Stabilization Characteristics of Transitional and Weakly Turbulent Lifted Jet Diffusion Flames

This work reports experimental results from laminar-to-turbulent lifted jet diffusion flames. Characteristics of flame liftoff height, flame base structure, cold and reacting jet flow fields, and velocity fields near the flame base during the transitional process were obtained. The flame transition and stabilization characteristics are the focus of the present study. Results show that the lifted flame transition is mainly caused by the interaction between turbulent jets and flame base. As the transition begins, folding and distortion of the flame base occur. With the increase in Reynolds number (Re), the extent to which flame base is distorted increases constantly due to influences of more vortices. Simultaneously, at this transitional stage, the liftoff height decreases with Re. At different co-flow air velocities (U-air), the mean liftoff height with Re are qualitatively the same. While as U-air is increased, the critical transitional Reynolds number (in which the flame transition begins) decreases. In the later transitional regime within a range of Re, both the cold and reacting jets exhibit intermittent breakup. After jet breakup, flame base immediately becomes turbulent. When the intermittent flow in the transitional stage gradually becomes less turbulent, flame base still remains turbulent for some time, within which flame and flow can be evidently decoupled. By statistical analysis, the integral length scale and the root mean square (r.m.s.) velocity fluctuation near the lifted flame base were obtained, and then the turbulent flame speed can be estimated. Experimental evidence provided for stabilization mechanism of the original transitional flames was confirmed by the balance between the local flow velocity and the flame speed. However, the difference between the two velocities gradually increases with the Reynolds number, suggesting some new stabilization characteristics. When the upstream eddy contains flammable fuel/air mixture, the flame base responds with moving upstream after ignition. On the contrary, when the mixture in the eddy is nonflammable, the flame base would move downstream.