powder. The combined powders were suspended in hexanes to allow for ultrasonic
mixing. Ultrasonic mixing improves homogeneity by reducing agglomerates.
The
suspended powders were placed under cyclic ultrasonic vibration with a Misonix
Sonicator 3000 for 10 seconds on and 10 seconds off for 2 minutes total. Cyclic operation
reduces the maximum thermal load on the sample. After mixing the suspended particles
are placed in a glass pan inside a fume hood to dry. The final dry powder mixture was
removed with an electrically grounded metal brush and placed into a glass vial.
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Table 2. Actual mass percents of Al and Teflon for mixtures
Φ
1
1.5
2
80% Pure Powder
%Al
%C2F4
30%
70%
39%
61%
46%
54%
4 Al + 3(C2 F4 ) → 4 AlF3 + 6C
(1)
(F O )
Φ=
(F O )
(2)
Actual
Balanced
Where: F = Mass of the fuel
0 = Mass of the oxidizer (C2F4)
The SEM micrograph, shown in figure 2, was taken of the final aluminum/Teflon
reactive material to verify homogeneity and the lack of agglomerates. Micrographs are of
an aluminum/Teflon pellet that was pressed to 50% of theoretical maximum density
(TMD). TMD (Equation (3)) is the bulk density of a composite material assuming there
are no voids. A Hitachi S-570, medium resolution, tungsten filament, scanning electron
microscope was used.
TMD =
ρ Al M Al + ρ C F M C F + ρ Al O M Al O
2 4
2 4
2 3
(3)
2 3
M total
Where: ρ = bulk density & M = mass
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REAL code was used to show that the balanced equation was a valid
approximation of the actual reaction. In Table 3, as the pressure increases the products
approach the assumed and ideal balance reaction. In an underwater reaction, the pressure
will increase as a function of depth (hydrostatic pressure), gas production, and reaction
rate. Watson (2007) showed that the aluminum/Teflon reaction was shown to have a
flame propagation rate of over 700 meters per second for confined burns. The confined
burn apparatus that was used consisted of loose powder in polycarbonate tubes (108mm
long with a 3.2 mm inner diameter) ignited with nickel/chromium wire. These faster
reactions have less time for the gas expansion therefore will build up pressure allowing
for reaction to occur at higher pressures.
Table 3. REAL code results for increasing pressure
Pressure, atm
1
10
100
1000
10000
Balanced Equation
AlF3
C
Other products Temp, K
62.33% 14.18% 23.49%
3513.3
64.98% 14.10% 20.92%
3835.9
67.68% 14.45% 17.88%
4189.1
70.51% 14.45% 15.04%
4326.2
77.62% 16.81%
5.57%
4726.6
82.34% 17.66%
0.00%
2.2 Experimental Setup
The underwater combustion experiments were conducted inside of an acrylic
tank. The tank was 4 inches (~10 cm) long, 4 inches wide and 7 inches (~18 cm) tall as
shown in Figure 4. The tank was filled with 1 liter of distilled water to ensure to constant
hydrostatic pressure between experiments and to reduce impurities from the water that
may influence the reaction chemistry. 1 inch, clear acrylic cubes (Delvies Plastics Inc.)
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were positioned at the bottom of the tank. The cubes had a flat bottomed, 3/16 inch
(~0.16 cm) diameter, and 3/16 inch deep hole milled into the top. Approximately 50mg
of powder mixture are placed along with a small length of 33-gage Nickel/chromium
(Omega engineering, 80% nickel/20% chromium) alloy wire inside the hole.
Nickel/chromium wire was used because it has a high melting temperature along with a
high electrical resistance. The 33-gage wire was used because the smaller wire will heat
the surroundings quicker than a large gage wire. Also by using a smaller wire, which will
take up less volume, allows for easier loading of the sample and more accurate TMD
calculations. The sample was ignited by supplying 15 volts of AC current to the wire
with a variac voltage transformer.
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Figure 4. Tank setup including the sample block (a), ignition wire leads (b), and underwater blast
pressure sensor (c).
2.3 Data Acquisition
Data was collected from high-speed video and pressure transducer signals.
A schematic of the test setup is shown below in Figure 5.
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Figure 5. Overall test setup with the high speed camera, tank, and fiber optic light guide
The reactions were recorded with a high speed digital camera (Vision Research,
Phantom 7) at 5000 frames per second (fps). The optimal camera settings for this analysis
were as follows: Resolution at 512x512 pixels, exposure time at 180µs, minimum
(Extreme Dynamic Range) exposure time at 100µs, and the f-stop at the “16” setting on
the 55mm Nikon lens. The Extreme Dynamic Range exposure time helps the image from
being over saturated during the reaction while still allowing the darker portions of the
reaction event to be filmed. Figure 6 shows an example of the video recorded.
