# Experimental visualization of aluminum ignition and burning close to a solid propellant burning surface

Thomas Geoffrey Decker, Robin William Devillers, Stany Gallier

<figure><img src="image_1.png" /></figure>

## To cite this version:

Thomas Geoffrey Decker, Robin William Devillers, Stany Gallier. Experimental visualization of aluminum ignition and burning close to a solid propellant burning surface. ECM 2023, Coria UMR 6614, French Combustion Institute, Apr 2023, Rouen, France. hal-04120551

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# Experimental visualization of aluminum ignition and burning close to a solid propellant burning surface

Thomas Geoffrey Decker*<fnref n="1" />, Robin William Devillers<fnref n="1" />, and Stany Gallier<fnref n="2" />

## Abstract

Visible images of burning solid propellants loaded with aluminum particles are presented. The images are recorded at high acquisition frame rate (up to 33kHz) with excellent sharpness, enabling the study of aluminum ignition and combustion close to the propellant burning surface. The article focuses on two aspects: the aluminum aggregate ignition and the alumina breaking and migration on the aggregate surface. The article presents new ways of studying aluminum agglomeration and combustion by applying image processing algorithms on state-of-the-art image series.

## 1 Introduction

Solid Rocket Motors (SRM) are widely used in military and civilian applications. They are highly reliable, require little maintenance, and are readily available even when stored for a long time. A solid propellant is a solid material used to generate thrust by its combustion. It is composed of a fuel binder and oxidizing particles. Metal particles, usually aluminum, are added to increase the propulsion performances with additional combustion heat release. While the fuel and the oxidizer burn at the propellant surface, the aluminum particles burn in the gas flow produced by the fuel/oxidizer combustion. The aluminum particles are subject to complex phenomena prior to their combustion in the gas flow [1, 2]. They follow an accumulation-aggregation-agglomeration mechanism [3, 4]. The particles accumulate in the solid propellant pockets [5], they aggregate while being attached to the burning surface and finally agglomerate when they ignite on the burning surface or in the gas flow. The aluminum particles are referred as aggregates (on the burning surface, with a coral-like shape, eventually morphing into a spherical one), and agglomerates (in the gas flow, with a spherical shape). The resulting droplets can reach hundreds of micrometers in diameter [6], while the virgin particle size is usually between 5 and 50µm. An oxide (alumina) cap is attached to the burning aluminum droplet. The droplet combustion produces heat as well as alumina, with some parts being evacuated in the gas flow as smoke while another portion increases the size of the oxide cap.

The inert alumina droplets produced during the aluminum combustion influences the stability and the performances of the SRM, depending on their size [7, 8]. The particles often follow a trimodal distribution [9] when captured with collection techniques ([2, 10, 11]). The lower and higher modes of the size distribution are respectively attributed first to alumina smokes and second to the aluminum agglomerates and coarse oxide, but the intermediate mode (4 – 8µm) is not fully

understood [9], and doubts remain about the production of those alumina droplets. The alumina production mechanism during the aluminum droplet combustion is considered in many studies [12, 13]. But its production close to the surface when the aluminum is still attached to the propellant has never been studied to the authors' knowledge. No model for oxide-cap formation during the aggregation-agglomeration mechanism has been published. This formation is crucial because the alumina migration to form an oxide cap influences the aluminum ignition on the burning surface while the size of the resulting oxide cap influences the aluminum combustion along the droplet lifetime [12, 14, 15].

Two phenomena associated with the alumina production and oxide cap formation on the burning surface are studied with image processing: the aluminum aggregate ignition and the alumina layer breaking and migration. This study shows new ways of studying physical phenomena associated with aluminum agglomeration and combustion in solid propellants.

## 2 Experimental setup and data

Shadowgraphy was used to visualize accurate details above the solid propellant surface during combustion [16]. Here a similar optical set-up was used without back illumination to improve the contrast between aluminum and alumina. A high-speed camera acquires frames to target physical phenomena on the solid propellant burning surface. The acquisition rate was adjusted up to 33kHz, and the recorded images size depends on the frame rate. The spatial resolution is about 3.2µm/px.

