- Email: [email protected]
Experimental investigation on the condensed combustion products of aluminized GAP-based propellants
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.1 (1-11)
Aerospace Science and Technology ••• (••••) ••••••
1
Contents lists available at ScienceDirect
67 68
2 3
Aerospace Science and Technology
4
69 70 71
5
72
6
www.elsevier.com/locate/aescte
7
73
8
74
9
75
10
76
11 12 13
Experimental investigation on the condensed combustion products of aluminized NEPE propellants
16 17
81
Science and Technology on Combustion, Internal Flow and Thermo-structure Laboratory, Northwestern Polytechnical University, Xi’an, 710072, China
83 85
a r t i c l e
i n f o
a b s t r a c t
86 87
21 23 24 25
Article history: Received 29 January 2019 Received in revised form 5 June 2019 Accepted 26 November 2019 Available online xxxx
26 27 28 29 30 31
82 84
19
22
79
Huan Liu, Wen Ao ∗ , Peijin Liu, Songqi Hu, Xiang Lv, Dongliang Gou, Haiqing Wang
18 20
78 80
14 15
77
Keywords: Agglomeration Condensed combustion products Aluminum combustion NEPE propellants Solid rocket motor
32 33 34 35 36 37 38
The condensed combustion products (CCPs) of solid propellants significantly affect the combustion and internal flow inside the solid rocket motor. A constant-pressure quench vessel was used to collect the condensed combustion products of an aluminized NEPE propellant. The effects of freezing medium, quench distance, chamber pressure and virgin aluminum size on the physicochemical properties of the condensed combustion products were studied. The typical size distribution of the condensed combustion products is in three modes, 1 ∼ 2 μm, 20 ∼ 30 μm, ∼300 μm and their sizes vary from 0.3 to 600 μm, corresponding to smoke oxide particles and agglomerates. Freezing medium is found to have little impact on the particle size, while the agglomeration ratio in water is larger than nitrogen and argon. In contrast to the freezing medium, pressure does affect the size of CCPs, but has little effect on the agglomeration ratio. Agglomeration ratio decreases with the increase of quench distance, which has no significant effect on the particle size. Agglomeration is found to increase first and then decreases with the increase of virgin aluminum size. The influence mechanism of freezing medium and virgin aluminum size on CCPs is proposed. The combustion-agglomeration map is particularly obtained, which shows that freezing medium and virgin aluminum particle size have more profound influence on the agglomeration than quench distance and pressure. The low-pressure condition presents poorest performance with high fraction of agglomerates, large agglomerate size and low combustion efficiency. Results of this work are expected to provide better insight in the combustion of solid propellant and solid rocket motor. © 2019 Elsevier Masson SAS. All rights reserved.
88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104
39
105
40
106
41
107 108
42 43
1. Introduction
44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59
Aluminum powder is usually added to solid propellants to enhance their energetic performance and provide the damping of combustion instabilities. However, aluminum particles accumulate on the burning surface of the solid propellant and roll up into agglomerates on or near the burning surface [1–4]. The agglomerates result in alumina product particles which are bigger than the initial aluminum used. These condensed particles are responsible for problems such as two phase flow loss in the nozzle, slag accumulation inside the rocket motor, erosion of nozzle and incomplete combustion. To accurately estimate any of these effects, it is essential to gain deep insight in the mechanism and the influence factors of aluminum agglomeration. Agglomeration of aluminized propellants has been studied widely mainly based on the cinephotomicrography or quenchcollection method. Babuk et al. [5,6] has made measurements
60 61 62 63 64 65 66
*
Corresponding author. E-mail address: [email protected] (W. Ao).
https://doi.org/10.1016/j.ast.2019.105595 1270-9638/© 2019 Elsevier Masson SAS. All rights reserved.
of condensed combustion products size distribution for different propellant formulations under different pressures. Their results showed that the agglomerate size varies inversely with the propellant burning rate. Sambamurthi et al. [7] used different propellant formulations for measuring the average size of the agglomerates under different mean pressures. It indicated that the agglomeration of aluminum on the burning surface in arrays was dictated by the particle size combinations of ingredients which resulted in the distribution of aluminum in propellant microstructure. Liu et al. [8] investigated the size distribution of the condensed phase products of solid propellant. It was found that the addition of an organic fluoride compound to solid propellant will generate smaller diameter CCPs due to sublimation of AlF3 . Galfetti et al. [9] studied nano-aluminized propellants and it was compared with corresponding micro-aluminized propellants. The results showed that nano-aluminized propellants show larger steady burning rate, without significant change in pressure sensitivity, and lower agglomeration phenomena in combustion products. Liang et al. [10] investigated the ignition and heterogeneous combustion characteristics of boron–aluminum blend by a laser ignition testing system.
109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
JID:AESCTE
AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.2 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
2
1 2 3
No.
4 5 6 7
67
Table 1 The propellant compositions used in present work.
N1 N2 N3
Al
68
AP (330 ∼ 340 μm)
RDX (100 μm)
GAP
Additive
69 70
DAl / μm
Wt/%
Wt /%
Wt/%
Wt/%
Wt/%
13 29 40
18 18 18
42 42 42
15 15 15
23.5 23.5 23.5
1.5 1.5 1.5
71 72 73 74
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
And oxidation products of aluminum with different morphology were found in the CCPs. Li et al. [11] designed a laser ignition experimental device for testing the aluminum–magnesium fuel-rich propellant. The results displayed that the ignition delay time was decreased with the increase of oxygen concentration and pressure. Chen et al. [12] used digital in-line holography to experimentally quantify the three-dimensional position, size, and velocity of aluminum particles during combustion of solid-rocket propellants. In the experiments, increasing initial propellant temperature was shown to enhance the agglomeration of nascent aluminum at the burning surface, resulting in ejection of large molten aluminum particles into the exhaust plume. Korotkikh et al. [13] presented the results of measurement of the burning rate of aluminized composite solid propellants and parameters of sampled CCPs including their particle size distribution, chemical and phase composition. Kalman et al. [14] used time-resolved synchrotron x-ray imaging to view aluminum agglomerate formation in situ at relevant rocket pressures. The agglomerate formation at motor-relevant pressures in real time with unprecedented fidelity, providing critical data for understanding the combustion of aluminized solid rocket propellants was observed. More recently, our group has conducted a series work on the agglomeration behavior of solid propellants [15–17]. It was observed that accumulation, aggregation, and agglomeration of aluminum particles at pressures of 3 and 5 MPa for NEPE propellants were similar to that at pressures below 1 MPa. A constant-pressure quench vessel was adopted to collect the CCPs and the morphology and chemical compositions were studied. A model of aluminum agglomeration on the burning surface of composite propellants was established. Based upon previous work, the characteristics of the agglomeration of the CCPs depend on the propellant formulations and environmental conditions. In general, the agglomerates are influenced by pressure [5,6,18–20], the size of the aluminum particles and their percentage [18,21–23], the size of AP particles [5,18, 23], binder properties [21], distance from the burning surface [24], burning atmosphere [25] and so on. At present, however, there are still some unknowns about the CCPs collection method. Study on the contrast effect of liquid collection and gas collection method is missing. Secondly, a large number of literatures investigated propellants based on isoprene rubber or HTPB as a fuel binder. Few work devoted to systematic studies on the condensed combustion products and agglomeration behaviors of NEPE propellants which are also widely used in industry nowadays. It is necessary to get experimental information about the condensed combustion products of NEPE propellant, because the binder properties were suggested to affect the agglomeration process [5]. Third, the effect of virgin aluminum size on CCPs is still unclear. Sambamurthi obtained the results that the agglomerate diameter D49 was found to increase first and then decrease with increasing virgin aluminum size [7]. However, Liu found that an increase in the virgin aluminum size lead to a decrease in the mean agglomerate size [18]. In this paper, a constant-pressure quench technique is used to collect the condensed combustion products of a series of NEPE propellants, followed by the analysis of the collections. Particle size distribution, morphology and compositions of the condensed combustion products are obtained. Effects of freezing medium, quench
distance, chamber pressure and virgin aluminum size on the agglomeration properties are discussed in detail.
75 76 77 78
2. Experimental
79 80
2.1. Propellant
81 82
Experiments were conducted with NEPE Propellants. The main ingredients used in the present study include aluminum, AP, RDX, and GAP binder, as given in Table 1. The propellant was tailored to 26 mm diameter, 23 mm long cylindrical strand, as shown in Fig. 1(c). The propellant strand was an end-burning grain with the flank and one of the end faces completely inhibited. The weight of strand was about 23 g.
83 84 85 86 87 88 89 90
2.2. Experimental method
91 92
The constant-pressure quench vessel is used to collect the condensed combustion products. The quench vessel is schematically shown in Fig. 1(a). It is a thick-walled cylindrical steel chamber of 1180 mm length and 235 mm internal diameter. The top and bottom ends of the chamber are closed with flanges. The top flange is welded with propellant specimen holder (combustion chamber). The combustion chamber and the top flange are equipped with two pressure transducers, one igniter and two solenoid valves for atmosphere gas inlet and combustion gas exhaust. Pressure chamber is filled with quench liquid (water) or cooling gases (nitrogen and argon). An orifice plate (Fig. 1(b)) made of steel is installed at the bottom of the quench vessel. The exhaust gas/water flow during propellant burning can be varied by modifying the orifice size. The pressure can be kept constant by choosing proper orifice plate. Black powder (3 g) is employed to ignite the propellant grain. During the test, firstly, the initial pressure in the apparatus is set up by filling high-pressure nitrogen or argon. The orifice diameter is determined by thermodynamic calculation. The orifice diameter calculation steps are as follows. Propellant mass flow:
qp =
π 4
qg
ρg
(1)
95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 114 115
=
π
(2)
ρg
4
117 118 120 121
·D
116
119
qp
Where Q g is the propellant gas volumetric flow; q g is the propellant gas mass flow; ρ g is the propellant gas density. Volume flow formula of orifice plate:
Q o = Cd ·
94
113
· ρ p · D 2p · r
Where q p is the propellant mass flow; ρ p is the propellant density; D p is the burning surface diameter; r is the static burning rate. Propellant gas volumetric flow:
Qg =
93
2
122 123 124 125
2 p
ρm
126
(3)
Where Q o is the volume flow of orifice plate; C d is the orifice plate coefficient; D is the orifice plate diameter; p is the differential pressure between combustion chamber pressure and atmospheric pressure; ρm is the density of discharge media.