Figure 6. Example of a single bubble reaction with 14 milliseconds between each frame
It was difficult to get enough light into the camera’s CCD without also oversaturating the image at high frame rates. To alleviate the problem a fiber optic light guide
(Cole-Parmer Illuminator 41720) was used to focus intense light where it was needed.
The fiber optic light guide enhances the lighting conditions to improve imaging before
and after the reaction event was self illuminating. Secondly, it was important for the
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surface of the bubble to be visible during the reaction. A white sheet of paper is used as a
background on the side opposite of the camera. The white background improved contrast
allowing the interface of the bubble to be monitored more accurately. Figure 7 shows the
setup under lighted conditions.
Figure 7. Lighted conditions with the high speed camera (a), tank setup (b), fiber optic light guide
(c), and variac transformer (d)
The high speed video was analyzed by using the Phantom software shown in
Figure 8. Radius data was collected by using the pressure sensor as a scale (4.2 mm) and
clicking on two points on the edge of the bubble. An origin was set at the center of the
top of the hole in the acrylic block (the intersection of the two lines in the figure). One
point was taken from where the top of the bubble intersects the vertical origin line. This
point would give a diameter from the top of the bubble to the top. The vertical radius is
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the quotient of the diameter is divided by two. The second point was taken from the
surface of the bubble at the furthest point to the right. The second point gives the side
radius.
Figure 8. Phantom software for radius analysis.
These points were averaged to find the final radius curve used as seen in Figure 9.
The maximum radius was defined as the average radius corresponding to when the
maximum side radius occurs. The maximum side radius was used because as the bubble
migrates upwards the side radius decreases while the top radius continues to increase.
19
Texas Tech University, Shawn C. Stacy, May 2008
25
Radius, mm
20
15
10
Avg
Side
5
TOP
0
0
0.01
0.02
0.03
0.04
0.05
0.06
Time, seconds
Figure 9. The radius is obtained from two points per frame on the surface of the bubble. The points
are averaged for the final radius curve.
Along with the high speed video, pressure transducer data was collected. The data
was recorded with an underwater blast pressure sensor (138A33, PCB Piezotronics Inc.),
along with a signal conditioner (483A22, PCB Piezotronics Inc.), and data acquisition
board (BNC-2110, National Instruments) at 10 kHz. The sensor was placed at a slight
angle, approximately 1cm over the sample cube, and approximately 3/4cm away from the
hole. The pressure sensor was 4.2 mm (.17 inch) in diameter allowing it to be placed
close to the small sample without being a large obstruction.
2.4 REAL Code Calculations
Estimated values for heat of combustion, adiabatic flame temperature, and gas
generation were calculated using the REAL code (Belov, 2005). REAL is a chemical
equilibrium modeling program designed for high temperatures (6000K) and high
20
Texas Tech University, Shawn C. Stacy, May 2008
pressures (600-800 MPa) (Belov, 2004). One of three different equations of state (EOS)
can be chosen for calculations: Ideal gas, 3 coefficients virial EOS or, an EOS proposed
by Nedostup (1979). Virial EOS was used for these calculations because it gave the best
results at the temperatures and pressures of these reactions.
The heat of combustion is calculated by using the default template provided in
REAL and inputting desired composition. The heat of combustion template included
options to fine-tune the specific volume and specific internal energy. Iterations of REAL
code calculations were done to determine the specific internal energy that will give
similar results as Fischer and Grubelich (1998) for a range of various reactions. The
specific internal energy was then used to calculate the heat of combustion for the
balanced aluminum/Teflon reaction. The heat of combustion was calculated to be 10,709
kJ/kg for aluminum/Teflon mixtures with pure aluminum and 8856 kJ/kg for 80% pure
aluminum. Experimental differential thermal analysis (DTA) results presented by
Cudzillo and Trxcinski (2001) for aluminum/Teflon at 26.5% aluminum (slightly fuel
lean) were lower at 7,540 kJ/kg but only 57% of the aluminum reacted. Their experiment
analyzed the mixture of aluminum (50 micron diameter) and Teflon particles (less than
40 microns in diameter) at a heating rate of 10 Kelvin per second. Assuming that only
57% of the Teflon reacted with the 57% of the aluminum, the REAL code heat of
combustion would be 6104 kJ/kg. The heat of combustion was calculated 8755 kJ/kg
with only 57% aluminum and 100% of the Teflon. Real code gave a close approximate to
21
Texas Tech University, Shawn C. Stacy, May 2008
the experimental data as the experimental results fall between the two heat of combustion
values.
