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Agglomerates, smoke oxide particles, and carbon inclusions in condensed combustion products of an aluminized GAP-based propellant
Acta Astronautica 129 (2016) 147–153
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Agglomerates, smoke oxide particles, and carbon inclusions in condensed combustion products of an aluminized GAP-based propellant ⁎
Wen Ao, Peijin Liu , Wenjing Yang Science and Technology on Combustion, Internal Flow and Thermal-structure Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Aluminum Solid propellants Condensed combustion products Agglomeration Solid rocket motor
In solid propellants, aluminum is widely used to improve the performance, however the condensed combustion products especially the large agglomerates generated from aluminum combustion significantly affect the combustion and internal flow inside the solid rocket motor. To clarify the properties of the condensed combustion products of aluminized propellants, a constant-pressure quench vessel was adopted to collect the combustion products. The morphology and chemical compositions of the collected products, were then studied by using scanning electron microscopy coupled with energy dispersive (SEM-EDS) method. Various structures have been observed in the condensed combustion products. Apart from the typical agglomerates or smoke oxide particles observed before, new structures including the smoke oxide clusters, irregular agglomerates and carbon-inclusions are discovered and investigated. Smoke oxide particles have the highest amount in the products. The highly dispersed oxide particle is spherical with very smooth surface and is on the order of 1– 2 µm, but due to the high temperature and long residence time, these small particles will aggregate into smoke oxide clusters which are much larger than the initial particles. Three types of spherical agglomerates have been found. As the ambient gas temperature is much higher than the boiling point of Al2O3, the condensation layer inside which the aluminum drop is burning would evaporate quickly, which result in the fact that few “hollow agglomerates” has been found compared to “cap agglomerates” and “solid agglomerates”. Irregular agglomerates usually larger than spherical agglomerates. The formation of irregular agglomerates likely happens by three stages: deformation of spherical aluminum drops; combination of particles with various shape; finally production of irregular agglomerates. EDS results show the ratio of O to Al on the surface of agglomerates is lower in comparison to smoke oxide particles. C and O account for most element compositions for all the carbon inclusions. The rough, spherical, strip shape and flake shape carbon-inclusions are believed to be derived from the degradation products of the binder or oxidizer, while the fiber silk is possibly the combustion product of fiber inside the heat insulation layer of the propellants. Images of products at different pressures reveal high pressure reduces the degree of agglomeration. The chemical compositions, size range and content of all the observed structures are given in this paper. Results of our study are expected to provide better insight in the working process of solid rocket motor.
1. Introduction Aluminum powders (typically of 5–20 µm radius) are usually added to solid rocket propellants to obtain high burning rates and increase the energetic performance. But the presence of aluminum can result in the formation of condensed combustion products (CCPs) which significantly influences the working process of the motor system: losses in specific impulse, slag deposition in the motor chamber, effects of combustion products on the inert motor components, and stability of motor operation. Gaining deep insight in the CCPs properties is a key problem for researchers to understand the combustion and internal
⁎
flow mechanism in the solid rocket motor. It is well known that agglomeration of aluminum takes place in the surface layer of the burning propellant: aluminum particles may coalesce at the propellant burning surface, which results in agglomerates that are larger than the initial virgin aluminum used. CCPs containing these large agglomerates then leave the burning surface, release into the gas phase and interact with the flow. So far, CCPs of aluminized propellants have been studied widely mainly based on the quench-collection method. Sambamurthi [1] used a plume quenchparticle collection setup to study aluminum agglomeration in solidpropellant combustion. Combustion products flowed to an ethanol pool
Corresponding author. E-mail address: [email protected] (P. Liu).
http://dx.doi.org/10.1016/j.actaastro.2016.09.011 Received 29 April 2016; Received in revised form 31 August 2016; Accepted 9 September 2016 Available online 10 September 2016 0094-5765/ © 2016 IAA. Published by Elsevier Ltd. All rights reserved.
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in which the burning agglomerates were quenched and collected. Glotov [2] developed a technique that frozen the combustion products by mixing with a concurrent cold flow of an inert diluents gas. Babuk [3,4] employed a constant-volume bomb to collect the particulate combustion products. Quenching of CCPs was accomplished using either alcohol or inert gas. Since the mass of the propellant samples was less than 2 g, the pressure in the bomb was constant within 10% during combustion. Jayaraman [5] and Anand [6] adopted the same set-up for agglomerate quench-collection. The ethanol bath was placed just 3 mm below the virgin propellant surface. The characteristics of the agglomeration process depend on the propellant formulations and environmental conditions. As a rule, the agglomerates are governed by the size of the parent aluminum particles and their percentage [1,4,6– 10], the size of AP particles [3,6,8], fraction of coarse and fine AP particles [1,5,7,9], oxidizer types [3,4,11], binder properties [4], distance from the burning surface [12], burning atmosphere [13], acceleration effects [14], chamber pressure [1,3,5,7,8,12,15,16] and burning rate of the propellant [6,10]. Recently, new methods were developed to explore agglomeration reduction, such as using tailored, mechanically activated aluminum/polytetrafluoroethylene composite particles [17], low-density polyethylene inclusion modified aluminum [18], porous aluminum [19], and nano-aluminum [5,20,21] as replacements for reference aluminum powders in propellants. As for the properties of CCPs, it is now generally accepted that CCPs in the burning of aluminized propellants of various classes can be classified into two basic categories: agglomerates and smoke oxide particles (SOPs). The agglomerates were found to be large spherical particles, while SOPs were massive particles having a regular, spherical shape and consisting of aluminum oxide [15,22]. More interestingly, Doi [23] found that the shape of luminous flame around agglomerated Al particles was dependent on the size of agglomerates. When the ratio of the luminous flame diameter to the agglomerated Al particles diameter was 1.54–1.71, the luminous flames were perfectly spherical. Otherwise, the luminous flames were elliptical. However, the available experimental data does not permit the discovery of the physical aspects of CCPs. A number of problems have not been studied sufficiently, including (i) not enough work devoted to systematic studies of the specific morphology and formation of nonspherical agglomerates, which seemed to abound in CCPs [17], (ii) few papers are concerned with the structures of carbon inclusions contained in CCPs (iii) according to published results, most papers investigated propellants based on isoprene rubber or HTPB as a fuel binder. It is necessary to obtain experimental information about the CCPs of NEPE (nitrate ester plasticized polyether) propellant which are also widely used at present, because the binder properties were suggested to affect the agglomeration process [3]. In this paper, we adopt a modified constant-pressure quench technique to collect the condensed combustion products of burning propellant, followed by the systematic, physical and chemical analysis of the collections. Morphology and compositions of various structures in CCPs are obtained, followed by the analysis of connection of the products and combustion mechanism of propellants. Such information, in combination with the available materials, would serve the purpose of clarifying the physical picture of aluminized propellants burning process in solid rocket motor.
Fig. 1. Schematic of the constant-pressure propellant combustion vessel.
