Energetic Materials Technology

Purdue University

2014

Link: https://www.me.psu.edu/sfnm/Publications1_files/Purdue/1-s2.0-S0010218013003039-main.pdf

Travis R. Sippel, Steven F. Son, Lori J. Groven

Abstract

In solid propellants, aluminum is widely used to improve performance, yet theoretical specific impulse is still not achieved largely because of two-phase flow losses. Losses could be reduced if aluminum particles quickly ignited, more gaseous products were produced and if upon combustion, aluminum particle breakup occurred. To explore this, tailored, fuel-rich, mechanically activated composite particles (aluminum/polytetrafluoroethylene, Al/PTFE 90/10 and 70/30 wt.%) are considered as replacements for reference aluminum powders (spherical, flake, or nanoscale) in a composite solid propellant. The effects on burning rate, pressure dependence, and aluminum ignition, combustion, and agglomeration are quantified. Using microscopic imaging, it is observed that tailored particles promptly ignite at the burning surface and appear to breakup into smaller particles, which can increase the heat feedback to the burning surface. Replacement of spherical aluminum with Al/PTFE 90/10 wt.% does not significantly affect propellant burning rate. However, Al/PTFE 70/30 wt.% increases the pressure exponent from 0.36 to 0.58, which results in a 50% increase in propellant burning rate at 13.8 MPa. This increased pressure sensitivity is consistent with more kinetically controlled combustion that occurs from smaller burning metal particles near the surface. Combustion products were quench collected using a new, liquid-free technique at 2.1 and 6.9 MPa and were measured. Both Al/PTFE 90/10 and 70/30 wt.% composite particles reduce the coarse product fraction and diameter. The most significant reduction occurs from 70/30 wt.% particle use, where average coarse product diameter is 25 lm, which is smaller than the original, average particle size and is also smaller than the 76 lm products collected from reference spherical aluminized propellant. This is a 66% decrease in agglomerate diameter or a 96% decrease in volume compared to agglomerates formed from reference spherical aluminum. Smaller diameter condensed phase products and more gaseous products will likely decrease two-phase flow loss and reduce slag accumulation.

Abstract

Aluminum particles ranging from 2 to 100 μm were subjected to the flow of detonation products of a stoichiometric mixture of hydrogen and oxygen at atmospheric pressure. Luminosity emitted from the reacting particles was used to determine the reaction delay and duration. The reaction duration was found to increase as d n with n ≈ 0.5, which is more consistent with kinetically controlled reaction rather than the classical diffusion-controlled regime. Emission spectroscopy was used to estimate the combustion temperature, which was found to be well below the flow temperature. This fact also suggests combustion in the kinetic regime. Finally, the flow field was modeled with a CFD code, and the results were used to model analytically the behavior of the aluminum particles.

May 2020

Link: https://docs.lib.purdue.edu/open_access_theses/1275

Trevor J. Fleck

Abstract

From its advent in the 1980s until the 2000s, many of the advances in additive manufacturing (AM) technology were primarily focused on the development of various 3D printing techniques. During the 2000s, AM came to a juncture where these processes were well developed and could be used effectively for rapid prototyping purposes; however, the ability to produce functional components that could reliably perform in a given system had not been fully achieved. The primary focus of AM research since this juncture has been to transition AM from a rapid prototyping technique to a legitimate means of mass manufacturing enduse products. In order to make this happen, two significant areas of research needed to be advanced. The first area focused on advancing the limited selection and functionality of the materials being used for AM. The second area focused on the characterization of the end-use products comprised of these new materials. The primary goals of the work described in this document are to substantially further the field of the additive manufacturing by developing new functional materials and subsequently characterizing the resultant printed components. The primary focus of the first two chapters (Chapters 2 and 3) is to further characterize an energetic material system comprising of aluminum (Al) particles embedded in a polyvinylidene fluoride (PVDF) binder, which has been shown to be compatible with AM. This material system has the ability to be implemented as a lightweight multifunctional energetic structural material (MESM); however, significant characterization of its structural energetic properties is needed to ensure reliable MESM performance. First, variations of a previously demonstrated Al/PVDF filament were investigated in order to determine the effect of material constituents on the structural energetic properties of the material. Seven different Al/PVDF formulations, with various particle loadings and particle sizes, were considered. The modulus of elasticity and ultimate strength for the seven formulations were obtained via quasi-static tensile testing of 3D printed dogbones. The energetic performance was quantified via burning rate measurements and differential scanning calorimetry (DSC) of 3D printed samples. Next, variations in the AM process were made and the effect of print direction on the same properties was determined. Once viable MESM performance was quantified, representative structural elements were printed in order to demonstrate the ability to create structural energetic elements. During quasi-static tensile testing, it was observed that aligning the load direction perpendicular to the print direction of the component resulted in inferior mechanical properties. This reduction in mechanical properties can be attributed to the lack of continuity at material interfaces, a well studied phenomena in AM. This phenomena is the primary focus of the next two chapters (Chapters 4 and 5), which investigate the polymer healing process as it pertains to fusion-based material extrusion additive manufacturing, also known as fused filament fabrication (FFF). In the context of the FFF process, the extent of the polymer healing, or lack thereof, at the layer interface is known to be thermally driven. Chapter 4 quantifies the relationship between the reduction in mechanical properties and the temperature of the previously deposited layer at the time the subsequent layer is deposited. This relationship gives insight into which parameters should be closely monitored during the FFF process. The following chapter investigates incorporating plasma surface treatment as a means to improve the reduced mechanical properties seen in Chapter 3 and 4. As plasma surface modification can affect various stages of the polymer healing process, a variety of experiments were completed to determine which mechanisms of the plasma treatment were significantly affecting the mechanical properties of the FFF components. The thermal history was analyzed and it was hypothesized that enhanced diffusion at the layer interface was not a significant contributor to, but a rather a detractor from, the improved mechanical properties in this system. A variety of tests investigating how the plasma treatment was affecting the wettability of the surface were performed and all of the tests indicated that the wettability was increased during treatment and was likely the driving mechanism causing the improvement seen in the mechanical properties. These tests give some initial insight into how to successfully pair plasma treatment capabilities with FFF systems and give insights into how that plasma treatment can affect the polymer healing process in FFF applications.

December 2017

Link: https://docs.lib.purdue.edu/open_access_theses/1275

Trevor J. Fleck

Abstract

The work discussed in this document seeks to utilize traditional additive manufacturing techniques to selectively deposit energetic materials. The goal was to gain a fundamental understanding of how to use commonplace 2D inkjet printing and 3D fused deposition technology to selectively deposit reactive materials. Doing so provides the ability to manipulate the geometry, as well as composition, of the energetic material during the manufacturing process. Achieving this level manipulation and control has shown to be nontrivial, if not impossible, using traditional manufacturing methods. The ability to change the geometry of the energetic material at will greatly increases the ability of these energetic materials to be integrated with a wide range of systems, such as transient electronics. To create a transient electronic device, a destruction mechanism and an initiation system need to be integrated with electronic components. Experiments in this document investigate nanothermites for their ability to serve as this destruction mechanism. Nanothermites were prepared at various equivalence ratios and syringe deposited onto silicon substrates. The resultant destruction was shown to vary with the equivalence ratio of the material. A wide range of substrate destruction was demonstrated, varying from disintegration to only charring the wafer. Materials prepared near stoichiometric conditions were shown to disintegrate the silicon substrates completely. As the equivalence ratio was raised, less severe destruction was observed. The ability inkjet print these nanothermites provides the geometric control necessary to incorporate them into electronic components. An ink formulation process was explored in an attempt to create a fuel and an oxidizer ink, which could be inkjet printed simultaneously to create a nanothermite. Separate inks allow for the equivalence ratio, and therefore the resultant destruction, to be selectively tuned during the additive manufacturing process. Additionally, this gives the advantage of only needing two largely inert, shelf stable inks, instead of having to develop a new ink for every desired destruction level. Various candidate inks were formulated using different loadings and combinations of surfactants. Polyvinylpyrrolidone was shown to be the surfactant best suited for holding both aluminum and copper (II) oxide nanoparticles in suspension over time. These inks both showed reasonable shelf stability as well as viable reactivity when stoichiometric nanothermite samples were prepared using on-chip mixing. With respect to 3D printed energetic materials, fused deposition methods were used to print a fluoropolymer based energetic material which could be used as a multifunctional reactive structure. A reactive filament comprising of a polyvinylidene fluoride (PVDF) binder with 20% mass loading of aluminum (Al) was prepared using a commercial filament extruder and printed using a Makerbot Replicator 2X. The printing performance of the energetic samples was compared with standard 3D printing materials using metrics such as bead-to-bead adhesion and the surface quality of the printed samples. The reactivity and burning rates of the filaments and the printed samples were shown to be comparable. This result is imperative for fused deposition modeling to be used as a viable manufacturing method of energetic materials. In total, this document lays some of the groundwork necessary for additive manufacturing to be adopted as a viable method for the selective deposition of energetic materials. Going forward these methods can be used to integrate energetic materials in a manner not possible using traditional manufacturing methods.

