Formation of bubbles and microexplosions in burning boron agglomerates

Abstract

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)n and B2O3 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 B2O3, B2O2, 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.