densed combustion products from the process of propellant sample burning. Compared to previous experimental quenching system [1,3,11,17], the improvement of this apparatus designed and built by us is its “constant-pressure” nature, since pressure has significant effect on aluminized propellant combustion, especially on the agglomeration characteristics of aluminum [5,15]. To avoid the chemical reaction between high temperature aluminum particles and water, here quenching of products was accomplished using an inert gas (nitrogen) as in Babuk's work [3]. A schematic of this apparatus is shown in Fig. 1. It is a thick-walled cylindrical steel chamber of 0.8 m length and 300 mm internal diameter. The top and bottom ends of the chamber are closed with flanges. The top flange is equipped with propellant specimen holder, pressure transducer, igniter and solenoid valve for nitrogen gas inlet and combustion gas exhaust. An orifice plate made of steel with an orifice in the center is installed at the bottom of the vessel. The exhaust gas flow during propellant burning can be varied by changing the orifice size. The balance of burning gas injection and exhaust is achieved by choosing proper pore plate so that the pressure can be kept constant. The test pressure is in the range of 5.5–9 MPa. The analysis work is mainly based on the result of 7 MPa. To study the pressure effect on the CCPs, the results of 5.5 MPa, 7 MPa and 9 MPa are compared. Each operating condition was tested at least twice to check the reproducibility. The repeatability is within 5% based on the mean-mass diameter D43 of the CCPs size distributions obtained in different runs at the same condition. The combustion products flowed downward during the burning of the specimen, and the particles got quenched in the nitrogen gas. All the products were supposed to fall on the bottom of the vessel. However, a part of the fine particles may remain suspended in the gas after the end of burning. To settle the suspended particles in the bottom, the vessel was allowed to stand still for two hours before release of gas pressure. All the particles were washed with cooling water and transferred to a bucket. The collections were then washed by ethanol to remove impurities. Part of the samples were directly used for size test, whereas part of them were vacuum dried to powders for SEM analysis. A SEM (Zeiss Supra55) coupled with an EDS equipment was used for the acquisition of electron images and the determination of elemental compositions (punctual and mapping) of the samples. EDS distribution maps of the elements were obtained for a better interpretation of the data. The samples were deposited on the surface of an aluminum pin for SEM analysis. As the samples were in powder form, they were glued to the surface using a special carbon adhesive tape for SEM analysis. The particle size distribution in the slurry is directly measured by laser diffraction particle size analyzer (Malvern
2. Experimental A typical industrial propellant based on energetic glycidyl azide polymer (GAP) as a fuel binder was chosen for this investigation. The propellant consisted of 18 wt% aluminum (18 µm), 17 wt% ammonium perchlorate (AP), 44 wt% hexanitrohexaazaisowurtzitane (CL-20), and the rest were 21 wt% GAP, nitroglycerine (NG) and burning rate catalyst. The propellant specimen was tailored to 26 mm diameter, 30 mm long cylindrical strand. A constant-pressure quench vessel was used to collect the con148
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Fig. 3. SEM image and EDS chemical map of highly dispersed oxide particles.
Fig. 2. SEM overview images of typical condensed combustion products show agglomerates and smoke oxide particles distributed in the products. Particles in the products are distributed around three modes: ~0.6 µm, ~4 µm, ~30 µm.
Mastersizer-2000) using a small amount of representative sample from the slurry similar to Ref. [17]. Each sample was tested twice and the averages of the values are reported in this paper. 3. Results and discussions 3.1. Overview of CCPs It is commonly agreed that the condensed products consist of agglomerates and SOPs, even close to the burning-propellant surface [4]. The size of agglomerates is usually very large, which may reaches hundreds or even thousands of micrometers, whereas the SOPs have sizes of the order of 1 µm. The SEM overview images of typical CCPs clearly prove the above results (Fig. 2). The particles in the combustion products are distributed around three modes: ~0.6 µm, ~4 µm, ~30 µm. The distribution is obtained by averaging the test results of two different samples at the same operating condition. The trimodal size distribution in the present study is consistent with Jeenu' work [22], in which the three modes are ~1 µm, ~4 µm, ~70 µm. According to the mechanism of aluminum combustion in propellants proposed by Price [24], mode ~0.6 µm particles are smoke oxide particles, primarily formed by the oxidation of aluminum vapor; mode ~4 µm are supposed to be the residual oxide particles formed from the molten oxide caps on the aluminum particles or agglomerates at its burn out, which can also be considered as one type of smoke oxide particles; and mode ~30 µm are the large agglomerates. It should be noted that in contrast to Jeenu's work [22], the smoke oxide particles mode is smaller. This can be attributed to the higher combustion gas temperature (above 3700 K compared to 3375 K) and pressure (7 MPa compared to 3 MPa) of our experiment, which leads to faster evaporation rate of aluminum drop. In addition, the presence of rough carbon-inclusion particles (discussed in Section 3.4), with a size order of hundreds nm or even smaller, may makes the overall size of the combustion products become smaller.
Fig. 4. SEM image of smoke oxide cluster.
exceed 100 µm. This increases the mean size of residual oxide particles occurring from size distribution test, i.e., the size of residual oxide particles is commonly on the order of 1–3 µm, as seen in Fig. 4, but the actual size distribution mode is 4 µm. The result is exactly in agreement with the inference of Jeenu [22], who suggested that a considerable amount of mode 4 µm particles should be formed by the coalescence of smoke oxide particles. When in a high-temperature region with temperature above the melting point of alumina (2328 K), the molten alumina particles can coalesce to form larger particles, while when in a low temperature region below the melting point of alumina, the alumina particles solidify and do not coalesce. Besides, Jeenu [22] also noticed that the high residence time of particles in high-temperature region could lead to higher amount of mode 4 µm particles in the combustion products. Accordingly, it can be concluded that the formation of large SOCs in our study is mainly ascribed to the high ambient temperature and long residence time of smoke oxide particles in the combustion chamber. A close view of the micromorphology of a typical highly dispersed oxide particle around 1 µm is shown in Fig. 3. The particle is spherical with a very smooth surface. Electron dispersive spectroscopy of the particle surface indicates it is made up of aluminum, oxygen, and gold. The atomic ratios of aluminum to oxygen is not in accordance with the theoretical value 2:3, because of EDS detecting limitation of light elements (H, C, O, etc.). Au element is derived from the gold spray process, required to improve the electroconductivity of the sample for SEM analysis.
3.2. SOPs To find out the specific micromorphology of various structures, the SOPs, agglomerates and other typical structures in the CCPs are to be analyzed individually through SEM-EDS analysis. SOPs are presented in a large amount in CCPs from the results of size distribution test. In general, SOPs are highly dispersed in the products (Fig. 2 and Fig. 3), as also observed by Babuk [3] and Glotov [25] who called them as highly dispersed oxide particles (HDO). From Fig. 4, however, it can be seen that the products contain many smoke oxide clusters (SOC) which have not been noted in previous work. These clusters are formed by the aggregation of the dispersed oxide particles. Their sizes are much larger than the initial smoke oxide particles, and the largest cluster can even
3.3. Agglomerates Two types of agglomerates, spherical pattern and irregular pattern, are observed in CCPs. The spherical agglomerates are large spherical particles (term as spherical agglomerates, SAG), as shown in Fig. 5. The surface structure is similar to Jeenu's result [22], that many cracks and 149
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Fig. 5. SEM image and EDS chemical map of typical spherical agglomerates.
burrows can be seen on the shell surface. Two mechanisms can be adopted to explain this phenomenon. First, the agglomerates may be not completely transformed to molten droplets before they get quenched, which makes the particles are spherical in shape but with imperfections on the surface [26]. These incompletely molten particles are termed as aggregates, as mentioned by Maggi [16,27], which are composed of partially molten aluminum particles. Another possibility is relevant to the ignition nature of aluminum droplets. The aluminum droplet is gradually covered by a thin protective alumina coating (thickness < 10 µm) during the heat up stage in a oxidizing atmosphere [28]. As the temperature continues to increase, the oxide coating will breakdown leading to ignition. So the cracks and burrows on the surface may correspond to the breakdown of the oxide coating. It should be noted that a number of broken spherical particles are found in the products (Fig. 6), which reflects a occurrence of collision or collapse to the particles. Three types of spherical agglomerates have been observed, as shown in Fig. 6. The first type is represented by a hollow oxide shell, which is typically embedded with one or more small Al particles. Agglomerates of this type are rarely directly observed in the samples we collected. However, it can not be determined that the number of hollow shell agglomerates is small unless the inside morphology of every spherical agglomerate particle is examined. The second type is a metal drop with an oxide cap on its surface. This agglomerate type is also described in the literature [3,15]. Obviously, the relative size of the metal and oxide cap is changing during different combustion stages. The final oxide cap radius is predicted to be approximately 70% of the original aluminum particle radius according to King's numerical simulation [29]. The third type is solid spherical particle. A broken particle is purposely shown in Fig. 6 to tell that the particle is solid inside. The content of this type in spherical agglomerates seems to be quite large compared with the former two types. The difference of morphology and amount among the above three types of spherical agglomerates is related to ambient temperature of aluminum drop combustion, which depends on the temperature of the combustion gas of propellant. Generally, part of the aluminum oxide appears in the form of smoke at the condensation zone, inside which the aluminum drop is burning. This condensation zone is an infinitesimally thin layer where final oxidation to liquid Al2O3 and oxide condensation occur [29]. Accordingly, rapid quenching would result in the hollow shell structure. However, if the ambient temperature is beyond the boiling point of Al2O3 (3273 K), then the liquid Al2O3 layer is supposed to evaporate quickly and the hollow shell disappears. This can interpret that there are just few “hollow agglomerates” in the products since the ambient gas temperature is above 3700 K in our study. When the aluminum oxide deposits on an accumulating cap at the aluminum particle surface, the “cap agglomerates” forms, as observed in Fig. 6. However, the ambient temperature is so high that most of the aluminum oxide would evaporate directly, rather than deposit.
