Experimental study of the settling and melting characteristics of equiaxed crystals in a falling tube

The morphology evolution of NH₄Cl equiaxed crystal settling in a falling tube filling with its superheated aqueous solution is studied. The effects of superheating and initial crystal size on settling rate and melting velocity are analyzed. The results show that for a non-spinning equiaxed crystal, it will transform from a "quasi-symmetrical" morphology to "quasi-delta" morphology, and for a spinning equiaxed crystal, it is more likely to sustain its initial "quasi-symmetrical" morphology. By analyzing the drag coefficients of equiaxed crystals settling in the solution at different superheating degrees, it is found that higher superheating leads to a smoother shape of the equiaxed crystal, thus increasing its settling rate. For a large equiaxed crystal, higher complexity in shape and increase in settling velocity will lead to a higher melting velocity. In the settling process of crystal in superheated melt, the solute transport condition on the melting interface is weakened by the gradually reducing the settling velocity, resulting in a relatively steady melting velocity for a certain equiaxed crystal.