MILLER THESIS

THE PENNSYLVANIA STATE UNIVERSITY
THE SCHREYER HONORS COLLEGE
DEPARTMENT OF MECHANICAL ENGINEERING
SPECIES EVOLUTION AND TEMPERATURE PROFILE OF HAN DROPLETS
DURING LASER ASSISTED PYROLYSIS
Approved:
JENNIFER JANE MILLER
Spring 1998
A thesis
submitted in partial fulfillment
of the requirements
for a baccalaureate degree with honors
in Mechanical Engineering
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i : ·. - i r i { / I - ...._ •' . ' ,t.,
I • {l 1l1 ,u ;! ' . i (_ I I ~ '( I
Thomas A. Litzinger J j
Thesis Supervisor ·
Date:
H~Date:
Honors Advisor
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ACKNOWLEDGMENTS
This project would not have been possible without the support of many
others. Primarily, I would like to thank Professor Tom Litzinger for his ceaseless
encouragement and countless contributions to my academic career. His efforts
include (but are certainly not limited to) my first exposure to thermodynamics,
undergraduate teaching internship, recommendation to and advising during the
NSF Summer Research Program, thesis advising, and career advice. Dr.
YoungJoo Lee and Mr. Gautam Kudva should know that I appreciate the many
hours they spent showing me around the laser lab and answering my questions;
their patience did not go unnoticed. Professor Joe Sommer has also provided
patient assistance, reassurance, and motivation through the past four, long
years.
I would also like to thank Dr. Francine Battaglia for contributing her
invaluable perspective. Thanks, Francine, for helping me to keep it together.
Last, but dearest to my heart, are my parents and family. Mom and Dad,
your unwavering support through every mistake, as well as the success, has
been priceless. You cannot possibly know how precious you are to me. I love
you.
ii

TABLE OF CONTENTS
ACKNOWLEDGMENTS.................................................................... ii
LIST OF FIGURES............................................................................ iv
LIST OF TABLES.............................................................................. v
ABSTRACT....................................................................................... vi
1. INTRODUCTION AND MOTIVATION............................................... 1
1.1 Introduction ....................................................................................... 1
1.2 Motivation.......................................................................................... 2
2. RELATED LIQUID GUN PROPELLANT AND HAN STUDIES......... 4
2.1 XM46................................................................................................. 4
2.2 HAN.................................................................................................. 7
2.3 Droplets............................................................................................. 9
2.4 Laser Ignition..................................................................................... 13
3. EXPERIMENTAL SET-UP AND PROCEDURE................................ 14
3.1 Experimental Setup........................................................................... 14
3.2 Species Measurement and Analysis ................................................. 17
3.3 Temperature Measurements............................................................. 18
4. RESULTS AND DISCUSSION.......................................................... 19
4.1 Droplet Explosion.............................................................................. 19
4.2 Q,ualitative Observations................................................................... 20
4.3 Temperature and Species Profiles.................................................... 21
4.4 Droplet Diameter............................................................................... 25
5. SUMMARY AND SUGGESTIONS FOR FUTURE WORK................ 28
6. REFERENCES.................................................................................. 30
iii

LIST OF FIGURES
Figure
1. REGENERATIVE LIQUID PROPELLANT GUN CONCEPT............. 2
2. STRAND BURNER SCHEMATIC..................................................... 6
3. C02 LASER LABORATORY SETUP................................................ 15
4. THERMOCOUPLE SETUP............................................................... 19
5. TEMPERATURE PROFILE AND SPECIES EVOLUTION................ 22
6. DROPLET DIAMETER and TEMPERATURE PROFILES
AS A FUNCTION OF TIME............................................................... 26
iv

LIST OF TABLES
Figure Page
1. PROPELLANT COMPOSITIONS...................................................... 11
v

ABSTRACT
Combustion characteristics and related chemical processes were
investigated for liquid hydroxylammonium nitrate (HAN) droplets. 500-800 µm
HAN droplets were suspended from Type K thermocouples in an argon
environment at 1 atmosphere and decomposed by a C02 laser at 50 W/cm2 . A
triple quadrupole mass spectrometer (TOMS) was applied for species
measurements in the gas phase above the droplets. Temperature of the droplet
experienced two plateaus at -140-175 °C and -270 °C during the evolution of
excess H20 and decomposition of HAN, respectively. Although there was no
visible flame observed during the HAN pyrolysis, a brown gas evolved and the
surface of the droplet pulsated. Species evolution of N02, N20, NO, N2, and H20,
during HAN decomposition was consistent with previous experimental results,
although HONO, 0 2, and HN03 were not detected. The coincidence of species
appearance with temperature is discussed. Also analyzed was the diameter of
the droplet during the latter HAN decomposition stage with respect to species
evolution and the d2-law.
vi

1. INTRODUCTION AND MOTIVATION
1.1. Introduction
In the field of liquid propellant guns and rocket propellants, HAN-based
propellants have attracted considerable attention due to their high energy-to-
mass content. There have been many recent studies of combustion
characteristics of HAN (hydroxylammonium nitrate) and HAN-based propellants,
specifically XM46*, for use by the U.S. Army with the next generation artillery
howitzer.1 Also, HAN-based monopropellants are being explored by NASA to
provide low cost, reliable, high-performance spacecraft propulsion while bearing
in mind environmental and safety concerns.2 Behaviors examined include
temperature and pressure profiles, ignitability, flame behavior, chemical reactivity
with other substances, environmental effects, and species evolution, to name a
few.
Since liquid gun propellant is injected into the gun (see Figure 1 on
following page for conceptual illustration), liquid propellant interest can be further
narrowed to spray combustion modeling. Droplet combustion behavior is
necessary to model spray combustion; relevant areas of study include
evaporation, ignition, and combustion characteristics.
•XM46 used to be commonly referred to as LGP 1846, LP 1846, LGP46, and
LP46 for "Liquid Gun Propellant" and "Liquid Propellant," respectively. The terms
are used interchangeably.
1