127 128 129 130 131 132
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.3 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
3
1
67
2
68
3
69
4
70
5
71
6
72
7
73
8
74
9
75
10
76
11
77
12
78
13
79
14
80
15
81
16
82
17
83
18
84
19
85
20
86
21
87 88
22 23
89
Fig. 1. Constant pressure CCPs collection system. (a) Quench vessel, (b) Orifice plate, (c) Propellant, (d) Pressure curve.
90
24 25 26 27
Test No.
Chamber pressure/ MPa
T1 T2 T3 T4 T5 T6 T7 T8 T9
7 7 7 7 7 5 9 7 7
28 29 30 31 32 33 34 35 36
91
Table 2 The scheme of tests.
92
Propellant
N2
N1 N3
Aluminum size/ μm
Burning rate/ mm/s
Freezing substance
Quench distance/ mm
Orifice diameter/ mm
93
29 29 29 29 29 29 29 13 40
5.6 5.6 6.1 5.7 5.5 4.7 6.5 5.6 6.8
N2 Ar H2 O H2 O H2 O H2 O H2 O H2 O H2 O
– – 210 420 740 210 210 210 210
2.55 2.55 2.55 2.55 2.55 3.01 2.45 2.52 2.64
95
39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
96 97 98 99 100 101 102 103
37 38
94
Relationship between volumetric flow of gas and volume flow of discharged media:
Q g = Qo
(4)
Combined with the above equations, the orifice diameter D could be obtained. In addition, reasonable trigger time of pneumatic control valve, ignition and drain valve is set to achieve proper experiment result. Pressure is required to maintain constant by exhaust liquid or gas when propellant burns at the same time. Pneumatic control valve is closed after the propellant combustion. After about half an hour, electromagnetic valve is opened to exhaust gas. The pressure within the container will decrease to atmospheric pressure. Then, the slurry (condensed combustion products) along with the quench liquid is transferred to barrels. The typical pressure curve in the experiment is shown in Fig. 1(d). It could be seen that the pressure holding device works normally, the parameter setting is reasonable, the pressure is stable during working time (Pressure fluctuation is less than 5%). The suspensions need to be precipitated for 48 hours. The bottom portion of mixed liquid of 500 ml in the barrel is collected to beakers. The collections are washed by ethanol for several times to remove impurities. Then the solids which are contained by the collections are separated by a centrifuge. The solids are dried at 70◦ C under vacuum for 24 hours before analysis. Each operating condition is tested at least twice to check the reproducibility [15]. The repeatability is within 5% based on the mean-mass diameter D 43 of the CCPs size distributions obtained in different runs.
Four operational parameters that affect the agglomeration properties of the propellants are involved in our experiments. They are freezing medium, quench distance, chamber pressure and virgin aluminum size, individually. The scheme of experimental test is given in Table 2. The uncertainty of measured burning rate is estimated to be 0.2 mm/s.
104 105 106 107 108 109 110
2.3. Product analysis
111 112
A scanning electron microscope (Zeiss Supra 55) is used to observe the microstructure of CCPs. The particle size distribution in of CCPs is directly measured by laser diffraction particle size analyzer (Malvern Mastersizer 2000) using a small amount of representative sample. The sample quality is about 0.1 g for measuring the particle size distribution. The obscuration is kept between 10% and 20% in order to obtain accurate particle size distribution data. The chemical composition of the CCPs is performed by using the X-ray diffractometer (Empyrean).
113 114 115 116 117 118 119 120 121 122
3. Results and discussions
123 124
3.1. Overview of microstructures
125 126
Condensed combustion products are characterized by scanning electron microscopy. The SEM overview image of condensed combustion products are shown in Fig. 2. CCPs are in the size range 0.3 to 600 μm and the size distribution is multimodal. The particles in the CCPs are distributed around three modes: ∼2 μm, 20 ∼ 30 μm and 300 μm. In general, it is agreed that the condensed products
127 128 129 130 131 132
JID:AESCTE
4
AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.4 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
1
67
2
68
3
69
4
70
5
71
6
72
7
73
8
74
9
75
10
76
11
77
12
78
13
79
14
80
15 16
Fig. 2. SEM image of overall condensed combustion products.
81 82
17
83
18
84
19
85
20
86
21
87
22
88
23
89
24
90
25
91
26
92
27
93
28
94
29
95
30
96
31
97
32
98
33
99
34
100 101
35 36
Fig. 3. SEM image and EDS chemical map of typical agglomerates. (a) Spherical particle with oxide cap, (b) Collision of two agglomerates.
102
37
103
38
104
39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
consist of smoke oxide particles and agglomerates [21]. The size of smoke oxide particles is in the order of 1 μm, whereas the agglomerates is usually very large, which may reach hundreds of micrometers. The trimodal particle size distribution in the current research is consistent with Ao’ work and Jeenu’ work [15,24]. Mode ∼2 μm particles are smoke oxide particles formed by the oxidation of aluminum vapor. Mode ∼30 μm particles are supposed to be the residual oxide particles, which are formed from the molten oxide caps on the aluminum particles or agglomerates at its burn out. Finally, mode ∼300 μm particles are the large agglomerates. A close view of the micromorphology of typical smoke oxide particle, residual oxide particle and agglomerate is shown in Fig. 2. The smoke oxide particle is spherical with a smooth surface, as well as the typical residual oxide particle is also smooth. Oppositely, the surface of the agglomerate is very rough. Different types of agglomerates were discovered in CCPs by Ao et al. [15], including “hollow agglomerates”, “cap agglomerates”, “solid agglomerates”, and “irregular agglomerates”. Similar structures can be found in the present NEPE propellant, as shown in Fig. 3. Fig. 3(a) is a spherical particle with oxide cap, which is described as a metal drop with an oxide cap on the surface. The electron dispersive spectroscopy results indicate the smaller part is oxide cap since the oxygen content is higher. Interestingly, the collision of two agglomerates is observed in Fig. 3(b). It may be formed by a collision of two spherical agglomerates along the reactive flow during the combustion process.
3.2. Effect of freezing medium
105 106 107
Fig. 4 shows the size distribution of CCPs in different mediums. Usually the size distribution of CCPs has a boundary D c which divides the CCPs into fine particles and agglomerates. As mentioned above, fine particles contain smoke oxide particles and residual oxide particles. Till far, there is still no rigorous definition of D c . Considering that large agglomerates are of our mainly concern, and according to Jackson’s work [5], D c is set to be 100 μm in the present work. A comparison of the fine particle distributions shows that for all the three mediums, it is possible to distinguish two modes in the intervals 1 ∼ 2 μm and 14 ∼ 30 μm. For agglomerates, there is a mode in the interval 290 ∼ 360 μm. The fraction of agglomerates in CCPs, mag /mCCPs , is proposed to evaluate the agglomeration characteristics. To simplify operation, it is assumed that CCPs are completely oxidized. This assumption is reasonable because little reactive aluminum is detected in the final products (Table 3). The mag /mCCPs are 0.0945, 0.1095, 0.1712 in N2 , Ar and H2 O, respectively. The agglomeration in H2 O is obviously stronger compared to N2 and Ar. According to literature [6], the effect of freezing medium on the particle size distribution of the CCPs is related to its heat transfer performance. The thermal conductivity of nitrogen (0.026 W/m/K) and argon (0.018 W/m/K) is much smaller than that of water (0.599 W/m/K) at 300 K and atmospheric pressure, which means that the cooling effect of the two gases is much lower than that of water. The combustion of aluminum particles depends on the combustion environment when
108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.5 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
5
1
67
2
68
3
69
4
70
5
71
6
72
7
73
8
74
9
75
10
76
11
77
12
78
13
79
14
80
15
81
16
82
17
83
18
84
19
85
20
86
21
87
22
88
23
89
24
90
25
91
26
92 93
27 28
94
Fig. 4. The particle size of different freezing mediums.
29
95
30
96
31
97
Table 3 Phase composition of CCPs sampled in different freezing mediums.
32 33
Freezing medium
34 35
N2 Ar H2 O
36 37 38
98
Content of the crystal phase, wt.%
Combustion efficiency
γ -Al2 O3
θ -Al2 O3
AlN
Al
41 46 47
52 41 45
7 5 5
– 8 3
η
43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
102 103 104 105 106
40 42
100 101
100 % 86.1 % 94.1 %
39 41
99
aluminum particles burn. In high temperature environment, the combustion of aluminum particles would be sufficient, resulting in a small amount of agglomerates. Therefore, the cooling effect of nitrogen and argon is lower than water, leading to a higher combustion environment temperature, and ultimately yielding fewer ag agglomerates. Besides, the mean diameter of agglomerates D 43 for N2 , Ar and H2 O are 276 μm, 293 μm, 274 μm, while the mean difi D 43
ameter of SOPs are 15 μm, 20 μm, 19 μm, respectively. These results mean the mean diameter of agglomerates and SOPs are insensitive with freezing medium. Interestingly, it may be concluded that the freezing medium only affects the agglomeration ratio, but has little impacts on the particle size. X-ray diffraction patterns of CCPs with different freezing mediums are shown in Fig. 5. γ -Al2 O3 , θ -Al2 O3 , AlN and Al are found in the products. During the oxidation of aluminum, the original oxide transforms into γ -Al2 O3 at 550◦ C [26]. Further temperature increase results in continuous growth of the θ -Al2 O3 . The generation of AlN needs to be special attention. It is supposed that AlN is from the reaction of Al and N2 . N2 can react with aluminum when the temperature reaches 2300 K [27]. In general, propellant combustion temperature is higher than 3000 K. So the freezing gas nitrogen must react with aluminum particles to produce AlN. In Ar environment, AlN is still detected, although the content of AlN is less than that in N2 . This suggests that AlN is yielded both by the reaction between Al and nitrogen-containing substances in propellant and Al/N2 reaction. The combustion efficiency of alu-
minum in the propellant is the ratio of the amount of alumina generated by actual combustion to the amount of alumina produced by the theoretical complete combustion. The combustion efficiency of aluminum in the propellant η is approximate calculated, as listed in Table 3. When the freezing medium is nitrogen, there is no aluminum in the CCPs. It can be inferred that the nitrogen environment promotes the combustion of aluminum compared with the argon environment. In general, the mechanism of different freezing medium on the agglomeration behavior is shown in Fig. 6.
