Liftoff heights of turbulent non-premixed flames in co-flows diluted by CO2/N2

Abstract

The objective of this study was to propose a new model for the prediction of the liftoff heights of turbulent flames diluted by the entrainment of burned gases. In combustion furnaces with the internal recirculation of burned gases, mixtures of fuel and oxidizer are diluted with recirculated burned gases through entrainment into the gas jets. We focused on the effects which dilution resulting from entrainment has on the stabilization mechanism of lifted flames. In order to investigate the effects of dilution on liftoff height, we employed a concentric burner incorporating fuel, oxidizer and co-flow gas nozzles. The recirculated burned gas was simulated by co-flow air diluted with either N2 or CO2 gas. Liftoff heights were observed to increase with decreasing O2 concentrations in the co-flow gas when maintaining a constant O2 concentration in the oxidizer, due to dilution resulting from entrainment of the diluted co-flow gas. The liftoff heights obtained with co-flow gases diluted by CO2 were greater than those obtained when diluting with N2 due to both thermal and chemical dilution effects. The conventional premixed model was not able to predict the liftoff trends observed in this study and we therefore propose a modified premixed model which takes into account the dilution effect resulting from entrainment. In this model, the amount of entrained co-flow gas is evaluated according to the self-similarity law of a round jet. Non-dimensional liftoff heights based on this modified model exhibit excellent linear correlation with non-dimensional fuel gas velocities, even when various co-flow gases are used for dilution. The conventional large eddy model was also modified in the same manner and the results obtained from the modified model exhibit satisfactory correlation.