Fig. 6. Sketch and morphology of various types of spherical agglomerates: hollow shell (top), oxide cap (middle), solid sphere (bottom).
Therefore, the amount of “solid agglomerates” is predominant among these three types. Compared to spherical agglomerates, the irregular agglomerates (term as IAG) are more widely distributed in products. However, the morphology of this type of agglomerates has been reported only once before [17]. These agglomerates are very irregular in shape with a large amount of small oxide particles covered on the surface (Fig. 2 and Fig. 7). Some part of the surface is flat, indicating the flow of molten material, possibly aluminum. The irregular agglomerates are usually found to have larger sizes than spherical agglomerates. The largest one can reach even more than 250 µm, while the spherical agglomeration particles are typically around 50 µm. Taking into account for the irregular shape and large size, the formation of the irregular agglomerates can be mainly ascribed to the aggregation of spherical agglomerates. The irregular shape indicates that the spherical agglomerates turn irregular as they move along with the burning gas. Fig. 8 presents the deformation of a spherical agglomerate which takes place through three paths. Firstly, due to contaminant entrainment during formation on the propellant surface, agglomerated droplets may have a heterogeneous composition (this has been confirmed in Section 3.4). Such agglomerates would exhibit spinning, jetting, fragmentation, or explosions during the combustion stage [30]. This may result in an irregular shape of the spherical droplets, as shown in Fig. 8 path 1. Secondly, the combustion characteristic of aluminum particle itself makes it become irregular. The oxide would become liquid and accumulates as a cap on the free liquid aluminum surface during the combustion, which distorts the distribution of gasification velocity, temperature and other quantities around the particle. Also, the oxide cap can cause jetting and fragmentation of the particle, and finally leads to the distortion of the profiles around the particle (Fig. 8 path 2). In addition, the spherical drops seem to experience collision or collapse during flow along with the burning gas, as mentioned before(Fig. 8 path 3). In a word, spherical particles would change to irregular via various routines. Besides, aluminum particles may leave the burning surface as aggregates with nonspherical shape [27]. Subsequently, liquid irregular particles from either agglomerated or non-agglomerated aluminum, together with the spherical agglomerates, coalesce by certain complicated particle-particle interactions since the temperature inside the internal flow field exceeds the melting point of Al2O3 [31]. Eventually, large irregular agglomerates are created as this aggregated particle 150
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Fig. 9. SEM images and EDS chemical maps of rough and spherical carbon-inclusions.
Fig. 7. SEM image and EDS chemical map of irregular agglomerates.
of Gibbs free-energy principle. It should be stressed that the calculation is based on a long reaction time range in which the chemical equilibrium can be reached. The reaction time of the compositions in real combustion chamber is not long enough to make sure the chemical equilibrium is reached. A typical HTPB-based propellant was calculated as well, for comparison, which contains 70 wt% AP, 12 wt% HTPB, and 18 wt% Al. Table 1 gives the calculated results, and only those compositions with content higher than 1% are presented. The oxygen balance of HTPB-based propellant is higher than GAP-based propellant. This might explain the appearance of considerable amounts of carbonaceous materials for the propellant in this study compared to common HTPB-based propellants. Another possible reason is that GAP-propellants are prone to produce soot according to Kubota's work [32], where pure GAP produces around 30% solid C. It is also seen from Table 1 that the condensed combustion products for both propellants only contain 8.0% Al2O3(l), but no C(s). The absence of solid products may be ascribed to the relatively higher combustion temperature in calculation, whereas in practical environment, the adiabatic combustion temperature cannot be reached due to the heat loss. Moreover, the incompleteness of the chemical equilibrium also leads to the presence of solid carbonaceous particles in a real situation. In addition to the above carbon-inclusion particles, strip shape carbon-inclusion (SSC) and flake shape carbon-inclusion (FSC) are observed in Fig. 10. It should be noted that these two structures are only found on the surface of certain large irregular agglomerates. This might be a result of the fact that Babuk [3] stated that agglomerates contained other ingredients (the binder or its degradation products and the oxidizer) in addition to the metal may be nonspherical. Additionally, silk-like fabrics can be seen in the products with 1 mm length and 10 µm diameter (Fig. 11). The main elemental compositions of the fabric is carbon and oxygen. This oxide fiber silk (OFS) is believed to be the combustion product of fiber in the heat insulation layer covered around the propellant specimen. Table 2 summarizes the nine distinct types of structures observed in the condensed combustion products, they are HDO, SOC, SAG, IAG, RCPs, SCPs, SSC, FSC, OFS, respectively. The chemical compositions, size range, and relative content of various structures are also given. The condensed particles, liquid or solid, can contribute particle damping effect to the combustion instability inside rocket motor. The amount of particle damping is dependent on the mass fraction of particles in the flow field, the physical property of particles and, most importantly, particle size. The classical relationships for particle damping are based on the theory of Temkin and Dobbins [33]. Particle damping by a small concentration of additive may be approximately given by:
Fig. 8. Schematic of the formation process of irregular agglomerates containing (a) deformation of spherical aluminum drops; (b) combination of particles with various shape; (c) irregular agglomerates generate.
burns. In comparison to SOPs particles, the ratio of O to Al on the surface of both SAG and IAG is lower, indicating that the degree of oxidation reaction is not that complete. The ratio of O to Al for IAG is higher than SAG. This may suggest the specific surface area of IAG is larger than that of SAG, hence the oxidation reaction efficiency is higher. C is an indication for the existence of degradation products from the binder or oxidizer.
3.4. Carbon inclusions Considerable amounts of particles shown in Fig. 9 are found distributed in the products, which have not been studied ever before, as far as we are aware. These particles are obviously different from SOPs, which are non-spherical with extremely rough surface structure, and have a smaller size of 500 nm. EDS result shows carbon accounts for most element compositions. It is suggested that these particles are derived from the degradation products from the binder or oxidizer. Although the atom ratio of carbon reaches as high as 95%, we cannot conclude that these are carbon particles because hydrogen cannot be detected in this study using EDS method. Among the rough carboninclusion particles (RCPs), spherical carbon-inclusion particles (SCPs) are observed. The spherical carbon-inclusion particles is very similar to smoke oxide which is spherical with smooth surface, but EDS analysis differs the two types that carbon content in the former is 76%, whereas no carbon is found in the latter. The amount of the carbonaceous particles is depending on the firing conditions and propellant formulation. To further clarify the reasons of the presence of carbonaceous particles, the oxygen balance and combustion products of the propellant were calculated by Chemical Equilibrium with Applications program (CEA), based on minimization
(
α = − Cm ω /2 where
151
)( ωτ)/⎡⎣⎢ 1 + ( ωτ) ⎤⎦⎥ 2
(1)
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Table 1 Comparison between the calculated oxygen balance and compositions of combustion products of GAP-based and HTPB-based propellant. Propellant
Pressure (MPa)
Oxygen balance
Combustion products (molar content %)
GAP-based
7
1.15
HTPB-based
7
1.27
CO 31, N2 24.5, H2 16.0, Al2O3(l) 8.0, H 7.7, H2O 5.0 H2 27.2, CO 22.0, H2O 13.0, HCl 12.6, Al2O3(l) 8.0, N2 7.8, H 4.0, CO2 1.3, Cl 1.2
Fig. 12. Estimated particle damping versus various particle morphologies in the condensed combustion products, by setting Cm=0.1.
size was adopted during calculation for each type of particle. It is apparent that SOC, SAG and IAG have a highest damping, while the damping for the other structures is all below 10 s−1. The results manifest that particle size plays a crucial role in particle damping, and particles with size between 25 and 130 µm can sufficiently suppress an instability ranged in 0–2000 Hz. 3.5. Pressure effect
Fig. 10. SEM images and EDS chemical maps of strip shape and flake shape carboninclusions.
