Ignition kernel formation and lift-off behaviour of jet-in-hot-coflow flames

The stabilisation region of turbulent non-premixed flames of natural gas mixtures burning in a hot and diluted coflow is studied by recording the flame luminescence with an intensified high-speed camera. The flame base is found to behave fundamentally differently from that of a conventional lifted jet flame in a cold air coflow. Whereas the latter flame has a sharp interface that moves up and down, ignition kernels are continuously being formed in the jet-in-hot-coflow flames, growing in size while being convected downstream. To study the lift-off height effectively given these highly variable flame structures, a new definition of lift-off height is introduced. An important parameter determining lift-off height is the mean ignition frequency density in the flame stabilisation region. An increase in coflow temperature and the addition of small quantities of higher alkanes both increase ignition frequencies, and decrease the distance between the jet exit and the location where the first ignition kernels appear. Both mechanisms lower the lift-off height. An increase in jet Reynolds number initially leads to a significant decrease of the location where ignition first occurs. Higher jet Reynolds numbers (above 5000) do not strongly alter the location of first ignition but hamper the growth of flame pockets and reduce ignition frequencies in flames with lower coflow temperatures, leading to larger lift-off heights.