The adiabatic flame temperature and gas generation were calculated by inputting
the desired composition and modeling the reaction in a combustion chamber at constant
pressure (1 atm). The specific enthalpy of the system was also inputted. The specific
enthalpy was determined in the method similar to the specific internal energy for the heat
of combustion
22
Texas Tech University, Shawn C. Stacy, May 2008
Chapter III
Results
3.1 Gas Generation
The modified aquarium test provides a method to measure gas generation. The
gas/water interface allows the radius to be tracked and assuming the bubble is spherical,
the volume can be calculated.
Table 4 shows the experimental results for the burn times and maximum radii for
15 different samples. The equivalence ratios (Φ), sample mass and block numbers are
also displayed. Blocks 26-28 were not used for maximum radius calculations. The
multiple bubbles would erroneously show the radius values to be estimated lower than
they actually were. Multiple bubbles were defined as a stream of 2 or 3 large bubbles.
The burn times for those blocks were also shown to be longer than the rest of the
samples. Since the radius results are not used for 26-28 blocks and block 29 has no video
there are 2, 4, and 5 results for the 1, 1.5, and 2 equivalence ratios, respectively. The
largest radius was shown to be 22.6 millimeters for a fuel rich sample.
Table 4. Experimental results
23
Feb-08
Oct-07
Texas Tech University, Shawn C. Stacy, May 2008
Block# EqR, Φ % TMD Sample Mass, mg Burntime, sec
18
1
17.3%
31.2
0.132
19
1
16.2%
29.5
0.116
20
1.5
11.5%
27.7
0.065
23
1.5
12.3%
30
0.070
25
2
15.8%
29.2
0.068
31
2
15.1%
28.6
0.072
26
1
14.3%
26.1
0.176
27
1
15.2%
28.3
0.196
28
1
14.9%
27.1
0.170
29
1.5
15.2%
34.3 no video
30
1.5
14.8%
31
0.100
34
1.5
15.2%
29.7
0.067
35
2
14.8%
28.6
0.065
37
2
15.3%
36.7
0.067
38
2
14.1%
26.1
0.119
24
Max. Radius, mm
18.3
18.0
17.7
19.2
19.4
18.8
Multiple bubbles
Multiple bubbles
Multiple bubbles
no video
16.7
17.6
20.4
22.6
19.5
Texas Tech University, Shawn C. Stacy, May 2008
To better understand how aluminum/Teflon stoichiometrically the radius results
were plotted in Figure 10. The bubble radius increases with equivalence ratio from the
balanced to the most fuel rich samples (Φ=1 to Φ=2). It also seems like there may be a
small increase from the balanced samples to the slightly more fuel rich (Φ=1.5) ones.
24
Max Radius, mm
23
22
21
20
19
18
17
16
15
0.5
1
1.5
2
2.5
Powder Equivalence Ratio, Φ
Figure 10. Maximum radius data from high speed videos showing the maximum radius increases
with powder equivalence ratio.
The sample mass would increase the bubble radius because there are more
reactants
to
product
gas.
Calculations
25
in
the
appendix
(
Texas Tech University, Shawn C. Stacy, May 2008
A.4 Normalizing Radius Data) normalize the radius data with moles and mass of the
sample. The bubble radius was normalized by calculating the volume of the assumed
spherical bubble and dividing by mass (Figure 11).
Volume of Bubble, m^3/kg
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
0.5
1
1.5
2
2.5
Equivalence Ratio, Φ
Figure 11. Maximum bubble volume
3.2 Pressure
The pressure data was correlated to the visual data by assuming there was no
pressure increase at the sensor until the bubble reached it. In detonations there would be
an initial shockwave followed by a slow decay until the bubble reached the sensor (Cole,
1948). For the deflagrations there is no shockwave therefore the first pressure increase
would be the interface of the bubble. The pressure at the maximum radius is shown in
figure 10. It cannot be determined if there is any pressure change at the maximum radius
based on this data.
26
Texas Tech University, Shawn C. Stacy, May 2008
5
4.5
4
Pressure, kPa
3.5
3
2.5
2
1.5
1
0.5
0
0
0.2
0.4
0.6
0.8
1
Tim e, sec
Figure 12. A representative pressure profile for one second
1.8
Bubble Pressure, kPa
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0.5
1
1.5
2
2.5
Equivalence Ratio, Φ
Figure 13. Experimental pressure results at maximum radius
3.3 Observations
In addition to the radius and pressure data, observations that were more
qualitative were made. The first observation was that micron aluminum was difficult to
27
Texas Tech University, Shawn C. Stacy, May 2008
ignite using resistance heating in this setup. A small amount of powder would ignite and
quench before completely burning as seen in Figure 14. Figure 14 illustrates small
amounts of lightly packed micron aluminum and Teflon powder that did not completely
burn. The powder immediately around the wire would ignite then quench.
