Dynamics and structure of detonations in stratified product-gas diluted mixtures

The dynamics and structure of detonations in stratified, product-gas diluted mixtures are exam-ined using high resolution two-dimensional simulations with detailed chemistry. A stoichiometric methane/oxygen/nitrogen mixture with a product-gas dilution level similar to that found in rotating deto-nation engines is considered. A parametric analysis of the stratification size is performed. For stratification sizes larger than the induction length lind, the traditional detonation cell structure is lost and triple-points are absent. For stratification sizes an order of magnitude larger than lind, large regions of unburned, cold gases are found far behind the detonation front, and are subsequently consumed through an isobaric combustion process initiated by scalar mixing. This mechanism results in a deficit of energy provided to the detonation and to a significant reduction in the overall detonation velocity. A one-dimensional model using ZND theory is constructed to gain insights on the transient detonation structure. This model predicts a slower detonation velocity through the cold gases, which is supported by the results from the simulations with large stratifica-tion sizes. In these cases, the detonation structure is well predicted by the model in the regions that undergo ignition by shock compression, i.e., for intermediate and high levels of product-gas dilution. In the absence of triple-points, the detonation structure and dynamics can be locally considered as quasi-one-dimensional. The model and simulation results have a larger discrepancy for small stratification sizes, implying that phe-nomena other than adiabatic shock compression, involving heat and/or mass transfer, might play a role to ignite the reactants.& COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.