in the flame is probably a bimolecular reaction of HCI04 with radicals, not a monomolecular decomposition reaction, and the constant of monomolecular decomposition of HCIO~ (the "Levy constant") cannot be extrapolated to the region of high temperatures in the flame.

LITERATURE CITED

2. 3, 4.

5.

6.

7.

8.

G. A. Heath and G. S. Pearson, llth Symp. (International) on Combustion, The Combustion Institute (1967). G. J. Williams and R. G. Wilklns, Combust. Flame, 21, 325 (1973). J. C. Biordi, C. P. Lazzara, and J. F. Papp, Combust. Flame, 23, 73 (1974). G. A. McD. Cummings and A. R. Hall, 10th Symp. (International) on Combustion. The Combustion Institute (1975). R. M. Fristrom and A. A. Westenberg, Structure of Flame [Russian translation], Metal- lurgiya, Moscow (1969). J. O. Hirschfelder, C. Curtiss, and R. Bird, Molecular Theory of Gases and Liquids, Wiley (1964). J. Papp, C. Lazzara, and J. Biordi, Chemical Flame InhlbitlonUsing MBMS. RI 8019. US Bureau of Mines (1975). J. B. Levy, J. Phys. Chem., 66, 1092 (1962).

AGGLOMERATION OF ALUMINUM PARTICLES IN CONDITIONS OF

NONSTEADY HEATING

A. A. Razdobreev, A. I. Skorik, Yu. V. Frolov, and V. A. Ermakov

Investigation of the agglomeration of metallic particles is stimulated by the need to solve a number of problems associated with the combustion of highly metallized systems. Elucidation of the main features of this phenomenon would offer the possibility of more completely discovering the mechanism of individual stages of the processes characteristic of SVS, pyrotechnics and powder metallurgy and, on this basis, of developing methods of regula- tion and means of increasing the effectiveness of their investigation.

In [1, 2], on the basis of an analysis of experimental data on the interaction of con- tacting particles in hlgh-temperature heating, it was shown that the fusion of particles of metals such as aluminum, magnesium, and their alloys is a multistage process, and cannot be described by laws establlshed in the physics of agglomeration. The aim of the present work is the further investigation of the laws governing the coalescence of aluminum particles in conditions of nonsteady heating.

In the work, spherical aluminum particles of diameter from 200 to 480 ~m were used. The impurity content in the metal did not exceed 0.01%. In the experiments, two particles 4 were placed at the upper end of a quartz fiber 3 of diameter up to 800 ~m (Fig. i). The particles were heated by the beam from a laser based on carbon dioxide 8, focused by a lens i0 to a spot of diameter 500-1500 Bm. The radiation intensity at the point where the particle is placed may be regulated in the range from i00 to i0,000 W/cm a. An SKS-I hlgh-speed motion- picture camera i was used to record the process of coalescence. Recordings were made at speeds of up to 4500 frames/sac through the optical system of an MBS-2 microscope 2 with two- to tenfold magnification. Simultaneously with the cinerecordlng, visual observations of the process may be made through the second barrel of the microscope. These observations are particularly useful for low flux densities. The optical axis of the microscope and the cinecamera were positioned perpendicular to the vertical plane which the heated particles occupy. The optical axis of the laser was inclined at an angle of 85 ~ to this plane. This experimental geometry ensured sufficiently uniform heating of the two particles and the clne- recording of processes occurring at the point of contact of the particles in optimal fore- shortening. So that the process could be fol!owed from the very beginning of heating, the

Tomsk' Translated from Fizika Goreniya i Vzryva, Vol. 17, No. 6, pp. 63-67, November- December, 1981. Original article submitted May 26, 1980.

0010-5082/81/1706-0637507.50 �9 1982 Plenum Publishlng Corporation 637

~5

9

~7

Fig. i. Experimental apparatus.

particles were illuminated by an incandescent lamp 6. The apparatus used in the work was described in more detail in [3].