107 108 109 110 111 112 113 114 115 116 117
3.3. Effect of quench distance
118 119
Fig. 7 shows the size distribution of CCPs in different quench distances. An analysis of the fine particle distributions shows that it is possible to distinguish two modes in the intervals 2 μm and 20 ∼ 23 μm. For agglomerates, only one mode is present in the interval 290 ∼ 320 μm. The mag /mCCPs are 0.1712, 0.1657, 0.1334 for 210 mm, 420 mm and 740 mm, respectively. The agglomeration ratio decreases slightly with the increase of quench distance. In the combustion of agglomerate, a part of the oxide is formed in the gas-phase state and then diffuses into the environment where it condenses to form SOPs [28]. This means some SOPs are produced by the burning of agglomerates. Agglomerates can burn for longer periods of time due to increased quench distance, yielding more SOPs. This result is in accordance with literature [24], where the
120 121 122 123 124 125 126 127 128 129 130 131 132
JID:AESCTE
6
AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.6 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
1
67
2
68
3
69
4
70
5
71
6
72
7
73
8
74
9
75
10
76
11
77
12
78
13
79
14
80
15
81
16
82
17
83
18
84
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
85
Fig. 5. The XRD of different freezing mediums. (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)
86
Fig. 6. Scheme of the CCPs evolution in different freezing mediums.
percentage of large particles (mode 70 μm) reduces with increase in quench distance. For the quench distance of 210 mm, 420 mm ag and 740 mm, the mean diameter of agglomerates D 43 are 274 μm, ag 273 μm, 279 μm. The mean diameter of agglomerates D 43 is basically the same due to the combustion behavior of agglomerates. According to previous study [29], the combustion law for agglomerated aluminum droplets in SRM condition is D 1.0 = D 10.0 -20t. Moreover, the velocity of agglomerates about 100 μm is less than 1.0 m/s [17]. It can be calculated that the combustion distance for an agglomerate particle 600 μm is less than 30 mm, much smaller than the quench distance used in this study. In consequence, most of the agglomerates should burn out before reaching the quench distance since they are less than 600 μm in this study. As a result, quench distance has little effect on the agglomerates size since their initial values are the same. Furthermore, quench distance also has no significant effect on the mean diameter of SOPs, 19 μm, 20 μm, 20 μm for the three conditions.
87 88
X-ray diffraction patterns of CCPs with different quench distances are shown in Fig. 8. The CCPs include γ -Al2 O3 , θ -Al2 O3 , AlN and Al. When the quench distance is 210 mm, 420 mm and 740 mm, the combustion efficiency of aluminum η are 94.1%, 94.1% and 96.0%, respectively (Table 4). The results indicate that there is little difference in aluminum combustion efficiency corresponding to different quenching distances. From the above discussion, one may conclude that most particles have stopped burning before they reach the distance of 210 mm. Only a small amount of large agglomerates would continue burning after they reach 210 mm.
89 90 91 92 93 94 95 96 97 98 99
3.4. Effect of pressure Fig. 9 shows the size distribution of CCPs in different pressures. It is possible to distinguish two modes in the intervals 1 ∼ 2 μm and 14 ∼ 22 μm by analyzing the fine particle distributions. For
100 101 102 103 104 105
40
106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130 131
65 66
Fig. 7. The particle size of different quench distances.
132
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.7 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
increase in burning rate could possibly reduce the agglomerate size. As the burning rate rises, the burning gas velocity near the burning surface increases, leading to the agglomerates being taken away from the burning surface more easily. The mean diameter of SOPs decreases slightly with the increase of pressure. In contrast to the freezing medium factor, pressure does affect the size of CCPs, whereas has little effect on the agglomeration ratio. X-ray diffraction patterns of CCPs under different pressures are shown in Fig. 10. It should be noted that a certain amount of α -Al2 O3 is detected in 9 MPa. Although the γ -Al2 O3 phase was found to be completely changed into α -Al2 O3 phase up to 900◦ C during oxidation [30], only little α -Al2 O3 is present in this work. This may be resulted from the phase transformation during the rapid cooling process, which is different from the heating in oxidation. According to Table 5, when the pressure is 5 MPa, 7 MPa and 9 MPa, the combustion efficiency of aluminum η are 85.0%, 94.1% and 100%, respectively. The combustion efficiency increases with the increase of pressure since aluminum particle gets more reactive under high pressure conditions.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
7
Fig. 8. The XRD of different quench distances.
33 34
agglomerates, there is a one mode in the interval 290 ∼ 320 μm. The mag /mCCPs are 0.1658, 0.1712, 0.1667 for 5 MPa, 7 MPa and 9 MPa, respectively. This indicates that the agglomeration ratio is little affected by pressure changes. The mean diameter of agglomag erates D 43 for 5 MPa, 7 MPa and 9 Mpa are 288 μm, 274 μm, 258 μm. The agglomerate size decreases as the pressure increases. It is evident that the burning rate increases with pressure increase. As the chamber pressure is increased from 5 MPa to 9 MPa, the corresponding burning rate is from 4.7 mm/s to 6.5 mm/s. An
37 38
69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 87 88
With the increase of initial aluminum size, the mean diameter ag of agglomerates D 43 are 207 μm, 274 μm, 236 μm, respectively. It ag is shown in Fig. 11 that the mean diameter of agglomerate D 43 increases first and then decreases with the size of initial aluminum in propellant. Sambamurthi et al. [7] and Liu et al. [17] also obtained the similar trends. As the aluminum content in propellant strand is the same for various aluminum sizes, the number den-
89 90 91 92 93 94 95 96 97
Table 4 Phase composition of CCPs sampled in different quench distances. Quench distance
35 36
68
86
3.5. Effect of virgin aluminum size
31 32
67
210 mm 420 mm 740 mm
98 99
Content of the crystal phase, wt.%
Combustion efficiency
γ -Al2 O3
θ -Al2 O3
AlN
Al
47 41 40
45 52 51
5 4 7
3 3 2
η
100 101
94.1 % 94.1 % 96.0 %
102 103 104
39
105
40
106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130 131
65 66
Fig. 9. The particle size of different pressures.
132
JID:AESCTE
AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.8 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
8
cles will be small. The aluminum particles have little opportunity to concentrate before ignition in such conditions. So that large aluminum particles also form small agglomerates. Intermediatesize aluminum particles form large agglomerates because that they have a relatively large number density and they can’t be taken away easily from the burning surface. They will facilitate the agglomeration of aluminum particles. Based on the above analysis, the distribution of aluminum in the propellants microstructure and the burning gas on particles near burning surface are the main factors affecting agglomeration in different virgin aluminum sizes. The mag /mCCPs are 0.1181, 0.1712, 0.1162 with increasing initial
1 2 3 4 5 6 7 8 9 10 11
fi D 43
12
aluminum size. The mean diameter of SOPs are 17 μm, 19 μm, 17 μm, respectively. The variation of agglomerate ratio versus initial aluminum size is the same as agglomerate size, whereas the
13 14 15
fi D 43
16 17 18 19 20
Fig. 10. The XRD of different pressures.
21 22 23 24 25 26 27 28 29 30 31
sity of aluminum particles will be large if the aluminum size is small. Aluminum particles can be concentrated easily at the surface of propellant. But the small particles are reactive, they are heated easily and taken away from the surface by the burning gas before agglomeration. This is the reason that small aluminum particles form small agglomerates. When the aluminum particle size is increased sufficiently, the number density of aluminum parti-
mean diameter of SOPs presents little change. Fig. 12 shows the X-ray diffraction patterns of CCPs with different aluminum size. Unlike the case of small particle size, a small amount of α -Al2 O3 is detected for 40 μm virgin Al, which implies virgin aluminum size may change the phase transformation dynamics of the oxide. It is unexpected from Table 6 that the combustion efficiency of the 13 μm virgin Al is the lowest, 83.0%, which is over 10% less than the other two cases. It may be because that the agglomerate size is smallest for 13 μm virgin Al propellant. Agglomerates are taken away from the burning surface easily resulting in short residence time across the combustion zone. This will cause relatively inadequate combustion for particles. The mechanism of the different aluminum particle size on the agglomeration is shown in Fig. 13.
Pressure
34 35 36 37
5MPa 7MPa 9MPa
68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
Table 5 Phase composition of CCPs sampled in different pressures.
97
32 33
67
Content of the crystal phase, wt.%
Combustion efficiency
γ -Al2 O3
θ -Al2 O3
α -Al2 O3
AlN
Al
44 47 46
43 45 32
– – 14
5 5 8
8 3 –
η
98 99 100
85.0% 94.1% 100%
101 102 103
38
104
39
105
40
106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130 131
65 66
Fig. 11. The particle size of different aluminum size.
132
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.9 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Fig. 12. The XRD of different aluminum size.
