Supersonic Close-coupled Atomization: Experimental Insights

Along with the growing economic importance of metal additive manufacturing by processes such as laser-based powder bed fusion, the demand for high-quality metal powders as the corresponding raw material is also increasing. These powders must have a variety of specific properties and be characterized, among other things, by a well-defined particle size distribution. They are primarily produced by the expensive supersonic close-coupled atomization of molten metals, which often results in a significant proportion of powder that is not suitable for additive manufacturing. In order to nevertheless meet the increasing demand and to increase the economic efficiency of the process, it is necessary to develop predictive modeling capabilities, which allow the process parameters to be specifically adjusted to a desired powder quality. In this thesis, the atomization of liquids by means of a generic supersonic close-coupled atomizer is investigated experimentally. The design of the atomizer is based on a real powder production plant. However, instead of molten metal, various substitute liquids are atomized to improve the accessibility for a variety of measurement techniques. In order to investigate the influence of the operational parameters on the atomization result, measurements are carried out using the phase Doppler measurement technique. This allows for determining the local particle size and velocity as well as their distributions. Insights into the atomization mechanisms are gained by different imaging techniques. The results of this thesis contribute to an improved understanding of the supersonic close-coupled atomization process. They show how the interaction between the gas flow and the liquid flow leads to the formation of the spray. Furthermore, they illustrate how the particle size distribution is influenced by the operational parameters and the physical properties of the liquid. They also provide insights into the mechanisms responsible for primary and secondary atomization. Consequently, this thesis provides a foundation for formulating novel models describing the supersonic close-coupled atomization process