SEAL CONTROL
PISTON INJECTION
PISTON
IGNITER
COMBUSTION
CHAMBER
Figure 1: Regenerative Liquid Propellant Gun Concept
1.2. Motivation
PROJECTILE
The characteristics of XM46 have been researched heavily and likewise
documented. Unfortunately, for many of its applications this information is
inadequate. Droplet behavior is different from bulk quantity behavior as there is
a higher surface area to mass ratio; rarely is a liquid power source utilized as a
stationary bulk mass, which is the manner in which the propellants have so far
been researched.
Although there exists an established body of knowledge regarding droplet
combustion, including models, an additional challenge has been proposed by the
increased interest in lasers as ignition sources for gun propulsion systems.
2

Lasers have many potential advantages. First of all, lasers would provide
consistent, repeatable, simultaneous or programmed time sequence multi-point
ignition.3 Conventional electrode systems are not capable of such reliable,
flexible alternatives. However, the use of low vulnerability propellants, by their
inherent stability, demands more effective ignition. Also, it is believed that
sequenced ignition along the charge length may decrease the appearance of
pressure waves.3 Further, problems such as radio interference and risk of
misfire due to induction caused by strong electromagnetic fields are eliminated
by the use of lasers.4
Although laser parameters such as power, energy, heat flux, and
wavelength as they apply to liquid propellants have been explored previously,
studies regarding its application to droplets have been difficult, to say the least.
This is because it is difficult to obtain experimental information without interfering
with the reaction itself. Droplet shape must not be compromised during the
experiment; otherwise the observed characteristics cannot be attributed to
droplet combustion. Also, the minute amount of products are difficult to collect.
This study attempts to quantify reaction phenomenon in droplets not
captured simultaneously by previous investigations. Namely, temperature profile
inside the droplet and species evolution above the droplet are measured during
pyrolysis by a C02 laser.
3

2. RELATED LIQUID GUN PROPELLANT & HAN STUDIES
2.1. XM46
The majority of available information about XM46 is documented in the
"Liquid Propellant XM46 Handbook." 5 This handbook is very useful, in that it
concisely presents information taken directly from hundreds of industry and
government references. General properties and basic characteristic behavior
are cited.
XM46 is a homogeneous mixture of HAN, TEAN, and water. TEAN
(triethanol ammonium nitrate), (HOCH2CH2}3NH+NQ3-, is fuel-rich while HAN is
oxygen-rich. They react stoichiometrically to produce carbon dioxide, nitrogen,
and water as shown in Equation 1.
7 (NH30H+No3-) + (HOCH2CH2)3NH+No3- ~
8 N2 + 6 C02 + 22 H20 (Eq. 1)
XM46 is a favored propellant because of its stable nature; it does not
sustain combustion in atmospheric pressure. Storage and transport are thus
safer. However, HAN starts to decompose at about 100°C producing HN03,
among other products. The presence of nitric acid is unfavorable because its
presence lowers the propellant performance and thermal stability, the two
characteristics that make XM46 so attractive. But long-term storage (>20 years)
4

is believed to be achievable between 30 and 65°C although studies are still in
progress.
The most critical storage issue is XM46's reactivity with transition metal
ions such as copper, iron, and nickel. HAN reacts with the ions, lowering the
decomposition activation energy. For that reason, materials compatibility is an
issue for not only storage and processing, but also for research. Regardless of
material properties that are documented, research still must be done to ascertain
propellant behavior under specific conditions.
Closed chamber (constant volume) studies are done to assess pressure
fluctuations during combustion and occasionally to obtain temperature profiles.
In a study done by Klingenburg, Knapton and Watson three different HAN-based
propellants were ignited with an electrically heated nichrome wire that was
immersed in the liquid. 6 XM45 and XM46 were among the propellants tested.
Effects of pre-pressurization on required ignition energy and temperature
profiles, and contamination by 4 metallic ions were explored. The maximum
pressure could not be quantified as the chamber was designed with a "blowout"
feature at 100 MPa. Temperatures reached well above 2400K when blowout
occurred and pressure profiles were obtained. Nickel and aluminum decreased
ignition delay, but copper and iron did not affect decomposition in the closed
chamber.
Often, research of liquid as well as solid propellants is done in what is
called a strand burner. Figure 2 is the schematic of a strand burner used by
5

Vosen to study the characteristics of XM46. 7 Strand burners are helpful in that
they give the liquid the "strand" shape.
Ceramic - - - - •
Electrodes ____.
Top View
5mm-.j
Electrodes _.,___i---i
Propellant
Side View
30mm
Figure 2: Schematic of Strand Burner used by Vosen.7
6
Quartz
Quartz
40mm

