Experimental Study of Buoyancy Effect on Transitional Jet Diffusion Flames ormalsize

The transition to turbulence of coflow propane-fueled jet diffusion flames was investigated experimentally with varying air coflow velocities, and the buoyancy-suppressing effect of coflow was analyzed to reveal the buoyancy effect on the transition and stability of diffusion flames. Compared to jet flames in quiescent air, a coflow with relatively high velocity can suppress the influence of buoyancy on transitional flames, and make the critical nozzle Reynolds number of transition to turbulent flame increasing, i.e., the transition process is delayed by coflow. When the coflow velocity is small, however, the critical Reynolds number stays almost the same. In the transitional regime, diffusion flames are characterized by periodic oscillation, and the oscillation amplitude decreases with increasing coflow velocity. As coflow velocity increases further, the periodicity of flame oscillation disappears eventually, and presents a randomness. Additionally, the experiments show that a larger jet nozzle results in a greater critical Reynolds number of flame transition. Such an observation is explained in terms of the local fuel jet properties taking into account of the influence of diffusion flame.