Characteristics of turbulent lifted flames in coflow jets with initial temperature variation

Characteristics of turbulent lifted flames in coflow jets have been investigated experimentally by varying the initial temperature of coflow air up to 900 K. In the turbulent jet regime, the liftoff height increased linearly with jet velocity and decreased with initial temperature. The liftoff velocity and reattachment velocity scaled by the stoichiometric laminar burning velocity remained constant regardless of the variation in the initial temperature. The behaviors of liftoff height and blowout velocity have been investigated based on the premixed flame model and the large-scale mixing model for turbulent lifted flames. In the prediction of liftoff height, the premixed flame model was found to be effective regardless of initial temperature. In the case of the large-scale mixing model, the thermal diffusivity evaluated at initial temperature showed much improved correlation than that evaluated at the adiabatic flame temperature which was originally adopted in the model. This result implies that the mixing in the unburned region between the nozzle exit and lifted flame base controls the flame stabilization as compared to the view of reentrained burnt gas in the large-scale mixing model. In predicting the blowout velocity, the effect of buoyancy needs to be considered in both models for the cases with initial temperature variation. The effect of buoyancy can be reconfirmed through the prediction of liftoff height at blowout.