Delineating the effects of ignition propensity and Markstein number on deflagration-to-detonation transition

Although the process through which a deflagration transitions to a detonation (DDT) has been studied for over a century, it is not well -understood how the propensity for DDT varies among different combustible mixtures. We investigate the effect of two fundamental mixture properties, ignition delay ( tau ign ) and Markstein number ( Ma ), on this stochastic process. Experiments were conducted in a smooth cylindrical tube, and the critical state (characterized by the velocity of the reacting front) at which a detonation formed from an accelerating deflagration was measured. The novelty of the study lies in the selection of mixture compositions that enabled changing only one relevant combustion property at a time and hence isolating the sensitivity to that property. Experiments with mixtures with similar deflagrative properties but different tau ign revealed that the critical state is not measurably sensitive to ignition propensity. Mixtures studied with different thermo-diffusive properties revealed that Ma has a strong effect on the critical state. Specifically, thermo-diffusively-unstable mixtures (with low Ma ) underwent DDT at significantly lower velocities. Furthermore, experiments were performed with and without a spiral to isolate the effect of flame wrinkling and turbulence. The spiral -affected flames underwent DDT at lower velocities, similar to thermo-diffusively-unstable mixtures. This observed effect of thermo-diffusive nature of the mixture and turbulence on DDT propensity is not predicted by existing models.