Investigation of flashback characteristics coupled with combustion instability in turbulent premixed bluff body flames using high-speed OH-PLIF and PIV

This study investigated the flashback phenomenon coupled with self-excited combustion instability in turbulent premixed bluff-body flames using the pressure fluctuation measurement, high-speed OH-PLIF, and PIV techniques. Previous studies investigating flashback in a bluff-body have found that the flame moves back and forth around the trailing edge of the bluff-body; however, the phenomenon in which the flame propagates beyond the bluff-body has not been sufficiently studied. Therefore, this study focused on understanding a strong flashback, which can damage the upper section of a combustor and which is vulnerable to heat due to flame propagation over the front of the bluff-body. The combustion instability frequency resulting from changes in the combustion length occurred within the range of the resonance frequency of the combustor, thereby confirming that thermal-acoustic combustion instability occurred in the combustor. When the strong flashback occurs, an instantaneous adverse pressure gradient is formed within a combustion instability cycle. Consequently, the generated reverse flow pushed the flame attached at the trailing edge of the bluff-body to the upstream from the bluff-body. The flame propagated rapidly along the side of the bluff-body by the influence of the boundary layer flow and the decreased quenching distance. This propagated flame became the ignition source at the front tip of the bluff-body and generated the flame surface that propagated in all directions; thus, it was found to be the primary cause of the increase in the flashback distance. The flame flashback distance also varied depending on the combustor length and the initial flow condition. An attempt was made to concentrate the measured data of flashback distance under various conditions into a single line, and turbulence intensity and combustion instability frequency were the dominant factors that impacted the flashback distance.