Large eddy simulation of transient leading edge propagation in a turbulent lifted hydrogen jet flame

Abstract

A lifted turbulent jet flame involves partial premixing and exhibits tribrachial flame structure at its leading edge (LE). Its propagation from the initial sparking location towards the final stabilisation height has rich physics. Large eddy simulation (LES) with flamelet-based reaction rate closure for partially premixed combustion is employed to study this propagation. The initial kernel grows radially in the rich region, and it is skewed by the oncoming flow as it is convected downstream. A LE is formed as the flame propagates radially into lean mixtures with low streamwise velocities. This LE encounters the lean limit, while a core flame continues to develop closer to the jet centreline, where the mixture reactivity and flow velocity are significantly large. Eventually, this core flame overtakes the LE in the lean mixture and upstream propagation ensues. The LE propagates mostly in the lean mixture, as the streamwise velocity in the vicinity of the jet core is high, although occasional rapid propagation is observed as the core flame encounters highly reactive mixture due to turbulence. Hence, the two flame branches are competing to form the LE. This behaviour is quite different to propagation in a lifted methane jet flame due to the increased reactivity and wider flammability limits of hydrogen. Turbulence plays a fundamental role during propagation for the formation of new upstream flame kernels, which evolve from pockets of hot reactants and fresh mixture. These kernels may be convected downstream causing the LE to recede. Eventually, the LE reaches a stationary state, and the flame root stabilises at a position where the burning mass flux is balanced by the flame normal advective mass flux. The LE does not encounter the value of the extinction dissipation rate for the mixture fraction during its evolution towards the stabilisation and at this location.