Figure 14. Examples of unburned micron aluminum and Teflon reactions
A second observation was that fuel rich aluminum and Teflon mixtures
intrinsically have less carbon in the final products. The carbon forms a thin layer on the
surface of the bubble. This thin layer reduces visibility through the surface of the bubble
as can be seen in the balanced mixture for figure 15. In the fuel rich bubble there was less
carbon on the surface of the bubble and the bubble is more transparent.
28
Texas Tech University, Shawn C. Stacy, May 2008
Figure 15. Illustration showing the influence of carbon on bubble transparency
Another important observation was that after being submersed a thin layer of air
would be visible on the surface of the aluminum and Teflon powder. The thin layer of air
can be seen as a small bubble (before the reaction is initiated) in the center of Figure 16.
When aluminum and MoO3 powder were placed under the same conditions the powder
would completely disperse in the water. The thin layer of air acts as barrier preventing
water interacting with the powder sample.
Figure 16. Air bubble before reaction
3.4 TMD Results
Bubble shape was roughly dependent on percent TMD (Theoretic Maximum
Density). As the percent TMD decreases the bubble shape becomes less chaotic, as
shown in Table 5; the high density samples burned as a series of bubbles and low density
ones burned as single bubbles. Furthermore, the mid-ranged TMD samples show a
transition where the sample will be either a ball of bubbles or a few large bubbles. In
addition, burn times were dependent on bubble shape. For jets of bubbles/multiple
bubbles, the burn duration was .260 to .3 seconds while for the single bubble the burn
29
Texas Tech University, Shawn C. Stacy, May 2008
times were more than five times faster at .049 to .052 seconds. The TMD and burn time
results are also shown in Figure 17 to better illustrate the increase. Finally, the percent
TMD analysis experiments were not used to find the bubble radius because of the large
differences between the samples’ mass.
EqR, Φ % TMD Sample Mass, mg Burntime, sec
1
41.6%
17.3
0.260
1
42.2%
17.1
0.300
1
35.9%
17.2
0.300
1
35.5%
17.4
0.158
1
16.8%
10.2
0.127
1
20.8%
9.1
0.052
1
17.8%
10.6
0.103
1
16.3%
10.6
0.049
Hydrodynamics
Jet of Bubbles
Jet of Bubbles
Multiple Bubbles
Ball of Bubbles
Mushroom
Single Bubble
Mushroom
Single Bubble
0.35
0.3
Burn Time, sec
Low TMD Mid. Hi.
Table 5. Percent TMD and bubble shape results
Block#
Top7
Right1
Top8
Right3
Right5
Right6
Right7
Right8
0.25
0.2
0.15
0.1
0.05
0
0%
10%
20%
30%
40%
Percent TMD
Figure 17. %TMD and burn time analysis
30
50%
Texas Tech University, Shawn C. Stacy, May 2008
Figure 18 illustrates the visual difference in percent TMD and bubble shape. Dark
clouds of carbon and gas from were previous bubbles have collapsed can be seen for the
40% and 35% TMD pictures. The defining difference between the mushroom and single
bubble is that the mushroom bubble would grow separated from the block.
Figure 18. From left to right: the jet of bubbles, the ball of bubbles, the mushroom shaped bubble,
and finally the single bubble that is used for the radius analysis.
It should also be noted that for the jet of bubbles in high TMD samples
oscillations are visible in bubbles that separate from main column of bubbles. The period
of oscillation was shown to be .0028 seconds (2.8 milliseconds) in Figure 19 and Figure
20.
31
Texas Tech University, Shawn C. Stacy, May 2008
8
7
Radius, mm
6
5
4
3
2
1
0
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
Time, seconds
Figure 19. Radius oscillations for block right1
8
7
Radius, mm
6
5
4
3
2
1
0
0.02
0.021
0.022
0.023
0.024
0.025
0.026
0.027
0.028
0.029
Time, seconds
Figure 20. Detailed radius oscillations for time .02 to .03 seconds for block right1
32
0.03
Texas Tech University, Shawn C. Stacy, May 2008
Chapter IV
Discussion
4.1 Underwater Testing of Deflagrations
Distinct and complete oscillations were difficult to capture in large single bubble
reactions produced from deflagrations. In single bubble deflagrations, the bubble would
slow its growth rate as it expanded and give the appearance of pausing. Deflagration
reactions are defined by a combustion wave that propagates at subsonic speeds while
detonations are defined by a combustion wave at supersonic speeds (Kuo, 2005). The
key difference for a hydrodynamical analysis is that deflagrations are slower than
detonations.