December 2016

Link: https://docs.lib.purdue.edu/open_access_theses/886/

Michael S. Powell

Abstract

Aluminized ammonium perchlorate composite propellants (APCP) form large molten agglomerated particles that can result in poor combustion performance and increased two-phase flow losses. Quantifying agglomerate size distributions is important for assessing these losses for different types of aluminum fuels that can help improve rocket performance. It is highly desirable to measure particle sizes in-situ using non-intrusive optical methods, rather than conventional particle collection, which can have large uncertainties. Regular high-speed microscopic imaging suffers from a limited depth of field. Digital inline holography (DIH) is an alternative approach that results in 3D information through numerical reconstruction. In this paper, DIH approach was used with two orthogonal viewing angles for simultaneous particle imaging and velocity measurements. Furthermore, two imaging speeds (4 Hz vs. 4,000 Hz) were compared to characterize biasing. DIH results were contrasted with high-speed visual imaging and conventional particle collection. All techniques were in agreement that ejected particles were larger than initial constituent particles. However, DIH allows for the acquisition of much less experimental data for statistically significant data sets when compared to videography and more accurately sizes agglomerates than particle collection. Low-speed DIH is found to be subject to biasing due to multiple counting of larger particles with slower velocities staying in the field of view. A model was employed to correct the velocity biasing was performed by adjusting the data based on size and velocity correlations. This was partially successful to reduce biasing of sizes for the low speed DIH data.

December 2009

DOI: https://pubs.acs.org/doi/10.1021/jp905175c

Jeremiah D. E. White, Robert V. Reeves, Steven F. Son, Alexander S. Mukasyan

Abstract

The influence of short-term (5-15 min) highly energetic ball milling on the ignition characteristics of a gasless heterogeneous Ni-Al reactive system has been investigated. By using Al-Ni clad particles (30-40 micron diameter Al spheres coated by a 3-3.5 micron layer of Ni, that corresponds to a 1:1 Ni/Al atomic ratio), it was shown that such mechanical treatment leads to a significant decrease in the self-ignition temperature of the system. For example, after 15 min of ball milling, the ignition temperature appears to be approximately 600 K, well below the eutectic (913 K) in the considered binary system, which is the ignition temperature for the initial clad particles. Thus, it was demonstrated that the thermal explosion process for mechanically treated reactive media can be solely defined by solid-state reactions. Additionally, thermal analysis measurements revealed that mechanical activation results in a substantial decrease in the effective activation energy (from 84 to 28 kcal/mol) of interaction between Al and Ni. This effect, that is, mechanical activation of chemical reaction, is connected to a substantial increase of contact area between reactive particles and fresh interphase boundaries formed in an inert atmosphere during ball milling. It is also important that by varying the time of mechanical activation one can precisely control the ignition temperature in high-density energetic heterogeneous systems.

May 2010

DOI: 10.1021/jp1018586

Alexander S Shteinberg, Ya-Cheng Lin, Steven F Son, Alexander S Mukasyan

Abstract

High temperature (>1000 K) reaction kinetics in the stoichiometric (1:1 by molar ratio) Al-Ni system was investigated by using the, so-called, electrothermal analysis (ETA) method. ETA is the only technique that allows studying kinetics of a heterogeneous gasless reaction at temperatures above the melting points of the precursors. Special attention was focused on methodological aspects of the ETA method. Two different reaction systems were studied: (i) initial Al/Ni clad particles; (ii) the same powders but after 15 min of high energy ball milling. Analysis of the obtained results leads to the conclusion that such mechanical treatment decreases the apparent activation energies of the reaction in the Ni-Al system, from 47 +/- 7 kcal/mol for the initial powder to 25 +/- 3 kcal/mol after ball milling. Comparison of these data with those reported previously was also made.

January 2013

DOI: https://doi.org/10.1016/j.combustflame.2012.10.001

Christopher R. Zaseck, Steven F. Son, Timothée L. Pourpoint

Abstract

In this paper we report the burning rate characteristics of hydrogen peroxide and micron-aluminum propellants. Theoretical calculations show that the sea level specific impulse of this simple binary mixture is comparable to standard composite propellant. In addition, the aluminum particle size, hydrogen peroxide concentration, and mixture ratio can be adjusted over a flat peak performance regime to attain specific thrust profiles and durations. We measured the burning rates in a windowed pressure vessel at pressures ranging from 7 to 14 MPa. Results show that mixture burning rates span from 0.5 to 4.5 cm/s at 7 MPa with power law burning rate pressure exponents ranging from 0.33 to 1.07. In this study, we focus a statistical analysis on the determination of the most influential variables affecting the burning rate and apply a thermal analysis to determine the combustion regimes of these mixtures. The statistical analysis provided a multivariate regression model for the logarithm of the burning rate with a correlation coefficient of 0.93. The model suggests aluminum diameter is the most important factor affecting the overall burning rate, and H2O2 concentration as the most influential variable on the burning rate pressure dependence. The burning rate dependence on theoretical combustion temperature shows two distinct combustion regimes attributed to kinetic and diffusion controlled combustion. A simple thermal analysis confirms the experimental burning rate pressure dependence observed for these two regimes.

2018

Link: https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2444&context=open_access_theses

Gabriel Diez

Abstract

Aluminum-lithium (Al-Li) alloys have demonstrated a mechanism to improve composite propellant performance by reducing agglomerates through microexplosions. In addition, use of AlLi significantly reduces hydrochloric acid production in ammonium perchlorate based propellants while also improving theoretical performance. Full combustion characterization (e.g., at various pressures) of the Al-Li based propellant has not been performed previously. Measurement of the aluminum-lithium composite propellant’s burning rate and quantification of agglomerate production at various pressures is presented. Agglomerate size of the aluminum-lithium appeared to be smaller at lower pressures than at higher pressures, likely due to increased microexplosions at low pressures. Additionally, at high pressures the aluminum-lithium did appear to produce larger agglomerates than the aluminum, but upon closer inspection it was observed that the majority of these large agglomerates were liquid metal that had splashed off of the melt layer rather than condensed phase oxide products. This biased the aluminum-lithium samples towards larger agglomerate sizes without clear evidence the larger agglomerates would not burn given greater residence time and distance from the surface. Results show a pressure exponent of 0.29 for a composite propellant using aluminum-lithium powder sieved to 25-40 µm and 0.39 for a propellant using aluminum-lithium powder as-received. The difference in pressure exponents for the two powder sizes could be attributed to the greater microexplosivity increasing the burning rate at low pressures.

May 2022

DOI: https://www.researchgate.net/publication/360862668_On_the_Use_of_Fluorine-Containing_Nano-Aluminum_Composite_Particles_to_Tailor_Composite_Solid_Rocket_Propellants

Kyle Uhlenhake, Omar R. Yehia, Andrew Noel, Brandon C. Terry

Abstract

The burning rate of solid propellants is an important factor for optimizing rocket motors and improving performance. The enhanced burning rate can increase thrust and reduce a propulsion system‘s overall size and weight. In this study, a novel nano‐aluminum/THV composite additive was prepared and introduced into a solid ammonium perchlorate/polybutadiene composite solid rocket propellant to enhance its burning rate. The morphology of the composite particle additive and its effects on combustion were characterized. The use of small quantities (<15 wt.%) of the additive resulted in a burning rate enhancement of up to 2.1 times that of the conventional coarse aluminized propellant with a specific impulse loss of only 3 seconds, and as much as 4.7 times enhancement with a predicted loss of 22 seconds in theoretical specific impulse. Some of this loss may be recovered by the improved combustion efficiency in smaller rocket motors because the additive was shown to significantly reduce the aluminum agglomeration at the propellant burning surface and reduce the size of reaction products which may reduce two‐phase flow losses. The additive also provides wide burning rate tailorability, favorable for motor, grain, and thrust curve design. The burning rate enhancement mechanism is thought to be a physical cratering mechanism governed by the burning rate disparity between the binder/oxidizer system and the nano‐aluminum/fluoropolymer additive and not a chemical catalytic effect.

December 2016

DOI: https://docs.lib.purdue.edu/open_access_dissertations/975/

Hatem Mohamed Belal

Abstract

Agglomeration reduction techniques are important field in solid propellant industry, Large agglomeration results in excessive two phase losses. Tailored composite particles has been applied to tailor aluminum particle ignition and combustion. In this research, mechanical activated aluminum magnesium powders are synthesized, tested in both laser ignition using CO2 and propellant. Prepared powders categorized into particle size that suitable for propellant application. Laser ignition tests showed that the prepared powder are more reactive than magnalium which has the same Al:Mg weight ratio. Agglomeration capturing showed that the prepared powder produce much less than neat aluminum or even similar physical mixture of aluminum and magnesium. The burning rate of propellant using the prepared powder is increased.