Numerous studies show pressure has significant effect on aluminum agglomeration, that increasing pressure reduces agglomeration. This is because high pressure can reduce the residence time of aluminum particles on the propellant surface, and a short residence time does not allow complete agglomeration [1,5,7,8,16,22]. As expected, a high degree of agglomeration has been seen in the lowerpressure experiment (Fig. 13), and the diameters of the agglomerates are also larger than that of higher-pressure. Similar trend can be noted from the size distributions of condensed combustion products at different pressures. As shown in Fig. 14, the third mode is approximately 50 µm, 30 µm, and 20 µm under 5.5 MPa, 7 MPa, and 9 MPa, respectively. Because the third mode corresponds to agglomerates, this indicates that increased pressure raises the mean size of the agglomerates in the CCPs. Moreover, the result shows a decrease in maximum size occurring in the size distribution with increasing pressure. This may imply that the content of irregular agglomerates is lower under high-pressure conditions. As the pressure increases from 5.5 MPa to 7 MPa, the contents of the first and second mode are getting lower, which suggests a lower total content of RCPs, SCPs and SOPs. The size distribution of these two modes for 9 MPa is basically the same as 7 MPa.
Fig. 11. SEM image and EDS chemical map of oxide fiber silk.
( )
τ = ρD 2 / 18μ
(2)
Cm=particle concentration, ω=angular frequency, τ=particle relaxation time, D=particle size, ρ=particle density, µ=gas viscosity (typical value of 0.00065 poise). Fig. 12 shows the particle damping for the nine particle morphologies in the frequency range of 0–2000 Hz. An estimated mean particle
4. Conclusions In summary, this work presents the morphology and compositions
Table 2 Various types of structures in condensed combustion products of aluminized propellants with their properties. No.
Name
Description
Main compositions
Size
Relative contents
1 2 3 4 5 6 7 8 9
HDO SOC SAG IAG RCPs SCPs SSC FSC OFS
Highly dispersed oxide Smoke oxide cluster Spherical agglomerates: hollow agglomerates; cap agglomerates; solid agglomerates Irregular agglomerates Rough carbon-inclusion particles Spherical carbon-inclusion particles Strip shape carbon-inclusion Flake shape carbon-inclusion Oxide fiber silk
Al2O3 Al2O3 Al2O3, Al Al2O3, Al C, O C, O C, O C, O C, O
~1 µm 5~50 µm 20~50 µm 10~250 µm ~500 nm ~500 nm ~1 µm ~1 µm ~1 mm
Very high High Low Low High Low Low Low Very low
152
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Fig. 13. SEM overview images of condensed combustion products at different pressures. The amount of agglomerates is decreased as pressure increases. [5] K. Jayaraman, S.R. Chakravarthy, R. Sarathi, Quench collection of nano-aluminium agglomerates from combustion of sandwiches and propellants, Proc. Combust. Inst. 33 (2011) 1941–1947. [6] K.V. Anand, A. Roy, I. Mulla, K. Balbudhe, K. Jayaraman, S.R. Chakravarthy, Experimental data and model predictions of aluminium agglomeration in ammonium perchlorate-based composite propellants including plateau-burning formulations, Proc. Combust. Inst. 34 (2013) 2139–2146. [7] N.S. Cohen, A. Pocket, Model for aluminum agglomeration in composite propellants, AIAA J. 21 (1983) 720–725. [8] T.K. Liu, Experimental and model study of agglomeration of burning aluminized propellants, J. Propuls. Power 21 (2005) 797–806. [9] J.C. Mullen, M.Q. Brewster, Reduced agglomeration of aluminum in wide-distribution composite propellants, J. Propuls. Power 27 (2011) 650–661. [10] K. Takahashi, S. Oide, T. Kuwahara, Agglomeration characteristics of aluminum particles in AP/AN composite propellants, Propellants Explos. Pyrotech. 38 (2013) 555–562. [11] O.G. Glotov, Condensed combustion products of aluminized propellants. IV. Effect of the nature of nitramines on aluminum agglomeration and combustion efficiency, Combust. Explos. Shock 42 (2006) 436–449. [12] O.G. Glotov, Condensed combustion products of aluminized propellants. II. Evolution of particles with distance from the burning surface, Combust. Explos. Shock 36 (2000) 476–487. [13] O.G. Glotov, Condensed combustion products of aluminized propellants. III. Effect of an inert gaseous combustion environment, Combust. Explos. Shock 38 (2002) 92–100. [14] V.A. Babuk, V.A. Vasil’ev, A.N. Potekhin, Experimental investigation of agglomeration during combustion of aluminized solid propellants in an acceleration field, Combust. Explos. Shock 45 (2009) 32–39. [15] V.A. Babuk, I.N. Dolotkazin, A.A. Glebov, Burning mechanism of aluminized solid rocket propellants based on energetic binders, Propellants Explos. Pyrotech. 30 (2005) 281–290. [16] F. Maggi, A. Bandera, L.T. DeLuca, Approaching solid propellant heterogeneity for agglomerate size prediction, in: Proceedings of the 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit (AIAA Paper No. 2010-6751), Nashville, TN, USA, 2010 [17] T.R. Sippel, S.F. Son, L.J. Groven, Aluminum agglomeration reduction in a composite propellant using tailored Al/PTFE particles, Combust. Flame 161 (2014) 311–321. [18] T.R. Sippel, S.F. Son, L.J. Groven, S. Zhang, E.L. Dreizin, Exploring mechanisms for agglomerate reduction in composite solid propellants with polyethylene inclusion modified aluminum, Combust. Flame 162 (2015) 846–854. [19] Y. Yavor, V. Rosenband, A. Gany, Reduced agglomeration in solid propellants containing porous aluminum, Proc. Inst. Mech. Eng. G – J. Aerosp. Eng. 228 (2014) 1857–1862. [20] K. Jayaraman, K.V. Anand, S.R. Chakravarthy, R. Sarathi, Effect of nano-aluminium in plateau-burning and catalyzed composite solid propellant combustion, Combust. Flame 156 (2009) 1662–1673. [21] K. Jayaraman, S.R. Chakravarthy, R. Sarathi, Accumulation of nano-aluminium during combustion of composite solid propellant mixtures, Combust. Explos. Shock 46 (2010) 21–29. [22] R. Jeenu, K. Pinumalla, D. Deepak, Size distribution of particles in combustion products of aluminized composite propellant, J. Propuls. Power 26 (2010) 715–723. [23] R. Doi, T. Kuwahara, Combustion of aluminum particles near the burning surface in AP/ AN composite propellants, Propellants Explos. Pyrotech. 40 (2015) 765–771. [24] E. Price, Comments on role of aluminum in suppressing instability in solid propellant rocket motors, AIAA J. 9 (1971) 987–990. [25] O.G. Glotov, V.A. Zhukov, Evolution of 100-A mu m aluminum agglomerates and initially continuous aluminum particles in the flame of a model solid propellant. I. Experimental approach, Combust. Explos. Shock 44 (2008) 662–670. [26] E. Price, Combustion of metalized propellants, Fundam. Solid Propellant Combust. 90 (1984) 479–514. [27] F. Maggi, S. Dossi, L.T. DeLuca, Combustion of metal agglomerates in a solid rocket core flow, Acta Astronaut. 92 (2013) 163–171. [28] V.S. Kanian, J.C. Rifflet, F. Millot, G. Matzen, I. Gokalp, Influence of nitrogen in aluminum droplet combustion, Proc. Combust. Inst. 30 (2005) 2063–2070. [29] M.K. King, Aluminum combustion in a solid rocket motor environment, Proc. Combust. Inst. 32 (2009) 2107–2114. [30] J.C. Melcher, H. Krier, R.L. Burton, Burning aluminum particles inside a laboratory-scale solid rocket motor, J. Propuls. Power 18 (2002) 631–640. [31] J. Hijlkema, Y. Prevot, M. Prevost, V.V. Mironov, Particle size distribution measurements in the keldysh research centre experimental setup at ONERA, in: Proceedings of the 47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, San Diego, California, 2011. [32] N. Kubota, T. Sonobe, Combustion mechanism of azide polymer, Propellants Explos. Pyrotech. 13 (1988) 172–177. [33] S. Temkin, R.A. Dobbins, Attenuation and dispersion of sound by particulate-relaxation processes, J. Acoust. Soc. Am. 40 (1966) 317–324.