Commonly used to obtain burning rates, strand burner tests are an
accepted standard among combustion experts. The experimental configuration
allows for convenient ignition and thermocouple placement options. Furthermore,
lateral quartz windows can be used for visual access to qualitatively study
propellant behavior during combustion, such as flame behavior and liquid-gas
interface.
A strand burner can be placed into a larget pressurized chamber so that
effects of pressure on combustion can be studied. Vosen reported that burning
rate, flame behavior, and energy release are heavily dependent upon pressure.
But the most striking of Vosen's conclusions is that XM46 combustion occurs in 2
steps. First is the liquid phase decomposition of HAN, followed by the
decomposition of molten TEAN droplets. Consequently, he concludes that the
HAN decomposition rate governs the overall decomposition of XM46.
2.2 HAN
In view of HAN's significant role in the combustion of XM46, it is likewise
of interest to study the decomposition of HAN. Once HAN behavior is
understood, we can further understand the synergy between HAN and TEAN in
XM46 decomposition. Therefore, most subsequent studies involve both HAN
and XM46.
t Pressure chamber size is large relative to sample so that pressure changes due
to sample deflagration do not significantly alter environmental pressure.
7

Although HAN can exist as a solid, it is most commonly found in an
aqueous solution. An overall chemical process for HAN decomposition is written
as:
NH3QHN03 + <j>H20 ~
aHN03 + (<j>+b) H20 + c N02 + d N2 + eN20 + /NO (Eq. 2)
where (a+2b)=4, 0 ~a ~ 2, 3a + b ~ 4, and 8/5 ~ b ~ 2.8
Cronin and Brill used metallic ribbon filaments in their research of HAN. 9
A type E thermocouple was spot welded to the filament upon which the samples
were measured from a calibrated microsyringe. They used rapid-scan Fourier
Transform Infrared spectroscopy to characterize product concentrations evolved
from the samples. Unfortunately, the infrared inactive products 0 2 and N2 could
not be quantified. Also, steam (H20) was not quantified because of difficulties in
calibration. The exclusion of N2 and H20, in particular, is unfortunate, as they
are possible products in the HAN reactions. Cronin and Brill conducted their
experiments at pressures ranging from 15 psi to 1000 psi in an Argon
environment with heating rates varying from 50 to 400 °cs·1•
In summary of their results, they believe the formation of N20 and N02
following HN03 indicates an early step as the proton transfer from NH30H+ to
N03-. This is believed to be an endothermic reaction. Further, water plays a
major role in the decomposition of HAN at this point in the reaction as altering
8

the amount affects product concentration oscillation during this phase. Possibly,
the initial vaporization of water cools the reactants. A second, exothermic stage
begins at about 180°C with a sharp decrease of HN03.
Cronin and Brill's research is principally noteworthy because they have
reported the HAN decomposition mechanism.
2.3 Droplets
Droplet behavior is difficult to research for many reasons, but primarily
due to the literally intangible nature of a pure droplet. Any change in surface
tension can significantly alter the droplet behavior. It is also believed that
droplets undergo subprocesses such as heat-up, vaporization, ignition, burning,
and extinction. 10 Designing an experimental setup and controlling the
environment to provide the proper atmosphere in which to study each process or
series thereof are formidable tasks.
Countless approaches to droplet maintenance, ignition, and data
collection have been attempted. Droplets can be studied in a microgravity
chamber, as freely falling articles, or suspended. Suspension can be achieved
from the end of a wire, on a wire loop, or across a horizontal wire. Droplet
combustion can be achieved by environmental means such as hot air flows,
electrically heating the suspension wire, or by direct ignition from a laser beam.
Each method has its own drawbacks and data collection is difficult in any case
due to the small size.
9

While microgravity chambers allow experimenters a few hundred extra
precious milliseconds during which to observe the falling droplet by decreasing
downward acceleration, relative velocity between the droplet and the ambient
atmosphere is reduced. Relative velocity affects droplet behavior by creating a
drag force that elongates the droplet and by introducing boundary layer effects.
While the spherical shape is desired to determine an ideal droplet combustion
model, droplets are not perfectly spherical in realistic conditions. Freely falling
droplets are closer to actual circumstances, but phenomenon measurement is
difficult.
Droplet suspension makes possible temperature and species evolution
measurement. Droplets have even been suspended in immiscible liquids. 11
Again, each of these methods provides different advantages but the approach
should be tailored to the application of the researched subject. For example,
HAN droplet decomposition temperatures are recorded anywhere from 120 to
230° C for different experimental techniques. 12 It follows that droplet
characteristics must be considered with the method in which they were
determined.
Beyer studied pressure effects on freely falling HAN, LGP 1845*, and LGP
1846 droplets.12 He closely controlled droplet size by creating them in a
piezoelectric ceramic cylinder with a small orifice. Size was regulated by the
magnitude of voltage pulse used to expel the droplet. Hot gases were provided
t LGP 1845 and LGP 1846 are similar in composition and are, therefore, often
studied together. See Table 1 for a comparison of their makeup.
10

by a methane with air or nitrous oxide flame and injected into a separate
chamber through which the droplets fell. A strobe light backlit the droplets which
were then photographed. Measurements of droplet diameter versus time were
made and fitted to the d2-law§. Also, the phenomena of droplet explosions were
observed and discussed.
Table 1: Composition of propellants7.
Density
Composition Concentration
Propellant (wt.%) (kmo/elm3)
(kglm3)
HAN TEAN H20 HAN TEAN H20
XM45 (LGP45) 1455 63.2 20.0 16.8 9.57 1.36 13.57
XM46 (LGP46) 1437 60.8 19.2 20.0 9.1 1.30 15.95
Beyer continued his studies with LP 1846 droplets, only this time
suspending droplets from a fused silica fiber while a thermocouple captured
temperature measurements 1 mm above the droplet. 13 A solid propellant was
electrically ignited by a nichrome filament creating a moderately turbulent hot air
flow over the droplet. This ignition type is of interest because it more closely
simulates gas composition in a gun environment. No attempt was made to
characterize temperature inside droplet.
§The "d2-law" is a classical model for droplet combustion stating that droplet
diameter decreases linearly with time. This law, developed with many simplifying
assumptions, also states that a droplet's liquid phase temperature during
combustion is uniformly at its boiling point.
11

