浮力效应对射流扩散火焰转捩特性影响的实验研究

对不同空气伴流速度下丙烷层流射流火焰向湍流火焰的转捩过程进行实验观测，分析伴流对火焰转捩行为及稳定性的影响.相对于静止环境中的射流火焰，较大速度的伴流可以减小浮力效应对射流扩散火焰转捩过程的影响，使火焰发生转捩的临界Reynolds数（Re_cr）增大，即火焰推迟转捩.但当伴流速度较小时，Re_cr保持不变，转捩过程中的射流火焰发生周期性振荡，振荡幅度随着伴流速度的增大而减小，继续增大伴流速度，火焰振荡的周期性最终消失，转而呈现随机性.实验还发现，喷嘴直径较大的扩散火焰的Re_cr更大.考虑到火焰对燃料射流局部流动状态的影响，对此现象进行了解释. 

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.