High-speed OH-PLIF imaging of extinction and re-ignition in non-premixed flames with various levels of oxygenation

This paper presents high-speed (HS) images of OH-PLIF collected at a repetition rate of 5 kHz along the entire length of turbulent, pilot-stabilised flames approaching global blow-off. Measurements are made in flames of compressed natural gas, CNG (similar to Sydney flames L, B, M [1]), CNG–air (similar to Sandia flames D and E [4]) as well as CNG–O2 flames with varying O2 concentrations. Although the HS-OH-PLIF images are only qualitative, they are found to be of sufficient spatial and temporal resolution to relay the evolution of extinction and re-ignition events. Three types of structures, common to all the flames studied here, are identified: ‘breaks’, ‘closures’ and ‘growing kernels’. Events of ‘breaks’ are counterbalanced by the occurrence of ‘closures’ which reconnect the flame sheet and maintain stable combustion particularly in the upstream regions of flames. Applying simple but effective data processing tools, it is found that the rate at which ‘breaks’ grow in the flames is faster that the rate at which they close. The measured speeds as well as the differential between the rates of growth and closures are consistent with the behaviour of edge-flames implying that these structures may play a significant role in the dynamics of extinction/re-ignition. Other contributors to re-ignition are found to be ‘growing kernels’ advected from upstream regions in the flames. As the flames approach global blow-off, these kernels become the main mechanism for re-ignition further downstream and their rate of growth is shown to vary significantly although the factors affecting such growth need to be explored further.