Flame behavior and flame-induced flow in a closed rectangular duct with a 90° bend

A 3-D premixed laminar flame propagation, ignited at one end in a closed rectangular duct with a 90° bend, has been investigated by both experimental and numerical methods. A high-speed camera was used to capture the flame shapes and propagation speed in the experiments, and a computational fluid dynamics (CFD) code KIVA-3V was employed to simulate the 3-D transient laminar reacting flow in the numerical studies. The results showed that: (1) a flame tends to avoid the inner and outer surfaces when it propagates through the first 45° of the bend and this phenomenon was termed “flame shedding”; (2) the numerical and experimental results were in good agreement for the basic physical phenomena, such as the tulip flame and “flame shedding”; (3) a flame could behave as a thrust resource for the unburned mixture to generate or induce very complex flow fields which could facilitate an environment that subsequently affects flame behavior. This interaction between the flame and the flame-induced flows, once the flame front begins to flatten, results in a higher dynamic pressure acting forward on the corner portions of the flame front than the middle and therefore a tulip-shape flame could be formed; (4) “flame shedding” is due to bend effects: the presence of a streamwise-oriented vortex pair near the inner surface of the bend and the high-pressure region near the outer surface.