Differential diffusion effects on turbulent premixed bluff-body stabilized flames near lean blow-off

The effects of differential diffusion on the flow dynamics and structure of a turbulent premixed bluff-body stabilized flame near lean blow-off have been investigated experimentally. A range of different fuels and decomposed ammonia fuel blends with different Lewis numbers (larger or smaller than unity) and preferential diffusivities have been used to compare lean blow-off limits (LBO). Simultaneous high-speed PIV and OH-PLIF (operated at 10kHz) have been employed to obtain time-averaged and time-resolved flame images, in which flame front curvatures, stretch rates and other turbulent flow characteristics can be obtained. Lean blow-off occurred by increasing the air flow rates at a constant rate. The LBO of ethylene occurred at leaner conditions than that of methane. In addition, as the hydrogen volume fraction increases in the NH3/H2/N2-air blends, the flame becomes more stable due to preferential diffusion effects. The near blow-off flame structure is compared at the same blow-off velocity but varying equivalence ratios, and is observed to change depending on the fuel blend. The crucial role played by differential diffusion effects in the propagation of turbulent flames near lean blow-off will be highlighted, and the relevant physical mechanisms will be discussed.