UNIVERSITY OF CINCINNATI
Date:______
I, ______, hereby submit this work as part of the requirements for the degree of: in:
It is entitled:
This work and its defense approved by:
Chair: ______
Heat Transfer Studies of a Pyrotechnic Event and its Effect on Fuel Pool Ignition
A thesis submitted to the
Division of Research and Advanced Studies
of the University of Cincinnati
in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE
in the Department of Mechanical, Industrial and Nuclear Engineering
of the College of Engineering
2005
by
Ravi Prasad
B.E., Marine Engineering and Research Institute, Kolkata, India
Committee Chair: Dr. Peter J. Disimile
ABSTRACT
The local temperature field associated with a pyrotechnic event has numerous implications, particularly in the area of aircraft survivability. This study determined the temperature distribution within the vicinity of the fireball of a pyrotechnic event. A low cost simulation
methodology has been developed to provide further understanding of this type of event from data
involving both the geometric and thermal state of a generated incendiary cloud. The study
discusses the temperature measurement methodology, the temperature distribution within and outside a fireball volume. The study also provides empirical evidence that the measured
temperature, over the short duration of the pyrotechnic event, cannot accurately be determined
from the size and radiant emission of the light emitted. The second phase of the study examines
the ignition of a fuel pool by the pyrotechnic event. Ignition height for various fuel pool
temperatures has been determined. These studies showed that the size and radiation emitted by
the incendiary cloud, above the fuel pool, does not correlate with the height at which the fuel
pool ignites.
ACKNOWLEDGEMENTS
I would like to thank the funding support from the Department of Defense, The Joint Aircraft
Survivability (JAS) program office. I would like to thank Dr. Peter J. Disimile, Associate
Professor at the Dept. of ASE & EM and my thesis advisor, for his excellent guidance, support
and patience during the course of this dissertation. I would like to thank Dr. Norman Toy,
Professor at the University of Surrey UK, for his valuable insights and suggestions during critical
junctures of the research. I would also like to thank Dr. Michael Kazmierczak, Associate
Professor at the Dept. of MINE, and Dr. Milind Jog, Associate Professor at the Dept. of MINE, for agreeing to be on the defense committee and reviewing my thesis and contributing to its improvement.
I would also like to thank Mr. Curtis Fox, Research Associate at the Dept. of ASE & EM, for having helped in the various stages of the research. I appreciate the guidance of Mr. Doug Hurd,
Junior Research Associate at the Dept. of MINE, at the Mechanical Engineering workshop. I
would like to thank Mr. Bo Westheider, Instrumentation Specialist at the Dept. of MINE, for
providing the electric supply equipment. Special thanks to Mr. Jeremy Dusina, Graduate Student at the Dept. of ASE & EM, for having provided an unending supply of thermocouples. I would like to thank all my friends for their moral support and having made my stay at the University of
Cincinnati enjoyable. I would like to appreciate the support shown to me by the members of UC-
FEST lab.
Thank you.
TABLE OF CONTENTS
1 INTRODUCTION ...... 9
1.1 Problem Description ...... 9
1.2 Objective...... 11
1.3 History and Background...... 12
2 EXPERIMENTAL METHODS...... 14
2.1 Development stage of the Experimental Rig ...... 14
2.2 Manufacture of the Experimental Rig...... 19
2.3 Development stage of the Test Piece...... 24
2.4 Manufacture of the Test Charge ...... 28
2.4.1 Mold for cylindrical test charge...... 28
2.4.2 Support Rod with Heating Element ...... 28
2.4.3 Composition of test charge ...... 29
2.4.4 CPM ...... 30
2.4.5 Labmix ...... 31
2.4.6 Electric match composition...... 32
2.4.7 Mixing Method...... 33
2.4.8 Casting Process...... 34
2.5 Experimental Procedure...... 34
2.5.1 Fixing the test charge...... 34
2.5.2 Alignment of thermocouple ...... 35
2.5.3 Setting up the data acquisition system...... 36
1 3 TEMPERATURE MEASUREMENTS...... 38
3.1 Theory of Thermocouples...... 38
3.1.1 Seebeck effect...... 38
3.1.2 Thermocouple definition...... 38
3.2 Thermocouples...... 38
3.2.1 Types of thermocouples...... 38
3.2.2 Thermocouple selection criteria...... 38
3.2.3 Thermocouple selected...... 40
3.3 Calibration and Data acquisition...... 43
3.3.1 Effect of Additional Junctions ...... 43
3.3.2 Software compensation...... 44
3.3.3 Data acquisition system ...... 45
3.3.4 LabVIEW software ...... 45
4 TEMPERATURE FIELD ANALYSIS ...... 48
4.1 Pyrotechnic theory...... 48
4.1.1 Definition ...... 48
4.1.2 Composition...... 48
4.1.3 Reaction Process...... 48
4.2 Heat Release calculations ...... 50
4.2.1 Heat Release from test charge made of CPM ...... 50
4.2.2 Heat Release from test charge made of Labmix ...... 52
4.2.3 Heat Release from Heating element ...... 54
4.3 Sparkler temperature profile analysis ...... 55
2 4.4 CPM profile analysis...... 60
4.5 Labmix profile analysis...... 61
4.6 Comparison of profiles ...... 63
5 IMAGING...... 65
5.1 Development of the procedure...... 65
5.2 Equipment for setup...... 66
5.3 Imaging Theory...... 67
5.4 Image Analysis...... 69
6 FUEL POOL IGNITION STUDIES...... 76
6.1 Development of the Procedure...... 76
6.2 Experimental Set up...... 77
6.2.1 Fuel ...... 77
6.2.2 Test Charge...... 77
6.2.3 Electric Ignition system...... 78
6.2.4 Hot Plate...... 78
6.2.5 Arrangement ...... 78
6.3 Experimental Procedure...... 78
6.3.1 Fuel pool evaporation rate ...... 78
6.3.2 Fuel Ignition Test Process...... 80
6.4 Fuel Properties...... 81
6.4.1 Flash Point...... 81
6.4.2 Auto Ignition Temperature (AIT) ...... 82
6.5 Background...... 83
3 6.6 Results and Discussion...... 85
7 Conclusion ...... 100
8 References...... 101
4 LIST OF FIGURES
Figure 1.1: Shot of an API through target panel...... 10
Figure 1.2: Cross section of an API...... 10
Figure 2.1: Initial experimental rig idea ...... 14
Figure 2.2: L-shaped frame...... 16
Figure 2.3: U-shaped frame ...... 17
Figure 2.4: Stable arrangement for temperature measurement...... 19
Figure 2.5: Mount for fixing the micrometers ...... 20
Figure 2.6: Block to fix the micrometers perpendicular to each other ...... 21
Figure 2.7: Traversing section ...... 21
Figure 2.8: Angle with extension arms...... 22
Figure 2.9: Extension arms with fixing screw to hold charge ...... 23
Figure 2.10: Final arrangement of experimental rig ...... 23
Figure 2.11: sparkler cut to 40mm length...... 24
Figure 2.12: Ground material...... 25
Figure 2.13: Magnet with iron filings...... 25
Figure 2.14: Spherical test piece...... 26
Figure 2.15: Bullet Mold...... 26
Figure 2.16: Mold for test charge ...... 28
Figure 2.17: Support rod with heating element ...... 29
Figure 2.18: Test charge with electric match...... 30
Figure 2.19: x-axis alignment ...... 35
5 Figure 2.20: z axis alignment...... 36
Figure 3.1: Comparison of R and K type temperature profile ...... 41
Figure 3.2: Response time of K-type thermocouple ...... 43
Figure 3.3: Additional Junctions...... 44
Figure 3.4: Interface of input parameters...... 46
Figure 3.5: Screen shot of output...... 47
Figure 4.1: Burning pyrotechnic composition [6]...... 49
Figure 4.2: Typical temperature profile...... 56
Figure 4.3: Temperature profile in the x axis ...... 58
Figure 4.4: Temperature profile in the +/-z axis...... 58
Figure 4.5: Comparison of trend lines ...... 59
Figure 4.6: Temperature distribution for CPM...... 60
Figure 4.7: Temperature distribution for Labmix...... 62
Figure 5.1: Color Filter properties ...... 66
Figure 5.2: Temperature distribution across candle flame w/o color filter...... 70
Figure 5.3: Temperature distribution across candle flame with color filter ...... 70
Figure 5.4: Burning test piece made of CPM ...... 71
Figure 5.5: Burning test piece made of CPM with filter...... 72
Figure 5.6: Comparison of diameters for test charge made of CPM ...... 73
Figure 5.7: Test piece made of Labmix w/o filter ...... 73
Figure 5.8: Test piece made of Labmix with filter ...... 74
Figure 5.9: Comparison of flame and light regions for test charge made of Labmix...... 74
Figure 6.1: Sketch of a fuel pool setup ...... 76
6 Figure 6.2: Ignition at a height of 90mm ...... 86
Figure 6.3: Ignition at a height of 25mm ...... 86
Figure 6.4: Ignition at a height of 30mm and fuel pool at 25 0C ...... 89
Figure 6.5: Ignition at a height of 20mm and fuel pool at 25 0C ...... 90
Figure 6.6: Ignition at a height of 30 mm and fuel pool at 50 0C ...... 92
Figure 6.7: Ignition at a height of 25mm and fuel pool at 50 0C ...... 92
Figure 6.8: Ignition at a height of 40mm and fuel pool at 75 0C ...... 93
Figure 6.9: Ignition at a height of 35mm and fuel pool at 75 0C ...... 93
Figure 6.10: Ignition at a height of 65mm and fuel pool at 100 0C ...... 95
Figure 6.11: Ignition at a height of 60mm and fuel pool at 100 0C ...... 95
Figure 6.12: Ignition at a height of 90mm and fuel pool at 125 0C ...... 96
Figure 6.13: Ignition at a height of 150mm and fuel pool at 125 0C ...... 97
Figure 6.14: Change in evaporation rate of fuel pool with change in temperature for different
heights...... 98
Figure 6.15: Ignition height with change in fuel temperature ...... 99
7 LIST OF TABLES
Table 1: Mass and dimensions...... 29
Table 2: Chemical composition of CPM ...... 31
Table 3: Mass percentages for CPM...... 31
Table 4: Chemical composition of Labmix ...... 32
Table 5: Chemical composition of Electric match...... 32
Table 6: Thermocouple junction selection...... 39
Table 7: Heat of formation of compounds...... 50
Table 8: Molecular weight of test components...... 50
Table 9: Comparison of composition and heat release ...... 63
Table 10: Temperature variation with distance ...... 64
8 1 INTRODUCTION
1.1 Problem Description
Cabin fire and safety protocols on commercial aircraft rely heavily on the knowledge of fire
dynamics, temperature distributions, and thermal mass transfer of specific chemical components
of the fuel and the source. If this cabin fire was initiated because of a short duration pyrotechnic
(SDP) event, then the scenario is further complicated, and is even more so if the event is in close
proximity to a pool of fuel. In both the military and commercial aviation fields, the knowledge of
temperature distributions associated with SDP’s is far from complete. The measurements of high
temperatures produced by near instantaneous combustion, are difficult to obtain and challenging to understand. For example, high temperatures reached within the fireball from a small, highly volatile explosion or SDP has not been measured with certainty. Based on initial research in this study, the ignition of fuel by an incendiary device, such as an armor-piercing incendiary (API) projectile and the temperature field it produces, has not been exhaustively covered. It is widely accepted that the energy is released uniformly and produces a homogeneous high temperature field. This temperature field dimensions are considered to be similar in size to the emitted spectral energy in the visible wavelength range. The API projectiles were not used for testing in the laboratory due to lack of proper equipment and high operating cost. In order to gather data about the fireball around the API projectile, a simulator needed to be developed and used in a laboratory setting. To develop an accurate simulator, the composition and structure of the API had to be examined.
The trajectory of a small arms API passing through a 6.35mm thick target panel is shown in
Figure 1.1. Figure 1.1 and Figure 1.2 has been borrowed with permission from Dusina [1]. An
API round is composed of three main parts as shown in Figure 1.2.
9 1. The solid steel core
2. The windscreen, made of Aluminum alloy
3. Enclosed within the windscreen is an incendiary material
Figure 1.1: Shot of an API through target panel.
Figure 1.2: Cross section of an API.
Once fired, the projectile impacts against the panel. The windscreen is effectively ‘peeled’ back as the API round passes through the target. The friction against the incendiary material is
10 sufficient to cause ignition. Figure 1.1 shows that the trajectory of this API moving from left to
right. The impact produces a large area of visible spectral energy, about 355mm in diameter. In a
concurrent study by Dusina [1], the resulting study showed the cloud did not provide a uniform
temperature distribution throughout. To examine this temperature field further in a laboratory
setting, an API simulator was developed to study the SDP event.
1.2 Objective
This study challenged the validity of the generally accepted notion of uniform temperature
distribution around a pyrotechnic event. In addition, the influence of this event on the ignition of
a fuel pool was also studied. Another aim of this study was to provide a low cost methodology
utilizing non-ballistic commercial pyrotechnic material (CPM) and laboratory prepared
chemicals (Labmix) to simulate the API tests. To accomplish these goals, a simulator that
provided a temperature field comparable to that of an API was developed.
