Nuclear shell model for the ignition process of boron particle with binary surface oxide layer

Abstract

Abstract The combustion of a single boron particle contains three continuous macroscopic processes, including ignition delay, ignition and combustion. Studies show that among these processes, the intermediate ignition process has the most complicated kinetic mechanism. On the other hand, conventional models have serious shortcomings to simulate this process. The main challenges in this regard are indeterminate initial thickness of the surface oxide layer, incomplete diffusion mechanism in the surface oxide layer, and neglected effects of the pressure and particle size on the control mechanism. In order to overcome the drawbacks of existing ignition models, a comprehensive kinetic ignition model is developed in the present study. To this end, the dual-beam focused ion beam micro/nanofabrication system is applied to etch and cut the heat-treated boron particles into slices. Then, the scanning transmission electron microscope with collateral energy dispersive spectrometer is applied for analyzing the microstructure and elementary composition of the slice. It is found that the average thickness of the surface oxide layer is higher than earlier assumed values, and the bidirectional diffusion mechanism of boron and oxygen in the liquid boron oxide is confirmed. During the model development, seven heterogeneous global reactions and two vapor-phase reactions are used in total for analyzing the kinetic mechanisms. In the present study, different indicators, including the vaporization process, O2 reactions, and H2O reactions on the internal and external surfaces of the binary oxide layer are investigated. Moreover, the conservation laws of mass and energy are used for the establishment of governing equations. Then, the accuracy of the developed model is evaluated by comparing the obtained results for the ignition time with that of the experiment, which shows a reasonable consistency between them. Then, the proposed model is applied to investigate the ignition characteristics of boron particles. Effects of different parameters, including the environment temperature, O2 concentration, and H2O concentration are studied on the ignition time. It is concluded that the established model, which is named as the nuclear shell (N-S) model is a powerful scheme to better understand the ignition mechanisms and characteristics of boron particles and can be further applied in the field of computational fluid mechanics.