Analysis of confined spray processes for powder production

Solid particles or powders may be produced in spray ocesses like spray drying or spray polymerizatio n. The tailoring of the particle and powder properties tha t are produced within spray processes is influenced by various unsteady transport processes in the dispersed multi phase spray flow in a confined spray chamber. In th is context differently scaled spray structures in a confined s pray environment have been analyzed. The basic setu p of the study consists of a twin-fluid atomizer central top -spraying in a confined spray chamber. The atomizer gas can be heated up to 500°C, providing enhanced disintegr a ion and drying conditions. In contrast to a tradi tional spray drying process by using heated atomization gas in t he twin-fluid atomizers no superimposed massflow of hot dry air is applied. Thus energy may be saved. Mixing ph enomena of momentum, energy and species in the near region of the nozzle play an even bigger role for thi s ot-gas atomization and drying process than in a conventional spray dryer. Large scale spray structures in enclos ures like: • the oscillation of the spray plume, • the shear flow instabilities at the spray edge, • gas entrainment and recirculation zones and • droplet cluster formation greatly influence the heat and mass transfer for in dividual droplets within the spray cone. These spra y structures are significantly influenced by the nozzle’s operat ional parameters (air-liquid ratio, atomizer gas pr essure, feed fluid, etc.). Another process parameter which has b een scarcely investigated so far is the geometry of the spray chamber. Therefore differently shaped conical spray chamber designs with square cross sections have be en analyzed experimentally and numerically. Numerical simulations of the bounded spray flow hav e been carried out using Reynolds-Averaged-NavierStokes models (RANS) and Large-Eddy-Simulation (LES ) models with lagrangian particle tracking for the droplet motion. The trajectory of droplets/particles in the gas has been tracked with respect to their env onment to estimate the drying conditions. Especially the loca l droplet concentration and the turbulent motion ch aracteristics of the gas phase and its interaction with the dropl ets have been analyzed. While the spray structures outside the spray cone flow like recirculation and entrainment zones of gas can be well predicted by RANS models, the prediction of large scale turbulent structures within the spray cone is realistic to be realized only wit h DetachedEddy-Simulation (DES) and LES models that are more complex due to the modeling and computation of the initialization and propagation of vortex patterns. Synthetic perturbation generators for the inflow co nditions have been applied to induce realistic vortex patterns an d to promote the unsteadiness of the spray flow. In the focus of the LES investigations is the spatial distribution of droplets within the spray. The specific process shows significant clustering of droplets, which is induced by th e coherent flow structures within the spray. The nu merical results show good agreement with Particle-Image-Vel ocimetry (PIV) measurements in a lab scale process under isothermal conditions. The Influence of the spray c hamber design on the droplet/particle-gas interacti ons will be analyzed. The project is part of the SPP 1423 Proze ss-Spray: „Herstellen funktionaler Feststoffpartike ln in Sprühverfahren: Von den Anforderungen an das Pulver und an seine Eigenschaften zum geeigneten Prozess“ funded by the German Research Foundation (DFG). * Corresponding author: lampa@iwt.uni-bremen.de