Beyer attempted to obtain the apparent droplet diameter by computer
edge fitting of the droplet with a circle, but his results are inconclusive. Although
drop diameter did oscillate, there was no internal bubble formation or explosion.
Because of the burning solid propellant, it was unclear whether the droplet
ignited; he was unable to describe even qualitatively the actual physical events
on the droplet surface during decomposition as either evaporation or ablation.
One major conclusion made by Beyer was the difficulty of capturing and
quantifying droplet data.
Zhu and Law studied freely falling LP 1845 droplets in much the same
manner as Beyer. 14 Motivation for the study is that liquid temperature affects
initiation and intensity of combustion. The droplets were ignited in a hot
postcombustion zone of a flat.flame, droplet image was frozen by stroboscopic
backlighting, and captured by a video camera. Temperature of the flow was
known, but not exact droplet temperature. Explosions were observed and
discussed, as well as gasification rates derived from fitting the results to the d2-
law. Zhu and Law concluded that gasification rates increased with ambient
temperature and the resulting increase in liquid-phase exothermic reaction rate.
A large majority of droplet knowledge is with regard to droplet size,
ambient temperature, and ambient pressure as droplet parameters such as
temperature are hard to measure. No research including species evolution of
decomposing droplets was found. Moreover, the general impression to be taken
from droplet combustion research is the difficulty of handling the small, fragile
droplets and quantifying reaction phenomena.
12

2.4 Laser Ignition
Laser-assisted combustion casts a new dimension to propellant research.
In addition to the added benefits of laser ignition in real-world applications
enumerated in Section 1, laser ignition widens research opportunities. For
example, XM46 will not ignite at atmospheric pressure, but laser assistance
allows ignition and sustained combustion. Vast quantities of literature exist
regarding laser-assisted ignition and combustion, but of particular interest is that
with respect to XM46, HAN, and droplets.
Considerable research of laser assisted combustion of XM46 and its
components has previously been done in this facility. Lee and Litzinger have
documented the behavior of XM46 and its ingredients, HAN and TEAN, under
high heating rate conditions by a C02 laser.15 Flame behavior, temperature
profiles, and species measurements were obtained for 0.2-0.3 cm3 samples
which were held in a small glass container. For HAN, H20 was initially observed
with very small amounts of N20, NO, N02, and HONO. A second stage of
decomposition consists of evolution of significant amounts of H20, N02, N2, 0 2,
NO, and N20. The first stage is believed to be primarily due to the evaporation
of water from HAN solution, followed by the characteristically evolving HAN
species. Only vaporized water and brown gas were apparent during low heat
flux (100 W/cm2) while a white HAN flame appeared during decomposition using
400 WI cm2•
13

3. EXPERIMENTAL SET-UP AND PROCEDURE
3.1 Experimental Setup
The schematic diagram of the experimental setup is shown in Figure 3.
The C02 laser beam was aligned with a helium-neon laser beam; the latter red
beam allowed for proper aiming of the invisible C02 laser beam. Before entering
the test chamber, the beam was deflected from the laser by a silicon mirror. The
beam then passed through a zinc selenide expanding lens. The expanding lens
could be moved vertically on a track (not shown in figure) to control the beam
area at the sample location and thus obtain various levels of incident heat flux.
The laser beam entered the aluminum test chamber pressurized with argon
through a KCI window in the top of the chamber. The test chamber is built with
two fittings: one provides argon from a pressurized bottle while the other
provides gas flow out induced by a vacuum pump. Also, fittings purchased from
CONAX Buffalo Co. specially designed for sealing signal-transmitting wires, carry
the thermocouple signals from the chamber.
The C02 laser used was changed from one with 800 W of power to 200
W, both in continuous mode, due to renovation of experimental facilities.
However, no significant change of results was found in our experiments. Our
experiments were performed at a fixed heat flux of-50 W/cm2•
14

Mirror
11
11
II
II
II
I
I
C02 Laser
Plexiglass
Window
n KCL Window
Macro Lens
PULNIX
Camera
VCR
Sample
(Refer to Figure
4 for detail)
Thermocouple
Leads
II III
11
I I
I I
I I
I I
Pre-Amplifier
Nicolet
Oscilloscope
Test Chamber
Microprobe
Extrel Triple
Quadrupole Mass
Spectrometer
(TQMS)
Mass Spectrometer
Electronic Control
Unit
Hewlett Packard
Universal Source
DT 2823
D/A-A/D Board
486
Personal Computer
Figure 3: Schematic diagram of the experimental setup for the species and
temperature measurements of HAN droplets.
15

