U01 - Generation of Spherical Microscopic Samples with Single Droplet Solidification
Generation of Spherical Microscopic Samples
In subproject U01 a method for the flexible and reproducible generation of spherical micro samples for the method "Farbige Zustände" is developed. The high-temperature single-drop generator established in the project extends the temperature spectrum from 1200 °C to 1600 °C and allows for the first time the application of single-drop processes for steels and other alloys in this temperature range [Link]. For the high-throughput method "colored states", it can be used to produce spherical micro-samples of various diameters of various alloys on aluminum base (600 - 800 °C), copper base (1000 - 1200 °C) and steels (1300 - 1600 °C).
The synthesis of such samples with a high reproducibility requires first of all a high reproducibility of drop formation and separation. The experimental determination and modeling of different drop formation modes is therefore an essential part of the subproject.
After droplet separation, the droplet solidifies rapidly during a free fall in an inert gas atmosphere (nitrogen, argon, helium) over a distance of up to 6.5m before being collected in a liquid quenching medium (usually water or oil) and further cooled. In order to set suitable cooling conditions, a cooling model has been developed which couples the motion of the droplet in the gas atmosphere with its cooling by convection and radiation. It is to be noted here that the correlations available up to now for the drag coefficient of a sphere lose their validity at high temperature differences between the sphere surface and the gas atmosphere. By means of a CFD parameter study, a temperature-dependent correction of the established Schiller-Naumann correlation for the resistance coefficient could be worked out [Link]. It was shown that at high temperature differences the flow around a sphere is strongly influenced by the gas expansion of the surrounding gas, which cannot be represented by a choice of reference temperature for the material values between sphere surface and ambient temperature alone. For heat transfer, on the other hand, the known correlation according to Ranz & Marshall could be confirmed even for high temperature differences.