Combining laser ignition tests with quenched particle cross-sectional analyses, this study explores the formation of bubbles and microexplosions in burning boron agglomerates, and discusses the plausible mechanisms behind the microexplosion (expansion–rupture) phenomenon. High-resolution images reveal that these phenomena are caused by bubble nucleation and growth within the molten droplet. Based on the timescale, it is categorized into weak expansion and strong expansion phenomena. The expansion rate of droplets during the weak expansion process mostly ranges between 0.006 m/s and 0.4 m/s, while during the strong expansion process, it predominantly falls between 0.4 m/s and 1 m/s. An increase in ambient pressure results in the delayed occurrence and reduced frequency of the expansion–rupture phenomenon. When the ambient pressure exceeds 3 atm, the expansion phenomenon no longer occurs. In addition, a decrease in the initial diameter of the agglomerates lowers the overall occurrences of expansion–rupture. Analysis of the quenched particle cross-sections reveals that the size and density distributions of the pores within the boron agglomerates reduce gradually during the combustion process. In addition, the surface oxide layers of certain pores are removed during combustion. Estimates of the vaporization rates of (BO) and BO support the speculation that the expansion of molten boron droplets is attributed to the evaporation of boron oxide layer. In this process, the molten boron acts as a heating surface to evaporate the boron oxide trapped inside the droplet, thus producing BO, BO, and BO, and leading to droplet growth and tearing of the liquid film. Analysis of bubble growth indicates that the merging of adjacent bubbles to reach the critical nucleation size is probable the starting point of the expansion process. These results can be helpful in promoting the combustion efficiency of boron within the combustion chamber.

中文翻译：

---

燃烧硼团块时形成气泡和微爆炸

本研究将激光点火测试与淬火颗粒横截面分析相结合，探讨了燃烧硼团块中气泡和微爆炸的形成，并讨论了微爆炸（膨胀-破裂）现象背后的合理机制。高分辨率图像显示，这些现象是由熔滴内的气泡成核和生长引起的。根据时间尺度，可分为弱膨胀现象和强膨胀现象。弱膨胀过程中液滴的膨胀速率大多在0.006 m/s~0.4 m/s之间，而强膨胀过程中液滴的膨胀速率主要在0.4 m/s~1 m/s之间。环境压力的增加会导致膨胀破裂现象的延迟发生和频率降低。当环境压力超过3个大气压时，不再发生膨胀现象。此外，团聚体初始直径的减小降低了膨胀破裂的总体发生率。对淬火颗粒横截面的分析表明，在燃烧过程中，硼团聚体内孔的尺寸和密度分布逐渐减小。此外，某些孔隙的表面氧化层在燃烧过程中被去除。 (BO) 和 BO 的蒸发速率的估计支持了熔融硼滴的膨胀归因于氧化硼层的蒸发的推测。在此过程中，熔融硼作为加热表面，使液滴内捕获的氧化硼蒸发，从而产生BO、BO、BO，并导致液滴生长和液膜撕裂。对气泡生​​长的分析表明，相邻气泡的合并达到临界成核尺寸可能是膨胀过程的起点。这些结果有助于提高硼在燃烧室内的燃烧效率。