Two experimental methods of investigation were used to accomplish the goals.
1. Thermometry using high speed thermocouples.
2. Image analysis from video recordings of SDP events using color filters.
The thermometry method was useful, from a quantitative standpoint, in determining the temperature profile inside the luminous region. The color filter method provided qualitative support by analysis of the fireball. This tested the validity of the accepted notion of a uniform temperature field. Ultimately, the effect of the fireball from the simulator, on a pool of fuel was investigated to assess its ignitability. The simulator was placed at different heights from the fuel pool surface, and the ignition height was examined. Also image analysis was used to examine the distance from the center of the fireball where ignition of the fuel pool occurred.
11 1.3 History and Background
The use of pyrotechnic compositions can be found in numerous fields and products like rocket propellants, highway flares, and entertainment. Initially, the composition of the incendiary material was not provided by the U.S. Air Force (USAF). A similar material which could create an intense fireball was sought in the commercial market. It was found that these types of high intensity flashes or fireballs were created by products found in the fireworks industry. A
literature search [2 to 17] for the chemical formula of these products gave numerous results
under the common name of flash compositions.
The ability to measure the temperature of a fireball is far from trivial. Several methods
have been used in the past to measure the temperature. Of these methods, the most notable, used
optical pyrometers in which the brightness of a flame is compared to the brightness of an
incandescent filament in order to determine the flame temperature [8]. Other types of pyrometers
have also been used, with limited success. Some examples are cinephoto pyrometer,
photoelectric photometer and color photometer [8].
In comparison, temperature measurements of complex combustion within non-uniform
temperature zones have been done successfully using the line-reversal method. This method may
provide intermediate temperature distributions within the various zones of a flame. The line
reversal method involves the use of emitters within the flame that emit characteristic spectral
lines as seen through a spectroscope [18]. However, in this method, the temperature
measurements only accounted for the average temperature across a flame region and the
variations within the inner zones were unaccounted. Likewise, temperature fields with relation to
flame height have been measured using a cinephoto pyrometer [8]. Detailed temperature
distributions were unattainable since this method could not provide measurements at intervals
12 under 10mm. In order to satisfy the goals of the experiment, it was considered necessary to adopt
a simplistic approach, using high-speed thermocouples. Such temperature measurements have
been performed earlier at a single fixed position in a closed system on different pyrotechnic
materials, notably Sb/KMnO4 (Antimony/Potassium permanganate) [19], and Pd/Al
(Palladium/Aluminum) mixtures [20]. Temperature profiles have also been measured using W-
0 Re (Tungsten/Rhenium) thermocouples for temperatures above 2000 C in Mo/KClO4
(Molybdenum/Potassium perchlorate) material [21], and W/KClO4/BaCrO4 (Tungsten/Potassium
perchlorate/Barium chromate) material [22], but again only at a fixed position in the system.
Bare bead W-Re type thermocouples are capable of measuring temperatures in excess of 2000
0C. However, they are very prone to oxidation that can lead to large errors. Therefore, these
thermocouples have to be used within inert environments to prevent the Tungsten from
oxidizing. Given these complications, it was decided to measure the temperature using R and K
type thermocouples that measure maximum temperatures upto 1450 0C and 1250 0C respectively.
In addition to the temperature measurements, it was considered prudent to utilize digital video
analysis of the pyrotechnic event in order to observe the area of the spectral radiation. This
method was further enhanced with the use of optical filters to suppress a significant portion of
the visible wavelength region. This allowed the hotter zones of the event, in the near infrared
region, to be captured. The temperature measurement method determines the temperature
quantitatively within an event. The video analysis supports this qualitatively, by taking optical
data. This has the effect of differentiating between the fireball and the inner reaction zone.
13 2 EXPERIMENTAL METHODS
2.1 Development stage of the Experimental Rig
Initially an experimental setup which would be used as a simulator for the API was contrived.
Initially the setup was conceptualized as a ceramic dish in which the incendiary composition was
to be ignited, shown in Figure 2.1.
Ceramic Incendiary composition dish Thermocouple
Heating element
Figure 2.1: Initial experimental rig idea
Holes drilled at the lower edge would allow the passage of the heating element coil. A thin stainless steel plate with diameter slightly less than the inner diameter of the ceramic dish would
separate the incendiary composition and heating element. A different set of holes drilled along
the walls of the dish would allow the introduction of the incendiary material. The temperature of
the burning material would then be measured by the thermocouples arranged around the
incendiary material. As a preliminary step, the ignition of various compositions was attempted
using the heating coil connected to the power supply. Many of the compositions failed to ignite
with the heating coil, even though they ignited when a candle flame was introduced. Due to the
14 difficulties in ignition and repeatability, this experimental method was discarded and alternate
ways were considered.
Before a new experimental setup could be designed, it was important to establish a final
composition, similar to the API, to perform the experiments. In order to do this, pre-mixed
chemicals found in the fireworks industry sold under commercially popular names like stars,
fountains, sparklers, etc. were examined. Among these, the sparklers were found to be the safest
and required least energy for ignition when the candle flame was introduced. The sparklers also
had a near cylindrical structure and the burning rate was slow and controlled. The chemical
composition of these commercially available pre-mixed chemicals could not be obtained from
the manufacturers due to proprietary concerns. A literature search for the compositions of
sparklers was performed. A balanced chemical equation of the major components for the gold
sparkler was found in reference [5]. Calculation of the stoichiometric ratio of the reactants and
the heat release from this reaction is given in section 4.2.
In order to observe the maximum temperature rise that could be attained by a burning
sparkler, it was ignited using a butane lighter flame. Different types of thermocouples available
at the lab were tested using a portable temperature meter. A difference of 400 0C was found
between the bare bead, 40 gauge (0.0787mm) K-type thermocouple and a similar sheathed
thermocouple. Since values higher than 1000 0C could not be measured using the hand held
meter, a data acquisition system was setup where the voltage generated by the thermocouples
was transmitted via an A/D converter into a computer. This was then displayed as temperature
data using LabVIEW software. The thermocouple was placed close to the surface of the burning
sparkler to observe the maximum temperature that the thermocouple could measure. It was found that, at a temperature of 1415 0C, the thermocouple bead melted.
15 Since the sparkler had a cylindrical structure, temperature was measured at different points along the length of the sparkler for the same radial distance from the surface of the sparkler. This procedure helped in reducing the number of tests required to achieve repeatability.
A test was conducted to confirm the uniformity of the measured temperature along the length of the sparkler at a particular radial distance from the sparkler. For this, an L-shaped frame, with one leg slightly longer than the sparkler length, was made See Figure 2.2 for a picture of the frame.
Drilled hole Grooves
Figure 2.2: L-shaped frame.
At the center of the shorter leg, a hole was drilled which could support the sparkler rod. The longer leg had 5 equidistant grooves in which the thermocouples could fit. The radial distance between the thermocouple bead and the sparkler surface was adjusted under a microscope. An examination of the resulting temperature profiles led to the following conclusions;
1. Variations were found in the measured temperature profile for the 5 thermocouples.
2. The measured temperature decreased towards the fixed end of the sparkler length due to a
slight taper in the body of the sparkler.
16 3. Variations were found in temperature profiles due to cantilever type bending of the free end
of the sparkler while burning (thus increasing the distance between the sparkler and the
thermocouple).
4. Variations were found in the temperature profiles as a result of non-uniform surface of the
sparkler due to pits and bumps along the length.
A slightly different arrangement was designed to overcome the above mentioned factors. A U-
shaped frame as shown in Figure 2.3 was designed. This arrangement had two equal sized legs.
Grooves Fixing piece piece Fixing
Figure 2.3: U-shaped frame
The thermocouples were placed in the grooves cut on the legs of the U–Frame. The lower end of
the sparkler rod passes through a groove at the base of the frame. The upper end of the rod passes
through a fixing piece that prevents bending of the sparkler rod. The grooves in the setup were such that the thermocouples were arranged at closer distance when compared to the L-Frame fixture. The slight diametrical variations (taper) along the length of the sparkler were reduced.
The surface of the sparkler was made uniform by sanding it with a fine emery paper to remove
17 the pits and bumps. Measuring the temperature at fixed radial locations, the following
observations were made;
1. Variations in temperature along the length of the sparkler, though lesser than the previous
case, were still found.
2. Variations were found in the temperature profiles due to sagging of the center of the sparkler
rod from its high length/diameter ratio.
3. Variations were found in the temperature profiles due to non-uniform spatial distribution of
particles in the sparkler, due to the effect of air bubbles inside the material. Sanding was
performed to remove the pits and bumps on the surface but the air bubbles were randomly
distributed over the entire volume of the sparkler.
In order to overcome these limitations, a test piece was manufactured which is discussed in section 2.3. As discussed earlier, the radial distance between the thermocouple bead and the sparkler surface was adjusted under a microscope. The microscope had a minimum reading of
0.1 mm. To increase this accuracy and also to firmly fix the test charge, a stable arrangement, as shown in Figure 2.4, was designed and fabricated in the workshop. This helped in eliminating the differences in the temperature profile that occurred due to the variations in the distance between the thermocouple bead and the sparkler surface.
18
Micrometers
Figure 2.4: Stable arrangement for temperature measurement
This arrangement is described in detail in the following section. This arrangement has 0.001mm accuracy in the x, y and z directions.
2.2 Manufacture of the Experimental Rig
This fixed arrangement for temperature measurements was developed to avoid errors that were
caused by placing the thermocouple at a fixed distance from the surface of the test charge. A
bench-type sliding micrometer was used as a base to mount the other parts of the set up. This
sliding type micrometer has a least count of 0.001mm in the horizontal axis. This micrometer has
a bridge, with holding screws, which can slide over the threaded portion along the horizontal
axis. Since accuracy was required in all the 3 directions, two more micrometers with a least
count of 1 micron were used so that the thermocouple could be located at a particular spatial
point accurately. To fix the other two micrometers, a mount was machined out of an Aluminum
block, shown in Figure 2.5.
19
Mount
Figure 2.5: Mount for fixing the micrometers
The mount was designed in such a way that the location of the bolts in the extension arm of the micrometer matched with those of the mount. The figure also shows the dimensions of the mount. A second support block was machined. This allowed the two micrometers to be fixed perpendicular to each other, see Figure 2.6. A flat rectangular piece (traversing section) was machined which acted as a base for fixing the thermocouples, as shown in Figure 2.7. This was named as the traversing block.
20
Support Block
Figure 2.6: Block to fix the micrometers perpendicular to each other
Traversing Block
Figure 2.7: Traversing section
21
Extension arms
Angle
Figure 2.8: Angle with extension arms
Further, an arrangement was required to fix the position of the test charge. An aluminum angle piece was machined to fit the width of the fixed bench type micrometer, shown in Figure 2.8.
This machined angle piece was also attached to the top face of the bench type micrometer.
Further, two extension arms were also machined for fixing the test charge. The dimensions of the extension arms are shown in Figure 2.9.
22
Extension arm Test charge +z
+y +x
Figure 2.9: Extension arms with fixing screw to hold charge
The vertical part of the angle was drilled at different heights so that the height of the extension arms could be varied. The final arrangement of the experimental rig can be seen in Figure 2.10.
Figure 2.10: Final arrangement of experimental rig
23 2.3 Development stage of the Test Piece
As discussed in section 2.1, variations in the temperature profiles were present as a result of large
length/diameter ratio. To overcome this, the sparkler length was reduced to 40mm. The support
rod, running along the longitudinal axis of the sparkler, was bent on both ends by 900 (Figure
2.11). As shown in the figure, the bent portion of the rod was inserted into grooves that were drilled on the support plate of the experimental set up rig.
Figure 2.11: sparkler cut to 40mm length
The thermocouple was fixed to the traversing block (Figure 2.7) of the rig and the
distance between the sparkler and the thermocouple bead was controlled from the wheel of the
micrometer. Tests were performed and the temperature profile were measured and plotted. This
is further discussed in section 4.3. Another reason for the variation in temperature profile, the non-uniform spatial distribution of particles in the sparkler material, could not be prevented.
Remolding of the sparkler material was considered, to remove non-uniformity. This was done by first breaking up the sparkler material from the support rod and then crushing the solid pieces in a mortar and pestle. Once the material was ground into a fine powder, it was wetted with distilled water to make it into a tacky paste, which could be molded into any required shape and size. The
24 powdered material was divided into two batches. One batch was introduced to a powerful magnet, which could separate the iron filings from the other components, and the second was not. This removal of the iron filings can be seen in Figure 2.12 and Figure 2.13.
Figure 2.12: Ground material
Figure 2.13: Magnet with iron filings
A tacky paste was made out of the powdered material from each batch. This paste was then
remolded into spherical balls. It was hypothesized that the spherical shape would give a uniform
flame. These spherical balls, shown in Figure 2.14, were initially cast in a round bullet mold
25 shown in Figure 2.15. The support rods were inserted through the center of the sphere while still
in the mold.
Figure 2.14: Spherical test piece
Figure 2.15: Bullet Mold
The spherical test pieces were allowed to dry for a few hours and then the mold was opened to release the sphere. The initial trials resulted in spheres with small craters due to shrinkage and drying. This was reduced by changing the tackiness of the paste. The tackiness of the paste was varied by changing the water percentage in the mixture.
