Modeling of liftoff heights of non-premixed turbulent flames in co-flows having various temperatures and O2 concentrations

Lifted flames in combustion furnaces are diluted with burned gas entrained into the fuel jet. The reduced concentrations of reactants resulting from this dilution increase the liftoff height, while the associated temperature increase decreases the height. The aim of the present study was to develop a premixed model capable of predicting the variation in the liftoff height resulting from entrainment. A triple concentric burner incorporating fuel gas, oxidizer and co-flow gas nozzles was employed to simulate a combustion furnace. Prior to combustion tests, the fraction of the fuel gas in the non-reactive jets forming on the burner was determined, to allow an evaluation of parameters affecting the entrainment rate of the co-flow gas. The flame liftoff height above the burner was found to increase with decreases in the O-2 concentration in the co-flow gas and was decreased with increases in temperature. Three premixed models were examined: a conventional premixed model, a DP1 model including only the effect of decreasing reactant concentrations and a DP2 model including the effects of both decreasing concentrations and temperature increases. Validations of these models demonstrated that the conventional model failed to predict variations in the liftoff height at a variety of co-flow gas O-2 concentrations and temperatures. The DP1 model also provided insufficient correlations between the bulk velocity and liftoff height, such that the correlation line at a high co-flow gas temperature separated from that at room temperature. In contrast, the DP2 model provided excellent correlations in conjunction with different virtual origin positions.