Lifted flames on fuel jets in co-flowing air

The three principal theories for the stabilization of lifted flames on turbulent jets of fuel are reviewed in the light of the most recent flame imaging experiments in the literature. Most of these experiments have been conducted with a small co-flow of air, but the observations are relevant to lift-off with higher ratios of co-flowing air to fuel jet velocity. The similarity solutions for jets in co-flow are developed, and data from a variety of fluid dynamic sources are assessed to yield the governing parameters for mean flow, turbulence and mixture fraction. New data for lifted flames on a methane jet in diffusing streams of co-flowing air are then presented. These data provide essential information on the intermittency, and on the properties of the jet conditioned on the presence of turbulent fluid. However, the co-flow lifts the flame to stabilize in better-mixed regions than in its absence. The ‘premixture’ model is confirmed for this situation, in which the lift-off heights were more than 20 jet diameters and where there is little intermittency at the stabilization radius. Nevertheless, mixing data for this geometry in the absence of a flame show that, with lift-off heights less than 20 jet diameters, the base of the flame would have been in the outer regions of the jet where the mixture of fuel in air only reaches stoichiometric proportions intermittently, with the passage of large eddies. Trading on many papers from the recent literature where this was the case, both experimental and computational insights as to the processes in this region are reviewed. A question remains about how ignition is maintained in these experiments with low turbulent lift-off. It is hypothesized that the mechanism is the diffusive heating of the slowly moving surrounding air which then provides an energy store for the incoming eddies. Further time-resolved observations of reaction zone and high temperature gas structure are required to test this model.