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21XV International Symposium on Self-Propagating High-Temperature Synthesis
EFFECT OF THERMOCAPILLARY FLOW OF MELT ON COMBUSTION OF A BINARY GASLESS MIXTURE
V. G. Prokofev*"A, A. I. Kirdyashkin", V. D. Kitler", and O. V. Lapshin"
aTomsk Scientific Centre, Tomsk, 634021 Russia bTomsk State University, Tomsk, 634050 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10135
The combustion of gasless powder mixtures is accompanied by the melting and convective flow of one or several components of a heterogeneous system in the matrix of refractory components and reaction products. The capillary flow of the melt is due to the action of surface tension forces in a porous medium. The first time, an experimental study of the capillary flow of a liquid metal and its effect on the combustion of gas-free systems was considered in [1] using the example of a powder mixture of titanium with carbon. In [2], approximate analytical estimates were obtained of the effect of capillary spreading on the propagation of a combustion wave in gas-free systems within the framework of the model of reactionary cells. The effect of the dispersion of reagents on the transition of the combustion regime from diffusion to capillary was considered in [3, 4]. In [5], the "anomalous" dependences of the burning rate on the size of titanium particles were obtained for powder systems Ti + Si and Ti + Fe, indicating that along with diffusion and capillary mass exchange, there is a convective mixing mechanism of the system components. An experimental study of powder mixtures by methods of high-speed video and dynamic pyrometry in [6] showed an "anomalous" increase in the burning rate with increasing size of aluminum particles. The authors explained this effect by the transition from the diffusion to hydrodynamic combustion regime with the development of convective aluminum melt flows. The effect of Marangoni capillary convection on the combustion of oxide systems and the separation of the metal and oxide phases was considered in [7]. An analytical and numerical study of the effect of thermocapillary convection on heat transfer in a gasless combustion wave with a meltable reagent for the inertialess flow of the liquid phase is considered in [8].
In this paper, we consider a model of combustion of a binary mixture, one of the components which is a low-melting metal. The synthesis reaction scheme can be represented as (1 - v)A(solid) + vB(solid, liquid) = P (solid), where v is the mass (stoichiometric) concentration B in the reaction product P.
The main goal of solving the problem was to calculate the burning rate of the A + B binary mixture (Fig. 1), as the main integral characteristic of high-temperature synthesis. Plots of U(a) are non-monotonic. The maximum burning rate shifts towards mixtures with excess B as a result of the influence of thermocapillary convection. The burning rate of the stoichiometric mixture (a = 1) is lower than for a non-stoichiometric mixture with an excess melting component a =1.1 (Fig. 1, curves 2, 3). The effect appears at a relatively high rate of thermocapillary melt flow ro. However, the calculated burning temperature of the stoichiometric mixture was always higher than the burning temperature of the mixture of non-stoichiometric composition and practically did not depend on the rate of convective flow. Similar "anomalous" dependences of the burning rate on the size of aluminum and titanium particles were obtained experimentally for Ni + 31.5% Al and (FeO + 20% Al) + mAhO3 systems in [6] and the Ti + 37% Si mixture in [5]. The maximum value of concentration B in the heating zone was about 70% higher than the initial concentration of this component. Then component B is consumed in a chemical reaction and its concentration drops.
ISHS 2019 Moscow, Russia
Fig. 1. The dependence of the burning rate on the coefficient of excess a of component B and on the scale of the flow velocity ro: ro = 0, 2 ro = 0.6, 3 ro = 0.8.
Thus, it is shown that it is possible in principle to burn mixtures with an excess of the low-
melting component at a higher rate than when burning a mixture of stoichiometric composition.
The research was supported by the Russian Foundation for Basic Research, (project
no. 19-03-00081).
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