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

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)% H2 + 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 O2 and Xe in the front compared to the equilibrium behind the wave and the subsequent significant acceleration of the chemical interaction of O2 and H2 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% H2 + 5% O2 + 85% He. In addition, the results of the performed numerical simulation with mixtures (75 – q)% H2 + 25% air + q% Xe (q = 0.25, 0.5), taking into account the rotational relaxation of О2 and N2 for conditions close to the conditions of the aforementioned experiments, showed the possibility of a nonmonotonic shift of the detonation threshold.