In underwater explosives the reactants are quickly converted into a high pressure
gas bubble that expands and oscillates. The oscillations occur because of the pressure
difference between the ambient surroundings and the inside of the bubble. These
oscillations are used to calculate the bubble, potential, kinetic, and energy lost from the
damping effects of the water. The equations used to calculate these quantities are all
based on the assumptions that the pressure is less than hydrostatic pressure at the
maximum radius, the maximum pressure is during the minimum radius and that reaction
is complete before the radius reaching its maximum (Cole, 1948; Trilling, 1951;
Vokurkal, 1987; Bocksteiner, 1996; Menon & Mihir, 1998). None of the assumptions
correlate to the results seen for the aluminum/Teflon deflagrations.
33
Texas Tech University, Shawn C. Stacy, May 2008
However, there was oscillatory behavior captured in high speed videos for some
of the small bubbles in high TMD samples. The periods of the oscillation were brief
compared to the overall length of the reaction. The period of oscillation was .0028
seconds (2.8 milliseconds) of a 0.3 second total burn time for a high TMD sample. For
explosives the burn time is shorter than the period of oscillation. In other research with
the detonation of explosives, the time scales of initial oscillation were on the order of 10100 milliseconds, but reaction was finished before end of the first period of oscillation
(Cole, 1948; Bocksteiner, 1996; Menon & Mihir, 1998). The difference in the total burn
time for detonations and deflagrations implies the reaction is obscuring the oscillations by
continued production of gas for deflagrations. Continued gas production could also
explain why the pressure at the maximum radius is greater than the hydrostatic pressure
as seen in Figure 13.
Furthermore, it is possible that the test setup plays a role in the slower gas
production. The sample is ignited in a cylindrical hole inside of an acrylic block. The gas
products are ejected into the surrounding environment as a jet. In other research (Cole,
1948; Menon & Mihir, 1998; Aochi et al., 2000) the sample charge was suspended in the
center of the tank and the gas expands radially outward. Limiting the gas to one direction
could slow the gas expansion allowing the oscillations to initiate while the gas is still
expanding thereby masking them.
An analogy for the difference between the deflagrations and other two
classifications of combustion in underwater reactions is the pendulum example suggested
34
Texas Tech University, Shawn C. Stacy, May 2008
in the introduction. An illustration of this analogy can be seen in Figure 21. The
detonation or explosion would be similar to releasing the weight of a pendulum from as
high as possible. The deflagration on the other hand, would be similar to having the
weight hanging freely and continually applying a force (wind for example) in one
direction. The oscillations for the explosion example would be very apparent, while in the
deflagration the oscillations would be reduced by the continued force.
Figure 21. Pendulum Analogy
4.2 Hydrophobic Teflon
The thin layer of air on the aluminum/Teflon shows that it is hydrophobic (little to
no water affinity) while aluminum/MoO3 is not. To quantify the water affinity of Teflon,
the sessile drop technique can be used (McHale et al., 2001; Blossey, 2003; Mao et al.,
2005; Van Der Wal & Steiner, 2007). A water drop is placed on the surface of the
material and the contact angle between the surface and the water drop is measured. The
maximum contact angle (180º) for a spherical drop will form for super-hydrophobic.
35
Texas Tech University, Shawn C. Stacy, May 2008
Conversely, for a super-hydrophilic material the drop would completely spread out with
no contact angle. The contact angle of smooth Teflon and water was reported as 110-118º
(Van Der Wal & Steiner, 2007).
Van Der Wal and Steiner (2007) have shown it is possible form a superhydrophobic surface made of Teflon. They sintered a thin layer of 200 nm Teflon
particles and micron scale polystyrene beads together on a glass surface. The polystyrene
was removed with chloroform leaving a porous Teflon matrix. Van Der Walm et al.
reported that their porous Teflon surface is more hydrophobic than a smooth Teflon
surface; they reported a maximum contact angle with the porous Teflon as approximately
171º. They also prepared a sample without polystyrene to have a solid surface that had a
contact angle of 125º. Having a rough, porous surface increases the contact angle for
Teflon, therefore a Teflon powder may also increase contact angle.
To understand the implication of the sessile drop results on underwater surfaces
the hydrophobic phenomena must be understood. For rough surfaces it could be assumed
that air is forming a thin layer on the surface of the material so the water only touches the
peaks of the Teflon. There are two main forces to be taken in to account, the surface
tension of the water and the water/solid interaction. The water/solid interaction can be
thought of as the surface friction or surface adhesion between the water and Teflon.
Having little contact with the solid means there is less surface area to provide friction to
the water. Teflon is also a low fiction surface further decreasing the water/solid
interaction, thereby ensuring the surface tension of the water the dominate force in the
36
Texas Tech University, Shawn C. Stacy, May 2008
droplet. The surface tension holds the water together to keep water from permeating and
air from escaping the material. Marmur (2006) used similar assumptions to these to
theoretically show that underwater surfaces can be super-hydrophobic if the surface was
rough enough. He used Gibbs surface energy analysis to show that the rougher surfaces
would keep water from touching less of the material therefore provide a more
hydrophobic surface.