MA Al/Mg powders as long as with comparable physical mixture are applied in propellant formulation with AP/HTPB. In order to quantify the effect of changing Mg percent. Burning rate is measured from videos captured for strand burning in windowed pressure vessel, also the agglomeration was capturing using special setup. The results showed that MA powder increase burning rate and this increase reach maximum at 50% Mg, while propellant using physical mixture of Al/Mg show constant or little decrease in burning rate. In addition, the MA powder show lower agglomeration size in comparison to neat aluminum propellant or physical mixture with the same Mg percent. The lowest agglomeration sizes were for MA50. However, equilibrium calculation showed 4 sec losses in specific impulse, so MA 70 was chosen as a compromise between low agglomeration size at the minimum loss in specific impulse.

Magnalium is an alloy of aluminum and magnesium and it is known for its ease of ignition and high oxidation energy content. It has been used as a metal fuel to increase burning rates of composite modified double base (CMDB) and ammonium perchlorate (AP) composite propellants. However, the ignition temperature is larger than the comparable mechanically activated (MA) Al-Mg powder.
Mechanical milling was performed on magnalium powders and modifications of structure and morphology of the alloy during milling were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The prepared magnalium powder was used in a solid propellant, which showed higher burning rates than those containing as-received magnalium. Furthermore, milled magnalium showed higher agglomeration reduction than both as received magnalium as well as MA Al-Mg powders.

Extend the application of mechanical alloying of aluminum to other metals with extreme difference in melting/ boiling temperature, the first is Zirconium which is a long time candidate in solid propellant community. The ease of zirconium ignition and the micro-explosive behavior shown by neat zirconium particles promote its usage in agglomeration reduction effort. the other metal is Indium, which has very low melting point compared to other metal, this may open the possibility of earlier reaction of aluminum particles at or near propellant surface resulting in less pre ignition time which reduce agglomeration tendency.

MA of 90% Aluminum and 10% of Zirconium or 10% Indium using High energy ball milling, particle characterization using SEM/FIB, XRD and DSC/TGA are performed, burning rate and agglomeration size analyses of solid propellant using sieved MA-powder are done. The results showed that the both MA Al-ZR and MA Al-In ignite in laser beam which verify change in reactivity from neat aluminum with its protective alumina coating. However, burning rate results show no change in burning rate from neat aluminum, also the prepared material shows no reduction in agglomeration sizes.

DOI: doi.org/10.1002/prep.202200204

Kyle E. Uhlenhake, Diane N. Collard, Alexander C. Hoganson, Alex D. Brown, Sara Fox, Metin Örnek, Jeffrey F. Rhoads and Steven F. Son

Abstract

Aluminum (Al) and polyvinylidene fluoride (PVDF) composites can be used in many unique applications in the field of energetic materials. Due to their low melting point, many PVDF composites can be additively manufactured. However, more research is needed to better understand the ignition and combustion of these materials. Manipulating the size of aluminum (Al) particles in Al/PVDF composites can drastically alter the burning rate, ignition characteristics, and possibly flame temperatures. This study characterizes the ignition of additively manufactured microand nano- Al/PVDF composites through hotwire ignition tests. Both nAl and μAl particles were mixed in PVDF at 20 wt.% and additively manufactured into disks via fused filament fabrication. The nAl/PVDF printed disks ignited at a minimum ignition power (MIP) of 4.1 W compared to the 9.5 W required for ignition of μAl/PVDF disks. At identical powers, ignition delays were significantly shorter for nAl/PVDF disks. Additionally, while the nAl/PVDF disks reacted instantaneously at any power above their MIP, the μAl/PVDF disks exhibited smaller, localized flames before complete combustion, if the power was below 17.5 W. Flame temperatures were estimated through three-color pyrometry and compared to thermochemical equilibrium calculations. While theoretical flame temperatures for nAl/PVDF and μAl/PVDF are 2023 K and 2314 K respectively, both samples burned near 2000 K when measured using pyrometry.

October 2022

DOI: https://www.sciencedirect.com/science/article/abs/pii/S001021802200267X?via%3Dihub

Diane N. Collard, Kyle E. Uhlenhake, Metin Örnek, Jeffrey F. Rhoads and Steven F. Son

Abstract

Solid propellants are employed in a range of applications from the inflation of airbags to propulsion systems for rockets. The ignition of solid propellants must be carefully controlled and modified on a per-use basis due the specific ignition requirements of each application. Using tailored photoreactive materials as a source of ignition for solid propellants, or other energetic materials, could reduce the added weight and risk of traditional initiators and result in safer, more effective solid rocket motor ignition systems. This study demonstrates the tunability of the ignition delay and propagation properties of optically-sensitive, nearly full density reactive aluminum/polyvinylidene fluoride (Al/PVDF) films and additively manufactured igniters. A single printed layer of pure nano-aluminum (nAl) at ideal stoichiometry in PVDF was found to flash ignite, but frequently yielded delayed transitions in steady propagation from the igniter to the propellant. To improve the continuity and steadiness of the transition, fuel particle size, igniter thickness, and a combination of layers of nAl and micron-sized aluminum (μAl) were investigated. In printed igniters with layers of μAl, only a single layer of nAl was needed to flash ignite the material and propagate to the layers of μAl without delay. For igniters cast onto strands of ammonium perchlorate composite propellant, continuous ignition was achieved with a single layer of nAl printed atop a triple layer of μAl for the flash-activated igniters and a single layer of nAl printed atop a single and triple layer of μAl for laser-driven igniters. The nAl/PVDF layer enabled good flash or laser ignition sensitivity, while the μAl/PVDF produced more sustained heat transfer to produce a reliable ignition process.

October 2020

DOI: https://arc.aiaa.org/doi/10.2514/1.B37848

Gabriel A. Diez, Timothy D. Manship, Brandon C. Terry, Ibrahim E. Gunduz and Steven F. Son

Abstract

Aluminum (Al)/lithium (Li)-alloy-based fuels can potentially improve composite propellant performance and reduce hydrochloric acid formation. Shattering microexplosions have been observed in Al–Li-based composite propellants at 0.1 MPa; however, combustion characterization of Al–Li-based propellant as a function of pressure has not been performed previously. Measurement of the burning rate of an Al–Li composite propellant and quantification of agglomerate production near the propellant surface at various pressures are presented in this work. Al–Li particle agglomeration, determined to be unconsumed Al–Li, increased with increasing pressure, suggesting that microexplosions were inhibited at higher pressures. Burning rate experiments demonstrated a plateau burning rate effect that occurred in propellant with fine grade (mean diameter: 17  μm) Al–Li particles, whereas the as-received Al–Li-containing (mean diameter: 53  μm) propellant maintained a constant pressure exponent of about 0.39 over all pressures tested. The finer Al–Li propellant had a pressure exponent of 0.59 at pressures below about 4 MPa and a pressure exponent of 0.11 above 4 MPa. Surface imaging of the Al–Li propellant showed a distinctive condensed phase reaction on the surface, which became more prominent with the finer Al–Li particles and at higher pressures: a potential source of the plateau burning rate effect.

2021

DOI: https://www.sciencedirect.com/science/article/abs/pii/S1540748920302492?via%3Dihub

Morgan D. Ruesch, Austin J. McDonald, Garrett C. Mathews, Steven F. Son, Christopher S. Goldenstein

Abstract

Understanding the temperature of aluminized, composite-propellant flames is critical to achieving robust rocket motor designs and developing accurate, predictive models for propellant combustion. This work presents measurements of (1) the temperature of CO (within the flame bath gas) and (2) the temperature of AlO (located primarily within regions surrounding the burning aluminum particles) within aluminized, composite-propellant flames as a function of height above the burning propellant surface. Three aluminized, ammonium-perchlorate (AP), hydroxyl-terminated polybutadiene (HTPB) composite propellants with varying aluminum particle size (nominally 31 �m, 4.5 �m, or 80 nm) and one non-aluminized AP-HTPB propellant were studied while burning in air at 1 atm. A wavelength-modulation-spectroscopy technique was utilized to measure CO temperature and mole fraction via mid-infrared wavelengths and a conventional AlO emission-spectroscopy technique was utilized to measure the temperature of AlO. The bath-gas temperature varied significantly between propellants, particularly within 2 cm of the burning surface. The propellant with the smallest particles (nano-scale aluminum) had the highest average temperatures and far less variation with measurement location. At all measurement locations, the average bath-gas temperature increased as the initial particle size of aluminum in the propellant decreased, likely due to increased aluminum combustion. The results support arguments that larger aluminum particles can act as a heat sink near the propellant surface and require more time and space to ignite and burn completely. On a time-averaged basis, the temperatures measured from AlO and CO agreed within uncertainty at near 2650 K in the nano-aluminum propellant flame, however, AlO temperatures often exceeded CO temperatures by  ≈ 250 to 800 K in the micron-aluminum propellant flames. This result suggests that in the flames studied here, and on a time-averaged basis, the micron-aluminum particles burn in the diffusion-controlled combustion regime, whereas the nano-aluminum particles burn within or very close to the kinetically controlled combustion regime.

