Detonation onset mechanisms and fundamental combustion properties in unobstructed channels

We examine the relationship between fundamental combustion properties (FCP) and detonation onset (DO) mechanisms in a square, unobstructed channel. Hydrogen and hydrogen/methane mixtures in oxygen, with and without nitrogen, helium, or argon dilution are tested. Two DO mechanisms were identified: (i) short, symmetric flames with a single normal precursor wave forming far ahead of the main flame leading to DO after reflected/transmitted shock collisions, and (ii) long, corner-inclined flames with multiple non-normal precursor waves resulting in DO from collisions between supersonic combustion structures propagating close to walls/corners and reflected shocks. Results show that the DO mechanisms identified are successfully grouped using the FCP boundaries used to classify flame morphologies during flame acceleration (FA) highlighting a direct link between flame morphology during FA and DO mechanisms. Distributions of DO on the channel cross-section are also provided.