Gaseous Detonations. IX. A Study of the Reaction Zone by Gas Density Measurements

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Abstract
An x‐ray absorption photometer capable of measuring gas densities with extreme rapidity and reasonable accuracy has been developed to study the reaction zone in detonation waves. At low initial gas pressures this zone is readily observable as a density peak at the wave front lasting a few μsec. In general the results agree with the hydrodynamic theory of the detonation wave. The observed densities at the end of the reaction zone (the Chapman‐Jouguet state) compare well with the results of approximate equilibrium calculations and the observed shape of the density profile qualitatively confirms kinetic predictions. However, the observed peak densities are substantially lower than expected and the initial chemical reaction rates are faster. The most plausible explanation at present seems to be a lack of equilibration between translational and internal degrees of freedom in the shocked gas, lasting long enough for a substantial progress of the chemical reactions to take place. The duration of the reaction zone in the mixtures studied is inversely proportional to the initial pressure of the detonating gas. This is true of the stoichiometric mixtures of hydrogen and oxygen which were studied most intensively. On varying the composition of these mixtures it is found that the duration is inversely proportional to the partial pressure of hydrogen and is independent of that of oxygen. An increase of the total pressure by the addition of a rare gas accelerates the reaction. Nitrogen and water vapor act as inert additives. Replacement of some hydrogen by carbon monoxide does not alter the duration of the reaction zone. On the other hand, when small quantities of hydrogen are added to stoichiometric carbon monoxide‐oxygen mixtures, the reaction times are found to vary as PH2—½. The addition of nitrogen has little effect on these mixtures but carbon dioxide proves to be a strong inhibitor. Methane reacts with oxygen at a rate which is comparable with that observed for hydrogen, while acetylene oxidation is very much faster.

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