STABILIZING MECHANISM OF DIFFUSION FLAME FORMED IN AN AXISYMMETRIC IMPINGING JET

利用新型轴对称冲击射流燃烧器系统，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。燃烧器由一个圆形喷嘴和嵌入在撞击板中的多孔盘组成，用于喷射燃料。然后，用实心圆盘封闭多孔板的一部分，形成边缘火焰作为反扩散火焰的孔。当固体盘直径di小于喷嘴直径D时，在撞击射流的停滞区形成边缘火焰。对于di>D，在轴对称壁面射流中建立了边缘火焰。滞流和壁面射流的结论如下：1。当di<D时，4e火焰在质量上表现出与纯扩散火焰相同的吹散特征，即边缘火焰具有临界停滞速度梯度，超过该梯度火焰将永远无法稳定。这意味着在停滞流动中带孔的扩散火焰的爆炸是由于边缘火焰区反应速率的化学限制所致。随着孔直径的增大，放空临界值逐渐减小。对于di>D，边焰转变为边焰与预混焰的三重焰，最后三重焰随着气流速度的增大而熄灭。在过渡过程中，边缘火焰区的放热量增加。火焰位置的决定因素由边缘火焰与固体壁面的热相互作用转变为三重火焰燃烧速度与可燃混合物速度之间的流体动力学平衡。

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.