An Experimental Approach to Vortex-Flame Interaction in Practical Turbulent Fields

本文通过实验研究了平面预混剪切流中具有相干结构的火花点燃火焰的涡-火焰相互作用过程，以及细柱棒阵列后尾迹中的卡门涡街，以阐明相邻涡流对涡爆发的影响。本研究采用丙烷-空气混合气，其当量比不同。所得结果总结如下：(1)当火花在大尺度有组织涡流中心点火时，火焰轴向传播速度快于径向传播速度，表明涡流运动增强了火焰的传播速度。在涡流内完成燃烧后，一对火焰向上游和下游相邻边界区域突出，沿流动方向传播。剪切流固有的卷升运动促进了其传播速度。如果火花在相邻涡流的边界处点燃，火焰沿着两种混合流之间的接触面延伸并变成s形。结果表明，流动方向上的火焰速度与轴向上的火焰速度几乎相等。当s型火焰的尖端到达相邻涡流处时，涡流爆发再次增强了火焰的轴向传播。结果表明：涡旋-火焰相互作用存在两种类型；一种是涡流爆破，另一种是涡流增压，这两种增强机制的结合构成了湍流燃烧的关键因素之一。(2)细圆柱棒阵列后的混合尾迹中产生的火花点燃的年轻火焰受到卡门涡多次袭击的强烈冷却和拉伸。因此，在弱混合气中，旋涡-火焰的相互作用并不一定能增强火焰的传播，反而会导致火焰的熄灭。

In this investigation the processes of vortex-flame interaction of a spark-ignited flame with either coherent structure in the plane premixed shear flow or Karman vortex street in the wake behind a fine cylindrical rod array are experimentally examined, in order to clarify effects of adjacent eddies on the vortex bursting. Propane-air mixture is used in this investigation with an equivalence ratio varied. The results obtained are summarized as follows.(1) In case of the spark ignition at the center of a large scale organized eddy, a flame propagates faster in the axial direction than in the radial one, indicating that the vortex motion enhances the propagation velocity. After completing combustion within an eddy, a pair of flames protrude to adjacent boundary regions on upstream and downstream sides and propagate in the flow direction. The propagation velocity is promoted by the rolling-up motion inherent in the shear flow. In case of the spark ignition at the boundary between adjacent eddies, a flame extends along the contact surface between two mixture streams and becomes S-shaped. It is found that the flame velocity in the flow direction is almost equal to that in the axial direction. When the tips of S-shaped flame reach adjacent eddies, the vortex bursting again enhances the axial flame propagation. It is concluded that there exist two kinds of vortex-flame interactions; one is the vortex bursting, the other is the vortex boosting, and that combination of these two enhancement mechanisms constitutes one of the key factors to the turbulent combustion.(2) A spark-ignited young flame generated in the mixture wake behind a fine cylindrical rod array suffers from the intense cooling and stretching by multiple attacks of Karman vortex. The vortex-flame interaction, therefore, can not always augment flame propagation, but lead to flame extinction in the weak mixture.