26 When ignition tests on the spheres were conducted, a major difficulty was observed.
During ignition by a butane lighter flame, the sphere burnt with a jet like ejection of flame from its bottom. This was the location of maximum heat transferred from the lighter. The flame front moved towards the top and interior of the sphere then consuming the entire sphere. This gave a non-spherical burning pattern countering the earlier stated assumption, of uniform burning.
The ignition by a lighter flame had many disadvantages. They were;
1. Non-uniform heat transfer to the surface of the test charge.
2. Amount of heat energy transferred to the test charge was unknown.
To overcome this difficulty an electric ignition system was devised so that a more controlled and uniform heat transfer could be achieved. This is discussed in section 2.4.6.
Since the spherical shaped test charge was not effective in generating a uniform burning pattern, it was discarded, and a cylindrical shape similar to the sparkler was considered. The different factors that caused non-uniform burning and large scatter in data due to the structure and dimension of the sparkler had to be avoided in this new test piece. The following factors were important in the creation of the cylindrical test charge;
1. Change in diameter along the length of the sparkler
2. Large length to diameter ratio leading to sagging
3. Non-uniform surface of the sparkler due to pits and bumps along the length
4. Variations due to non-uniform spatial distribution of particles in the sparkler, such as the
effect of air bubbles inside the material
In order to minimize the effect of the above factors, a mold was developed to manufacture the cylindrical shaped test charge.
27 2.4 Manufacture of the Test Charge
2.4.1 Mold for cylindrical test charge
The dimensions of the manufactured test charge needed to be consistent for each test. A mold, as
shown in Figure 2.16, was designed to shape the test material. Cylindrical pipes made of 6061-
T6 Al Alloy [43] of 4mm internal diameter were cut to small pieces of length 15mm each. The burrs were filed off and the edges rounded. A total of 50 molds were made so that the test pieces could be formed in batches of 50.
Figure 2.16: Mold for test charge
2.4.2 Support Rod with Heating Element
The support rod, as shown in Figure 2.17, was made of steel. Its dimensions and weight are
given in table 1. The test charge was ignited using a heating element that was connected to a DC
power supply. The heating element consisted of a 50.8mm (2 inch) long, 0.16mm diameter (34
gauge) Nichrome wire with a resistance of 2.73 Ohms. The center of the heating element was
coiled around the support rod. An insulating tape was wrapped around the support rod and then
the heating coil was wound over it to avoid electrical contact. The weight of this support rod with heating coil and tape is given in Table 1.
28
Heating element
Support Rod
Figure 2.17: Support rod with heating element
Nichrome Rod + Nichrome Rod + Nichrome Rod Mold (Shell) wire wire + tape wire + tape + Mold Weight 0.5 0.008 0.53 0.19 0.072 (gms) Error (gms) 0.025 0.000 0.025 0.005 0.03 Length 60 50.8 - 15 - (mm) ID = 4mm Diameter 1.18 0.16 - OD = - (mm) 4.7625mm Table 1: Mass and dimensions
2.4.3 Composition of test charge
The test charge basically consisted of two components
1. The main charge
2. The electric match
29
Main Charge Electric Match
Support Rod Heating Element
Figure 2.18: Test charge with electric match
The structure of the test charge could be seen in Figure 2.18. The heating element could be seen embedded in the electric match composition or ignition initiator. The flat surface of the match composition is in contact with the test composition or main charge around which the temperature distribution was to be measured. The diameter of the test charge is 4mm and the length is 15mm of which 3mm of the length is spanned by the electric match. The manufacturing procedure for this test charge is described in the following sections.
2.4.4 CPM
Prior to obtaining the composition of the API from the USAF, the commercially available pyrotechnic material (CPM) was used as the composition for the main charge. As described earlier, the initial set of temperature profiles were obtained for test charges made of remolded commercially available pyrotechnic material, obtained from sparklers. This remolded test charge composition will be referred to as CPM in the rest of the document. The remolding process was
30 similar to that done for the spherical test charge as described in section 2.3. It was found that the total weight of the material was reduced by 25% once the iron filings were removed.
To ensure that the particles were of uniform size, the powdered material was sifted through a 200-mesh sieve. The size of the openings of a 200-mesh sieve is 0.074mm. The magnet was used to remove any residual iron filings from the powder. This powder was then wetted with distilled water to make it into a tacky paste so that it could be easily molded into the cylindrical shell. The details of the composition for the CPM test charge are given in Table 2.
Also the details of the amount of each composition used for the preparation of the final mixture are given in Table 3.
Charge Chemical Formulae Quantity (by wt) Mesh size Barium Nitrate Ba(NO ) 74% <200 Main Charge 3 2 Aluminum Al 24% <200 (CPM) Binder (Dextrin) (C6H10O5)n 2% <200
Table 2: Chemical composition of CPM
Weight (gms) Fuel Oxidizer Binder Water CPM 14.0 - 5.32 (38%) Electric Match 1.32 1.65 0.03 0.6 (20%) Labmix 7.61 7.92 0.32 3.17 (20%) Table 3: Mass percentages for CPM
2.4.5 Labmix
Once the composition of the API was obtained from the USAF, the individual components were procured and were mixed in the lab. This composition, prepared in the laboratory, will be referred to as Labmix in the rest of the document. Barium Nitrate was used as the oxidizer. It had to be ground from its initial crystalline state to a fine powder. The ground powder was passed through the openings of a 200-mesh sieve to obtain a fine powder with grain size less than
0.074mm. Magnalium powder with a grain size of less than 0.074mm was used as the fuel.
31 Magnalium is a mixture of Aluminum and Magnesium powder in equal proportions. In order to obtain the final composition, the oxidizer and fuel were mixed with a binder material to obtain a homogeneous composition. The details of the mixing process are given in section 2.4.7. Also, the details of the Labmix composition are given in Table 4.
Charge Chemical Formulae Quantity (by wt) Mesh size Barium Nitrate Ba(NO ) 50% <200 Main Charge 3 2 Magnalium Mg-Al 48% 200 (Labmix) Binder (Dextrin) (C6H10O5)n 2% <200
Table 4: Chemical composition of Labmix
2.4.6 Electric match composition
The electric match consisted of two components
1. The heating element (described in section 2.4.2)
2. Match composition
Charge Chemical Formulae Quantity (by wt) Mesh size
Potassium Chlorate KClO3 55% <200 Electric Lead Thiocyanate Pb(SCN)2 44% <200 match Binder (Dextrin) (C6H10O5)n 1% <200
Table 5: Chemical composition of Electric match
Initially the ignition of both CPM and the Labmix composition was attempted using a heating element connected to a DC source. It was found the heat transferred to the main charge by this heating element, was not sufficient to initiate ignition. A substance that ignited at a lower temperature and released sufficient energy to ignite the main composition was chosen for the electric match. The details of this composition are given in Table 5. The oxidizer of this composition was Potassium Chlorate. This was ground from its crystalline state and then sieved to obtain a 200-mesh powder. The fuel was powdered Lead Thiocyanate, which was also ground
32 and sieved to a 200-mesh powder. The fuel and oxidizer were mixed together using the mixing
process described in section 2.4.7 to obtain a homogeneous composition. A binder was required
to bind the different components of the electric match and also to bind it to the main charge after drying. Dextrin was used as the binder. The binder was dissolved in water and applied to the mixture to obtain a tacky paste. The details of the amount of chemicals used are given in Table 3.
2.4.7 Mixing Method
The following method was employed to obtain uniform mixing of the individual components.
The method was common for mixing of components of the main charge as well as the electric
match.
1. Prior to mixing, the major ingredients, the fuel and oxidizer, were sieved separately to
remove any clumps. This was repeated until all the clumps were broken and the ingredients
could pass through the sieve.
2. A 210mm by 297mm (A4) size, sheet of paper was spread out on a flat surface.
3. The chemicals were placed at the center of the paper and mixed with a wooden spoon.
4. Two opposite corners of the paper were picked up and they were alternately lifted so the
chemicals rolled together. This process was alternated with the other two corners.
5. The mixture was now sieved onto another A4 size paper sheet.
6. The sieving process was repeated at least 5 times to obtain a homogenous mixture.
7. Small clumps that wouldn’t pass through the sieve were broken up by applying gentle
pressure with the back of a wooden spoon or a piece of paper.
Once the chemicals had been mixed thoroughly, the final mixture was wetted with distilled water
to make a tacky paste that could be molded easily in the cylindrical shell. The details of the
amount of each composition used for the preparation of the final mixture, are given in Table 3.
33 2.4.8 Casting Process
Initially, the weights of different components like the shell and support rod were measured and are shown in table 1. The wet material, CPM or Labmix, was then filled into the mold shell and weighed. This filled material was compacted using a lab made wooden press so that the material was compressed to occupy a length of 12mm inside the shell. Now, the support rod was inserted into the main composition through the central axis of the shell. The coiled portion of the heating element around the support rod was positioned in the 3mm open space inside the shell and above the main charge. A weighed amount of the match composition was formed above the main charge composition such that the heating coil was embedded in this region. The final weight of the cast was measured and it was allowed to dry in ambient conditions. The weight of the dried test charge was measured and compared to the wet test charge. The weight of the wet test charge was 0.365gms. The main charge accounted for 0.32gms, and the electric match, accounted for
0.045gms. After a period of 6 hours, the test charge was removed from the shell and allowed to dry for a further 24 hours. Finally, the weight of the dried test charge was measured and found to be 0.267gms. The difference in weights was due to the evaporated water which accounted to
0.098gms.
2.5 Experimental Procedure
2.5.1 Fixing the test charge
The manufactured test charge was positioned so that both ends of the support rod were placed in the extension arms of the experimental rig. The support rod was fixed in place by tightening set screws present in the extension arms. Wires of the heating element, partially embedded in the test charge, were facing downwards. This was done to ease the access of electric connectors to the heating element and to reduce obscuration of visualization of the burn process.
34 2.5.2 Alignment of thermocouple
Three thermocouples were set up around the test charge such that the temperature field could be
mapped. For sake of convenience the axis perpendicular to the longitudinal axis of the test charge in the horizontal plane is considered as the x axis. The axis perpendicular to the x axis in the vertical direction is taken as the z axis. Upward direction in the z axis is taken as +z axis and downward direction is taken as –z axis. The first thermocouple was set up along the x axis of the test charge by fixing it to the center of the traversing block (Figure 2.7) on the experimental rig.
The thermocouple was attached to a steel rod such that there was no sag in the portion extending from the traversing block. To ensure that the thermocouple was aligned with the x axis, the following procedure was adopted. Using the z axis and x axis micrometers, the bead of the thermocouple was adjusted to touch the top surface of the test charge.
+z
3mm 2mm x axis 4mm
-z
Figure 2.19: x-axis alignment
From this position, as shown in Figure 2.19, the bead was moved 3mm away from the test charge
in the direction of x axis. Since the diameter of the test charge was known to be 4mm, the thermocouple was moved half way or 2mm in the direction of –z axis using the micrometer. This
ensured that the bead was in the x axis of the test charge. To adjust the distance along the x axis,
35 the bead was again made to touch the surface of the test charge. Then, it was moved away from the test charge to the required test distance using the horizontal micrometer.
To set the thermocouples in the +z axis, the z axis micrometer was adjusted such that the thermocouple in the x axis was moved by a distance equal to the test distance in the direction of the –z axis (Figure 2.20). Now, the thermocouple for the +z axis was fixed to the traversing block such that its bead touched the top surface of the test charge. The x axis thermocouple was restored to its original position by moving it in the +z direction.
+z
Step 2 (Fix thermocouple to traversing block)
x axis 4mm Step 1 Step 3
-z Traversing block
Figure 2.20: z axis alignment
Thus, the test distance for the thermocouple in the +z axis was set. The same procedure was followed to adjust the test distance for the thermocouple in the –z axis.
2.5.3 Setting up the data acquisition system
Extension wires connected the thermocouples to the data acquisition system. LabVIEW software was used to convert the voltage generated by the thermocouple to a temperature data. At the beginning of each test, various parameters such as sampling rate, sample size and type of
36 thermocouple were set. During the experiment, the acquisition system was triggered immediately after electric match circuit was closed to capture the entire temperature profile.
37 3 TEMPERATURE MEASUREMENTS
3.1 Theory of Thermocouples
3.1.1 Seebeck effect
Thomas J. Seebeck discovered that an electric current flows in a closed circuit of two dissimilar
metals when one of the two junctions is heated with respect to the other. This is the basic
principle behind the functioning of a thermocouple.
3.1.2 Thermocouple definition
A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals,
joined together at one end, which produce a small unique voltage at a given temperature.
3.2 Thermocouples
3.2.1 Types of thermocouples
Thermocouples are often used in research in a wide variety of industries. They measure in wide
temperature ranges and can be relatively rugged. Thermocouples are available in different
combinations of metals or calibrations. The four most common types are J, K, T and E. Each type has a different temperature range and environment, although the maximum temperature
varies with the diameter of the wire used in the thermocouple.
3.2.2 Thermocouple selection criteria
The following criteria were used in selecting a thermocouple for the current set of experiments:
1. Temperature range: The operating range of a thermocouple is the temperature range over
which the thermocouple will perform satisfactorily, with negligible error in the output signal.