Fig. 14. Size distributions of condensed combustion products at different pressures.
of various structures in the condensed combustion products of aluminized propellants. Nine typical structures have been observed, and some of them have not, to our knowledge, been observed or further studied before, including the smoke oxide cluster, irregular agglomerates and carbon-inclusions. The smoke oxide cluster is formed by the aggregation of dispersed oxide particles. Considerable amount of rough carbon-inclusion particles can be seen in the products, with few spherical carbon-inclusion particles distributed among them. On the surface of certain agglomerates, strip-shape carbon inclusions and flake-shape carbon inclusions are discovered. Spherical agglomerates are observed in the form of “hollow agglomerates”, “cap agglomerates”, and “solid agglomerates”. The formation of different spherical agglomerates is analyzed to be related to the ambient temperature. The size of irregular agglomerates is much larger, which is believed to be produced by the recombination and coalescence of fragments of fused aluminum droplets in the combustion chamber. The lower atomic ratios of aluminum to oxygen on the surface of agglomerates compared to SOPs suggest that the agglomerates is incompletely oxidized. The content of agglomerates in the CCPs is found to decrease with increasing pressure. Current efforts are focused on the properties of CCPs and the effect of various factors on agglomeration of aluminum particles. Future work will explore strategies to reduce agglomeration using a variety of techniques. Acknowledgments This work was supported by “National Nature Science Foundation of China” (51506181; 51276150) and “Fundamental Research Funds for the Central Universities” (3102015ZY083; 3102014ZD0032). References [1] J.K. Sambamurthi, E.W. Price, R.K. Sigman, Aluminum agglomeration in solid-propellant combustion, AIAA J. 22 (1984) 1132–1138. [2] O.G. Glotov, V.Y. Zyryanov, Condensed combustion products of aluminized propellants. I. A technique for investigating the evolution of disperse-phase particles, Combust. Explos. Shock 31 (1995) 72–78. [3] V.A. Babuk, V.A. Vasilyev, M.S. Malakhov, Condensed combustion products at the burning surface of aluminized solid propellant, J. Propuls. Power 15 (1999) 783–793. [4] V.A. Babuk, V.A. Vassiliev, V.V. Sviridov, Propellant formulation factors and metal agglomeration in combustion of aluminized solid rocket propellant, Combust. Sci. Technol. 163 (2001) 261–289.
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Agglomerates, smoke oxide particles, and carbon inclusions in condensed combustion products of an aluminized GAP-based propellant ⁎
Wen Ao, Peijin Liu , Wenjing Yang Science and Technology on Combustion, Internal Flow and Thermal-structure Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Aluminum Solid propellants Condensed combustion products Agglomeration Solid rocket motor
In solid propellants, aluminum is widely used to improve the performance, however the condensed combustion products especially the large agglomerates generated from aluminum combustion significantly affect the combustion and internal flow inside the solid rocket motor. To clarify the properties of the condensed combustion products of aluminized propellants, a constant-pressure quench vessel was adopted to collect the combustion products. The morphology and chemical compositions of the collected products, were then studied by using scanning electron microscopy coupled with energy dispersive (SEM-EDS) method. Various structures have been observed in the condensed combustion products. Apart from the typical agglomerates or smoke oxide particles observed before, new structures including the smoke oxide clusters, irregular agglomerates and carbon-inclusions are discovered and investigated. Smoke oxide particles have the highest amount in the products. The highly dispersed oxide particle is spherical with very smooth surface and is on the order of 1– 2 µm, but due to the high temperature and long residence time, these small particles will aggregate into smoke oxide clusters which are much larger than the initial particles. Three types of spherical agglomerates have been found. As the ambient gas temperature is much higher than the boiling point of Al2O3, the condensation layer inside which the aluminum drop is burning would evaporate quickly, which result in the fact that few “hollow agglomerates” has been found compared to “cap agglomerates” and “solid agglomerates”. Irregular agglomerates usually larger than spherical agglomerates. The formation of irregular agglomerates likely happens by three stages: deformation of spherical aluminum drops; combination of particles with various shape; finally production of irregular agglomerates. EDS results show the ratio of O to Al on the surface of agglomerates is lower in comparison to smoke oxide particles. C and O account for most element compositions for all the carbon inclusions. The rough, spherical, strip shape and flake shape carbon-inclusions are believed to be derived from the degradation products of the binder or oxidizer, while the fiber silk is possibly the combustion product of fiber inside the heat insulation layer of the propellants. Images of products at different pressures reveal high pressure reduces the degree of agglomeration. The chemical compositions, size range and content of all the observed structures are given in this paper. Results of our study are expected to provide better insight in the working process of solid rocket motor.
1. Introduction Aluminum powders (typically of 5–20 µm radius) are usually added to solid rocket propellants to obtain high burning rates and increase the energetic performance. But the presence of aluminum can result in the formation of condensed combustion products (CCPs) which significantly influences the working process of the motor system: losses in specific impulse, slag deposition in the motor chamber, effects of combustion products on the inert motor components, and stability of motor operation. Gaining deep insight in the CCPs properties is a key problem for researchers to understand the combustion and internal
⁎
flow mechanism in the solid rocket motor. It is well known that agglomeration of aluminum takes place in the surface layer of the burning propellant: aluminum particles may coalesce at the propellant burning surface, which results in agglomerates that are larger than the initial virgin aluminum used. CCPs containing these large agglomerates then leave the burning surface, release into the gas phase and interact with the flow. So far, CCPs of aluminized propellants have been studied widely mainly based on the quench-collection method. Sambamurthi [1] used a plume quenchparticle collection setup to study aluminum agglomeration in solidpropellant combustion. Combustion products flowed to an ethanol pool
Corresponding author. E-mail address: [email protected] (P. Liu).
http://dx.doi.org/10.1016/j.actaastro.2016.09.011 Received 29 April 2016; Received in revised form 31 August 2016; Accepted 9 September 2016 Available online 10 September 2016 0094-5765/ © 2016 IAA. Published by Elsevier Ltd. All rights reserved.