April 2018

DOI: https://doi.org/10.2514/1.B36859

Michael S. Powell, Ibrahim W. Gunduz, Weixiao Shang, Jun Chen, Steven F. Son, Yi Chen and Daniel R. Guildenbecher

Abstract

Aluminized ammonium perchlorate composite propellants can form large molten agglomerated particles that may result in poor combustion performance, slag accumulation, and increased two-phase flow losses. Quantifying agglomerate size distributions are needed to gain an understanding of agglomeration dynamics and ultimately design new propellants for improved performance. Due to complexities of the reacting multiphase environment, agglomerate size diagnostics are difficult and measurement accuracies are poorly understood. To address this, the current work compares three agglomerate sizing techniques applied to two propellant formulations. Particle collection on a quench plate and backlit videography are two relatively common techniques, whereas digital inline holography is an emerging alternative for three-dimensional measurements. Atmospheric pressure combustion results show that all three techniques are able to capture the qualitative trends; however, significant differences exist in the quantitative size distributions and mean diameters. For digital inline holography, methods are proposed that combine temporally resolved high-speed recording with lower-speed but higher spatial resolution measurements to correct for size–velocity correlation biases while extending the measurable size dynamic range. The results from this work provide new guidance for improved agglomerate size measurements along with statistically resolved datasets for validation of agglomerate models.

February 2017

DOI: https://www.sciencedirect.com/science/article/abs/pii/S0010218016303078?via%3Dihub

Mario A. Rubio, I. Emre Gunduz, Lori J. Groven, Travis R. Sippel, Chang Wan Han, Raymond R. Unocic, Volkan Ortalan, Steven F. Son

Abstract

Aluminum particles are widely used as a metal fuel in solid propellants. However, poor combustion efficiencies and two-phase flow losses result due in part to particle agglomeration. Recently, engineered composite particles of aluminum (Al) with inclusions of polytetrafluoroethylene (PTFE) or low-density polyethylene (LDPE) have been shown to improve ignition and yield smaller agglomerates in solid propellants. Reductions in agglomeration were attributed to internal pressurization and fragmentation (microexplosions) of the composite particles at the propellant surface. Here, we explore the mechanisms responsible for microexplosions in order to better understand the combustion characteristics of composite fuel particles. Single composite particles of Al/PTFE and Al/LDPE with diameters between 100 and 1200 µm are ignited on a substrate to mimic a burning propellant surface in a controlled environment using a CO2 laser in the irradiance range of 78–7700 W/cm2. The effects of particle size, milling time, and inclusion content on the resulting ignition delay, product particle size distributions, and microexplosion tendencies are reported. For example particles with higher PTFE content (30 wt%) had laser flux ignition thresholds as low as 77 W/cm2, exhibiting more burning particle dispersion due to microexplosions compared to the other materials considered. Composite Al/LDPE particles exhibit relatively high ignition thresholds compared to Al/PTFE particles, and microexplosions were observed only with laser fluxes above 5500 W/cm2 due to low LDPE reactivity with Al resulting in negligible particle self-heating. However, results show that microexplosions can occur for Al containing both low and high reactivity inclusions (LDPE and PTFE, respectively) and that polymer inclusions can be used to tailor the ignition threshold. This class of modified metal particles shows significant promise for application in many different energetic materials that use metal fuels.

January 2012

DOI: https://www.researchgate.net/publication/269621306_Combustion_of_bimodal_aluminum_particles_and_ice_mixtures

Terrence L. Connell, Jr., Grant A. Risha, Richard A. Yetter, Vigor Yang, & Steven F. Son

Abstract

The combustion of aluminum with ice is studied using various mixtures of nano- and micrometersized aluminum particles as a means to generate high-temperature hydrogen at fast rates for propulsion and power applications. Bimodal distributions are of interest in order to vary mixture packing densities and nascent alumina concentrations in the initial reactant mixture. In addition, the burning rate can be tailored by introducing various particle sizes. The effects of the bimodal distributions and equivalence ratio on ignition, combustion rates, and combustion efficiency are investigated in strand experiments at constant pressure and in small lab-scale [1.91 cm (0.75 in.) diameter] static firedrocket-motor combustion chambers with center-perforated propellant grains. The aluminum particles consisted of nanometer-sized particles with a nominal diameter of 80 nm and micron-sized particles with nominal diameters of 2 and 5 μm. The micron particle addition ranged from 0% to 80% by active mass in the mixture. Burning rates from 1.1 (160 psia) to 14.2 MPa (2060 psia) were determined. From the small scale motor experiments, thrust, C*, Isp, and C* and Isp efficiencies are provided. From these results, mechanistic issues of the combustion process are discussed. In particular, overall lean equivalence ratios that produce flame temperatures near the melting point of alumina resulted in considerably lower experimental C* and Isp efficiencies than equivalence ratios closer to stoichiometric. The infstitution of micron aluminum for nanometer aluminum had little effect on the linear burning rates of Al/ice mixtures for low-mass infstitutions. However, as the mass addition of micron aluminum increased (e.g., beyond 40% 2-μm aluminum in place of 80-nm aluminum), the burning rates decreased. The effects of bimodal aluminum compositions on motor performance were minor, although the experimental results suggest longer combustion times are necessary for complete combustion.

September 2013

DOI: https://www.researchgate.net/publication/269567051_CuOAl_Thermites_for_Solid_Rocket_Motor_Ignition

David A. Reese; Darren M. Wright; Steven Son

Abstract

A safe, fast, repeatable means of ignition is required for lab-scale solid rocket motor test experiments. To prevent early burn data from being obscured, an energetic compound with a high reaction rate is needed. A thermite mixture based on micron copper (II) oxide and micron aluminum was chosen for this purpose. This work investigates the efficacy of CuO/Al thermites for lab-scale rocket motor ignition, including experiments on safety testing, packaging, initiation, igniter size determination, and hot fire testing. The end result is a safe, inexpensive, efficient, and readily available method of igniting lab-scale solid rocket combustors.

July 2011

DOI: https://www.researchgate.net/publication/268479247_Further_Development_of_an_Aluminum_and_Water_Solid_Rocket_Propellant

David E. Kittell, Timothée L. Pourpoint, Lori J. Groven, and Steven F. Son

Abstract

Nanoscale aluminum and water has been used as a stepping stone towards in-situ rocket propellants and as a testbed for nanoenergetic composite propellants. A baseline formulation of nanoscale aluminum and water was developed and demonstrated with a sounding rocket flight in 2009. Performance of the propellant was not optimized, hence a reformulation was sought with an emphasis on improved safety and more efficient combustion. The chosen reformulation is a bimodal powder distribution of 70 wt.% Novacentrix 80 nm Al and 30 wt.% Valimet 2 μm Al at an equivalence ratio of 0.813 (optimized for sea level Isp). The mixture also includes 3 wt.% ammonium dihydrogen phosphate, to inhibit the slow reaction of nanoaluminum with water, and 1 wt.% polyacrylamide to improve material suspension. Ammonium dihydrogen phosphate can protect nanoaluminum in solution for several hours, but degradation can occur while mixing, and pH increases from slightly acidic to basic with increased mixing time and temperature. The stress of mixing might be removing the coating and exposing nanoaluminum to water. It is also shown that nanoaluminum reacts faster in basic aqueous solutions than in solutions with neutral pH. Static motor tests reveal that propellant formulations with neutral pH provide better performance. Implementations of shorter mixing times and reduced temperatures are used to control the pH of the propellant, resulting in increased Isp values of as much as 30%. © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

April 2011

Link: https://apps.dtic.mil/sti/tr/pdf/ADA546818.pdf

Grant A. Risha, Terrence L. Connell, Jr, Michael Weismiller, Richard A. Yetter, 5d. PROJECT NUMBER Dilip S. Sundaram,  Vigor Yang, Tyler D. Wood, Mark A. Pfeil, Timothee L. Pourpoint, John Tsohas and Steven F. Son

Abstract

Frozen solid propellants based on nAl and ice mixtures have been studied. In particular, the burning rate was measured for frozen aluminum ice mixtures, a model was developed for the combustion of aluminum water mixtures, small scale motor firing tests were reported to examine chemical efficiency and performance, an internal ballistics analysis of the combusting aluminum and ice motor grain was developed using a lumped-parameter model for motor development, and the results from an initial sounding rocket launch using a nonoptimized frozen aluminum-water grain were attained. In addition, various additives to the nAl and ice mixture, such as alane, ammonia borane, and hydrogen peroxide, were surveyed with initial studies to investigate overall combustion behavior and motor performance.

