Effects of Burned Gas Dilution on the Liftoff Height of Turbulent Non-Premixed Flames (Effects of Reduced Reactant Concentrations through the Burned Gas Dilution)

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Objective of this study is to propose a new method to predict liftoff heights of turbulent non-premixed flames in combustion furnaces. At the first stage of this study, we investigated effects of the burned gas dilution of fuel jet on the liftoff height. The burner used consists of co-axial nozzles from which fuel, oxidizer and co-flow gas are issued, respectively. The co-flow gas surrounding the fuel jet simulates burned gases with low oxygen concentration in combustion furnaces. The fuel jet entrains the co-flow gas in the unburned mixing region of the base of lifted flame. The entrainment of co-flow gas decreases the oxygen concentration of mixtures, and hence the laminar burning velocity of premixed flame formed at the flame base decreases. The lower burning velocity lifts up the flame base. Edge flame model and premixed flame model fail to reproduce this phenomenon of liftoff height due to the dilution through co-flow gas entrainment. A dilution ratio is introduced to quantify the magnitude of the dilution. It has found that the dilution ratio can generalize liftoff heights of the turbulent flames diluted through co-flow gas.