Two solid propellants are studied (P1 and P2). Their compositions are similar. Both include a trimodal repartition of Ammonium Perchlorate (AP) particles, a hydroxyl-terminated polybutadiene (HTPB) binder and approximately 20% of aluminum particles. They only differ in the initial size of the aluminum particles. The P1 initial aluminum particles size is approximately 30µm while the P2 is approximately 15µm. In order to capture the aluminum particles or agglomerates as clearly as possible, propellants are beveled. The cam-

<footnote n="1">¹DMPE, ONERA, Université Paris Saclay, Palaiseau, F-91123, France</footnote>

<footnote n="2">²ArianeGroup SAS, Le Bouchet Research Center, 91710 Vert le Petit, France</footnote>

<footnote>*Corresponding author: thomas.decker@onera.fr
Proceedings of the European Combustion Meeting 2023</footnote>
era views the propellant slightly from the top with an
angle around 45°. Heated aluminum and alumina are
clearly visible because of their incandescence.

The *P1* propellant is studied at two initial pressures,
5 and 10 *bar*. It is recorded at 14kHz in order to cap-
ture the aluminum ignition of the aggregates. The *P2*
propellant is studied at one initial pressure, 5 *bar*. It
is recorded at 33kHz in order to capture the aluminum
breaking and migration. Image examples are presented
in figures 1, 2, and 3.

<figure><img src="image_2.png" /><figcaption>Figure 1: Image of the *P1* propellant at the initial pressure of 5 *bar*. Agglomerates are visible in the gas flow, and aggregates (igniting or not) on the burning surface.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2: Image of the *P1* propellant at the initial pressure of 10 *bar*.</figcaption></figure>

The surface on the image of the *P1* propellant at
10 *bar* is brighter than at 5 *bar* (the experimental setup
is kept identical). The solid propellant flames are closer
to the surface with increasing pressure. The aluminum
close to the surface is therefore hotter, i.e. brighter.

<figure><img src="image_4.png" /><figcaption>Figure 3: Image of the *P2* propellant at the initial pressure of 5 *bar*. Aggregates transforming into agglomerates are visible on the left and in the middle.</figcaption></figure>

**3 Image processing**

**3.1 Aggregates detection**

The first studied phenomenon is the aluminum ag-
gregates ignition on the burning surface of the propel-
lant. The objective is to automatically detect and track

the aluminum aggregates and agglomerates on the pro-
pellant surface and in the gas flow. The number of cap-
tured images for the *P1* propellant at the initial pressures
of 5 and 10 *bar* being approximately 2000 and 2500, au-
tomatic algorithms are needed. The following process-
ing steps were used:

Step 1 - Top-hat filtering

The Top-hat transform [17] extracts elements from
given images. The transform depends on a structuring
element, usually a disk. The bright objects with much
smaller sizes are considered as part of the background.
Therefore, only the aggregates with sufficient sizes can
be easily detected.

Step 2 - Thresholding

A simple thresholding of the resulting image enables
the detection of the aggregates and agglomerates on the
burning surface. The threshold choice is a compromise
in order to avoid merging separate aggregates into a sin-
gle detected object.

Figure 5 shows steps 1 and 2 for the *P1* propellant at 5 *bar*, compared to the color scale limited original image shown in figure 4.

<figure><img src="image_5.png" /><figcaption>Figure 4: Image of the *P1* propellant at the initial pressure of 5 *bar* with a limited color-scale.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5: Thresholding following the Top-hat transform appli-
cation on the image of the *P1* propellant at the initial pressure
of 5 *bar*.</figcaption></figure>

Steps 3 and 4 - Detection and tracking

The resulting thresholded image is binary. The ag-
gregates and agglomerates are easily detected. Metrics
are automatically calculated, such as the area, the circu-
larity, the mean and the maximum intensity.

The resulting detections are tracked on successive
images to analyze the behavior of aggregates on the
solid propellant surface. Thanks to the significant frame
rate, a simple Intersect-Over-Union (IOU) threshold is
employed. Two tracked aggregates at 5 and 10 *bar* can
be visualized in figures 6a and 6c).

## 3.2 Alumina detection

The second studied phenomenon is part of the aggre-
gate ignition, it is the alumina breaking and migration.
The objective is to detect and track alumina on the sur-
face of aluminum aggregates. In order to study the phe-
nomenon, the frame rate is much higher, 33kHz. We fo-
cus here on one sequence of approximately 300 frames
where the alumina encapsulating an aggregate breaks up
and migrates.

A Top-hat transform is applied, followed by a thresholding. An example is shown in figure 7 for propellant P2 at 5 bar with (a) Top-Hat transform and (b) Threshold image. Figure 7 zooms on a igniting aggregate on the propellant surface.

The detection algorithm can segment the main alu-
mina surface surrounding the aluminum collapsing into
a lobe (on the left-hand side) but also isolated alumina
portions moving on the aluminum surface (on the right-
hand side) that will eventually reach the forming oxide
cap.