2011

DOI: https://doi.org/10.1063/5.0067523

Allison R. Range; Nicole R. McMindes; Jacob Morris; Bryce A. Geesey; Jeffrey F. Rhoads

Abstract

This work seeks to explore the macroscale, thermomechanical response of polymer-bonded composite energetic materials in their inert form to high-frequency mechanical excitation in the range of 1–100 kHz. Cylindrical samples were fabricated according to a mock PBXN-109 formulation, consisting of hydroxyl-terminated polybutadiene, mock RDX material (sucrose), and varying ratios of spherical aluminum powder. Experiments were performed utilizing laser Doppler vibrometry and infrared thermography in order to analyze the thermal and mechanical response of the samples when mechanically excited using a piezoelectric shaker. Thermal analysis of the samples revealed temperature rises on the order of ⁠, with several samples reaching within a 15 min experiment. Generally, formulations containing a higher weight percentage of aluminum additive content tended to exhibit greater temperature increases than those with pure sucrose embedded in the binder. The investigation presented herein serves as an advancement toward the complete characterization of these composite materials in this frequency range.

Texas Tech University and NSWC WD, China Lake CA & others

April 2008

DOI: https://doi.org/10.1016/j.combustflame.2007.11.014

Matt Jackson, Michelle L. Pantoya, Walt Gill

Abstract

This study details the characterization and implementation of a burner designed to simulate solid propellant fires. The burner was designed with the ability to introduce particles (particularly aluminum) into a gas flame. The aluminized flame conditions produced by this burner are characterized based on temperature and heat flux measurements. Using these results, flame conditions are quantified in comparison to other well-characterized reactions including hydrocarbon and propellant fires. The aluminized flame is also used to measure the burning rate of the particles. This work describes the application of this burner for re-creating small-scale propellant flame conditions and as a test platform for experiments that contribute to the development of a particle combustion model, particularly in propellant fires.

March 2021

DOI: https://www.researchsquare.com/article/rs-289403/v1

Quan Tran, Michelle L. Pantoya, Igor Altman

Abstract

Experiments were designed to investigate two regimes of metal particle combustion: fast and slow burning regimes. Stress-altering aluminum particles had been shown to produce a distinctly faster burning rate compared to untreated aluminum particles. The root cause for the differences in burning rate had been unclear. In this study, stress-altered and untreated aluminum particles were reacted as dispersed powder in a closed bomb calorimeter designed to monitor the transient temperature changes resulting from energy release upon combustion. The product residue was analyzed for size and species concentration. Results showed metastable γ-alumina that is associated with nano-oxide formation was in substantially higher concentration for stress-altered particle reactions that produced greater energy transfer rates. The increased energy transfer rate corresponded to higher radiant energy emission owing to condensation of nano-oxide particles. This study justifies condense-luminescence as a means for increasing the energy release rate of aluminum particles. By strategically altering metal fuels to control a formation of nano-oxide particles upon combustion, appreciable increases in the radiant energy flux can transform energy release rates.

March 2024

DOI: https://doi.org/10.1016/j.combustflame.2024.113310

Igor Altman, Michelle L. Pantoya

Abstract

Recently demonstrated high values of radiative loss during metal combustion require a new approach to describing dust flames. In particular, given strong light emission, the adiabatic flame temperature (AFT) concept is not applicable due to high radiative losses. Accordingly, global flame characteristics such as an expansion factor become unrelated to the AFT. That expansion factor can be directly inferred from the flame geometry, and therefore, its analysis offers a simple path to justify the need for a detailed energy balance in metal dust combustion. The analysis can also provide insight into peculiarities of the temperature distribution within the flame. In the current paper, previously published data on flow velocities in a metal dust flame are used for the expansion factor analysis. A relatively low value of the obtained expansion factor is reconciled with the advanced comprehension of metal particle combustion.

March 2023

DOI: https://doi.org/10.1016/j.csite.2023.102809

Harrison Jones, Pascal Dube, Quan Tran, Michelle L. Pantoya, Igor Altman

Abstract

Metal combustion is a process accompanied by strong light emission. Correspondingly, radiative loss can significantly affect the overall energy balance, and needs to be considered in the global numerical models describing metal dust combustion. In this work, we experimentally estimated the fraction of radiative loss during aluminum (Al) dust combustion by studying the heat release in a modified constant volume bomb calorimeter that enabled the additional measurement of pressure. The previously developed method of dispersing powder ensured nearly 100% combustion efficiency. The contribution of the combustion energy to heating the gas inside the calorimeter bomb was determined by analyzing the measured pressure traces and found to be measurably lower than 100%. The energy loss was attributed to radiant heat transfer from burning metal particles to the bomb wall. Aluminum powders with median size ranging from 4 μm to 100 μm were studied. The estimated fraction of radiative loss depended on the particle size. Radiative loss saturated at nearly 50% for larger particles and gradually reduced with the particle size decrease below 20 μm. We related the observed radiative loss to a recently introduced process that occurs during metal combustion, namely condense-luminescence. The results shown here have important implications for the role of radiant energy exchange in metal particle combustion and will transform future approaches to harnessing metal oxidation energy for a multitude of applications.

January 2022

DOI: https://iopscience.iop.org/article/10.1088/1361-6501/ac47bc/meta

Quan Tran, Igor Altman, Pascal Dube, Mark Malkoun, R Sadangi, Robert Koch and Michelle L Pantoya

Abstract

Off-the-shelf calorimeters are typically used for hydrocarbon-based fuels and not designed for simulating metal powder oxidation in gaseous environments. We have developed a method allowing a typical bomb calorimeter to accurately measure heat released during combustion and achieve nearly 100% of the reference heat of combustion from powder fuels such as aluminum. The modification uses a combustible organic dispersant to suspend the fuel particles and promote more complete combustion. The dispersant is a highly porous organic starch-based material (i.e. packing peanut) and allows the powder to burn as discrete particles thereby simulating dust-type combustion environments. The demonstrated closeness of measured Al heat of combustion to its reference value is evidence of complete metal combustion achieved in our experiment. Beyond calorific output under conditions simulating real reactive systems, we demonstrate that the calorimeter also allows characterization of the temporal heat release from the reacting material and this data can be extracted from the instrument. The rate of heat release is an important additional parameter characterizing the combustion process. The experimental approach described will impact future measurements of heat released during combustion from solid fuel powders and enable scientists to quantify the energetic performance of metal fuel more accurately as well as the transient thermal behavior from combusting metal powders.

July 2022

DOI: https://www.sciencedirect.com/science/article/abs/pii/S0010218022000736?via%3Dihub

Andrew R. Demko, Kevin J. Hill, Elektra Katz Ismael, Alan Kastengren

Abstract

Particle interactions with the binder melt layer are a major factor in the combustion efficiency and stability of the propellant in a solid rocket motor. Metal particles tend to agglomerate on the surface of burning solid propellants, inhibiting the combustion process. Therefore, reduction of molten aluminum agglomeration in a solid rocket motor is vital to the improvement of solid rocket propulsion system performance. In other work, quenched samples have been used to study the impact of how metal particles alter the flow properties of the molten propellant surface. In-situ optical measurements have also been attempted for these particle-condensed phase interactions, but with little success due to the opacity and strong gradients within the flame. This study expanded on previous work using synchrotron–based x-rays to directly image these aluminum agglomerates as they interacted with the binder. X-ray absorption and phase contrast in the images allowed for the direct measurement of the particle size in combusting propellants in-situ at typical rocket pressures. Particle size distributions were collected in an optically accessible combustion vessel over a pressure range of 1.4- to 6.9-MPa (200- to 1000-psig). This investigation measured the aggregate sizes ranging from 300-to 650-µm at a 6.9 MPa chamber pressure. Interestingly, the lowest binder melting temperature produced the longest aluminum chaining at 1.1 mm and the highest binder melting temperature produced the second longest chain at 524 µm long. The study observed aluminum interactions with the binder melt layer on the propellant surface that may be a contributing factor in a plateau propellant. Additionally, data pointed to possible relationships between binder viscosity, burning rate, aluminum clusters, and chain formation in a solid propellant. The study concluded with collecting data on the molten aluminum volumetric changes in a propellant environment, thus defining a starting particle diameter for particle regression rates. Collecting the regression rates also provided information on the evaporation rate of aluminum, which burned in the propellant flame. All of these data are critical for understanding the combustion of aluminum-based solid rocket propellants and outlines knowledge gaps that need additional data.

