B. S. Seplyarskii

Experimental investigation of combustion of a gasless pelletized mixture of Ti + 0.5C in argon and nitrogen coflows

Combustion of a pelletized mixture of titanium and carbon black placed in a quartz tube and exposed to a flow of argon or nitrogen is studied. The gas flow (cocurrent filtration) is provided by a fixed pressure gradient at the inlet and outlet of the tube, which did not exceed 1 atm. The possible modes of combustion of pelletized mixtures related to the presence of a more complex hierarchy of scales (micro, macro, and meso) compared to that of powder mixtures (micro, macro) are analyzed. A comparison is made of the burning rates of powder and pelletized mixtures. An increase in the burning rate when using pelletized mixtures was found experimentally. It is shown that the gas coflow through the pelletized mixture of Ti + 0.5C leads to an increase in the burning rate. It is established that the propagation of the flame front of the pelletized mixture of Ti + 0.5C in flows of nitrogen and argon is controlled by different reactions. In contrast to combustion of powder mixtures of Ti + 0.5C, in combustion of pelletized mixtures of Ti + 0.5C in a nitrogen flow, only one front is observed. It is proved that radiation plays a significant role in the propagation of the combustion front in the pelletized mixture of Ti + 0.5C.

Combustion of the gasless system Ti + 0.5C in a nitrogen coflow

Combustion of mixtures of titanium with carbon black of bulk density placed in a quartz tube with nitrogen purging (concurrent filtration) is examined. The nitrogen flow is provided by a fixed pressure difference not exceeding 1 atm at the ends of the mixture. The burning rate is determined as a function of the amount of titanium nitride added to the initial mixture and of the pressure at the ends of the sample. The tests show that a nitration front is formed during combustion of a Ti + 0.5C (carbon black) powder mixture in the nitrogen flow, in addition to a carbidization front. Various modes of propagation of the carbidization and nitration fronts are found and described. A classification of combustion modes of the Ti + 0.5C (carbon black) powder mixture in the nitrogen flow is proposed.

Combustion of bulk density powder mixtures in a coflow of inert gas: 1. The Ni-Al system

For combustion of bulk density N + Al mixtures an a coflow of argon, the burning velocity Uc was found to markedly depend on the flow rate. The effect (unexplainable in terms of classical combustion theory) has been rationalized in terms of the convective-conductive theory. Preliminary thermal treatment of green mixtures in vacuum was found to increase the initial value of Uc and to decrease the effect of pressure difference ΔP on the extent of flow-induced increase in the burning velocity.

Influence of granulation on combustion of 2Ti + C mixtures

65 Previously, SHS reactions in 2Ti + C and Ti + C bulk density mixtures in a coflow of inert gas have been studied in [1]. Variation in pressure difference afforded to affect burning velocity and hence product structure. These results also confirmed the predictions of con vective–conductive combustion theory as for abnor mal dependence of burning velocity on sample diam eter [2]. But a main difficulty of the above experiments was low gas permeability of powder blends, especially finely dispersed ones.

Effect of added reactive agents on the flame propagation velocity in rich hydrogen-air mixtures

An approximate analytical method for evaluating the efficiency of the action of an inhibitor on the velocity and propagation limits of a flame in rich hydrogen-air mixtures with small amounts of added propylene and isobutylene inhibitors is proposed. The method is based on the model of a narrow reaction zone and the distinct features of the branching chain mechanism of hydrogen oxidation reactions. Using this method, it is shown that the occurrence of flame propagation limits at higher concentrations of added reactive agents (inhibitors) is caused by the existence of a positive feedback between the flame front velocity and the number of active combustion sites, which break down in the inhibitor-added reaction. According to this feedback, the action of the inhibitor decreases the flame temperature and flame velocity.

Dependence of burning rate on sample size in the Ni + Al system

Burning rates were measured for samples of the starting mixture of Ni + Al and the same mixture subjected to mechanical treatment in argon and then in water. The dependences of the burning rate on the diameter of the samples of the original and mechanically activated mixtures are similar. The burning rate passes through a maximum as the diameter increases from 8 to 12 mm. It is found that the burning rate of 270–360 µm thick films obtained by rolling the starting mixture of Ni + Al and the mixture mechanically activated and then activated in water (dispersed) is 4–20 times the burning rate of cylindrical samples 8–12 mm in diameter, pressed from the same powders. The data obtained in this study were explained using a convective-conductive model of combustion-wave propagation.

Effect of batch pelletizing on a course of SHS reactions: An overview

Such a simple technological trick as pelletizing (granulating) green powder mixtures (popular in conventional powder technology) has seldom been used in SHS technology. We have demonstrated that pelletizing green powder mixtures can markedly affect the parameters of various SHS reactions (SHS in a mode of infiltration-mediated combustion, SHS with a reduction stage, combustion of thermit mixtures, SHS in aluminum melt) and properties of their products.

Combustion of bulk density powder mixtures in a coflow of inert gas: 4. Ti-Si and Zi-Al systems

Combustion of bulk density Ti-Si and Zi-Al mixtures an a coflow of argon was studied in the presence/absence of (1) applied pressure difference δP along the direction of wave propagation, (2) thermal treatment of green samples in vacuum, and (3) blowing agent (borax). The data on variation of the burning velocity Uc were rationalized in terms of the convection-conduction combustion theory for heterogeneous condensed systems.

Combustion of Cr2O3 + Al powder mixtures in a coflow of inert gas: 5. Effect of green density

Combustion of Cr2O3 + Al powder mixtures in a coflow of inert gas (Ar) was investigated upon variation in green density in the presence/absence of blowing agents (borax, baking soda). The results were rationalized in terms of the convection-conduction theory for combustion in heterogeneous condensed systems.

Combustion of cylindrical Ti + 0.5C compacts: Influence of mechanical activation, thermovacuum degassing, and ambient pressure

We explored the influence of mechanical activation (MA), thermovacuum degassing (TVD), and ambient pressure on burning velocity and sample shrinkage/elongation for SHS reactions in cylindrical Ti + 0.5C powder compacts. Thermovacuum degassing of Ti + 0.5C mixtures was found to increase the burning velocity (2-fold) and sample shrinkage. Mechanical alloying decreased the burning velocity and gave large (3-fold) sample elongation. The results can be readily rationalized in terms of conduction-convection combustion theory.