21 22
3.6. Agglomeration-combustion map
23 24 25 26 27 28 29 30 31 32
The generalization of the agglomeration characteristics from the above experimental results is presented in Fig. 14, showing the weak and high agglomeration trends. Weak agglomeration trend is characterized by the location of the point in the left lower corag ner of diagram, which has small value of agglomerate size D 43 and small mass fraction of agglomerates in CCP mag /mCCPs . Accordingly, the right upper corner in diagram corresponds to the high agglomeration trend. All the four factors, freezing medium, quench distance, pressure and virgin aluminum size do have effect
9
on the agglomeration behavior during the propellant combustion, either on agglomerate size or agglomeration ratio. It is obviously from the map that the freezing medium and virgin aluminum particle size have more profound influence on the agglomeration than quench distance and pressure. Water collection has totally different agglomeration properties in comparison with gas collection, e.g. water collection belongs to high agglomeration whereas N2 and Ar collection is weak agglomeration. This helps us to know that the agglomeration results from different experiments cannot be compared identically if their collection mediums are different. The most important conclusion from the map is about the effect of virgin aluminum size. All the five conditions located at the high agglomeration side are corresponding to the virgin aluminum of 29 μm. This suggests the intermediate-size virgin aluminum is the key factor leading to undesired agglomeration, which can give a reference to the practical industrial formulation design. To reveal the relationship between combustion and agglomeration of aluminized propellants, the agglomeration-combustion map is presented in Fig. 15. It is arbitrarily prescribed that 90% is high combustion efficiency since rapid cooling is rigorous to propellant burning. The combustion efficiency is highest with operating conditions T1&T7, since nitrogen environment and high pressure can promote the combustion of aluminum particles. It should be noted that T8 has low combustion efficiency with small agglomeration ratio and agglomerate size. This suggests the residence time significantly affects the combustion of agglomerates according to previous analysis. To our special concern, T6 present poor performance with high fraction of agglomerates, large agglomerate size and low combustion efficiency. This implies low-pressure conditions are adverse to the combustion and agglomeration of aluminized propellants.
35 36
Table 6 Phase composition of CCPs sampled in different aluminum size. Aluminum size
37 38 39 40
68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99
33 34
67
13 μm 29 μm 40 μm
100 101
Content of the crystal phase, wt.%
Combustion efficiency
γ -Al2 O3
θ -Al2 O3
α -Al2 O3
AlN
Al
55 47 41
33 45 45
– – 4
3 5 7
9 3 3
η
102 103
83.0% 94.1% 94.1%
104 105 106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130 131
65 66
Fig. 13. Scheme of the agglomeration with different aluminum size.
132
JID:AESCTE
AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.10 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
10
crease of aluminum size. Spatial topology of aluminum inside the propellant is a key factor to agglomeration. 6) Freezing medium and virgin aluminum particle size have more profound influence on the agglomeration than quench distance and pressure. The low-pressure condition is adverse to the combustion and agglomeration of aluminized propellants.
1 2 3 4 5 6
Declaration of competing interest
8
None declared.
10
Acknowledgements
12
This work was funded by the “Natural Science Foundation of Shanxi Province” (Grant No. 2018JQ5112) and “Fundamental Research Funds for the Central Universities” (Grant No. 3102018ZY003).
14 15 16 17 19
References Fig. 14. The agglomeration trends of NEPE propellants.
21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
Fig. 15. The Agglomeration-Combustion map of NEPE propellants.
40
4. Conclusions
42
45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
72 74 76 78 80 81 82 83 84
18
44
71
79
13
43
70
77
11
41
69
75
9
39
68
73
7
20
67
1) The condensed combustion products are in the size range 0.3 to 600 μm and the size distribution is multimodal. The CCPs are distributed with three peaks: 1 ∼ 2 μm, 20 ∼ 30 μm, ∼300 μm. Mode ∼2 μm particles are smoke oxide particles. Mode ∼30 μm particles are residual oxide particles. Mode ∼300 μm particles are agglomerates. The smoke oxide particle and residual oxide particle are spherical with a smooth surface. However, the surface of the agglomerate is very rough. In general, the compositions of the CCPs include γ -Al2 O3 , θ -Al2 O3 , α -Al2 O3 , AlN and Al. 2) The agglomeration ratio in H2 O is stronger compared to N2 and Ar. The nitrogen environment promotes the combustion of aluminum compared with the argon environment. Freezing medium has little impacts on the mean diameter of agglomerates and SOPs. 3) The ratio of SOPs increases with the increase of quench distance. Quench distance has no significant effect on the mean diameter of agglomerates and SOPs. 4) Agglomeration ratio is insensitive with pressure. Pressure only affects the particle size that the mean diameters of agglomerates and SOPs decrease with the increase of pressure. The combustion efficiency increases as pressure increases. 5) Both the mean diameter of agglomerates and agglomeration ratio are found to increase first and then decrease with the in-
85 86
[1] A. Gany, L.H. Caveny, Agglomeration and ignition mechanism of aluminum particles in solid propellants, Symp. (Int.) Combust. 17 (1979) 1453–1461. [2] T.K. Liu, H.C. Perng, S.P. Luh, F. Liu, Aluminum agglomeration in ammonium perchlorate/cyclotrimethylene trinitramine/aluminum/hydroxyterminated polybutadiene propellant combustion, J. Propuls. Power 8 (1992) 1177–1184. [3] V.A. Babuk, V.A. Vassiliev, V.V. Sviridov, Formation of condensed combustion products at the burning surface of solid rocket propellant, in: V. Yang, T.B. Brill, W.Z. Ren (Eds.), Solid Propellant Chemistry, Combustion, and Motor Interior Ballistics, in: Progress in Astronautics and Aeronautics, vol. 185, AIAA, 2000, pp. 749–776. [4] T.L. Jackson, J. Buckmaster, F. Najjar, New aluminum agglomeration models and their use in solid-propellant-rocket simulations, J. Propuls. Power 21 (2005) 925–936. [5] 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. [6] 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. [7] J.K. Sambamurthi, E.W. Price, R.K. Sigmant, Aluminum agglomeration in solidpropellant combustion, AIAA J. 22 (1984) 1132–1138. [8] Z. Liu, S. Li, M. Liu, D. Guan, X. Sui, N. Wang, Experimental investigation of the combustion products in an aluminised solid propellant, Acta Astronaut. 133 (2017) 136–144. [9] L. Galfetti, L.T. DeLuca, F. Severini, G. Colombo, L. Meda, G. Marra, Pre and post-burning analysis of nano-aluminized solid rocket propellants, Aerosp. Sci. Technol. 11 (2007) 26–32. [10] D. Liang, R. Xiao, J. Liu, Y. Wang, Ignition and heterogeneous combustion of aluminum boride and boron–aluminum blend, Aerosp. Sci. Technol. 84 (2019) 1081–1091. [11] L.B. Li, X. Chen, O. Musa, C.S. Zhou, M. Zhu, The effect of pressure and oxygen concentration on the ignition and combustion of aluminum–magnesium fuel-rich propellant, Aerosp. Sci. Technol. 76 (2018) 394–401. [12] Y. Chen, D.R. Guildenbecher, K.N.G. Hoffmeister, M.A. Cooper, H.L. Stauffacher, M.S. Oliver, E.B. Washburn, Study of aluminum particle combustion in solid propellant plumes using digital in-line holography and imaging pyrometry, Combust. Flame 182 (2017) 225–237. [13] Alexander G. Korotkikh, Oleg G. Glotov, Vladimir A. Arkhipov, Vladimir E. Zarko, Alexander B. Kiskin, Effect of iron and boron ultrafine powders on combustion of aluminized solid propellants, Combust. Flame 178 (2017) 195–204. [14] Joseph Kalman, Andrew R. Demko, Bino Varghese, Katarzyna E. Matusik, Alan L. Kastengren, Synchrotron-based measurement of aluminum agglomerates at motor conditions, Combust. Flame 196 (2018) 144–146. [15] W. Ao, P. Liu, W. Yang, Agglomerates, smoke oxide particles, and carbon inclusions in condensed combustion products of an aluminized GAP-based propellant, Acta Astronaut. 129 (2016) 147–153. [16] W. Ao, X. Liu, H. Rezaiguia, H. Liu, Z. Wang, P. Liu, Aluminum agglomeration involving the second mergence of agglomerates on the solid propellants burning surface: experiments and modeling, Acta Astronaut. 136 (2017) 219–229. [17] X. Liu, W. Ao, H. Liu, P. Liu, Aluminum agglomeration on burning surface of NEPE propellants at 3-5 MPa, Propellants Explos. Pyrotech. 42 (2017) 260–268. [18] T.K. Liu, Experimental and model study of agglomeration of burning aluminized propellants, J. Propuls. Power 21 (2005) 797–806. [19] F. Maggi, A. Bandera, L.T. DeLuca, Approaching solid propellant heterogeneity for agglomerate size prediction, in: 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA Paper No. 2010-6751, Nashville, TN, USA, 2010.
87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.11 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
1 2 3 4 5 6 7 8 9 10 11 12 13
[20] K. Jayaraman, S.R. Chakravarthy, R. Sarathi, Quench collection of nanoaluminium agglomerates from combustion of sandwiches and propellants, Proc. Combust. Inst. 33 (2011) 1941–1947. [21] 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. [22] J.C. Mullen, M.Q. Brewster, Reduced agglomeration of aluminum in widedistribution composite propellants, J. Propuls. Power 27 (2011) 650–661. [23] 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. [24] R. Jeenu, K. Pinumalla, D. Deepak, Size distribution of particles in combustion products of aluminized composite propellant, J. Propuls. Power 26 (2010) 715–723.
11
[25] O.G. Glotov, Condensed combustion products of aluminized propellants. III. Effect of an inert gaseous combustion environment, Combust. Explos. Shock Waves 38 (2002) 92–100. [26] M.A. Trunov, M. Schoenitz, X. Zhu, E.L. Dreizin, Effect of polymorphic phase transformations in Al2 O3 film on oxidation kinetics of aluminum powders, Combust. Flame 140 (2005) 310–318. [27] A. Mukasyan, Combustion synthesis of nitrides: mechanistic studies, Proc. Combust. Inst. 30 (2005) 2529–2535. [28] V.A. Babuk, V.A. Vasilyev, Model of aluminum agglomerate evolution in combustion products of solid rocket propellant, J. Propuls. Power 18 (2002) 814–823. [29] 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. [30] S. Hasani, M. Panjepour, M. Shamanian, The oxidation mechanism of pure aluminum powder particles, Oxid. Met. 78 (2012) 179–195.