September 2022

DOI: https://doi.org/10.1016/j.jaecs.2022.100080

Quan Tran, Michelle L. Pantoya, Igor Altman

Abstract

Thermal processing of aluminum (Al) particles such as annealing followed by rapid quenching had been previously shown to affect single metal particle burning rates. This study extends single particle combustion to a global material-based energy exchange model. Experiments were designed to investigate the global energy exchange resulting from Al powder suspensions processed to induce different (fast and slow) burning regimes. Thermally processed and untreated Al particles were reacted as suspended powder in a closed bomb calorimeter. The calorimeter monitored the transient temperature changes resulting from energy release upon powder combustion. The product residue was analyzed for species concentration using X-ray diffraction. Results link the phase fractions of the aluminum oxide combustion products with global radiant fluxes in the calorimeter system. Metastable alumina associated with nano-oxide formation is in substantially higher concentration for thermally processed powder reactions and also produces greater energy transfer rates. The increased energy transfer rates correspond to higher radiant energy emission which may result from condensation energy associated with nano-oxide particle formation. This study qualifies condense-luminescence as a means for increasing the energy release rates of aluminum particles. By strategically altering metal fuels to control formation of nano-oxide particles upon combustion, appreciable increases in the radiant energy flux can transform energy release rates.

McGill University

October 2020

DOI: https://www.tandfonline.com/doi/abs/10.1080/00102202.2020.1820496?journalCode=gcst20

Frederic Blais, Philippe Julien, Jan Palecka, Samuel Goroshin, Jeffrey M. Bergthorson

Abstract

Measurements of how the flame speed in suspensions of metal fuels depends on the initial temperature of the unburned mixture are important for understanding the role of mixture preheat by radiation heat transfer. This preheat can be an important stabilization mechanism for metal dust flames in energetic devices. A newly constructed counter-flow flat-flame metal dust burner allows for the measurement of burning velocities in aluminum-air suspensions preheated to temperatures up to 524 K using Particle Image Velocimetry (PIV). The experimental method was verified by measuring and comparing burning velocities in preheated methane-air mixtures at different fuel-oxygen equivalence ratios with previous experimental data and theoretical predictions. Whereas the flame speed in methane-air mixtures increases by 2.75 times with an increase in temperature to about 524 K, the flame speed in aluminum-air mixtures increases by less than 2 times over the same temperature interval. The independence of the adiabatic flame temperature of aluminum-air flames on the initial temperature of the mixture suggests practically constant reaction rates either for kinetically- or diffusion-controlled aluminum combustion. Thus, the observed moderate dependence of the aluminum-air flame speed on mixture initial temperature can be explained by a cumulative effect of the increased heat diffusivity, decrease in the amount of heat required to reach the particle ignition temperature, and increased flame sensitivity to preheat due to discrete source effects discussed in recent flame models.

June 2017

DOI: https://doi.org/10.1016/j.combustflame.2017.03.006

Michael Soo, Samuel Goroshin, Nick Glumac, Keishi Kumashiro, James Vickery, David L. Frost, Jeffrey M. Bergthorson

Abstract

Imaging emission spectroscopy, spatially resolved laser-absorption spectroscopy, and particle image velocimetry (PIV) are applied to a flat flame stabilized in a suspension of micron-sized aluminum. The results from the combination of diagnostics are used to infer the combustion regime of the particles and to estimate the characteristic combustion time of the suspension. It is observed that the reaction zone of the flame in stoichiometric aluminumair suspensions exhibits strong self-reversal of the atomic aluminum emission lines. These lines also exhibit high optical depths in both emission and absorption spectroscopy. The strong self-reversal and high optical depths indicate high concentrations of aluminum vapor within the reaction zone of the flame at multiple temperatures. These features provide evidence of the formation of vapor-phase micro-diffusion flames around the individual particles in the suspension. In aluminum-methane-air flames, the lack of self-reversal and lower optical depths of the aluminum atomic lines indicate the absence of vapor-phase micro-diffusion flames, and point to a more heterogeneous, and likely kinetically-controlled, particle combustion regime. The reaction zone thickness is estimated from the spatially resolved profiles of aluminum resonance lines in both absorption and emission through the flame. The emission measurements yield a reaction zone thickness on the order of 1.7±0.3 mm in aluminum-air flames, and the absorption measurements yield a thickness on the order of 2.3±0.5. It is demonstrated that the combination of the combustion zone thickness measurement, flame temperatures determined from molecular AlO emission spectra, and particle velocity measurements from the PIV diagnostic permits an estimation of the burning time in the suspension. The burning time in stoichiometric aluminum-air suspensions using the suite of diagnostics is estimated to be on the order of 0.7 milliseconds.

2021

DOI: 10.1016/j.proci.2020.09.017

Jan Palečka, Judy Park, Samuel Goroshin, Jeffrey M Bergthorson

Abstract

This paper introduces a novel Hele-Shaw cell apparatus to be used for the study of propagation and stability phenomena in heterogeneous flames. In particular, the apparatus is used to experimentally examine the coupling/decoupling of dual-front flames propagating in suspensions of micron-size aluminum particles in propane-air gas mixtures at varying gas equivalence ratios and aluminum concentrations. The results show that the thermal coupling that exists between the primary propane-air flame front and the secondary aluminum flame front is a strong function of the rate of reaction, and of the temperature, of the secondary front and much less dependent on the reaction rate or temperature of the primary front. It is also shown that flame instabilities in hybrid aluminum-propane-air flames significantly increase the flame surface area, enhance the propagation rate, and can also exhibit complex interactions with front coupling.

2021

DOI: https://doi.org/10.1016/j.pecs.2022.100994

Samuel Goroshin, Jan Palečka, Jeffrey M. Bergthorson

Abstract

This paper critically reviews the theoretical and experimental literature regarding the fundamental aspects of flames in nonvolatile solid fuel suspensions. Unlike volatile fuels that form continuous premixed gaseous flame sheets, flame fronts in nonvolatile suspensions are driven by heterogeneous reactions localized on the surface, or near the surface, of individual particles. Practically all peculiarities of heterogeneous flames can be linked to this “flame-inside-the-flame” combustion front structure. These localized reactions enable particles to self-heat and transition from kinetically to diffusion-limited heterogeneous reaction during the process of particle ignition. After ignition, burning particles behave as individual diffusion micro-reactors that are insensitive to the bulk gas temperature and overall heat loss from the system. Relatively small quenching distances of the flame in suspensions, long plateaus in the dependence of burning velocity on fuel concentration stretching to very fuel-rich mixtures, and the discrete flame propagation regime, where burning velocity is insensitive to particle combustion time and the flame-front structure is rough and nonuniform, are all manifestations of particle ignition and combustion in the diffusion-limited regime. This review summarizes the key experimental evidence of laminar flame structure and flame speed from a variety of experimental apparatus both in the laboratory and under microgravity conditions, and interprets these results in terms of relatively simple theoretical models. Heterogeneous flames are observed to exhibit many of the thermodiffusive and hydrodynamic instabilities of homogeneous flames, as well as several new instabilities that arise from the multiphase nature of the fuel and particle ignition and extinction. Flames of binary mixtures of heterogeneous fuels, or gaseous and solid fuel mixtures, are also reviewed and it is shown that a simple model based on matching the flame speed between thermally interacting fronts can capture the key physics. Finally, the last chapter of the review discusses why the important or even crucial role of radiation heat transfer predicted by theoretical models for flames in suspensions is not supported by the available experimental evidence. It is argued that large spatial scales of radiation heat transfer do not permit separation of the radiation transfer problem from boundary conditions and flow configuration, making one-dimensional flame models that include radiation inadequate for the description of flames in the laboratory and even in relatively large unconfined dust clouds.

2021

Link: https://escholarship.mcgill.ca/concern/theses/9w032781h

Geoffrey Chase, David Frost

Abstract

The detonation of suspensions of nanometric and flake aluminum powders mixed with aqueous solutions of dilute hydrogen peroxide (H2O2) was experimentally investigated. The nano-Al powder was coated with 8–10 wt% Viton and had a nominal diameter of 91 ± 27 nm. The flake-Al powder was coated with 10 wt% trimethylolpropane trimethacrylate and had a surface area of 5.09 m2/g. Detonation velocity and cylinder wall expansion tests were conducted in aluminum- and PVC-encased charges measured with shock pins and photonic Doppler velocimetry. Mixtures containing aqueous solutions of 10–20 wt% H2O2 detonated at 2.9–3.5 km/s, with variations in the detonation velocity attributed to variations in time-dependent density. Mixtures containing 10 wt% H2O2 solutions did not consistently detonate, indicating that porosity from hydrogen peroxide decomposition has a sensitizing effect. Mixtures containing only distilled water, 5 or 7.5 wt% H2O2 solutions, dilute ammonium nitrate solutions, glass microballoons, or micron-scale spherical aluminum powder failed to sustain detonation, but could potentially sustain detonation with optimization of the shot processing.

