Extending Excitation Time for Detonation Prevention: Mixture Effect in Dimethyl Ether Auto-Ignition

Abstract

Undesired detonation development is an obstacle in the development of modern combustion systems based on auto-ignition. Excitation time is one of two time scales that affect detonation development. It describes the time interval, during which heat is released. Extending excitation time decreases the propensity to detonation development by inhibiting the coupling between heat release and pressure waves emerging from reactivity gradients, which are often present in technical systems. As excitation time is mixture-dependent, mitigation of detonation development is possible through mixture tailoring. This work investigates the underlying physico-chemical processes that are responsible for the effect of dilution and equivalence ratio on excitation time. The numerical investigation is performed for dimethyl ether/air mixtures at 15 bar, which feature multistage ignition depending on initial temperature. The resulting nonmonotonous evolution of the heat release rate requires to adapt the analysis methods and utilize a novel excitation time definition. Diluted and off-stoichiometric mixtures feature longer excitation times compared to undiluted stoichiometric mixtures, which is favorable for decreasing the detonation propensity of a mixture. The results demonstrate that excitation time is mainly controlled by reactions that affect reactivity and the production of important intermediate species, which are related to the underlying heat release chemistry. Dilution impacts excitation time by thermal effects, related to the diluent's heat capacity, and chemical effects, such as scavenging of important radicals by third-body collision of the diluent. The current work illuminates which physico-chemical processes extend the excitation time when mixture composition changes, which supports future work on mixture tailoring for mitigation of detonation development.