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in which the burning agglomerates were quenched and collected. Glotov [2] developed a technique that frozen the combustion products by mixing with a concurrent cold flow of an inert diluents gas. Babuk [3,4] employed a constant-volume bomb to collect the particulate combustion products. Quenching of CCPs was accomplished using either alcohol or inert gas. Since the mass of the propellant samples was less than 2 g, the pressure in the bomb was constant within 10% during combustion. Jayaraman [5] and Anand [6] adopted the same set-up for agglomerate quench-collection. The ethanol bath was placed just 3 mm below the virgin propellant surface. The characteristics of the agglomeration process depend on the propellant formulations and environmental conditions. As a rule, the agglomerates are governed by the size of the parent aluminum particles and their percentage [1,4,6– 10], the size of AP particles [3,6,8], fraction of coarse and fine AP particles [1,5,7,9], oxidizer types [3,4,11], binder properties [4], distance from the burning surface [12], burning atmosphere [13], acceleration effects [14], chamber pressure [1,3,5,7,8,12,15,16] and burning rate of the propellant [6,10]. Recently, new methods were developed to explore agglomeration reduction, such as using tailored, mechanically activated aluminum/polytetrafluoroethylene composite particles [17], low-density polyethylene inclusion modified aluminum [18], porous aluminum [19], and nano-aluminum [5,20,21] as replacements for reference aluminum powders in propellants. As for the properties of CCPs, it is now generally accepted that CCPs in the burning of aluminized propellants of various classes can be classified into two basic categories: agglomerates and smoke oxide particles (SOPs). The agglomerates were found to be large spherical particles, while SOPs were massive particles having a regular, spherical shape and consisting of aluminum oxide [15,22]. More interestingly, Doi [23] found that the shape of luminous flame around agglomerated Al particles was dependent on the size of agglomerates. When the ratio of the luminous flame diameter to the agglomerated Al particles diameter was 1.54–1.71, the luminous flames were perfectly spherical. Otherwise, the luminous flames were elliptical. However, the available experimental data does not permit the discovery of the physical aspects of CCPs. A number of problems have not been studied sufficiently, including (i) not enough work devoted to systematic studies of the specific morphology and formation of nonspherical agglomerates, which seemed to abound in CCPs [17], (ii) few papers are concerned with the structures of carbon inclusions contained in CCPs (iii) according to published results, most papers investigated propellants based on isoprene rubber or HTPB as a fuel binder. It is necessary to obtain experimental information about the CCPs of NEPE (nitrate ester plasticized polyether) propellant which are also widely used at present, because the binder properties were suggested to affect the agglomeration process [3]. In this paper, we adopt a modified constant-pressure quench technique to collect the condensed combustion products of burning propellant, followed by the systematic, physical and chemical analysis of the collections. Morphology and compositions of various structures in CCPs are obtained, followed by the analysis of connection of the products and combustion mechanism of propellants. Such information, in combination with the available materials, would serve the purpose of clarifying the physical picture of aluminized propellants burning process in solid rocket motor.
Fig. 1. Schematic of the constant-pressure propellant combustion vessel.
densed combustion products from the process of propellant sample burning. Compared to previous experimental quenching system [1,3,11,17], the improvement of this apparatus designed and built by us is its “constant-pressure” nature, since pressure has significant effect on aluminized propellant combustion, especially on the agglomeration characteristics of aluminum [5,15]. To avoid the chemical reaction between high temperature aluminum particles and water, here quenching of products was accomplished using an inert gas (nitrogen) as in Babuk's work [3]. A schematic of this apparatus is shown in Fig. 1. It is a thick-walled cylindrical steel chamber of 0.8 m length and 300 mm internal diameter. The top and bottom ends of the chamber are closed with flanges. The top flange is equipped with propellant specimen holder, pressure transducer, igniter and solenoid valve for nitrogen gas inlet and combustion gas exhaust. An orifice plate made of steel with an orifice in the center is installed at the bottom of the vessel. The exhaust gas flow during propellant burning can be varied by changing the orifice size. The balance of burning gas injection and exhaust is achieved by choosing proper pore plate so that the pressure can be kept constant. The test pressure is in the range of 5.5–9 MPa. The analysis work is mainly based on the result of 7 MPa. To study the pressure effect on the CCPs, the results of 5.5 MPa, 7 MPa and 9 MPa are compared. Each operating condition was tested at least twice to check the reproducibility. The repeatability is within 5% based on the mean-mass diameter D43 of the CCPs size distributions obtained in different runs at the same condition. The combustion products flowed downward during the burning of the specimen, and the particles got quenched in the nitrogen gas. All the products were supposed to fall on the bottom of the vessel. However, a part of the fine particles may remain suspended in the gas after the end of burning. To settle the suspended particles in the bottom, the vessel was allowed to stand still for two hours before release of gas pressure. All the particles were washed with cooling water and transferred to a bucket. The collections were then washed by ethanol to remove impurities. Part of the samples were directly used for size test, whereas part of them were vacuum dried to powders for SEM analysis. A SEM (Zeiss Supra55) coupled with an EDS equipment was used for the acquisition of electron images and the determination of elemental compositions (punctual and mapping) of the samples. EDS distribution maps of the elements were obtained for a better interpretation of the data. The samples were deposited on the surface of an aluminum pin for SEM analysis. As the samples were in powder form, they were glued to the surface using a special carbon adhesive tape for SEM analysis. The particle size distribution in the slurry is directly measured by laser diffraction particle size analyzer (Malvern
2. Experimental A typical industrial propellant based on energetic glycidyl azide polymer (GAP) as a fuel binder was chosen for this investigation. The propellant consisted of 18 wt% aluminum (18 µm), 17 wt% ammonium perchlorate (AP), 44 wt% hexanitrohexaazaisowurtzitane (CL-20), and the rest were 21 wt% GAP, nitroglycerine (NG) and burning rate catalyst. The propellant specimen was tailored to 26 mm diameter, 30 mm long cylindrical strand. A constant-pressure quench vessel was used to collect the con148
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Fig. 3. SEM image and EDS chemical map of highly dispersed oxide particles.
Fig. 2. SEM overview images of typical condensed combustion products show agglomerates and smoke oxide particles distributed in the products. Particles in the products are distributed around three modes: ~0.6 µm, ~4 µm, ~30 µm.
Mastersizer-2000) using a small amount of representative sample from the slurry similar to Ref. [17]. Each sample was tested twice and the averages of the values are reported in this paper. 3. Results and discussions 3.1. Overview of CCPs It is commonly agreed that the condensed products consist of agglomerates and SOPs, even close to the burning-propellant surface [4]. The size of agglomerates is usually very large, which may reaches hundreds or even thousands of micrometers, whereas the SOPs have sizes of the order of 1 µm. The SEM overview images of typical CCPs clearly prove the above results (Fig. 2). The particles in the combustion products are distributed around three modes: ~0.6 µm, ~4 µm, ~30 µm. The distribution is obtained by averaging the test results of two different samples at the same operating condition. The trimodal size distribution in the present study is consistent with Jeenu' work [22], in which the three modes are ~1 µm, ~4 µm, ~70 µm. According to the mechanism of aluminum combustion in propellants proposed by Price [24], mode ~0.6 µm particles are smoke oxide particles, primarily formed by the oxidation of aluminum vapor; mode ~4 µm are supposed to be the residual oxide particles formed from the molten oxide caps on the aluminum particles or agglomerates at its burn out, which can also be considered as one type of smoke oxide particles; and mode ~30 µm are the large agglomerates. It should be noted that in contrast to Jeenu's work [22], the smoke oxide particles mode is smaller. This can be attributed to the higher combustion gas temperature (above 3700 K compared to 3375 K) and pressure (7 MPa compared to 3 MPa) of our experiment, which leads to faster evaporation rate of aluminum drop. In addition, the presence of rough carbon-inclusion particles (discussed in Section 3.4), with a size order of hundreds nm or even smaller, may makes the overall size of the combustion products become smaller.
Fig. 4. SEM image of smoke oxide cluster.
exceed 100 µm. This increases the mean size of residual oxide particles occurring from size distribution test, i.e., the size of residual oxide particles is commonly on the order of 1–3 µm, as seen in Fig. 4, but the actual size distribution mode is 4 µm. The result is exactly in agreement with the inference of Jeenu [22], who suggested that a considerable amount of mode 4 µm particles should be formed by the coalescence of smoke oxide particles. When in a high-temperature region with temperature above the melting point of alumina (2328 K), the molten alumina particles can coalesce to form larger particles, while when in a low temperature region below the melting point of alumina, the alumina particles solidify and do not coalesce. Besides, Jeenu [22] also noticed that the high residence time of particles in high-temperature region could lead to higher amount of mode 4 µm particles in the combustion products. Accordingly, it can be concluded that the formation of large SOCs in our study is mainly ascribed to the high ambient temperature and long residence time of smoke oxide particles in the combustion chamber. A close view of the micromorphology of a typical highly dispersed oxide particle around 1 µm is shown in Fig. 3. The particle is spherical with a very smooth surface. Electron dispersive spectroscopy of the particle surface indicates it is made up of aluminum, oxygen, and gold. The atomic ratios of aluminum to oxygen is not in accordance with the theoretical value 2:3, because of EDS detecting limitation of light elements (H, C, O, etc.). Au element is derived from the gold spray process, required to improve the electroconductivity of the sample for SEM analysis.