Several metrics are automatically calculated, such as the area, the circularity, the mean and the maximum intensity. The main alumina surface is easily tracked over time, as the largest detection. We follow other alumina detections with the IOU calculation. Here, the minimum IOU is fixed very low because of the rapid motion of the alumina zones compared to their size. Figure 8a shows the evolution of the main alumina layer that initially surrounds the whole liquid aluminum, and figure 9a shows successive positions of a single alumina portion.

# 4 Results and discussion

## 4.1 Aggregates ignition

Here are comments on the aggregate ignition on the
surface by analyzing two aggregates tracked other time
(figures 6a and 6c). The metrics are plotted in figure 6b.

The growth of the detected areas of the two aggre-
gates on the burning surface (see images 1 to 4 from fig-
ure 6a and images 1 to 3 from figure 6c) can be observed
with the area. Only fragments of the aggregated are de-
tected for the first detection because the overall aggre-
gate is not bright enough at this early heating stage. The
detections grow with time as more and more portions of
the aggregates become sufficiently bright.

The increase in temperature of a tracked aggregate
is not only synonymous with an increase of the detected
area but also a steady increase in the mean intensity (see
images 1 to 4 from 6a and 1 to 3 from figure 6c). The
mean intensity slowly increases from 2.5 to 4.0ms for
the aggregate at 5 bar (blue plot) while the intensity in-
crease happens from 1.1 to 2.0ms at 10 bar (red plot).
The intensity spike happening around 0.3ms in the red
plot corresponds to a burning droplet passing nearby in
its upwards movement.

When the aggregates are ignited and transformed

into agglomerates, they establish a diffusion flame with
the surrounding gas (see image 6 from 6a and image 5
from figure 6c). The diffusion flame establishment cor-
responds to an increase in area, an increase in mean in-
tensity and a decrease in circularity. Simultaneously, the
newly formed agglomerates leave the surface and move
in the gas flow. The movement in the gas flow is visi-
ble with the vertical velocity increase starting at 5ms at
5 bar and 4.2ms at 10 bar.

The metrics are calculated for each tracked aggre-
gate on the burning surface detected for at least five
frames. 786 aggregates were tracked at 5 bar and 1259
at 10 bar. Two estimates are calculated for each tracks

* The track duration time gives the estimate of the ignition time $\tau$

* A mean intensity $I_{mean}$ is calculated over the complete track

Distributions for the two estimated are presented in figures 10a and 10b. To avoid visualizing the threshold effect with a spike of distribution on the lowest values, the distribution of the ignition time is presented as a cumulative. The ignition time $\tau$ is globally lower at 5 bar than at 10 bar. The aggregates ignition is faster with increasing pressure. A shorter ignition delay leads to less agglomeration. It is consistent with the physics of aluminum agglomeration in solid propellants, agglomerates are smaller with increasing pressure [18, 19].

The mean intensity $I_{mean}$ is globally increasing with
increasing pressure. Propellant flames get closer to the
burning surface with increasing pressure, leading to a
more efficient aggregate heating (i.e. brighter detec-
tions).

Overall, tracking aluminum aggregates on the burn-
ing surface enables the study of physical phenomena as-
sociated to aluminum agglomeration, ignition, and com-
bustion on the solid propellant burning surface. It helps
understand the importance of the aggregates’ tempera-
ture on its ignition. The study of the detected area shows
that the aggregates are not homogeneous in temperature.
Data also show the importance of temperature in the ag-
gregates ignition. With increasing pressure, the solid
propellant flames are closer to the burning surface, the
aggregates are hotter, resulting in a decreasing ignition
time.

A more quantitative study is required to compare the
ignition time of the aggregates to their residence time on
the propellant surfaces. Models would have to include
ignition delay if its time range is comparable to the res-
idence time on the burning surface. A more detailed
study could put into equation the ignition time depend-
ing on physical parameters such as pressure, the propel-
lant constitution or the aggregate size.

## 4.2 Alumina breaking and migration

The second focus is on the alumina breaking and mi-
gration of an aluminum aggregate attached to the solid
propellant burning surface. The alumina breaking and
migration exposes the liquid aluminum to the surround-
ing oxidizing gas. Therefore, the liquid aluminum starts
to burn and establishes a diffusion flame (see images 4
<figure><img src="image_7.png" /><figcaption>Figure 6: Tracking and physical metrics of two aggregates/agglomerates of the P1 propellant at the initial pressures of 5 and 10 bar.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 7: (a) Top-hat transform application on the zoomed image on an aggregate of the P2 propellant at the initial pressure of 5 bar, (b) Thresholding of the resulting image</figcaption></figure>

to 6 from figure 8a). We focus on two tracks, the main alumina surface that initially surrounds the whole aluminum aggregate and an isolated alumina portion moving rapidly on the liquid alumina surface. Metrics associated with the two tracks are presented in figures 8b and 9b.