A droplet of 13 M (13 molar) HAN was suspended at the junction point of
a thermocouple connected to flexible copper leads. These copper leads were
easily adjustable for optimum placement of the droplet with respect to the mass
spectrometer microprobe. Droplets varied in size from 500-800 µm in diameter
and the distance between the droplets and the microprobe was on the order of 1..
2 droplet diameters.
The quartz microprobes were shaped in our lab from quartz tubing to have
25 µm orifices and a -45° from the horizontal bend at the end. This shape has
been designed, tested, and perfected specifically for use in this lab for previous
experimentation. The tubing, from Quartz Scientific, Inc., is placed in a lathe
operating a low rpm, heated by a torch, and pulled to create a narrow neck. The
tubing is then cut at the neck and the remaining tip ground to obtain the desired
orifice size. The microprobe captures species profiles of the gaseous products
above or next to the reacting HAN droplet with a sampling rate of 200 amu/sec.
Data acquisition software on 486 personal computer allows for continuous
sweeps over a range of molecular weights or for monitoring of up to 12 specific
weights. The mass spectrometer is a C-50 triple quadrupole mass spectrometer
(TQMS) from Extrel. The mass spectrometer is capable of ranges from 1-500
amu and therefore can capture all anticipated species. The gaseous species
collected by the microprobe are drawn into the first pole where they are ionized
by electron impact. They then proceed to the detection system from which the
signals are amplified for collection by the software. There are three vacuum
16

pumps providing vacuum to the TQMS: a Leybold pump for the primary chamber
and two turbo-molecular pumps from Balzers for the main TQMS chamber.
A high-quality Plexiglas window installed in one side of the test chamber
allowed direct image photography during the droplet combustion by a Pulnix
video camera with Nikon macro lens. The camera recorded the images at 35-40
times magnification with a spatial resolution of about 100 µm. Droplet sizes were
determined during frame-to-frame viewing of the video recordings.
3.2 Species Measurement and Analysis
Based on previous publications and liquid propellant studies done in this
lab, initial assumptions were made concerning the products of decomposing
HAN. From these assumptions we chose specific individual molecular weights
for which to test. For HAN it was found that there was only one possible
gaseous species for each given mass, therefore further differentiation of each
mass was not performed.
Sensitivity coefficients of measured species are acquired by calibration of
the TQMS with gases of know concentration. The coefficient is determined by
relating the signal intensity to the known concentration. Water was heated and
vaporized by the C02 laser to obtain the sensitivity coefficient for its vapor state.
An ionization energy of 22 eV is needed to obtain acceptable intensities;
however, it contributes to fragmentation of some species. As a result, some
species contribute to signals for lower masses. For example, N02 fragments and
increases the signal for masses 44 and 30. Such fragmenting is quantified
17

during calibration and the fragmentation signals are subtracted from the total
intensities at those masses.
3.3 Temperature Measurements
Temperatures of the droplets were measured using chromel-alumel
thermocouples manufactured by Omega Engineering Co. 50 and 75 µm wires
were used in this study, with the droplet suspended at the junction point of a
horizontal wire pictured on following page in Figure 4. The thermocouple was
held in place by copper alligator clips attached to flexible copper leads. The
copper leads were connected by banana adapters to leads running into a
preamplifier. Voltage signals were displayed and recorded either on a Nicolet
Oscilloscope or LabVIEW software.
Initial attempts to suspend the droplets from platinum-rhodium
thermocouples, fabricated in our laboratory to have very small junction points,
were unsuccessful; the droplets simply failed to remain on the wires. The
droplets did, however, hang on the Omega Co. 25 µm thermocouples, probably
due to the increased junction point surface area. With limited success
suspending the droplets from 25 µm thermocouples, small droplets yielded a
very weak species signals from the mass spectrometer. The results discussed in
the next section are, therefore, from 50 and 75 µm Type K thermocouples.
18

copper
alligator clips
rdroplet on
thermocouple
junction point
Figure 4: Droplet Suspension Setup
4. RESULTS AND DISCUSSION
4.1. Droplet Explosion
Banana
Adapters
Pre-Amplifier
Nicolet
Oscilloscope
During initial experiments, when a suitable heat flux had not yet been
determined, droplets appeared to briefly boil violently before exploding or falling
from the thermocouple. Since the scope of our study did not include explosion
mechanisms, the occurrence of such intense droplet explosions was a nuisance.
Previously discussed by Beyer,12 mechanisms for droplet explosions are
not yet fully understood. There are three possible causes. One possibility is that
the liquid could be transforming into a gas phase with subsequent pressure
19

buildup in the interior of the droplet. This gas bubble balloons the droplet until it
either explodes or simply falls off the wire. Another possibility is that, in
multicomponent drops, a concentration gradient of components appears in the
drop as the most volatile components evaporate off the surface rapidly. The
liquid diffusion rates of the less volatile components are slower than the
evaporation rate of more volatile component. The slower component nucleates
in the center where it becomes superheated, possibly on the thermocouple itself.
A third possibility is that chemical reactions inside of the droplet produce
gaseous products.
HAN does not "ignite," rather it decomposes into several gaseous
products. As H20 is the major constituent and earliest product, water vapor
could be nucleating inside the droplet as suggested by the second possibility.
Also, recorded images reveal formation of internal gas bubbles that swell and
either move to the surface of the droplet or burst, as previously discussed by Call
et al.14 Lowering the laser beam heat flux reduced this event; however, it is still
unwise to conclude which of the aforementioned mechanisms presented itself in
this study. Although it did not eliminated microexplosions altogether, a heat flux
of 50 W/cm2 provided a sufficient decomposition rate without expelling the
droplet from the thermocouple.
4.2. Qualitative Observations
There was no visible flame at the selected heat flux of 50 W/cm2• This
lack of flame makes it difficult to distinguish between vaporization and actual
burning which does occur for HAN at higher heating rates.15 However, a faint
20

