Local quenching phenomena of lean turbulent premixed flame formed in a wall stagnation flow

Lewis number effect on the local quenching phenomena of lean methane/air and propane/air turbulent premixed flames formed in a wall stagnation point flow have been studied experimentally. The stretch rate due to flow divergence was 60 s-1, while the turbulent intensity of velocity fluctuation in the approach flow was 0.38 m/s. The flame front behavior was taken by using laser tomographic technique with a 16 mm high speed camera. Near the extinction limit, the flame front reaches the cooled wall. Then, the flame is locally quenched, though it is globally stable. The local quenching occurs when the concave curvature to the unburnt gas is close to the cooled wall. The local quenching is sensitive to the Lewis number in that its probability of lean methane/air flame (Le>1) is more than ten times that of propane/air flame (Le>1). This suggests that the interaction of the cooled wall and flame front plays an important role in the local quenching of lean methane/air flame. When the cusp flame front reaches the cooled wall, the duration of local quenching time is short. In this case, the local quenching area may be small, and the global extinction does not occur. On the other hand, when the small curvature flame front reaches the cooled wall, the time is long. Then, the local quenching area may be large, and the local quenching become a trigger to develop the global extinction.