The general picture of the coalescence of aluminum particles is expediently considered for the example of the agglomeration of particles of di&meter 480 ~m, heated by a flux of intensity 1000 W/cm 2. The experimental data are presented Fig. 2 (curve 5) in the coordin- ates of time and the relative diameter of the contact region d = dc/doD where d c is the diameter of the contact region and do is the diameter of the initial particles, and in cinephotographs (Fig. 3a).

In the initial stages of heating, as in [4], increase in particle diameter, the appear- ance of strongly emitting regions on the particle surface, and change in the reflective properties of the surface are observed. At a definite time To, an initial contact region of diameter ~o is formed between the particles. The values of To and do are sufficiently well reproduced in parallel experiments~ and in the present case are 42,10 -3 and 0o12, respec- tively. Further heating leads to slow, almost linear growth of the contact region at a rate

--4 v c = 3o10 rel. units/see. After the contact region reaches a relative diameter of 0.15- 0.20, increase in its rate of growth is observed, up to a maximum Vma x = 200,10 -4 tel. units/ sec at 6 = 0.81-1.00, after which the rate decreases. The process ends with the formation of a single spherical particle of diameter close to 1.26do and its ignition. The ignition and combustion of the particle which forms are analogous to previous descriptions [4]. The stage of slow growth of the contact region is characterized by the formation of regions with increased intensity of luminescence on the surface of the particle near its ends.

Change in flux intensity and particle diameter, while not changing the character of the agglomeration process, affect its quantitative characteristics (see Figs. 2 and 4).

Increasing the power of the flux to 7800 W/cm a leads to a reduction in To, and increase in 6o, v c and Vmax. In this case D transition from v C to Vma x occurs more sharply at small z and large d (Fig. 2; Table i). Particles heated by a flux of 10,000 W/cm 2 coalesce practically in one stage, so rapidly that the particle which forms in the initial stage is ellipsoidal in form, with a vertical dimension exceeding do by a factor of 1.4-1.6. Subse- quently, it takes on spherical form. Ignition, as a rule, occurs after complete coalescence of the particles. However, at fluxes of 7800 and I0,000 W/cm a, the appearance of a flame is observed before the end of coa!escence~ at d = 0.85-0.90. With reduction in power of the flux, To rises, while do and the growth rate of the contact region decreases in all stages. The change in growth rate of the contact region in the course of heating occurs more smoothly.

Overall, in the flux range investigated~ ~o, vc, and Vma x rise with increase in flux power according to a near-linear law, while the change in To is described by the expression: To = a/q n, where n depends on the particle diameter, and varies from 0.42 for particles with d = 480 Bm to 0.58 for particles with d = 200 ~m, while a is not greatly dependent on the diameter.

Decrease in particle diameter has little influence on ~o, increases v C and Vmax, and leads to reduction in To according to a near-llnear law (Fig. 4; Table i).

In order to understand the mechanism of agglomeration, it is of great interest to know the particle temperature of different stages of coalescence. Accordingly, experiments were conducted with simultaneous cinerecording of the heated particles and measurement of the particle temperatures by means of a 50-Vm tungsterf-rhenium thermocouple. For convenience of

638

,b ~ 6/

0,.i CJ/., 0 Ioo 200 ~,msec

Fig~ 2 Fig, 3

Fig. 2. Agglomeration kinetics of aluminum particles of diam- eter 480 ~m in fluxes of 200 (7), 400 (6), i000 (5), 2000 (4), 4500 (3), 7800 (2) and 10,000 W/cm ~ (i).

Fig. 3. Cinephotographs of coalescence Of aluminum particles heated by a flux of i000 W/cm'.

working, particles in the form of cylinders of diameter 680 and length 700 ~m were used. One (or two) of the particles were cut up to half its diameter, the microthermocouple Junction was placed in the cut, and it was tightly shut. The signal from the thermocouple was re-

corded by an N-700 loop oscillograph ii. The experimental diagram was as in Fig. 1. In this case, the particles came into contact along the generatrlx of a cylinder, and recordings were made from the directions of the ends. To mark the time of onset of particle heating, use

was made of two photodiodes 9 illuminating a special incandescent lamp 4 when the photogate 7 was open. The signal from one of them switched on the timer of the SKS-1 camera while the other was connected to one of the N-700 loops. This allowed the dependences T(~) and ~(T) to be compared over time, with an accuracy of up to 5 msec.

