Effect of gas flow mixing uniformity on the flashback characteristics of turbulent jet flames

[Objective] Modern gas turbine combustors often adopt premixed combustion technology for lower NOx emissions. In premixed combustion, the flame flashback is an important issue, particularly for hydrogen-rich fuels burned in micromix burners. The combustible gas in the micromix burner is mixed near the burner nozzle outlet, resulting in a nonuniform species mole fraction in micromix turbulent jets. This mixing-induced nonuniformity of the species deserves careful investigation for its impact on the flame flashback. [Methods] By designing four mixing modes (MM 1-4) of H2, CO, and air, this work experimentally studied the effect of gas flow mixing conditions on the flashback characteristics of turbulent premixed jet flames. MM 1: H2, CO, and air directly enter the nozzle through a straight tube. MM 2: H2 and CO flow into a section of PVC (Polyvinyl chloride) pipe through a tee joint and then mix with air through a straight tube. MM 3: H2, CO, and air directly enter the nozzle through a Venturi tube. MM 4: H2, CO, and air enter a chamber equipped with flow conditioning components (sintered metal plate, glass balls, etc.) and then enter the nozzle through a Venturi tube. From MM 1 to MM 4, the degree of the corresponding mixing uniformity of H2, CO, and air increased. The flashback phenomenon is captured using a high-speed camera integrat with the schlieren method. The flow field near the burner exit is measured using particle image velocimetry. [Results] The results showed that the onset of the flame flashback at different fuel/air mixing modes always occures near the burner wall, similar to the classical “boundary layer flashback” phenomenon. High-speed camera images indicated that the near-stoichiometric premixed flame was blue, surrounded by a diffusion flame layer before flashback. For the worst mixing mode, the region of this diffusion flame layer where the flashback starts appeared orange, indicating higher hydrogen concentration in this region. Upon changing to better mixing conditions, the orange diffusion flame disappeared, and the starting point of the flashback was not specific on one side. Under this fuel-lean condition, no surrounding diffusion flame layer appeared, and thus the mixing uniformity could not be directly evaluated through the flame color. As the mixing uniformity was improved, the flashback velocity decreased. The velocity distributions near the burner exit at different mixing modes were top hat-shaped, suggesting that the influence of the mixing conditions on the velocity distribution was unclear. [Conclusions] The mixing-induced nonuniformity of the species has little effect on the flame flashback mechanism. Schlieren images clearly distinguish the onset and position of the flashback. The flashback process observed from schlieren images agrees well with the high-speed camera result. The flashback flow velocity differs among mixing modes, and the flashback is more likely to occur when the mixing condition is worse. The effect of mixing conditions on the flashback is mainly due to the change in the flame propagation speed near the burner rim. The results of this study shed light on the antiflashback burner design. © 2023 Press of Tsinghua University. All rights reserved.