Reactant transport in combustion

The study of the Mo-B and Ta-C systems without direct contact of reactants shows that formation of MoB and TaC is under way both in vacuum and in argon at high temperatures close to that for the combustion front (Figures 3-5). During prolonged runs (5 min and more) some sections of the molybdenum wire react almost completely, whereas others remain unreacted. MoB is formed only on the metal surface, and TaC only on the carbon surface. The gas-phase transfer is the only mode of reactants transport in these experiments. Moreover, one can suppose that the transport is unidirectional in the Mo-B system and that a two-way process occurs in the Ta-C system. 

Thermochemistry of the Combustion Wave. In order to clarify the mechanism of the gas-phase reactants transport in the Mo-B and Ta-C systems and to obtain the data for... 

Diffusion in solids does not ensure the experimentally observed velocity of combustion wave propagation in the systems which are traditionally considered as gasless and burned in the mode of solid flames (gasless solid-state combustion). The phenomenology of indirect interactions, the thermochemistp and dynamics of the gas-phase carriers formation, as well as their participation in the reactants transport are studied in the systems Mo-B and Ta-C. The distributions of the main species in the gas phase of the combustion wave are measured in situ with the use of a dynamic mass-spectrometry (DMS) technique which allows for high temporal and spatial resolution. The detailed chemical pathways of the processes were established. It was shown that the actual mechanism of combustion in the systems under study is neither solid state nor gasless and the reactions are fiilly accomplished in a narrow front. 

Besides their catalytic role, electrodes collect (anode) or supply (cathode) the electrons involved in the electrochemical reactions, and should consist of materials of high electrical conductivity. Continuous electron supply (or removal) is necessary for the electrochemical reactions to proceed, resulting in a constant electron flow from the anode to the cathode. At the same time, the electrolyte, by transporting reactants in the form of ionic species, completes the cell circuit. The electro-combustion of hydrogen sustains a difference in the chemical potentials of the electro-active species (conducting ions) between both electrodes, which is the... 

Neglecting influences of transport phenomena on the chemical reaction rate, we may hypothesize that the distance downstream at which the flame first appears equals the product of the chemical reaction time and the mean convective velocity for the element of the combustible mixture that first reacts. The mean convective velocity for this element is roughly (uj -h u /l. The reaction time can be estimated from the reaction rate Wp (mass per unit volume per unit time produced) for the principal reactant F namely, the reaction time is roughly i/( —w ). Employing the definition of the reaction rate co given in equation (1-43), we then find that the flame development length is roughly... 

More recently, the cross-flow filtration combustion configuration (Fig. 29), commonly used for powder production, was investigated (Dandekar et al., 1990). Within the reactor, a rectangular container holds the solid reactant powder, and the gaseous reactant is transported from the surroundings to the front not only by longitudinal flow from above, but also by countercurrent flow from ahead of the front. The assumptions used in the model were as follows ... 

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