Typical time dependences of the temperature and 6, obtained on heating the particles by a flux of 930 W/cm 2, are shown in Fig. 5. Note that the conditions of contact of the cylin- drical particles and their heat transfer with the surrounding medium differs from the condi- tions realized in the experiments with spherical particles. In connection with this, only the most general conclusions which follow from the given data and, it is suggested, do not depend on the particle shape are noted. The formation of the initial contact region occurs at a temperature of about 900~ after melting of the agglomerating particles is complete. Increase in the growth rate of the contact region is observed in the temperature range 1200- 1350~ and agglomeration of the particles is complete at 1700-1800~ The latter conclusion is confirmed by the results of measuring the surface temperature T S of the particles at the moment of coalescence and the appearance of the flames obtained using optical methods of temperature measurement. In these experiments, T S = 1740-1920~ at different points of the surface~

Increase in flux density =o 8000 W/cm 2 leads =o increase in ~he rate of ~emperature rise of the particles, hut did not have a pronounced effect on the temperature of formation, of the initial change in growth rate of the contact region, and of the end of coalescence of the particles. Thus, the results of temperature measurements show that aluminum particles

*Measurements of the surface temperature by an optical method were performed with the partici- pation of S. S. Smolyakov and V. V. Pozdeev, co-workers of the Siberian Physicotechnlcal Institute.

639

0,4

0 fO0 200 %reset

Fig. 4. Agglomeration kinetics of aluminum particles of diameter 200 (i), 370 (2), and 480 ~m (3) in a ~ flux of 1000 W/cm~.

agglomerate in the liquid state in a temperature range lower than the melting point of the oxide film. Taking this into account, the features of the agglomeration process observed in the present work may be explained on tt basis of the following concepts.

In the heating of aluminum particles, the difference in expansion coefficients of the metal and the oxide film leads to periodic rupture of the latter. After the melting point of aluminum is reached, rupture of the oxide film is accompanied by the escape of liquid metal to the particle surface, oxidation of the fine droplets in contact with the air, and "mending" of the oxide film [5]. When a sufficiently large droplet of metal reaches the particle surface close to its point of contact with the other particle, in the presence of oxidant, interaction of the droplet with the surface of the latter particle is possible. Since liquid aluminum wets the oxide [6], the primary result of this interaction will be the formation of the initial contact region. As a result of further hlgh-temperature reac- tion of liquid aluminum with its oxide, rupture of the oxide film is possible, and the formation of contact between the llquld-metal cores of the interacting particles.

Subsequently, agglomeration occurs according to the law of coalescence of liquid drops. The "absorption" of a small particle by a larger particle in experiments with particles of different diameter (Fig. 3b) may be taken, in particular, as confirmation of this. The presence of a solid oxide film on the surface of the particles of liquid metal significantly hinders their coalescence, and has the result that the time for the agglomeration of aluminum particles considerably exceeds the coalescence time of liquid drops of similar size [7]. With rise in particle temperature, the oxide film is weakened [8]; the increase in growth rate of the contact region observed in the experiments is associated with this. Increase in the temperature at which the particles are heated leads to reduction in the time required to reach the temperatures at which formation the contact region occurs, increases the mechanical stress in the oxide film, and intensifies its rupture. The result of this is the above-noted reduction in To and increase in 6o ~nd v with rise in heat-flux intensity and decrease in size of the interacting particles.

For direct confirmation of the role of the oxide film in the agglomeration of aluminum, experiments were performed with particles preliminarily heated a= 600~ in an atmosphere of air for 50 h. According to [9-11], the thickness of the oxide film after such treatment may reach 0.2 Bm. As is seen from the results in Fig. 6 (curves 3 and 4), increase in thickness of the oxide film slgnlflcantly reduced the rate of growth of the contact region. Also in Fig. 3, data are given on the agglomeration of the initial and oxide-free particles in an atmosphere ofhellum, i.e., in conditions elimlna=ing the possibility of =he formation and growth of an oxide film in the course of the experiment. Comparison of curves 1 and 2 with curves 3 and 4 shows that the growth rate of the connecting region is considerably higher in these conditions than in an oxidizing atmosphere.

