Effect of dilution on the liftoff of non-premixed jet flames

Effects of fuel dilution on the characteristics of lifted, non-premixed jet flames have been studied experimentally by using nitrogen as a diluent in the fuel stream. Depending on the nozzle diameters and the degree of dilution, the liftoff behavior can be classified into three different modes: (1) both the liftoff and blowout happen in the laminar regime, (2) the liftoff occurs in the laminar regime, while the blowout takes place after the fluid undergoes the transition to turbulence, and (3) both the liftoff and blowout occur in the turbulent regime. For the transition case (i.e., case 2), the flame is lifted off in the laminar regime, and the liftoff heightincreases rapidly until the height becomes comparable to the breakup length of the jet. At this point, both the liftoff height and breakup length decrease slowly with the jet velocity. As the jet velocity increases further toward the critical Reynolds number for transition to turbulence, there is a sudden decrease in the liftoff height. After this transition, the liftoff height increases linearly with the jet velocity. In the laminar regime, the liftoff height scaled with the nozzle diameter squared can be correlated with the jet velocity, and the preferential diffusion between the fuel and diluent is observed to play a significant role for a diluted jet. In the turbulent regime, the linear dependence of the liftoff height on nozzle diameter has been shown by a correlation between the nozzle diameter, the level of dilution, and the jet velocity.