Numerical Study of the Detonation Wave Structure in Ethylene-oxygen Mixtures

We examine a transition from a weakly to a highly unstable regime of a cellular detonation in stoichiometric ethylene-oxygen systems with varied dilution. The structure and propagation of cellular detonations is calculated using two-dimensional, time-dependent, reactive Euler fluid-dynamics algorithm. A dynamically adapting mesh is used to resolve reaction zones, shocks, contact surfaces, and vortices in flow. A simplified chemical model with Arrhenius kinetics is used for all mixtures. Effects of dilution are modeled by varying the adiabatic index γ and molecular weight of matter. Results are compared to experimental data obtained in a detonation tube by simultaneous visualization of a chemical species (OH), density gradients in the reaction zone, and to soot foil records. Due to a strong sensitivity of the post-shock temperature to variations of γ, the degree of chemical - fluid dynamics coupling inside the detonation structure varies significantly with dilution. Variations of the equation of state with dilution can account for and could be the main mechanism explaining a wide range of detonation behavior observed in the experiments