Thus, the present work has investigated the agglomeration of aluminum particles in condl- tions of nonsteady heating. The influence of the particle size, the thickness of the oxide film, the oxidation conditions in the course of heating, and the intensity of the heat flux on the kinetics of the process has been established. The temperature of the interacting particles at different stages of the process has been measured. The results obtained have been discussed, on the basis of the hypothesis that agglomeration of the aluminum particles

640

2000 V T,~

I

IO00

Y

6

~ 0,6

X--X-XAg(

d f,2

0,8

0,4

0 I00 200 % msec o

I f J2 'l

1OO 200 % msec

Fig. 5 Fig. 6

Fig. 5. Changes in temperature (i) and relative diameter of contact region (2) in the agglomeration of aluminum particles.

Fig. 6. Agglomeration kinetics of initial (3) and oxide-free (4) aluminum particles in an atmosphere of air and helium (1 and 2, respectlvely). Theaparticle diameter is 480 ~m, and the flux density 1000 W/cm . [This caption doesn't seem to correspond to the description of the figure in the text.]

TABLE i.

% % ...o

200

260

23

OOOl 9 5ooi 6 8oo I 4

]lo ooo I 3

30 000 / 22

~000 t6 4 50O / to 7 8001 7

0,t5 0,15 0,t3 0,15 0,t8 0,20 0,34

0,tt 0.t3 032 0,t3 0,t5 0,19

i.5 218 i,9 t,4 4,5 5,9

0,9 t .0 51o

15 370

40 88 88

172

48O t8 24 3O 30

70

I 400[ 50 0,]3 0,4 000[ 6 0,6 0,i3

38 0,18 i , i 8 7 0 , 2 , .

2001 74 10,05 0,2 4001 57 0,09 0,4

i 0001 42 ! 0,t2 0,3 2 0001 34 I 0,13 0,8 4 5001 22 i 0,15 2,2 7 8001 17 020 2,6

lO 0001 13 0',2i 5,0 I

22 3i

t20 72

8 i2 20 20 24 59 77

occurs as a result of the interaction of their metallic cores, which are in the molten state. The retarding influence of the solid oxide film covering the particles before complete co- alescence is demonstrated.

LITERATURE CITED

i. V. D. Gladun, Yu. V. Frolov, a n d L. Ya. Kashporov, Fiz. Goreniya Vzryva, 13, No. 5, 705 (1977) .

2. A. A. Razdobreev, A. I. Skorlk, and Yu. V. Frolov, in: Materials of the Twelfth All-Unlon Conference on Problems of the Evaporation, Combustion, and Gas Dynsmics of Disperse Sys- tems. Abstracts [in Russian], Odessa (1976), p. 33.

3. A. A. Razdobreev and I. I. Bukatyi, Izv. Vyssh. Uchebn. Zaved., Fiz., No. 4, 155 (1973). 4. A. A. Razdobreev, A. I. Skorik, and Yu. V. Frolov, Fiz. Goreniya Vzryva, 12, No. 2, 203

( 1 9 7 6 ) . 5. P. Kofstad, High-Temperature Oxidation of Metals [Russian translation], Mir, Moscow

(1969). 6. G. V. Samsonov (ed.), Physicochemical Properties of Oxides (Handbook) [in Russian],

Metallurgiya, Moscow (1978). 7. W. D. Kingeri, J. Appl. Phys., 26, 1205 (1955). 8. P. Harper, C. Shepard, and A. Dorn, Acta Met., 6, 509 (1958) 9. Zh. Benar (ed.), Oxidation of Metals (Handbook)--[In Russian] i Vol. 2, Metallurgiya,

Moscow (1969). 10. N. Pilling and J. Bedworth, J. Inst. Met., 29, 429 (1923). 11. J. Herenguel and J. Boghen, Rev. Met., 51, 265 (1954).

641