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The early stages of hydrogen-air flame dynamics and the physical mechanism oftulip flame formation were studied using high-resolution numerical simulationsto solve the two-dimensional fully compressible Navier-Stokes equations coupledwith a one-step chemical model, which was calibrated to obtain the correct thelaminar flame velocity-pressure dependence. The formation of tulip flames wasinvestigated for a flame ignited by a spark or by a planar ignition andpropagating to the opposite closed or open end. For a flame ignited by a sparkon-axis at the closed end of the tube and propagating to the opposite closed oropen end, a tulip flame is created by a tulip-shaped axial velocity profile inthe unburned gas flow near the flame front caused by the rarefaction wave(s)created by the flame during the deceleration stage(s). It is shown that, in atube with both closed ends, this mechanism of tulip flame formation also holdsfor flames initiated by planar ignition. The deceleration stages in the case ofplanar ignition are caused by collisions of the flame front with pressure wavesreflected from the opposite end of the tube. In the case of a flame initiatedby planar ignition and propagating toward the open end, the mechanism of tulipflame formation is related to the stretching of the flame skirt edges backwardalong the side wall of the tube due to wall friction, which leads to theformation of bulges in the flame front near the tube walls. The bulges grow andfinally meet at the axis of the tube, forming a tulip-shaped flame. Regardlessof the method of flame initiation at the closed end, no distorted tulip flameis formed when the flame propagates to the open end of the tube.

On the mechanism of "tulip flame" formation: the effect of ignition sources