Flame acceleration and detonation initiation around a T-shaped bifurcation

We experimentally studied the flame behavior of H2-O2-N2 premixed gas around a T-shaped bifurcation in a piping system with an inner diameter of 20 mm. In the experiments, we measured the flame speed as a function of the path length from the igniting spark plug using ion probes and observed self-emission images using a highspeed camera. The speed and disturbance of the flame entering the bifurcation were varied by installing various numbers of obstacle rods into the pipe just downstream of the igniting spark plug. We compared the effects of the bifurcation on the flame propagation between two cases: the flame propagating straight through the T-shaped bifurcation and the flame turning 90 degrees at the T-shaped bifurcation. In both cases, the flame was accelerated around the T-shaped bifurcation. Especially, in the case of the flame turning 90 degrees at the T-shaped bifurcation, the flame acceleration was remarkable and, in some cases, a detonation was initiated immediately downstream of the T-shaped bifurcation. The observed remarkable flame acceleration is considered to be ascribed to the recirculation region formed by the separation of the unburned-gas flow downstream of the T-shaped bifurcation, and governed by the ratio between the ignition delay time of the mixture of the burned and unburned gases and the sound-crossing time for the pipe diameter which is the only length scale of a long pipe.