67 68 69 70 71 72 73 74 75 76 77 78 79
14
80
15
81
16
82
17
83
18
84
19
85
20
86
21
87
22
88
23
89
24
90
25
91
26
92
27
93
28
94
29
95
30
96
31
97
32
98
33
99
34
100
35
101
36
102
37
103
38
104
39
105
40
106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130
65
131
66
132
[m5G; v1.261; Prn:29/11/2019; 12:50] P.1 (1-11)
Aerospace Science and Technology ••• (••••) ••••••
1
Contents lists available at ScienceDirect
67 68
2 3
Aerospace Science and Technology
4
69 70 71
5
72
6
www.elsevier.com/locate/aescte
7
73
8
74
9
75
10
76
11 12 13
Experimental investigation on the condensed combustion products of aluminized NEPE propellants
16 17
81
Science and Technology on Combustion, Internal Flow and Thermo-structure Laboratory, Northwestern Polytechnical University, Xi’an, 710072, China
83 85
a r t i c l e
i n f o
a b s t r a c t
86 87
21 23 24 25
Article history: Received 29 January 2019 Received in revised form 5 June 2019 Accepted 26 November 2019 Available online xxxx
26 27 28 29 30 31
82 84
19
22
79
Huan Liu, Wen Ao ∗ , Peijin Liu, Songqi Hu, Xiang Lv, Dongliang Gou, Haiqing Wang
18 20
78 80
14 15
77
Keywords: Agglomeration Condensed combustion products Aluminum combustion NEPE propellants Solid rocket motor
32 33 34 35 36 37 38
The condensed combustion products (CCPs) of solid propellants significantly affect the combustion and internal flow inside the solid rocket motor. A constant-pressure quench vessel was used to collect the condensed combustion products of an aluminized NEPE propellant. The effects of freezing medium, quench distance, chamber pressure and virgin aluminum size on the physicochemical properties of the condensed combustion products were studied. The typical size distribution of the condensed combustion products is in three modes, 1 ∼ 2 μm, 20 ∼ 30 μm, ∼300 μm and their sizes vary from 0.3 to 600 μm, corresponding to smoke oxide particles and agglomerates. Freezing medium is found to have little impact on the particle size, while the agglomeration ratio in water is larger than nitrogen and argon. In contrast to the freezing medium, pressure does affect the size of CCPs, but has little effect on the agglomeration ratio. Agglomeration ratio decreases with the increase of quench distance, which has no significant effect on the particle size. Agglomeration is found to increase first and then decreases with the increase of virgin aluminum size. The influence mechanism of freezing medium and virgin aluminum size on CCPs is proposed. The combustion-agglomeration map is particularly obtained, which shows that freezing medium and virgin aluminum particle size have more profound influence on the agglomeration than quench distance and pressure. The low-pressure condition presents poorest performance with high fraction of agglomerates, large agglomerate size and low combustion efficiency. Results of this work are expected to provide better insight in the combustion of solid propellant and solid rocket motor. © 2019 Elsevier Masson SAS. All rights reserved.
88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104
39
105
40
106
41
107 108
42 43
1. Introduction
44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59
Aluminum powder is usually added to solid propellants to enhance their energetic performance and provide the damping of combustion instabilities. However, aluminum particles accumulate on the burning surface of the solid propellant and roll up into agglomerates on or near the burning surface [1–4]. The agglomerates result in alumina product particles which are bigger than the initial aluminum used. These condensed particles are responsible for problems such as two phase flow loss in the nozzle, slag accumulation inside the rocket motor, erosion of nozzle and incomplete combustion. To accurately estimate any of these effects, it is essential to gain deep insight in the mechanism and the influence factors of aluminum agglomeration. Agglomeration of aluminized propellants has been studied widely mainly based on the cinephotomicrography or quenchcollection method. Babuk et al. [5,6] has made measurements
60 61 62 63 64 65 66
*
Corresponding author. E-mail address: [email protected] (W. Ao).
https://doi.org/10.1016/j.ast.2019.105595 1270-9638/© 2019 Elsevier Masson SAS. All rights reserved.
of condensed combustion products size distribution for different propellant formulations under different pressures. Their results showed that the agglomerate size varies inversely with the propellant burning rate. Sambamurthi et al. [7] used different propellant formulations for measuring the average size of the agglomerates under different mean pressures. It indicated that the agglomeration of aluminum on the burning surface in arrays was dictated by the particle size combinations of ingredients which resulted in the distribution of aluminum in propellant microstructure. Liu et al. [8] investigated the size distribution of the condensed phase products of solid propellant. It was found that the addition of an organic fluoride compound to solid propellant will generate smaller diameter CCPs due to sublimation of AlF3 . Galfetti et al. [9] studied nano-aluminized propellants and it was compared with corresponding micro-aluminized propellants. The results showed that nano-aluminized propellants show larger steady burning rate, without significant change in pressure sensitivity, and lower agglomeration phenomena in combustion products. Liang et al. [10] investigated the ignition and heterogeneous combustion characteristics of boron–aluminum blend by a laser ignition testing system.
109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
JID:AESCTE
AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.2 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
2
1 2 3
No.
4 5 6 7
67
Table 1 The propellant compositions used in present work.
N1 N2 N3
Al
68
AP (330 ∼ 340 μm)
RDX (100 μm)
GAP
Additive
69 70
DAl / μm
Wt/%
Wt /%
Wt/%
Wt/%
Wt/%
13 29 40
18 18 18
42 42 42
15 15 15
23.5 23.5 23.5
1.5 1.5 1.5
71 72 73 74
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
And oxidation products of aluminum with different morphology were found in the CCPs. Li et al. [11] designed a laser ignition experimental device for testing the aluminum–magnesium fuel-rich propellant. The results displayed that the ignition delay time was decreased with the increase of oxygen concentration and pressure. Chen et al. [12] used digital in-line holography to experimentally quantify the three-dimensional position, size, and velocity of aluminum particles during combustion of solid-rocket propellants. In the experiments, increasing initial propellant temperature was shown to enhance the agglomeration of nascent aluminum at the burning surface, resulting in ejection of large molten aluminum particles into the exhaust plume. Korotkikh et al. [13] presented the results of measurement of the burning rate of aluminized composite solid propellants and parameters of sampled CCPs including their particle size distribution, chemical and phase composition. Kalman et al. [14] used time-resolved synchrotron x-ray imaging to view aluminum agglomerate formation in situ at relevant rocket pressures. The agglomerate formation at motor-relevant pressures in real time with unprecedented fidelity, providing critical data for understanding the combustion of aluminized solid rocket propellants was observed. More recently, our group has conducted a series work on the agglomeration behavior of solid propellants [15–17]. It was observed that accumulation, aggregation, and agglomeration of aluminum particles at pressures of 3 and 5 MPa for NEPE propellants were similar to that at pressures below 1 MPa. A constant-pressure quench vessel was adopted to collect the CCPs and the morphology and chemical compositions were studied. A model of aluminum agglomeration on the burning surface of composite propellants was established. Based upon previous work, the characteristics of the agglomeration of the CCPs depend on the propellant formulations and environmental conditions. In general, the agglomerates are influenced by pressure [5,6,18–20], the size of the aluminum particles and their percentage [18,21–23], the size of AP particles [5,18, 23], binder properties [21], distance from the burning surface [24], burning atmosphere [25] and so on. At present, however, there are still some unknowns about the CCPs collection method. Study on the contrast effect of liquid collection and gas collection method is missing. Secondly, a large number of literatures investigated propellants based on isoprene rubber or HTPB as a fuel binder. Few work devoted to systematic studies on the condensed combustion products and agglomeration behaviors of NEPE propellants which are also widely used in industry nowadays. It is necessary to get experimental information about the condensed combustion products of NEPE propellant, because the binder properties were suggested to affect the agglomeration process [5]. Third, the effect of virgin aluminum size on CCPs is still unclear. Sambamurthi obtained the results that the agglomerate diameter D49 was found to increase first and then decrease with increasing virgin aluminum size [7]. However, Liu found that an increase in the virgin aluminum size lead to a decrease in the mean agglomerate size [18]. In this paper, a constant-pressure quench technique is used to collect the condensed combustion products of a series of NEPE propellants, followed by the analysis of the collections. Particle size distribution, morphology and compositions of the condensed combustion products are obtained. Effects of freezing medium, quench
distance, chamber pressure and virgin aluminum size on the agglomeration properties are discussed in detail.
75 76 77 78
2. Experimental
79 80
2.1. Propellant
81 82
Experiments were conducted with NEPE Propellants. The main ingredients used in the present study include aluminum, AP, RDX, and GAP binder, as given in Table 1. The propellant was tailored to 26 mm diameter, 23 mm long cylindrical strand, as shown in Fig. 1(c). The propellant strand was an end-burning grain with the flank and one of the end faces completely inhibited. The weight of strand was about 23 g.
83 84 85 86 87 88 89 90
2.2. Experimental method
91 92
The constant-pressure quench vessel is used to collect the condensed combustion products. The quench vessel is schematically shown in Fig. 1(a). It is a thick-walled cylindrical steel chamber of 1180 mm length and 235 mm internal diameter. The top and bottom ends of the chamber are closed with flanges. The top flange is welded with propellant specimen holder (combustion chamber). The combustion chamber and the top flange are equipped with two pressure transducers, one igniter and two solenoid valves for atmosphere gas inlet and combustion gas exhaust. Pressure chamber is filled with quench liquid (water) or cooling gases (nitrogen and argon). An orifice plate (Fig. 1(b)) made of steel is installed at the bottom of the quench vessel. The exhaust gas/water flow during propellant burning can be varied by modifying the orifice size. The pressure can be kept constant by choosing proper orifice plate. Black powder (3 g) is employed to ignite the propellant grain. During the test, firstly, the initial pressure in the apparatus is set up by filling high-pressure nitrogen or argon. The orifice diameter is determined by thermodynamic calculation. The orifice diameter calculation steps are as follows. Propellant mass flow:
qp =
π 4
qg
ρg
(1)
95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 114 115
=
π
(2)
ρg
4
117 118 120 121
·D
116
119
qp
Where Q g is the propellant gas volumetric flow; q g is the propellant gas mass flow; ρ g is the propellant gas density. Volume flow formula of orifice plate:
Q o = Cd ·
94
113
· ρ p · D 2p · r
Where q p is the propellant mass flow; ρ p is the propellant density; D p is the burning surface diameter; r is the static burning rate. Propellant gas volumetric flow:
Qg =
93
2
122 123 124 125
2 p
ρm
126
(3)
Where Q o is the volume flow of orifice plate; C d is the orifice plate coefficient; D is the orifice plate diameter; p is the differential pressure between combustion chamber pressure and atmospheric pressure; ρm is the density of discharge media.
