Explanation of Increase in Combustion Velocity of Ti + C Powder Mixture upon Dilution with Nickel Using Convective–Conductive Combustion Model

For the first time, a comparative study of the macrokinetic combustion parameters for granular and powder Ti + C and (Ti + C) + 20% Ni mixtures with variation in Ti particle sizes from 31 to 142 µm was carried out. It was found that the combustion velocity of (Ti + C) + 20% Ni powder mixture is 2–3 times higher than that of Ti + C mixture, in spite of the lower combustion temperature. The data obtained contradict theoretical concepts about the dependence of the combustion velocity on the maximum temperature, which leads to a formal negative value of the activation energy of combustion. In the convective–conductive model of combustion, these unusual results are explained by the strong effect of impurity gas release on the combustion velocity. For Ti + C and (Ti + C) + 20% Ni compositions, the conditions for heating particles of powder mixtures in the combustion wave warm-up zone were experimentally confirmed. The values of the reaction front velocity inside the granules were calculated using values of combustion velocities of samples with granules 0.6–1.7 mm in diameter for different sizes of Ti particles. They turned out to be several times higher than combustion velocities of powder mixtures with the same composition. The ratio of the values of the combustion velocity of the substance of the granules to the burning front velocity in the powder mixture can serve as a quantitative measure of the effect of the release of impurity gases on the burning velocity of powder mixtures. For both mixture compositions, the same power function ~d^–0.9 approximates dependences of the combustion velocity inside the granules on the Ti particle size, which indicates the leading role of the Ti + C reaction in the propagation of the combustion wave.