vapor could be observed during the beginning of some videos, which evolved
into a brownish vapor which appeared on all videos. The start of evolution of the
brown vapor was very gradual so its beginning could not be determined.
During the first half of the droplet lifetime, the surface pulsated. The
droplet would swell, either due to liquid expansion or internal gasification,
followed by an abrupt decrease in diameter. This cycle was repeated several
times, consistent with the microexplosion mechanism discussed by Call et al. 14
The entire droplet would expand with a uniform increase in dimension, followed
by a deformation as the internal bubble moves to the surface and ruptures the
droplet. At low heat fluxes our droplet remained on the thermocouple throughout
these events, rather than fragmenting into smaller droplets, as in Gall's study.
Occasionally a discernible bubble appeared in the center. However, due to
inadequate video recording resolution and lack of backlighting, such events were
not distinctly visible.
. During the last -.5 seconds of droplet lifetime the microexplosion
phenomenon was not observed. It is possible that the size fluctuations existed
but were small relative to the video resolution, and therefore undiscernible. The
droplet continued an overall decrease in diameter until it finally disappeared.
4.3. Temperature and Species Profiles
Temperature and species profiles obtained had consistent and repeatable
trends, although exact values varied slightly from test to test. Species and
temperature profiles from a typical experiment are shown in Figure 5. Over the
lifetime of the droplet, 4 different stages are observed.
21

1400 I I
~50 >--Stage : I
• ... •....__ - -12'
I300 f - - <D ;~ -- I
I
250
I
200 I
150
1ult...~~
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1100 /
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50 v I
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0.0 0.2 0.4 0.6
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1.4
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;. r'~ ... -
I
I
'I
I
0.2 0.4 0.6
t
ti ••
0.8
. ~
. ,,.
'
0.8
i:
3' J
~ .I·-
ir.~
./'
1.0
~
1.0
:remp •c
1(£
-··
1
I
I
1.2 1.4 1.6
Time [sec]
!
1--%H20
I
: I
I I
' .-:- .. -.........I
.
I
I
I
1.2
~ . i~..,,.
1.4 1.6
Time[sec]
1--%NOI
1..,._%N2 I
.
I ~
'I
I
1.8 2.0 .2
i
I
- ... .
tl''k# L
"'"'' . -.. ..~Jl1
1.8 2.0 2.2
~
t ., I
I o.3
I 0.2
10.1
. 0 ~! J.• I •"I A. •... A.~ ,_ - . ........ ""'M :l.l::l.J Jt.ji
"' IUI
i.Aljft 4 11111
.
0.0 0.2 0.4 0.6 0.8
I0.:
I 0.6 I
1.0 1.2 1.4 1.6
Time[sec]
l--%N02
1---%N20
1.8 2.0 2.2
•
I
I
.l
!
'
I
I
i
I
2.4 2.6 2.8
I
I
I I
I
I I
t i I
1• ~ • +I rl. 1
I ~ f1. 'i 'f,, If~ ' II
• -2.4 2.6 2.8
" ~
II Tl I
TINll II•
2.4 2.6 2.8
I
1
I
I
iI
I
3.0
I
i
i
'
i
:
I• I
. I
I
I
3.o I
i
I
I 0.4
•.ft _]MIt rt ~ I ••
I
0.2
i 0
I
0.0
~
i;111••.11t !
••
0.2 0.4
- A ..
0.6 . 0.8 1.0 1.2
. . •--
~7i ''
......,1 n
~·~ m1r
1.4 1.6 1.8 2.0 2.2
Time(sec)
.,........--------------! ELEMENT BALANCE
0.8
0.6
0.4
0.2
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Time [sec]
I
u
.,
2.4 2.6 2.8
2.4 2.6 2.8
Figure 5: Representative Temperature Profile and Species Evolution of HAN droplet
decomposed by C02 laser at 50 W/cm2 and 1 atm.
22
3.0 I
3.0

lower H20 content during lasing would compound this phenomenon.
Unfortunately, no conclusions could be drawn regarding the quantity of H20
evaporated prior to lasing because droplet size was not monitored during droplet
creation. Instead, size was determined from the video image mere seconds prior
to lasing; up to 10 minutes lapsed between droplet creation and the start of any
given experiment.
After the first plateau, the temperature again sloped up for -.2 seconds,
for a third characteristic stage, to a level plateau at 260-275°C where it remained
for the remaining lifetime of the droplet. There was also no correlation between
thermocouple size and temperatures at either plateau.
Coinciding with the temperature increase of stage 3 at -0.8 seconds, the
mole fractions start to include other species expected for HAN. After the
temperature reaches the second plateau at -1 second, the element balance
quickly converges to that expected for HAN: .4 for hydrogen and oxygen and .2
for nitrogen, indicating that the actual HAN decomposition occurs during this
second temperature plateau. Dominant species consist of N02, N20, NO, and
N2• While previous experiments in this lab reveal 0 2 and possibly HONO as HAN
decomposition products, this study did not conclusively indicate those products.
In spite of our good atom balance without 0 2 and HONO, it is possible that
these products existed in amounts below a detectable limit. Furthermore, during
calibration it was determined that TQMS is insensitive to HN03 which is known to
be a product of HAN decomposition.9
24