The main alumina surface follows a first period of ~7ms where it keeps a circular shape but seems to have a variable temperature (i.e. a variable mean intensity in figure 8b). The exothermic production of alumina is a possible explanation of the increased temperature (i.e. an increased mean intensity in figure 8b between 2.5 and 5ms) prior to the alumina breaking. Price et al. [1] demonstrated that the encapsulating alumina cracks due to the expanding liquid aluminum. The liquid aluminum finds itself partially exposed, resulting in the production of additional alumina.

Then, the alumina surface breaks up on the top of the aggregate (where it is supposed to be hotter), a local temperature increase occurs due to the released liquid aluminum combustion, and the circular alumina surface retracts. The alumina break up happens at ~7ms. The alumina may move on the aluminum surface as small portions. The phenomenon is very rapid, as shown in
<figure><img src="image_9.png" /><figcaption>Figure 8: Tracking and physical metrics of the main alumina surface of an aggregate of the P2 propellant at the initial pressure of 5 bar ().</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 9: Tracking and physical metrics of a moving alumina portion of an aggregate of the P2 propellant at the initial pressure of 5 bar ().</figcaption></figure>

figure 9b (the total duration of the alumina movement being only a few ms). The alumina may also be ejected as small drops (see images 3 and 4 from figure 9a).

The small tracked alumina portion also has a variable temperature (i. e. a variable mean intensity) as shown in figure 9b. It seems that its temperature decreases when it is moving from the top of the liquid aluminum to the main alumina surface, meaning that the aluminum aggregate has a vertical temperature gradient. It confirms the analysis in the previous section that the aggregate temperature is not homogeneous.

The movement of alumina on the liquid aluminum stops at ~14ms. The reason attributed here is that the aluminum of this aggregate does not produce enough heat. The alumina produced on the liquid aluminum surface tempers the combustion and slowly passivates the aggregate. It is visually observed that some igniting aggregates can be passivated. Another ignition needs to occur to finalize the transition from aggregate to agglomerate.

Overall, significant alumina production occurs on the liquid aluminum surface, during the residence of the aluminum aggregate on the solid propellant burning surface. The produced alumina then migrates into the oxide cap. Therefore, a portion of the final residue is produced when the aluminum aggregate is attached to the propellant burning surface. This study shows the importance of physical phenomena associated to aluminum on a solid propellant surface, influencing the size of the

resulting alumina cap, and affecting the stability of a SRM.

## 5 Conclusion

Visible images of burning solid propellants loaded with aluminum particles were presented. Physical phenomena associated with alumina production were studied thanks to high-speed recording, excellent spatial resolution, good image clearness, and new algorithms using a Top-hat transform.

Different stages of the aggregated aluminum ignition were shown at two initial pressures, 5 and 10 bar. Aggregate temperature increases before it ignites and transforms into a spherical agglomerate. It was found that the ignition time decreases with increasing pressure. The mean intensity of the aggregates increases with increasing pressure, meaning that the aggregates are hotter on the burning surface. Increased pressure leads to closer solid propellant flames to the burning surface and the aggregates.

The alumina breaking and migration were observed thanks to a test at a very high frame rate of 33kHz. An alumina portion was even tracked on a liquid aluminum droplet. Small aluminum/alumina drops were observed being ejected in the gas flow during the surrounding alumina surface migration. A significant production of alumina occurs on the liquid aluminum surface. To the authors' knowledge, those phenomena were never studied in the literature. This study shows the importance of the
<figure><img src="image_11.png" /><figcaption>Figure 10: Distributions for the tracked aggregates at 5 and 10 bar. (a) Mean Intensity distribution, (b) Ignition time cumulative distribution</figcaption></figure>

phenomena highlighted in the domain of SRM stability.

The study showed the importance of the aggregates' temperature during their ignition. The study also showed that aggregates ignite with a delay, depending on pressure. With refined data analysis, quantitative data about the temperature gradient depending on pressure or aggregate size could be calculated. Its influence on the aluminum aggregate ignition delay could also be investigated. Quantitative data on the aggregate temperature gradient and its ignition delay will be useful for future aluminum agglomeration models.

## 6 Acknowledgment

Thomas Decker's PhD is financed by ONERA and ArianeGroup. S. G. thanks DGA (French Procurement Agency) for funding.

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