For the current set of experiments, a temperature range with a high upper measuring limit
was desired.
38 2. Operating Environment: The operating temperature and external environment can affect the
performance of the thermocouple. The present test conditions required thermocouples whose
output signals were not affected by the particle and smoke emissions from the burning test
charge.
3. Cost/Performance Ratio: The cost of the thermocouple increases with the accuracy of the
thermocouple. The desire is to achieve a balance between the cost of the thermocouple and
the accuracy of the measurement.
4. Thermocouple junction selection: Each thermocouple must utilize a measuring junction and a
reference junction at two different temperatures. The measuring junction is generally at the
higher of the two temperatures and the reference junction is at ambient. The measuring
junction is placed near or on whatever is to be measured and the referenced junction is
connected either to a controller or a temperature indicator. Different kinds of measuring
junctions are used with respect to measuring requirements. This can be seen in table 6. The
junction selected for the experiment is discussed in section 3.2.3.
Response Type of Junction Advantages Disadvantages time Reliable and rugged Ungrounded Slow Sluggish response time construction Sheathed Useful for electrically Grounded Normal conductive metallic Noise injection sheaths Low thermal mass Prone to damages in a Bead weld Fast increases response time corrosive environment Exposed Corrosive failure and Useful in high speed Butt weld Fastest physical or mechanical measurements damage Table 6: Thermocouple junction selection
5. Response Time: The response time constant is the time required for a thermocouple to reach
63.2% of step change in temperature of a surrounding media. Five time constants are
39 required for the sensor to approach 100% of the step change value. Typical response time for
thermocouples range from hundreds of a second all the way up to 5 seconds, depending on
the size of the thermocouple and the junction employed. The response time for the
thermocouple used is discussed in section 3.2.3.
3.2.3 Thermocouple selected
The thermocouple used for this experiment was 0.0787mm thick (40 gauge), exposed, bead weld junction, K-type thermocouple. The K-type thermocouple consists of 2 wires, which are insulated from each other except at the region of the bead, placed inside a sheath of 3 mil thickness (0.0762mm). The positive wire is made of Chromel, which is a composition of 90%
Nickel and 10% Chromium. The negative wire is made of Alumel, a composition of 95% Nickel,
2% Aluminum, 2% Manganese and 1% Silicon.
The advantages of using the K-type thermocouples are
1. They have the highest temperature range (-200 0C to 1250 0C) among the most commonly
used types of thermocouples.
2. They are one of the cheapest available types.
The manufacture of the thermocouple as reported by Dusina [1] is given below. Spools of pre-insulated wires were bought and cut to required lengths. On one end of the thermocouple the insulation was removed for a length of 15mm and the exposed wires were twisted together. This twisted end was placed in a thermocouple welder which welded the two wires together to form a bead. Proper fusion of the wires and the size of the beads are ensured by adjusting the current settings on the thermocouple welder, used in the construction of thermocouples.
Other types of thermocouples such as R-type and the C-type (this name is not yet approved by the ANSI) were also tested. These types are rated to have a much higher temperature
40 measuring range. The R-type thermocouple can measure temperature upto 1768 0C, while the C-
type thermocouple can measure a maximum temperature of 2320 0C. The R-type thermocouple
consists of an alloy of 87% Platinum and 13% Rhodium as the positive wire, while the negative
wire is made of 100% Platinum. The C-type thermocouple consists of an alloy of 95% Tungsten
and 5% Rhenium as the positive wire, while the negative wire consists of 74% Tungsten and
26% Rhenium. The temperature profiles and response times of a 36 gauge (0.127mm) thick R- type thermocouple were compared with a 40 gauge K-type thermocouple under the same
settings. This can be seen in Figure 3.1.
1400 R-type TC K-type TC 1200
1000
800
600
400 Temperature, deg C 200
0 0 2000 4000 6000 8000 10000 12000 Time, msecs
Figure 3.1: Comparison of R and K type temperature profile
It was found that the temperature measured by the R-type thermocouple was slightly lower than
that of the K-type thermocouple, though the response times for both the thermocouples were
identical. The R-type thermocouple was not chosen for the following reasons:
1. Contamination of the measuring bead at high temperatures leading to large errors in
measurements. This could not be controlled as the burning test charge emitted hot gases
during the reaction.
41 2. High Cost to Performance ratio
It was also considered worthwhile to compare the temperature profiles and response times of the
K-type thermocouple with the C-type thermocouple due to its high measuring range. A 36 gauge bare bead, unsheathed, C-type thermocouple was available. Every attempt to sheath it resulted in damage to the thermocouple near the bead. Due to this difficulty in handling, the C-types were not used for further testing.
The other reasons for not choosing the C-types were
1. The tungsten wires require an inert atmosphere as they oxidize at high temperatures
2. These wires are very brittle and handling thin wires of 40 gauge thickness is extremely
difficult. Hence a compromise between response times (related to the thickness of the
wires) and handling had to be reached.
Tests were performed using shock tube to measure the response times of the K-type
thermocouple. The procedure and results obtained from these tests has been discussed in detail
by Dusina [1]. A brief description is given below. The thermocouple whose response time was to
be measured was fixed at one end of the shock tube. A lighted candle was placed below the
thermocouple. The two ends of the shock tube were separated by a thin plastic diaphragm. High
pressure air was passed through the other end such that the diaphragm broke resulting in shock
waves. The flame of the candle was put off by the shock waves.
42
600
500 Tmax 400
300
200 Temperature, deg C 100
0 0 1000 2000 3000 4000 5000 6000 7000 Time, millisecs
Figure 3.2: Response time of K-type thermocouple
The drop in the thermocouple temperature was recorded. This data was used to calculate the response time of the thermocouple. The response time was calculated using the data by applying the following formula;
st Time (0.368 Tmax) – Time (Tmax) = 1 time constant
where Tmax is the recorded maximum temperature just before the shock. The response time
calculated for the 40 gauge K-type thermocouple was 10 milliseconds [1].
3.3 Calibration and Data acquisition
3.3.1 Effect of Additional Junctions
A problem arises when measuring the voltage across a dissimilar metal junction. Consider the
case when two additional thermocouple junctions form where the wires connect to the voltmeter
(Figure 3.3). If the wire leads which connect to the voltmeter are made of alloy "C", then there exist thermal emf’s at the A-C and B-C junctions as these two junctions generate voltages
proportional to the temperature of the junction and the voltage-to-temperature relationship for
that type of junction.
There are two approaches to solve this problem,
43 1. Make corrections for the thermocouples (or extra junctions) formed by the connection to
the voltmeter, or
2. Use a reference junction at a known temperature.
The voltage contributions from the extra junctions can be subtracted from the measured voltage to get the actual voltage corresponding to the junction A-B. This has been accomplished using the LabVIEW software. The reference or compensating junctions have not been used externally in the experimental setup.
Additional Junctions
Measuring A C Junction
V
B C Figure 3.3: Additional Junctions
3.3.2 Software compensation
In this method, after the reference junction temperature is measured, the software is programmed to add the appropriate voltage value to the measured voltage and eliminate the problem due to the effect of additional junctions. LabVIEW performs the software compensation for the data acquisition system which is being used by built in routines. The thermocouple temperature was calculated using the direct voltage addition method, when the measured voltage and the reference junction temperature were given.
44 Direct voltage addition method: In this method, the measured reference junction temperature is
converted into the equivalent voltage using polynomials that assume a reference of 0 0C. These polynomials are of the form,
2 n T = a0 + a1v + a2v + ... + anv
where v is the thermocouple voltage in volts, T is the temperature in degrees Celsius, and a0 through an are coefficients that are specific to each thermocouple type.
This voltage is added to the measured value to give the voltage that the thermocouple will
produce if the reference temperature was 0 0C. This new value of voltage is converted into
temperature to give the correct temperature of the thermocouple junction.
3.3.3 Data acquisition system
The system uses a National Instruments PXI - E6040E data acquisition device. This device
features 16 analog input channels with 12-bit resolution.
3.3.4 LabVIEW software
The software was programmed, using its user friendly interface, such that different types of
thermocouples can be accommodated at a single input channel. The DAQ was setup for three such channels. This approach was adopted so that different types of thermocouple like K, R, and
C can be tested through the same channel. A screen shot of the LabVIEW interface showing the
required input parameters can be seen in Figure 3.4
45
Figure 3.4: Interface of input parameters
The important parameters for the acquisition of data were scan rate, number of samples and type of thermocouple. In order to record data for 15 seconds at a sampling (scan) rate of 100Hz, the scan rate was set at 100 and the number of samples were set at 1500. The type of thermocouple and number of thermocouples used were chosen from the T/C type selector box. A screen shot of the output, showing the plot, can be seen in Figure 3.5. The recorded data points were plotted using MS Excel to get the resultant graphs.
46
Figure 3.5: Screen shot of output
47 4 TEMPERATURE FIELD ANALYSIS
4.1 Pyrotechnic theory
4.1.1 Definition
The word “Pyrotechnic” refers to a chemical mixture of oxidizing and reducing agents capable of
reacting exothermically. Such mixtures are used to produce light, heat, smoke or gas in different fields involving propellants, highway flares, and entertainment.
4.1.2 Composition
The major constituents that make up a pyrotechnic composition are fuel, oxidizer and binder.
Other components are also included to perform special functions in different applications such as in highway flares to produce color and light, and in firework shows to produce different effects.
As mentioned earlier, the fuel used for the current set of experiments was Magnalium, a 50-50
mixture of Magnesium (Mg) and Aluminum (Al) in the case of Labmix. The oxidizer used was
Barium Nitrate and the binder was Dextrin. The details of the composition can be found in
sections 2.4.4 and 2.4.5.
4.1.3 Reaction Process
Consider a reacting pyrotechnic composition as shown in Figure 4.1, the reaction zone that
moves along the length of the composition is called a “combustion wave”.
48 Unreacted Reaction material products
a’ b’ Ignition A C stimulus Adjucent applied layer being here c’ d’ heated as reaction Reaction zone nears zone B
Figure 4.1: Burning pyrotechnic composition [6]
Figure 4.1 shows the path of a typical combustion wavefront through the pyrotechnic material. In this figure, zone A is the unreacted materials, zone C is the reaction products and zone B represents the reaction zone. The difference between the zone A and zone C is the combination of atoms which change through zone B. At B, the composition reacts, generating an amount of heat called the Heat of Combustion. This is equal to the difference between the heats of formation of material at A and at C [6]. A part of this heat increases the temperature of C, which slowly starts to dissipate by radiation, conduction or convection. A part of the heat is turned back to A, through the boundary a’b’, supplying layer with the required activation energy for the reaction to continue. The process of propagation of the flame along the length of the material was explained in detail by Blunt [37]. He explained that the convective and radiative losses become more important than the conductive losses near the reacting layer a’b’. This provides information on the major modes of heat transferred from the reaction zone to the measuring thermocouple.
49 4.2 Heat Release calculations
4.2.1 Heat Release from test charge made of CPM
Consider the reaction between Barium Nitrate and Aluminum as in the CPM composition. The reaction [5] proceeds exothermically where equation (1) shows the balanced equation,
3Ba(NO3)2 + 10Al Æ 3BaO + 5Al2O3 + 3N2 ↑ (1)
Chemical Formulae Chemical Name Heat of Formation Δ = − Ba(NO3)2 Barium Nitrate H f 992KJ / mol Δ = − BaO Barium oxide H f 548.1KJ / mol Δ = − Al2O3 Aluminum oxide H f 1675.7KJ / mol Δ = − MgO Magnesium oxide H f 601.6KJ / mol Δ = Al Aluminum H f 0KJ / mol Δ = Mg Magnesium H f 0KJ / mol Δ = N2 Nitrogen gas H f 0KJ / mol
Table 7: Heat of formation of compounds
Chemical Formulae Molecular weight Ba(NO3)2 261.34gms/mol Al 27gms/mol Mg 24.31gms/mol Table 8: Molecular weight of test components
Equation (1) is a balanced equation where 3 moles of the oxidizer Ba(NO3)2 are required to react with 10 moles of the fuel Al to result in a complete combustion reaction. Using Table 8 the molecular weight of the reactants is calculated as follows;
In gram equivalents,
1 mole of Ba(NO3)2 equals 261.34 grams
1 mole of Al equals 27 grams therefore, 3 moles of Ba(NO3)2 = 261.34 × 3 = 784.02 and 10 moles of Al = 270 grams
50 To calculate the heat release per gram of reactant, the molecular weight of the reactants need to be calculated first.
Molecular weight of Reactants = 261.34 × 3 +10 × 27
= 1054.02 gms.