January 2006

DOI: 10.1063/1.2263484

Article

David L. Frost, Samuel Goroshin, Jeff Levine, Robert Ripley, and Fan Zhang

Abstract

The critical conditions for the ignition of spherical aluminum particles dispersed during the detonation of long cylindrical explosive charges have been investigated experimentally. The charges consist of packed beds of aluminum particles (Valimet, CA), ranging in size from 3 -115 mum in diameter, and saturated with sensitized liquid nitromethane. The ignition conditions depend on both the charge and particle diameters, which govern the thermal history of the particles as they are dispersed within the conically expanding products. For a given charge diameter, the most reactive particles correspond to an intermediate size (˜55 mum dia). For this particle size, with increasing charge diameter the particle reaction behavior progresses through several distinct regimes: i) no particle reaction, ii) reaction at isolated spots, iii) reaction in distinct radial bands, and iv) continuous reaction of the particle cloud. In each case, a separation between the detonation front and the onset of aluminum reaction is always observed. To determine the point of particle ignition, visible radiation from the charge is recorded, through a slit, with a 3-color pyrometer and with a line spectrometer, with the wavelengths chosen to overlap the AlO emission lines.

March 2021

DOI: https://doi.org/10.1002/prep.202000328

David L. Frost, John-Mark Clemenson, Samuel Goroshin, Fan Zhang, Michael Soo

Abstract

The detonation of a metalized explosive generates a fireball that has a spatially non-uniform distribution of particle concentration and gas temperature. The transient gas temperature field must be probed with ruggedized spatially- and temporally-resolved diagnostics. The use of in-situ thermocouples for temperature measurements within multiphase fireballs is demonstrated. Although the thermocouple temperature lags behind the local gas temperature, the transient gas temperature is assessed by modeling the sensor assuming first-order response and using two analysis methods: (1) when the thermocouple temperature trace reaches a local extrema, the thermocouple temperature is instantaneously equal to the local gas temperature, and (2) reconstructing the gas temperature trace using multiple co-located thermocouples of different lag responses. The temperature history within the fireball at various distances is presented for charges consisting of packed beds of particles saturated with liquid nitromethane. The results for reactive particles (Al, Ti, Zr) are compared with non-reactive particles (Fe), as well as homogeneous NM charges. For NM charges, a maximum gas temperature of about 1100 K occurs at times on the order of 100’s of milliseconds, less than the temperature of the burning soot in the fireball (∼1900 K). With Al particles, the gas temperature is spatially non-uniform due to particle jetting and non-uniform particle combustion, but gas temperatures up to about 1800 K are recorded for times up to 0.5 s, less than the temperature of the burning particles (∼2700 K). Inert particles act as a heat sink and the thermocouple temperatures recorded did not exceed 400 K.

Other Universities and Research Centers

January 2013

DOI: https://doi.org/10.1016/j.combustflame.2012.12.011     

Yasmine Aly, Mirko Schoenitz, Edward L. Dreizin

Abstract

Adding aluminum to propellants, pyrotechnics, and explosives is common to boost their energy density. A number of approaches have been investigated that could shorten aluminum ignition delay, increase combustion rate, and decrease the tendency of aluminum droplets to agglomerate. Previous work showed that particles of mechanically alloyed Al·Mg powders burn faster than similarly sized particles of pure aluminum. However, preparation of mechanically alloyed powders with particle sizes matching those of fine aluminum used in energetic formulations was not achieved. This work is focused on preparation of mechanically alloyed, composite Al·Mg powders in which both internal structures and particle size distributions are adjusted. Milling protocol is optimized to prepare equiaxial, micron-scale particles suitable for laboratory evaluations of their oxidation, ignition, and combustion characteristics. Oxidation of the prepared powders is studied using thermo-analytical measurements. Ignition is characterized experimentally using an electrically heated filament setup. Combustion is studied using a constant volume explosion setup for the powder cloud combustion, and a laser ignition setup for characterization of combustion rates and temperatures for individual particles. For the prepared materials, ignition and combustion characteristics are compared to those of pure Al. It is observed that the mechanically alloyed powders ignite at much lower temperatures than Al. Once ignited, the particles burn nearly as fast as Al, resulting in an overall improvement of the combustion performance.

2008

Link: https://www.osti.gov/servlets/purl/1144885

Marcia A. Cooper and Anita M. Renlund

Abstract

The effects that dispersed aluminum particles in explosives have on performance are of continuing interest to the enhanced blast community. Two experimental studies have been conducted to parametrically investigate the material response of aluminized explosive mixtures to shock and detonation. It is commonly known that the particle-particle shear forces are highest at shock loadings characteristic of detonation providing optimum conditions for removal of the oxide layer from the metal particles which is necessary to initiate chemical reaction with neighboring species. By conducting experiments at both shock and detonation conditions, a range of inter-particle forces and their effect on performance may be explored. The first study utilized a gas gun with projectile velocities between 0.4 and 1.2 km/s to subject a lowdensity (64-68% TMD) pressed pellet of aluminized HMX to shock loading. The VISAR-measured particle velocities were recorded. For relatively low impact velocities less than 0.7 km/s, the aluminum particles reduced the sensitivity of the mixture as compared to pure HMX. For the higher impact velocities, evidence of rapid reaction is observed. An unreactive pressure-velocity analysis was completed that shows the initial post-shock particle velocities can be predicted from the material Hugoniots assuming the aluminum particles remain inert. The second study involved detonating high-density (>90% TMD) pressed pellets of aluminized HMX, RDX and Composition B in air and inert gas environments. The broadband emission and filtered emission in the wavelength of aluminum oxide emission were recorded. Comparisons between materials show greater aluminum oxide emission in the Composition B mixtures and in air environments.

August 2020

DOI: https://etda.libraries.psu.edu/files/final_submissions/21951

Garett Foster

Abstract

Aluminum is an energy-dense metal that reacts exothermically with oxygen, in addition to a range of other oxidizers, making it a potentially useful fuel for thermal propulsion and power applications. Fine aluminum dusts, with particle diameters on the order of micrometers, can be aerosolized and mixed with a gaseous oxidizer to produce dust flames from which heat can be extracted. The reactive properties of aluminum, combined with its natural abundance and long-term stability, make it a prospective replacement for fossil fuels in some energy or power systems. In order for the potential utility of aluminum-air dust flames to be assessed, understanding of the fundamental combustion behavior, such as the burning velocity and temperature, is necessary. Laminar aluminum-air dust flames have been previously studied at atmospheric pressure to measure the burning velocity and flame temperature. Typically, the effects of varying stoichiometry and particle size are reported. However, limited information exists on the effects of pressure and turbulence, which are often present in practical systems. This thesis will investigate aluminum-air dust flames with a focus on the effect of polydisperse particle size distributions, pressure, and turbulence intensity on the resulting burning velocity. Temperature fields of aluminum-air dust flames will also be investigated and reported for the first time. An experimental system was developed that allows metal dust flames to be observed within an optically-accessible high-pressure chamber. Using a high-speed camera, aluminum-air flames were imaged through two narrowband interference filters (700 nm and 900 nm). The images were used to calculate burning velocity along with a two-dimensional temperature field of the flame. All experiments were performed with fuel-rich aluminum-air mixtures. Three micron-scale aluminum powders with different size distributions of particles were tested at atmospheric pressure. The laminar burning velocity was found to increase with decreasing average particle size. A single size distribution of particles was tested at pressures ranging from atmospheric to ~105 psi. Laminar iv burning velocity was found to decrease with increasing pressure (P), with an approximate proportionality to P-0.6 over the range of pressures tested. This was potentially due to the decrease in interparticle spacing and enhanced particle-particle interactions as pressure increased. A single particle size distribution was also tested at four different turbulence levels. The turbulence intensity at each level was measured using particle image velocimetry in a representative non-reacting flow. Burning velocity was found to increase with increasing turbulence intensity (TI), with an approximate proportionality to TI1.0 over the range of conditions tested. Two-dimensional temperature fields were measured for two size distributions of particles and at elevated pressures up to ~105 psi using two-color ratio pyrometry. However, due to spectrally-dependent scattering/absorption and unknown emissive properties of the collected incandescent signal, only relative comparisons between the temperature measurements could be made.

July 2021

Link: https://www.osti.gov/servlets/purl/1809919

Marcia A. Cooper

Abstract

Ignition and material response properties of aluminumized HMX heterogeneous explosive mixtures were explored in a series of planar impact experiments performed over multiple years. This work expands on previous work studying material response to impact in single-component HMX granular materials. The addition of nanometric aluminum is shown to affect the ignition sensitivity and growth to reaction from impact. The gas gun test results are presented here varying parameters of particle size, shock strength, and aluminum mass fraction.

