Modeling on transient microstructure evolution of solid-air solidification process under continuous cooling in liquid hydrogen

The safety hazard brought by the oxygen-rich solid formed by the solidification of air in liquid hydrogen cannot be ignored. A numerical model describing the evolution of dendrite during solidification of nitrogen-oxygen binary solutions in liquid hydrogen is developed. By introducing the reduction factor, the growth anisotropy of the six-fold symmetric dendrite simulated by the Cartesian grid is effectively reduced. The reliability of the model is verified by comparing the tip growth rate of dendrites with six-fold symmetry with analytical models. On this basis, the microstructure evolution and solute segregation of solid-air dendrites are investigated. The results show that with the increase of the cooling rate, the dendrite growth rate accelerates while aggravating the solute segregation, and the outer edge of the dendrite is more likely to reach the oxygen-rich state. The improvement in solute diffusibility enables dendrites to reach an oxygen-rich state at larger sizes, but also accelerates dendrite growth. The initial composition has little effect on the microstructure evolution and growth rate of the solid-air dendrite. However, in the presence of forced convection, the solid-air dendrite morphology loses its symmetry and makes the upstream dendrite arms reach an oxygen-rich state at a smaller size. This study helps to understand the air solidification process in liquid hydrogen and provides theoretical guidance for the safety research of liquid hydrogen system.