Effects of additives on marginal detonation in gases

Abstract

Detonation has been studied under marginal conditions in various mixtures of hydrogen with oxygen, with additions of argon, nitrogen or organic molecules as a third component. Propagation was studied in mixtures in a long cylindrical tube exposed to the initiating impact from a detonating mixture of 2H 2 + O 2 . Time intervals for the passage of detonation past successive probes were recorded to 0.01 /us accuracy. At 1 atm pressure and 20 °C, binary mixtures of hydrogen with oxygen showed limits of detonation at 15.3 % H 2 and 91.0% H 2 . These limits were narrowed by additions of argon, nitrogen or iodomethane. Additions of furan or of water vapour had complex effects. Benzene had little effect. From the results, the marginal conditions that determine whether detonation will be propagated in a mixture or whether it will decay into an explosion wave, can be correlated with thermodynamic and kinetic characteristics of the reactions whose energy liberation sustains the detonation wave. In binary mixtures of oxygen with hydrogen, it appears that the rate of liberation of energy becomes inadequate to sustain detonation, when the nominal temperature and pressure behind the shock front fall below critical values. Much of the effect of the additives studied can be attributed to their thermal properties, which modify the peak temperature and pressure and thereby influence the chemical reactions that liberate energy. There is evidence that internal degrees of freedom of additives such as nitrogen do not participate appreciably in energy exchange immediately behind the shock front. Specific chemical intervention of the additives in these reactions cannot be excluded on the basis of present evidence, but appears to be subsidiary to thermal effects inhibiting the propagation of detonation.