In order to compare the composition percentages of the CPM with the Labmix, the stoichiometric ratio of the reactants is calculated as follows;
The percentage composition, by weight, of Al can be calculated from the following;
Weight of Aluminium in the reactant 10× 27 = = 25.62 % Total Molecular weight of the reactants 1054.02
3× 261.34 % composition of Ba(NO3)2 = = 74.38 % 1054.02
In this reaction, Nitrogen gas is formed instead of Nitrogen dioxide (NO2), due to the high initial temperature (> 550 0C) and large heat release. The heat released by this reaction is calculated by using the following relationship
Δ = ΣΔ − ΣΔ H R H f ( products) H f (reac tants) where
Δ H R = Heat released/absorbed for the entire reaction
ΣΔ H f ( products) = Summation of the heat of formation of the products
ΣΔ H f (reac tan ts) = Summation of the heat of formation of the reactants
Therefore using Table 7 for inputs,
ΔH = (−548.1× 3 −1675.7 × 5)KJ = −10,022.8KJ f ( product)
ΔH = −992× 3KJ = −2976KJ f ( reac tan t)
Δ = − H R 7046.8KJ
51 Negative sign indicates that the reaction is exothermic (heat released)
− 7046.8 Energy released per gram of reactant = 1054.02
= - 6.6856 KJ/gm.
= - 1.6 Kcal/gm.
The heat release above was calculated for 1 gram of the reactants. The actual weight of the CPM in the test charge was 0.267 grams. Therefore the actual heat release for 0.267 grams of the mixture is computed below.
= 0.267gms × (-1.6) Kcal/gm
= -0.4272 Kcal
Again, a negative sign indicates heat is released from the reaction or exothermic reaction
4.2.2 Heat Release from test charge made of Labmix
For the Labmix composition with a component ratio as given below,
Ba(NO3)2 = 50%
Al = 24%
Mg = 24%
Neglecting the 2% accounting towards the binder composition, the heat release calculations were performed as follows:
For 1 gm of the reactants the various constituents in grams
Barium Nitrate is 0.5 gms,
Aluminum is 0.24 gms and
Magnesium is 0.24 gms.
Since,
1 mol of Barium Nitrate consists of 261.34 gms,
52 1 gm consists of 1/261.34 mols of barium nitrate or
0.5 gms of barium nitrate consists of 0.5/261.34 mols = 0.0019132 mols of barium nitrate
Similarly,
0.24 gms of Al consist of 0.24/27 mols = 0.0088889 mols and
0.24 gms of Mg consist of 0.24/24.31 mols = 0.0098725 mols respectively.
Balancing the moles in the reaction, we get
0.0019132Ba(NO3)2 + 0.0088889Al + 0.0098725Mg + 0.0068198O2 Æ 0.0019132BaO +
0.0044444Al2O3 + 0.0098725MgO + 0.0019132N2 ↑ where atmospheric oxygen has been used to balance the reactant side, i.e., the reactant side is rich and hence atmospheric oxygen is being consumed by the reactants for complete oxidation.
Heat of the reaction can be calculated by
ΔH = (−548.1× 0.0019132 −1675.7 × 0.0044444 − 601.6 × 0.0098725)Kj f ( Pr oduct)
ΔH = −992 × 0.0019132Kj f ( Reac tan t)
Δ Hence H R = - 12.5375 Kj/gram of reactants
Δ Or H R = - 2.9965 Kcal/gram
The heat release above was calculated for 1 gram of the reactants, neglecting the 2% weight of the binder in the calculations. The actual weight of the incendiary material in the API as obtained from the USAF was 0.972 grams. The heat release calculations for this are as follows:
0.972 × 0.98 = 0.95256 gms = actual weight of the reacting compounds for which the heat release is calculated, since the weight of the binder is 2% of the API
Therefore the actual heat release for 0.972 gms of the mixture
= 0.95256gms × (-2.9965) Kcal/gm
= -2.854 Kcal
53 Again a negative sign indicates heat is released from the reaction or exothermic reaction.
The weight of the Labmix composition, which is similar to the API, was 0.267 gm. The actual heat release is calculated as follows:
0.267 × 0.98 = 0.2616 grams = actual weight of the reacting components - weight of the binder.
Therefore, the actual heat release for 0.267 gms of the mixture
= 0.2616gms × (-2.9965) Kcal/gm
= -0.784 Kcal
4.2.3 Heat Release from Heating element
The amount of heat generated in the Nichrome wire to ignite the electric match was calculated as follows.
Length of Nichrome wire = 2 inches or 50.8mm
Diameter = 0.16mm
Resistance = 2.73 ohm.
A DC power supply was used to energize the heating element. The heating element or resistance was connected across the 5 Volt supply through which current of 1.5 amp was passed. The output power is given by
P = I2 × R where, P = Power dissipated in the resistor in Watts (W)
I = Current flowing in the resistor in Amperes (A)
R = Resistance of heating element in Ohms ( Ω )
Hence the power dissipated was
P = 1.52 × 2.73 = 6.1425 W
Or
54 P = 6.1425 Joules/sec
However, this is the total heat lost over the entire length of the resistor or heating coil. Since only a length of 8mm of the heating coil was present inside the electric match composition, the power dissipated needed to be calculated for this portion of the element.
Pmatch = (6.1425× 8)/50.8 = 0.9673 Joules/sec
Or
Pmatch = 0.2312 Calories/sec
Since the exact products of the reaction between the components of the match composition could not be found, the theoretical heat release for the match composition could not be calculated.
4.3 Sparkler temperature profile analysis
In the initial phase of the experiment, tests were performed on commercially available sparklers to study the feasibility of investigating the temperature profile around an intense heat source. The diameter of the test sparkler was 2.8mm and the length was reduced from 150mm to 40mm. A typical temperature profile taken at a distance of 1.2mm from the surface of the test charge in the horizontal axis is shown in Figure 4.2. The voltage generated by the thermocouple was transmitted via an A/D converter to the computer. This was converted into temperature using
LabVIEW. Data was recorded continuously at a sampling frequency of 100 Hz for 15 seconds, as the flame passed across the thermocouple bead. The capture generated 1500 data points. A steady rise in temperature was observed as the flame approached the bead. The maximum temperature was recorded at the closest distance between the flame and the thermocouple bead.
A typical temperature profile measured at 1.2mm from surface of a 2.8mm diameter burning test piece along the x axis can be seen in Figure 4.2. The temperature, due to the moving around of the flame, seemed to exhibit a pulsating behavior around the maximum region and also as the
55 flame moved away from the thermocouple. Also, it can be seen that the temperature rise to the maximum is faster than the temperature drop.
1400 1200 1000 800 600 400
Temperature, deg C deg Temperature, 200 0 0246810121416 Time, seconds
Figure 4.2: Typical temperature profile
An explanation for the pulsating behavior of pyrotechnic systems was given by Wasmann [39].
The major factors affecting pulsating behavior are,
1. The competition between the various chemical reactions occurring in the composition
2. Physical factors like heat flow, heat accumulation, intermittent vaporization processes in
the test composition
Pulsating behavior also occurs as a combination of the above mentioned factors. The pulsating burning behavior occurring as a result of competition in the chemical reaction can be explained.
An alternate between a dark cycle and a light cycle occurs when the composition has two different fuels. The fast reacting fuel reacts with the oxidizer without much energy release in the form of light to form the dark cycle. The light cycle is caused due to the large energy release, including light emission, from the oxidation of the slow reacting fuel. The rate of oxidation of the slow reacting component increases rapidly after the dark cycle due to the increase in the
56 reaction surface area, and also due to the presence of oxidative gases from the dark cycle, trapped in the micro-porous structure of the surface. This dual cycle process seems to cause the pulsations in reactions with multiple components, with variable rate of reactions, as fuel. In the case of sparklers, though a clear dark and light zones was not observed, the intensity of the emitted light could be seen to vary. The pulsating behavior is observed as a change in emitted light intensity. In the chemical composition of the sparkler, the effect of the minor components has not been considered. These minor components can affect the reaction rate by reacting with the oxidizer, hence resulting in the down phase of the pulsation cycle.
Physical properties like viscosity and volatility can also affect the burning process to create a pulsation effect. Gol’binder et al [40] explained the physical processes involved when an explosive composition with components of different volatility was heated. In the case of the sparkler, since there are no volatile components involved, the properties like viscosity and volatility do not affect the pulsating behavior.
Assuming that the flame is stationary, the temperature that the bead measures is equal to the maximum temperature that is recorded from the moving flame. Hence, the maximum temperature at successive intervals from the surface was required for each test. A plot of the maximum temperatures at different intervals from the surface of the 2.8mm diameter test piece along the x axis is shown in Figure 4.3. Measurements were plotted for a range of 1.15mm to
2.4mm in increments of 0.05mm along the x axis. Also measurements were plotted, along the +/- z axis, for a range of 1.2 to 2.5mm in increments of 0.1mm. The maximum temperatures along the +/-z axis can be seen in the Figure 4.4.
57 1500 Measured 1400 Trendline 1300 1200 1100 1000 900 800 700 Max. temperature, deg C deg temperature, Max. 600 11.522.5 Distance from surface of test sample, mm
Figure 4.3: Temperature profile in the x axis
1500 1400 1300 1200 1100 1000
900 +z axis 800 -z axis +z axis Trendline 700
Max. temperature, deg C temperature, Max. -z axis Trendline 600 11.522.5 Distance from surface of test sample, mm
Figure 4.4: Temperature profile in the +/-z axis
From Figure 4.4, it can be observed that a drastic drop in temperature occurs between 1.2mm to
2.5mm in both the z and x axis. Comparing the curve-fitted temperature profiles in the x and z axis as shown in Figure 4.5, it can be seen that the drop in the temperature along the x axis is
58 much higher than that in the z axis. Also, the profile suggests that the drop in the +z axis is less than that in the –z axis.
1500 Horizontal 1400 Upwards 1300 Downwards 1200 1100 1000 900 800 700
Max. temperature, deg C C deg temperature, Max. 600 11.522.5 Distance from surface of test sample, mm
Figure 4.5: Comparison of trend lines
The following factors could explain this seemingly different behavior:
1. In the region very close to the surface of the test piece, the jet-like ejection of particles from
the burning composition could overshadow the natural convection in the lower region that
would occur if there were no such ejection. This would reduce the differences in the
temperature profile, along the +z and –z axis, that occur as a result of natural convection.
2. In the reaction zone, as the solids melt, there is a possibility of a shift in position downwards
due to their weight before complete combustion can occur. This can result in a relatively
higher temperature in the –z axis. This effect becomes pronounced when the linear burning
speed, of the reaction zone moved along the length of the test piece, is low. This behavior
cannot be observed for the Labmix with a burn speed of 12mm/sec but can be observed for
the CPM and sparkler with a burn speed of 2mm/sec.
3. Effect of convection of the surrounding air
59 Using K-type thermocouples, within a distance of 1.1mm from the surface of the 2.8mm diameter test piece, temperatures data was not attainable due to melting of the thermocouples at around 1415 0C. The temperature measurements within this range could only be measured by R and C type thermocouples, which have higher measuring range.
4.4 CPM profile analysis
The same set of measurements was repeated for a test piece of 4mm diameter and 15mm in length. Temperature measurements were taken along the x and z axis as performed earlier. In this case, the burnout of the K-type thermocouples occurred at around 2mm from the surface of the test piece, which is much farther down the axis, compared to the 2.8mm diameter test piece.
Figure 4.6 shows the profile around the 4mm diameter test species. It can be observed, near the surface, the profile in the –z axis is higher than that in the +z axis. This follows a trend similar to that of the 2.8mm diameter test species. The hypothesized reasons for this type of behavior are assumed to be similar to those of discussed for the 2.8mm diameter.
+z axis 1600 x axis -z axis 1400 Power (+z axis) Power (x axis) 1200 Power (-z axis) 1000 800 600 400 200 0 Max. temperature, deg C 1.8 2.4 3 3.6 4.2 4.8 5.4 Distance from surface, mm
Figure 4.6: Temperature distribution for CPM
60 The primary reason for the scatter in the data was due to the continuous pulsations in the flame when the premixed composition burned. The reasons for pulsation were discussed earlier. The other factors that cause scattering could be derived from the following initial assumptions:
• Each test piece has the same density/composition
• Burning is uniform – following a ring like pattern along the length of the test piece
• Test piece is unaffected by external conditions
An explanation of the assumptions is offered below,
While molding the test piece, care was taken to maintain the weight of each test piece within +/-
2% error. While compacting the composition into the mold shell, the dimensions were maintained with an error of +/- 2%, so as to minimize the final variations in density.
The ignition of the main charge was assumed to be transmitted instantly from the initiator
(electric match). It was also assumed that the flame uniformly propagates the thermal energy across the entire cross-section of contact with the main charge. This thermal energy is sufficient to ignite the material in the contact area and generate sufficient energy to produce flame propagation. A ring like burning pattern was assumed. If the flame does not cover the entire cross-section, an irregular burning pattern results. The difference in the surface burning speed and the burning speed in the radial direction of the test piece also causes deviations from the ring like burning pattern.
Due to the controlled nature of the experiment, ambient conditions, such as variations in temperature and forced convection, were assumed to be absent.
4.5 Labmix profile analysis
The above procedure of temperature measurement was repeated for a composition similar to the armor piercing incendiary (API) composition, named Labmix, as defined in Table 4. The heat
61 release of this new mixture was approximately twice that of the CPM as shown in the calculations done in section 4.2. The dimensions of the test piece used for this composition were the same as that of the 4mm diameter test piece used earlier.
In this case, the melting of the K-type thermocouples occurred at around 20mm from the surface of the test piece in the x axis. This burnout distance also differed in the z axis. A drop in temperature, similar to that observed in the CPM, with increasing distance can be observed in
Figure 4.7 for the Labmix of diameter 4mm.