3.2. SOPs To find out the specific micromorphology of various structures, the SOPs, agglomerates and other typical structures in the CCPs are to be analyzed individually through SEM-EDS analysis. SOPs are presented in a large amount in CCPs from the results of size distribution test. In general, SOPs are highly dispersed in the products (Fig. 2 and Fig. 3), as also observed by Babuk [3] and Glotov [25] who called them as highly dispersed oxide particles (HDO). From Fig. 4, however, it can be seen that the products contain many smoke oxide clusters (SOC) which have not been noted in previous work. These clusters are formed by the aggregation of the dispersed oxide particles. Their sizes are much larger than the initial smoke oxide particles, and the largest cluster can even
3.3. Agglomerates Two types of agglomerates, spherical pattern and irregular pattern, are observed in CCPs. The spherical agglomerates are large spherical particles (term as spherical agglomerates, SAG), as shown in Fig. 5. The surface structure is similar to Jeenu's result [22], that many cracks and 149
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Fig. 5. SEM image and EDS chemical map of typical spherical agglomerates.
burrows can be seen on the shell surface. Two mechanisms can be adopted to explain this phenomenon. First, the agglomerates may be not completely transformed to molten droplets before they get quenched, which makes the particles are spherical in shape but with imperfections on the surface [26]. These incompletely molten particles are termed as aggregates, as mentioned by Maggi [16,27], which are composed of partially molten aluminum particles. Another possibility is relevant to the ignition nature of aluminum droplets. The aluminum droplet is gradually covered by a thin protective alumina coating (thickness < 10 µm) during the heat up stage in a oxidizing atmosphere [28]. As the temperature continues to increase, the oxide coating will breakdown leading to ignition. So the cracks and burrows on the surface may correspond to the breakdown of the oxide coating. It should be noted that a number of broken spherical particles are found in the products (Fig. 6), which reflects a occurrence of collision or collapse to the particles. Three types of spherical agglomerates have been observed, as shown in Fig. 6. The first type is represented by a hollow oxide shell, which is typically embedded with one or more small Al particles. Agglomerates of this type are rarely directly observed in the samples we collected. However, it can not be determined that the number of hollow shell agglomerates is small unless the inside morphology of every spherical agglomerate particle is examined. The second type is a metal drop with an oxide cap on its surface. This agglomerate type is also described in the literature [3,15]. Obviously, the relative size of the metal and oxide cap is changing during different combustion stages. The final oxide cap radius is predicted to be approximately 70% of the original aluminum particle radius according to King's numerical simulation [29]. The third type is solid spherical particle. A broken particle is purposely shown in Fig. 6 to tell that the particle is solid inside. The content of this type in spherical agglomerates seems to be quite large compared with the former two types. The difference of morphology and amount among the above three types of spherical agglomerates is related to ambient temperature of aluminum drop combustion, which depends on the temperature of the combustion gas of propellant. Generally, part of the aluminum oxide appears in the form of smoke at the condensation zone, inside which the aluminum drop is burning. This condensation zone is an infinitesimally thin layer where final oxidation to liquid Al2O3 and oxide condensation occur [29]. Accordingly, rapid quenching would result in the hollow shell structure. However, if the ambient temperature is beyond the boiling point of Al2O3 (3273 K), then the liquid Al2O3 layer is supposed to evaporate quickly and the hollow shell disappears. This can interpret that there are just few “hollow agglomerates” in the products since the ambient gas temperature is above 3700 K in our study. When the aluminum oxide deposits on an accumulating cap at the aluminum particle surface, the “cap agglomerates” forms, as observed in Fig. 6. However, the ambient temperature is so high that most of the aluminum oxide would evaporate directly, rather than deposit.
Fig. 6. Sketch and morphology of various types of spherical agglomerates: hollow shell (top), oxide cap (middle), solid sphere (bottom).
Therefore, the amount of “solid agglomerates” is predominant among these three types. Compared to spherical agglomerates, the irregular agglomerates (term as IAG) are more widely distributed in products. However, the morphology of this type of agglomerates has been reported only once before [17]. These agglomerates are very irregular in shape with a large amount of small oxide particles covered on the surface (Fig. 2 and Fig. 7). Some part of the surface is flat, indicating the flow of molten material, possibly aluminum. The irregular agglomerates are usually found to have larger sizes than spherical agglomerates. The largest one can reach even more than 250 µm, while the spherical agglomeration particles are typically around 50 µm. Taking into account for the irregular shape and large size, the formation of the irregular agglomerates can be mainly ascribed to the aggregation of spherical agglomerates. The irregular shape indicates that the spherical agglomerates turn irregular as they move along with the burning gas. Fig. 8 presents the deformation of a spherical agglomerate which takes place through three paths. Firstly, due to contaminant entrainment during formation on the propellant surface, agglomerated droplets may have a heterogeneous composition (this has been confirmed in Section 3.4). Such agglomerates would exhibit spinning, jetting, fragmentation, or explosions during the combustion stage [30]. This may result in an irregular shape of the spherical droplets, as shown in Fig. 8 path 1. Secondly, the combustion characteristic of aluminum particle itself makes it become irregular. The oxide would become liquid and accumulates as a cap on the free liquid aluminum surface during the combustion, which distorts the distribution of gasification velocity, temperature and other quantities around the particle. Also, the oxide cap can cause jetting and fragmentation of the particle, and finally leads to the distortion of the profiles around the particle (Fig. 8 path 2). In addition, the spherical drops seem to experience collision or collapse during flow along with the burning gas, as mentioned before(Fig. 8 path 3). In a word, spherical particles would change to irregular via various routines. Besides, aluminum particles may leave the burning surface as aggregates with nonspherical shape [27]. Subsequently, liquid irregular particles from either agglomerated or non-agglomerated aluminum, together with the spherical agglomerates, coalesce by certain complicated particle-particle interactions since the temperature inside the internal flow field exceeds the melting point of Al2O3 [31]. Eventually, large irregular agglomerates are created as this aggregated particle 150
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Fig. 9. SEM images and EDS chemical maps of rough and spherical carbon-inclusions.
Fig. 7. SEM image and EDS chemical map of irregular agglomerates.
of Gibbs free-energy principle. It should be stressed that the calculation is based on a long reaction time range in which the chemical equilibrium can be reached. The reaction time of the compositions in real combustion chamber is not long enough to make sure the chemical equilibrium is reached. A typical HTPB-based propellant was calculated as well, for comparison, which contains 70 wt% AP, 12 wt% HTPB, and 18 wt% Al. Table 1 gives the calculated results, and only those compositions with content higher than 1% are presented. The oxygen balance of HTPB-based propellant is higher than GAP-based propellant. This might explain the appearance of considerable amounts of carbonaceous materials for the propellant in this study compared to common HTPB-based propellants. Another possible reason is that GAP-propellants are prone to produce soot according to Kubota's work [32], where pure GAP produces around 30% solid C. It is also seen from Table 1 that the condensed combustion products for both propellants only contain 8.0% Al2O3(l), but no C(s). The absence of solid products may be ascribed to the relatively higher combustion temperature in calculation, whereas in practical environment, the adiabatic combustion temperature cannot be reached due to the heat loss. Moreover, the incompleteness of the chemical equilibrium also leads to the presence of solid carbonaceous particles in a real situation. In addition to the above carbon-inclusion particles, strip shape carbon-inclusion (SSC) and flake shape carbon-inclusion (FSC) are observed in Fig. 10. It should be noted that these two structures are only found on the surface of certain large irregular agglomerates. This might be a result of the fact that Babuk [3] stated that agglomerates contained other ingredients (the binder or its degradation products and the oxidizer) in addition to the metal may be nonspherical. Additionally, silk-like fabrics can be seen in the products with 1 mm length and 10 µm diameter (Fig. 11). The main elemental compositions of the fabric is carbon and oxygen. This oxide fiber silk (OFS) is believed to be the combustion product of fiber in the heat insulation layer covered around the propellant specimen. Table 2 summarizes the nine distinct types of structures observed in the condensed combustion products, they are HDO, SOC, SAG, IAG, RCPs, SCPs, SSC, FSC, OFS, respectively. The chemical compositions, size range, and relative content of various structures are also given. The condensed particles, liquid or solid, can contribute particle damping effect to the combustion instability inside rocket motor. The amount of particle damping is dependent on the mass fraction of particles in the flow field, the physical property of particles and, most importantly, particle size. The classical relationships for particle damping are based on the theory of Temkin and Dobbins [33]. Particle damping by a small concentration of additive may be approximately given by:
Fig. 8. Schematic of the formation process of irregular agglomerates containing (a) deformation of spherical aluminum drops; (b) combination of particles with various shape; (c) irregular agglomerates generate.
burns. In comparison to SOPs particles, the ratio of O to Al on the surface of both SAG and IAG is lower, indicating that the degree of oxidation reaction is not that complete. The ratio of O to Al for IAG is higher than SAG. This may suggest the specific surface area of IAG is larger than that of SAG, hence the oxidation reaction efficiency is higher. C is an indication for the existence of degradation products from the binder or oxidizer.