127 128 129 130 131 132
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.3 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
3
1
67
2
68
3
69
4
70
5
71
6
72
7
73
8
74
9
75
10
76
11
77
12
78
13
79
14
80
15
81
16
82
17
83
18
84
19
85
20
86
21
87 88
22 23
89
Fig. 1. Constant pressure CCPs collection system. (a) Quench vessel, (b) Orifice plate, (c) Propellant, (d) Pressure curve.
90
24 25 26 27
Test No.
Chamber pressure/ MPa
T1 T2 T3 T4 T5 T6 T7 T8 T9
7 7 7 7 7 5 9 7 7
28 29 30 31 32 33 34 35 36
91
Table 2 The scheme of tests.
92
Propellant
N2
N1 N3
Aluminum size/ μm
Burning rate/ mm/s
Freezing substance
Quench distance/ mm
Orifice diameter/ mm
93
29 29 29 29 29 29 29 13 40
5.6 5.6 6.1 5.7 5.5 4.7 6.5 5.6 6.8
N2 Ar H2 O H2 O H2 O H2 O H2 O H2 O H2 O
– – 210 420 740 210 210 210 210
2.55 2.55 2.55 2.55 2.55 3.01 2.45 2.52 2.64
95
39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
96 97 98 99 100 101 102 103
37 38
94
Relationship between volumetric flow of gas and volume flow of discharged media:
Q g = Qo
(4)
Combined with the above equations, the orifice diameter D could be obtained. In addition, reasonable trigger time of pneumatic control valve, ignition and drain valve is set to achieve proper experiment result. Pressure is required to maintain constant by exhaust liquid or gas when propellant burns at the same time. Pneumatic control valve is closed after the propellant combustion. After about half an hour, electromagnetic valve is opened to exhaust gas. The pressure within the container will decrease to atmospheric pressure. Then, the slurry (condensed combustion products) along with the quench liquid is transferred to barrels. The typical pressure curve in the experiment is shown in Fig. 1(d). It could be seen that the pressure holding device works normally, the parameter setting is reasonable, the pressure is stable during working time (Pressure fluctuation is less than 5%). The suspensions need to be precipitated for 48 hours. The bottom portion of mixed liquid of 500 ml in the barrel is collected to beakers. The collections are washed by ethanol for several times to remove impurities. Then the solids which are contained by the collections are separated by a centrifuge. The solids are dried at 70◦ C under vacuum for 24 hours before analysis. Each operating condition is tested at least twice to check the reproducibility [15]. The repeatability is within 5% based on the mean-mass diameter D 43 of the CCPs size distributions obtained in different runs.
Four operational parameters that affect the agglomeration properties of the propellants are involved in our experiments. They are freezing medium, quench distance, chamber pressure and virgin aluminum size, individually. The scheme of experimental test is given in Table 2. The uncertainty of measured burning rate is estimated to be 0.2 mm/s.
104 105 106 107 108 109 110
2.3. Product analysis
111 112
A scanning electron microscope (Zeiss Supra 55) is used to observe the microstructure of CCPs. The particle size distribution in of CCPs is directly measured by laser diffraction particle size analyzer (Malvern Mastersizer 2000) using a small amount of representative sample. The sample quality is about 0.1 g for measuring the particle size distribution. The obscuration is kept between 10% and 20% in order to obtain accurate particle size distribution data. The chemical composition of the CCPs is performed by using the X-ray diffractometer (Empyrean).
113 114 115 116 117 118 119 120 121 122
3. Results and discussions
123 124
3.1. Overview of microstructures
125 126
Condensed combustion products are characterized by scanning electron microscopy. The SEM overview image of condensed combustion products are shown in Fig. 2. CCPs are in the size range 0.3 to 600 μm and the size distribution is multimodal. The particles in the CCPs are distributed around three modes: ∼2 μm, 20 ∼ 30 μm and 300 μm. In general, it is agreed that the condensed products
127 128 129 130 131 132
JID:AESCTE
4
AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.4 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
1
67
2
68
3
69
4
70
5
71
6
72
7
73
8
74
9
75
10
76
11
77
12
78
13
79
14
80
15 16
Fig. 2. SEM image of overall condensed combustion products.
81 82
17
83
18
84
19
85
20
86
21
87
22
88
23
89
24
90
25
91
26
92
27
93
28
94
29
95
30
96
31
97
32
98
33
99
34
100 101
35 36
Fig. 3. SEM image and EDS chemical map of typical agglomerates. (a) Spherical particle with oxide cap, (b) Collision of two agglomerates.
102
37
103
38
104
39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
consist of smoke oxide particles and agglomerates [21]. The size of smoke oxide particles is in the order of 1 μm, whereas the agglomerates is usually very large, which may reach hundreds of micrometers. The trimodal particle size distribution in the current research is consistent with Ao’ work and Jeenu’ work [15,24]. Mode ∼2 μm particles are smoke oxide particles formed by the oxidation of aluminum vapor. Mode ∼30 μm particles are supposed to be the residual oxide particles, which are formed from the molten oxide caps on the aluminum particles or agglomerates at its burn out. Finally, mode ∼300 μm particles are the large agglomerates. A close view of the micromorphology of typical smoke oxide particle, residual oxide particle and agglomerate is shown in Fig. 2. The smoke oxide particle is spherical with a smooth surface, as well as the typical residual oxide particle is also smooth. Oppositely, the surface of the agglomerate is very rough. Different types of agglomerates were discovered in CCPs by Ao et al. [15], including “hollow agglomerates”, “cap agglomerates”, “solid agglomerates”, and “irregular agglomerates”. Similar structures can be found in the present NEPE propellant, as shown in Fig. 3. Fig. 3(a) is a spherical particle with oxide cap, which is described as a metal drop with an oxide cap on the surface. The electron dispersive spectroscopy results indicate the smaller part is oxide cap since the oxygen content is higher. Interestingly, the collision of two agglomerates is observed in Fig. 3(b). It may be formed by a collision of two spherical agglomerates along the reactive flow during the combustion process.
3.2. Effect of freezing medium
105 106 107
Fig. 4 shows the size distribution of CCPs in different mediums. Usually the size distribution of CCPs has a boundary D c which divides the CCPs into fine particles and agglomerates. As mentioned above, fine particles contain smoke oxide particles and residual oxide particles. Till far, there is still no rigorous definition of D c . Considering that large agglomerates are of our mainly concern, and according to Jackson’s work [5], D c is set to be 100 μm in the present work. A comparison of the fine particle distributions shows that for all the three mediums, it is possible to distinguish two modes in the intervals 1 ∼ 2 μm and 14 ∼ 30 μm. For agglomerates, there is a mode in the interval 290 ∼ 360 μm. The fraction of agglomerates in CCPs, mag /mCCPs , is proposed to evaluate the agglomeration characteristics. To simplify operation, it is assumed that CCPs are completely oxidized. This assumption is reasonable because little reactive aluminum is detected in the final products (Table 3). The mag /mCCPs are 0.0945, 0.1095, 0.1712 in N2 , Ar and H2 O, respectively. The agglomeration in H2 O is obviously stronger compared to N2 and Ar. According to literature [6], the effect of freezing medium on the particle size distribution of the CCPs is related to its heat transfer performance. The thermal conductivity of nitrogen (0.026 W/m/K) and argon (0.018 W/m/K) is much smaller than that of water (0.599 W/m/K) at 300 K and atmospheric pressure, which means that the cooling effect of the two gases is much lower than that of water. The combustion of aluminum particles depends on the combustion environment when
108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.5 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
5
1
67
2
68
3
69
4
70
5
71
6
72
7
73
8
74
9
75
10
76
11
77
12
78
13
79
14
80
15
81
16
82
17
83
18
84
19
85
20
86
21
87
22
88
23
89
24
90
25
91
26
92 93
27 28
94
Fig. 4. The particle size of different freezing mediums.
29
95
30
96
31
97
Table 3 Phase composition of CCPs sampled in different freezing mediums.
32 33
Freezing medium
34 35
N2 Ar H2 O
36 37 38
98
Content of the crystal phase, wt.%
Combustion efficiency
γ -Al2 O3
θ -Al2 O3
AlN
Al
41 46 47
52 41 45
7 5 5
– 8 3
η
43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
102 103 104 105 106
40 42
100 101
100 % 86.1 % 94.1 %
39 41
99
aluminum particles burn. In high temperature environment, the combustion of aluminum particles would be sufficient, resulting in a small amount of agglomerates. Therefore, the cooling effect of nitrogen and argon is lower than water, leading to a higher combustion environment temperature, and ultimately yielding fewer ag agglomerates. Besides, the mean diameter of agglomerates D 43 for N2 , Ar and H2 O are 276 μm, 293 μm, 274 μm, while the mean difi D 43
ameter of SOPs are 15 μm, 20 μm, 19 μm, respectively. These results mean the mean diameter of agglomerates and SOPs are insensitive with freezing medium. Interestingly, it may be concluded that the freezing medium only affects the agglomeration ratio, but has little impacts on the particle size. X-ray diffraction patterns of CCPs with different freezing mediums are shown in Fig. 5. γ -Al2 O3 , θ -Al2 O3 , AlN and Al are found in the products. During the oxidation of aluminum, the original oxide transforms into γ -Al2 O3 at 550◦ C [26]. Further temperature increase results in continuous growth of the θ -Al2 O3 . The generation of AlN needs to be special attention. It is supposed that AlN is from the reaction of Al and N2 . N2 can react with aluminum when the temperature reaches 2300 K [27]. In general, propellant combustion temperature is higher than 3000 K. So the freezing gas nitrogen must react with aluminum particles to produce AlN. In Ar environment, AlN is still detected, although the content of AlN is less than that in N2 . This suggests that AlN is yielded both by the reaction between Al and nitrogen-containing substances in propellant and Al/N2 reaction. The combustion efficiency of alu-
minum in the propellant is the ratio of the amount of alumina generated by actual combustion to the amount of alumina produced by the theoretical complete combustion. The combustion efficiency of aluminum in the propellant η is approximate calculated, as listed in Table 3. When the freezing medium is nitrogen, there is no aluminum in the CCPs. It can be inferred that the nitrogen environment promotes the combustion of aluminum compared with the argon environment. In general, the mechanism of different freezing medium on the agglomeration behavior is shown in Fig. 6.
