惰化防爆作为可燃气体燃爆防控的常用的措施之一，有效降低爆炸破坏力的同时也降低了安全防控的经济成本。然而，当被惰化的可燃气体被点爆后，较低的扩展速度使得火焰受到的浮力作用显著，浮力存在及显著程度将影响火焰的形状及传播特性，使得可燃气体的惰化受到一定影响，因此，为了对惰化情况下浮力对球形火焰的影响，本文对不同含量CO₂的甲烷球形膨胀火焰浮力及胞状化特性展开了研究，可为工业生产过程中的惰化防爆技术提供指导。

      本文基于20L球形实验系统对不同惰化程度下的甲烷燃爆火焰特性进行爆炸实验，同时利用高速纹影仪对火焰进行捕捉，分析球形火焰在爆炸初期火焰在浮力影响下的变化，以及爆炸后期胞状火焰特性和压力变化，最后利用Fluent软件对甲烷惰化情况下燃爆过程进行重现，以弥补实验无法获得的数据。研究结果表明：惰化程度影响火焰的浮力显著程度，而浮力作用的不同使初燃阶段火焰呈现正球形，椭球形及“w”形。添加CO₂可增加容器内整体密度，使火焰已燃区和未燃区内的密度差增大，浮力增强，表现为向上扩展速度大于水平与向下扩展速度且浮力率变大；同时，添加CO₂使发展阶段胞状火焰形成时间被延迟。若CO₂含量较大，火焰的拉伸率明显减小，马克斯坦长度为正值，其火焰更稳定，导致胞状火焰的形成被有效抑制。在火焰发展阶段，因壁面反射压力与火焰自身膨胀的“对抗作用”，火焰将出现“涟漪”现象，惰化后容器内爆炸压力在短期内分布不均，“涟漪”在容器上部更明显。通过分析爆炸压力数据发现爆炸超压上升速率与胞状火焰有直接关系，其表现为超压上升速率越大时，蜂窝状的胞格最密集。

       对数值模拟的爆炸超压，火焰形状及火焰扩展半径与实验结果进行对比，发现模拟与实验有较好的一致性。通过分析实验和模拟典型火焰的流线，可以发现火焰的形状主要受到涡流的诱导，其中涡的形成时间对火焰的形状有直接关系，涡流形成越早，火焰变形越早且越显著。而火焰面内部最初点火区速度一直最大是导致火焰出现“w”形的直接原因。添加少量的CO₂使得燃爆火焰不稳定性增强，更易转捩分形而形成胞状火焰。

Inerting technology is one of the commonly used measures for the prevention and control of combustible gas combustion and explosion, which effectively reduces the destructive power of explosions and the economic cost. However, when the combustible gas inerted is ignited, the lower expansion speed makes the buoyancy effect of the flame significant, which will affect the shape and propagation characteristics of the flame. The inerting effect will be affected to a certain extent. Therefore, to investigate the effect of buoyancy on the spherical flame in the case of inerting, this paper carried out a study on the instability and cellularization characteristics of methane spherical expansion flame, which can provide guidance for the inerting explosion-proof technology in the industrial production process.

    Based on the 20L spherical experimental system, the experiment of methane combustion and explosion under different inerting degrees was conducted. At the same time, flame is captured by schlieren, and the shapes of flames under buoyancy at the beginning of the explosion and cellular process of flame in the later stages of the explosion are analyzed. Then, numerical reproduce the explosion process to make up for the data that cannot be obtained in the experiment. The buoyancy effect caused the flame to appear in the shape of spheroid, ellipsoid and “w” in the initial combustion stage. Adding CO₂ can increase the overall density in the container, which increases the density difference between the burned area and the unburned area of ​​the flame, the buoyancy effect was increased. The upward expansion speed is greater than the horizontal and downward expansion speeds and the buoyancy rate becomes larger; at the same time, add CO₂ delays the formation of cellular flame in the development stage. The stretching rate of the flame is significantly reduced when CO₂ much more added, and the Markstein length is positive, the flame is more stable and the formation of cellular flame is effectively suppressed. During the flame expansion process, due to the “antagonism” between the wall emission pressure and the flame expansion, the flame will appear “rippling”. After inerting, the explosion pressure in the container will be unevenly distributed in a short time, which makes the “rippling” will be more obvious on the upper part of the container. By analyzing the explosion pressure data, it is found that the explosion overpressure rising rate is directly related to the cellular flame. The higher the overpressure rising rate, the densest honeycomb cells.

     The numerical simulation of explosion overpressure, flame shape and flame expansion radius are compared with the experimental results, and it is found that the simulation is in good agreement with the experiment. By analyzing the experiment and simulating the streamline of a typical flame, it can be found that the shape of the flame is mainly affected by the vortex. The formation time of the vortex is directly related to the shape of the flame. The earlier the vortex is formed, the earlier and more significant the flame deformation. The maximum velocity in the initial ignition zone inside the flame surface is the direct cause of the “w” shape of the flame. Adding a small amount of CO₂ enhances the instability of burning and explosion flames, and it is easier to transform fractals to form cellular flames.

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