3.4. Carbon inclusions Considerable amounts of particles shown in Fig. 9 are found distributed in the products, which have not been studied ever before, as far as we are aware. These particles are obviously different from SOPs, which are non-spherical with extremely rough surface structure, and have a smaller size of 500 nm. EDS result shows carbon accounts for most element compositions. It is suggested that these particles are derived from the degradation products from the binder or oxidizer. Although the atom ratio of carbon reaches as high as 95%, we cannot conclude that these are carbon particles because hydrogen cannot be detected in this study using EDS method. Among the rough carboninclusion particles (RCPs), spherical carbon-inclusion particles (SCPs) are observed. The spherical carbon-inclusion particles is very similar to smoke oxide which is spherical with smooth surface, but EDS analysis differs the two types that carbon content in the former is 76%, whereas no carbon is found in the latter. The amount of the carbonaceous particles is depending on the firing conditions and propellant formulation. To further clarify the reasons of the presence of carbonaceous particles, the oxygen balance and combustion products of the propellant were calculated by Chemical Equilibrium with Applications program (CEA), based on minimization
(
α = − Cm ω /2 where
151
)( ωτ)/⎡⎣⎢ 1 + ( ωτ) ⎤⎦⎥ 2
(1)
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Table 1 Comparison between the calculated oxygen balance and compositions of combustion products of GAP-based and HTPB-based propellant. Propellant
Pressure (MPa)
Oxygen balance
Combustion products (molar content %)
GAP-based
7
1.15
HTPB-based
7
1.27
CO 31, N2 24.5, H2 16.0, Al2O3(l) 8.0, H 7.7, H2O 5.0 H2 27.2, CO 22.0, H2O 13.0, HCl 12.6, Al2O3(l) 8.0, N2 7.8, H 4.0, CO2 1.3, Cl 1.2
Fig. 12. Estimated particle damping versus various particle morphologies in the condensed combustion products, by setting Cm=0.1.
size was adopted during calculation for each type of particle. It is apparent that SOC, SAG and IAG have a highest damping, while the damping for the other structures is all below 10 s−1. The results manifest that particle size plays a crucial role in particle damping, and particles with size between 25 and 130 µm can sufficiently suppress an instability ranged in 0–2000 Hz. 3.5. Pressure effect
Fig. 10. SEM images and EDS chemical maps of strip shape and flake shape carboninclusions.
Numerous studies show pressure has significant effect on aluminum agglomeration, that increasing pressure reduces agglomeration. This is because high pressure can reduce the residence time of aluminum particles on the propellant surface, and a short residence time does not allow complete agglomeration [1,5,7,8,16,22]. As expected, a high degree of agglomeration has been seen in the lowerpressure experiment (Fig. 13), and the diameters of the agglomerates are also larger than that of higher-pressure. Similar trend can be noted from the size distributions of condensed combustion products at different pressures. As shown in Fig. 14, the third mode is approximately 50 µm, 30 µm, and 20 µm under 5.5 MPa, 7 MPa, and 9 MPa, respectively. Because the third mode corresponds to agglomerates, this indicates that increased pressure raises the mean size of the agglomerates in the CCPs. Moreover, the result shows a decrease in maximum size occurring in the size distribution with increasing pressure. This may imply that the content of irregular agglomerates is lower under high-pressure conditions. As the pressure increases from 5.5 MPa to 7 MPa, the contents of the first and second mode are getting lower, which suggests a lower total content of RCPs, SCPs and SOPs. The size distribution of these two modes for 9 MPa is basically the same as 7 MPa.
Fig. 11. SEM image and EDS chemical map of oxide fiber silk.
( )
τ = ρD 2 / 18μ
(2)
Cm=particle concentration, ω=angular frequency, τ=particle relaxation time, D=particle size, ρ=particle density, µ=gas viscosity (typical value of 0.00065 poise). Fig. 12 shows the particle damping for the nine particle morphologies in the frequency range of 0–2000 Hz. An estimated mean particle
4. Conclusions In summary, this work presents the morphology and compositions
Table 2 Various types of structures in condensed combustion products of aluminized propellants with their properties. No.
Name
Description
Main compositions
Size
Relative contents
1 2 3 4 5 6 7 8 9
HDO SOC SAG IAG RCPs SCPs SSC FSC OFS
Highly dispersed oxide Smoke oxide cluster Spherical agglomerates: hollow agglomerates; cap agglomerates; solid agglomerates Irregular agglomerates Rough carbon-inclusion particles Spherical carbon-inclusion particles Strip shape carbon-inclusion Flake shape carbon-inclusion Oxide fiber silk
Al2O3 Al2O3 Al2O3, Al Al2O3, Al C, O C, O C, O C, O C, O
~1 µm 5~50 µm 20~50 µm 10~250 µm ~500 nm ~500 nm ~1 µm ~1 µm ~1 mm
Very high High Low Low High Low Low Low Very low
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Fig. 14. Size distributions of condensed combustion products at different pressures.
of various structures in the condensed combustion products of aluminized propellants. Nine typical structures have been observed, and some of them have not, to our knowledge, been observed or further studied before, including the smoke oxide cluster, irregular agglomerates and carbon-inclusions. The smoke oxide cluster is formed by the aggregation of dispersed oxide particles. Considerable amount of rough carbon-inclusion particles can be seen in the products, with few spherical carbon-inclusion particles distributed among them. On the surface of certain agglomerates, strip-shape carbon inclusions and flake-shape carbon inclusions are discovered. Spherical agglomerates are observed in the form of “hollow agglomerates”, “cap agglomerates”, and “solid agglomerates”. The formation of different spherical agglomerates is analyzed to be related to the ambient temperature. The size of irregular agglomerates is much larger, which is believed to be produced by the recombination and coalescence of fragments of fused aluminum droplets in the combustion chamber. The lower atomic ratios of aluminum to oxygen on the surface of agglomerates compared to SOPs suggest that the agglomerates is incompletely oxidized. The content of agglomerates in the CCPs is found to decrease with increasing pressure. Current efforts are focused on the properties of CCPs and the effect of various factors on agglomeration of aluminum particles. Future work will explore strategies to reduce agglomeration using a variety of techniques. Acknowledgments This work was supported by “National Nature Science Foundation of China” (51506181; 51276150) and “Fundamental Research Funds for the Central Universities” (3102015ZY083; 3102014ZD0032). References [1] J.K. Sambamurthi, E.W. Price, R.K. Sigman, Aluminum agglomeration in solid-propellant combustion, AIAA J. 22 (1984) 1132–1138. [2] O.G. Glotov, V.Y. Zyryanov, Condensed combustion products of aluminized propellants. I. A technique for investigating the evolution of disperse-phase particles, Combust. Explos. Shock 31 (1995) 72–78. [3] V.A. Babuk, V.A. Vasilyev, M.S. Malakhov, Condensed combustion products at the burning surface of aluminized solid propellant, J. Propuls. Power 15 (1999) 783–793. [4] V.A. Babuk, V.A. Vassiliev, V.V. Sviridov, Propellant formulation factors and metal agglomeration in combustion of aluminized solid rocket propellant, Combust. Sci. Technol. 163 (2001) 261–289.
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