Blow off mechanism in turbulent premixed bluff-body stabilised flames with pre-vaporised fuels

The lean blow off (LBO) limits and flame structure of turbulent premixed flames were investigated with pre-vaporised liquid fuels in a bluff-body burner. Ethanol, heptane, and two kerosenes (fuels "A2" and "C1" from the National Jet Fuel Combustion Programme) were used. In order to facilitate comparisons to gaseous-fueled flames, results were also obtained from methane flames. The flame structure was studied with OH* chemiluminescence and CH2 O-PLIF imaging far from and close to blow off. The measured LBO limits indicate that the single-component fuels in this burner are more resilient to blow off than the kerosene fuels. Furthermore, a correlation based on a Damköhler number, which is proportional to the laminar flame speed, does not lead to the successful collapse of the different fuels, indicating that the heavy hydrocarbon fuels experience a different LBO mechanism than the simpler fuels. Average OH* chemiluminescence images of the ethanol and heptane flames are qualitatively similar to that from methane with an M-shape flame brush close to blow off. In contrast, most of the OH* chemiluminescence from the A2 flame is found further above the bluff-body indicating weaker burning near the bluff body, which is not evident in the gaseous-fueled and lighter single-component liquid fueled flames. Both single-and multi-component fuels exhibit significant broadening of their CH2 O-layers as blow-off is approached, with the average thickness increasing from about twice to nearly six times the unstrained laminar flame value. Also, an increasing amount of CH2 O-LIF signal is observed within the recirculation zone, which is consistent with prior and present results obtained from methane flames. Overall, the thickness and appearance of the CH2 O-layers are qualitatively similar between the single-and multi-component fuels; however, the kerosene fuels tend to exhibit wider CH2 O-layers. Additionally, these fuels tend to possess more isolated pockets of CH2 O-LIF signal within the recirculation zone, which suggests that a larger amount of partially-combusted fluid enters it. This result is consistent with the observation that these heavy hydrocarbon flames tend to burn less vigorously near the bluff body than the simpler-fueled flames. The results indicate that at LBO conditions, fuel effects may result in a complex behaviour that can not be simply understood through high-temperature chemistry concepts such as the laminar flame speed