Convective and conductive combustion modes for granular $\rm Ti$–$\rm C$–$\rm B$ mixtures. Determination of the heat transfer coefficient between filtering gas and granules

Combustion regularities of the bulk density $(100 - x)(\rm Ti + \rm C) - x(\rm Ti + 2\rm B)$ system in powder and granular forms are analyzed. It is shown that the powder mixture combustion velocity as a function of the $\rm Ti + 2\rm B$ content has a nonmonotonic pattern, which is due to the effect the impurity gas release has on the combustion process. For a granular mixture, a monotonic dependence having two characteristic sections is obtained. The fact that a growth in the combustion velocity is observed, when the $\rm Ti + 2\rm B$ content is more than $60$ wt $\%$, made it possible to draw a conclusion that the conductive combustion mode changes for the convective one. For the convective combustion mode, it is shown that a decrease in the content of a gasifying addition in the mixture (granulation with ethanol) leads to an unexpected result: the system combustion velocity increases for compositions with $x = 90$ and $100$ wt $\%$. For compositions with the $\rm Ti + 2\rm B$ content more than $60$ wt $\%$, the combustion velocity under the conditions, when impurity gas is removed through the sample lateral surface, is determined for the first time. This made it possible to exclude the influence of convective heat transfer and determine the substance combustion velocity inside the granules. It is shown, based on the totality of available experimental data and the obtained calculation results, that the critical conditions, under which a transition takes place to the convective combustion mode for granules $1.7$ mm in diameter, correspond to the system with $x = 60$ wt $\%$, and those for granules $0.6$ mm in diameter, with $x = 80$ wt $\%$ It has been found that there is a significant difference between the predicted and experimentally obtained coefficients of heat transfer between gas glow and granules in the convective combustion mode. The main factor behind this difference is the chemically active medium of the compositions studied.