1400 +z axis x axis 1200 -z axis Power (-z axis) Power (x axis) 1000 Power (+z axis)
800
600
400
Max. temperature, deg C 200
0 19 21 23 25 27 29 31 Distance from test charge surface, mm
Figure 4.7: Temperature distribution for Labmix
From Figure 4.7, it can be seen that the temperature distributions are different from that of the
CPM. Less scatter can be observed for this temperature profile. The temperature profile for this composition seems to follow a predictable trend according to the laws of natural convection.
Also, the temperature along the +z axis has a higher value than that along the –z axis. The temperature distributions at the top and the bottom of the charge are different, with the temperature at the top being approximately 350 0C higher. Importantly, for both the
62 compositions, a drastic drop in temperature was observed as the thermocouple was moved away from the surface of the test charge. This showed that the temperature inside the uniformly luminous fireball is not the same but drops drastically towards the periphery of the fireball. It provides evidence that the heat release rate has a significant effect on the temperature distribution, even when the two compounds have similar chemical proportions.
4.6 Comparison of profiles
The heat released by both the CPM and Labmix are compared to that of the API in Table 9. It is clear that the heat released by the Labmix is the same as that for the API since the composition used for the Labmix was the same as that of the API. The heat release calculations for the CPM and the Labmix can be found in sections 4.2.1 and 4.2.2.
Chemical Formulae API CPM Labmix Barium Nitrate Ba(NO3)2 50% 74.38% 50% Aluminum Al 24% 25.62% 24% Magnesium Mg 24% - 24% Binder Dextrin 2% - 2% Heat Release (Kcal/gm) 2.99 1.6 2.99 Table 9: Comparison of composition and heat release
The CPM has a heat release value of 1.6 Kcal/gm which is approximately half that of the API heat release. This value was obtained by calculating the heat release using equation (1). The presence of additives in the CPM, would still reduce the heat release value. A comparison of temperature variations for different orientations is given in Table 10.
63 Sparkler, 2.8mm CPM, 4mm Labmix, 4mm
diameter diameter diameter +z axis 200 590 580 Temperature drop, x axis 600 940 680 degree C -z axis 300 750 620 Distance, mm 1.3 3.2 10
Table 10: Temperature variation with distance
From the table, it can be inferred that the maximum drop in temperature for all the three sets of experiments occurs along the x axis. The minimum drop is observed in the +z axis. The reason can be attributed to the flow of gases, due to natural convection, in the +z axis. The temperature drop in the –z axis is between the temperature drop values in the other two directions. The reason for the relatively lower temperature drop in the –z axis can attributed to interference with natural convection from jet-like ejections.
64 5 IMAGING
5.1 Development of the procedure
As seen earlier, temperature measurements clearly indicated a large drop in temperature over a very short distance from the surface of the test charge. Though this data was sufficient to prove the observed flash was not of uniform temperature throughout, a qualitative way was sought.
Also, a visual interpretation of the spatial temperature gradient over the observed flash region was required. Video recordings of the burning test piece were used to observe the size of the flash region. The brightness of the observed flash region saturated the recording medium and a uniform ball of fire was captured by the camera. This high intensity light, generated in the exothermic reaction of the premixed chemicals in the main charge, is called “white out”. Due to this, not enough information was available about the extent of the reaction zone. Temperature profile around a reaction zone was required to be studied. A burning object whose reaction zone can be clearly observed was a candle. The observed flame in a candle or other sources is the region where the combustion or reactions between different species occur. Smyth et al [41] has shown the temperature distribution away from the surface of a diffusion flame. A steep fall in temperature could be observed at a very short distance away from the flame surface or reaction zone. Since a drastic drop in the temperature profile in our set of experiments was observed, the presence of a reaction zone was expected in the former.
The use of neutral density filters was considered to gain information about the reaction zone. This reduced the overall intensity of light recorded by the camera. The observed region became smaller, and there was no clear distinction of the extent of the reaction zone. The use of color filters with different regions of peak wavelengths and transmission intensities was considered. The color filters were arranged in front of the camera to observe the burning test
65 charge. This gave sufficient qualitative evidence to support the temperature measurements observed earlier. The results are discussed in section 5.4. The following sections give brief discussions of the equipment and theory behind the color filter experiments.
5.2 Equipment for setup
In order to provide further information on the extent of the spectral region in a pyrotechnic event, image analysis was utilized with the aid of specific spectral filters. The combustion process was recorded using the 3-CCD Panasonic WVF250B series NTSC color video camera and displayed on a Panasonic CT-1331Y color monitor and then recorded using a Panasonic AG-7750 SVHS recorder. The images are transferred to the computer through the Adaptec VideOH USB device.
The visual data was captured using the associated software at 30fps. The rate of burning and flash intensity was observed from this data. The video camera had a shutter speed of 10ms and f- stop – 2.8. Neutral density filters built in the camera were used in conjunction with color filters to overcome the saturation of the recording medium due to the high intensity flash. The built in neutral density filters had a 12.5% reduction in intensity.
Figure 5.1: Color Filter properties
66 Figure 5.1 shows the sharp cut-off in the color filter in the spectral range of 580 – 610 nm used to distinguish the reaction zone. The color filter had a cut off 95% of the visible radiation below a wavelength of 600 nm. This resulted in an 80% transmission reduction over the wavelength range.
5.3 Imaging Theory
For a given set of reactants, an initial amount of energy called activation energy is required to initiate the reaction. In the excited state, a reaction will occur to form the anticipated products, with the liberation or absorption of energy. With respect to the energy released or absorbed, the reactions are classified as exergonic or endergonic reactions. The net energy released or absorbed is due to the change in the chemical composition of the reacting compounds. This released energy is primarily in the form of thermal radiation. The range of thermal radiation in the electromagnetic spectrum is from 0.1 micrometers to 100 micrometers. The visible region has a wavelength range from 0.4 to 0.7 micrometers. If the emitted energy has a wavelength in this region, then the burn of the test charge is observed as a bright flame. The emitted light could result due to two factors.
1. Energy released from excited electrons, in an energized atom, falling back to the ground state
2. Blackbody radiation emitted from products of combustion present in the high temperature
region
These factors are discussed below;
Standard flame tests are conducted for the identification of a metal in a compound by the release of characteristic colors. For an element to have a characteristic color in a flame test, it must have an exited electron that emits radiation in the visible part of the electromagnetic spectrum. This spectrum of radiation arising from the electron transitions within an atom is termed as atomic
67 spectrum. In the case of the pyrotechnic composition, Labmix or CPM, the fuel used was a combination of Aluminum and Magnesium powder. For both Aluminum and Magnesium, the flame test does not give any characteristic color. Aluminum and Magnesium ions are involved in the flame tests rather than their atoms, since their salt solutions are used for performing these tests. The atomic emission spectra of these ions are not present in the visible region of electromagnetic spectrum. The atomic spectrum for fuel atoms or ions could not be the reason for the bright flash from the burn of the test charge.
Faraday [42], in his lectures, had indicated that the presence of solid particles in a heat releasing reaction zone is the primary reason for the generation of light. From his experiments, he had demonstrated that the unburned carbon particles or soot become incandescent due to the heat from the reaction zone. This can also be seen from the pictures of the candle flame burning in microgravity [44]. In microgravity, the candle flame burns with a dim blue color unlike the bright yellow flame burning in normal gravity. The possible explanations given for this occurrence were
1. The soot had not formed because of the low temperature of the flame
2. Soot existed but was not luminous due to the low temperature of the flame
Faraday [42] also explained that the solid particles in the region of high temperature could be produced either during the reaction or could be premixed along with the reactants. The retainment of the solid state by these substances not involved in the reaction process was the primary cause for the release of light from the reaction. He also showed that a highly exothermic reaction (e.g., between Hydrogen and Oxygen) need not emit intense light.
Since the reaction between the fuel and oxidizer is exothermic, a large amount of heat is released along with the products of combustion. The particles of the products of combustion
68 present in this high temperature region act as near black bodies and emit radiation. Even before the final products are created, the short-lived species or particles from intermediate reactions start emitting black body radiation due to the high temperatures generated. Energy released as black body radiation is distributed over the entire visible wavelength range. This spectrum in which the radiation is distributed over a continuous range of wavelengths is known as the continuous spectrum. The color observed from this emitted radiation for a particular temperature is the average value of color in the visible region of energy within the radiation curve. At higher temperatures, this color becomes white due to the presence of the visible region of radiation inside the emitted wavelength range. The products of combustion emitting electromagnetic radiation do not behave like black bodies but rather like grey bodies. Since the particles emitted in the intermediate reactions are not known for both the CPM and the Labmix, their emissivity could not be calculated.
The products of combustion are at a very high temperature due to the continuous generation of heat, within the flame. Once the products leave the region of flame, they continue to travel away from the test charge with their temperature dropping rapidly. Outside the flame region, the intensity and temperature of the radiation is less and hence, the radiation curve outside the flame region is lower than that of the flame region. To better display the results, a filter cutting off the low intensity regions and only allowing the higher intensity radiation to pass through was desired.
5.4 Image Analysis
To assess the ability of this filter to provide a clear definition of the ignition process from a CPM charge, it was necessary to establish how the filter would reduce the area of a flame event. This was achieved by imaging a candle flame with and without the filter present. The images of the
69 candle flame with the temperature distribution with and without color filters are shown in Figure
5.2 and Figure 5.3 respectively.
Figure 5.2: Temperature distribution across candle flame w/o color filter
Figure 5.3: Temperature distribution across candle flame with color filter
70 This experiment demonstrated the ability of this filter to record the extent of the flame in both the radial and axial directions within acceptable limits. With the filter present, a reduction in width was 12% and in height, was 7%. The flame is three-dimensional and the outer edges of the flame will be considerably weaker than the internal structure of the flame.
Figure 5.4: Burning test piece made of CPM
A frame-by-frame analysis of the video recording of the burning test piece made from the
CPM was accomplished. The images indicate a region of constant brightness or saturation around the burn area as shown in Figure 5.4. This suggests the burning process produces a large fireball with uniformly high temperature throughout.
However, as the burning composition was premixed and molded into a region much smaller than the fireball region, the reaction zone of the propagating flame in the test piece must occupy an area equal to or slightly larger than the cross sectional area of the test piece. This region of bright light saturates the recording medium and hence very little information can be attained regarding the reaction zone from the Figure 5.4, and its uniformity.
Application of the color filter resulted in most of the wavelengths of visible light to be removed. Figure 5.5 shows an area of red light indicated by the outer circle, which is the region
71 of the bright white light of Figure 5.4, and an inner region of the reaction zone. Recall, the reaction zone is the region where high temperatures are reached and is visible as flame.
Figure 5.5: Burning test piece made of CPM with filter
A question arose regarding the influence of the smoke emitted, during the burning of the test charge, on the captured image. The tests conducted by Blunt [37] on different pyrotechnic compositions, clearly showed that the dominant wavelength emitted for each test was not affected by the emitted smoke. Less than 0.5% variation in dominant wavelength was observed for the various test compositions.
As different images of the burning process were examined, it was found that the images taken with the application of the above filter showed the flash region varied in size due to previously explained pulsations. The size of the inner flame region also varied. The diameters of the flame and flash regions were calculated using GIMP (GNU Image Manipulation Program); a versatile image processing software. The image observed in Figure 5.4 is that of the intense flash region using CPM as the test charge is of approximately 40mm diameter. Figure 5.5 shows that the actual flame diameter, as observed through the specified filter, to be approximately 8mm.
72 Figure 5.6 indicates the spatial variation in diameters of the flash region with that of the flame region follows a linear trend.
70
60
50
40
30
Light Diameter, mm Diameter, Light 20
10
0 45678910111213 Flame Diameter, mm
Figure 5.6: Comparison of diameters for test charge made of CPM
Region of constant brightness
Figure 5.7: Test piece made of Labmix w/o filter
73
Region of flame
Region of constant brightness
Figure 5.8: Test piece made of Labmix with filter
Image analyses of tests were conducted on charges made from Labmix. The filter as described above was used. The results are shown in Figure 5.7 and Figure 5.8.
300
250
200
150
100
50
Observed Flash Diameter, mm 0 0 102030405060 Flame Diameter, mm
Figure 5.9: Comparison of flame and light regions for test charge made of Labmix
74 From the image analysis, a relationship was derived between the flame diameter to that of the flash diameter of the Labmix, as shown in Figure 5.9. In this case, there appears to be, to 1st order approximation, a near linear relationship of flash to flame ratio of about 5:1. This showed a clear distinction between the high temperature region and the low temperature region inside the fireball, initially assumed to be uniform. This result helped in qualitatively supporting the temperature measurements conducted in the previous section.
75 6 FUEL POOL IGNITION STUDIES
6.1 Development of the Procedure
The ability of a pyrotechnic device to ignite and sustain a fire over a fuel pool or a fuel storage area is not well understood. The purpose of conducting this experiment was to qualitatively and quantitatively study the effect of an API burning over a pool of fuel. This series of tests were devised to provide answers to a range of questions. For example,
1. How does the height of charge above a fuel pool affect ignition and sustainment?
2. How does the temperature of the fuel pool affect ignition and sustainment?
3. Does ignition of the fuel pool occur due to the entire flash region or the reaction zone?
In order for this study to be undertaken, a simple set-up was considered. The charge was set at some height (H) above a small pool of Kerosene. The test piece, made of Labmix, was set up such that its long axis was in line with the viewing axis of the camera. The lower surface of the horizontally suspended test piece was kept at a fixed height from the center of fuel pool.
