Flame acceleration and detonation onset in narrow channels: simultaneous schlieren visualization

Deflagration-to-detonation-transition (DDT) experiments were performed in a fully optically accessible smooth narrow channel using simultaneous schlieren visualization. Stoichiometric H2-O2 mixtures with various N 2 dilution levels were used to assess the effect of expansion ratio on flame acceleration (FA) and detonation onset (DO). Results show that the flame topologies and flame-shock complexes that form prior to transition differ and the corresponding DO modes become less random as dilution is increased. The widely accepted symmetric assumptions during the very early stages of flame propagation are verified whereas complex asymmetric structures are unraveled and described during FA and DO. Using the information collected from the additional visualization direction, the role of asymmetries during FA are highlighted, and maps of the spatial likelihood of detonation onset on the channel's cross section are built. Conclusive experimental evidence of the role played by walls and corners in the formation of ignition centers/hot spots on DDT is provided. The value of simultaneous schlieren visualization in providing unique insight into the reacting front and wave topologies present during the entire DDT evolution is, for the first time, demonstrated