107 108 109 110 111 112 113 114 115 116 117
3.3. Effect of quench distance
118 119
Fig. 7 shows the size distribution of CCPs in different quench distances. An analysis of the fine particle distributions shows that it is possible to distinguish two modes in the intervals 2 μm and 20 ∼ 23 μm. For agglomerates, only one mode is present in the interval 290 ∼ 320 μm. The mag /mCCPs are 0.1712, 0.1657, 0.1334 for 210 mm, 420 mm and 740 mm, respectively. The agglomeration ratio decreases slightly with the increase of quench distance. In the combustion of agglomerate, a part of the oxide is formed in the gas-phase state and then diffuses into the environment where it condenses to form SOPs [28]. This means some SOPs are produced by the burning of agglomerates. Agglomerates can burn for longer periods of time due to increased quench distance, yielding more SOPs. This result is in accordance with literature [24], where the
120 121 122 123 124 125 126 127 128 129 130 131 132
JID:AESCTE
6
AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.6 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
1
67
2
68
3
69
4
70
5
71
6
72
7
73
8
74
9
75
10
76
11
77
12
78
13
79
14
80
15
81
16
82
17
83
18
84
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
85
Fig. 5. The XRD of different freezing mediums. (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)
86
Fig. 6. Scheme of the CCPs evolution in different freezing mediums.
percentage of large particles (mode 70 μm) reduces with increase in quench distance. For the quench distance of 210 mm, 420 mm ag and 740 mm, the mean diameter of agglomerates D 43 are 274 μm, ag 273 μm, 279 μm. The mean diameter of agglomerates D 43 is basically the same due to the combustion behavior of agglomerates. According to previous study [29], the combustion law for agglomerated aluminum droplets in SRM condition is D 1.0 = D 10.0 -20t. Moreover, the velocity of agglomerates about 100 μm is less than 1.0 m/s [17]. It can be calculated that the combustion distance for an agglomerate particle 600 μm is less than 30 mm, much smaller than the quench distance used in this study. In consequence, most of the agglomerates should burn out before reaching the quench distance since they are less than 600 μm in this study. As a result, quench distance has little effect on the agglomerates size since their initial values are the same. Furthermore, quench distance also has no significant effect on the mean diameter of SOPs, 19 μm, 20 μm, 20 μm for the three conditions.
87 88
X-ray diffraction patterns of CCPs with different quench distances are shown in Fig. 8. The CCPs include γ -Al2 O3 , θ -Al2 O3 , AlN and Al. When the quench distance is 210 mm, 420 mm and 740 mm, the combustion efficiency of aluminum η are 94.1%, 94.1% and 96.0%, respectively (Table 4). The results indicate that there is little difference in aluminum combustion efficiency corresponding to different quenching distances. From the above discussion, one may conclude that most particles have stopped burning before they reach the distance of 210 mm. Only a small amount of large agglomerates would continue burning after they reach 210 mm.
89 90 91 92 93 94 95 96 97 98 99
3.4. Effect of pressure Fig. 9 shows the size distribution of CCPs in different pressures. It is possible to distinguish two modes in the intervals 1 ∼ 2 μm and 14 ∼ 22 μm by analyzing the fine particle distributions. For
100 101 102 103 104 105
40
106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130 131
65 66
Fig. 7. The particle size of different quench distances.
132
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.7 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
increase in burning rate could possibly reduce the agglomerate size. As the burning rate rises, the burning gas velocity near the burning surface increases, leading to the agglomerates being taken away from the burning surface more easily. The mean diameter of SOPs decreases slightly with the increase of pressure. In contrast to the freezing medium factor, pressure does affect the size of CCPs, whereas has little effect on the agglomeration ratio. X-ray diffraction patterns of CCPs under different pressures are shown in Fig. 10. It should be noted that a certain amount of α -Al2 O3 is detected in 9 MPa. Although the γ -Al2 O3 phase was found to be completely changed into α -Al2 O3 phase up to 900◦ C during oxidation [30], only little α -Al2 O3 is present in this work. This may be resulted from the phase transformation during the rapid cooling process, which is different from the heating in oxidation. According to Table 5, when the pressure is 5 MPa, 7 MPa and 9 MPa, the combustion efficiency of aluminum η are 85.0%, 94.1% and 100%, respectively. The combustion efficiency increases with the increase of pressure since aluminum particle gets more reactive under high pressure conditions.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
7
Fig. 8. The XRD of different quench distances.
33 34
agglomerates, there is a one mode in the interval 290 ∼ 320 μm. The mag /mCCPs are 0.1658, 0.1712, 0.1667 for 5 MPa, 7 MPa and 9 MPa, respectively. This indicates that the agglomeration ratio is little affected by pressure changes. The mean diameter of agglomag erates D 43 for 5 MPa, 7 MPa and 9 Mpa are 288 μm, 274 μm, 258 μm. The agglomerate size decreases as the pressure increases. It is evident that the burning rate increases with pressure increase. As the chamber pressure is increased from 5 MPa to 9 MPa, the corresponding burning rate is from 4.7 mm/s to 6.5 mm/s. An
37 38
69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 87 88
With the increase of initial aluminum size, the mean diameter ag of agglomerates D 43 are 207 μm, 274 μm, 236 μm, respectively. It ag is shown in Fig. 11 that the mean diameter of agglomerate D 43 increases first and then decreases with the size of initial aluminum in propellant. Sambamurthi et al. [7] and Liu et al. [17] also obtained the similar trends. As the aluminum content in propellant strand is the same for various aluminum sizes, the number den-
89 90 91 92 93 94 95 96 97
Table 4 Phase composition of CCPs sampled in different quench distances. Quench distance
35 36
68
86
3.5. Effect of virgin aluminum size
31 32
67
210 mm 420 mm 740 mm
98 99
Content of the crystal phase, wt.%
Combustion efficiency
γ -Al2 O3
θ -Al2 O3
AlN
Al
47 41 40
45 52 51
5 4 7
3 3 2
η
100 101
94.1 % 94.1 % 96.0 %
102 103 104
39
105
40
106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130 131
65 66
Fig. 9. The particle size of different pressures.
132
JID:AESCTE
AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.8 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
8
cles will be small. The aluminum particles have little opportunity to concentrate before ignition in such conditions. So that large aluminum particles also form small agglomerates. Intermediatesize aluminum particles form large agglomerates because that they have a relatively large number density and they can’t be taken away easily from the burning surface. They will facilitate the agglomeration of aluminum particles. Based on the above analysis, the distribution of aluminum in the propellants microstructure and the burning gas on particles near burning surface are the main factors affecting agglomeration in different virgin aluminum sizes. The mag /mCCPs are 0.1181, 0.1712, 0.1162 with increasing initial
1 2 3 4 5 6 7 8 9 10 11
fi D 43
12
aluminum size. The mean diameter of SOPs are 17 μm, 19 μm, 17 μm, respectively. The variation of agglomerate ratio versus initial aluminum size is the same as agglomerate size, whereas the
13 14 15
fi D 43
16 17 18 19 20
Fig. 10. The XRD of different pressures.
21 22 23 24 25 26 27 28 29 30 31
sity of aluminum particles will be large if the aluminum size is small. Aluminum particles can be concentrated easily at the surface of propellant. But the small particles are reactive, they are heated easily and taken away from the surface by the burning gas before agglomeration. This is the reason that small aluminum particles form small agglomerates. When the aluminum particle size is increased sufficiently, the number density of aluminum parti-
mean diameter of SOPs presents little change. Fig. 12 shows the X-ray diffraction patterns of CCPs with different aluminum size. Unlike the case of small particle size, a small amount of α -Al2 O3 is detected for 40 μm virgin Al, which implies virgin aluminum size may change the phase transformation dynamics of the oxide. It is unexpected from Table 6 that the combustion efficiency of the 13 μm virgin Al is the lowest, 83.0%, which is over 10% less than the other two cases. It may be because that the agglomerate size is smallest for 13 μm virgin Al propellant. Agglomerates are taken away from the burning surface easily resulting in short residence time across the combustion zone. This will cause relatively inadequate combustion for particles. The mechanism of the different aluminum particle size on the agglomeration is shown in Fig. 13.
Pressure
34 35 36 37
5MPa 7MPa 9MPa
68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
Table 5 Phase composition of CCPs sampled in different pressures.
97
32 33
67
Content of the crystal phase, wt.%
Combustion efficiency
γ -Al2 O3
θ -Al2 O3
α -Al2 O3
AlN
Al
44 47 46
43 45 32
– – 14
5 5 8
8 3 –
η
98 99 100
85.0% 94.1% 100%
101 102 103
38
104
39
105
40
106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130 131
65 66
Fig. 11. The particle size of different aluminum size.
132
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.9 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Fig. 12. The XRD of different aluminum size.
21 22
3.6. Agglomeration-combustion map
23 24 25 26 27 28 29 30 31 32
The generalization of the agglomeration characteristics from the above experimental results is presented in Fig. 14, showing the weak and high agglomeration trends. Weak agglomeration trend is characterized by the location of the point in the left lower corag ner of diagram, which has small value of agglomerate size D 43 and small mass fraction of agglomerates in CCP mag /mCCPs . Accordingly, the right upper corner in diagram corresponds to the high agglomeration trend. All the four factors, freezing medium, quench distance, pressure and virgin aluminum size do have effect
9
on the agglomeration behavior during the propellant combustion, either on agglomerate size or agglomeration ratio. It is obviously from the map that the freezing medium and virgin aluminum particle size have more profound influence on the agglomeration than quench distance and pressure. Water collection has totally different agglomeration properties in comparison with gas collection, e.g. water collection belongs to high agglomeration whereas N2 and Ar collection is weak agglomeration. This helps us to know that the agglomeration results from different experiments cannot be compared identically if their collection mediums are different. The most important conclusion from the map is about the effect of virgin aluminum size. All the five conditions located at the high agglomeration side are corresponding to the virgin aluminum of 29 μm. This suggests the intermediate-size virgin aluminum is the key factor leading to undesired agglomeration, which can give a reference to the practical industrial formulation design. To reveal the relationship between combustion and agglomeration of aluminized propellants, the agglomeration-combustion map is presented in Fig. 15. It is arbitrarily prescribed that 90% is high combustion efficiency since rapid cooling is rigorous to propellant burning. The combustion efficiency is highest with operating conditions T1&T7, since nitrogen environment and high pressure can promote the combustion of aluminum particles. It should be noted that T8 has low combustion efficiency with small agglomeration ratio and agglomerate size. This suggests the residence time significantly affects the combustion of agglomerates according to previous analysis. To our special concern, T6 present poor performance with high fraction of agglomerates, large agglomerate size and low combustion efficiency. This implies low-pressure conditions are adverse to the combustion and agglomeration of aluminized propellants.