During the first stage, the temperature inside the illustrative droplet
increased until -.3 sec where it leveled off between 140-150°C. During this
temperature rise, H20 mole fraction increased from zero to one. Other high,
erratic mole fractions indicated at the very beginning of this stage are erroneous.
Although in reality these are residual gases existing in the chamber whose
amounts are relatively small compared with Argon present, they are represented
as a percentage of expected species detected by the TQMS. Because no real
products have yet evolved, these residual gases comprise the entire mole
percentage.
A plateau with a minimal slope characterizes the second stage. Starting
at 140-150°C, and for -0.5 seconds following, the temperature gradually
increased to 150-175°C.
Species evolution indicates that this first plateau corresponds mainly to
evaporation of excess H20 from the HAN solution; H20 is the primary species
with a mole fraction almost equal to 1. Although other species are visible at this
stage, their relatively low mole fractions suggest that little of the water indicated
at this stage is related to HAN decomposition. Rather, the water is from the
solvent in the13 M HAN solution.
The test-to-test variance in exact temperature on this first plateau could
be due to the evaporation of H20 from the droplet prior to lasing; Beyer warned
of the difficulties of such evaporation due to the high surface area to volume ratio
of droplets.16 We would expect that a higher temperature to indicate a higher
HAN content in the solution. Further, H20 cools the sample during reaction. A
23

Toward the end of the second plateau and just as the element balance
reflects pure HAN, the droplet extinguishes and the temperature profile spikes
and settles to a constant temperature due to the heat flux from the laser.
4.5 Droplet Diameter
Measurements were taken of droplet diameter as a function of time and a
representative profile is shown in Figure 6. It is impossible to determine from the
video the exact time of lasing initiation. However, temperature data collection by
the Nicolet oscilloscope is trigger simultaneously with the laser. Therefore, the
event initiation on video is interpolated by comparing droplet lifetime from
temperature profiles with droplet disappearance in the recording.
After a short, relatively stable period the droplet diameter begins to vary.
The surface eventually boils erratically, swelling to as much as twice the initial
diameter. The square of the diameter as a function of time finally settles into a
trend more closely resembling the linear behavior of a droplet adhering to the d2-
law.
The stable period at the beginning is probably due to a heat-up phase.
During this period, although the droplet diameter does vary, this initial
fluctuations are minor with respect to the violent boiling concurrent with the first
half of the second temperature plateau. Since the temperature increase is
simultaneous with the onset of violent pulsation, it is possible that the water
diffusion rate did not keep pace with water vaporization. Instead, the water
vapor nucleated inside of the droplet. But since actual HAN decomposition also
25

occurred at this point, the irregular droplet behavior might also be due to HAN
reactions.
0 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
Time [sec]
;: 0. 02 -;--+---t-----l---+---+----t--t-r--t-t----k---+-t----t----+---+---~------1
><
E o.o1 5 -1----+---l-----1----1---~""'+"""----l-+1-l----1.-=-~-+--~'"""---1---+-------l
E
:: 0.01 +--+---t----t------.....r-+----+---+----t---i.r+---+--i...-........--+----l
i..
~ 0.005 +--+---t----1---+--+--+----+---t----l---+---+---'~+...---I
E
~ 0 -!----i---+--+---+---+----+---+---+---+--+--1----t--+-I
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
Time [sec]
Figure 6: Typical HAN droplet diameter variation as a function of time. (Shown
with temperature profile of same droplet for discussion purposes.)
One major precept of the d2-law is that temperature of the condensed
phase is a constant during the linear behavior. Although temperature stabilizes
at two separate values during the HAN droplet decomposition, it would be
difficult to resolve the law with respect to the first half of the decomposition
26

period, that of water, due to the erratic size fluctuations. But when we can be
relatively certain that HAN decomposition is occurring, the fluctuations are
smaller. Since the temperature at this point is a constant and all species
evolution data suggest actual HAN decomposition, it may be suitable to apply
the d2-law to droplet behavior over this range.
Unfortunately, the extent to which the thermocouple interferes with the
droplet behavior is unknown. Although the droplet maintains a spherical shape
in almost every frame, occasionally the droplet will assume an elliptical shape-
appearing to cling to the thermocouple at the end of its lifetime. The effect of the
thermocouple could render droplet diameter measurements in this study
erroneous. Further, inadequate resolution on the video recordings makes it
impossible to determine the c9ndensed phase boundary.
Gasification rates for HAN droplets at high pressure have previously been
recorded.12 However, these results do not reveal at what specific point in the
droplet lifetime the measurements were taken. Although Zhu and Law document
gasification rates for XM46,11 our new understanding of separate temperature
constants raise the question whether it is appropriate to fit a constant gasification
rates for the entire droplet lifetime. Perhaps two or more gasification constants
are possible for separate stages of decomposition.
In light of our understanding of the droplet decomposition process, we can
more accurately approach application of the d2-law in later studies.
27