Kerosene was chosen as fuel as it was readily available and also safer to use in the laboratory than other aviation fuels. Kerosene was filled up to 2mm deep in a 150mm diameter container.
This arrangement is shown in Figure 6.1.
TEST CHARGE
CAMERA H
FUEL POOL STAND
Figure 6.1: Sketch of a fuel pool setup
76 Electric match was used to ignite the test piece and the burning process above the fuel pool was recorded using the camera. This process was repeated for several heights of the fuel pool below the test piece. The fuel pool was maintained at the ambient temperature of 25 0C for all the experiments.
Initially, the test piece was placed at three different heights of 178mm (7inches), 90mm
(3.5inches) and 25mm (1 inch) above the fuel pool and the combustion was recorded using the video camera. For the 7-inch height, no reaction or flash was observed from the fuel pool. A similar result was obtained for the 3.5-inch height. But the 1-inch height resulted in a flame from the surface of the fuel pool that burned for a very short duration of 0.2 seconds.
To obtain more accurate results, this experiment was again performed with variations in the height of the test charge in steps of 10mm. Also, each experiment was repeated 6 times to check for repeatability of the obtained results. Experiments in which the fuel pool temperature was varied were conducted to see the variation in ignition height.
6.2 Experimental Set up
The experimental setup used for the fuel pool ignition tests is described below. The setup includes the fuel, test charge, electric ignition system and the hot plate, discussed as follows:
6.2.1 Fuel
Dyed kerosene was used as the fuel, which was filled up to height of 1.4mm in a 150mm diameter container. Kerosene was chosen as it was readily available and also safe to use in laboratory conditions.
6.2.2 Test Charge
The 4mm diameter test charge was used for the fuel pool ignition studies. The manufacturing procedure for the test charge is described in the earlier section.
77 6.2.3 Electric Ignition system
The heating element of the test charge was connected to a power supply that supplied 5 Volt DC at 1.5 amperes and was operated with a push button switch.
6.2.4 Hot Plate
A uniformly heated hot plate was used to heat the fuel vessel to a maximum temperature of 125
0C from the bottom. It was a 200mm diameter 316 stainless steel horizontal disk with a capability of surface temperatures upto 700 0C produced by a 180mm CALROD heating coil embedded within the heater assembly [46].
6.2.5 Arrangement
The test piece was set up such that its axis was in line with the viewing axis of the camera as shown in Figure 6.1. The bottom surface of the horizontally suspended test piece was kept at a fixed height from the center of top surface of the fuel pool. This fuel pool container was placed on the hot plate. The hot plate was preheated to the required set temperature. Electric ignition was used to ignite the test piece and the burning process above the fuel pool was recorded through the camera. This process was repeated for varying heights of the fuel pool below the test piece. The minimum height for ignition of the fuel pool was observed and recorded.
6.3 Experimental Procedure
6.3.1 Fuel pool evaporation rate
The rate of evaporation of fuel varies for fuel pools set at different temperatures. It was expected that providing a heat source (test charge) in the vicinity a fuel pool would increase its rate of evaporation. Hence a change in evaporation rate would indicate that ignition had occurred. The evaporation rates of the fuel for different temperatures were determined experimentally. This was achieved by first measuring the weight of fuel in the container at a given fuel temperature
78 and then recording its change over a time interval. Five fuel temperatures were chosen; 25 0C, 50
0C, 75 0C, 100 0C and 125 0C. All these were below the threshold of Kerosene’s auto ignition temperature of 210 0C [26].
The evaporation rate test was carried out by first heating a hot plate to the required temperature and allowing the entire surface of the plate to reach the set value. Once the plate reached the set temperature, the container with the fuel pool was placed on the hot plate. The problem encountered with the hot plate was that it took a long time for the core temperature of the plate to stabilize at the required temperature.
For the plate at 25 0C, the weight was recorded every ten minutes. For higher temperatures, the recording time interval was reduced as the evaporation rate was high and the amount of fuel involved was less. A problem encountered when measuring the evaporation rate was, at higher temperatures of 50 0C, 75 0C, 100 0C and 125 0C, the fuel pool container temperature dropped whenever it was removed from the hot plate to measure the remaining weight. The temperature drop and the loss of fuel vapor during the fuel measurement time interval resulted in errors in the evaporation rates.
To overcome this problem, the fuel pool evaporation was measured for larger time intervals by restarting the experiment and increasing the time interval for every measurement.
For example, for a plate temperature of 100 0C, initially the fuel pool was placed on the hot plate for 1 minute and the final weight was measured. Then the pool was cooled to ambient conditions and the weight was increased to the original value. Now the pool was placed on the hot plate for
2 minutes and the final weight was measured. This was done for a time interval of up to 6-8 minutes. This prevented the problem of fuel vapor loss and drop in fuel temperature that would occur during the time taken for measuring the fuel weight.
79 A second problem introducing error in the measurement of evaporation rates, involved the time required by the fuel pool to reach the set temperature of the hot plate. Since the fuel pool at ambient conditions was placed on the pre heated hot plate, which was set at the test temperature, there was a time lag before the pool could reach the set test temperature. This time lag was measured by placing a K-type thermocouple in the fuel pool and recording the time for it to reach the test temperature. During this time interval, the temperature of the fuel pool increases from ambient to the set temperature. Due to this the fuel pool evaporation rate increases continuously until it reaches the set temperature. Hence, the amount which vaporized during this time lag only gives the average evaporation rate.
6.3.2 Fuel Ignition Test Process
Pre-Ignition
Once the evaporation rates of the fuel were measured for different temperatures, arrangement was made to perform the fuel pool ignition tests. The fuel temperatures for which the ignition tests were performed were 25 0C, 50 0C, 75 0C, 100 0C, and 125 0C. The hot plate was initially maintained at 25 0C. The test charge was set up above the center of the hot plate. The electric ignition system was connected to the test charge through the stand supporting the test charge.
The weight of the fuel in the container was measured on a weighing scale. This weighed pan was placed on the hot plate and the height of the test charge above the fuel pool surface was adjusted to test-height value using a steel ruler. The fuel pan was removed and allowed to cool to ambient temperature. The fuel container was put back on the weighing scale and the weight was now replenished to the original value. This was performed to prevent the loss of weight due to vaporization of fuel while setting the height of the test charge. In the imaging experiments, the
80 camera was adjusted to capture the burn process and the hardware was lined up such that the video could be digitally streamed directly into the computer.
Ignition
The weighed fuel pan was placed on the preheated hot plate. The fuel pan was left on the hot plate for a period of 150 seconds so that the temperature of the fuel in the pan equalizes with the surface temperature of the hot plate. For the fuel temperature of 25 0C, the aforementioned procedure was relaxed as the fuel temperature was already at ambient. In experiments involving imaging, the video capture was initiated and the electric supply switch was closed to ignite the test charge. The video capture was continued until the test piece had burnt completely and the fuel pool changes observed.
Post Ignition
Once the charge had burned out completely, the fuel pool was observed for ignition. If the fuel pool did not ignite or the ignition was not sustained, then the fuel pan was removed from the hot plate and the weight of fuel measured. If the fuel pool had ignited and sufficient vapor were generated such that the flame sustained, then the flame was extinguished by cutting off the air supply to the fuel pool pan. After two minutes of keeping the pan with air cut off, the fuel in the pan was weighed again. The entire process was repeated for each test height over all the given temperatures to check for repeatability.
6.4 Fuel Properties
6.4.1 Flash Point
The common definition of flash point is stated as the lowest temperature at which the liquid or solid produces sufficient vapor to ignite momentarily on the introduction of a heat source. The
National Fire Protection Association (NFPA) definition of flash point [27] is:
81 “The flash point of a liquid is the minimum temperature at which the liquid gives
off sufficient vapor to form an ignitable mixture (mixture within the flammable
range) with air, near the surface of the liquid or within the test vessel used.”
Flash point is generally measured by two methods, namely, the closed-cup or open-cup method.
A general correlation between the open cup flash point and closed cup flash point was given by the engineering division of the Associated Factory Mutual Fire Insurance Companies [31].
T FP (open cup) = 1.12 T FP (closed cup) + 7.1
For the present test of fuel placed in a pan open to the atmosphere, the flash point by the open cup method was chosen, since the conditions for this method closely match the conditions of the current experiment. The flash point for the dyed kerosene is 100 deg F [26].
6.4.2 Auto Ignition Temperature (AIT)
Auto ignition temperature is the minimum temperature required by a substance to cause self sustained combustion without an external source of heat. Unlike the case with gases, heating fuels in an open pool does not cause ignition. When the temperature is increased, the fuel evaporation rate increases and the fuel is converted to vapor rapidly. When all of the fuel is vaporized, there is no liquid fuel left and hence no vapor for ignition to occur. Hence, raising the temperature of the fuel does not spark auto ignition of liquid fuels. For measuring auto ignition temperature of liquid fuels in open atmospheric conditions, different methods have been established. In all the methods employed [18], the vessel or container was first preheated to the test temperature. Liquid fuel at ambient temperature was then dropped or injected into this hot vessel and observed for flash. AIT is determined by the fuel’s reactivity rather than the volatility as in flash point.
82 6.5 Background
Earlier fuel pool ignition tests have been performed on jet fuels by Atkinson and Eklund [24] to test their ignitability under two different test conditions – an open pool and a shrouded one.
The fuels used were JP – 4 and JP – 5 which were different grades of aviation kerosene. A pilot flame, 10J spark and a hot Nichrome wire were used as ignition sources. From the results of their tests they concluded that
1. The minimum fuel temperature for ignition increases as the height of the energy source
increases.
2. For both types of fuel, the minimum fuel temperature required for ignition is lower for
the open flame igniter than for the spark igniter.
3. The fuel ignition height is higher for JP – 5 than for JP – 4 type of fuel.
The difference in the ignition heights for the two fuels under consideration was primarily due to the difference in the physical properties of the fuels. Due to the lower flash point, lower specific gravity and higher vapor pressure, JP – 4 ignited at a lower height compared to the JP -5. In general, the conditions required for a liquid to ignite are [18]:
1. Evaporation of the fuel must occur first. It is accepted that fuel in the liquid state cannot
undergo ignition. Combustion occurs only when the vapors of the fuel are oxidized and
not the liquid fuel.
2. Fuel vapor must be present above the liquid in sufficient concentration.
3. Oxidizer must be mixed with the fuel in adequate concentration.
4. A localized heat source is required in the region of this fuel concentration.
For the present test conditions, with the fuel open to the atmosphere, a concentration gradient of the fuel vapor was present above the fuel surface. Experiments conducted by Hirano et al [25]
83 using methanol as fuel and Ishida et al [32] using octane as fuel showed that, for open fuel pools, fuel concentrations with an equivalence ratio of approximately one exists only in a narrow boundary layer above the fuel surface. It was also shown that the concentration of fuel vapor decreases as the height above the fuel surface increases.
The fuel vapor concentration values must be within certain limits for ignition to occur. These are known as the lower flammability limit (LFL) and upper flammability limit (UFL). The values are generally expressed as percentage of fuel, by volume, in air. For Kerosene, this range is between 0.7% and 5% [26]. The igniter is required to be present in this region of flammability for ignition to occur. If the igniter is placed very close to the surface of the fuel pool, most of the igniter energy may be transferred to the fuel pool to heat the liquid fuel. The minimum distance an igniter could be placed, above the fuel pool surface, within which ignition does not occur is termed as quench layer height [29].
Considering again the experiments performed by Atkinson and Eklund [24], their results indicated that the minimum fuel temperature required for ignition varies with the type and energy of igniter. The fuel temperature for ignition depends on the energy released from an igniter. The minimum energy required to ignite the fuel vapor between the flammability limit is known as the minimum ignition energy (MIE). The MIE in turn depends on the droplet diameter of the fuel.
Burgoyne et al [33] showed that the combustion behavior of aerosols is identical to the vapor of that substance if the droplet diameter is less than 10 microns. Hence flash point tests for a particular fuel depended on the igniter height and also the igniter energy [28].
The extensive review on ignition and flame spread by Ross [29] summarizes that the flash point measured for each fuel should include the height and ignition energies of the igniter. Also, the igniter should not be a pilot flame as the high energy involved can change the fuel surface
84 temperature. But, in the present experiments, the fuel was ignited by a pyrotechnic device which was a source of high energy.
As implied by the flammability limits described earlier, the burning test charge must ignite the fuel vapor at a specific height range, where the fuel concentration is within the LFL and
UFL. Accordingly, if the fuel vapor concentration is higher than its UFL then ignition must not occur. The results of the present experiment do not indicate any effect due to the UFL, as ignition was seen to occur at heights very close to the surface of the fuel pool. This is further discussed at the end of this section
6.6 Results and Discussion
Initially, the experiment was performed with the fuel pool at ambient conditions and the test charge suspended above the pool at three different heights. For the 178mm and 90mm heights, no combustion was observed. For a height of 25mm from the surface of the pool, the fuel vapor suspended above the pool was ignited for a very short interval of time (6 video frames). The sequence of these frames is shown in Figure 6.2 and Figure 6.3. Individual pictures in a given figure were named as follows. In a figure with 6 pictures the naming pattern was a b c d e f
And in a figure with 4 pictures the naming pattern was a b c d
85
Figure 6.2: Ignition at a height of 90mm
Figure 6.3: Ignition at a height of 25mm
In Figure 6.2 a-e, the fireball of the test charge grows in size to a maximum and then decays.
