Heteroaggregate Formation Dynamics in the Dispersed Phase – Modeling, Simulation, and Experiments

• Applicants: Professor Dr. Sebastian Kaiser; Dr.-Ing. Irenäus Wlokas
• Subject Area: Mechanical Process Engineering ; Chemical and Thermal Process Engineering ; Fluid Mechanics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 462475304

Hetero-aggregates are composite particles formed by two or more phases. A possible process route is via mixing of aerosols. The functionality of hetero-aggregates is determined by size, crystallinity, and composition of the primary particles, as well as the morphology of the aggregate and quality of the hetero-contacts. The formation of hetero-aggregates from aerosol particles is determined by the aerosol mixing and the collisions of the particles therein. However, for nanoparticles the observability of hetero-aggregate formation is limited due to their small size, and they exhibit transport properties that are challenging for a simulation. This project investigates the mixing of aerosol streams and the ensuing nanoparticle hetero-aggregation in flow experiments via in situ laser diagnostics and in numerical simulations of transport and particle dynamics. The principal flow configuration is an axisymmetric jet in a co-flow. Each of these two streams carries a different particle species. The jet can be laminar with a repeatable vortex ring created by periodic perturbations in the jet exit velocity (in the experiment, with a loudspeaker in the flow path to the jet), or, at higher flow velocities and without acoustic forcing, it can be a classic, round, turbulent jet. Mainly two aerosol material systems are considered: a system allowing particle-species-specific optical detection but without functionality of the hetero-aggregates, and TiO2/graphene nanoparticles - a system that is more challenging for the measurements but creates hetero-aggregates with a function: electrocatalytic activity. In the first system, laser-induced phosphorescence is used to image the distribution of the two particle species in the flow. They are tracked in the flow in additional experiments with high spatio-temporal resolution. In the second (functional-particle) system, we explore what methods can be employed to optically distinguish the two particle species to allow similar measurements as in the first (non-functional-particle) system. Additionally, in-situ and ex-situ characterization of the hetero-aggregates provides detailed information about the morphology and quality of hetero-contacts. The experiments are complemented by numerical investigations. In the first step, three-dimensional direct numerical simulation (DNS) of generic mixing layers is performed, later, large eddy simulations (LES) of the fully turbulent mixing using models derived from the DNS. The transport simulations are fully coupled to population balance equations for aggregate number concentration, volume, surface area, and average composition. The number and quality of hetero-contacts is computed from Monte Carlo simulations, with some of the models based on the measurements in experiments.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2289: Creation of synergies in tailor-made mixtures of heterogeneous powders: Hetero aggregations of particulate systems and their properties
• Co-Investigator: Professor Dr.-Ing. Andreas Kempf