STABILIZING MECHANISM OF DIFFUSION FLAME FORMED IN AN AXISYMMETRIC IMPINGING JET

采用轴对称冲击射流的新型燃烧器系统，对影响扩散火焰稳定机理的边缘火焰特性进行了实验研究。燃烧器由一个释放空气的圆形喷嘴和嵌入撞击板以喷射燃料的多孔盘组成。然后，通过用实心圆盘封闭多孔板的一部分，边缘火焰可以形成为反扩散火焰中的孔。当固体圆盘直径di小于喷嘴直径D时，在冲击射流的滞止区形成边缘火焰。对于di&gt；D，在轴对称壁面射流中建立了边缘火焰。定常流动和壁面喷流各自的结论如下。对于Di&lt；D，4E火焰在性质上表现出与纯扩散火焰相同的喷吹特性，即边缘火焰具有临界滞止速度梯度，超过该临界停滞速度梯度，火焰永远不能稳定。这意味着在静止流动中有孔洞的扩散火焰的喷出是由于边缘火焰区对反应速度的化学限制。随着孔径的增大，喷吹的临界值减小。对于di&gt；D，随着气流速度的增加，边缘火焰转变为边缘火焰和预混火焰的三重火焰，最后三重火焰熄灭。在过渡过程中，边缘火焰区的放热量增大。火焰位置的决定因素由边缘火焰与固壁之间的热相互作用转变为三重火焰燃烧速度与可燃混合气速度之间的流体动态平衡。

Characteristics of edge flames that dominate the stabilizing mechanisms of diffusion flames have been studied experimentally by using the novel burner system that utilized an axisymmetric impinging jet. The burner consisted of a round nozzle that issued air, and porous disk that was embedded in the impinging plate for fuel ejection. Then, by closing a part of the porous plate with a solid disk, the edge flame could be formed as a hole in the counter diffusion flame. When the solid disk diameter di was smaller than the nozzle diameter D, the edge flame was established in the stagnation region of the impinging jet. For di>D, the edge flame was established in the axisymmetric wall jet. The each conclusion for stagnation flow and wall jet is as follows.1. For di<D, the 4e flame shows qualitatively the same blowoff character as a pure diffusion flame, that is, the edge flame has the critical stagnation velocity gradient beyond which the flame can never be stabilized. It means that the blowoff of the diffusion flame with a hole in the stagnation flow is due to the chemical limitation on the reaction rate in the edge flame zone. Moreover the critical value of the blowoff decreases as the hole diameter is increased.2. For di>D, the edge flame turns into a triple flame that is edge flame with premixed flame, and finally triple flame blows out with increasing the air flow velocity. In the transition process, the amount of heat release in the edge flame zone gains. Moreover the determinant factor for flame position changes from the thermal interaction between the edge flame and the solid wall to the fluid dynamical balance between the burning velocity of triple flame and the flammable mixture velocity in that process.