Flame propagation along 90° bend in an open duct

Experiments of flame propagation in a small, open square duct including a 90 degrees bend were performed for a methane-air mixture using high-speed cine camera and schlieren techniques. In a slow flame propagation from open end to closed end, a blunt flame front shape is kept along the bend. However, in the case of closed- to open-end rapid propagation, from the bend the sharpened flame tip proceeds near the inner wall with succeeding flame front elongated towards the axial direction, and in serious cases transition to turbulent flame occurs. The latter propagation mode was examined in detail. The flame tip speed takes a large value in the bend. In the latter straight section, after a short deceleration period, the flame propagation speed (in axial direction) continuously increases far a while. The relation between the flame behavior and flow field ahead of the flame was investigated Further by observing time line movement in the unburned mixture. From the similarity between the flame front shape and time line shape variations, it was revealed that the flame behavior around the bend is determined primarily by the nature of unburned gas flow driven toward the open end. The observed time line movements were compared further with those obtained by numerical simulation based on two-dimensional incompressible inviscid or viscous cold flow models. It was shown that around the bend, the mixture flow pattern and also features of the flame behavior are determined primarily by potential flow characteristics. It was suggested that the temporary flame speed increase at the bend is mainly due to large axial flow velocity of unburned mixture near the inner wall, while the increase of flame propagation speed at the downstream straight section is primarily due to the increase of the flame area.