June 2023

DOI: https://doi.org/10.1016/j.combustflame.2023.112689

Michael J. Soo, Zachary E. Loparo, Rohit J Jacob, Brian T. Fisher

Abstract

A stabilized methane/air Bunsen flame is seeded with powder fuel mixtures containing various combinations of spherical, micron-sized aluminum (Valimet H-2 with nominal d50 = 3.5 µm and H-15 with nominal d50 = 20 µm) and silicon carbide (SiC) at different mass ratios over a ∼0-400 g/m3 concentration range. It is observed that in dispersions of only H-2 aluminum, a bright white aluminum flame front forms and couples to the methane-air flame, resulting in a sustained flame speed even at concentrations beyond 400 g/m3. In contrast, dispersions of only H-15 powder decrease flame speed rapidly and cause an open-tipped methane-air flame at concentrations beyond 200 g/m3, similar to inert SiC powder. When blended together into H-2:H-15 1:1 (mass ratio) mixture dispersions, an aluminum flame front forms and couples to the methane-air flame, with a flame speed comparable to unitary H-2 aluminum in contrast to mixtures of H-2:SiC 3:1 which produce no noticeable difference in flame speed from purely inert mixtures of SiC at nearly 200 g/m3. Mixtures of H-2:SiC 3:1 demonstrate an aluminum flame coupling to the methane flame, but with an increased separation between the fronts that was not observed in previous hybrid flames studies. The flame temperatures, flame coupling, and aluminum combustion efficiency behaviors are attributed to the effective amount of slowly reacting or inert solid material in the mixed powder fuels. The behaviors are consistent with a simple hybrid flames model, developed in previous work, where the effectively reduced heat of reaction of the powder fuel is unable to support sufficient heat feedback to the methane-air flame permitting effective secondary flame front formation and flame-coupling. From this understanding, a method for determining the relative energy contribution of a lower reactivity component in a fuel mixture when it is thermally driven by a higher reactivity fuel is demonstrated using the secondary front formation and flame coupling as a benchmark for reaching a certain heat of reaction of the fuel solid mixture. It is estimated that the H-15 component of a H-2:H-15 1:2 mixture contributes to approximately 55% of the thermal energy required to achieve the same behavior as a H-2:SiC 3:1 fuel mixture.

2009

DOI: https://doi.org/10.1016/j.proci.2008.06.205

Patrick Lynch, Herman Krier, Nick Glumac

Abstract

A study of the combustion times for aluminum particles in the size range of 3–11 lm with oxygen, car[1]bon dioxide, and water vapor oxidizers at high temperatures (>2400 K), high pressures (4–25 atm), and oxidizer composition (15–70% by volume in inert diluent) in a heterogeneous shock tube has generated a correlation valid in the transition regime. The deviation from diffusion limited behavior and burn times that could otherwise be accurately predicted by the widely accepted Beckstead correlation is seen, for example, in particles below 20 lm, and is evidenced by the lowering of the diameter dependence on the burn time, a dependence on pressure, and a reversal of the relative oxidizer strengths of carbon dioxide and water vapor. The strong dependence on temperature of burn time that is seen in nano-Al is not observed in these micron-sized particles. The burning rates of aluminum in these oxidizers can be added to predict an overall mixture burnout time adequately. This correlation should extend the ability of mod[1]elers to predict combustion rates of particles in solid rocket motor environments down to particle diameters of a few microns.

April 2022

https://doi.org/10.1364/OL.456342

K. A. Daniel, C. M. Murzyn, D. J. Allen, K. P. Lynch, C. R. Downing, and J. L. Wagner

Abstract

This work advances laser absorption spectroscopy with measurements of aluminum monoxide (AlO) temperature and column density in extreme pressure (P > 60 bar) and temperature (T > 4000 K) environments. Measurements of the AlO A 2Πi–X 2Σ + transition are made using a micro[1]electromechanical system, tunable vertical cavity surface emitting laser (MEMS-VCSEL). Simultaneous emission measurements of the AlO B 2Σ +–X 2Σ + transition are made along a line of sight that is coaxial with the laser absorption. Absorption temperature fits agree with emission spectra for a T =3200 K, P=9 bar case. In cases with T > 4000 K, P > 60 bar, absorption fits match the ambient temperature while emission fits over-estimate it, owing to high optical depths. These data juxtapose passive and active spectro[1]scopic methods and demonstrate the versatility of AlO laser absorption in high-pressure and high-temperature environments where experimental data remain scarce, and engineering models will benefit from refined measurements.

February 2023

DOI: https://doi.org/10.1016/j.combustflame.2022.112532

G. Foster, N.J. Kempema, J.E. Boyer, J.R. Harris, R.A. Yetter

Abstract

Aluminum is an energy-dense metal that reacts exothermically with a range of oxidizers, making it a potentially useful fuel for certain thermal energy and power applications (e.g., boilers, swirl-stabilized burners, rockets, etc.). In addition to its reactive properties, aluminum is naturally abundant and commercially available in fine powders that are relatively inexpensive and chemically stable. Fine aluminum dusts, with particle diameters on the order of micrometers, can be aerosolized and mixed with a gaseous oxidizer such as air to create stable dust flames and heat for a thermodynamic cycle. In order to realize the potential utility of aluminum-air dust flames, the fundamental combustion behavior, such as the burning velocity, must be understood. In this work, an experimental system was developed that allows metal dust flames to be observed within an optically-accessible pressurized chamber. Using a high-speed camera, fuel-rich premixed aluminum-air jet flames were imaged through narrowband interference filters, and the data were used to calculate burning velocity. A single polydisperse size distribution of particles was tested at pressures ranging from 1 to 7.2 bar. Burning velocity was found to decrease with increased pressure (P), with an approximate proportionality to P−0.6 over the range of pressures tested (∼P−0.3 for only the laminar flow conditions). This reduction in burning velocity with increased pressure is hypothesized to occur due to increased oxidizer density along with a decrease in interparticle spacing, which may result in asymmetric particle heating and increased competition for oxidizer.

December 2010

DOI: https://www.sciencedirect.com/science/article/abs/pii/S0032591010003645

Laila J. Jallo, Mirko Schoenitz, Edward L. Dreizin, Rajesh N. Dave, Curtis E. Johnson

Abstract

Surface modification of aluminum powders for the purpose of flow improvement was performed and several samples were prepared. Correlations between the flowability and reactivity for these powders as well as for the initial untreated aluminum powder were established. The powders were characterized using Scanning Electron Microscope (SEM), particle size distribution, angle of repose flowability test, Constant Volume Explosion (CVE) combustion test, and Thermo-Gravimetric Analysis (TGA). The surface modification of micron-sized aluminum powders was done by: (1) dry coating nano-particles of silica, titania and carbon black onto the surface of spherical aluminum powders and (2) chemically and physically altering the surface properties of the same powders with methyltrichlorosilane. All surface modifications improved flowability of the powders. CVE measurements indicate that powders with an improved flowability exhibit improved combustion characteristics if the powder treatment does not add an inert component to aluminum. The TGA results do not show significant differences in the reactivity of various powders. Based on combined flowability and CVE characteristics, the silane modified material gave the best results followed by the powders dry coated with carbon, titania and silica, respectively.

March 2020

DOI: https://doi.org/10.1016/j.combustflame.2020.03.012

Journal: Combustion and Flame 217(18):93-102

Demitrios Stamatis, Elliot Wainwright, Shashank Vummidi Lakshman, Michael S Kessler, Tim Weihs

Abstract

Micron-sized composite particles consisting of an Al-Mg alloy and Zr were produced via mechanical milling. Three different particle chemistries were prepared with varying ratios of the Al-Mg alloy to Zr. In addition, the prepared powders were size selected using mechanical sieves. Explosively launched combustion properties of these powders were independently measured as a function of the particle stoichiometry and particle size. Ignition temperatures were measured utilizing a heated filament experiment while combustion efficiency was characterized by measuring the dynamic pressure produced in a closed bomb in which the powder was explosively dispersed under fixed enthalpy conditions. Commercial Al powder, Valimet H-2, was also tested alongside these materials as a benchmark. High-speed video and thermocouple measurements were also obtained for the closed bomb experiments. We observed an increase in combustion efficiency from 30% to 80-90% in the composite materials compared to the pure Al. Furthermore , reaction products were collected and analyzed by powder x-ray diffraction to gain further insight into combustion efficiency and reaction pathways. We observed significant improvement of combustion under these experimental conditions, including higher quasi-static pressures and higher rates of pressure rise, with composite fuels compared to pure Al, even without a secondary oxidizer additive.

January 2019

DOI: https://www.researchgate.net/publication/330975798_Effects_of_aluminum_composites_on_the_regression_rates_of_solid_fuels

Christian Paravan, Marco Stocco, Simone Penazzo, Juxhin Myzyri, Luigi T. DeLuca and Luciano Galfetti

Abstract

Innovative, mechanically activated Al–polytetrafluoroethylene (PTFE) composites and ammonium perchlorate (AP) coated nano-sized aluminum (C-ALEX) were produced, characterized, and tested as solid fuel additives. The ballistics of fuel formulations based on hydroxylterminated polybutadiene (HTPB) was investigated in a microburner by a time-resolved technique for regression rate ( r f ) data reduction. Both Al-composites show promising results in terms of r f and mass burning rate enhancement. In particular, the C-ALEX showed a percent r f increase over the baseline (HTPB) of 27% at an oxidizer mass flux of 350 kg/(m ² s), without requiring dedicated dispersion procedures. This performance enhancement was nearly constant over the whole investigated range.

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