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The early stages of flames propagation in tubes with no-slip walls and theinversion of the flame front from a convex shape directed towards unburned gasto a concave shape with a cusp directed to the burned gas, known as a tulipflame, was investigated for closed and half-open tubes by solving the fullycompressible reactive Navier-Stokes equations with a one-step Arrheniuschemical model for the highly reactive hydrogen/air and slowly reactingmethane/air mixtures. The development of the tulip flame in hydrogen/airobtained in simulations with a one-step Arrhenius model was compared withsimulations using a detailed chemical model. It is shown that the inversion ofthe flame front and the onset of the tulip flame occurs due to rarefactionwaves generated by the decelerating flame when its surface was reduced due toextinguishing of the rear parts of the flame skirt at the sidewalls. Therarefaction waves reduce the flow velocity ahead of the flame, creating anuneven velocity profile, which facilitate the flame front inversion. In thecase of a flame propagating in a tube with both ends closed, the compressionwave reflected from the tube end opposite to the ignition end, also contributeto the inversion of the flame front. In the case of fast flames and arelatively short tube with both closed ends, the rarefaction waves may invertthe flow velocities in the unburned gas towards the propagating flame. Thetulip flame formation mechanism is a purely gas-dynamic phenomenon notassociated with flame instabilities or vortex motion, in agreement with theconclusion obtained in recent experimental studies [1].

Early Stages of Flame Propagation in Tubes with No-slip Walls and the Mechanism of Tulip Flame Formation