  The present work reports new experiments of detonation diffraction in a 2D
channel configuration in stoichiometric mixtures of ethylene, ethane, and
methane with oxygen as oxidizer. The flow field details are obtained using
high-speed schlieren near the critical conditions of diffraction. The critical
initial pressure for successful diffraction is reported for the ethylene,
ethane and methane mixtures. The flow field details revealed that the lateral
portion of the wave results in a zone of quenched ignition. The dynamics of the
laterally diffracting shock front are found in good agreement with the recent
model developed by Radulescu et al. (Physics of Fluids 2021). The model
provides noticeable improvement over the local models using Whitham's
characteristic rule and Wescott, Bdzil and Stewart's model for weakly curved
reactive shocks. These models provide a link between the critical channel
height and the critical wave curvature. The critical channel heights and global
curvatures are found in very good agreement with the critical curvatures
measured independently by Xiao and Radulescu (Combust. Flame 2020) in
quasi-steady experiments in exponential horns for three mixtures tested.
Furthermore, critical curvature data obtained by others in the literature was
found to provide a good prediction of critical diffraction in 2D. These
findings suggest that the critical diffraction of unstable detonations may be
well predicted by a model based on the maximum curvature of the detonation
front, where the latter is to be measured experimentally and account for the
role of the cellular structure in the burning mechanism. This finding provides
support to the view that models for unstable detonations at a meso-scale larger
than the cell size, i.e., hydrodynamic average models, are meaningful.
