The space mission Hetero-3D is planned to clarify the grain refinement mechanism of Ti6Al4V with titanium carbide (TiC) heterogeneous nucleation site particles; the mission involves melting and solidifying samples in the electrostatic levitation furnace in the International Space Station (ISS-ELF). This study aimed to confirm the optimal sample preparation process for preventing bubble formation in ISS-ELF experiments. Bubble formation can impede the observation of the nucleation behavior of the samples. In this study, TiC particles were prepared using two methods: pulverizing with a mortar and pestle or a ball mill and crusher mill. These methods produce two types of particles that differ in their compositions and distributions of particle diameters. The TiC particles pulverized with the mortar and pestle were smaller and contaminated by the material from which the mortar and pestle were manufactured. Furthermore, Ti6Al4V and TiC particles were sintered under two different conditions. Samples sintered under a lower pressure had higher porosity than those sintered under a higher pressure. Owing to these differences, samples comprising the TiC particles pulverized using the ball mill and crusher mill could prevent bubble formation in ground-based electrostatic levitation experiments; in contrast, samples comprising TiC particles pulverized using the mortar and pestle and sintered under a low pressure formed a bubble. In conclusion, the optimal sample preparation process for suppressing bubble formation involves 1) preventing contamination on the surface of the TiC particles, 2) improving the fluidity of TiC or sintering the particles under a high pressure, and 3) reducing the surface area of TiC by increasing the mean diameter.

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Fluid flow and heat transfer in levitated droplets were numerically investigated. Three levitation methods: electro-magnetic levitation (EML), aerodynamic levitation (ADL), and electro-static levitation (ESL) were considered, and conservative laws of mass, momentum, and energy were applied as common models. The Marangoni effect was applied as a velocity boundary condition, whereas heat transfer and radiation heat loss were considered as thermal boundary conditions. As specific models to EML, the Lorentz force and Joule heat were calculated based on the analytical solution of the electromagnetic field. For ADL model, besides the Marangoni effect, the flow driven by the surface shear force was considered. For ADL and ESL models, the effect of laser heating was introduced as a boundary condition. All the equations were nondimensionalized using common scales for all three levitations. Numerical simulations were performed for several materials and droplet sizes, and the results were evaluated in terms of the Reynolds number based on the maximum velocity of the flow in the droplet. The order of magnitude of Reynolds numbers was evaluated as Re ∼ 10⁴ for EML, Re ∼ 10³ for ADL, and Re ∼ 10¹ for ESL. Based on the simulation results, we proposed simple formulas for predicting the Reynolds number of droplet internal convection using combinations of nondimensional numbers determined from physical properties of the material and the driving conditions. The proposed formulas can be used as surrogate models to predict the Reynolds numbers, even for materials other than those used in this study.

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Space debris removal using laser (Laser Debris Removal (LDR)) is a promising method for the removal of debris smaller than 100mm. LDR utilizes the thrust generated by laser ablation. In previous studies, the thrust was measured using pendulums, so thermally and mechanically isolated environments like space debris were not reproduced. To measure the thrust with high resolution and low noise in a debris-like environment, we developed an Electrostatic Levitator (ESL), which electrostatically levitates metal samples and irradiates laser beams. By combining a real-time controllable high-speed camera with image processing methods, positions of a circular sample with 1 mm radius are detected by the 1 μm and levitated within an error of 5μm. As the result of which a levitated aluminum sphere was irradiated with a pulsed laser beam, the sphere exhibited a damped oscillation in the horizontal direction. By fitting the oscillation with the motion of a spring-mass-damper system, the average thrust force generated on the sphere was estimated as 8.0×10−7N.

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