4.3 Gas Generation
The results presented in Figure 10 and Figure 11 show the experiment is
repeatable and can be correlated with theoretical REAL code results. Gas production
REAL code results for aluminum/Teflon without and with water (50% of the mass) are
shown in Figure 22. The reactive mixture produces more gas with water than the without.
Also as the reactive material becomes more fuel rich the amount of gas produced
increases.
37
Texas Tech University, Shawn C. Stacy, May 2008
Product Volume, m^3/kg
8
7
6
5
4
3
2
1
With 50% Water
Without Water
0
0
1
2
3
4
5
6
Powder Equivalence Ratio, Φ
Figure 22. REAL code calculation for the Al/Teflon without and with 50% water. The equivalence
ratio is only calculated for the Al/Teflon powder.
To estimate the percentage of water reacting with the powder mixture the
stoichiometric reactions for aluminum/Teflon and aluminum/water are used (Equation (1)
and Equation (3), respectively). First the excess aluminum in the fuel rich
aluminum/Teflon mixtures was determined, and then the water necessary to react
stoichiometrically with that excess aluminum. The percentage of water for total reaction
mass was determined to be less than 10% for the samples in this experiment. 50% water
was chosen for REAL code calculations to give improved contrast between the volume
increase with and without water gas generation. These calculations are shown in
appendix
(
38
Texas Tech University, Shawn C. Stacy, May 2008
A.3 Calculations for Excess Water).
2 Al + 3H 2 O → Al 2 O3 + 3H 2
(4)
The excess aluminum in the fuel rich mixtures is likely reacting with water
producing hydrogen gas (among other gases), thereby increasing the volume of gas
produced from the reaction. To validate this hypothesis, the thermodynamic properties of
the gas products must be understood. The volume is a function of the moles of gas
produced, temperature, and the inverse of pressure. The ideal gas law can be used to
estimate these relationships.
To begin with, a possible reason for an increase in volume is an increase in
temperature. REAL code calculations (Figure 23) were done to show adiabatic flame
temperature for various mixtures of aluminum, Teflon, and water at constant pressure
(1atm). To help determine how to model the reaction the videos were studied. Based on
the video evidence, aluminum and Teflon began reacting inside the block and the reacting
mixture is forced upwards by the expanding gas. In the bubble of expanding gas, excess
aluminum fuel has a chance to react with water. Two extremes and a transition condition
were modeled to determine the flame temperature/equivalence ratio gradient. One of the
extremes was where the aluminum/Teflon mixture reacts alone without the influence of
water. Without the influence of water the flame temperature decays (2126 to 1315K) over
the range of equivalence ratios used in the experiments (φ=1, 1.5, and 2). For the second
extreme, the mixture reacts with 50% by mass o f water and increases (1088 to 1717K)
39
Texas Tech University, Shawn C. Stacy, May 2008
over the same range. Finally, the temperature was approximately constant over the range
of the experiments for powder mixed with 25% water. In the tank setup, aluminum and
Teflon can be treated as a premixed reaction and those products are able to react with the
water as a diffusion controlled reaction. The flame temperature decreases with increasing
equivalence ratio (Figure 23). Therefore, temperature is not increasing the volume of
Adiabatic Flame Temperature, K
reaction.
4000
3500
3000
2500
2000
1500
Al+Teflon (without water)
1000
Al+Teflon (with 25% water)
500
Al+Teflon (with 50% water)
0
0.5
1.5
2.5
3.5
4.5
Powder Equivalence Ratio, Φ
Figure 23. REAL code calculation of adiabatic flame temperature for different mass percents of
water
Secondly, pressure can account for the volume change. There was a large pressure
range for the fuel rich mixtures and the there are only two data points for the balanced
mixtures. From the results there was no pressure decrease, but it cannot be stated that
there was a pressure increase for fuel rich mixtures. The pressure was not decreasing,
40
Texas Tech University, Shawn C. Stacy, May 2008
therefore pressure cannot be a reason for the increased volume of the bubble for fuel rich
mixtures.
Finally, the excess gas for fuel rich mixtures may be hydrogen gas released when
the extra aluminum fuel reacts with surrounding water. Aochi et al. (2000) reacted
aluminum and potassium chlorate KClO3 mixtures underwater and collected the products.
Products were analyzed with gas chromatography. Figure 24 was produced using their
table of results. The total gas volume increases for the fuel rich mixture, while it remains
constant for fuel lean and balanced mixtures. Also, the amount of hydrogen gas produced
increases with aluminum content. In addition other gases can be produced such as CO,
CO2, and methane from the reaction with the carbon and liberated oxygen.
