Two-Fluid Atomization and Spray Impact
Ramona Strob  1@  , Tejas Babaria  1@  , Gerhard Schaldach  1@  , Markus Thommes  1@  
1 : TU Dortmund University, Laboratory of Solids Process Engineering

The possible applications of the spray-impact are versatile. For medical inhalers, in flame spectrometers or included in a spray dryer, the spray-impact can be used to produce smaller droplets in the small micrometer range. The mechanisms and phenomena after a single-droplet or spray-impact emerging from a single-fluid nozzle were examined by several researchers and the droplet formation mechanisms are well-understood.This work focuses on the combination of a two-fluid nozzle and the spray-impact on a sphere. A two-fluid nozzle already produces a small amount of fine droplets (>5 µm) under moderate pressure conditions. However, the distribution is usually broad. Spray-impact can be used to increase the mass flow of the fine fraction. In addition, with two-fluid nozzles, the droplets are directly dispersed into the gas flow. The spray-impact was investigated experimentally and the influence of the atomizing parameters on the impact outcome was examined. Two in-house build two-fluid nozzles with internal mixing are applied which show a good geometrical similarity. With the small-scale nozzle, an aerosol with a mass median droplet diameter of about 12 µm was produced under moderate conditions (Δp=5 bar). A multimodal distribution was measured after impact on a sphere (diameter 11 mm). However, an increased mass flow rate of small droplets (2 and 3 µm) was observed after impact for low liquid-to-gas mass flow ratios. For a detailed evaluation of the spray-impact outcome, a large-scale nozzle was applied, which produced an aerosol with a mass median diameter of about 80 µm (Δp=1 bar). In the large-scale set-up, a sphere with a diameter of 55 mm was used. After impact, the droplet size distribution was analysed at different regions around the sphere. There, the measurement position around the sphere after impact has no effect on the droplet size distribution. Compared to the aerosol before impact, the droplet size was reduced by the spray-impact. In addition, a smaller droplet size for decreased liquid-to-gas mass flow ratio and increased gas pressure was measured. The liquid film produced on the sphere surface during the impact influences the impact outcome. A characteristic film could be visualized during the experiments with the large-scale nozzle. However, a large variation in thickness was determined. Finally, different droplet formation mechanisms such as splashing and crown formation were observed. The influence of different impact conditions on the mechanisms were analysed. The phenomena and mechanisms were compared to the investigations observed with single-fluid nozzles described in the literature.

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