35 36
Table 6 Phase composition of CCPs sampled in different aluminum size. Aluminum size
37 38 39 40
68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99
33 34
67
13 μm 29 μm 40 μm
100 101
Content of the crystal phase, wt.%
Combustion efficiency
γ -Al2 O3
θ -Al2 O3
α -Al2 O3
AlN
Al
55 47 41
33 45 45
– – 4
3 5 7
9 3 3
η
102 103
83.0% 94.1% 94.1%
104 105 106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130 131
65 66
Fig. 13. Scheme of the agglomeration with different aluminum size.
132
JID:AESCTE
AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.10 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
10
crease of aluminum size. Spatial topology of aluminum inside the propellant is a key factor to agglomeration. 6) Freezing medium and virgin aluminum particle size have more profound influence on the agglomeration than quench distance and pressure. The low-pressure condition is adverse to the combustion and agglomeration of aluminized propellants.
1 2 3 4 5 6
Declaration of competing interest
8
None declared.
10
Acknowledgements
12
This work was funded by the “Natural Science Foundation of Shanxi Province” (Grant No. 2018JQ5112) and “Fundamental Research Funds for the Central Universities” (Grant No. 3102018ZY003).
14 15 16 17 19
References Fig. 14. The agglomeration trends of NEPE propellants.
21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
Fig. 15. The Agglomeration-Combustion map of NEPE propellants.
40
4. Conclusions
42
45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
72 74 76 78 80 81 82 83 84
18
44
71
79
13
43
70
77
11
41
69
75
9
39
68
73
7
20
67
1) The condensed combustion products are in the size range 0.3 to 600 μm and the size distribution is multimodal. The CCPs are distributed with three peaks: 1 ∼ 2 μm, 20 ∼ 30 μm, ∼300 μm. Mode ∼2 μm particles are smoke oxide particles. Mode ∼30 μm particles are residual oxide particles. Mode ∼300 μm particles are agglomerates. The smoke oxide particle and residual oxide particle are spherical with a smooth surface. However, the surface of the agglomerate is very rough. In general, the compositions of the CCPs include γ -Al2 O3 , θ -Al2 O3 , α -Al2 O3 , AlN and Al. 2) The agglomeration ratio in H2 O is stronger compared to N2 and Ar. The nitrogen environment promotes the combustion of aluminum compared with the argon environment. Freezing medium has little impacts on the mean diameter of agglomerates and SOPs. 3) The ratio of SOPs increases with the increase of quench distance. Quench distance has no significant effect on the mean diameter of agglomerates and SOPs. 4) Agglomeration ratio is insensitive with pressure. Pressure only affects the particle size that the mean diameters of agglomerates and SOPs decrease with the increase of pressure. The combustion efficiency increases as pressure increases. 5) Both the mean diameter of agglomerates and agglomeration ratio are found to increase first and then decrease with the in-
85 86
[1] A. Gany, L.H. Caveny, Agglomeration and ignition mechanism of aluminum particles in solid propellants, Symp. (Int.) Combust. 17 (1979) 1453–1461. [2] T.K. Liu, H.C. Perng, S.P. Luh, F. Liu, Aluminum agglomeration in ammonium perchlorate/cyclotrimethylene trinitramine/aluminum/hydroxyterminated polybutadiene propellant combustion, J. Propuls. Power 8 (1992) 1177–1184. [3] V.A. Babuk, V.A. Vassiliev, V.V. Sviridov, Formation of condensed combustion products at the burning surface of solid rocket propellant, in: V. Yang, T.B. Brill, W.Z. Ren (Eds.), Solid Propellant Chemistry, Combustion, and Motor Interior Ballistics, in: Progress in Astronautics and Aeronautics, vol. 185, AIAA, 2000, pp. 749–776. [4] T.L. Jackson, J. Buckmaster, F. Najjar, New aluminum agglomeration models and their use in solid-propellant-rocket simulations, J. Propuls. Power 21 (2005) 925–936. [5] 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. [6] 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. [7] J.K. Sambamurthi, E.W. Price, R.K. Sigmant, Aluminum agglomeration in solidpropellant combustion, AIAA J. 22 (1984) 1132–1138. [8] Z. Liu, S. Li, M. Liu, D. Guan, X. Sui, N. Wang, Experimental investigation of the combustion products in an aluminised solid propellant, Acta Astronaut. 133 (2017) 136–144. [9] L. Galfetti, L.T. DeLuca, F. Severini, G. Colombo, L. Meda, G. Marra, Pre and post-burning analysis of nano-aluminized solid rocket propellants, Aerosp. Sci. Technol. 11 (2007) 26–32. [10] D. Liang, R. Xiao, J. Liu, Y. Wang, Ignition and heterogeneous combustion of aluminum boride and boron–aluminum blend, Aerosp. Sci. Technol. 84 (2019) 1081–1091. [11] L.B. Li, X. Chen, O. Musa, C.S. Zhou, M. Zhu, The effect of pressure and oxygen concentration on the ignition and combustion of aluminum–magnesium fuel-rich propellant, Aerosp. Sci. Technol. 76 (2018) 394–401. [12] Y. Chen, D.R. Guildenbecher, K.N.G. Hoffmeister, M.A. Cooper, H.L. Stauffacher, M.S. Oliver, E.B. Washburn, Study of aluminum particle combustion in solid propellant plumes using digital in-line holography and imaging pyrometry, Combust. Flame 182 (2017) 225–237. [13] Alexander G. Korotkikh, Oleg G. Glotov, Vladimir A. Arkhipov, Vladimir E. Zarko, Alexander B. Kiskin, Effect of iron and boron ultrafine powders on combustion of aluminized solid propellants, Combust. Flame 178 (2017) 195–204. [14] Joseph Kalman, Andrew R. Demko, Bino Varghese, Katarzyna E. Matusik, Alan L. Kastengren, Synchrotron-based measurement of aluminum agglomerates at motor conditions, Combust. Flame 196 (2018) 144–146. [15] W. Ao, P. Liu, W. Yang, Agglomerates, smoke oxide particles, and carbon inclusions in condensed combustion products of an aluminized GAP-based propellant, Acta Astronaut. 129 (2016) 147–153. [16] W. Ao, X. Liu, H. Rezaiguia, H. Liu, Z. Wang, P. Liu, Aluminum agglomeration involving the second mergence of agglomerates on the solid propellants burning surface: experiments and modeling, Acta Astronaut. 136 (2017) 219–229. [17] X. Liu, W. Ao, H. Liu, P. Liu, Aluminum agglomeration on burning surface of NEPE propellants at 3-5 MPa, Propellants Explos. Pyrotech. 42 (2017) 260–268. [18] T.K. Liu, Experimental and model study of agglomeration of burning aluminized propellants, J. Propuls. Power 21 (2005) 797–806. [19] F. Maggi, A. Bandera, L.T. DeLuca, Approaching solid propellant heterogeneity for agglomerate size prediction, in: 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA Paper No. 2010-6751, Nashville, TN, USA, 2010.
87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
JID:AESCTE AID:105595 /FLA
[m5G; v1.261; Prn:29/11/2019; 12:50] P.11 (1-11)
H. Liu et al. / Aerospace Science and Technology ••• (••••) ••••••
1 2 3 4 5 6 7 8 9 10 11 12 13
[20] K. Jayaraman, S.R. Chakravarthy, R. Sarathi, Quench collection of nanoaluminium agglomerates from combustion of sandwiches and propellants, Proc. Combust. Inst. 33 (2011) 1941–1947. [21] 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. [22] J.C. Mullen, M.Q. Brewster, Reduced agglomeration of aluminum in widedistribution composite propellants, J. Propuls. Power 27 (2011) 650–661. [23] 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. [24] R. Jeenu, K. Pinumalla, D. Deepak, Size distribution of particles in combustion products of aluminized composite propellant, J. Propuls. Power 26 (2010) 715–723.
11
[25] O.G. Glotov, Condensed combustion products of aluminized propellants. III. Effect of an inert gaseous combustion environment, Combust. Explos. Shock Waves 38 (2002) 92–100. [26] M.A. Trunov, M. Schoenitz, X. Zhu, E.L. Dreizin, Effect of polymorphic phase transformations in Al2 O3 film on oxidation kinetics of aluminum powders, Combust. Flame 140 (2005) 310–318. [27] A. Mukasyan, Combustion synthesis of nitrides: mechanistic studies, Proc. Combust. Inst. 30 (2005) 2529–2535. [28] V.A. Babuk, V.A. Vasilyev, Model of aluminum agglomerate evolution in combustion products of solid rocket propellant, J. Propuls. Power 18 (2002) 814–823. [29] 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. [30] S. Hasani, M. Panjepour, M. Shamanian, The oxidation mechanism of pure aluminum powder particles, Oxid. Met. 78 (2012) 179–195.
67 68 69 70 71 72 73 74 75 76 77 78 79
14
80
15
81
16
82
17
83
18
84
19
85
20
86
21
87
22
88
23
89
24
90
25
91
26
92
27
93
28
94
29
95
30
96
31
97
32
98
33
99
34
100
35
101
36
102
37
103
38
104
39
105
40
106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130
65
131
66
132