Determining the efficiencies of airborne droplet collectors in spray drift analysis using CFD.

Type of content
Theses / Dissertations
Publisher's DOI/URI
Thesis discipline
Mechanical Engineering
Degree name
Master of Engineering
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Volume Title
Williams, Reid Thomas

Airborne sampling collectors are commonly used in spray drift analysis to measure the concentration of an airborne product, such as pollen or spray, by collecting the mass of the product during the sampler's operation. To help improve the accuracy of spray drift analysis, the collection efficiencies of airborne droplet sampling collectors were evaluated using computational fluid dynamics (CFD). The collection efficiency is defined as the ratio of the measured concentration to the actual concentration of the airborne product in the sampled air. This study aimed to evaluate the effectiveness of the string, flat plate, and Rotorod air sampling collectors for spray drift analysis. The collection efficiency of each sampler was modelled for a range of droplet sizes (25 − 1000 μ𝑚) and atmospheric boundary layer (ABL) conditions (𝑧0=0.02 − 3.08 𝑚), which resulted in a mean airspeed range observed at the height of the samplers of 0.340 – 5.120 m/s. The Stokes number was used as the metric for analysis, and measurements of the collection efficiency were obtained using Lagrangian particle tracking on precomputed flow fields. The results demonstrated that the string sampler exhibited more consistent collection efficiencies across the simulated Stokes range in comparison to the other samplers. Additionally, it demonstrated good agreement with prior correlations and expressions, whereas the flat plate and Rotorod deviated from their respective correlations. Collection efficiencies exceeding 100% were observed for all samplers, particularly at high Stokes numbers, which had not been previously explored. This was attributed to the samplers processing a larger area of upstream flow due to their influence on the surrounding flow field, as well as the trajectory of the spray cloud through the sampling region. When processing the results through an impaction splashing model to account for mass collection losses, the certainty of the sampler's measurements significantly reduced when the splashing criterion was reached. This effect was most pronounced with the Rotorod, which reached the splashing criterion at a low Stokes number due to its rotational motion. This study contributes to a better understanding of the performance of different air sampling collectors, which can aid in improving the measurements of spray drift and spray control measures.

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