700
Total Gas
Hydrogen Gas
Volume x10^-6 m^3
600
500
400
300
200
100
0
0
0.5
1
1.5
2
2.5
Sample Equivalence Ratio, Φ
Figure 24. Results showing gas production increases with fuel rich mixtures and most of the gas
production increase is from hydrogen gas (Aochi, 2000).
41
Texas Tech University, Shawn C. Stacy, May 2008
The temperature, pressure, and gas results together show that the volume changes
are likely from excess gas production. The volume should be decrease with temperature
and stoichiometry. Also, the pressure was not decreasing, so it can not account for the
volume increase. Finally, the only property that can increase the volume is that there are
more moles of gas in the bubble.
Maximum radius was used to find the energy required to displace the water
around the bubble. This energy, shown in Equation (5), is based on the hydrostatic
pressure at the depth of reaction and volume of the bubble (Cole, 1948). The equation
assumes that the bubble pressure is less than the hydrostatic pressure at the maximum
radius. The bubble volume was approximated as spherical. To compare the energy
between different samples, the energy was divided by the sample mass. Determining
displacement energy has an advantage of being able to predict the size of the bubble at
different depths by solving for maximum radius at different hydrostatic pressures.
Edisplacement
3
4 πP
hydrostatic Rmax
3
=
M sample mass
(5)
42
Texas Tech University, Shawn C. Stacy, May 2008
Chapter V
Conclusions
The modified aquarium test provides a unique diagnostic technique to study the
hydrodynamics of reactive material deflagrations underwater. The hydrophobic and gas
generation properties of Teflon particles and the combustion properties of nano-scale
aluminum powder allow for the aluminum/Teflon mixture to be studied in this method.
Bubble radius and pressure were recorded and analyzed to show that gas
production increases with fuel rich mixtures. Excess aluminum for fuel rich reactions
reacts with the surrounding water and produces more gas. Product analysis has shown
that there is hydrogen produced in the reaction and the only hydrogen available is in
surrounding water. The results were validated with the REAL thermal equilibrium code
and results from other researchers. There was also a correlation between the percent
theoretical maximum density of the sample and the hydrodynamics of the bubble. The
gas production dynamics transition with increasing density from a quick single bubble
burn to a longer duration of small bubbles.
43
Texas Tech University, Shawn C. Stacy, May 2008
Chapter VI
Future Work
There are some improvements that could be made for this experimental setup.
First, having the pressure sensor outside of the bubble and having the bubble grow into
the sensor can cause problems. There is the effect of the water/gas interface on the
surface of the sensor and the pressure concentrations due to jet of products in the center
of the bubble both erroneously influence the pressure data. A better way to find the
pressure inside the bubble would be to have multiple pressure sensors outside of the
bubble. The data from the pressure sensors could be plotted on a pressure versus distance
plot. The data curve fitted and the bubble pressure at zero distance can be determined.
Future work would also include expanding the experiment. One change that could
be made is to find other materials to use in place of Teflon. The hydrophobic feasibility
of new materials could be determined by the sessile drop technique. Also it may be
interesting to reproduce bubble energy experiments by igniting hanging pellets from
within. Hanging pellets were tried initially for this work but the pellet would travel away
from the initial position. If the pellets were stationary the bubble would also be more
spherical. The spherical assumption would be more valid in that case than in this work.
Also by hanging the pellets it will eliminate the effect of the surface of the block on the
shape of the bubble.
