The effect of argon dilution on the stability of acetylene/oxygen detonations

Experimental observations indicate that dilutions with large amounts of argon lead to stable detonations having a regular cellular structure with only weak transverse waves. In the present study, the stabilizing effect of argon dilution in acetylene/oxygen detonations is investigated numerically by detailed numerical simulations of one-dimensional time-dependent pulsating detonations with a realistic seven-step chemistry model. The results show that heavy argon dilution in the mixture leads to single-frequency small-amplitude regular oscillations of the shock front pressure. As the dilution is decreased, the detonations become unstable, characterized by larger amplitude oscillations. The stabilizing role of argon is further investigated by analyzing the reaction zone structure of the steady Zeldovich-Von Neumann-Doring detonation with varying degrees of argon dilution. For the same characteristic induction lengths, the dilution with argon leads to lower temperatures in the reaction zone and slower exothermic reaction rates, thus rendering the reaction zone structure less temperature sensitive and more stable to hydrodynamic fluctuations. The present unsteady numerical simulations also indicate that with argon dilution less than approximately 70%, a one-dimensional time-dependent detonation cannot self-propagate. Below this limit, due to the low-velocity excursions of the pulsating leading shock, the reactions are quenched and detonation failure occurs. This fundamental limit reveals that a one-dimensional shock-induced ignition mechanism in an unstable detonation is insufficient to account for the ignition and propagation mechanism in multidimensional detonations. These observations are consistent with recent experiments performed in porous wall tubes where, as the transverse waves were eliminated, the detonation could not self-sustain in the one-dimensional limit. The experimental stability limit, at which the transverse waves begin to play the dominant role, also corresponding to the loss of regularity in the cellular pattern, agrees very well with the stability limit determined numerically in the present study.