Anne Geppert

The gas atomization process for metal powder production is strongly influenced by the thermodynamic and fluid-dynamic conditions of the atomizing gas. In this study, stainless steel (316L) powders produced using a close-coupled gas atomizer (CCA) with heated nitrogen and argon were investigated. A systematic experimental campaign was conducted by varying gas type, gas pressure, and gas temperature to analyze their impact on the particle size distribution and morphology. Numerical gas-flow simulations were performed to characterize the dependence of the gas velocity field in the atomization region on the applied process parameters. Based on the experimental and numerical results, classical correlations for characteristic particle size prediction have been adapted for CCA hot gas atomization by introducing an additional temperature scaling term (T_G/T_R)^t. The modified correlations improved the prediction accuracy compared with the original formulations. For the Lubanska correlation, the mean prediction error could be significantly reduced. Furthermore, general linear models and a complementary analysis of covariance (ANCOVA) were used to evaluate the associations of gas temperature, gas type, and gas pressure and the derived flow parameters with the particle size distribution. The combined experimental–numerical approach provides further insight into hot gas atomization of metal melts and supports the optimization of process parameters for tailored metal powder production.

The distribution of metalworking fluid (MWF) during a stationary variant of the vibration-assisted drilling using minimum quantity lubrication is analyzed. Two numerical approaches are employed to simulate the MWF transport: a droplet-based Eulerian-Lagrangian-Eulerian model for spray supply, and a volume-of-fluid method for jet-like MWF delivery. The simulations reveal distinct deposition patterns: spray supply results in a broad wetting of the bore ground from the drill center up to approximately three quarters of the radius, while jet supply leads to wetting of the outermost region. Drill rotation significantly affects jet-based wetting but has minor influence on spray supply. The findings highlight the complementary strengths of both supply strategies and point to an optimization goal for minimum quantity lubrication in vibration-assisted drilling: the tailored combination of spray and jet supply to target specific regions of the chisel edge and cutting edge for improved lubrication and tool performance.

We study silicone oil B3 droplet impacts onto films of the same fluid with submerged cubic structures with side lengths, heights, and separation distances of 0.1, 0.2, and 0.5 mm. The structures are either barely submerged or submerged in a film that rises to 0.2, 0.3, or 0.5 mm above the pillar tops. The impact Weber numbers range between 270 and 1 100. We use high-speed imaging from four perspectives simultaneously to record the experiments. Experimental results are compared with numerical simulations. We show that the underlying structure size can affect the droplet impact process significantly and that the droplet impact morphology is affected differently from either a film on a smooth surface or dry surface structures. Streaks are observed in the side view, which mainly result from azimuthal variations of the crown thicknesses due to differing flow resistance in the crown crater. For barely submerged structures, this causes symmetric crown detachment. Furthermore, we show that the crown height and lifetime depend on the film height above the pillar tops. Finally, a correlation to predict the maximum crown height for fully and barely submerged structures is presented.

Single droplet impacts onto thin wall-films are a common phenomenon in many applications. For sufficiently high impact velocities, the droplet impact process consists of three phases, i.e., initial contact stage, droplet deformation with radial momentum transfer inducing an upward rising lamella, and crown propagation. Here, we present the results of a combined numerical and experimental study focusing on the early dynamics of the impact process. Specifically, the effects of the initial droplet shape, wall-film thickness, and contact line motion are analyzed. Prior to impact, an oblate spheroidal droplet shape was observed. Using direct numerical simulation, we show that the droplet shape affects the impact dynamics only during the first two phases, as it is one of the key parameter influencing the correct prediction of the impact zone. The contact line propagation is described by a square-root-time dependence R¯CL=ατ for both, dry and wetted surfaces. On dry surfaces, the advancement of the contact line is determined by the rolling motion of the truncated droplet. On wetted surfaces, the value of the α-parameter is controlled by two concurrent effects, namely, rolling motion and wall-film inertia. For impact onto thin films, the rolling motion prevails. With increasing wall-film height, the droplet penetrates into the soft substrates and wall-film inertia becomes the controlling factor. These insights into the early impact dynamics on wetted surface are important for the formulation of a unified modeling approach.