44
Texas Tech University, Shawn C. Stacy, May 2008
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Cudzillo, S. & Trzcinski, W.A. (2001). Metal/Polytetrafluoroethylene Pyrolants, Polish
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Fischer, S.H. & Grubelich, M.C. (1998). Theoretical Energy Release of Thermites,
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Texas Tech University, Shawn C. Stacy, May 2008
Appendix A
Supplementary Results and Calculations
A.1 Supplemental REAL Results
C7H5N3O6
(TNT)
2Al+Fe2O3
2Al+MoO3
2Al+3H2O
4Al+3C2F4
0
200
400
600
800
kJ/mole
Figure 25. Heat of Combustion (kJ/mole)
48
1,000
1,200
Texas Tech University, Shawn C. Stacy, May 2008
C7H5N3O6
(TNT)
2Al+Fe2O3
2Al+MoO3
2Al+3H2O
4Al+3C2F4
0
5,000,000 10,000,000 15,000,000 20,000,000 25,000,000 30,000,000
kJ/m^3
Figure 26. Heat of Combustion (kJ/m3)
C7H5N3O6
(TNT)
2Al+Fe2O3
2Al+MoO3
2Al+3H2O
4Al+3C2F4
0
2,000
4,000
6,000
8,000
kJ/kg
Figure 27. Heat of Combustion (kJ/kg)
49
10,000
12,000
Texas Tech University, Shawn C. Stacy, May 2008
Table 6. REAL code results for Teflon and 80% pure aluminum
Φ
Heat of Comb., kJ/kg
0.5
5628
1
8856
1.5
8045
2
7404
2.5
6885
Gas Gen., %mass
88.56%
85.82%
86.14%
86.12%
87.35%
Temp, K
2583.3
3513.3
3473.5
3126.4
2383.8
Table 7. REAL code volume product results for Al + Teflon with a mass percentage of water
Φ No Water
0.5
2.56
1
3.77
1.5
3.98
2
3.97
2.5
3.41
3
2.78
4
2.40
5
2.07
Gas Generation, m^3/kg
50% Water 25% Water 10% Water
3.10
4.46
4.47
3.77
5.15
5.07
4.79
4.77
4.58
5.51
4.69
3.86
6.03
4.61
3.75
6.40
4.54
3.66
6.84
4.51
3.24
6.65
4.17
2.62
Table 8. REAL code adiabatic flame temperature results for Al + Teflon with a mass percentage of
water
Φ
0.5
1
1.5
2
2.5
3
4
5
Adiabatic Flame Temperature, K
No Water 50% Water 25% Water 10% Water
2583
941
1634
2441
3513
1088
2027
2749
3474
1442
1902
2429
3126
1717
1925
1952
2384
1931
1940
1953
1860
2099
1951
1956
1871
2326
2065
2013
1800
2327
2122
2028
50
Texas Tech University, Shawn C. Stacy, May 2008
A.2 Supplemental Experimental Results
Table 9. Experimental results for displacement energy, bubble growth rate, and pressurization rate
Initial Bubble
Displacement Avg Bubble Pressurization
Energy
Growth Rate Rate
Block#
kJ/kg
m/s
kPa/s
18
83.75
0.89
54.36
19
83.77
1.06
118.79
20
85.58
1.15
84.08
23
99.90
1.91
130.25
25
105.92
1.45
86.65
31
98.26
1.58
69.65
26
N/A
N/A
19.84
27
N/A
N/A
16.47
28
N/A
N/A
26.76
29
N/A
N/A
13.04
30
63.42
0.43
49.89
34
77.88
1.65
80.73
35
125.85
1.84
103.12
37
133.98
1.41
134.37
38
120.35
0.92
54.62
51
Texas Tech University, Shawn C. Stacy, May 2008
A.3 Calculations for Excess Water
Water required =
Alexcess
(A1)
F
 
 O  Balanced Al + H 2O
(A2)
Alexcess = AlΦ + AlΦ =1
Table 10. Percent of water required for comparison in REAL code results
Φ
1
1.5
2
2.5
Al excess Water Req.
0%
0%
25%
6%
50%
10%
75%
14%
52
Texas Tech University, Shawn C. Stacy, May 2008
A.4 Normalizing Radius Data
The radius data was normalized to reduce the influence of radius. To understand
the how the radius increases with increasing mass, maximum radius and sample mass
were plotted in Figure 28. For the most fuel rich (Φ=2) and the balanced (Φ=1) reactions
it can be seen that the radius increases with the sample mass.
24
Max Radius, mm
23
22
21
20
19
18
Φ=1
Φ=1.5
Φ=2
17
16
15
20
25
30
35
40
Sample Mass, mg
Figure 28. Data from Table 4 plotted to show the effect of sample mass
To reduce the effect of sample mass, the radius divided by the sample mass and
was plotted in Figure 29. The influence of the sample mass was overcompensated as the
largest sample radius (fuel rich sample) is now as low as the fuel balanced reactants.
53
Texas Tech University, Shawn C. Stacy, May 2008
Specific Radius, m/kg
800
750
700
650
600
Φ=1
Φ=1.5
Φ=2
550
500
20
25
30
35
40
Sample Mass, mg
Figure 29. First attempt normalizing radius data
By calculating the volume (assuming the bubble is spherical) and dividing
by the mass the radius can be normalized (Figure 30). There is still a slight trend for the
volume increasing with mass, but it is less than the experimental error of the data. The
volume was also multiplied by the molar weight to get the volume per moles (Figure 31).
54
Specific Volume, m^3/kg
Texas Tech University, Shawn C. Stacy, May 2008
1.4
1.2
1
0.8
0.6
0.4
Φ=1
Φ=1.5
Φ=2
0.2
0
20
25
30
35
40
Sample Mass, mg
Volume, m^3/mole
Figure 30. Second attempt normalizing radius data
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0.0004
Φ=1
Φ=1.5
Φ=2
0.00045 0.0005
0.00055 0.0006
0.00065
Moles of Reactant
Figure 31. Second attempt normalizing radius data in terms of volume per moles
55
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