Figure 6.2 f shows the frame when the test charge has just burned out. It can be seen that ignition of the fuel pool had not occurred. In Figure 6.3 a-d, the fireball of the test charge grows in size to a maximum and then decays. Pictures e and f show that the fuel pool had ignited. The heat generated by this short duration vapor flame was not sufficient to propagate to the fuel pool to sustain the burn. The short duration (0.2 seconds) of the flame above the pool was due to the
86 insufficient amount of heat energy released by the flame to vaporize the fuel pool for continuous burning.
The fuel ignition test was performed for different fuel pool temperatures, and for each temperature the test piece was suspended at various heights from the surface of the fuel pool and electrically ignited. The test piece burned for a period of around 1 second (28-32frames) which was visualized using the CCD Camera.
It was also observed that for the various heights from which the test piece was fired, the observed fireball seemed to touch or even engulf the fuel pool surface for each case. This showed that the seemingly hot fireball does not have sufficient heat energy to transfer to the fuel pool or the temperature near the circumference of the fireball was not high enough. As seen in the earlier experiments conducted to measure the temperature inside the fireball, a continuous drop in temperature was observed as the distance increased from the core of the fireball.
The fuel temperature of the hot plate for the first set of temperatures was 25 0C. The test charge was set up at a height of 80mm from the surface of the fuel pool. Since no ignition was observed, the test charge was set closer to the pool in steps of 10mm each test run. At a height of
30mm above the pool, it was observed that the fuel pool was momentarily ignited as shown in
Figure 6.4. Notably, the test temperature is less than the flash point of the fuel in consideration.
For fuel pools at a temperature lower than their flash point, neither ignition nor flashing should occur on the introduction of an igniter [18]. This is assumed to be true even on the introduction of a large heat source to the fuel surface. The reason for this involves thermally induced fluid motions in the fuel pool. On heating the fuel pool surface locally by an igniter, a convective current is set up in the fuel pool [29]. This is due to the presence of lower fuel temperature in the peripheral region of the fuel pool. This liquid-phase convection slows down the process of
87 ignition by removing heat from the igniter. Also the gas-phase flow field is affected by this liquid-phase convection [36]. Thermocapillary and buoyancy forces are the major driving forces for the liquid phase convection to occur.
The fuel pool depth in the vessel of our experimental set up was 1.4mm. In thin layers of liquid, the possibility of ignition was reduced due to,
1. Thermocapillary-driven flow that deformed the fuel film under the igniter [34].
2. The heat lost through the bottom of the vessel [35].
Though the process of ignition is affected by the liquid phase convection, ignition itself would occur only at the gas-phase above the fuel surface. Buoyancy in the gas-phase convected the heat away from the igniter, but it had a counter effect of bringing in the fuel vapor towards the igniter. Convective air flow above the fuel pool surface in a quiescent atmosphere is caused in two ways [23]:
1. Transfer of heat to the layer of air above the hot fuel pool surface causing the air to
convect upward
2. Convective flow around an ignition source such as an open flame causes the air below the
fuel source to flow over the fuel pool surface and rise upwards.
On ignition, the flame propagates along the surface of the pool and downwards towards the fuel pool surface.
As it can be seen from Figure 6.4a - d, figure c and d show that the vapor above the fuel pool gets ignited for a short interval of time. In this case, the charge appears to have completely engulfed the fuel and its container throughout the period. However, the actual fireball emanating from the charge was not close enough to provide sufficient heat for the pool to maintain a
88 significant fuel evaporation rate sustaining a fuel burn scenario. Although the fuel vapor above the pool did ignite, this extinguished once the initial vapor had been consumed.
Figure 6.4: Ignition at a height of 30mm and fuel pool at 25 0C
The experiment was repeated for the same height to confirm the observation of momentary ignition. Also, the experiment for a test charge height of 40mm was repeated several times to ensure that there was no ignition at that height.
This led to the question about the height at which the fuel pool will sustain ignition for a particular fuel temperature. For the present set of experiments, the criterion for sustained ignition was continued burning after the initial ignition. The burning was considered to have sustained if continued burning occurred resulting in the consumption of the entire fuel pool. Since the previously conducted experiments indicated ephemeral ignition, the height of the test charge was taken closer to the surface of the fuel pool to check for flame sustainment. This experiment was
89 performed for a pool temperature of 25 0C with the test charge at a height of 20mm from the pool surface. It was observed that the fuel pool ignited and also the resultant flame continued to burn as shown in Figure 6.5.
Figure 6.5: Ignition at a height of 20mm and fuel pool at 25 0C
Once again, the incendiary flash enveloped the fuel and its container. However, this time the charge was able to transfer sufficient heat to ignite the fuel vapor at an earlier stage in its development. Also, it was able to transfer heat energy to a larger area of the fuel pool due to proximity of the test charge to the pool surface. This assisted in evaporating additional fuel from the surface to assist in the sustenance of the fire. This test was repeated to confirm the sustainment of the flame on the fuel pool at 20mm for a pool temperature of 25 0C and check for repeatability.
Further, experiments were performed to check the sustainment of the fuel pool for a test charge height of 25mm. It was observed that flame sustainment did not occur for this height. It
90 could be derived from the above set of experiments that for a fuel pool temperature of 25 0C ignition occurs at a height less than 40mm and ignition along with flame sustainment occurs at a height less than 25mm. This behavior by the fuel pool film seemed to agree with the conclusions of Armendariz and Matalon [45]. When the heat released to the fuel pool is less or when the igniter is away from the surface of the fuel pool, the thermocapillary effects on the surface are dominant. This results in a depression caused on the top surface of the fuel pool which eventually causes the fuel pool to rupture. When the heat released from the igniter is sufficiently high, as in the case of the test charge, the film surface gains heat from the gas phase and the thermocapillary forces help in stabilizing the film.
The second set of experiments was performed for a new fuel pool temperature of 50 0C.
Beginning with a height of 80mm, the experiments were performed similar to the earlier ones at
25 0C. It can be seen from Figure 6.6, that at a test charge height of 30mm from the surface of the fuel pool, ignition occurred but did not sustain. On further decreasing the height, it was observed that ignition leading to flame sustainment occurred at a height of 25mm from the pool surface for this temperature (Figure 6.7). Hence, the height for ignition to occur at this fuel temperature is anywhere less than 40mm and the height for sustained burning is anywhere less than 30mm.
For a fuel temperature of 75 0C, the ignition height and flame sustainment height were less than 50mm and 40mm respectively (Figure 6.8 and Figure 6.9).
91
Figure 6.6: Ignition at a height of 30 mm and fuel pool at 50 0C
Figure 6.7: Ignition at a height of 25mm and fuel pool at 50 0C
92
Figure 6.8: Ignition at a height of 40mm and fuel pool at 75 0C
Figure 6.9: Ignition at a height of 35mm and fuel pool at 75 0C
93 An unexpected behavior was observed at a fuel pool temperature of 100 0C. It was found that for a height of up to 62mm above the pool surface, there was no ignition (Figure 6.10). Once the height was reduced to 60mm, not only did the fuel get ignited but the ignition was sustained, as shown in Figure 6.11. This indicated that the temperature for ignition was the same as the temperature for sustainment for this particular case.
From the above experiments, it can be seen that the evaporation rate of the fuel depends on two factors
1. Temperature of the fuel pool
2. Heat transferred from the test charge to the fuel surface
In the low temperature cases, when the fuel was at 25 0C and 50 0C, the heat transferred from the test charge set at a height of 30mm was sufficient to burn the vapor already suspended above the fuel pool. This burning vapor could not transfer the required amount of heat energy to generate sufficient vapor for sustained burning. Also, the temperature of the fuel pool itself was not high enough to vaporize the fuel in the pan. All these factors create ignition without sustainment at lower temperatures.
94
Figure 6.10: Ignition at a height of 65mm and fuel pool at 100 0C
Figure 6.11: Ignition at a height of 60mm and fuel pool at 100 0C
95 Proceeding with the fuel pool experiments, the test for a higher temperature of 125 0C, as shown in Figure 6.12, was performed, beginning with a height of 80mm. It was observed that the evaporation rate was high enough for the fuel to ignite and for sustained burning to occur. The test was repeated for different heights of 90, 110 and 150mm with the fuel pool maintained at
125 0C (Figure 6.13).
Figure 6.12: Ignition at a height of 90mm and fuel pool at 125 0C
96
Figure 6.13: Ignition at a height of 150mm and fuel pool at 125 0C
It was found that for all of the above mentioned heights at a pool temperature of 125 0C, the fuel pool had ignited and the flame sustained. Due to the intensity of the flash, it could not be observed as to exactly when the ignition had occurred. The video showed when the burning test charge is dying or is in its last stages of burn, the fuel pool was already ignited and burning. This could be observed due to the decrease in the intensity of the flash. Further it could be said that the ignition had occurred much earlier in the duration of the burn. If the burning process of the test charge is observed it could be seen, as the flame propagated along the length of the test charge the intensity of the flash grows. The charge reaches a maximum and then decays. Hence the ignition of the test charge would have occurred when the intensity of the test charge was increasing or was at its maximum.
97 It was also observed that for all of the test temperatures of the fuel pool, ignition had occurred when the test charge was placed within the quench layer height above the fuel pool. In this region the fuel vapor concentration exceeded the UFL, but still ignition was observed. This could be explained by considering the size of the pilot flame or test charge flame. The test charge released a large amount of energy and also the flame from this igniter covered a large area above the fuel pool. Hence penetration of the heat source into the flammability region of the vapor above the fuel pool is likely to have occurred. Even if this had not occurred, the gas-phase convection around this large flame could have introduced air into the fuel rich region to dilute the fuel vapor to flammable proportions.
1.2
1
0.8 Region of fuel pool fire sustainment on ignition 0.6 Fuel Temp = 25C Fuel Temp = 50C Fuel pool ignition 0.4 but no Fuel Temp = 75C sustainment Fuel temp = 100C Fuel Temp = 125C 0.2 Fuel evaporation rate, gms/min rate, Fuel evaporation 0 0 2 4 6 8 1012141618 Height above the fuel pool surface, cm
Figure 6.14: Change in evaporation rate of fuel pool with change in temperature for different heights
Figure 6.14 shows the change in fuel evaporation rate with height for different temperatures. The results of these tests showed that the evaporation rate was approximately 0.025 gm/min, 0.045 gm/min, 0.13 gm/min, 0.24 gm/min for each of the respective temperatures upto 100 0C. From
98 this it can be expected as the temperature of the fuel is increased, the evaporation rate is markedly increased. Also the evaporation rate increased at a particular height of the test charge for a given fuel pool temperature. A sudden increase in the evaporation rate for a set fuel temperature at a particular height indicated that ignition had taken place. An interesting behavior was observed at lower temperatures. The fuel ignition and flame sustainment occurred at two different heights for the same fuel pool temperature. As the fuel temperature was increased to
100 0C, the difference converged and ignition resulted in sustainment of the burn. It was also observed that at a fuel temperature of 125 0C, the fuel ignited irrespective of the height of the test charge above the fuel pool surface. This indicated that for a particular temperature between 100
0C and 125 0C, the fuel pool ignition becomes independent of the height of ignition. It can also be said, above a particular evaporation rate of fuel, ignition becomes independent of the height of the igniter. This is shown in Figure 6.15.
16 14 Flame sustainment after ignition No flame sustainment after ignition 12 10 8 cms 6 4 2 0 Height above fuel pool surface,Height fuel pool above 0 25 50 75 100 125 150 Temperature of fuel pool, deg C
Figure 6.15: Ignition height with change in fuel temperature
In all the cases, the charge appears to have completely engulfed the fuel and its container throughout the period. However, the actual fireball emanating from the charge was not close
99 enough to provide sufficient heat to the fuel to maintain an evaporation rate from the fuel and thereby sustain a fuel burn scenario.
7 Conclusion
Ignition of a fuel pool by an incendiary device producing a fireball occurs as the pool surface moves towards the core of the fireball. Ignition does not occur when the circumferential regions of the fireball touch the fuel pool kept at ambient temperatures. A simulated API incendiary charge has been developed that produces a fireball. It has an inner (hot) region and a lower temperature light intensity (outer) region. Temperature profiles have been taken in this latter region to-date. This shows how the temperature profile emanates from the core of the incendiary.
Studies have been conducted whereby the charge has been supported over a pool of fuel, in this case Kerosene, at different heights. This established how such a fireball can ignite and sustain a fire. It has been found that although the fireball appears to engulf the fuel pool it has to be within a certain dimensional range before the fuel vapors are ignited. Even then, the fuel fire may not be self-sustaining. Further studies are progressing with regard to how the temperature of the fuel may influence these height parameters.
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