.....
5. SUMMARY AND SUGGESTIONS FOR FUTURE WORK
5.1. SUMMARY
An experimental methodology was designed to suspend HAN droplets so
as to obtain condensed phase temperature and species evolution during
decomposition by C02 laser beam. HAN droplets were suspended from Type K
thermocouples and pyrolyzed by a C02 laser at 50 W/cm2. Temperature profiles
of the droplets were obtained, with two distinct plateaus. The first plateau at
temperatures between 140-175°C coincided with the evaporation of water from
the HAN solution. A second plateau, consistently occurring at -260-275°C
corresponded to the decomposition of HAN.
Species detected during HAN decomposition include N02, N20, N2, NO,
and H20, as expected. However, the possibility exists that HN03, HONO, and 0 2
also occurred at undetectable amounts.
After the majority of the water was evaporated, the square of the droplet
diameter settled into a linear decomposition rate.
5.2. . FUTURE WORK
The natural progression continues this work into droplet experimentation
with XM46 and TEAN solutions. Since it is believed that HAN decomposition
controls the XM46 reaction, comparisons could be made among the temperature
profiles and species evolution of all three samples.
Later work should more carefully ·consider droplet creation in an effort to
control the size, possibly utilizing a piezoelectric droplet maker to this end.
Unfortunately, the difficulty comes not in creating the droplet, but in persuading
28

the entire droplet to hang from a thermocouple. With such minute droplets, even
the smallest moisture clinging to a needle could create inaccuracies in metered
sample size. If droplet size at conception is known, then the amount of H20
evaporated prior to lasing could be determined.
If temperature and species evolution profiles are known throughout
droplet lifetime, the d2-law could be applied to droplet decomposition over
appropriate ranges of the droplet lifetime, namely constant temperature ranges.
Also for this research video recording quality would need to be improved to
provide higher resolution for the diameter measurement. Backlighting would
make internal gasification easier to observe. For future study we have installed
in this lab an LED which lights simultaneous with laser initiation so that exact
timing details of event are known. In addition it may be helpful to utilize a
computer program to edge-fit the droplet diameter to a circle, as in Bayer's
studies.13
It may be insightful to compare accurate droplet diameter results with
similar experiments utilizing convective, rather than radiative, heating. This
would determine if internal vaporization were greater due to the radiative heating
of the inside of the droplet, which would not occur in a convective heating
situation.
29

REFERENCES
1. W. F. Oberle, and G. P. Wren, "Burn Rates of LGP 1846 Conditioned
Ambient, Hot, and Cold," Technical Report BRL-TR-3287, U.S. Army Ballistic
Research Laboratory, Aberdeen Proving Ground, MD, October 1991.
2. R. S. Jankovsky, "HAN-Based Monopropellant Assessment for Spacecraft,"
Technical Report AIAA 96-2863, American Institute of Aeronautics and
Astronautics, NASA Lewis Research Center, Cleveland, OH, July 1996
3. R. Beyer, L.-M. Chang, and B.E. Forch, "Laser Ignition of Propellants in
Closed Chambers," Technical Report ARL-TR-1055, U.S. Army Research
Laboratory, Aberdeen Proving Ground, MD, April 1996.
4. F.B. Carleton, N. Klein, K. Krallis, and F.J. Weinberg, "Initiating Reaction in
Liquid Propellants by Focused Laser Beams," Combustion Science and
Technology, Vol. 88, 1992, pp 33-41.
5. Dowler, Warren L., Ferraro, Ned W., "Liquid Propellant XM46 Handbook," Jet
Propulsion Laboratory, California Institute of Technology, Pasadena, CA, July
29, 1994.
6. G. Klingenberg, J. D. Knapton, and C. Watson, "Investigation of the
Combustion of Liquid Gun Propellants in Closed Chambers," Propellants,
Explosives, Pyrotechnics, Vol. 12, 1987, pp 133-136.
7. S. R. Vosen, "The Burning Rate of HAN-Based Liquid Propellants," Technical
Report SAND88-8600, Combustion Research Facility, Sandia National
Laboratories, Livermore, CA, February 1988.
8. B. D. Shaw and F. A. Williams, "A Model for the Deflagration of Aqueous
Solutions of Hydroxylammonium Nitrate," 24th Symposium (International) on
Combustion, The Combustion Institute, 1992, pp1923-1930.
9. J. T. Cronin and T. B. Brill, "Thermal Decomposition of Energetic Materials
29-The Fast Thermal Decomposition Characteristics of a Multicomponent
Material: Liquid Gun Propellant 1845, "Combustion and Flame, Vol. 74,
1988, pp 81-99.
10.T. L. Jiang and W. Hsu, "Comparison of Droplet Combustion Models in Spray
Combustion," Journal of Propulsion and Power, Vol. 9, 1993 pp 644-646.
30

11. D. L. Zhu, C. K. Law, "Aerothermochemical Studies of Energetic Liquid
Materials: 1. Combustion of HAN-Based Liquid Gun Propellants Under
Atmospheric Pressure," Combustion and Flame, Vol. 70, 1987, pp 333-342.
12. RA. Beyer, "Atmospheric Pressure Studies of Liquid Propellant Drops in Hot
Flows," Technical Report BFL-TR-3038, U.S. Army Ballistic Research
Laboratory, Aberdeen Proving Ground, MD, April 1996.
13. R. A. Beyer, "Continuing Studies of Liquid Propellant Drops in Hot, High-
Pressure Environments," Technical Report BFL-TR-3038, U.S. Army Ballistic
Research Laboratory, Aberdeen Proving Ground, MD, September 1989.
14. C. Call, D. L. Zhu, C. K. Law, and S. C. Deevi, "Combustion and
Microexplosion of HAN-Based Liquid Gun Propellants at Elevated
Pressures," Journal of Propulsion, Vol. 13, 1997, pp 448-450.
15.T. A. Litzinger, Y. J. Lee, "Combustion Chemistry of HAN, TEAN, and XM46,"
33rd JANNAF Combustion Meeting, CPIA Pub. 653, Vol. 1, 1996, pp. 171-
183.
16. R. A. Beyer, private communication with author, June 1997.
31

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