Dependence of the Knock Threshold of an H2-Air Mixture With Small Xe Additives

Abstract

The effect of a small addition of Xe on the conditions for the occurrence of detonation in an incident shock wave (SW) with a change in its intensity is studied. The experiments are carried out in a shock tube (ST) with mixtures (75 -q)% H 2 + 25% air + q% Xe, where q = 0, 0.25, and 0.5. The addition of Xe leads to a shift in the detonation threshold to the region where the conditions are more unfavorable for its occurrence. Moreover, decreasing x from 0.5 to 0.25 causes a stronger shift in the detonation threshold; i.e., dependence x became nonmonotonic. This effect is due to a strong increase in the frequency of high-energy collisions of O 2 and Xe in the front compared to the equilibrium behind the wave and the subsequent significant acceleration of the chemical interaction of O 2 and H 2 behind the front. It is a consequence of the occurrence of a specific translational nonequilibrium in the wave front. This is indicated by the results of similar studies of the effect of replacing a small amount with an inappropriate amount of Xe on the conditions for the onset of detonation for mixtures of 10% H 2 + 5% O 2 + 85% He. In addition, the results of the performed numerical simulation with mixtures (75 -q)% H 2 + 25% air + q% Xe (q = 0.25, 0.5), taking into account the rotational relaxation of О 2 and N 2 for conditions close to the conditions of the aforementioned experiments, showed the possibility of a nonmonotonic shift of the detonation threshold.