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55THERMALLY COUPLED PROCESSES IN A COMPOSITE GRANULAR MIXTURE (Ni + Al)-(Ti + C)
B. S. Seplyarskii*", R. A. Kochetkov", T. G. Lisina", and N. I. Abzalov"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10152
As is known, practical implementation of thermally coupled SHS reactions [1] encounters the problem of scaling associated with non-uniform warmup/ignition, evolution of impurity gases [2], and separation of combustion products. A best way to go about solving the problem may turn the use of granulated green compositions [3-5].
In this work, we explored coflow combustion of composite (Ni + Al)-(Ti + C) granules in which rapidly burning Ti-C granules played the role of donor while slowly burning Ni-Al granules, the role of acceptor.
Commercial powders of Ti (PTM-1 brand), carbon black (P-308), Al (ASD-4), and Ni (PNE-1) were used as reagents. Preparation of granules, experimental setup, and measurement procedure were described in detail elsewhere [3]. The content of donor Ti + C mixtures in composite granules was 46 wt %.
In the absence of gas flow, the burning velocity of composite mixtures (12.5 mm/s) is much higher than that of acceptor ones (3 mm/s). In view of this, we assumed that, within the combustion front, the Ni + Al granules behaved as an inert additive. To check the validity of this assumption, in control experiments we replaced them by the inert Al2O3 granules of the same size and weight ratio, i.e. 54% AhO3 to 46% (Ti + C). Such a mixture could not be ignited altogether, thus implying that, in the 54% (Ni + Al)-46% (Ti + C) mixture, the Ni + Al granules ignited within the combustion front. This was verified in the experiments conducted in the combustion geometry shown in Fig. 1. In this case, a column of Ti + C granules (black) was separated by a 3-mm layer of Ni + Al granules (gray). After ignition at the top, the combustion wave propagated through the Ni + Al barrier without delay but at strong deceleration accompanied by a marked decrease in visual luminosity (see Fig. 1b). The layers emittance equalized in 5 s after the end of wave propagation (Fig. 1c), this time period being shorter than the time required for sample cooling down to room temperature. It is clear that, in uniform donor-acceptor mixtures, the time required for temperature equilibration should be several times shorter and, above all, depend only on granular size.
Fig. 1. Propagation of combustion front through a column of Ti + C granules (black) with a 3-mm interlayer of Ni + Al granules (grey): (a) green structure, (b) just after the end of wave propagation, and (c) afterglow in 5 s afterwards (without gas flow).
ISHS 2019 Moscow, Russia
It is worth noting that the products formed upon combustion of granulated composite mixtures did not sinter and could be manually separated (by color) into individual grains probably due to the fact that TiC is not wetted with NiAl.
In the theory of filtration combustion [6], burning velocity Uf is represented as a linear function of gas flow rate Q through sample sectional area s as Uf = U0 + cgpgQ/scsps, where U0 is burning velocity at Q = 0, cg and cs are heat capacity of gas and of porous solid, respectively, Pg and ps are gas density and the bulk density of granulated sample. In calculations, we assumed that s = 2 cm2; ps and cs are 0.8 g/cm3 and 560 J/(kg-K) for Ti + C granules; and 1.7 g/cm3 and 590 J/(kg-K) for Ni + Al ones. Table 1 presents the measured (U) and calculated (Uf) burning velocities in constituent and composite granules for different Q.
As follows from Table 1, the measured (U) values for Ti + C granules grow faster with increasing Q than the calculated (Uf) ones. This is indicative of the involvement of reactive gas in the process of granules igniting from the surface [7]. For Ni + Al granules, the calculated values better agree with the measured ones (cf. Table 1).
Table 1. Measured (U) and calculated (Uf) burning velocities in constituent and composite granules at different Q._
Ni + Al Ti + C 54%(Ni + Al)-46%(Ti + C)a
Tad 1 = 1912 Kb Tad = 3290 K b T ad = 2518 Kb
Q, U, Uf, Q, U, Uf, Q, U, Tic TYlTYl/c Uf, mm/s
L/h mm/s mm/s L/h mm/s mm/s L/h mm/s
0 3 3 0 22 22 0 12.5 12.5
630 (Ar) 4.5 3.8 750 (Ar) 25 24 720 (Ar) 16.8 15.7
220 (N2) 3 3.3 200 (N2) 23 22.8 200 (N2) 15.5 13
700 (N2) 6.2 4.2 650 (N2) 36 24.5 720 (N2) 24 14,4
1000 (N2) 8 4.7 1050 (N2) 45 26.1 1100 (N2) 28.5 15.4
aWeight percentage. bTERMO software (www.ism.ac.ru/thermo)
In case of composite 54% (Ni + Al)-46% (Ti + C) granules in Ar, the gain in U was low even for Q = 600-700 l/h, in full agreement with theoretically predicted Uf values (see Table 1). The combustion products formed in Ar retained their shape, were brittle, and easily disintegrated into individual grains. This finding looks unexpected in view of the fact that (a) Tad for composite granules (2518 K) is markedly higher than the melting point of NiAl (1912 K) and (b) the presence of the liquid phase facilitates sample shrinkage [7, 8].
In principle, the U(Q) function for composite granules at Q <1100 l/h can be approximated by a straight line steeper than it follows from the filtration combustion theory. It could be assumed that, for high Q, heat exchange between donor and acceptor granules might take place behind the combustion wave, so that the Ni + Al granules would ignite and burn after passage of the visible combustion front. However, thorough inspection of still frames of combustion in granulated (Ni + Al)-(Ti + C) mixtures did not show any signs for formation of a double-front combustion mode at any Q.
The formation of brittle product in combustion of 54% (Ni + Al)-46% (Ti + C) mixtures in the presence of nitrogen flow can be explained by the formation of titanium nitride on the surface of Ti + C granules, thus preventing their sintering. Our XRD results confirm that the combustion product contains TiC and NiAl. The absence of nitride and carbonitride phases can be ascribed to low sensitivity of the method.
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